Chromotograph Analyzer
Short Description
Design...
Description
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Evaluate Gas Chromatograph Analyzers
Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning : The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.
Chapter : Instrumentation File Reference: PCI10803
For additional information on this subject, contact J.R. Van Slooten on 874-6412
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Content
Page
INTRODUCTION ............................................................................................................................ 1 EVALUATING GAS CHROMATOGRAPH ANALYZERS............................................................. 2
Function of Gas Chromatograph Analyzers................................................................ 2 Separation Theory ......................................................................................... 3 Practical Implementation................................................................................ 7 General Applications ................................................................................................. 8 Refineries....................................................................................................... 8 Gas Separation Plants ...................................................................................11 HARDWARE ELEMENTS AND COMPONENT SEPARATION TECHNIQUES OF GAS CHROMATOGRAPH ANALYZERS .............................................................................. 15
Gas Chromatograph Analyzer Configuration.............................................................15 Oven/Temperature Control.......................................................................................16 Isothermal.....................................................................................................16 Programmed Temperature.............................................................................16 Carrier Gas Supply System .......................................................................................17 Carrier Gases ................................................................................................17 Flow/Pressure Regulation .............................................................................18 Detectors .................................................................................................................19 Thermal Conductivity Detectors....................................................................20 Ionization Detectors......................................................................................24 Flame Ionization ...........................................................................................24 Photo ionization............................................................................................26 Flame Photometric........................................................................................27 Electron Capture...........................................................................................27 Columns...................................................................................................................28 Adsorption Columns .....................................................................................29 Partition Columns .........................................................................................29 Packed Columns ...........................................................................................30 Capillary Columns.........................................................................................30 Valves ......................................................................................................................30
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Functions......................................................................................................30 Types of Sample Valves................................................................................32 Electronic Controllers...............................................................................................35 Timing..........................................................................................................36 Measurement Data Processing Functions ......................................................36 System Diagnostics......................................................................................42 Component Separation Techniques...........................................................................44 Backflush......................................................................................................44 Heartcut .......................................................................................................46 Column Stepping ..........................................................................................47 Trap/Bypass..................................................................................................48 Programmed Temperature.............................................................................50 GAS CHROMATOGRAPH NETWORK CONFIGURATIONS ..................................................... 52
Gas Chromatograph/Process Control Computer Interface.........................................52 Data Transmission ........................................................................................52 Hardware Variations.....................................................................................54 Practical Considerations of Serial Communication Techniques ......................57 Use of Personal Computers ......................................................................................59 Data Processing and Reporting .....................................................................59 SQC Techniques ...........................................................................................60 Network Capabilities ................................................................................................62 Control Capabilities ......................................................................................62 Architecture and Operation of Various Networks..........................................63 Network Integrity and Redundancy...............................................................65 Network Topology .......................................................................................66 Multi-Analyzer Integration............................................................................67 INSTALLATION, OPERATIONAL, AND MAINTENANCE CONSIDERATIONS...................... 69
Installation Considerations........................................................................................69 Installation Data (Items 7 through 20)...........................................................69 Operational Considerations.......................................................................................73 Sample Supply Data (Items 21 through 28)...................................................73 Process Data (Items 29 through 38) ..............................................................77
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Sample Return Data (Items 39 through 46) ...................................................79 Analyzer Return Data (Items 47 through 50).................................................80 Output Signal Data (Items 51 through 54) ....................................................80 Performance Data (Items 55 through 58) ......................................................81 Special Data Section (Items 69 through 72) ..................................................85 Stream Composition Data (Item 73)..............................................................85 Maintenance Considerations .....................................................................................86 Calibration Data (Items 59 through 62).........................................................86 Maintenance Data (Items 63 through 68) ......................................................87 EVALUATING GAS CHROMATOGRAPH ANALYZERS IN RELATION TO REQUIREMENTS FOR SPECIFIC APPLICATIONS.................................................................... 90
Refinery Application.................................................................................................90 Fractionation Tower .....................................................................................90 REFERENCES ............................................................................................................................... 98 WORK AID 1: RESOURCES USED TO EVALUATE GAS CHROMATOGRAPH ANALYZERS IN RELATION TO REQUIREMENTS FOR SPECIFIC APPLICATIONS........................................... 99
Work Aid 1A: Resources Used to Evaluate the Instrument Air Pressure and Quality for Gas Chromatograph Analyzers ..................................99 Work Aid 1B: Resources Used to Evaluate the Measurement Response Time for Gas Chromatograph Analyzers..........................................100 Work Aid 1C: Resources Used to Evaluate the Phase of the Process Sample for Gas Chromatograph Analyzers.......................................101 GLOSSARY ................................................................................................................................. 102 ADDENDUM A: GUIDELINES FOR EVALUATING PROCESS ANALYZERS........................ 104
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List Of Figures
Page
Figure 1: Basic Chromatography Process ................................................................ 1 Figure 2: Types of Chromatography........................................................................ 1 Figure 3: Block Diagram of a Typical Gas Chromatograph Analyzer ....................... 2 Figure 4: Establishment of Equilibrium Between a Gas Sample and a Liquid Solvent ................................................................................ 3 Figure 5: Simplified Example of a Single Sample Gas Component in a Column........ 4 Figure 6: Plot of Component A Equilibria vs. Number of Theoretical Plates (13) ..... 5 Figure 7: Typical Chromatogram............................................................................. 6 Figure 8: Gas Chromatograph ................................................................................. 7 Figure 9: Applications of Gas Chromatograph Analyzers......................................... 8 Figure 10: Typical Distillation Tower Process Flow Diagram .................................... 9 Figure 11: Typical Gas Chromatograph Analyzer in a Gasoline Blending System......11 Figure 12: Expander Gas Plant.................................................................................12 Figure 13: Configuration of Gas Chromatograph Analyzer .......................................15 Figure 14: Isothermal and Programmed Temperature Profiles ..................................16 Figure 15: Properties of Common Carrier Gases(3) ..................................................17 Figure 16: Recommended Carrier Gas Cylinder Installation......................................18 Figure 17: Guide to the Selection of Gas Chromatograph Detectors.........................19 Figure 18: Thermal Conductivities of Typical Gases - k in [cal/(sec)(cm2)(°C/cm) x 10-6] at 38° C..................................................20 Figure 19: Two Thermal Conductivity Detectors in a Gas Chromatograph Analyzer .................................................................20 Figure 20: Wheatstone Bridge..................................................................................21 Figure 21: Thermistor Bead Detector.......................................................................22 Figure 22: Filament Elements in a Measuring Cell Block(5)......................................23 Figure 23: Ionization Detector(4) .............................................................................24 Figure 24: Compounds That Give Little or No Response to the Flame Ionization Detector(4) ..................................................................24 Figure 25: Flame Ionization Detector(4) ..................................................................25
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Figure 26: Methanator .............................................................................................25 Figure 27: Air Treater..............................................................................................26 Figure 28: Photo Ionization Detector .......................................................................26 Figure 29: Schematic of a Flame Photometric Detector ............................................27 Figure 30: Electron Capture Detector ......................................................................28 Figure 31: Enlargement of Support and Liquid Stationary Phase ..............................29 Figure 32: Packed Column.......................................................................................30 Figure 33: Atmospheric Referencing Valve (ARV)...................................................31 Figure 34: Typical Rotary Valve ..............................................................................32 Figure 35: Typical Slider Valve for Gases (1) ...........................................................33 Figure 36: Slider Valves for Liquids(1) ....................................................................33 Figure 37: Six-Port Plunger Diaphragm Valve..........................................................34 Figure 38: Transport Injection Valve........................................................................35 Figure 39: Peak Detection........................................................................................36 Figure 40: Slope Gating ...........................................................................................37 Figure 41: Example of Baseline Shift........................................................................38 Figure 42: Selecting Bottom Line of Peak ................................................................38 Figure 43: Identification of Unknown Peaks by Use of Standard ..............................39 Figure 44: Shoulder Peak.........................................................................................40 Figure 45: Peak Resolution ......................................................................................41 Figure 46: Illustration of Column and Solvent Efficiency(4) .....................................42 Figure 47: Diagnostic Functions of Electronic Controller .........................................43 Figure 48: Ideal Backflush(1) ...................................................................................44 Figure 49: Real Backflush(1)....................................................................................45 Figure 50: Valve Configuration for Backflush to Vent Using a Single Column(4) .....45 Figure 51: Backflush to Vent Using Two Columns...................................................46 Figure 52: Heartcut Procedure .................................................................................47 Figure 53: Reverse Column Step..............................................................................48 Figure 54: Trap/Bypass............................................................................................49
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Figure 55: Constant Temperature Technique............................................................50 Figure 56: Programmed Temperature.......................................................................50 Figure 57: Schematic of Programmed Temperature Gas Chromatograph Analyzer ...51 Figure 58: Analog Circuit.........................................................................................52 Figure 59: Basic Serial Representation .....................................................................54 Figure 60: Voltage Amplifier for Serial Communication ...........................................56 Figure 61: Pareto Chart............................................................................................57 Figure 62: Normal Distribution ................................................................................61 Figure 63: Typical Analyzer Network.......................................................................62 Figure 64: Typical Master/Slave Network ................................................................64 Figure 65: Typical Masterless Network....................................................................65 Figure 66: Network Topology..................................................................................66 Figure 67: Electronics Interface Device....................................................................68 Figure 68: Ras Tanura Plant 40 Depropanizer Process Flow Diagram ......................91 Figure 69: Instrument Specification Sheet for an Gas Chromatograph Analyzer........92 Figure 70: Typical Analyzer Manufacturer’s Data Sheet for a Gas Chromatograph Analyzer...............................................................93 Figure 71: Typical Process Gas Chromatograph Analyzer Specific Application Data Sheet for Depropanizer Product Application ................94 Figure 72: Depropanizer Overhead Application Gas Chromatograph Analyzer Sample Handling System Diagram ............................................95 Figure 73: Process Analyzer Evaluation Flow Chart ...............................................104 Figure 74: Instrument Specification Sheet for Process Analyzers............................107
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INTRODUCTION Chromatography is the physical process of separating the various components of a gas or vapor mixture into pure fractions of each component. The chromatography process is a batch process in which a small sample of a multicomponent mixture is transported by a mobile phase through a long, narrow tube called a column (Figure 1). The column contains an absorbent material called the stationary phase. As the components in the sample mixture migrate through the column, they are separated by the stationary phase based on the differences in their chemical and physical properties. The sample exits the column as individual components, which are grouped together.
Figure 1: Basic Chromatography Process
There are four types of chromatography, all of which are classified by the types of mobile and stationary phases that they use (Figure 2). The mobile phase may be a liquid or a gas. The stationary phase can be a liquid or a solid. This module will focus on gas-liquid chromatography, which will be simply referred to as gas chromatography. Mobile Phase Liquid Liquid Gas Gas
Stationary Phase Liquid Solid Liquid Solid
Figure 2: Types of Chromatography
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EVALUATING GAS CHROMATOGRAPH ANALYZERS Function of Gas Chromatograph Analyzers The function of gas chromatograph analyzers is to separate and analyze very small amounts of components that are in multicomponent streams. The analysis is both quantitative and qualitative. A quantitative analysis determines now much of each component is in the sample. A qualitative analysis provides information about the identity of the components. Figure 3 shows a block diagram of a typical gas chromatograph analyzer. The carrier gas carries the sample through the column and to the detector. The sample valve collects a precise measured amount of sample and injects the sample into the carrier gas stream. The column separates the sample into individual components. The detector, which is located at the end of the column, senses the individual components or bands as they elute off the column and pass through it. The detector generates a signal, which is amplified and plotted as a chromatogram.
Figure 3: Block Diagram of a Typical Gas Chromatograph Analyzer
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Separation Theory
Gas chromatography involves the use of an inert carrier gas to transport a multicomponent sample past a stationary phase (non-volatile solvent) in the column. The sample is partitioned between the carrier gas and the solvent. The solvent retains each sample component for a period of time based on the component’s solubility in the solvent. In order to understand the interaction between the sample and the solvent, it is necessary to review how gases dissolve in liquids. Figure 4A shows the introduction of a pure gas sample into a closed vessel that contains air and a liquid solvent. In Figure 4B, the gas sample dissolves in the solvent to a point of equilibrium where the tendency for more gas to dissolve is balanced by the tendency for some of the dissolved gas to come out of solution.
Figure 4: Establishment of Equilibrium Between a Gas Sample and a Liquid Solvent
The ratio of the amount of the gas in the air and the amount of gas in the liquid is called the partition coefficient, K, where K=
Concentration of gas in the liquid phase Concentration of gas in the gas phase
K indicates the degree of solubility of the gas. In Figure 4, it was assumed that K=1, that is, the gas sample is evenly distributed in the air and liquid phases. If more gas is introduced into the vessel, half of the gas would dissolve to re-establish equilibrium (up to the point of saturation).
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The situation inside of a gas chromatograph column is much more complex than the previous example because the gas sample contains several components and the sample is constantly transported over the stationary phase by a carrier gas. It is necessary to describe what happens to one of the gas components (Component A) in order to understand how all of the components in the sample are separated. To understand how the column operates, imagine the column as a series of closed vessels (Figure 5). Assume that the partition coefficient of Component A in the solvent is KA=1 (equally soluble in the carrier gas and solvent). (1) After the carrier gas pushes Component A into the first sealed vessel, Component A is allowed to reach equilibrium between the carrier gas and the solvent (2). After equilibrium is reached, the carrier gas is allowed to push the remaining sample in the gas phase into the next vessel (3). After the carrier gas pushes the sample out of the first vessel, equilibrium between the gas and liquid phase is established in both the first and second vessels (4). The carrier gas pushes the sample component from the first and second vessels into the second and third vessels (5). Equilibrium is re-established in the three vessels (6). Notice that the component concentrations in the leading and trailing vessels are lower than the concentration in the intermediate vessel.
Figure 5: Simplified Example of a Single Sample Gas Component in a Column
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The imaginary vessels in this example are analogous to the number of theoretical plates, which was originally proposed to model the performance of distillation columns. If the procedure in Figure 6 is allowed to continue over five vessels (theoretical plates), the results would be as shown in Figure 6. The equilibria for the five theoretical plates are shown graphically as the solid line.
Figure 6: Plot of Component A Equilibria vs. Number of Theoretical Plates (13)
If the procedure is repeated over 11 and 21 steps, the resulting concentrations would appear as the dashed lines in Figure 6. In each case, the sample component is distributed through all the plates with the maximum concentration at the center plate. Notice that the distribution curve becomes narrower as the number of theoretical plates increases. The narrower the peak, the more efficient the separation of the Component A. For large numbers of plates, the center of the distribution curve starts to resemble the characteristic shape of a chromatogram peak (Figure 7).
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Figure 7: Typical Chromatogram
The greater the solubility of each component in the sample, the longer its retention time, tR, in the column. Thus, the sample is separated into individual components as it flows through the column. The width of the peak, W, is length of the base line that intersects the two tangents to the peak. The width of the curve at one-half the height of the peak is represented by the variable W0.5. The number of theoretical plates that are required for separation is calculated from the chromatogram measurements by using the expression: t N = 5.54 R W0.5
2
where: N tR W0.5
= = =
number of theoretical plates retention time of the component the width of the peak at half height
The column efficiency is expressed best as a quantity called the plate height, or the height equivalent to a theoretical plate, H, which has a dimension of distance. The plate height is related to the peak width from the chromatogram; therefore, it is possible to calculate the plate height as shown in the expression: L W H = 0.5 16 t' R
2
where: H L W0.5 t'R
= = = =
plate height length of the column peak width at half the peak height the retention time corrected for transit time of an unabsorbed solute
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Practical Implementation
Figure 9 shows an analysis of a C3-C5 blend, which might be used in a refinery. The separation in this example is good. The column is 3.5m in length, N was calculated to be 1498, and H was calculated to be 1.001 mm. The analysis time, or cycle time, for this sample is 3 minutes. Complete component separation is sometimes sacrificed for cycle requirements of the process.
Figure 8: Gas Chromatograph
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General Applications Gas chromatograph (GC) analyzers measure the percent concentration or ppm of the components that make up gas streams, equilibrium vapor streams, and liquid streams that can be vaporized without affecting the stream composition. This information is used for process monitoring and process control. Some of the applications of gas chromatograph analyzers are shown in Figure 10 and they are described on the following pages.
Category
Applications
Refineries
Distillation towers, fuels blending
Gas Separation Plants
Product quality control, measurement of BTU content
Figure 9: Applications of Gas Chromatograph Analyzers
Refineries Distillation Towers - One of the most common applications for a process gas chromatograph is to
provide compositional data to the control system of a distillation tower. A distillation tower is used to separate a chemical process stream into two or more product streams. Figure 11 shows a diagram of a crude distillation unit, which separates crude oil into various blends by distilling the crude into fractions according to boiling point. First, a stabilizer fractionates to total crude to remove butanes and lighter components. Second, the stabilized crude is fed to a distillation tower which operates at atmospheric pressure. The overhead stream from the tower, light straight run naphtha, is used in gasoline blending or it is further processed in an isomerization unit. The bottoms product from the tower is fed to a vacuum crude tower.
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Figure 10: Typical Distillation Tower Process Flow Diagram
In order of increasing boiling points, the main products (side draws) from a typical distillation tower are as follows: Fuel Gas
The fuel gas consists mainly of methane and ethane. In some refineries, propane is included in the fuel gas stream. This stream also called a "dry gas." (C1, C2, C3).
Wet Gas
The wet gas stream contains propanes and butanes as well as some methane and ethane. The propanes and butanes are separated to be used for LPG and, in the case of butane, for gasoline blending. (C2, C3, C4).
LSR Gasoline
The Light Straight Run (LSR) gasoline is desulfurized and used in gasoline blending or processed in an isomerization unit to improve the octane number before blending. (C5, C6).
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HSR Gasoline
The Heavy Straight Run (HSR) gasoline cuts are typically used for catalytic reformer feed to produce high octane reformate for gasoline blending and aromatics recovery. (C7 to C10).
Kerosene
The kerosene stream is treated and then sent to the blending pool for sale as kerosene product. (C9 to C15).
Diesel
The diesel cut is treated and then sent to the bending pool for sale as diesel fuel. (C13 to C18).
Gas Oils
The Light Gas Oil (LGO) and Heavy Gas Oils (HGO) are processed in a hydrocracker or catalytic cracker to produce gasoline, jet, and diesel fuels. The light vacuum and heavy vacuum gas oils (LVGO and HVGO) can also be used as feedstocks for lubricating oil processing units. (C13 to C45 ).
Residuum
The vacuum tower bottoms can be processed in a visbreaker coker, or deasphalting unit to produce heavy fuel oil or cracking and/or lube base stocks. For asphaltic crudes, the residuum can be processed further to produce road and/or roofing asphalts. (C40 and up). Analyzer Placement in Crude Oil Distillation Unit
Analyzer No. 1
Stream
2
HSR Naphtha
3
Kerosene
4
Diesel
LSR Naphtha
Component Analyzed Simulated Distillation Simulated Distillation Simulated Distillation Simulated Distillation
Purpose Insure proper separation of LSR Naphtha from lighter material. Insure proper separation of HSR and kerosene. Insure proper separation of kerosene from naphtha and diesel. Insure proper separation of diesel from kerosene and gas oil.
Fuels Blending - Gasolines that are produced by straight-run distillation, catalytic cracking,
hydrocracking, and reforming must be blended into various grades of products that will perform well under varying weather conditions, altitudes, and engine compressions. The gasoline blending unit blends the various upstream products according to formulations to meet these product demands.
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The critical properties specified for gasoline are vaporization and combustion. These properties are measured as the Reid Vapor Pressure (RVP), Vapor to Liquid Ratio (V/L), and Knock Intensity or Octane Number. These measurements have traditionally been made with physical property analyzers; however, their high maintenance requirements and imprecise results limit their effectiveness. The measurements can now be made with a programmed temperature GC analyzer equipped with fused silica open tubular capillary column and utilizing a flame ionization detector. This GC analyzer measures gasoline components by separating them according to their boiling points. The boiling points are related to the volatility of the gasoline, which is the key property for combustion. Although the distillation boiling points do not provide complete information, they do provide an indication that can be used to ensure that the key parameters are met. The 10% off boiling point is related to the RVP of the final product. The 50% off boiling point is related to the V/L point. A typical example of a gas chromatograph analyzer in a 10% off boiling point application is shown in Figure 12.
Figure 11: Typical Gas Chromatograph Analyzer in a Gasoline Blending System
Gas Separation Plants Product Quality Control - When process analyzers are used to determine whether or not a process
stream conforms to product specification, the application is classified as product quality control. Although final certification of a product is usually made by laboratory analyzers, on-line analysis of product quality control avoids potential off-spec operation.
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In many instances, process are operated to produce product that has a higher quality than is required by specification in an effort to prevent off-spec product. Producing higher quality product leads to more costly operation and results in product giveaway. Product quality control with on-line process analyzers allows more efficient process operation not only by maintaining onspec operation but also by minimizing product giveaway. Expander gas plants (Figure 13) are designed to recover ethane in large quantities for feed stock to ethylene plants. Typically, expander gas plants take gas out of a natural gas pipeline and return the residue gas to the pipeline. The plants are billed for shrinkage, which is the BTU decrease between the feed and the returned residue gas. The residue gas must be returned at the same or higher pressure as it was taken out of the pipeline. The typical expander plant has three sites for analysis by a process gas chromatograph. The first analysis site is the feed to the plant in order to determine the composition of the feed gas and to calculate its BTU content, which is shown in the following example. After the analysis, the gas is compressed from about 500 to 800 psi pressure. A valve splits the gas to either a reboiler medium to warm up the demethanizer or to feed the demethanizer.
Figure 12: Expander Gas Plant
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The gas in the feed line to the demethanizer is cooled as it goes through a cold gas to gas heat exchanger. The gas then goes through an inlet separator to remove condensed liquids and through an expander to quickly drop its pressure to about 150 psig. At this point, the gas is extremely cold (-100°F) and liquids begin to form. These liquids are fed into the top of the demethanizer tower. The warmer liquids that condensed in the inlet separator are fed into the middle of the tower. The overhead product stream that leaves the top of the tower is mostly methane with some ethane. After the overhead stream exchanges heat with the feed gas, it is recompressed and trimmed to the exact pressure of the gas pipeline. The plant tries to produce an ethane and heavier fraction with a certain methane to ethane ratio spec going to the NGL product pipeline. Consequently, the most important analysis site is the Demethanizer bottoms in order to control the methane to ethane ratio. The overhead stream is the final sample site in the gas plant where the ethane and the BTU content of the gas is measured. The ethane measurement shows how well the plant is operating and the BTU content of the gas determines the shrinkage across the plant. A number of plants will simply try to use a calorimeter to measure the BTU content of the gas; however, the BTU content of the gas can be changed by the amount of nitrogen or carbon dioxide in the gas. A process chromatograph has a distinct advantage since it both determines composition and measures the BTU of the overhead stream. Measurement of BTU Content - Gas chromatographs provide on-line measurement of the BTU
content of gas mixtures by separating and measuring the concentrations and heating values of all the components in the sample. The ideal BTU content of the gas mixture is calculated by totaling the product of the mole fraction of each component and its ideal BTU value. A sample calculation is shown below.
Component Nitrogen Carbon Dioxide Methane Ethane Propane iso-Butane n-Butane iso-Pentane n-Pentane Hexane +
Concentration (mole %) 2.39 2.53 85.83 4.99 2.51 0.50 0.50 0.25 0.25 0.25 100.00
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Mole Fraction 0.0239 0.0253 0.8583 0.0499 0.0251 0.0050 0.0050 0.0025 0.0025 0.0025 1.0000
BTU Value (BTU/ft3) 0.0 0.0 1012.1 1773.0 2523.3 3260.7 3269.8 4009.7 4018.9 0323.6
BTU (BTU/ft3) 0.0 0.0 868.7 88.5 63.3 16.3 16.3 10.2 10. 1 13.3 1086.7 BTU/ft3
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The BTU value is referred to as "ideal" because it has not been corrected for errors that are caused by the less than ideal behavior of gases. Since some gases are more compressible than others, the compressibility of the individual gases needs to be accounted for in order to get a more accurate BTU value. The compressibility factor (or Z factor) is then applied to the ideal BTU to arrive at a real BTU. The sample calculation below assumes a Z factor of 0.9974. Real BTU =
ideal BTU 1086.7 = = 1089.5BTU/ft 3 Z factor 0.9974
Gas chromatographs may perform satisfactorily when they are used to measure simple gas mixtures like natural gas; however, complex mixtures require longer cycle times. In the case of BTU control systems for furnaces and boilers, long cycle times are not acceptable.
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HARDWARE ELEMENTS AND COMPONENT SEPARATION TECHNIQUES OF GAS CHROMATOGRAPH ANALYZERS Gas Chromatograph Analyzer Configuration The configuration of a typical gas chromatograph is shown in Figure 13. The electronics section controls the automatic timing of the analyzers batch process, and it controls the sample system. With a smart sample system, the electronics section can monitor its condition. The chromatograph oven, which is controlled by the electronics section, can either be isothermal or have a programmed temperature ramp. The oven is controlled to maintain the temperature of the sample valve, the columns, and the detector. The sample conditioning section is one of the most critical parts of the analyzer system. An improperly designed sample system is one of the primary reasons for analyzer failures and high maintenance costs. With modern electronics, the sample system is being monitored so that if a failure occurs, the sample system is shut down to protect the analyzer. Carrier gas, which is usually stored in pressurized cylinders, is supplied to the analyzer through a two-stage regulator.
Figure 13: Configuration of Gas Chromatograph Analyzer
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Oven/Temperature Control The operation of the oven is critical to the reproducible and reliable operation of the analyzer because it has a dramatic effect on the retention times and column performance. The temperature of the oven is also critical to the components of the oven. The sample valve’s temperature must be hot enough to vaporize a liquid sample quickly, but not hot enough to thermally decompose the sample. The column temperature should be hot enough to achieve the desired component separation within the desired cycle time; however, the temperature should be low enough to extend the life of the columns. The detector’s temperature must be hot enough to avoid condensation of the sample. The oven temperature can be maintained at a constant value (isothermal), or the oven temperature may be increased over time (programmed temperature). The operations of isothermal ovens and programmed temperature ovens are described below. Isothermal
The isothermal oven must maintain a constant temperature (Figure 15) over the entire column, and it should be able to maintain the temperature to ±0.1°C. The temperature gradient (the difference between the highest and lowest mean temperatures) should be kept to a minimum. The operating range of the oven should be from 5-10° above ambient up to 400°C. Programmed Temperature
Temperature programming is used to separate gas components with a wide range of boiling points. The oven temperature is gradually increased after the sample is injected to speed up the elution of the components with high boiling points. The programmed temperature technique is used mostly in the laboratory; however, some manufactures offer the option for process GCs. The column is heated at rates that vary from 0.25°C/min to 20°C/min (Figure 14). The oven should be able to heat the column from ambient temperature to 400°C within 40 minutes. The programmed temperature oven should be able to maintain the actual temperature to ±2°C of the desired temperature.
Figure 14: Isothermal and Programmed Temperature Profiles Saudi Aramco DeskTop Standards
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Carrier Gas Supply System The function of a carrier gas system is to supply a clean, dry, and constant pressure of carrier gas to the analyzer. The choice of carrier gas primarily depends on the type of detector that is used (although for best column efficiency nitrogen would be preferred). For example, thermal conductivity detectors require the use of helium of hydrogen because they provide the maximum thermal conductivity difference (sensitivity) between the carrier and the sample components. Nitrogen is used when hydrogen in the sample needs to be measured, which provides maximum sensitivity for the detector. When hydrogen needs to be measured along with other components, two carrier gases (nitrogen and helium or hydrogen) and two detectors are recommended. Nitrogen is for the hydrogen measurement, and helium or hydrogen is for measurement of the other components. Carrier Gases
Carrier gases carry the sample components from the sample valve through both the column and the detector. The goal of the carrier gas is to provide a stable transport and detection medium for the sample components. The carrier gas should be: • • •
inert to avoid reaction with the sample or solvent pure and dry appropriate for the detector
The properties of common carrier gases are shown in Figure 15. Helium, hydrogen, and nitrogen are the most commonly used carrier gases.
Argon Carbon dioxide Helium Hydrogen Nitrogen Oxygen a At 99.6°C b At 100.5°C c At 99.74°C
Molecular Weight 39.95 44.01 4.00 2.016 28.01 32.00
Thermal Conductivity λ x 105 at 100°C (g-cal/sec-cm-°C) 5.087 5.06 39.85 49.94 7.18 7.427
Viscosity η x 10-6 100°C (µP) 270.2a 197.2 234.1 104.6b 212.0 248.5c
Figure 15: Properties of Common Carrier Gases(3)
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Flow/Pressure Regulation
Reliable operation of the gas chromatograph is dependent on a carrier gas flow rate that is accurately controlled. Varying carrier flow rates will effect retention times of the components as well as affect the stability of the detector. Pressure regulators that are not affected by ambient temperature changes should be used. Carrier gas pressure should be supplied to the analyzer at about 15 psi above column head pressure to provide optimum conditions for the pressure regulator at the analyzer. Two cylinders of carrier gas should be used for each analyzer, and they should be installed as shown in Figure 16. The cylinders are installed so that one cylinder can be replaced without affecting the carrier gas flow through the analyzer.
Figure 16: Recommended Carrier Gas Cylinder Installation
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Detectors The detector responds to the gases that emerge from the column. In practice, detectors operate on a deferential principal; that is, they show a constant base line with carrier gas flowing, generate a signal as the components pass, and then return to baseline. The signal that is generated is used to measure the amount of component that passed through the detector. Due to their stability in a harsh environment and their sensitivity to a wide range of components, the following detectors have become the limited choices in process gas chromatographs: • • • • •
thermal conductivity detectors (TCD) flame ionization detectors (FID) photo ionization detectors (PD) flame photometric detectors (FPD) electron capture detectors (ECD)
Thermal conductivity and flame ionization detectors are most commonly used. The type of detector that is selected is based on the type and concentration of the components to be measured. The detector is usually selected by the analyzer vendor. Figure 17 shows a guide that is used for selecting a detector based on the type and concentration of the component. All five detectors will be explained on the following pages.
Figure 17: Guide to the Selection of Gas Chromatograph Detectors
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Thermal Conductivity Detectors
Thermal conductivity is the ability of a substance to transmit heat by conduction. Thermal conductivity detectors use this thermodynamic property of gas to detect the components as they exit the column. As a general rule, the lower the molecular weight (MW) of a gas molecule, the higher is its thermal conductivity. To illustrate the differences in the thermal conductivities of gases, assume that two identical ingots of steel are heated to the same temperature. One ingot is placed in an atmosphere of pure helium (MW=4), and the other ingot is placed in an atmosphere of pure nitrogen (MW=28). The ingot of steel in the helium atmosphere will cool faster due to the higher thermal conductivity of helium. The thermal conductivity technique is appropriate for GC analysis because the thermal conductivity of the carrier gas (hydrogen or helium) is significantly different from the thermal conductivities of sample gas components (see Figure 18). The thermal conductivity of a gas changes slightly with the operating temperature. Argon n-Butane Ethane Ethylene Helium
44.22 40.91 54.50 52.07 368.63
Hydrogen Methane Nitrogen Oxygen Propane
458.72 85.54 64.06 65.91 45.46
Figure 18: Thermal Conductivities of Typical Gases - k in [cal/(sec)(cm2)(°C/cm) x 10-6] at 38° C Two thermal conductivity detector elements are used for stability. The two detectors are sometimes split into two pairs (Figure 19). One detector is used to measure the separated components and the reference detector is subjected to pure carrier. These two detectors should be identical and exposed to the same conditions. The detectors are then electrically connected in a Wheatstone bridge circuit, or in a constant temperature circuit.
Figure 19: Two Thermal Conductivity Detectors in a Gas Chromatograph Analyzer
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The Wheatstone bridge (Figure 20) is used to measure any change in the resistance of the elements. The bridge provides the best measurement results when it is balanced. The bridge is balanced when the measuring and reference gases have the same thermal conductivity characteristics. With carrier gas flowing across both detectors, the circuit is then balanced, or zeroed, electronically. Zeroing causes the circuit to force the reference detector to operate at the same temperature as the measuring detector. With a constant voltage applied and the circuit balanced, components cross the measuring element causing a change in its resistance. A corresponding increase in current, which is the basis for the output, will occur. Due to the increase in current, detectors using this circuit are susceptible to burning out. With a constant voltage detector circuit, as the resistance of the element changes, the voltage is adjusted to maintain the same current. The change in voltage becomes the basis for the output. This circuit dramatically improves the life of the elements. Thermal conductivity detectors are selected, typically, when the sample gas component concentrations are greater then 0.1 mole %. The detector element can either be a filament or a thermistor. The element that is chosen must respond accurately to variations in carrier gas impurities and sample component concentrations. The operation of the filament and the thermistor are described below.
Figure 20: Wheatstone Bridge
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Thermistor - Thermistors are sintered mixtures of manganese, cobalt, and nickel oxides, plus trace
elements, all of which give them the desired electrical properties. The thermistor, which is in the form of a small bead, is mounted on a platinum wire. The thermistor is then coated with glass to make it inert. For this reason, a thermal conductivity detector that uses a thermistor is often referred to as a thermistor bead detector. Figure 21 shows two thermistors in the measuring cell block. The two thermistors are a matched set. The two elements are heated by the electrical current, which flows through them. The measuring cell, in which the element resides, is connected to the effluent of the chromatograph column. The element is cooled, to some extent, by the carrier gas flow. As the gas components elute off the column, less of the heat generated by the thermistor is removed, and the resistance of the element changes. Because the gas component absorbs less of the heat than the carrier gas, an imbalance in the circuit will occur proportional to the change in thermal conductivity. This change is directly related to the concentration of the component. The output of the detector is plotted over time in the form of a chromatogram.
Figure 21: Thermistor Bead Detector
Thermistors are very sensitive; however, they have a limited temperature range and poor stability.
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Filament - Figure 22 shows a duel pair of filaments in a measuring cell. The four filaments are
heated by an electrical current that flows through them. The measuring element cell, where the filaments reside, is connected to the effluent of the chromatograph column. The filaments are cooled, to some extent, by the carrier gas flow. As the gas components elute off the column, less of the heat generated by the filament is removed, and the resistance of the element changes. Because the gas component absorbs less of the heat than the carrier gas, an imbalance in the circuit will occur proportional to the change in thermal conductivity. This change is directly related to the concentration of the component. The output of the detector is plotted over time in the form of a chromatogram.
Figure 22: Filament Elements in a Measuring Cell Block(5)
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Ionization Detectors
An ionization detector is used when trace amounts of gas molecules need to be measured. In general, the operation of ionization detectors is based on the fact that the electrical conductivity of a gas is proportional to the concentration of charged particles (ions) within the gas. Figure 23, shows an ionization detector with an unspecified ionizing source. The ionization source ionizes the molecules in the effluent gas from the column. The presence of ions within the electrode gap causes a current, I, to flow through a measuring resistor, R2. The resulting voltage drop E0 is amplified by an electrometer, and the signal is sent to a recorder. The "bucking voltage" is used to zero the circuit when only carrier is flowing.
Figure 23: Ionization Detector(4)
Two types of ionization detectors are used in gas chromatography, flame ionization and photo ionization. Both detectors are used to measure gas component levels from 0.5% to levels as low as a few ppb. Although both detectors operate on the same principals, there are several differences, which are described below. Flame Ionization
A flame ionization detector is component-specific, it measures low level organic compounds. Figure 24 shows a list of compounds to which an FID will not respond. He Ar Kr Ne
Xe O2 N2 CS2
COS H2S SO2 NO
N2O NO2 NH3 CO
CO2 H2O SiCl4 SiHCl3 SiF4
Figure 24: Compounds That Give Little or No Response to the Flame Ionization Detector(4)
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The hydrogen air flame in Figure 25 is the ionizing source. The sample gas components are mixed with the hydrogen fuel, and then they are burned. At the high temperature of the flame, compounds that contain carbon bonds break down into positive and negative ions and electrons. A metal grid (the Collector) surrounds the flame to collect the current that passes through the flame. The number of ions that are formed, and thus the current that is conducted, is roughly proportional to the number of carbon atoms in the flame.
Figure 25: Flame Ionization Detector(4) Flame ionization detectors can measure low level inorganic molecules such as carbon monoxide, carbon dioxide, carbonyl sulfide and formaldehyde by converting them to methane with the use of a methanator. Methanator - A methanator consist of 2 feet of 1/8” tubing, packed with a nickel-coated catalyst
(Figure 26). The methanator is placed in front of the detector. As the components leave the end of the column, they are readily converted to methane by the catalyst in the presence of hydrogen. The methane that is produced can then be measured using the flame ionization detector.
Figure 26: Methanator Saudi Aramco DeskTop Standards
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Air Treater - Carrier gas impurities (e.g., hydrocarbons) that are present between the electrodes
will cause a constant current to flow. This constant current is called the “background current.” It is desirable to minimize the background current so that small changes in current flow can be more easily detected. The best method of removing trace hydrocarbons from the instrument air is to use an air treater to heat the air in the presence of a catalyst, which converts the hydrocarbons to inert carbon dioxide (Figure 27).
Figure 27: Air Treater Photo ionization
The photo ionization detector (Figure 28) contains a sealed ultraviolet lamp with a specific energy (in eV). Lamps with energies of 9.5, 10.0, 10.2, 10.9, and 11.7 eV are available. As the gas components elute from the column, their UV-absorbing molecules are ionized by the ultraviolet light. Compounds whose ionization potentials are lower then the lamp’s ionization energy are affected. In the presence of the radiation the components become ionized. A pair of electrodes are located in a chamber, which is adjacent to the ultraviolet source. An electric potential is applied across the electrodes to create an electrical current. When the ions that were formed pass through the electrodes, a current output is generated proportional to the concentration of ions. The current from the detector is amplified and passed to the recorder.
Figure 28: Photo Ionization Detector Saudi Aramco DeskTop Standards
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Flame Photometric
Flame photometric detectors are used to measure trace quantities of compounds that contain sulfur. Figure 29 shows a schematic of a flame photometric detector. The sample gas stream from the column is mixed with a constant flow of hydrogen near the burner tip. Sulfur-containing compounds are burned in the flame to produce a blue light. The light is transmitted through an optical cable to a 394 nm band-pass filter. The band-pass filter isolates the desired wavelength range before the light reaches the photomultiplier tube. The photomultiplier tube produces an output current that is proportional to the square of the sulfur concentration. The output current is amplified, and it is converted to a voltage by an electrometer circuit.
Figure 29: Schematic of a Flame Photometric Detector Electron Capture
Figure 30 shows a schematic of an electron capture detector (ECD). The detector contains two electrodes through which the nitrogen carrier gas passes. One of the electrodes is treated with a radioisotope (tritium or nickel-63), which emits high-energy electrons as it decays. The electrons ionize the nitrogen molecules to produce an abundance of low-energy electrons in the carrier gas stream. These low-energy electrons, which are collected by the positively-charged electrode, produce a steady current. If the gas sample contains molecules that absorb the low-energy electrons, the amount of current is reduced. The loss of current is a measure of the electron affinity of the sample gas component. The ECD is very sensitive to certain molecules such as alkyl halides, conjugated carbonyls, nitriles, nitrates, and organometals; however, it is insensitive to hydrocarbons, alcohols, ketones, etc.
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Figure 30: Electron Capture Detector
Columns The separation of sample components occurs in the chromatograph column. There are two basic types of chromatograph columns, adsorption and partition columns. Variations of the types of columns are packed and capillary. Within these variations, hundreds of column materials are used, and others are constantly being developed. Although there are many columns, their basic operation is the same. The operation of these columns are described on the following pages.
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Adsorption Columns
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Adsorption columns are used in gas-solid chromatography for the separation of gases, such as nitrogen, oxygen, carbon dioxide, and hydrogen sulfide. These columns can also be used to separate hydrocarbons in the C1 to C3 range. Components are separated by their differences in adsorption, which can be defined as their tendency to adhere to the adsorbent. Packing materials are surface-active solids, such as activated alumina, charcoal, molecular sieves, silica gel, and synthetic zeolites. The mole sieve column separates the sample gas components by molecular size. The particles in the mole sieve column contain molecular-sized holes that trap small molecules that fit into them. A length of tubing acts as a series of sieves. The larger molecules pass through the column faster than the smaller molecules, which enter the pores. For petroleum service, adsorption columns have a limited range of application except when used in combination with other columns. Partition Columns
Partition columns are used in gas-liquid chromatography for separating complex hydrocarbon samples. The column packing is a granular solid (support), which is coated with a nonvolatile liquid stationary phase (Figure 31).
Figure 31: Enlargement of Support and Liquid Stationary Phase The packing exposes a large liquid surface to the vaporized sample components as they migrate through the column. The sample components are partitioned between the gas and liquid phases. The components that are least soluble in the liquid pass rapidly through the columns and emerge first. The components that are most soluble in the liquid are retained longer and emerge later. The more volatile components generally emerge earliest. Partition columns are very versatile because there are a large variety of liquids that can be used to obtain different separations. The granular solid support may be crushed firebrick, celite, or other solids of moderate surface area (1 to 4 square meters per gram). It may be treated to reduce residual adsorptive effects. The stationary liquid must have a very low vapor pressure at the operating temperature in order for the column to have a long service life.
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Packed Columns
Packed columns consist of tubing, packing material, and packing retainers. The tubing may be made of stainless steel, copper, or glass (fused silica). The tubing length ranges from less than 1 m up to 10 m with a bore diameter of 1.6, 3.2, 6.4, or 9.5 mm. The packing material contains solid particles (support), which are coated with a thin liquid stationary phase. The sample gas components are soluble in the stationary phase. The carrier gas molecules and the soluble sample gas molecules travel along different paths in the packing (Figure 32). As a result, the sample gas molecules have different residence times. The structure of the packing affects the column efficiency and the component retention times.
Figure 32: Packed Column Capillary Columns
Capillary columns have an open and unrestricted path for the carrier gas. The two most common types of capillary columns are the wall-coated open tubular (WCOT) and the support-coated open tubular (SCOT) columns. The WCOT column is a long narrow-bore tubing (0.25 mm I.D.) in which the inner wall is coated with a liquid stationary phase to about 1 µm in thickness. WCOT column lengths range from 50 to 150 m. The sample capacity of capillary columns is mainly determined by the thickness of the stationary phase. The major limitation of WCOT columns is their limited sample capacity. SCOT columns have more sample capacity than WCOT columns; therefore, SCOT columns are shorter (about 16 m in length). The inside wall of a SCOT column (0.5 mm I.D.) is coated with an inert porous layer, which is formed by chemical treatment or is deposited on the inside wall. The porous layer is coated with a liquid stationary phase. Valves Functions
There are different types of gas chromatograph valves, each of which has a separate function. These functions include • • •
sample injection column switching atmospheric referencing
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Sample Injection - Sample injection valves are located in the analyzer’s oven. A gas or liquid
sample from the sample conditioning system constantly flows through the sample valve and returns to the sampling system. The function of the valve is to trap a constant volume of sample and to periodically inject the sample into the flowing carrier gas stream. Column Switching - The function of column switching valves is to redirect the carrier gas flow
during an analysis cycle so that specific components are loaded onto different columns for further separation. Column switching valves are also used to reverse the flow of carrier gas through a column to backflush unwanted components off of the column. Atmospheric Referencing - Gas is a compressible fluid, and the number of molecules of sample in
the sample loop can be determined by using the ideal gas law. Because the volume and the temperature of the sample loop are constant, the number of moles that are trapped is solely a function of the pressure in the loop. A good practice to ensure repeatable results is to reference the sample loop pressure to atmosphere prior to sample injection. Atmospheric referencing is accomplished by stopping the flow of vapor sample just prior to sample valve actuation through the use of a sample shut off valve (SSO). At the same time, the atmospheric referencing valve (ARV) shown in Figure 33 is actuated. The valve actuation allows the trapped sample in the sample loop to equilibrate to atmospheric pressure.
Figure 33: Atmospheric Referencing Valve (ARV)
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Types of Sample Valves
The different types of sample valves include • • • •
rotary valves slider valves plunger diaphragm valves transport injection valves
Rotary valves are used to inject very small amounts of liquid sample (less than 1 µL), or they are
used as column switching valves for capillary columns. A typical rotary valve is shown in Figure 34. In the de-energized position (Figure 34A), the carrier gas flows through the valve to the column to perform the analysis. The sample stream flows through the metered volume so that the most representative sample will be available when the cycle is ready to repeat. When the next analysis cycle begins, the valve is energized (Figure 34B) so that it instantly rotates 60 degrees. The valve rotation transfers the metered volume to the carrier gas stream, which sweeps the sample into the column.
Figure 34: Typical Rotary Valve Saudi Aramco DeskTop Standards
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Slider valves - A typical slider valve for gases is shown in Figure 35. The valve transfers a metered
volume of the sample to the carrier gas stream by using a moving plate or slider.
Figure 35: Typical Slider Valve for Gases (1) Slider valves for liquids use a straight-through or cavity-type configuration (Figure 36). The straight-through configuration is faster than the cavity-type configuration.
Figure 36: Slider Valves for Liquids(1)
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Plunger Diaphragm Valves - Figure 37 shows a schematic of a six-port plunger diaphragm valve.
The six ports are arranged in a circular configuration. Between each pair of ports, there is a twoposition plunger that can open or seal the passage between the two ports. The six plungers are operated in two sets of three. Each set of plungers is controlled by a spring-loaded, air-actuated piston so that one set closes three passages between ports as the other set opens three passages between ports. The passages between Port 1 and 6, Ports 4 and 5, and Ports 3 and 2 are normally closed. The other three passages between Ports 1 and 2, Port 3 and 4, and Ports 5 and 6 are normally open. The two actuating pistons are spring-loaded in such a way that assures all six passages are momentarily closed during the switching operation. This momentarily closed position prevents unwanted mixing during the switching cycle. The plungers and diaphragm move only a few thousandths of an inch to permit flow through the passages. This small movement of the plungers and the diaphragm along with the absence of sliding seals that contact the process fluid, eliminates the abrasions that can cause sample loop volume changes and valve leakage, which are prevalent in sliding type valves. In addition, the small movement means the valves can switch in as little as 150 milliseconds to prevent smearing of the sample during injection.
Figure 37: Six-Port Plunger Diaphragm Valve
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Transport injection valves, or quill-type valves, inject a measured volume of sample through the use
of a grove carved in a cylindrical rod. The sample is collected in the grove and periodically injected into the carrier flow path as the rod is moved by the actuator. The collection section of the valve is located outside the chromatograph oven. The injection section of the valve is located in the chromatograph oven. With the use of a heater, the injector temperature can be raised to vaporize high boiling point samples.
Figure 38: Transport Injection Valve Although transport injection valves are more expensive, there are some benefits of using them. Samples that contain salts, that would cake on a valve during inject, are washed off when the valve quill is moved back to the flowing sample. Samples that contain a lot of solids, which would clog a slider valve or plunger valve, can be measured using a transport injection valve. With the use of a temperature control circuit, the injector temperature can be changed for different boiling point samples to insure complete vaporization. Electronic Controllers A gas chromatograph analyzer separates a sample into its component parts and then it detects the concentration of each component. For the analyzer to successfully perform these functions, the electronic controller must control the temperature, pressure, flows, and valve sequences within very close tolerances. State-of-the-art GC analyzers have dedicated controllers that are located at each analyzer. In this configuration, each analyzer can either have independent input/output capability or it can be connected to a centralized high-speed communication network. A network configuration also simplifies the analyzer’s installation because all of the analyzers in the plant can be connected by a strategically-placed set of wires. Saudi Aramco DeskTop Standards
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Timing
The timer controls the time of operation of the analyzer valves, the detector signal gating, the attenuator selection, the readout devices, the auto zero, and the alarms. Most timers in use today employ solid state electronics, which combine timing and sequencing operations. Controllers that utilize minicomputers or microprocessors are widely used. Computer-based controllers that use either full-scale minicomputers or microprocessors have added a dimension of versatility in data interpretation, presentation, troubleshooting, and programming aids. Measurement Data Processing Functions Peak Detection - Two methods are used to detect chromatogram peaks: peak height and peak
integration. The peak height measures the maximum height of the generated peaks. The peak integration measures the total area under the peak. Figure 39 illustrates a chromatogram with three peaks and key chromatogram terminology. The “Gate On” tick mark (1) indicates when electronic integration of the peak starts. Gate On start times are stored in a file in the electronic controller. Peak retention time (2) is the time that it takes the maximum height of peak to appear after the sample is injected at the start of the cycle. Each component in the sample has a unique retention time which is used to identify the component. The Gate Off tick mark (3) indicates when the integration ends. Peak height (4) is the maximum height of the detector signal above the baseline voltage. For sharp symmetrical peaks, peak height can sometimes be used as an indication of relative component concentrations in the sample. Gate On, residence time, and Gate Off times are corrected by using a calibration blend that contains known concentrations of the components of interest.
Figure 39: Peak Detection
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There are several types of gating options that are used by the electronic controller. These gating options include: time gating, slope gating, auto-gating, and retention gating. With time gating, the gate is forced “ON” or “OFF” at a specific time in the analysis cycle by the electronic controller as was shown in Figure 39. With slope gating (Figure 40), the controller activates the gates at a point where the detector signal, at front gate or back gate, changes at a greater rate than is specified by a threshold limit with respect to time. This technique can be used when a component’s retention time changes due to its natural characteristics or between two peaks that are not completely resolved. Slope gating is used by auto-gating and retention gating. Whichever option is used, it is assumed that the retention time for the known component does not change with respect to time.
Figure 40: Slope Gating
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Due to baseline shifts, peak resolution baseline corrections are made through the electronic controller. Figure 41 shows the surplus peak area caused by baseline shift.
Figure 41: Example of Baseline Shift There are several methods for correction for baseline shift or making corrections for unresolved components. Figure 42 shows some of the options used by the electronic controller to correct the peak area. The best option in most cases is baseline directly between front and back gate.
Figure 42: Selecting Bottom Line of Peak Saudi Aramco DeskTop Standards
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It is up to the chromatograph technician to choose the best option or combination of options for each component. Time gating, Slope gating, Threshold limit, Retention gating and which basing option to use as well. Peak Identification - Positive peak identification is one of the problems facing chromatography today. As the column technology changed with the use of high efficiency capillary column, the number of components that elute from the end of the column has also increased. The ability to identify all of these components did not increase with the technology.
There are three key factors that affect the ability to identify known components by their retention times—oven temperatures, carrier flow rate, and column stability. Oven temperature affects the retention time of a component. The hotter the oven temperature, the faster the component elutes. The use of microprocessors to control oven temperatures has resulted in very stable oven temperatures, which results in stable retention times. The carrier flow rate will affect the retention time of the component. The use of a high quality carrier regulator is important to maintain a constant pressure, which will result in a constant flow rate. The third factor that affects the retention time of a component is the stability of the column itself. Identification is made by comparing the retention time of the unknown component to that of a known blend, or standard sample, which is run under the same conditions (Figure 43).
Figure 43: Identification of Unknown Peaks by Use of Standard Saudi Aramco DeskTop Standards
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The analyzer’s electronics are capable of looking for components that elute at the specified retention time. If a component is not there, or if it has moved outside the specified time, the electronic controller can generate an alarm. The electronic controller is incapable of positively identifying a component or identifying an unknown component. This identification has to take place by the chromatograph technician. Unknown components that have similar retention times as the components of interest must be identified and eliminated to have an accurate analysis. These unknown components cause may be directly under the component of interest or show up as interference peaks, or shoulder peaks (Figure 44). The use of standard samples, or some other analytical technique to identify the unknown component, in chromatography is imperative. The interfering component may sometimes be removed through valve timing of the controller, or a new column study may be necessary.
Figure 44: Shoulder Peak
Peak Resolution - As the gas sample components migrate through the column, their zones always broaden. Separation of the components into discrete bands will occur only if they widen to a lesser degree than their peaks separate. The separation of two consecutive peaks is measured by the resolution, R. Resolution is defined as the peak separation, S, divided by the peak width, W (Figure 45A). For ideal peak separation, the peak separation and the peak width are equal, R = 1.0 (Figure 45B).
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Figure 45: Peak Resolution
The resolution of chromatogram peaks is related to two factors: column efficiency and solvent efficiency (Figure 46). The column efficiency concerns the peak broadening of an initially compact component band as it migrates through the column. The broadening results from the column design (column diameter, packing, etc.) and operating conditions. Solvent efficiency results from the interaction between the stationary phase and the sample gas components. Solvent efficiency is expressed as the ratio of the adjusted retention times (peak maxima).
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Figure 46: Illustration of Column and Solvent Efficiency(4) System Diagnostics
Electronic controllers for gas chromatograph analyzers perform the following functions: 1. 2. 3. 4. 5. 6. 7. 8. 9.
10.
Amplify and digitize the detector signal. Integrate the detector signal, identify the peaks, and calculate component concentrations. Automatically calibrate and update response factors. Activate sample and column valve switching on a cycle clock basis. Activate sampling system valves to switch between multiple sample streams or a calibration stream on a sequential basis. Control the oven temperature. Perform mathematical calculations on the measured component concentrations. Have an interface for operation, maintenance, and programming. Generate results in the form of: a. Printed reports b. Analog outputs c. Serial computer links to host computers Communicate with other analyzers, I/0 devices, and operator interfaces using a data network.
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do C b su T vg V m yS iw H IfcD rto e p O R An lLo a n A I/sfh u vp O m S b T kn yce lC ita rD g
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The interaction of these functions are shown in Figure 47.
Figure 47: Diagnostic Functions of Electronic Controller
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Component Separation Techniques Gas chromatograph analyzers must be designed so that the sample components of interest are completely separated from the unwanted components. Single component measurements can usually be achieved by using one column; however, two and three component separations require multiple columns. Multiple component separations greatly increase the complexity of the design and maintenance of GC analyzers. The uses of various component separation techniques are described below These component separation techniques include • • • • •
backflush heartcut column stepping trap/bypass programmed temperature
Backflush
Most gas samples contain components that are not separated in the chromatograph column or components that will not pass totally through the column in a reasonable length of time. For this reason, all sample components are removed from the column prior to the next measurement cycle by using the backflush technique. During backflush, the carrier gas flow through a column is reversed for a time T seconds, which is approximately equal to the time of the normal flow (Figure 48A). Under ideal backflush conditions, the chromatogram peaks of the components would recombine after T seconds and half of the backflush peak would be eluted from the column (Figure 48B).
Figure 48: Ideal Backflush(1)
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In reality, the column conditions during measurement and backflush are different. The peaks do not recombine during backflush as shown in Figure 49.
Figure 49: Real Backflush(1)
If only the lighter components are to be measured with a single column, the heavier (high boiling point) components can be backflushed to the vent by using the valve configuration shown in Figure 50. To reduce the time that is required for backflushing, the pressure of the purge gas is increased above the normal operating pressure of the column. Most applications today use some form of backflush to insure that the column is clean before the next analysis.
Figure 50: Valve Configuration for Backflush to Vent Using a Single Column(4)
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Complex multiple column arrangements are usually required to separate three or more components. Figure 51 shows the backflush to vent flow path configurations. The system consists of two columns, a column valve (CV), a sample valve (SV), and a sample shut off valve (SSO). Figure 51A shows the carrier flows path through the configuration while the system is in forward flow. Figure 51B shows the carrier flows path through the configuration while the system is in backflush.
Figure 51: Backflush to Vent Using Two Columns
Heartcut
The heartcut procedure is used when there are components the elute off the first column prior to the component of interest. It is also used when there is a component that would contaminate the second column or the detector if the component were allowed to flow forward. This application is typically used when low level components need to be measured on an FID and the larger components would saturate the detector. Figure 52A shows the heartcut application in the heartcut flow position. Figure 52B shows the heartcut application in forward flow.
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Figure 52: Heartcut Procedure
Column Stepping
The column stepping technique (Figure 53) is used to measure the heaviest components first. After the lighter components are allowed to pass through the first column, the carrier gas flow is reversed in the column. In this position, the first column is lined up so that it is in the third position in front of columns two and three. The reversed carrier flow sends the heavier components that remain in the first column to the detector.
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Figure 53: Reverse Column Step
Trap/Bypass
The trap/bypass procedure (Figure 54) is used to capture the lighter components that are harder to separate and to bypass the heavier components that are easier to separate. In this example, the lighter components that elute from column 1 are allowed to flow into column 2. The lighter components are then trapped, which allows the heavier components to pass via the column 2 simulator.
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Figure 54: Trap/Bypass
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Programmed Temperature
Temperature programming is the controlled change of the column temperature during the analysis. Temperature programming is used to separate gas components with a wide range of boiling points, which is a limitation of constant temperature techniques. At a constant temperature (Figure 55), components with low boiling points elute form the column so rapidly that their peaks overlap. The peaks of the higher boiling point components are flat and immeasurable. Sometimes, the high boiling point components do not elute from the column and they appear in a later analysis as baseline noise.
Figure 55: Constant Temperature Technique With temperature programming, a lower initial temperature is used so that the peaks of the lower boiling point components are well resolved (Figure 56). The oven temperature is gradually increased to speed up the elution of the components with high boiling points. The programmed temperature technique is used mostly in the laboratory; however, some manufactures offer the option for process GCs.
Figure 56: Programmed Temperature Saudi Aramco DeskTop Standards
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Generally, temperature programming is recommended if the boiling points of the components differ by 100°C or more. A schematic of a programmed temperature GC analyzer is shown in Figure 57. The configuration includes separate ovens for the sample injection valves, the columns, and the detector. It is desirable to heat the column at rates that vary from 0.25°C/min to 10°C/min. The oven should be able to heat the column from ambient temperature to 400°C within 40 minutes and it should be able to maintain the actual temperature to ±2°C of the desired temperature.. The injection valves and the detector remain at a constant temperature. The temperature of the detector must remain as constant as possible to avoid baseline shift and detector response changes. A flame ionization detector may be used because it is not sensitive to small temperature changes.
Figure 57: Schematic of Programmed Temperature Gas Chromatograph Analyzer
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GAS CHROMATOGRAPH NETWORK CONFIGURATIONS Gas Chromatograph/Process Control Computer Interface Data Transmission
Modern process analyzers provide a great deal of information, both about their analytical results and about the status of the analyzer itself. The oldest and still most prevalent method is a local display of results through an analog output. The drawback with an analog system is the lack of analyzer status information. With the development of microprocessor-based analyzers and control computers, the analyzers are able use serial communication to directly communicate with the control computer. Analog Data Transmission - Direct analog connection to the control computer has been the standard
for many years. In its simplest form, analog transmission is accomplished by causing an electronic amplifier to mirror the process variable. Figure 58 shows an analog signal connected to a recorder. When the analog signal is plotted on the recorder over time, a “trend” is developed.
Figure 58: Analog Circuit
There are several problems in using analog data transmission with a gas chromatograph. Gas chromatographs typically read multi-components on more than one stream. In this situation, each component has an analog output for each component on each stream. The control computer has to have a similar number of analog inputs available. The amount of hardware increases with the number of analyzers. The other problem is that there is a fixed range for each component. Figure 58 shows an example of an output ranged with a chromatograph. If the analyzer results fall outside these ranges in an upset condition, the results can no longer be seen on the trend. For these reasons, the use of serial communication may be a better choice.
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Example: A chromatograph is used to measure the concentrations of methane and ethane in a natural gas stream. Two current amplifiers are dedicated to the task of transmitting the results of the measurement. Both current amplifiers are capable of delivering a current no lower than 4 mA and no higher than 20 mA. If the methane ranges from 70% to 90% and the ethane ranges from 5% to 20%, the outputs of the current amplifiers represent the actual measurements as follows: Amplifier 1 4 mA means methane at 70% 12 mA means methane at 80% 20 mA means methane at 90% Amplifier 2 4 mA means ethane at 5% 12 mA means ethane at 12.5% 20 mA means ethane at 20% Notice that 12 mA is the mid-range of the current amplifiers (that is, halfway between 4 mA and 20 mA) and is therefore used to represent the halfway point of the process variables being measured. Serial Data Transmission - The availability of computer electronics has made serial communication a
popular and rapidly growing communication technique. This technique provides a means to overcome the limitations with analog communication systems. Furthermore, serial communication permits an analyzer to both send and receive data. The essence of serial communication hardware is a single digital electronics device, an electronic voltage or current amplifier. The electronics are capable of assuming only two conditions - off / on or lower / higher voltage level. By introducing a time element into the picture, the same electronic hardware can be used to express an unlimited amount of information either into or out of an analyzer system.
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Figure 59 demonstrates this technique. At a single point in time, the electronic hardware is in only one condition - high or low. But if a timer is started and the pattern of high / low voltages coming from the electronic hardware is watched, the pattern itself can be used to represent an item of complex information. For example, as shown in the figure, in the ASCII standard code interpretation, a pattern of 7 states is used to represent the alphabetic letter “C”. Thus, by starting a timer that synchronizes the signal source (such as the analyzer) and the listener (such as a printer) and by observing the pattern of signals that occurs during a specified time interval, it is possible for the analyzer to send the letter “C” to the printer. Similarly, if the timer is then restarted so that a different pattern is created by the analyzer in the next time interval, it is possible for the analyzer to send yet another letter to the printer. As shown in the figure, this could be the letter “B”.
Figure 59: Basic Serial Representation This technique makes it is possible for the analyzer to send written information. The key elements are (1) that the analyzer and the printer “agree” on the binary patterns that will represent the various letters, (2) that the printer and the analyzer use a consistent and identical time standard, and (3) that enough time is allowed to pass for information to be transmitted from one device to the other. Hardware Variations
In the next section, some practical considerations of serial communication will be discussed; however, it is first appropriate to consider some of the variations in hardware that are used for this purpose. Several systems are in common use. The older of these systems are: • •
RS-232-C voltage hardware 0/20 mA current loop hardware
Most newer systems are variations of these two systems.
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RS-232-C Convention - The RS-232 system is a convention that is generally accepted as a standard
hardware technique throughout industry. It uses voltage amplifiers to create the two voltage signal levels that are needed to represent the two binary conditions. The basic hardware has three signal wires, which transmit, receive, and ground. The transmit line sources voltages relative to the ground line, and the receive line observes voltages relative to the same ground line. The voltages in use are: -3 to -15 volts
means binary 1, or set, or on, or high
+3 to +15 volts
means binary 0, or reset, or off, or low
>15 volts
may cause hardware damage
-3 to +3 volts
is indeterminate and may cause error
The binary conditions are often expressed in three ways as indicated. It is significant to note that the high / low binary condition is the opposite of the actual voltage signal. The RS-232-C convention includes several optional signals besides transmit and receive. These include: •
RTS - Request to Send
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DSR - Data Set Ready
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CTS - Clear to Send
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DTR - Data Terminal Ready
These optional signals are also defined and are used for control of communication devices such as modems. A more complete discussion is beyond the scope of this module. Physical wiring is also often standardized under the RS-232 convention. A “D” shaped cable connector is used. In this system, pin 2 is the transmit line, pin 3 is the receive line and pin 7 is the common. The other pins are also defined in a standard manner. In general, a voltmeter or an oscilloscope can be used to observe RS-232 signal activity. Current Loop Convention - This technique was originally established as a means of operating mechanical teletype machines. As long as a signal current was flowing, moving components in a teletype machine would remain in one physical location. When the current in the signal line was interrupted, mechanical components would release. At rather slow speeds, it was possible to cause these mechanical fingers to activate appropriate keys on the mechanical typewriter of the teletype. The convention establishes two current levels as the operating levels that represent binary states:
1. 20 milliamps means binary 0 or space or reset 2. 0 milliamps
means binary 1 or mark or set
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While the technique has almost no present use in teletype activation, it has been retained as a de facto industry standard for several reasons. First, there was a large installed base of teletype equipment in the word and it was suitable for equipment manufacturers to make new equipment that was compatible with the old. Second, the current loop offers higher signal noise immunity that the newer RS-232-C convention. In general, there is wide latitude around the nominal 0 and 20 mA setpoints of the current loop convention. Third, current loops can be powered by current amplifiers that can operate into longer transmission lines. The voltage amplifiers of the RS-232 are limited to voltage losses that occur in the resistive wiring. The RS-232 specification itself limits line length to 50 feet (18 meters). Fourth, multiple receiving devices can usually be installed in series on a current loop. Multiple devices installed on a voltage line like RS-232 can overload the voltage drive capabilities of the electronic amplifiers. Fifth, current loop systems do not have to be referenced to a common voltage reference such as ground. Consequently, current loops are more likely to be immune to ground loop problems. Next to these advantages, there are some disadvantages of current loop systems. Such systems do not have well defined control signals to permit more complex device control such as those needed for modems. Because of electronic rise and fall times, current-loop based systems may be constrained to operate at slower speeds. Other Hardware Variations - Most modern communication systems support RS-232-C and current
loop conventions in standard configurations. Most modern systems also implement newer conventions including a later military specification, RS-422, and others. These newer conventions are based on voltage amplifiers that operate with differential line drivers (see Figure 60). Such systems usually depend on a 0 volts / 5 volts signal differential that switches polarity between two signal wires. A ground line may be provided as a noise shield, but this line is not part of the transmission loop. As a result, ground loops do not occur.
Figure 60: Voltage Amplifier for Serial Communication
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Additionally, modern transmission line drive circuitry supports a “third” state besides the binary high /low. This third state is “off” and in this condition, a transmitter on a transmission line is apparently “disconnected.” This operating mode permits several different devices to be connected to a single communication line in a “multi-drop” configuration (see Figure 61).
Figure 61: Pareto Chart Practical Considerations of Serial Communication Techniques
Serial communication systems are in common use because they offer the following advantages: • • •
The capability to combine both input and output functions into a system that is common with maintenance and control functions. The capability to transmit and receive large amounts of information with very little hardware cost. The ability to use standard hardware to connect two different systems from two different manufacturers.
However, for each of these advantages, there is a corresponding disadvantage. These are: • • •
The difficulty of training human operators to understand the function that is being performed at any one time. The requirement for very specialized hardware and potentially complex software, both of which lead to relatively high expense. The lack of standardization at any level above hardware, including software protocols and application software.
In systems in which an analyzer is to communicate with another computer, care should be taken and analog or discrete digital options should be considered. Saudi Aramco DeskTop Standards
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The following sections detail two of the most common problems that occur with serial communication. Software and Protocol - As was stated in a previous section, successful serial communication
requires agreement between the sending and receiving devices on the codes that will be used to represent information and the timing and time intervals through which the codes will be sent. In most systems, standard time intervals are defined and accepted in the industry. Expressed as ‘baud rate’ these frequencies specify the number of binary bits of information per second that are sent from one device to another. Standard baud rates are 300, 1200, 2400, 4800, 9600, and 19200 baud. Some other variations are occasionally used. Timing is often controlled by a bit synchronization sequence. For instance, if an alphabetic character requires 7 binary bits to represent it, then the sending device precedes these seven bits with a “start” bit to “wake up” the receiving device. Following the seven data bits is usually an error control bit, called parity, and one or two end bits. The start and end bits allow a known time interval for both transmitting and receiving devices to synchronize. Protocol agreement is much more difficult to establish. At the most rudimentary level, certain industry standards have been adopted which allow representation of alphabetic characters in a predetermined form. The most commonly accepted standard is ASCII. Alternatively, raw computer information is often transmitted as an image of the computer’s memory (i.e., in its direct binary form). Such information may be “received” by a device and still be of no value if the receiver does not “understand” the data. Such understanding is accomplished by the software in both the sending and receiving devices. Additionally, there must be rules as to which device may “talk” and which device must “listen” at any given point in time. There must be agreement about how long one device may talk before it must listen. These rules are implemented in the software in the sending and receiving devices. This software is referred to as communications “protocol.” Protocol software is usually complex and it is usually customized for each application because very few industrial standards exist. Fortunately, the analyzer industry has made some progress toward mutual agreement on protocol standardization. One of the more commonly accepted protocols was originally published by Gould, Inc. as part of the Modicon line of programmable logic controllers. The original protocol and various adaptations of it are referred to as the Modbus* protocol. (Modbus is a registered trademark of Gould, Inc.) Hardware - Although hardware conventions are well established and standardized, there are many
practical considerations in dealing with communications hardware. Normally, specialized equipment is required to test or troubleshoot such systems.
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As mentioned earlier, time is required to pass information from one device to another in serial fashion. In other words, information transfer from one point is delayed until data transfer from another point is completed. This is one of the biggest disadvantages of serial communication systems. Transfer is neither continuous nor is it instantaneous. To overcome the time delay problem, manufacturers have improved electronics to allow very high speed data transfers. Faster data transfer has reduced problems of delays in communication systems, but it has not eliminated them. For example, if several analyzers want to send their information to output devices, the analyzers must transfer data one at a time. Ultimately, the last analyzer to send its data may have to wait a long time. If a communications system is made faster, this delay is reduced, but it is never eliminated. In the process of making the systems faster, manufacturers have also made the hardware more difficult to maintain. It is not possible to use a voltmeter to observe communication that occurs at high speed. The best that can be done is to use the meter to troubleshoot simple short circuits and open lines in the communications wire. Additionally, although an oscilloscope might be used to observe voltage changes at high speed, an oscilloscope does not have the software to recognize and understand binary patterns. Specialized equipment, such as a serial data analyzer, may be used for this purpose. Although a serial data analyzer makes trouble shooting possible, it introduces a new problem. Now, maintenance personnel must understand the software protocol that the machines are using. Despite these difficulties, the advantages are strong and development continues on serial digital communication techniques. Problems are continuously being resolved and these systems are being made easier to use. Use of Personal Computers Data Processing and Reporting
The introduction of the microprocessor during the early 1970’s drastically improved instrumentation and process control. More specifically, the microprocessor provided a better means of programming the complex functions that the GC performs during each cycle (i.e., sample injection, column switching, peak detection, etc.). Manufacturers continue to develop more powerful and sophisticated GCs while improving the analyzers’ reliability and redundancy. One of the major areas of improvement involve the development of GC networks. The networks today allows the users to communicate with all of the analyzers in the system. With a personal computer connected to the analyzer network comes the ability for the analyzer, to not only receive and transmit information to the control computer, but for personal computer to receive and transmit the same information. All of the analyzers status and component information can now be gathered by a personal computer and used locally by the user.
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Personal computers have increased in power and capabilities to the point where they allow the user to perform the following functions: • • • • •
access the system backup and restore each analyzers program store analyzers results create historical trends monitor the overall system
Some computer systems allow the user to perform most maintenance tasks directly from the personal computer. Multiple personal computers can be connected to the system or remotely by way of phone lines and modems. Analytical monitoring systems allow live links to the analyzer network system, which allows live trending. With certain software systems, live information from the analyzer system such as process and calibration information can be gathered. With this information .procedures and analyzer documentation can be maintained at the personal computer. Statistical quality control (SQC) procedures and data gathering can now be automated by the personal computer. SQC Techniques
Statistical Quality Control (SQC) techniques are not only being applied to process control systems, but they are now being applied to the analyzers. SQC helps to take out the guess work of when the analyzer may need maintenance or how often the analyzer should be calibrated. With SQC, measurement data is collected, statistically analyzed, and displayed to prove the analyzer’s capability, or Cp. Cp is a number that shows the capability of an analyzer to repeat measurement on a consistent process stream. The specifications for GC analyzers are determined by the potential use of the analyzer, type of process stream to be analyzed, etc. These specifications are given in percentage of full scale. For example, if full scale is 10 units, the analyzer will be run near 5 (halfway) with a specification of ±0.5%. This means that the measurements should repeat within a band of 0.1 units. With the availability of personal computers connected to the analyzer network, the measurement data is gathered automatically. The technician’s can now have instant access to the analyzer statistical information. The use of a personal computer to automate the SQC task frees up the user to better maintain the analyzer system.
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Understanding the measure of variation, or standard deviation, is necessary to understand Cp. Standard deviation can be thought of as the average deviation from the average of a set of values. While this definition of standard deviation is not totally correct, it gives a good idea of how variation is measured. The formula for the average and standard deviation, σ, of a number is shown in Figure 62.
Figure 62: Normal Distribution
The formula says that standard deviation is the square root of the sum of the differences from average squared, divided by the number of data points. The variable “n” is the number of data points and Xi is the ith data point (reading from a cycle). A distribution like the normal distribution in Figure 60 will have 99.73% of the data within +/- 3 standard deviations of the average. This means that almost all of the data is covered by a 6 standard deviation spread. Cp is the ratio of the spread of the specification limits to the spread of the data (6 σ). The formula is as follows:
This ratio Cp is 1 if the data spreads as wide as the specifications. If the data spreads further than the specifications, the ratio will be less than 1. The best situation is where the Cp is greater than one, which means that all of the data fits within the specifications. An analyzer is considered acceptable if Cp is greater than 1 and the run chart doesn’t show any unusual readings or patterns. This means that the analyzer will repeat readings of a stable stream within the given percentage of full scale. A Cp of less than 1 is unacceptable, which indicates that corrections must be made to the analyzer of its operating conditions to bring it into conformance.
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Network Capabilities Control Capabilities
A new communications option available to process analyzer systems is the use of analyzer communication networks. While networking has been available to distributed control systems for many years, its use in process analyzer communications has only been present since the mid1980’s. The use of networks, or data highways as they are sometimes called, are meant to satisfy three emerging requirements in the process analyzer market: •
Reduce analyzer I/O quantity and complexity
•
Simplify analyzer maintenance and control
•
Allow sophisticated data archiving and retrieval
Reducing analyzer I/O quantity and complexity is in response to the dramatic growth in the use of process analyzers. As the number of field analyzers grew at a plant site, so did the number of I/O points that needed to be installed. Analyzer networks (Figure 63) allow the data from all connected analyzers to be gathered via a common set of hardware and cables. The network cable can then be brought to central control rooms, maintenance shops, and engineering offices. In these locations, the data can be dispersed into the desired format (e.g., printer logs, trend recorders, and alarm panels). While the initial costs for establishing a network in a plant is higher than traditional techniques, the savings in hardware and cabling can be tremendous as the plant’s analyzer count increases.
Figure 63: Typical Analyzer Network Saudi Aramco DeskTop Standards
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Another user requirement that increases the need for analyzer networks is the desire to simplify analyzer maintenance and control. With the increased use of process analyzers comes the increased need for analyzer maintenance. Networks provide the ability to centralize engineering and maintenance functions by using devices such as service panels and PC workstations connected to the network. Through the use of these devices, a technician or engineer can review and adjust an analyzer without having to physically visit each analyzer in the plant. An extension of the desire to make analyzer maintenance more efficient is the move toward more sophisticated data archiving and retrieval. Although process analyzer data is often being stored in distributed control systems, it is not always readily available to engineering and maintenance staffs. The ability to connect personal computers to the analyzer network allows direct collection of the information that is generated by the analyzers. Once this information is stored on disk, it can be used for generating preventive maintenance schedules, development of SQC and Statistical Process Control (SPC) programs and spotting trends in analyzer performance. Often the data is stored in a format that is compatible with commercial, off-the-shelf software packages. Architecture and Operation of Various Networks
Analyzer networks tend to fall into two categories: Master/Slave designs and Masterless designs. These two categories are also called by a wide variety of other names such as Central Manager Based, Polling Architecture, and Peer to Peer. All of these systems can be reduced down to two basic approaches based on how the system regulates the access of connected equipment that need to communicate on the network. Either centralized control (Master/Slave design) or distributed control (Masterless design) is used. Master/Slave Networks - The Master/Slave design uses a centralized communications controller to
regulate data flow and access onto the network (see Figure 64). The centralized controller is typically a personal computer. The computer is often called a network master or data manager because it dictates the availability of the network to all other hardware when data transfer is needed. All other devices on the network (analyzers, printers, service panels, PC workstations) are Slaves and can only communicate when they are given permission by the network master. In some designs, the Master device will “poll” each of the attached analyzers to collect data and then pass the data onto the destination device. The network master also typically acts as the central I/O device for connecting serial links to DCS systems, printers, and personal computers.
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Figure 64: Typical Master/Slave Network
The Master/Slave design is well suited for applications that require lower hardware costs. The electronics and software that are required to implement this technique are readily available. Modification of the process analyzer is minimized since it plays a passive role in the network communications. As a result, the Master/Slave design has become the most commonly used design with the largest selection of options. The Master/Slave Technique suffers from two serious disadvantages: lack of full redundancy and lack of system configuration flexibility. In the Master/Slave network design, there can only be one active network master. If the network master fails, all communication on the network is stopped. Also lost is all I/O originating from the network master. Some designs allow a second network master to be connected in a standby mode; however, this design often results in complicated wiring and switch over procedures between the network masters. There is also the possibility of loss of data during the switch from one network master to the other. The Master/Slave design also limits network configuration flexibility by restricting the use of personal computers, printers, and central I/O. Most designs require these devices to be connected directly to the network master or the designs even use the network master itself in the dual role of a PC workstation. These types of designs limit the number and location of these devices; however, for smaller installations this may not be a problem. Masterless Networks - In the Masterless network, each piece of equipment that connects onto the
network (analyzers, printers, service panels, personal computers, etc.) contains its own electronics and software to regulate communication onto the network (see Figure 65). No network master is required because each device cooperates with one another on network access through a defined “access contention” procedure. The primary purpose of the access contention procedure is to keep two devices from trying to talk on the network at the same time. When an analyzer has data, it wants to transmit the data onto the network. The analyzer follows the access contention procedure to insure that the network is available for use.
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Figure 65: Typical Masterless Network The Masterless design provides the stability and redundancy needed for larger systems and where important data is being handled. The primary advantage of the Masterless design is its inherently redundant nature. There is no “single point of failure” that will stop all network communication. The Masterless design also allows more flexibility in how the network is configured. Because there is no network master controlling communication, there is no problem with having system printers, personal computers, and I/O in numerous locations on the network. This flexibility is extremely beneficial to larger facilities and to those locations with maintenance, engineering, and operations departments in different locations. The disadvantage of the Masterless design is that its highway sophistication has resulted in more expensive hardware. As a result, installation costs are higher and the availability of hardware is limited. It can also be more difficult to troubleshoot the Masterless network for communications problems because there is no single device that monitors and interrogates the network. Network Integrity and Redundancy
One of the most important points a user needs to consider when evaluating an analyzer network is its ability to handle network failures without the loss of data. In the process environment, there is always the possibility that hardware will fail, that cable will be accidentally cut or shorted, and many other potential problems. A well designed analyzer network needs to be able to handle system failures without loss of data. This can be accomplished by a number of techniques such as redundant electronics, dual network cable sets, and automatic switch over on loss of communication. Saudi Aramco DeskTop Standards
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Some of the important questions to ask when evaluating and analyzer network are: • • • •
Is there any single piece of hardware that is needed for the network to operate? How is this single point of failure prevented from happening? What happens when the network cable is accidentally cut or shorted? How does the network handle devices that “lock” onto the network?
Network Topology
Due to the higher initial installation costs of analyzer networks over traditional techniques, the network must be able to handle the plant’s requirements for many years. Consequently, one must consider the ability and ease of expanding the network to handle future demands. The issues that are involved in network expansion include maximum network size, both physical and system capacity, cabling restrictions, and the ability to add multiple types of analyzers onto the network. While most plants will never put 100% of their process analyzers onto the analyzer network, there are certainly tangible benefits to putting as many analyzers on the network as practical. Nevertheless, the sheer length of cable needed to connect everything can be intimidating. Fortunately, most networks support 1 to 5 miles of cable with even higher lengths available as options. It is important to remember cabling restrictions when estimating cable requirements. For example, many networks require all equipment to be connected one after another (daisy-chained) so that everything is connected in a single long cable path (see Figure 66). Some designs require that the cable radiate out from a central point, often the network master, into a star configuration. The third and most flexible approach are networks that allow branching. Branching often results in the minimum amount of cable used to connect everything together.
Figure 66: Network Topology Saudi Aramco DeskTop Standards
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System capacity, also called unit loading, refers to the maximum number of devices and I/O that can be handled by the network. All networks have a maximum number of analyzers and devices that can be connected together. Many of the newer designs allow 100 or more analyzers to be connected into one system; however, the addition of future analyzers may require changes in existing equipment. One example is a Master/Slave design where the network master needs to be modified as new equipment is added. Also, new or replacement equipment may be needed to get the network up to maximum capacity. Multi-Analyzer Integration
The integration of multiple types of analyzers onto one network can be a problem because currently available process analyzer networks are developed by individual process analyzer manufacturers as part of their analyzer system. Consequently, the user is often limited in choices based on the type of process analyzers he is currently using. Fortunately, most designs allow the connection of other analyzers onto the network through the use of an electronics interface device (EID). Typically, the EID would be mounted near the analyzer and it would receive the analog and status signal that are generated by the process analyzer (see Figure 67). Once this information is in the EID, it is available for transmission via the network to a printer, a trend recorder, or any other type of I/O device. The EIDs are also capable of performing calculations as part of their data handling function. It is often possible to program the EIDs to initiate calibration of the attached analyzers through the use of relay contacts.
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Figure 67: Electronics Interface Device
Analyzers that are connected by EIDs generally have less access over the network than analyzers that are designed to connect directly to the network. It is not possible to get all of the operational information via the EID such as operating temperature, power supply voltage, baseline offset, calibration factors, etc. This lower level of access to information may or may not be a problem depending on the complexity of the analyzer.
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INSTALLATION, OPERATIONAL, AND MAINTENANCE CONSIDERATIONS Installation Considerations Installation considerations concern the physical installation of the analyzer, sample handling system, and peripherals. Due to the harsh environment in Saudi Arabia, the installation requirements for Saudi Aramco applications are more demanding than standard industrial application. Evaluation of a process analyzer requires a comparison of the requirements for the specific application that are given in an Instrument Specification Sheet (ISS) with the capabilities of the particular analyzer that are given in the Analyzer Manufacturer's Data Sheet (AMDS). In the case of a gas chromatograph analyzer (GC), a Specific Application Data Sheet may also be provided by the manufacturer. Addendum A provides a procedure and information to evaluate the general installation considerations for all process analyzers. The following section provides information to evaluate the specific installation considerations for GCs. Each topic is identified by the category and item number presented on the ISS in Figure 69. Installation Data (Items 7 through 20) Location (Item 7) - Gas chromatograph analyzers require side access for installation and
maintenance. Typically, an additional 12" to 18" (305 to 457 mm) must be provided on both sides of the analyzer. The location of the sample handling system must be evaluated relative to the location of the GC for optimum installation and maintenance. When sample handling systems are provided as an integral part of a GC, they are usually installed below the analyzer. The size of the sample handling system depends on the type and complexity of the application. If a large sample handling system is mounted below the analyzer, maintenance of both the GC and sample handling systems is more difficult. The top of the GC is too high and difficult to access for maintenance of electronic components without a ladder. In addition, the bottom of the sample handling system is near the floor, which makes it difficult to perform operation and maintenance on sample handling system components. Indoors/Outdoors (Item 8) - Gas chromatograph analyzers are rather sophisticated and delicate
instruments that are designed for optimum performance in a controlled environment. In general, GCs are not suitable for outdoor installation and they should be protected from extreme environmental conditions. Mounting Type (Item 9) - Gas chromatograph analyzers are always wall-mounted. Gas
chromatograph analyzers are moderately larger and heavier than many typical analyzers. Proper attention must be given to the type of wall mounting attachment to assure proper support of the size and weight of the GC.
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Enclosure Type (Item 10) - Gas chromatograph analyzers are only available with general purpose,
wall-mount enclosures. Because of the complexity of the analytical technique and the diversity of control system options, GCs are not available with optional enclosures. Ambient Temperature (Item 11) - Gas chromatograph analyzers are capable of operating over a wide range of ambient temperatures because the analytical section is in a temperature-controlled oven. Nevertheless, the stability of the GC measurement is dependent on a stable ambient temperature. A stable ambient temperature is necessary for a stable supply of utility gases (carrier and fuel) because their pressure control regulators are not located in the temperature controlled oven and their pressure setpoints are affected by temperature.
Normal changes in ambient temperature (typical day to night changes of 20-30°F, 11-16°C) usually do not affect the gas chromatograph measurements outside of the performance specifications. Wide changes in ambient temperature, such as can be expected for Saudi Aramco applications, can affect the accuracy of the gas chromatograph measurement. Thus, for Saudi Aramco applications, it is recommended that GCs be installed in climate-controlled shelters. Ambient Corrosion (Item 12) - Some GCs are available with additional protection (material of construction or coating) for locations with high ambient corrosion. In addition, with the sophisticated electronics used by GCs, conformal coating of electronic circuit boards is usually a standard option.
The primary approach that is used to protect GCs from ambient corrosion is purging. The analytical section of a GC is already protected by purging because it is located in the air-bath oven. Because the control section of the gas chromatograph is installed in a general-purpose enclosure, it is usually available with an air purge as a standard option for hazardous areas. Vibration (Item 13) - The standard thermal conductivity detectors for GCs are not usually affected
by normal plant vibration. The more sophisticated and sensitive detectors such as flame ionization, flame photometric, and photo ionization detectors can be adversely affected by vibration from normal equipment near the analyzer such as process pumps or air conditioning units that are mounted on the analyzer shelter wall near the GC. Hazardous Area Data (Items 14 and 15) - Gas chromatograph analyzers have two separate sections:
the analysis or oven section and the controller or electronics section. Usually, the only electrical components in the analysis section are the detector and the air-bath heater. Most detectors have explosion-proof enclosures because they operate directly with the process sample. Because the air-bath heater requires air flow, it is usually designed for hazardous areas by compliance with purge requirements. When detectors are not available with explosion-proof enclosures, the airbath heater is sometimes used to reduce the area classification within the oven to non-hazardous by compliance with purge, pressurization, and dilution requirements.
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The sample injection valve is usually installed inside the oven section of a standard GC. For lowpressure gas applications (less than 50 psig, 345 kPa) this configuration does not present any additional hazard risk. For high-pressure gas samples and applications with volatile liquid samples, additional consideration to the hazardous area classification inside the oven section must be given due to the potential hazard in the event of a leak. If the detector and air-bath heater are explosion proof or if they are rated for Division 1 hazardous locations, the potential risk is eliminated. If the detector and air-bath heater are only rated for Division 2 or if they must be purged to meet the hazardous area classification, addition precautions should be considered. The recommended solution to minimize the potential hazard for applications with a high-pressure gas or volatile liquid sample is to install the sample injection valve outside the oven section. This configuration is more complex and it can only be used in certain applications with the approval of the manufacturer. For volatile liquid samples, a quill-type sample injection valve can also be used to minimize the potential hazard because the sample injection valve is configured with the process sample outside the oven section. The temperature rating of the hazardous area classification is a necessary factor in a GC evaluation because of the operating temperature that is required for the air-bath heater. Although the temperature control setpoint of the oven compartment must be lower than the hazardous temperature rating, the hazardous temperature rating has a greater impact on the maximum operating or surface temperature of the air-bath heater element. This maximum operating temperature of the heater element determines how fast the oven can be heated after a cold start. For certain applications that have either a high oven temperature or low hazardous temperature rating, it can be hours before operating temperature control is obtained. Although not critical, this circumstance is a nuisance when maintenance is performed. In addition, the hazardous temperature classification can increase the cycle time of a programmed temperature application, such as simulated distillation, by limiting the maximum operating temperature of the air-bath heater element. The control sections for many GCs are non-incindive and they are suitable for Division 2 hazardous areas without purging. The control section for other GCs must be purged for hazardous areas. Non-incindive control sections must also be purged for the more severe requirements of a Division 1 hazardous area. Purging is usually available as a standard option for GCs. Purging is usually not an inconvenience for installation because instrument air is a required utility for operation of the GC. Required Utilities (Items 16 through 20) - With the temperature-controlled oven, GCs require a
greater amount of ac power than most process analyzers (typically 1,400 watts with a 20 amp circuit). It is important that the power circuit and wiring be sized correctly to prevent a low voltage condition during heater operation, which can prevent accurate temperature control and adversely affect operation of electronic components.
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Gas chromatograph analyzers require instrument air for operation. The supply pressure, consumption, and the quality of instrument air are more important factors to evaluate for GCs than for other types of process analyzers. Plant air is not an acceptable air supply for GCs. The instrument air supply pressure must be sufficient for both valve actuation and oven air supply. The supply pressure is most important for valve actuation because a higher pressure is required. If the instrument air supply pressure is too low, the valves will not actuate. An even greater problem occurs when the instrument air pressure is only marginal. In this case, the valves have sluggish actuation, which causes repeatability errors and false results. If the instrument air pressure is insufficient for proper valve operation, plant nitrogen or cylinder gas can be used. Only inert cylinder gas should be used. In most cases, the gas chromatograph carrier gas is helium or nitrogen which would be acceptable because the valve consumption is low. A separate set of gas cylinders would only be required for valve actuation if the carrier gas is not inert. The air-bath heater in a GC requires an instrument air flow that is unusually high (normally 3 scfm, 85 Lpm, minimum) and not common for instrumentation. In plants that have many GCs, the GCs are the most significant users of instrument air. It is necessary that the instrument air consumption required by the GCs be confirmed with the plant instrument air supply system. In addition, the instrument air supply piping and tubing to the GC must be sized correctly to prevent appreciable pressure drop at the required flow. Correct sizing of supply piping and tubing is particularly critical when the instrument air supply pressure is only slightly higher than the minimum pressure required for valve actuation. In this case, the pressure drop in the instrument air supply lines to the GC at the required flow can cause the pressure at the analyzer to be insufficient for valve actuation. Gas chromatograph analyzers are normally configured to use instrument air for both valve and oven supply. Typically, GCs have a single common instrument air supply connection. In addition, because the oven heater supply is air, the oven exhaust air vent is normally open to the outside of the oven enclosure. When plant nitrogen or cylinder gas is required for valve actuation, it is necessary to route separate supply connections to the gas chromatograph analyzer (usually a standard option with the analyzer). Although it is not recommended that plant nitrogen be used for the oven supply, nitrogen is sometimes used as a backup source for the instrument air supply. There are other cases where it may be advisable to use plant nitrogen as the heater supply to purge the oven section with an inert gas. Purging the oven with nitrogen also minimizes the potential hazard when the application involves a highly volatile or toxic sample. With the relatively high oven heater flow, a potential oxygen deficiency condition could occur if nitrogen is exhausted directly from the GC into an enclosed area. To prevent this potential hazard, the exhaust from the oven must be routed to a freely ventilated area (usually outside an enclosed analyzer shelter). Oxygen deficiency alarms and warning signs should also be installed to minimize the potential hazard in the event an oven door must be opened for maintenance.
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If the exhaust air from the oven is routed away from the GC, care must be taken to size the exhaust vent line large enough to prevent back-pressure in the oven section. The air-bath heaters in GCs require a relatively high air flow to maintain precise temperature control (typically + 0.05°F/0.03°C). Excessive restriction of the air flow from the oven section can prevent accurate oven temperature control. Gas chromatograph analyzers require instrument quality air. If the instrument air is even slightly dirty, or if it contains oil or water, the chromatograph valves will become contaminated and will not actuate properly. Eventually the valves will fail and require cleaning or replacement. In addition, even small amounts of oil or hydrocarbons in the air supply will burn on the heater element of the air-bath heater, which causes the element to deteriorate and eventually fail. It is also common to use instrument air with a catalytic air cleanup unit to supply air for the flame ionization or photometric detectors. The catalytic air cleanup unit is only designed for trace amounts of hydrocarbons or carbon monoxide (parts per million quantities as acceptable in instrument quality air). As with the oven heater, even small amounts of oil or hydrocarbons in the air supply will burn in the catalytic heater element and cause the element to deteriorate and eventually fail. Operational Considerations Operational considerations concern the process application and the ability of the process analyzer and the sample handling system to perform the required measurement. Evaluation of a process analyzer requires a comparison of the requirements for the specific application that are given in an Instrument Specification Sheet (ISS) with the capabilities of the particular analyzer that are given in the Analyzer Manufacturer's Data Sheet (AMDS) and the Specific Application Data Sheet (SADS). Addendum A provides a procedure and information to evaluate the general operational considerations for all process analyzers. The following section provides information to evaluate the specific operational considerations for GCs. Each topic is identified by the category and item number presented on the ISS in Figure 69. Sample Supply Data (Items 21 through 28) Process Tap Location (Item 21 and 22) and Line Length (Item 23) - The typical sample injection valve of
a GC has a small orifice, which can only accommodate relatively low flow. For gases, the maximum typical flow is approximately 100 ccm. For liquids, an even smaller flow is required at approximately 20 ccm. (An exception to this is the quill-type sample injection valve used for some liquid applications.) Because the flow requirement to the GC is low, a fast-loop bypass stream is required.
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For gas process samples, the flow and pressure to the GC is not an important factor when the atmospheric reference valve arrangement is used (see Item 25 and 26 below). The atmospheric reference valve arrangement affords a high degree of versatility in the use of GCs for diverse process sample tap conditions. For liquid applications, the process supply tap must be selected to provide sufficient differential pressure to maintain flow to the process sample return point. Phase (Item 24) - Gas chromatograph analyzers can operate with either a liquid or gas sample. It is
critical to identify the sample phase at the GC because different sample injection valves are required for gases and liquids. For most applications, the phase of the process sample at the GC is simply determined by the phase at the process sample tap and the appropriate phase in the sample handling system. For applications with a liquid process sample that can be handled as either a gas or liquid in the sample handling system (such as hydrocarbon samples that have ambient-temperature boiling points like butane), the natural tendency is to handle the sample as a gas. Although the GC is better suited to gas process samples, it is important to consider the following additional factors for the sample phase in these applications. The most important factor in determining the phase of the process sample at the GC concerns the calibration sample that is required for the application. For a gas phase sample injection, the gas calibration sample must have a dew point that is below the minimum ambient or storage temperature at the maximum supply pressure of the calibration sample cylinder. Because gas is compressible, calibration gas samples are supplied under pressure to provide a sufficient quantity of gas. As a result, the analyzer can be calibrated with the same standard over a period of time (usually 2-3 months). Standard high-pressure cylinders are pressurized to approximately 2,000 psig (13,789 kPa) to give the maximum quantity of calibration gas. For low-pressure applications, larger cylinders are used to increase the gas quantity; however, they can only be pressurized to 250 psig (1,724 kPa) maximum. Normally, a low-pressure gas calibration cylinder will have a minimum starting pressure of 200 psig (1,379 kPa) to provide a sufficient quantity of calibration gas. There are special cases where a lower pressure is required by the analyzer manufacturer. For liquid sample injection, the liquid calibration sample must have a bubble point that is above the maximum ambient or storage temperature at the minimum supply pressure of the calibration sample cylinder. Because liquid is non-compressible, calibration liquid samples are pressurized for supply to the analyzer with an external supply of inert gas that is commonly called a pressure pad. In addition, to prevent potential vaporization of light components during transport and storage, a minimum pressure pad is applied to the liquid calibration cylinder when it is initially supplied. Normally, a liquid calibration cylinder will have a minimum storage and operating pressure of 100 psig (690 kPa). The cylinder can only be pressurized up to 250 psig (1,724 kPa) maximum.
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The other factor in determining the sample phase at the GC injection valve is the oven temperature. Because the oven operates at an elevated temperature, which is determined and fixed by the measurement requirements, the sample phase in the GC must be evaluated with this additional temperature criteria. For liquid process samples that can be vaporized and handled as a gas, the dew point temperature of the sample must be lower than the oven temperature at the sample pressure in the sample injection valve. If the required oven temperature is not above the gas sample dew point, the operating pressure in the sample handling system should be reduced to lower the sample dew point. If it is not possible or practical to lower the gas sample pressure (usually because of sample effluent return constraints), the sample should not be vaporized and handled as a liquid. For liquid samples, the bubble point temperature of the sample must be higher than the oven temperature at the sample pressure in the sample injection valve. If the oven temperature is above the liquid sample bubble point and the temperature cannot be lowered due to application requirements, the liquid sample should be vaporized and handled as a gas. A unique situation can occur with the sample phase of hydrocarbon process samples that have boiling points in the ambient temperature range (primarily C3, C4, and C5 hydrocarbons). It is possible that the process sample is a gas at low pressure and above-ambient temperature such that a gas sample injection valve is required. However, the dew point of a calibration sample with these components would be above the ambient temperature at even low pressure. The only solution in this situation is to use a liquid calibration sample and vaporize the liquid in the sample handling system prior to introduction to the GC. The manufacturer will usually identify this unique situation where the phase of the calibration sample is different from the phase of the sample injection valve. Sample Supply Pressure (Items 25 and 26) - The sample supply pressure requirements for a GC
depend on the phase of the sample. Standard gas sample injection valves are designed to operate at moderate pressure, approximately 300 psig (2068 kPa) maximum. Optional gas sample injection valves are available for applications that require a high sample inject pressure up to approximately 1,500 psig (10350 kPa) maximum. Standard liquid sample injection valves are usually rated at a slightly higher pressure than the standard gas sample injection valve approximately 500 psig (3450 kPa) maximum. These valves are also available with an optional high-pressure configuration, approximately 1,500 psig (10350 kPa). Although it is important to verify that the sample supply pressure does not exceed the design pressure of the sample injection valve, there are certain applications where a minimum sample supply pressure must also be confirmed. Diaphragm-plunger sample injection valves require a minimum supply pressure for operation (typically 5 psig, 35 kPa). Sliding-plate or rotary-type sample injection valves do not have a minimum supply pressure requirement.
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For gas process samples, it is necessary to maintain a stable pressure in the sample injection valve because the sample is compressible and a change in the sample pressure will cause a change in the measurement. Because the pressure needs to be constant at the time of injection, only a constant inject pressure is required. A constant injection pressure is usually provided by the use of an atmospheric reference valve arrangement. The sample in the atmospheric reference valve is blocked and vented to atmospheric pressure. With the atmospheric reference valve arrangement, the flowing pressure in the sample injection valve can vary without adverse effect on the measurement. Thus, process sample effluent from GCs can be more readily routed to a process return point. With the atmospheric reference valve arrangement in gas applications, the sample supply pressure is not a limiting factor and vacuum applications are possible. When the supply pressure is a vacuum, special consideration must always be given to the type of sample injection valve that is used. In order to assure proper operation and prevent leakage that could contaminate the sample. The manufacturer must be consulted for appropriate design requirements when the sample supply to a GC is under a vacuum. For liquid process samples, the sample pressure is not a factor because liquids are not compressible. When a diaphragm-plunger, sliding-plate, or rotary-type sample injection valve is used; however, the pressure drop through the sample injection valve must be evaluated carefully. The orifice and port sizes of these types of sample injection valves are small and restrict liquid flow. In applications with a volatile liquid sample, the pressure drop through the injection valve can affect the sample phase integrity during injection. As a result, the measurement will not be repeatable. To eliminate this problem, the sample flow can be isolated before sample injection with a shut-off valve at the outlet of the sample injection valve to eliminate the flow instability by providing consistent sample inject conditions. Because the supply pressure is greater than the return pressure, isolating the sample at the outlet of the injection valve provides a higher pressure in the sample injection valve for greater stability. Sample Supply Temperature (Items 27 and 28) - The sample supply temperature requirements for a
GC are primarily limited by the injection valve’s materials of construction. Standard sample injection valves are designed to operate at moderate temperatures, approximately 250°F (122°C) maximum. Optional sample injection valves are available for applications that require a high sample inject temperature, approximately 450°F (234°C) maximum. Because sample injection valves with a higher temperature rating usually require a lower pressure rating, the temperature and pressure specifications of the sample injection valve must be evaluated together. For simulated distillation applications, the sample supply temperature must be lower than the initial boiling point of the process sample. In this case, it is important to recognize the difference between the initial boiling point, which is determined at atmospheric pressure, and the bubble point, which is determined at process pressure.
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Process Data (Items 29 through 38) Density or Specific Gravity (Item 29) - Sample density or specific gravity is not an important factor to
GCs. Molecular Weight (Item 30) - The sample molecular weight is not an important factor to GCs. Bubble Point or Dew Point (Item 31) - Gas chromatograph analyzers can operate with either a liquid
or gas process sample. Because the oven operates at an elevated temperature, the sample bubble or dew point in the GC must be evaluated with this additional temperature criteria. For gas process samples, the dew point temperature of the sample must be lower than the oven temperature at the sample pressure in the injection valve. If the required oven temperature is not above the gas sample dew point, the operating pressure in the sample handling system should be reduced to lower the sample dew point. If it is not possible or practical to lower the gas sample pressure (usually because of sample effluent return constraints), the sample should not be vaporized and handled as a liquid. For liquid samples, the bubble point temperature of the sample must be higher than the oven temperature at the sample pressure in the sample injection valve. If the oven temperature is above the liquid sample bubble point and the temperature cannot be lowered due to application requirements, the liquid sample should be vaporized and handled as a gas. Viscosity (Item 32) - Viscosity only applies to liquid process samples. For liquid samples, the
viscosity must be compatible with proper flow in the GC sample injection valve. Certain types of sample injection valves such as the sliding plate, rotary, and diaphragm plunger type valves have small ports and flow paths to minimize sample volume as required for liquid measurements. The sample viscosity of the liquid sample must be low enough to allow proper flow through the injection valve. For liquid samples with a high viscosity, the quill-type injection valve provides a larger flow passage for less restriction to a flowing liquid sample. Stability (Item 33) - In a standard gas chromatograph configuration, the sample injection valve is
located inside the temperature-controlled oven compartment. The process sample must be stable at the specified oven temperature. If the process is unstable at the elevated oven temperature, the sample injection valve can be installed outside the oven. This configuration is more complex and it can only be used in certain applications when approved by the manufacturer. For liquid samples, the quill-type sample injection valve is designed to keep the flowing sample outside the oven to prevent exposure of the process sample to the oven temperature.
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Contaminants (Item 34) - Contamination of GC columns changes the component separation
efficiency. The most common contaminants are substances that coat the sample injection valve and causes the sample inject volume to change. Additional contaminants are sample components that build up in the columns, usually heavy hydrocarbons. Column contamination is minimized by the use of heartcut and backflush valve configurations. When columns become contaminated, they can be reconditioned by purging with carrier gas for a period of time (normally hours but sometimes days) or the column must be replaced. Condensable (Item 35) - Condensable only applies to gas process samples. If the process sample is
condensable, the dew point temperature of the sample must be lower than the oven temperature at the sample pressure in the injection valve. If the required oven temperature is not above the gas sample dew point, the operating pressure in the sample handling system can be reduced to lower the sample dew point. Corrosive (Item 36) - For the GC, the material of the sample injection valve must be compatible
with the process sample. Because only a small quantity of process sample is injected for analysis, other components in the GC such as columns and detectors are usually constructed of general corrosion-resistant material such as stainless steel. In addition, a backflush valve configuration is normally used to prevent potentially corrosive sample components from coming in contact with sensitive analysis components such as the detector. Toxic (Item 37) - If the process sample is toxic, potential leaks in the GC would normally occur at the sample injection valve seal. The seal arrangement must be evaluated to determine the potential hazard in the event of a leak. Sliding-plate and rotary-type sample injection valves do not have configurations that would allow containment of a leak. Diaphragm-plunger valves can be configured with a “captive vent” to allow purging and venting of toxic samples, which may leak in the event of a break in the diaphragm seal. Quill-type sample injection valves used for liquid applications include a "stuffing box" that provides a double seal with a void space to allow venting or draining of process sample that might leak from the primary valve seal.
Except for the quill-type that is valve used in certain liquid applications, the sample injection valve for GCs is installed inside the oven section. Because the oven temperature is controlled with an air-bath heater, the potential hazard from a leak of toxic sample can be minimized by routing the exhaust air from the oven section to a safe area (usually outside an enclosed analyzer shelter). As previously mentioned, care must be used when routing the oven exhaust air from the chromatograph analyzer. The vent line must be sized properly to prevent back-pressure in the oven that would disrupt the operation of the oven temperature control. In addition, the oven enclosure is not designed to provide an air-tight seal. Back pressure in the oven compartment could also allow leaks of potentially toxic sample to escape from an oven compartment that is only partially sealed.
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Additional safety measures must be utilized for gas sample applications that have a significant toxic hazard in the event of a leak. For GCs that operate with a toxic gas sample under high pressure or where a low-pressure gas sample has a toxic component significantly above the TLV (threshold limit value), it is not sufficient to simply route the oven exhaust air to a safe area because maintenance personnel can still be exposed when the oven door is opened. Additional safety features such as installing a toxic gas sensor on the oven exhaust air vent line must be used to warn of a toxic leak inside the oven section. Alternately, the oven door can be locked to prevent opening unless personnel wear special protection apparatus or the GC is isolated and purged of the toxic sample. The same toxic hazard precautions that apply to the GC also apply to the sample handling system. Consequently, a special configuration can be used where the sample injection valve is installed in the sample handling system enclosure that is mounted directly adjacent to the GC. With the gas chromatograph oven section isolated from the sample handling system enclosure, special safety measures would only be required for the sample handling system. Because only a very small quantity of process sample (microliters) is injected into the gas chromatograph analysis section, special venting of the oven exhaust air and safety features for the oven section are not required. Particulates (Item 38) - The process sample that flows to GCs must be clean to avoid deposits in the
sample injection valve and cause the sample inject volume to change. Proper sample filtration in the sample handling system is also important for GCs because particulates are the primary cause of wear and plugging in the sample injection valve. Because sample injection valves have small ports and flow paths, the valves are susceptible to plugging from particulates . In addition, the sample injection valve is a moving device so it is more subject to wear and failure if particulates are present in the process sample. Due to the adverse affect of particulates on GCs, a “guard” filter (typically 7 microns) is normally included directly upstream of the sample injection valve. For process samples that have particulates, the design of the different types of sample injection valves is also a factor in the valve performance. For applications with particulate problems, the diaphragm-plunger injection valve can provide longer and more stable analysis results than sliding plate or rotary-type valves. Sliding-plate and rotary-type sample injection valves are adversely affected by particulates because they have moving surfaces that can be “scored” by the particulates. This form of abrasion is a particular problem in the gas chromatograph injection valve because it can allow “cross-port” leakage. Cross-port leakage is difficult to detect by very sensitive flow measuring devices and it will eventually result in incorrect measurements. The diaphragm-plunger injection valve is less susceptible to cross-port leakage and it is better suited for applications with high particulates. Sample Return Data (Items 39 through 46)
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Analyzer Return Data (Items 47 through 50)
The wide operating pressure ranges that are available for sample injection valves makes the GC well suited for gas and liquid applications where the analyzer sample effluent must be returned under pressure. For liquid applications, it is preferable to maintain the process sample at a pressure that is close to the pressure of the process supply point to prevent changes in the sample composition at the time of injection. Thus, it is usually possible to combine the analyzer sample return with the process return from the sample handling system. For this to be accomplished, there must be sufficient differential pressure to maintain the required analyzer supply flow. For gas process samples, it is only important to maintain a stable pressure in the sample injection valve at the time of injection. A constant injection pressure is usually provided by the use of an atmospheric reference valve arrangement . In atmospheric reference valve arrangements, the sample in the injection valve is blocked and vented to atmospheric pressure immediately prior to sample injection. With the atmospheric reference valve arrangement, the pressure in the sample injection valve can vary without an adverse effect on the measurement. Thus, the process sample effluent from the GC can be more readily routed to a process return point. Output Signal Data (Items 51 through 54) Measurement Signal (Item 51) - For GCs that are used in conventional measurement applications, a
milliamp analog output signal is generally required for transmission of measurement results. Because GCs can measure multiple components, they are designed to provide multiple output signals. In addition, GCs are available with a wide variety of output signals, which are configured for the specific application. Both non-isolated and isolated milliamp analog signals are available and can usually be configured as either self-powered or loop-powered with a simple connection or jumper wire change. Because GCs are microprocessor-controlled, direct digital communication of measurement signal data is common. Direct digital communication is a particular advantage with GCs because most applications involve multiple measurements, which would otherwise require many analog signal cables. Gas chromatograph analyzers can be tied together in a network (see Item 72) to allow all chromatograph signals to be directly transmitted to the process control system with a minimum of signal wiring and interface equipment cost. Another advantage of digital communication over analog involves the power supply. The power supply in a GC is only capable of driving a moderate number of milliamp analog signals (typically 8 to 10) in a typical application. If an application requires more output signals than the power supply can support, a separate output signal system with a power supply is required. By using digital communication, this additional cost and installation complexity at the GC is avoided.
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When direct digital communication is used, special protocols are required for correct transmission and receipt of the data. Of course, it is necessary to correctly identify the measurement signal (analyzer, stream, and component). In addition, with digital communication, the range or scaling factor associated with the measurement data must also be correctly identified to avoid misinterpretation of the data by the process control system. Status Signal (Item 52) - With the microprocessor controller, GCs are capable of performing internal
diagnostics to provide status alarms. Status alarms are always available for local indication but they can also be transmitted to the control system as alarm contact output signals. As with analog output signals, GCs are available with a wide variety of status alarms and output signals. In addition, external status signals from the sample handling system or utility gas supply can be connected as input signals into the control section of the GC. These external status signals are included as part of the diagnostic and status alarm functions. The direct digital communication capability of the GC also allows communication of status alarms, which reduces the digital signal wiring and control system hardware interface requirements. By communicating diagnostic and status signals with the measurement data, the process control system can check the analyzer performance to confirm the validity of measurement data. When GCs are connected together in a network configuration, operating and maintenance data for any analyzer can be accessed at any point on the network. Thus, gas chromatograph measurement and status data can be monitored at a centrally-located maintenance station, such as in the control building. For status conditions that require immediate attention, an alarm can be transmitted to the operator who can obtain more detailed information from the network interface. For status conditions that need only preventative maintenance, a simple report can be generated and stored for review during normal maintenance procedures. Limit Alarm Signals (Items 53 and 54) - Limit alarms are available as a standard option with GCs. Limit alarms can either be transmitted as digital output signals or transmitted with measurement data over the digital communication link. Performance Data (Items 55 through 58) Analyzer Response Time (Item 55) - Gas chromatograph analyzers utilize a batch-type analytical
technique where the analyzer response time is determined by the analysis cycle time. The analysis cycle time is normally the time associated with the chromatography process in the analysis section. For typical applications, the microprocessor controller makes data processing of the measurement results concurrent with the chromatograph analysis. Where special analytical results are calculated from the chromatograph measurement data (e.g., BTU and simulated distillation), the cycle time is slightly longer than the analysis time in order to yield the calculated results.
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The analysis cycle time for GCs is a direct function of the measurement requirements. Simple, single-component measurements require the shortest analysis cycle time. Because GCs are capable of separating and measuring many components in a process sample, there is a natural tendency to “want” as many component measurements as possible This tendency has the result of making many GCs more complex than is necessary with analysis cycle times that are needlessly long. In addition, complex analyses are difficult to operate and maintain which reduces the reliability of the measurement results. Component measurements should be restricted to the measurement that is “needed.” Except for specific applications, component measurements by GCs should be limited to 10 minutes. Analyses that require longer cycle times should either be simplified or performed by multiple GCs on the same sample stream. Sample Transport Lag Time (Item 56) - GCs are not used in applications that require a fast
measurement response time because the analysis cycle time for GCs is typically 5 to 10 minutes. Thus, it is also not necessary for GC applications to have a fast sample transport lag times, as is normally the case for analyzers that provide a continuous measurement. The combination of the analysis cycle time and sample transport lag time should to be considered together when determining the measurement response time required by the application. Unless a minimum measurement response time is required, it is not necessary for the same sample transport lag time to be disproportionately shorter than the analysis cycle time. It is also important to recognize that the sample transport time is always a factor in the overall measurement response time, even if sample continues to flow and purge the sample inject valve during an analysis cycle. When the sample transport lag time can be de-emphasized, the sample handling system can also be simplified. Although fast-loop bypass streams are required for GCs, the bypass flow rate is not as critical when a fast response time is not necessary. Decreasing the bypass flow minimizes the amount of process sample that is consumed or that must be reprocessed. Analyzer Sensitivity (Item 57) - Gas chromatograph analyzers are very versatile instruments for
making analytical measurements because the chromatographic technique is selective to the particular component(s) of interest. Gas chromatograph analyzers are also capable of measuring a wide range of concentration because different types of detectors can be used to perform the required measurement. Each type of detector has particular limits of measurement sensitivity. These limits are a function of the detector in general and on the particular response of the detector to the component of interest as follows: • • •
Thermal conductivity detectors are more sensitive to components that have a high thermal conductivity. Flame ionization detectors do not respond well to components that have a low ionization potential. Flame photometric or photo ionization detectors only respond to components at a specific wavelength of light.
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In general, gas chromatograph detectors have the following minimum range of operation: • • • •
Thermal Conductivity - 500 ppm Flame Ionization - 5 ppm Flame Photometric - 1 ppm Photo Ionization - 0.1 ppm
Thus, thermal conductivity detectors are used for high-range measurements and flame ionization detectors are used when the measurement range is low. Flame ionization and photo ionization detectors are used to measure specific components at a low range. Because each detector has a different response and sensitivity to different components, each measurement must be confirmed by the manufacturer. Analyzer 24-hour Drift (Item 58) - Because GCs use a batch-type analytical technique, the
measurement drift associated with an application is defined in terms of the repeatability of consecutive analysis results over time. The measurement repeatability associated with GCs is primarily a function of the complexity of the application and of the type of detector that is used for the measurement range. Detectors that are used for low range measurements are more sensitive and they are more likely to change performance over time. The more complicated applications require special column packing and complex valve switching functions that are also more susceptible to a change in performance over time. In general, the repeatability of gas chromatograph measurements is as follows: • • • • •
+ 0.5 % over 24 hours for full scale ranges from 2-100 % + 1 % over 24 hours for full scale ranges from 0.5-2 % + 2 % over 24 hours for full scale ranges from 50-500 ppm + 3 % over 24 hours for full scale ranges from 5-50 ppm + 5 % over 24 hours for full scale ranges from 0.5-5 ppm
Because each detector has a different response and sensitivity to different components and the complexity of the component measurement depends on the application, the repeatability for each measurement must be confirmed by the manufacturer. Although the inherent measurement drift for GCs is determined by the application, there are other external factors that affect repeatability. Some common problems that affect repeatability are as follows:
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•
Oven Temperature Control - Stable operation of the chromatographic analytical technique is obtained by precise temperature control by the air-bath heater (typically + 0.05°F/0.03°C). Factors that affect the air-bath heater such as supply voltage fluctuation, instrument air supply pressure fluctuation, or back-pressure on the exhaust air vent will affect the repeatability of the measurement. Poor instrument air quality (oil, water, or particulates) can also have an adverse affect on the oven temperature control by affecting heat transfer from the heater element.
•
Ambient Temperature - Although the oven section of a GC is well insulated and the temperature control of the air-bath heater is precise, rapid or wide swings of ambient temperature can affect the measurement stability. Ambient temperature primarily affects the control of the instrument air and carrier gas pressure because the pressure control regulators are not located in the temperature-controlled oven and their pressure setpoints are affected by temperature.
•
Sample Injection Volume - The volume of sample that is injected by the GC must remain constant to provide stable and accurate measurement results. For liquid samples, which are not compressible, it is only necessary to maintain a relatively constant temperature to obtain a constant sample volume. For gas sample, which are compressible, the sample pressure is important to maintaining a constant volume for each analysis cycle. Because the required precision of the pressure control is not easily obtained with pressure regulators, the atmospheric reference valve arrangement is used for almost all gas applications.
•
Column Loading or Deterioration - Any factor that affects the performance of the column (e.g., column packing deterioration or column loading) will potentially affect its measurement performance. Although a high operating temperature can be helpful at preventing column loading (along with backflush and heartcut valve configurations) it can also cause faster deterioration of the column packing material.
•
Sluggish Valve Actuation - Gas chromatograph valves that have slow or inconsistent operation can also affect the measurement repeatability. Valve performance is affected by the instrument air pressure or contamination of the valve actuator by oil, water, or particulates in the air supply.
•
Detector Contamination - Sample components that contaminate or build up on the detector can affect the repeatability of the measurement over time. Backflush and heartcut valve arrangements can prevent exposure of the detector to contaminating components.
•
Carrier and Fuel Gas Quality - Contaminants such as water, air, or hydrocarbons in the carrier gas can adversely affect the measurement stability in certain applications. In addition, contaminants in the fuel gas for flame detectors can also cause instability in the detector performance. Chromatographic grade carrier gas (99.9995% pure) and fuel gas must always be used with GCs.
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Special Data Section (Items 69 through 72) Detector (Item 69) - The selection of the type of detector for GC applications is determined by the
type of component(s) to be measured and the measurement range. Normally, the detector selection is straight forward and it is predetermined by the manufacturer. The selection of the type of detector can become an important factor for applications that require a wide or multiple measurement range. A typical example would be a fractionation tower where it is necessary to control an impurity in the overhead stream at a low range but also desirable to monitor the impurity concentration at a high range during an upset or startup condition. In this case, a flame ionization detector would not be accurate at the high range but a thermal conductivity detector would not have the sensitivity to measure the low range accurately. Because the application requires measurement accuracy at the low range and only a measurement trend at the high range, a flame ionization detector should be specified. When a flame-type detector is identified on the ISS, the availability of fuel gas from cylinders must be confirmed and the fuel gas should be identified as a consumable for the application. Carrier Gas (Item 70) - Gas chromatograph analyzers require a carrier gas for operation.
Identifying the type of carrier gas on the ISS provides a means to confirm that carrier gas cylinders are available for the GC installation. The carrier gas should also be identified as a consumable for the application. The grade of compressed gas that is used for the carrier gas is important. To provide the required purity for proper operation, only compressed gas that is rated as “chromatograph” grade or better should be used. Chromatographic grade compressed gas is certified to be 99.9995% pure. Specification of the proper grade for the carrier gas is important to assure that contaminants such as oxygen, nitrogen, moisture, and hydrocarbons are below levels that could affect the stability of the measurement. Columns (Item 71) - Gas chromatograph analyzers use either packed or capillary columns.
Typically, the type of column is determined by the manufacturer according to the application and measurement requirements. The type of column is given on the ISS to identify the column as a necessary spare part. Network (Item 72) - The network is specified on the ISS to identify if the GC is to be connected on
the direct digital communication network. Typically, the requirements for the network, network features, and data communication over the network are defined in a separate specification. Stream Composition Data (Item 73)
In most cases, the GC manufacturer must be consulted to determine if a particular measurement can be made for a specific application. Based on the application requirements, the manufacturer will provide the configuration and operating performance of the GC.
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Maintenance Considerations Installation and operation considerations are critical to the initial design and implementation of process analyzers and sample handling systems. Maintenance considerations concern the requirements for proper performance of the analyzer and sample handling system once it is put in operation. Maintenance of process analyzers is fundamental to the long-term success of the installation. Evaluation of a process analyzer requires a comparison of the requirements for the specific application that are given in an Instrument Specification Sheet (ISS) with the capabilities of the particular analyzer that are given in the Analyzer Manufacturer's Data Sheet (AMDS) and the Specific Application Data Sheet (SADS). Addendum A provides information to evaluate the general maintenance considerations for all process analyzers. The following section provides information to evaluate the specific maintenance considerations for GCs. Each topic is identified by the category and item number presented on the Instrument Specification Sheet (ISS) given in Figure 69. Calibration Data (Items 59 through 62) Manual or Automatic Calibration (Item 59) - Gas chromatograph analyzers are very sophisticated and
complex analytical instruments. The complexity of their operation makes GCs more susceptible to factors that affect the measurement accuracy. Thus, verification of calibration is required on a frequent basis, typically once per day. With the relatively long analysis cycle time (typically 5-10 minutes), manual calibration of a GC takes more time than the calibration of continuous-type analyzers. Fortunately, only span calibration is required as zero correction is performed automatically on each analysis cycle. Because of the frequency and time required to perform calibration, GC applications usually include automatic calibration verification. With the microprocessor controller, automatic calibration or calibration verification is a standard feature with GCs. For simple applications with a simple and stable calibration sample, automatic calibration of the gas chromatograph measurement(s) is an acceptable approach. For complex applications, or for applications that require a complex or potentially unstable calibration sample, automatic calibration can cause incorrect measurement data. Consequently, only automatic calibration verification should be used. Calibration verification is the same function as automatic calibration except that the measurement response factors are not automatically updated. During calibration verification, the component measurement value is compared to the known calibration sample component concentration that is manually entered into the calibration program. If the measured value is outside the preset error limit, an alarm is generated to alert maintenance personnel that a calibration error has been detected. Maintenance personnel can then evaluate whether the GC is operating properly, which only requires updating of the measurement response factors, or if maintenance is required to correct the analyzer’s operation. Manual updating of the calibration response factors prevents measurements from being made with incorrect data or faulty equipment. Saudi Aramco DeskTop Standards
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Lab Sample (Item 60) - Lab sample facilities are specified for the sample handling system when it is
desirable to verify the GC measurement with a laboratory analysis. Zero and Span (Items 61 and 62) - Gas chromatograph analyzers only require a calibration span
sample. Zero calibration is not required because the chromatograph measurement technique performs an automatic zero correction with the carrier gas as the zero baseline. For GCs, it is important that the span calibration sample includes any background components that must be separated from the component(s) of interest by the chromatographic technique. Calibration with the background components is required to obtain an accurate calibration of the GC measurement relative to the actual process operating conditions. Maintenance Data (Items 63 through 68) Accessibility (Item 63) - Gas chromatograph analyzers require open space on both sides to perform
maintenance. Space is needed to fully open the oven compartment door for complete access to all parts inside the oven. With a typical configuration, space is needed on the sides for access to utility gas supply components and, in some cases, for access to electrical connections. Due to the complexity of GCs, space for a workbench or portable working surface is also recommended near or adjacent to the analyzer to allow disassembly and maintenance of parts. Calibration Frequency (Item 64) - Gas chromatograph analyzers normally require calibration
verification on a daily basis. For certain simple applications, it may be determined that the calibration frequency can be extended to a longer period, but only after the operating performance has been verified. Routine Service (Item 65) - The routine service requirements for GCs is dependent on the
performance of the analyzer in the particular application. In general, the columns, valves, and detectors will function properly for 6 to 12 months. In certain difficult applications, more frequent maintenance or replacement of the columns, valves, or detector may be required. A schedule for routine service must be established for each GC application based on actual performance data. Consumables (Item 66) - Gas chromatograph analyzers require carrier gas for operation. In
addition, detectors that operate with a flame also require fuel gas. Both carrier and fuel gases are supplied in compressed gas cylinders. In general, GCs consume carrier and fuel gas at a rate of slightly less than one compressed gas cylinder per month. It is important that replacement or backup cylinders are available to assure uninterrupted operation of the GC. Carrier and fuel gases must be supplied from a manifold of compressed gas cylinders (minimum of two) to assure the continuous, uninterrupted gas flow to the GC. The gas cylinder manifolds must include both block valves and bleed valves to allow purging of the interconnecting tubing when gas cylinders are replaced. Saudi Aramco DeskTop Standards
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Burner air for flame detectors can be supplied from either compressed gas cylinders or from a clean instrument air supply. When instrument air is used for burner air supply, a catalytic air cleanup unit is required to eliminate any residual combustible material that would otherwise affect the performance of the flame detector. The AC power to the catalytic air cleanup unit should be switched separate from the GC so that the unit can remain heated and ready to operate when power is removed from the analyzer for maintenance. Instrument air can contain sulfur compounds from plant emissions that enter the instrument air system at the inlet. Consequently a catalytic air cleanup unit is not recommended for applications where low range sulfur compounds (less than 10 ppm) are measured with a flame photometric detector. Although the sulfur is oxidized in the catalytic air cleanup unit, there is still residual sulfur that causes background interference with the flame photometric detector. For these applications, burner air from compressed gas cylinders is recommended. Spare Parts (Item 67) - The most common spare part required for GCs is the column. Over time, packed columns become contaminated or deteriorate and they must be replaced. In general, the useful life of a packed column is at least 6 months. Fused-silica capillary columns are very fragile and break easily.
Because the valves in the GC are moving parts, they eventually wear out and fail. Diaphragmplunger type valves have longer operating lives because they do not have moving surfaces like sliding plate or rotary-type valves. Depending on the application, gas chromatograph valves typically function for 6 months or longer before maintenance is recommended. Gas chromatograph analyzer valves are supplied with spare parts to allow reconditioning and reuse. Gas chromatograph analyzer detectors can also become contaminated in time and require replacement of their elements. Spare filaments are recommended for thermal conductivity detectors; spare burner tips are recommended for flame detectors; and spare source or detector tubes are recommended for photo-type detectors. In most applications where instrument quality air is available, the oven air-bath heater should not require maintenance or replacement. In addition, air pilot solenoid valves should also not require maintenance or replacement. Spare oven heaters and air pilots should be available in case of a failure. Although the electronic components in the GC are not expected to fail or need replacement, spare components should be available to minimize analyzer down time. Electronic functions for GCs such as the microprocessor controller, detector electronics, and analog/digital input/output interfaces are typically supplied on separate boards to allow convenient replacement. Electronic boards do not fail often and they can be expensive to stock as spare parts; therefore, the quantity and type of spare electronics boards depends on the total number of GCs at a particular location.
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Training (Item 68) - Gas chromatograph analyzers are sophisticated and complicated instruments
that require a significant level of knowledge and experience to operate and maintain. Expertise is required for both the gas chromatograph analytical technique and operation programming. Training on the operation and maintenance of the GC is necessary for all technical personnel. The typical training program for GCs is a minimum of one week and up to two weeks for in-depth coverage.
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EVALUATING GAS CHROMATOGRAPH ANALYZERS IN RELATION TO REQUIREMENTS FOR SPECIFIC APPLICATIONS Refinery Application Fractionation Tower
Gas chromatograph analyzers are used to monitor the quality of separation by fractionation towers by identifying the concentrations of important components. A typical example of a gas chromatograph analyzer in a process control application is the depropanizer fractionator at the Saudi Aramco, Ras Tanura Plant 40 in Figure 68. As shown in the figure, propane and lighter hydrocarbon components exit the depropanizer in the overhead stream. Butane and heavier hydrocarbon components exit the depropanizer in the bottoms stream. Generally, process chromatographs are used to sample up to three locations; the overhead product, the bottoms product, and the feed stream. The measurement of the overhead and bottoms is directed toward controlling the loss of a valuable product or minimizing the presence of an impurity in a stream. A gas chromatograph (AT) is used on the overhead product stream to measure ethane and butane as impurities in the propane. It is preferred to measure the impurities in the overhead stream instead of the propane because a small change in the impurities can be measured with greater accuracy than a small change in the propane. For example, the change in concentration of an impurity from 3.1% to 3.3% can be measured with greater precision than a change of the major component from 96.9% to 96.7%. This information is used by the control system to monitor the tower performance against the setpoint. A typical Instrument Specification Sheet for the gas chromatograph analyzer in this application is shown in Figure 69. A typical Analyzer Manufacturer's Data Sheet (AMDS) for a gas chromatograph analyzer provides only general information and does not identify any specific measurement data (Figure 70). The manufacturer provides specific measurement data for each gas chromatograph application in a separate Specific Application Data Sheet (SADS). A typical SADS for the gas chromatograph analyzer is shown in Figure 71. A typical Sample Handling System Diagram for the gas chromatograph analyzer is shown in Figure 72.
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Figure 68: Ras Tanura Plant 40 Depropanizer Process Flow Diagram
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Saudi Arabian Oil Company Instrument Specification Sheet - Analyzer 1 2 3
General
Tag Number Application P & ID Number
* Depropanizer Prod. * Shelter No. *
4 5 6
(Draft Copy)
Instrument Type Manufacturer Model Number
Chromatograph
14 15 16 17 18 19 20
Hazardous Area Haz Temp Rating AC Power Inst. Air Pressure Nitrogen Pressure Cooling Water Pres. Steam Pressure
Cl. I, Gr. D, Div. 2 T2A 120 VAC, 60 Hz 80 psig N/A N/A N/A
* *
7 8 9 10 11 12 13
Location Indoors or Outdoors Mounting Type Enclosure Type Ambient Temperature Ambient Corrosion Vibration
21 22 23 24
Process Tap Location Line or Vessel No. Line Length Phase
Reflux Pump Disch.
Sample Supply Data
*
200 ft. Liquid
25 26 27 28
Pressure Normal Pressure Max | Min Temperature Norm Temp. Max | Min
360 psig 400 psig | 300 psig 130°F 150°F | 100°F
Process Data
29 30 31 32 33
Density or S. G. Molecular Weight Bubble Pt or Dew Pt Viscosity Stability
0.5 @ 60°F 44 160°F N/A Stable
34 35 36 37 38
Contaminants Condensable Corrosive Toxic Particulates
N/A N/A N/A N/A Pipe Scale & Rust
Sample Return Data
39 40 41 42
Process Tap Location Line or Vessel No. Phase Line Length
Reflux Pump Suct. * Liquid 200 ft.
43 44 45 46
Pressure Normal Pressure Max | Min Temperature Norm Temp. Max | Min
275 psig 300 psig | 250 psig 130°F 150°F | 100°F
Analyzer Return Data
47 48
Return Point Line Length
Flare 100 ft.
49 50
Pressure Normal Pressure Max | Min
3 psig 10 psig | 1 psig
Output Signal Data
51 52
Measurement Status Alarm
4 - 20 mA N/A
53 54
Limit Alarm 1 Limit Alarm 2
N/A N/A
Performance Data
55 56
Response Time Transport Lag Time
5 min (cycle time) 60 sec
57 58
Sensitivity 24 hr Drift
0.5% FS ± 1% FS
Calibration Data
59 60
Manual or Automatic Lab Sample
Manual N/A
61 62
Zero Span
N/A Cylinder
Maintenance Data
63 64 65
Accessibility Calibration Frequency Routine Service
Normal 1 day 6 month
66 67 68
Consumables Spare Parts Training
Carrier Gas Yes Yes
Special Data
69 70
Detector Carrier Gas
Thermal Cond. Hydrogen
71 72
Columns Network
Packed Data Hiway
73
Component Ethane Propane i-Butane n-Butane C5+
Installation Data
Indoors Wall Explosion-Proof 60 - 90°F N/A N/A
Unit % % % % %
Normal 0.7 97.4 1.2 0.7 Trace**
Minimum 0.1 95.0 0.2 0.1 Trace**
Maximum 3.0 99.0 5.0 3.0 0.1
Calibration Range 0-3% 0-5% 0-3%
Stream Composition Data
* Application specific information not required for this example. ** Trace means that the component is not normally present or that it is present in concentrations less than 0.005%.
Figure 69: Instrument Specification Sheet for an Gas Chromatograph Analyzer
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Process Gas Chromatograph Analyzer Analyzer Manufacturer Data Sheet Configuration Detector Oven Sample/Column Valve Columns
Thermal Conductivity, Flame Ionization, Flame Photometric, or Photo Ionization Isothermal or Programmed Temperature Plunger (standard), Rotary and Piston optional Packed (standard), Capillary optional
Performance Minimum Range (general)
Repeatability (general)
Cycle Time Sensitivity Linearity Temperature Control Temperature Effect Vibration Effect
Thermal Conductivity - 500 ppm (confirm with application) Flame Ionization - 5 ppm (confirm with application) Flame Photometric - 1 ppm (confirm with application) Photo ionization - 0.1 ppm (confirm with application) + 0.5% of full scale over 8 hours for 2-100 % (confirm with application) + 1% of full scale over 8 hours for 0.5-2 % (confirm with application) + 2% of full scale over 8 hours for 50-500 ppm (confirm with application) + 3% of full scale over 8 hours for 5-50 ppm (confirm with application) + 5% of full scale over 8 hours for 0.5-5 ppm (confirm with application) Application Dependent + 0.5% of full scale (confirm with application) + 2% of full scale (confirm with application) + 0.05 °F over specified ambient none (temperature controlled) negligible
Output Signal Analog Output Digital (alarm) Output Digital (stream) Output Data Hiway
4-20 mAdc or 2-10 Vdc (isolated optional) into 600 ohm (4 per board) SPDT, 0.5 amp at 120 Vac/dc (8 per board) SPDT, 0.5 amp at 120 Vac/dc (8 per board), 110 Vdc internal power Proprietary serial communication network (redundant pair cable)
Sample Requirements Sample Flow Sample Filtration Max Sample Pressure Max Sample Temperature Material of Construction
50-200 cc/min (max.) 5 micron (max.) 300 psig (2068 kPa) standard, 1500 psig (10350 kPa) optional 250 deg F (122 deg C) standard, 450 °F (234 °C) optional Stainless Steel and FEP Teflon, other material optional
Installation Configuration Mounting Dimensions
Weight Hazardous Class Ambient Temperature AC Power Instrument Air
Carrier Gas Corrosion Protection
Single Unit Enclosure Wall, requires 12" (305 mm) minimum side access Height - 42" (1070 mm) Width - 26" (660 mm) Depth - 14" (340 mm) 180 lbs (82 kg) Class I, Groups B/C/D, Division 2 - nonincindive Class I, Groups B/C/D, Division 1 - Type Y air purge 0-122 deg F (-18-50 deg C), weather protection required 120/240 Vac + 10%, 50/60 Hz, 14 amp (max) 50 psig (345 kPa) minimum for valves 25 psig (172 kPa) minimum for oven 3 scfm (85 Lpm) typical, 4 scfm (113 Lpm) for high-temperature applications Cylinder Nitrogen, Helium, or Hydrogen depending on application Tropicalization optional, Purge optional
Calibration Manual/Automatic Zero Span
Manual or Automatic Automatic Baseline Correction Standard Sample Cylinder
Figure 70: Typical Analyzer Manufacturer’s Data Sheet for a Gas Chromatograph Analyzer
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Process Gas Chromatograph Analyzer Specific Application Data Sheet
Specific Application Data Measurement Detector Carrier Gas Cycle Time Repeatability Sample Inject Phase
Ethane 0-3%, i-Butane 0-5%, n-Butane 0-3% Thermal Conductivity Hydrogen 5 minutes + 0.5% of full scale over 8 hours Gas
Oven Temperature Columns
150°F (66°C) Packed
Figure 71: Typical Process Gas Chromatograph Analyzer Specific Application Data Sheet for Depropanizer Product Application
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Figure 72: Depropanizer Overhead Application Gas Chromatograph Analyzer Sample Handling System Diagram
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To completely evaluate a gas chromatograph analyzer, use Addendum A and the installation, operation, and maintenance considerations for gas chromatograph analyzers that were described earlier. For the purpose of this depropanizer example, we will evaluate three considerations: 1. Is the instrument air pressure and quality acceptable for the GC analyzer? The instrument air supply has been confirmed to be clean, dry, and oil free. Item 17 of the ISS (Figure 69) for this application specifies that the instrument air pressure is 80 psig (552 kPa). The AMDS (Figure 68) for this particular analyzer states the following: • the valve air pressure requirement is 50 psig (345 kPa). • the oven air pressure requirement is 25 psig (172 kPa). • the oven air flow requirement is 4 SCFM (113 liters/min.). The pressure drop in the piping and tubing for the instrument air supply has been confirmed to be 2 psig (13.8 kPa) at the required oven flow rate. The instrument air pressure at the GC is equal to the available iinstrument air pressure (80 psig) minus the pressure drop in the piping and tubing (2 psig). The available instrument air pressure (78 psig) is greater than the valve air pressure requirement (50 psig); therefore, the instrument air pressure is high enough to operate the valves in the gas chromatograph analyzer. The oven air pressure and flow requirements from the AMDS are 25 psig and 4 SCFM, respectively. The required oven air pressure is less than the available instrument air pressure; therefore, the instrument air pressure is high enough to operate the gas chromatograph analyzer oven. 2. Do the analyzer response time and sample handling system lag time provide a measurement response time that is acceptable for this application? The ISS for this application specifies the following: • Item 55 states that the analyzer response time is 5 minutes. • Item 56 states that the sample transport lag time is 60 seconds. The SADS for this particular analyzer application states the following: • the analyzer cycle time is 5 minutes. The sample handling system diagram states that the sample transport lag time has been calculated to be 30 seconds.
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The required measurement response time is calculated as follows: MRTReq = =
required analyzer response time + required sample transport lag time 5 min + 1 min = 6 min
The actual measurement response time is calculated as follows: MRTAct
= =
specified analyzer response time + calculated sample transport lag time 5 min + 30 sec = 5.5 min
Because the actual measurement response time is less than the required measurement response time, the analyzer response time and sample handling system lag time provide a measurement response time that is acceptable for this application. 3. Is the phase of the process sample at the GC analyzer appropriate for this application? The ISS for this application specifies the following: • Item 24 states that the process sample supply is a liquid The SADS for this particular analyzer application states the following: • the sample inject valve is for a gas. • the oven operating temperature is 150°F (65.5°C) The sample handling system diagram contains the following information: • the liquid process sample is vaporized for delivery to the GC analyzer as a gas. • the sample supply pressure to the GC is 15 psig. At 15 psig the pew point of the sample is calculated to be 0 °F (-18°C). The minimum ambient temperature has been confirmed to be 25°F (-4°C). The calibration sample will be similar to the process sample and will also have a dew point of 0°F (-18°C) at 15 psig (103 kPa). In order to maximize the calibration cylinder life, the desired pressure in the calibration cylinder is raised to 200 psig (1,378 kPa) which raises the dew point to 120°F (49°C). Because the phase of the process sample is a liquid at the process sample tap and the liquid is converted to a vapor at 15 psig (103 kPa) in the sample handling system, a vapor sample valve in this application is acceptable. Because of the dew point of the calibration cylinder at the desired pressure, the calibration sample should also be a liquid that is vaporized in the sample handling system before it is delivered to the gas chromatograph analyzer.
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REFERENCES 1.
Clevett, K.J. Process Analyzer Technology. John Wiley & Sons. New York. 1986.
2.
Willard, H.H., Merritt, L.L. Jr., Dean, J.A., Settle, F.A., Jr. Instrumental Methods of Analysis, Sixth Edition. Wadsworth Publishing Co., Belmont, CA., 1981.
3.
Grob, R.L. Modern Practice of Gas Chromatography, Second Edition. John Wiley & Sons, Inc., New York, 1985.
4.
McNair, H.M., Bonelli, E.J. Basic Gas Chromatography. Varian. August 1968.
5.
American Petroleum Institute Recommended Practice 550, Fourth Edition, April 1983.
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WORK AID 1: RESOURCES USED TO EVALUATE GAS CHROMATOGRAPH ANALYZERS IN RELATION TO REQUIREMENTS FOR SPECIFIC APPLICATIONS Work Aid 1A: Resources Used to Evaluate the Instrument Air Pressure and Quality for Gas Chromatograph Analyzers This Work Aid contains guidelines to evaluate whether the instrument air pressure and quality are acceptable for the gas chromatograph analyzer. 1. Is instrument quality air available (not plant air)? Yes
Instrument air quality is acceptable for use with a gas chromatograph analyzer.
No
Stop. The instrument air quality is not is appropriate for use with a gas chromatograph analyzer. Either use an alternate source of instrument air or select another type of analyzer.
2. From the ISS, obtain the instrument air supply pressure (Item 17). 3. From the AMDS, obtain the valve air pressure requirement. 4. Is the valve air pressure requirement less than the instrument air pressure after the pressure drop in the air supply piping and tubing is taken into consideration? Yes
The instrument air pressure is high enough to operate the valves in the gas chromatograph analyzer.
No
Is plant nitrogen available at a pressure that is higher than the valve air pressure requirement? Yes
Use plant nitrogen to operate the gas chromatograph analyzer valves. Confirm that the analyzer is configured for separate oven and valve supply.
No
Use inert cylinder gas to operate the gas chromatograph analyzer valves. Cylinder gas can be supplied from the carrier gas cylinders (if inert) or a dedicated set of inert gas cylinders.
4. From the AMDS, obtain the oven air pressure and flow requirements. 5. Is the oven air pressure requirement less than the instrument air pressure after the pressure drop in the air supply piping and tubing is taken into consideration? Yes
The instrument air pressure is high enough to operate the gas chromatograph analyzer oven.
No
Stop. A gas chromatograph analyzer cannot be used in this application. Either use an alternate source of instrument air or select another type of analyzer.
6. Is the oven air flow requirement less than the instrument air supply capacity? Yes
The instrument air supply capacity is sufficient to operate the gas chromatograph analyzer oven.
No
Stop. A gas chromatograph analyzer cannot be used in this application. Either use an alternate source of instrument air or select another analyzer.
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Work Aid 1B: Resources Used to Evaluate the Measurement Response Time for Gas Chromatograph Analyzers This Work Aid contains guidelines to evaluate whether the gas chromatograph analyzer’s response time and the sample handling system’s transport lag time are acceptable for the required measurement response time. 1. From the ISS, obtain the required analyzer response time (Item 55) and the required sample transport lag time (Item 56). 2. From the Specific Application Data Sheet (SADS), obtain the specified analyzer response time. 3. From the sample handling diagram, obtain the calculated sample transport lag time. 4. Calculate the required measurement response time as follows: MRTReq = required analyzer response time (Item 55) + required sample transport lag time (Item 56) 5. Calculate the actual measurement response time as follows: MRTAct = specified analyzer response time (SADS) + calculated sample transport lag time 6. Is MRTAct < MRTReq? Yes
The analyzer response time and sample handling system lag time provide a measurement response time that is acceptable for this application.
No
The analyzer response time and sample handling system lag time do not provide a measurement response time that is acceptable for this application. Either simplify the gas chromatograph analyzer application or choose another analyzer. The application of the gas chromatograph analyzer can be simplified by reducing the number of components that are measured. If the number of measured components cannot be reduced, contact the gas chromatograph analyzer manufacturer for other options that can be considered to simplify the application.
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Work Aid 1C: Resources Used to Evaluate the Phase of the Process Sample for Gas Chromatograph Analyzers This Work Aid contains guidelines for evaluating whether the phase of the process sample should be a liquid or a gas at the gas chromatograph analyzer. 1. Is the phase of the process sample a gas at process sample supply tap? Yes
No
Is the dew point of the gas calibration sample below the minimum ambient storage temperature at the maximum supply pressure of the calibration sample cylinder? Yes
The gas calibration sample is acceptable for this application.
No
The gas calibration sample is not acceptable for this application. A liquid calibration sample should be used and vaporized in the sample handling system for delivery to the gas chromatograph.
Continue.
2. Is the bubble point temperature of the liquid process sample higher than the oven temperature at the sample inject valve? Yes
No
Is the bubble point of the liquid calibration sample above the maximum ambient or storage temperature at the minimum supply pressure of the calibration sample cylinder? Yes
The liquid calibration sample is acceptable for this application.
No
The liquid process sample must be vaporized in the sample handling system and handled as a gas. This is not a viable liquid application.
Is the dew point temperature of a vaporized process sample lower than the oven temperature at the sample inject valve? Yes
The vaporized liquid process sample can be handled as a gas for this application.
No
Can the oven temperature be raised above the dew point, without affecting the analysis? Yes
The liquid process sample can be handled as a gas.
No
This is not a viable gas application. For this application the sample may be vaporized and a liquid knock out used.
3. Is the dew point of the gas calibration sample below the minimum ambient storage temperature at the maximum supply pressure of the calibration sample cylinder? Yes
The gas calibration sample is acceptable for this application.
No
The gas calibration sample is not acceptable. For this application a liquid calibration sample should be used and vaporized in the sample handling system for delivery to the gas chromatograph analyzer.
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GLOSSARY analyzer response time
The length of time required for the analyzer to completely analyze a sample and produce an output signal.
analyzer sensitivity
The ratio of the change in the analyzer response with a corresponding change in the process sample.
annulus
A ring-like part.
background components
Components other than the component of interest in a mixture.
British Thermal Unit (BTU)
The amount of heat that is required to raise the temperature of 1 lb. of water from 60° to 61° F.
calorific value
The heat value of a gas at standard conditions (14.7 psia and 60°F).
carrier gas
A gas that provides a stable transport and detection medium for the sample components.
catalytic cracking
A petroleum refining process in which large alkanes are broken down by a catalyst into smaller, branched-chain alkanes suitable for use in gasoline.
chromatogram
The output from a gas chromatograph analyzer, which is displayed on a recorder.
column
A long, narrow tube that contains an absorbent material.
column loading
Deterioration or buildup of unwanted sample components in the column.
contaminant
A component in the process sample that will cause degradation or deterioration of the analyzer measurement.
frequency
The number of complete waves passing a point in a given amount of time.
Hertz (Hz)
The unit of frequency, given as the number of cycles per second; 1 Hz = 1 s-1.
hydrocracking
Catalytic cracking in the presence of hydrogen.
ideal gas
A gas in which the volume occupied by the gas molecules is negligible compared with the total volume of the system.
linearity
The extent to which any signal modification process, as detection, is accomplished without amplitude distortion.
LNG
Liquefied natural gas.
LPG
Liquefied petroleum gas.
mobile phase
The carrier gas.
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net calorific value
The gross calorific value that is generated by combustion less the latent heat that is carried by the products of combustion .
octane number
The octane quantity of a fuel defined as the fuel’s resistance to knock.
partition coefficient
The ratio of the amount of the gas in the air and the amount of gas in the liquid.
peak resolution
The peak separation divided by the peak width.
reforming
A process for upgrading naphthas into high-octane gasoline and petrochemical feedstocks.
Reid Vapor Pressure (RVP)
A measure of the vapor pressure of a sample of gasoline at 100°F. The results are reported in pounds. This test is usually carried out in accordance with ASTM Method D 323.
sample transport lag time
The amount of time that is required for a sample to travel from the sample tap to the analyzer.
slip stream
A small stream of solution drawn off from a larger solution stream.
specific gravity
The ratio of the density of a fluid to the density of water.
standard temperature and pressure
Standard conditions at a pressure of 14.7 psia and a temperature of 60°F.
stationary phase
The absorbent material inside the chromatograph column.
stoiciometrically
Pertaining to the substances that are in the exact proportions required for a given reaction.
thermal conductivity
Thermal conductivity is the intrinsic property of a material (gas, liquid, or solid) to transfer heat. (The quantity of heat in calories that is transferred in 1 second in a gas between two surfaces of area 1 cm2 that are placed 1 cm apart when the temperature difference between the two surfaces is 1°C.)
thermal cracking
A process that uses heat to break down the residue, or bottoms, from vacuum distillation and sometimes the heavy gas oils that result from catalytic cracking.
Vapor to Liquid Ratio (V/L),
The partition coefficient, usually designated as K, which is equivalent to y/xm where y is the mole fraction of a given component in the vapor phase that is in equilibrium with x, the tool fraction of the same component in the liquid phase. K is a function of temperature, pressure, and composition of the particular system.
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ADDENDUM A: GUIDELINES FOR EVALUATING PROCESS ANALYZERS This Addendum should be used as a general guideline for evaluating process analyzers. This addendum contains: a flow chart (Figure 78), which shows the guidelines for evaluating a process analyzer; an Instrument Specification Sheet (ISS), and the General Instructions for Evaluating Process Analyzers. The flow chart in Figure 78 graphically shows the general guidelines for evaluating a process analyzer. Each numbered item in the flow chart item is explained on the following page.
Figure 73: Process Analyzer Evaluation Flow Chart
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1.
Data is obtained from the Piping and Instrument Diagram (P&ID) and the Process Flow Diagram (PFD) concerning the application and process stream data, respectively.
2.
Data from the P&ID and PFD is used to complete an Instrument Specification Sheet (ISS), which is shown in Figure 79. The ISS specifies Saudi Aramco’s requirements for the analyzer and the sample handling system. Additional information concerning the installation must be determined separately based on the analyzer requirements and analyzer shelter considerations, which are given in Saudi Aramco Design Practice Number SADP-J-502, Analyzer Shelters.
3.
A process analyzer is evaluated for the application by comparing the requirements given in the ISS with the capabilities of a proposed analyzer as defined on the Analyzer Manufacturer’s Data Sheet (AMDS). The evaluation process can also include indirect information such as past experience with similar analyzers and applications as well as references from the manufacturer. This supplemental information is important in order to determine whether a successful measurement can be made. The proposed sample handling system is also evaluated on the basis of information on the ISS and the manufacturer’s data sheet.
4.
If a suitable analyzer is selected, the information on the ISS should be updated so that it becomes an accurate record of the analyzer installation. In addition, after the analyzer is placed in operation, the information on the ISS should be verified with actual operating conditions to confirm that the analyzer and sample handling system are properly designed for the application.
5.
If certain specifications on the ISS are not available features of a particular analyzer, the specifications on the ISS must be prioritized in order of importance. Exceptions to the specifications on the ISS may be required if the high priority features are available for the analyzer but some other low priority features are not available.
6.
If an alternate analyzer or an alternate analytical technique exists, the evaluation process is repeated.
7.
Sometimes, an analyzer that can comply with all of the measurement and installation requirements does not exist. In other instances, the process sample may not be compatible with the analyzer or a sample handling system may be required that is too complex to be successful. If an alternate analyzer or an alternate analytical technique does not exist, it should be determined whether or not the ISS can be revised.
8.
If the ISS can be revised to match the capabilities of the analyzer according to the Analyzer Manufacturer’s Data Sheet, the evaluation process is repeated.
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9.
If no analyzer and sample handling system can be approved, it may be necessary to reevaluate the PFD and P&ID to determine whether an alternate process measurement location can be used.
10.
If an alternate process measurement location can be used, the P & ID is revised and the evaluation process is repeated
11.
If an alternate process measurement location cannot be used, the installation should be aborted.
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Saudi Arabian Oil Company Instrument Specification Sheet - Analyzer 1 2 3
Tag Number Application P & ID Number
4 5 6
Instrument Type Manufacturer Model Number
7 8 9 10 11 12 13
Location Indoors or Outdoors Mounting Type Enclosure Type Ambient Temperature Ambient Corrosion Vibration
14 15 16 17 18 19 20
Hazardous Area Haz Temp Rating AC Power Inst. Air Pressure Nitrogen Pressure Cooling Water Pres. Steam Pressure
21 22 23 24
Process Tap Location Line or Vessel No. Line Length Phase
25 26 27 28
Pressure Normal Pressure Max | Min Temperature Norm Temp. Max | Min
29 30 31 32 33
Density or S. G. Molecular Weight Bubble Pt or Dew Pt Viscosity Stability
34 35 36 37 38
Contaminants Condensable Corrosive Toxic Particulates
Sample Return Data
39 40 41 42
Process Tap Location Line or Vessel No. Phase Line Length
43 44 45 46
Pressure Normal Pressure Max | Min Temperature Norm Temp. Max | Min
Analyzer Return Data
47 48
Return Point Line Length
49 50
Pressure Normal Pressure Max | Min
Output Signal Data
51 52
Measurement Status Alarm
53 54
Limit Alarm 1 Limit Alarm 2
Performance Data
55 56
Response Time Transport Lag Time
57 58
Sensitivity 24 hr Drift
Calibration Data
59 60
Manual or Automatic Lab Sample
61 62
Zero Span
Maintenance Data
63 64 65
Accessibility Calibration Frequency Routine Service
66 67 68
Consumables Spare Parts Training
Special Data
69 70
General
Installation Data
Sample Supply Data
Process Data
73
(Draft Copy)
| |
| |
|
71 72 Component
Unit
Normal
Minimum
Maximum
Calibration Range
Stream Composition Data
Figure 74: Instrument Specification Sheet for Process Analyzers Saudi Aramco DeskTop Standards
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The instructions on the following pages can be used to compare the specifications on Instrument Specification Sheet with the capabilities of the analyzer that are stated on the Analyzer Manufacturer's Data Sheet. The item numbers below refer to the Instrument Specification Sheet in Figure 79. Item No. Description General Data
Instruction
1
Tag number
The tag number of the analyzer as given on the Piping and Instrument Diagram (P & ID).
2
Application
The application describes how the analyzer is used and not where the analyzer is located.
3
P & ID number.
The P & ID number is provided to refer back to the P & I D that identifies the analyzer.
4
Instrument Type
The instrument type is specific to the application (e.g., infrared analyzer)
5
Manufacturer
The name of the manufacturer is recorded after selection to provide a record of the installation.
6
Model Number
The analyzer model number is recorded after selection of the analyzer from the Manufacturer's Data Sheet.
Installation Data 7
Location
Confirm that the physical space required for installation of the analyzer, sample handling system, and peripheral equipment is available at the location specified. The location defines many parameters related to the installation of the analyzer and its associated sample handling system which must each be addressed separately (see following items). The location generally defines the space available for the analyzer, sample handling system, and peripherals (calibration gas cylinders, etc.). Specific requirements for analyzer locations in Saudi Aramco applications are given in Section 3.3 of SADP-J-502, Analyzer Shelters. The location of the analyzer must also be evaluated relative to the location of the process sample tap. The distance that separates the analyzer and sample tap defines the sample transport line length (Item 23). This line length is then used to evaluate the sample transport lag time (Item 56) relative to the performance requirement based on the design of the sample handling system.
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Item No.
Description
Instruction
8
Indoors or Outdoors
Confirm that the analyzer is suitable for installation indoors or outdoors as specified. Analyzers that are located outdoors must be suitable for adverse conditions. Identification of the location as indoors or outdoors defines the relative environment for the analyzer installation. Outdoor locations are subjected to the extremes of the environment such as temperature, weather, dust, and corrosion. Indoor locations are protected from extreme environmental conditions but, for Saudi Aramco application, still have more severe requirements than most industrial applications. Identifying the location as indoor or outdoor will also help to explain the type of enclosure that is specified for the analyzer. Specific environmental conditions for Saudi Aramco applications, both indoors and outdoors, are given in Sections 13.1.1, 13.1.2, 13.2.1, 13.2.2, 13.2.3 and 13.2.4 of SAES-J-003, Basic Design Criteria.
9
Mounting Type
Confirm that the analyzer is available for mounting as specified. There are three mounting types for process analyzers: wall-mount, panelmount, and floor-mount. The type of mounting defines the space that is required for the installation of the analyzer. In addition, the type of mounting also defines the access that is required for maintenance of the analyzer. Wall-mounted analyzers do not provide any rear access so that all operation and maintenance is performed directly from the front. Panelmounted analyzers require rear access for installation and, in many cases, also require rear access for maintenance. Analyzers that are floor-mounted usually require maintenance from all four sides. If the only available space for installation of an analyzer requires wall-mounting, an analyzer that requires panel-mounting may fit into the space but access will be denied to the rear of the analyzer for maintenance.
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Item No.
Description
Instruction
10
Enclosure Type
Confirm that the analyzer is available with the type of enclosure specified. The enclosure type specifies what type of protection must be provided for the analyzer (corrosion-resistant, explosion-proof, etc.) In general, all equipment enclosures for Saudi Aramco applications must comply with the requirements given in SAES-J-003, Basic Design Criteria. The enclosure type must match the environmental conditions and hazardous area classification requirements given in Items 10 through 15. In addition, the enclosure may be required to provide personnel protection from exposure to toxic hazards. In some cases, it may be necessary to prioritize the type of enclosure based on the application. For example, in a hazardous area that can experience severe weather, an explosion-proof enclosure is more important than a weather-proof enclosure. On the other hand, if the installation is also highly corrosive, a purged weather-proof enclosure is more important than an explosion-proof enclosure. The type of enclosure specified on the ISS identifies the priority for the type of enclosure to be used for the application.
11
Ambient Temperature Confirm that the analyzer is capable of operating within the specified ambient temperature range. The ambient temperature limitations of the analyzer must be evaluated relative to the installation location. To minimize the effect of a changing ambient temperature, analyzers that are sensitive to temperature variation include internal temperature control. Due to the extreme outdoor temperatures, most analyzers are installed inside temperature controlled shelters (as defined by Items 7 & 8). The effects of relative humidity must also be considered with the ambient temperature. Specific relative humidity and ambient temperature design conditions for Saudi Aramco applications, both indoors and outdoors, are given in Sections 13.1.1, 13.1.2, 13.2.1, and 13.2.2 of SAES-J-003, Basic Design Criteria.
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Item No.
Description
Instruction
12
Ambient Corrosion
Confirm that the analyzer is provided with corrosion protection for the specified conditions. The analyzer enclosure must be suitable for the ambient corrosion. Outdoor locations are subject to more severe ambient corrosion than indoor locations. In addition, the affects of relative humidity, both indoor and outdoor, must be considered with the ambient corrosion. Alternately, a purge may be required with an explosion-proof enclosure to provide corrosion protection. Some analyzers are available with additional protection (material of construction or coating) for locations with high ambient corrosion. In some cases, it may be more important to utilize a purged, general-purpose enclosure for both corrosion and hazardous area protection than to use an explosion-proof enclosure for the hazardous area protection only. In addition to chemical corrosion, equipment in Saudi Aramco application must also be protected from dust. Specific ambient corrosion design conditions for Saudi Aramco applications, both indoors and outdoors, are given in Sections 13.2.3 and 13.2.4 of SAES-J-003, Basic Design Criteria.
13
Vibration
Confirm that the analyzer is capable of operating with the specified vibration. Process analyzers are designed to be rugged in order to operate continuously in the harsh plant environment. Most analyzers are usually unaffected by normal plant vibration. Excessive vibration from motors or compressors can adversely affect the stability of the process analyzer. The location of the analyzer relative to other equipment must be evaluated to determine the possible effect of vibration.
14 & 15
Hazardous Area and Temp. Rating
Confirm that the analyzer is suitable for the specified hazardous area classification. The process analyzer must be suitable for the specified hazardous area. There are three methods used to comply with the hazardous area requirements: enclosures that are rated to be explosion proof to prevent the propagation of an explosion outside the analyzer, enclosures that are purged and pressurized to prevent an explosion by eliminating the accumulation of explosive gas inside the analyzer enclosure, and analyzers that are rated as non-incendive and cannot cause an explosion. Most analyzers are available with general-purpose enclosures that can be purged and pressurized for hazardous areas. Many analyzers are available with explosion-proof enclosures for hazardous areas. Purged enclosures offer both electrical hazard and environmental protection. However, explosion-proof enclosures provide the safest installation for hazardous process samples.
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Item No.
Description
Instruction
16 - 20
Available Utilities
Confirm that the utilities required for operation of the analyzer and sample handling system are available at the installation location. The utilities required to operate the analyzer and sample handling system must be available at the location where it will be installed. AC power is usually required by all process analyzers. If the analyzer is purged and pressurized, instrument air or plant nitrogen may also be required. Depending on the application, other utilities (such as steam or cooling water) may be required for operation of the sample handling system.
Sample Supply Data - The sample supply data identifies the operating conditions of the process sample at the Sample Supply Tap. The sample handling system must be designed to provide the process sample to the analyzer under conditions that are compatible with the analyzer operation. 21
Process Tap Location
This information provides the actual physical location of the process sample supply tap. The process tap location identifies where the sample supply tap is physically located in the process. This allows the determination of the distance between the process tap and the analyzer which is given as the line length (item 34).
22
Line or Vessel No.
Information only.
23
Line Length
The sample transport line length is used to calculate the sample transport lag time. The distance that separates the analyzer and process sample tap defines the length of the sample transport line. The sample transport line length then determines the sample flow that is required to obtain the sample transport lag time. A fast-loop bypass is usually required to provide the transport lag time.
24
Phase
Confirm that the process analyzer is designed to measure the sample in the specified phase. Process analyzers can only operate on a single phase sample. The process sample must be maintained as either a gas or liquid in both the analyzer and sample handling system. A process sample that has a mixed phase cannot be measured accurately.
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Item No.
Description
Instruction
25
Pressure Normal
Confirm that the sample pressure is compatible with the analyzer operating pressure. If necessary, confirm, that proper pressure reduction or pressure enhancement is provided in the sample handling system. The sample handling system must provide a sample supply pressure and flow that are compatible with the analyzer design under both normal and extreme operating conditions (Item 26). Most gas analyzers operate at relatively low pressure and require reduction of the process pressure in the sample handling system. Many liquid analyzers can operate at high pressure which is more compatible with the normal process conditions. It is always desirable to operate the analyzer and sample handling system at a low pressure to minimize the exposure of personnel to hazard in the event of a leak.
26
Pressure Max | Min
Confirm that proper pressure control is provided under the range of possible operating pressures.
27
Temperature Norm
Confirm that the sample temperature is compatible with the analyzer operating temperature. If necessary, confirm, that proper temperature reduction is provided in the sample handling system. The sample handling system must provide a sample supply temperature that is compatible with the analyzer design under both normal and extreme operating conditions (Item 28).
28
Temp. Max | Min
Confirm that proper temperature control is provided under the range of possible operating temperatures
Process Data - The process data defines the physical properties of the process sample. The sample handling system must condition the process sample to be compatible with the analyzer. 29
Density or S. G.
Information only. The density or specific gravity of the process sample is used in the design of the sample handling system to determine sample flow and pressure drop through the system.
30
Molecular Weight
Information only. The molecular weight of the process sample is provided in addition to the density or specific gravity to allow the specific gravity of the sample to be determined under conditions that are different from the actual process conditions. This is useful for applications with gas samples where the process sample pressure is reduced in the sample handling system for delivery to the analyzer.
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Item No.
Description
Instruction
31
Bubble Pt. or Dew Pt.
Confirm that proper temperature control and heating are provided in the sample handling system to maintain the sample temperature above the dew point for gases or below the bubble point for liquids. The temperature and pressure of the process sample must not exceed the design specifications of the analyzer but must also be maintained within the limits of the sample bubble point for liquids and dew point for gases. For liquid samples, the temperature must either be low enough or the pressure maintained high enough to prevent gas bubbles from forming in the analyzer or sample handling system. For gases, the pressure must either be low enough or temperature maintained high enough to prevent condensation of liquid in the analyzer or sample handling system. A process sample that is not maintained at the required phase in the analyzer and sample handling system (gas or liquid) will not provide a measurement that is representative of the actual process stream.
32
Viscosity
Applies to liquid samples only. Confirm that the operating temperature of the analyzer and sample handling system maintains the sample viscosity for proper flow to the analyzer. Viscosity only applies to liquid process samples. Some liquid process samples become too viscous to flow unless they are heated. The process sample must be maintained at a temperature that provides a flowing viscosity in the analyzer and sample handling system.
33
Stability
Confirm that the sample is stable and will not change properties or composition in the sample handling system or in the process analyzer. The process sample must be stable at the operating temperature and pressure in the analyzer and sample handling system. It is important to identify components in the process sample that may react, polymerize, or degenerate under the operating conditions of the analyzer and sample handling system.
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Item No.
Description
Instruction
34
Contaminants
Confirm that none of the sample components will contaminate the analyzer and cause degradation or deterioration of the measurement. It is important to identify components in the process sample that will cause degradation or deterioration of the measurement. As opposed to corrosives or condensables that also have an adverse effect on the analyzer, contaminants are components that specifically affect the analyzer measurement due to the analyzer principle of operation. For example, contaminants for infrared and photometric analyzers would be any substance that coats the measurement cell windows and degrades the transmission of energy. Contaminants for electrolytic oxygen and moisture analyzers include any substance that reacts with the electrolyte and causes deterioration of the measurement sensor. If contaminants are present, they must either be removed from the process sample in the analyzer or sample handling system by filtration, absorption, or reaction, or the analyzer’s operation must compensate for the effect of the contaminant. If removal or compensation is not possible, an analyzer with an alternate type of measurement principle must be used.
35
Condensable
Applies to gas samples only. Confirm that none of the sample components will condense in the analyzer or sample handling system. Condensable only applies to gas process samples. If the process sample is condensable, the pressure must either be low enough or the temperature maintained high enough to prevent condensation of liquid in the analyzer and sample handling system.
36
Corrosive
Confirm that the materials of construction of the analyzer and sample handling system are compatible with the process sample. It is important to identify components in the process sample that are corrosive to materials that are commonly used. The materials in contact with the process sample must be suitable for the potentially corrosive components. This applies to both the analyzer and sample handling system. All materials must be evaluated, especially the elastomers that are used for seals and gaskets.
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Item No.
Description
Instruction
37
Toxic
If the sample is toxic, confirm that the design of the analyzer and sample handling system do not expose personnel to hazards in the event of a leak. The toxic nature of the process sample may not affect the operation of the analyzer and sample handling system, but it is important to identify toxic hazards relative to personnel exposure during operation and maintenance. If the process sample is highly toxic and presents a significant hazard to personnel, the design of the analyzer and sample handling system must be evaluated to determine the potential exposure risk in the event of a leak. If necessary, special precautions must be taken to prevent exposure of personnel to the process sample if a leak occurs. These precautions normally include installing the sample handling system in an enclosure and purging the analyzer and sample handling system with the purge exhaust routed to a safe area. Toxic sensors should also be included in the enclosures or on the purge vent line to warn personnel that a leak has occurred.
38
Particulates
If the process sample contains particulates, confirm that the analyzer is adequately protected with proper filtration in the sample handling system. Particulates in a process sample can obstruct the analyzer and sample handling system. In addition, particulates can interfere with the measurement of some analyzers (e.g., in infrared analyzers, particulates obstruct the transmission of the infrared beam). The sample handling system must include filtration of particulates according to the requirements of the analyzer. Applications that have high particulate loads must include self-cleaning or redundant filters to minimize maintenance.
Sample Return Data The sample return data identifies the operating conditions for the process return of the sample handling system fast-loop bypass stream. 39
Process Tap Location
Information only. Provides the actual physical location of the process sample return tap. The process tap location identifies where the sample return tap is physically located in the process. This allows the determination of the distance between the process tap and the analyzer which is given as the line length (Item 42).
40
Line or Vessel No.
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Item No.
Description
Instruction
41
Phase
Confirm that the phase of the sample return effluent is the same as the phase of the process at the sample return point. The phase of the sample return effluent must be the same as the process at the sample return point to prevent disruption of the process. The sample handling system must be designed with a sample bypass stream that operates with the same phase as the process at the specified sample return point. If the application requires the phase of the process sample to be changed for proper operation of the analyzer or sample handling system, an alternate sample return point be required.
42
Line Length
Confirm that the sample handling system is designed to accommodate the pressure drop in the sample return line under flowing conditions. The size of the sample return line must be evaluated with the line length to confirm that the pressure drop in the return line at the bypass stream flow is consistent with the sample handling system design. Excessive pressure drop could adversely affect the operation of the analyzer and sample handling system.
43
Pressure Normal
Confirm that the sample handling system is designed to operate at sufficient pressure to allow sample return at the specified pressure. The sample return pressure is also critical to the design of the fast-loop bypass stream in the sample handling system. The design of the fast-loop bypass stream must be evaluated with the sample return pressure (and pressure drop in the sample return line) to verify stable operation under both normal and extreme operating conditions (Item 44).
44
Pressure Max | Min
Confirm that the sample handling system is designed to operate under the specified range of sample return pressure.
45
Temperature Norm
Confirm that the sample return tubing and connection are rated for the sample return process temperature. The sample return tubing must be rated for operation at the process temperature of the sample return point under both normal and extreme operating conditions (Item 46).
46
Temp. Max | Min
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Item No. Description Instruction Analyzer Return Data - Although some analyzers can operate at high pressure, high pressure operation should be avoided unless no other option is available. High pressure operation is more common with liquid applications where low pressure return points are not usually available. High-pressure operation may also be required with gas applications where it is more appropriate to obtain higher sensitivity with a higher operating pressure than by using a longer measurement cell. Analyzers that are designed to operate at low pressure require a low pressure process return point. If a low-pressure process return point is not available, a sample recovery system is required. 47
Return Point
Information only. Identifies the physical location of the analyzer’s sample return point. The analyzer sample return point identifies where the analyzer effluent sample is to be routed. It is also used to determine the analyzer sample return line length (Item 48).
48
Line Length
Confirm that the analyzer and sample handling system are designed to accommodate the pressure drop in the sample return line under flowing conditions. The size of the analyzer sample return line must be evaluated with the line length to confirm that the pressure drop in the return line at the analyzer sample flow is consistent with the analyzer and sample handling system design. Excessive pressure drop could adversely affect the operation of the process analyzer and sample handling system.
49
Pressure Normal
Confirm that the analyzer is designed to operate at the sample return pressure. If the sample return pressure is greater than the analyzer operating pressure, confirm that a sample recovery system is provided. The analyzer sample return pressure is critical to the design of the analyzer effluent return stream in the sample handling system. The sample return pressure must be within the design limits of the process analyzer. The design of the analyzer effluent return stream must be evaluated with the analyzer sample return pressure (and pressure drop in the analyzer return line) to verify stable operation of the process analyzer under both normal and extreme operating conditions (Item 50). For liquid applications, the pressure in the analyzer is not critical to stable operation of the analyzer. For gas applications, the pressure in the analyzer must remain constant for stable operation. Pressure control in the analyzer and to the analyzer sample return point is critical for gas applications.
50
Pressure Max | Min
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Confirm that the sample handling system provides pressure control for the specified range of analyzer sample return pressure.
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Item No. Description Instruction Output Signal Data - The output signal data specifies the required output signals from the process analyzer. 51
Measurement
Confirm that the analyzer is provided with the specified output signal. For milliamp signals, confirm that the analyzer output signal is suitable for the specified loop impedance. For most analyzers, a milliamp analog signal is generally required and usually available. The output signal must be evaluated to determine if signal isolation is required and to confirm if the milliamp output signal can drive into the required impedance. In addition, the analog output signal must be specified to be either sourced (self-powered) or sinked (looppowered) according to the requirements of the process control system analog input.
52
Status Alarm
Confirm that the analyzer is available with status alarm contacts, if specified. For control systems, status alarms are needed to determine if an analyzer is on-line or off-line . The status alarm alerts the operator or control system that the analyzer is off-line and the data from the analyzer is not valid. Microprocessor-based analyzers contain internal diagnostics, which allow the analyzers to provide status alarms.
53 & 54
Limit Alarms
Confirm that the analyzer is available with measurement limit alarms, if specified. The limit alarms specify high or low (or high-high or low-low) analyzer measurement values that may indicate abnormal or hazardous process operating conditions. Certain applications require the analyzer to provide the limit alarms (e.g., hydrocarbon detection applications). Most often, the process control system generates the limit alarms internally.
Performance Data - The Performance Data specifies the performance requirements for the analyzer and sample handling system. The performance data must be met for the analyzer measurement data to be valid and usable. 55
Response Time
Confirm that the analyzer response time is within the response time specified for the application. The analyzer response time is the time required for the analyzer measurement to indicate a change in the sample composition at the inlet connection to the analyzer at the required analyzer sample flow. For continuous analyzers, the analyzer response time is primarily a function of the sample purge time as determined by the sample flow. For batch type analyzers, the response time is primarily a function of the cycle time of the analytical technique.
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Item No.
Description
Instruction
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Transport Lag Time
Confirm that the sample handling system provides a sample transport lag time within the specified time limit. The sample transport lag time is the time required for the process sample to be transported from the process sample tap to the analyzer. The sample handling system must be designed to provide the required sample transport lag time. The combination of the analyzer response time and the sample transport lag time determines the overall measurement response time; the time between an actual change in the process stream and the corresponding change in the analyzer measurement reading.
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Sensitivity
Confirm that the analyzer measurement provides the sensitivity specified. The analyzer sensitivity is the smallest change in the sample composition that the analyzer can detect. The analyzer sensitivity specification is used to determine the number of significant figures that can be applied to the analyzer measurement value. Specification of sensitivity is also used to indicate whether the measurement is critical. If the measurement is critical, it requires a high degree of accuracy and small changes can be important such as for process control. If the measurement is not critical, only general changes or trends are required such as in monitoring applications.
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24 hr Drift
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The 24-hour drift specifies the allowable error in the analyzer measurement value due to unstable operation over a 24 hour time period. The drift characteristic of the analyzer is used to determine how often the analyzer needs to be calibrated. If the analyzer can operate within the specified drift for a period longer than 24 hours, calibration will be required less frequently to maintain the measurement within the specified error limit. If the analyzer can only operate within the specified drift for a period shorter than 24 hours, more frequent calibration will be required. If the 24-hour drift of the analyzer is greater than the specification, more frequent calibration can be used to enhance the analyzer performance to meet the drift specification.
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Item No. Description Instruction Calibration Data - The Calibration Data section specifies maintenance information related to the calibration of the analyzer. Calibration is the most common form of maintenance required by analyzers. The requirements for calibration of the analyzer will define the most significant amount of material and labor cost related to maintenance of the process analyzer. 59
Manual or Automatic
If automatic calibration is specified, confirm that the analyzer is provided with automatic calibration capability. The drift characteristic of the analyzer is used to determine how often the analyzer will require calibration. Most analyzers are calibrated manually so that a technician can monitor the performance of the analyzer and make the necessary decisions relative to the required maintenance. For analyzers and applications that require frequent calibration, automatic calibration can be performed by the analyzer, if available, to reduce maintenance labor. Automatic calibration does increase the complexity and cost of the analyzer and sample handling system. Due to the complex electronics required, automatic calibration is usually only available with microprocessor-based analyzers.
60
Lab Sample
This information specifies lab sample facilities in the sample handling system. Lab or grab sample facilities are specified for the sample handling system when it is desirable to verify the analyzer measurement with a laboratory analysis. In certain cases, comparing the analyzer measurement with a laboratory measurement is the only approach that can be used to calibrate the analyzer. Lab sample facilities are critical for these applications.
61
Zero
Confirm space available for the zero calibration gas cylinder. Most analyzers require calibration of both the zero and span values for proper operation. This section specifies whether both zero and span calibration is required for the particular analyzer and if calibration sample will be supplied from cylinders (either liquid or gas depending on the application).
62
Span
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Confirm space available for the span gas cylinder.
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Item No. Description Instruction Maintenance Data - Although not a consideration for the initial cost or operation of the analyzer, long-term maintenance costs, both material and labor, must be included for a complete evaluation of the analyzer. 63
Accessibility
Confirm that the physical configuration and maintenance requirements for the analyzer are suitable for the location where it will be installed. The location and mounting configuration of the analyzer will define the accessibility available to the analyzer, sample handling system, and peripheral equipment. This accessibility must be evaluated relative to the long-term maintenance of the analyzer. Analyzers that require a significant amount of maintenance will require a greater degree of accessibility than those that do not require regular maintenance.
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Calib. Frequency
The frequency required for calibration of the analyzer is determined by the drift performance of the analyzer. Normally, the minimum frequency allowed for calibration of an analyzer is 24 hours or once per day. Hence, the drift specification is given as 24-hour drift. Analyzers that can perform within the drift specification for periods longer than 24 hours will require less frequent calibration. Analyzers that cannot perform within the drift specification for 24 hours will require more frequent calibration.
65
Routine Service
Confirm that the analyzer and sample handling system do not require routine maintenance in excess of what has been allocated. The routine service requirement defines the amount of maintenance time and the availability of spare parts that will be allocated for normal operation of the analyzer. Routine service is generally defined on a monthly basis. If it is anticipated that the analyzer will require more frequent routine service than specified, additional maintenance manpower must be allocated.
66
Consumables
Confirm that all consumables have been identified for the analyzer and sample handling system, if required. If consumables are required for operation of an analyzer, it must be identified so that maintenance manpower and consumable material will be allocated as part of the maintenance requirements. Consumables do not include zero and span gas cylinders that are identified separately.
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Item No.
Description
Instruction
67
Spare Parts
Confirm that spare parts have been specified and are available for the analyzer and sample handling system, if required. Spare parts are specified for all new analyzer applications. If identical analyzers already exist in the plant or facility, spare parts are probably already available and do not need to be supplied. If spare parts are not available or not available in sufficient quantities, they must be purchased with the analyzer. Spare parts must be provided for both the analyzer and sample handling system.
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Training
Confirm that training is available for the analyzer and sample handling system, if required. Training of operating and maintenance personnel is also usually required and specified for all new analyzer applications. If identical analyzers already exist in the plant or facility or if the analyzer is not complex, training may not be necessary. Training must be provided for both the analyzer and sample handling system.
Special Data - The Special Data Section is used to define specific requirements for the particular analyzer or application. Stream Composition Data - The Stream Composition Data section shows the composition of the process sample stream and the component(s) measured. 73
Component
Identifies each component in the process sample by chemical name or formula.
Normal Concentration
Identifies the normal concentration for each component in the process sample.
Min-Max Concentration
Identifies the possible range of concentration for each component in the process sample. If possible, the analyzer should operate within the performance specifications under the specified range of composition for the process sample. However, the minimum and maximum concentrations usually only occur during process startup and upset conditions when the accuracy of the analyzer is not as critical as during normal process operation. It is important that the analyzer be capable of operating under the range of possible process conditions, but it may not be necessary of the analyzer measurement to meet the performance requirements at the extremes of sample compositions. If variation in component concentration over the specified range does cause a problem with the normal analyzer measurement, the performance specifications should be evaluated separately for both normal and extreme conditions.
Calibration Range
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Defines measurement range for the analyzer.
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