Instrumentation Tubing and Their Connections-Nirbhay Gupta
Short Description
Descripción: This technical note was written by me while I was taking lectures for the graduate engineers in my organiza...
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TECDOC-01
SEPTEMBER 2008
TECHNICAL DOCUMENT ON INSTRUMENTATION TUBING AND THEIR CONNECTIONS
Instrumentation Technical Document Series
By: Nirbhay Gupta
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PREFACE Instrumentation design and construction is a very interesting proposition. One is supposed to know the electronics and electrical aspects as well as the mechanical aspects too. Instrumentation tubing is one such field where an instrumentation engineer has to don the robes of a mechanical engineer. In NPCIL, for a long time, it was felt that there is no single document that can cater to the needs of budding as well as practicing engineers when they want to search some information on instrumentation tubing and connections. Instrumentation tubing covers both Impulse tubes (sensing lines) as well as pneumatic tubes. Connections include tapping points, root valves and tube fittings. Usually one has to refer to myriad technical documents, codes and standards to s earch for a specific aspect of tubing design or construction. This technical note is an attempt to put all the information at one place. The efforts have been put to expose the reader to all the aspects of tubing and make him aware of all the developments in the world. A comprehensive list of all the reference documents is given at the end and they have been liberally used while preparation of this note was underway. Effort has been made to represent all the relevant information here however, enterprising readers will benefit even more if they peruse the reference documents directly. Attempt has been made to demonstrate analytically that if the design and installation practices are followed as per this note then the sensing line will meet the intent of class -I tubing. Readers may note that the word tube/tubing used here should be inferred as instrumentation tubing only limited to maximum 1” size. It may be noted that various tubing practices have not been discussed in this note. The detailed installation practices for various process measurements will be discussed in respective process measurement/field installation technical notes. However, salient issues common to all installations have been discussed in detail. Author is grateful to a large number of engineers with whom they had an opportunity to work with during their long career in NPCIL and on the way a lot of design aspects were concluded. Nirbhay Gupta 23rd September, 2008 Mumbai
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TABLE OF CONTENT Section 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.6.1 2.6.2 2.6.2.1 2.6.2.2 2.6.2.3 2.7 2.8 2.9 2.10 3.0 4.0 4.1 4.2 4.3 4.4 4.5 5.0 6.0 7.0 7.1
TITLE INTRODUCTION DIFFERENCE BETWEEN A PIPE AND A TUBE MAJOR ADVANTAGES OF TUBING OVER PIPING SYSTEMS TYPES OF TUBES GUIDELINES FOR SELECTION OF INSTRUMENTATION TUBES DIFFERENT SIZES OF TUBES CRITERIA FOR SELECTING THE SIZE OF A TUBE SELECTION AND DESIGN CRITERIA DESIGN OF TUBING AND TUBING SYSTEMS CLASS-I INSTRUMENTATION TUBING DESIGN REQUIREMENTS OF MATERIAL FOR INSTRUMENT TUBING/PIPING AS PER NB-2000 DESIGN REQUIREMENTS OF INSTRUMENT PIPING/TUBING AS PER SUBSECTION NC (NC 3600) PRESSURE DESIGN (INTERNAL PRESSURE) OF INSTRUMENT TUBING/ PIPING ANALYSIS CRITERION OF TUBING/PIPING SYSTEM ANALYSIS OF SS TUBES USED IN NPCIL WALL THICKNESS AND PRESSURE RATING OF DIFFERENT SIZES OF INSTRUMENT TUBING STRESS ANALYSIS OF TUBING SYSTEMS ANALYSIS FOR SUSTAINED MECHANICAL LOADS ANALYSIS FOR OCCASIONAL LOADS (LEVEL A&B SERVICE LIMITS) ANALYSIS FOR STRESS DUE TO THERMAL EXPANSION AND OTHER SUSTAINED LOADS CONSIDERATION FOR VARIOUS FORCES TUBE BENDING CONSIDERATIONS SPECIAL DESIGN ASPECTS TO MEET THE REQUIREMENTS OF CLASS-I TUBING AND TUBING SYSTEMS CONCLUSION TECHNICAL REQUIREMENTS OF SS TUBES PNEUMATIC TUBING ADVANTAGES OF USING COPPER TUBES DIFFERENT TYPES OF COPPER TUBES RECOMMENDATIONS FOR SELECTION OF A TYPE OF COPPER TUBE TECHNICAL REQUIREMENTS OF COPPER TUBE APPLICABLE INTERNATIONAL STANDARDS FOR COPPER TUBES ASTM TUBING SPECIFICATIONS OUTSIDE DIAMETER/WALL THICKNESS EMBEDDED PENETRATIONS METHODS OF CONNECTION OF INSTRUMENTATION TUBES WELDED JOINTS
Page No. 1 1 2 3 3 5 5 6 13 13 13 13 14 15 18 18 19 19 19 20 23 23 23 24 25 27 27 28 29 34 35 36 38 39 39 v
7.2 7.3 8.0 8.1 8.2 8.3 9.0 10.0 10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9 11.0 12.0 13.0 13.1 13.2 13.3 13.4 13.5 13.6 13.7 13.8 13.8.1 13.8.2 13.8.3 13.8.4 13.8.5 13.9 13.10 13.11 14.0 14.1 14.2 14.3 14.4
FLARED, FLARELESS AND COMPRESSION JOINTS THREADED JOINTS
GUIDELINES FOR TAKE OFF C ONNECTIONS FOR SENSING LINES LOCATION OF PRESSURE TAPS CONSIDERATIONS FOR PRESSURE TAP DESIGN RECOMMENDATIONS FOR PRESSURE TAP DESIGN GUIDELINES FOR ROOT VALVES INSTALLATION OF INSTRUMENTATION TUBING BEST PRACTICES FOR IMPULSE TUBE INSTALLATION SOME PRACTICAL GUIDELINES FOR TUBE LAYING AND BENDING TUBE BENDING CHECK LIST CHARACTERISTICS OF A WELL-MADE TUBING CIRCUIT COMMON CAUSES OF IMPERFECT BENDS ROUTING OF BENDS GUIDELINES FOR C OPPER TUBE INSTALLATION GUIDELINES FOR C OPPER TUBE BENDING COPPER TUBE JOINTS IMPULSE TUBE/SENSING LINE SUPPORT IMPULSE TUBE INSTALLATION THROUGH EPS TUBE FITTINGS REQUIREMENTS OF A TUBE FITTING CONSTRUCTION OF A TUBE FITTING TYPES OF TUBE FITTINGS FLARED FITTING FLARELESS BITE TYPE TUBE FITTING FLARELESS C OMPRESSION TYPE TUBE FITTING SINGLE FERRULE FLARELESS COMPRESSION TYPE TUBE FITTING
TWIN FERRULE FLARELESS COMPRESSION TYPE TUBE FITTING FERRULE AND ITS PURPOSE SWAGING OPERATION OF A TWIN FERRULE TUBE FITTING EFFECT OF TUBE THICKNESS ON SWAGING SAFETY PRECAUTIONS FOR TUBE FITTING INSTALLATION REPEATED ASSEMBLY AND DISASSEMBLY OF TUBE FITTING SPECIFICATION FOR SS TUBE FITTINGS SPECIFICATION FOR BRASS TUBE FITTINGS THREADS USED FOR TUBE FITTINGS EVOLUTION OF THREADS TYPE OF THREADS SIZES TAPER/PARALLEL THREADED JOINTS
39 40 41 41 42 43 44 45 45 48 50 54 55 57 60 60 61 62 64 65 65 67 68 68 69 69 70 71 72 73 74 78 80 82 83 85 87 87 87 88 89 vi
14.5 15.0 15.1 15.2 15.3 15.4 15.4.1 15.4.2 15.4.3 15.4.4 15.4.5 15.4.6 15.4.7 15.4.8 15.4.9 15.4.10 15.4.11 16.0
DRY SEAL NPTF THREADS WELDING METHODS 300 SERIES STAINLESS STEELS C1018 FITTINGS TIG WELDING ORBITAL TUBE WELDING ORBITAL WELDING EQUIPMENT REASONS FOR USING ORBITAL WELDING EQUIPMENT INDUSTRIAL APPLICATIONS FOR ORBITAL WELDING GENERAL GUIDELINES FOR ORBITAL TUBE WELDING THE PHYSICS OF THE GTAW PROCESS MATERIAL WELDABILITY WELD JOINT FIT-UP SHIELD GAS (ES) TUNGSTEN ELECTRODE WELDING BASICS AND SET-UP WELDING PARAMETER DEVELOPMENT References and Suggested Reading
93 96 96 96 97 98 99 99 100 101 102 102 103 104 105 106 109 116
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TECHNICAL DOCUMENT ON INSTRUMENTATION TUBING AND THEIR CONNECTIONS 2008
1.0 Introduction Impulse sensing lines are the lines containing process fluid which run between the sensing instruments and process tapping points, and are usually made of tubing/piping, valves and tube fittings.
1.1 Difference between a pipe and a tube The fundamental difference between pipe and tube is the dimensional standard to which each is manufactured. A tube is a hollow product of round or any other cross section having a continuous periphery. Round tube size may be specified with respect to any two, but not all three, of the following: Outside diameter, inside diameter, wall thickness; type K, L and M copper tube (See section6 for details) may also be specified by nominal size and type only. Dimensions and permissible variations (tolerances) are specified in the appropriate ASTM or ASME standard specifications. Generally tubing is specified by giving O.D. and wall thickness whereas pipes are specified by giving nominal diameter & wall thickness (NB and Schedule). A pipe is a tube with a round cross section conforming to the dimensional requirements for nominal pipe size as tabulated in ANSI B36.10, Table 2 and 4, and ANSI B36.19, Table 1. For special pipe having a diameter not listed in these tables, and also for round tube, the nominal diameter corresponds with the outside diameter.
Pipe versus Tubes Standard fluid line systems, whether for simple household use or for the more exacting requirements of industry, were for many years constructed from threaded pipe of assorted materials and were assembled with various standard pipe fitting shapes, unions and nipples. Such systems under high pressures were plagued with leakage problems besides being cumbersome, inefficient and costly to assemble and maintain. Therefore, the use of pipe in these systems has largely been replaced by tubing because of the many advantages it offers. Figure 11 Tubing provides simplified, free flow system
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TECHNICAL DOCUMENT ON INSTRUMENTATION TUBING AND THEIR CONNECTIONS 2008
Old Method Each connection is threaded ‐ requires numerous fittings – system not flexible or easy to install and service connections not smooth inside ‐ pockets obstruct flow. Modern Method ‐ Bendable tubing needs fewer fittings ‐ no threading required ‐ system light and compact ‐ easy to install and service ‐ no internal pockets or obstructions to free flow.
1.2 Major Advantages of Tubing over Piping Systems 1. Bending Quality ‐ Tubing has strong but relatively thinner walls; is easy to bend. Tube fabrication is simple. 2. Greater Strength ‐ Tubing is stronger as no threads are required for connection. No weakened sections from reduction of wall thickness by threading.
Figure 12: With no threading necessary, tubing does not require extra wall thickness
3. Less Turbulence ‐ Smooth bends result in streamlined flow passage and less pressure drop. 4. Economy of Space and Weight ‐ With its better bending qualities and a smaller outside diameter, tubing saves space and permits working in close quarters. Tube fittings are smaller and also weigh less. 5. Flexibility ‐ Tubing is less rigid, has less tendency to transmit vibration from one connection to another. 6. Fewer Fittings ‐ Tubing bends substitute for elbows. Fewer fittings mean fewer joints, fewer leak paths.
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TECHNICAL DOCUMENT ON INSTRUMENTATION TUBING AND THEIR CONNECTIONS 2008
7. Tighter Joints ‐ Quality tube fittings, correctly assembled, give better assurance of leak‐free systems. 8. Better Appearance ‐ Tubing permits smoother contours with fewer fittings for a professional look to tubing systems.
9. Cleaner Fabrication ‐ No sealing compounds on tube connections. Again no threading; minimum chance of scale, metal chips, foreign particles in system. 10. Easier Assembly and Disassembly ‐ Every tube connection serves as a union. Tube connections can be reassembled repeatedly with easy wrench action. 11. Less Maintenance ‐ Advantages of tubing and tube fittings add up to dependable, trouble‐free installations.
1.3
Types of tubes
Tubes can be categorized in different ways. 1. Categorization based on tube dimensional specifications: Tubes can be classified as a. Metric tubes, where dimensions are specified in mm units e.g. 10mm, 20 mm etc. b. Fractional tubes, where dimensions are specified in inch units e.g. ½”, ¾”, 1” etc. 2. Categorization based on material of tubes e.g. carbon steel tubes, PVC Tubes, Copper tubes, SS tubes, Inconel tubes, etc. 3. Categorization based on method of tube drawing i.e. welded and drawn, seamless etc.
1.4 Guidelines for selection of instrumentation tubes Proper Tubing Selection 1. Always Match Materials – S.S. Tubing should be used only with S.S. Fittings. The only exception to this rule is copper tubing with brass fittings. Mixing materials can cause galvanic corrosion.
Galvanic Corrosion (Electrochemical) All metals have a specific relative electrical potential. When dissimilar metals come in contact in the presence of moisture (electrolyte), a low intensity electric current flows from the metal having the higher potential to the metal having the lower potential. The result of this galvanic action is the corrosion of the metal with the higher potential (more anodic). (See Galvanic Series Chart)
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TECHNICAL DOCUMENT ON INSTRUMENTATION TUBING AND THEIR CONNECTIONS 2008
Figure13: Galvanic Series chart
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TECHNICAL DOCUMENT ON INSTRUMENTATION TUBING AND THEIR CONNECTIONS 2008
2. Select proper tubing hardness – Remember instrumentation tube Fittings are designed to work within specific hardness ranges. RB 90 maximum for S.S., RB 80 recommended. For proper swaging the hardness of the tube should be less than the hardness of the fitting.
3. Select proper tubing wall thickness –
Proper wall thickness is necessary to accommodate accepted safety factors relative to desired working pressures.
4. Tubing surface finish –
Always select tubing free of visible draw marks or surface scratches. If possible, cut off any undesirable sections. These “deep” scratches can cause leaks when attempting to seal low‐density gases such as argon, nitrogen, or helium. Proper surface finish ensures leak‐proof compression joint with fitting.
1.5
Different sizes of tubes Following tube sizes have been used in NPCIL NPPs SS Tubes (metric): 6 mm, 10mm, 12mm, 20mm and 25mm. SS tube (Fractional): ¼”, 3/8”, ½”, ¾” and 1”. Copper tubes (metric): 6mm, 10mm, 12mm, 20mm and 25mm. Copper tubes (Fractional): ¼”, 3/8”, ½”, ¾” and 1”.
1.6
Criteria for selecting the size of a tube The selection criteria for sizing the tube are as follows: • • •
The O.D. of the tubes/impulse tubes should be the same and not smaller than 6 mm even with clean liquids and non corrosive piping, owing to the chance of blockage after long service. If condensation is likely to occur or if gas bubbles are likely to be liberated, the O.D. should not be smaller than 10 mm. When long runs cannot be avoided, the internal diameter of impulse tubing/piping may be selected as per the following table‐1‐1:
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TECHNICAL DOCUMENT ON INSTRUMENTATION TUBING AND THEIR CONNECTIONS 2008
TABLE – 11 Pressure signal Inside Dia. in mm of impulse tubing/piping for different process transmission fluids distance Water/steam Wet air or gas Oil of low to Very dirty (meter) liquid or gas Dry air/gas med. viscosity 0 ‐ 16 7 to 9 13 13 25 16 ‐ 45 10 13 19 25 45 ‐ 90 13 13 25 38
•
1.7
As very long runs of impulse tubing/piping are not expected in our systems and also process fluid is expected to be clean, 10 mm OD tubing having I.D. of 7.6 mm has been found to be adequate, for pressure/ ΔP measurement except for some cases for level measurement in tanks/vessels using ΔP principle. Based on hold up, installation and material cost, radiation streaming considerations, higher size (>10 mm OD) tubing is not recommended for pressure/∆P measurement in primary/nuclear system in general.
Selection and Design criteria Following requirements should be met for impulse tubing for sensing the pressure/differential pressure signal for all types of process systems including for safety and safety related systems. The most important consideration in the selection of suitable tubing for any application is the compatibility of the tubing material with the media to be contained. Table 1‐2 lists common materials and their associated general application. Table 1‐2 also lists the maximum and minimum operating temperature for the various tubing materials. Properly designed tubing/piping based on service conditions, should only be used for sensing lines. The practice of mixing materials should be strongly discouraged. The only exception is brass fittings with copper tubing. Dissimilar materials in contact may be susceptible to galvanic corrosion. Further, different materials have different levels of hardness, and can adversely affect the fittings ability to seal on the tubing. The use of a particular type of tube for a specific usage depends on the application and the process condition. The following table briefly describes the application guidelines for a specific tube material.
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TECHNICAL DOCUMENT ON INSTRUMENTATION TUBING AND THEIR CONNECTIONS 2008
Table12
1. 2.
For operating temperatures above 800 °F (425 °C), consideration should be given to media. 300 Series Stainless Steels are susceptible to carbide precipitation which may lead to intergranular corrosion at elevated temperatures. All temperature ratings based on temperatures as per ASME/ANSI B313 Chemical Plant and Petroleum Refinery Piping Code, 1999 Edition.
Gas Service Special care must be taken when selecting tubing for gas service. In order to achieve a gas‐ tight seal, ferrules in instrument fittings must seal any surface imperfections. This is accomplished by the ferrules penetrating the surface of the tubing. Penetration can only be
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TECHNICAL DOCUMENT ON INSTRUMENTATION TUBING AND THEIR CONNECTIONS 2008
achieved if the tubing provides radial resistance and if the tubing material is softer than the ferrules. Thick walled tubing helps to provide resistance. Tables‐1‐3 to 1‐10 below indicate the minimum acceptable wall thickness for various materials in gas service. The ratings in white indicate combinations of diameter and wall thickness which are suitable for gas service. Acceptable tubing hardness for general application is listed in Table 1‐12. These values are the maximum allowed by the ASTM. For gas service, better results can be obtained by using tubing well below this maximum hardness. For example, a desirable hardness of 80 RB is suitable for stainless steel. The maximum allowed by ASTM is 90 RB.
System Pressure The system operating pressure is another important factor in determining the type, and more importantly, the size of tubing to be used. In general, high pressure installations require strong materials such as steel or stainless steel. Heavy walled softer tubing such as copper may be used if chemical compatibility exists with the media. However, the higher strength of steel or stainless steel permits the use of thinner tubes without reducing the ultimate rating of the system. In any event, tube fitting assemblies should never be pressurized beyond the recommended working pressure. The following tables (1‐3 to 1‐10) list by material the maximum suggested working pressure (in psi) of various tubing sizes. Acceptable tubing diameters and wall thicknesses are those for which a rating is listed. Combinations which do not have a pressure rating are not recommended for use with instrument fittings.
Table13: Fractional 316 or 304 STAINLESS STEEL (Seamless)
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TECHNICAL DOCUMENT ON INSTRUMENTATION TUBING AND THEIR CONNECTIONS 2008
Table14: Fractional 316 or 304 STAINLESS STEEL (Welded & Drawn)
Table15: Seamless Stainless Steel metric tubing
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TECHNICAL DOCUMENT ON INSTRUMENTATION TUBING AND THEIR CONNECTIONS 2008
Table16: Fractional Carbon Steel (Seamless) Tube OD in.
1/8 3/16 1/4 5/16 3/8 1/2 5/8 3/4 7/8 1 1 1/4 1 1/2 2
0.028
0.035
0.049
0.065
0.083
Tube Wall Thickness, in. 0.09 0.10 0.120 5 9
0.134
0.148
0.165
0.180
0.220
Working Pressure, psig Note: For gas service, select a tube wall thickness outside of the shaded area.
8000 10 200 5100 6 600 9600 3700 4 800 7000 9600 3 700 5500 7500 3 100 4500 6200 2 300 3200 4500 1 800 2600 3500 2100 2900 1800 2400 1500 2100 1600
5900 4600
5300
3700
4300
3200
3700
2700
3200
2100
2500
1800
2000 1500
510 0 430 0 370 0 290 0 240 0 170 0
4100 3200
3600
4000
4600
5000
2600
2900
3300
3700
4100
5100
1900
2100
2400
2700
3000
3700
Table17: Carbon Steel Metric tubing
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Table18: ALUMINIUM (SEAMLESS)
Table19: COPPER (SEAMLESS)
Table110: MONEL 400 (SEAMLESS)
Note: • • • •
All working pressures have been calculated using the maximum allowable stress levels in accordance with ASME/ANSI B31.3, Chemical Plant and Petroleum Refinery Piping or ASME/ANSI B31.1 Power Piping. All calculations are based on maximum outside diameter and minimum wall thickness. All working pressures are at ambient (72°F) temperature. Ratings in gray are not suitable for gas services.
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TECHNICAL DOCUMENT ON INSTRUMENTATION TUBING AND THEIR CONNECTIONS 2008
Systems Temperature Operating temperature is another factor in determining the proper tubing material. Copper and aluminum tubing are suitable for low temperature media. Stainless steel and carbon steel tubing are suitable for higher temperature media. Special alloys such as Alloy 600 are recommended for extremely high temperature (see Table 1‐2). Table 1‐11 lists de‐rating factors which should be applied to the working pressures listed in Table 1‐3 to 1‐10 for elevated temperature (see Table 1‐2). Simply locate the correct factor in Table 1‐11 and multiply this by the appropriate value in Tables 1‐3 to 1‐10 for the elevated temperature working pressure. Table-1-11 Temperature Derating Factors Temperature °F (°C) 100 200 300 400 500 600 700 800 900 1000 1100 1200
(38) (93) (149) (204) (260) (316) (371) (427) (486) (538) (593) (649)
Copper
Aluminum
1.00 .80 .78 .50
1.00 1.00 .81 .40
316 SS 1.00 1.00 1.00 .97 .90 .85 .82 .80 .78 .77 .62 .37
304 SS 1.00 1.00 1.00 .94 .88 .82 .80 .76 .73 .69 .49 .30
Steel
Monel 400
1.00 .96 .90 .86 .82 .77 .73 .59
1.00 .88 .82 .79 .79 .79 .79 .76
EXAMPLE: 1/2 inch x .049 wall seamless stainless steel tubing has a working pressure of 3700 psi @ room temperature. If the system were to operate @ 800°F (425°C), a factor of 80% (or .80) would apply (see Table 111 above) and the “at temperature” system pressure would be 3700 psi x .80 = 2960 psi
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TECHNICAL DOCUMENT ON INSTRUMENTATION TUBING AND THEIR CONNECTIONS 2008
Table‐1‐12 Material
Type
ASTM Tubing Spec.
Condition
Stainless Steel Copper
304, 316, 316L K or L
ASTM‐A‐269, A‐249, A‐ 213, A632 ASTM‐B75 B68, B88* (K or L) SAE‐J524b, J525b
Fully Annealed
Carbon 1010 Steel Aluminum Alloy 6061 Monel™ 400 Alloy C‐ C‐276 276 Alloy 600 600 Carpenter 20CB‐3 20™ Titanium Commercially Pure Grade 2
Soft Annealed Temper 0
Max. Recommended Hardness 90 RB
Fully Annealed
60 Max. Rockwell 15T 72 RB
ASTM‐A‐179 ASTM B‐210 ASTM B‐165 ASTM‐B‐622, B‐626
T6 Temper Fully Annealed Fully Annealed
56 RB 75 RB 90 RB
ASTM B‐167 ASTM B‐468
Fully Annealed Fully Annealed
90 RB 90 RB
ASTM B‐338
Fully Annealed
99 RB 200 Brinell Typical
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TECHNICAL DOCUMENT ON INSTRUMENTATION TUBING AND THEIR CONNECTIONS 2008
2.0 DESIGN OF TUBING AND TUBING SYSTEMS 2.1
CLASSI INSTRUMENTATION TUBING DESIGN In ASME Section III‐Division‐I sub‐section NB (Class I components), the design criterion/design requirements for instrument tubing has not been covered separately. Thus design guidelines given for small size of piping is being followed for Class I instrument tubing also. Also as the outside diameter of instrument tubing is being limited to 1” (25 mm); so any design concession permitted for lower size piping (
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