Compressor and Pump Curves

Compressor and Pump Curves

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Compressor and Pump Curves

© 1999 - AEA Technology plc - All Rights Reserved. ADV 1_2.pdf

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Compressor and Pump Curves

Workshop In this module, compressor and pump curves will be used to model the behaviour of simulated compressors and pumps. Using curves to model these unit operations allows HYSYS to accurately simulate actual plant equipment.

Learning Objectives Once you have completed this module, you will be able to:

• • • •

Specify and attach head and efficiency curves to compressors Use single and multiple curves to model compressors Attach head curves to pumps Accurately model existing plant equipment with HYSYS

Prerequisites Before beginning this module, you need to know how to:

• Define and import a fluid package • Add streams and unit operations to the PFD

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Compressor and Pump Curves

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Compressor Curves Using compressor curves in your HYSYS simulation allows you to accurately model existing plant equipment. You can determine if an existing compressor is able to meet the specifications of your process. Using compressor curves allows HYSYS to calculate heads and efficiencies that are dependant on the flow rate. If the flow rate through the compressor is known to be constant, a single flow rate and efficiency can be supplied. If, however, the flow rate is expected to change, using a compressor curve will allow HYSYS to calculate new heads and efficiencies based on the current flow rate. This results in greater accuracy in the simulation, and allows HYSYS to more closely model actual plant equipment.

Defining the Fluid Package Before we begin any simulation in HYSYS, we must first define the appropriate fluid package. 1.

Start a New Case and add a Fluid Package.

2.

Select the Sour PR EOS package

3.

Add the following components: H2O, H2S, CO2, C1, C2, C3, i-C4, and n-C4.

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Compressor and Pump Curves

Installing a Stream 1.

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Add a Material Steam to the PFD with the following data:

In This Cell...

Enter...

Name

Feed

Temperature

70 oC (160 oF)

Pressure

130 kPa (19 psia)

Molar Flow

500 kgmole/hr (1100 lbmole/hr)

Mole Fraction [H2O]

0.24

Mole Fraction [H2S]

0.07

Mole Fraction [CO2]

0.06

Mole Fraction [C1]

0.04

Mole Fraction [C2]

0.11

Mole Fraction [C3]

0.25

Mole Fraction [i-C4]

0.08

Mole Fraction [n-C4]

0.15

Compressor and Pump Curves

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Adding the Compressor 1. 2.

Compressor button

Add a Compressor to the PFD. Enter the following information on the Connections page:

In This Cell...

Enter...

Inlet Stream

Feed

Outlet Stream

Outlet

Energy Stream

Comp Duty

3.

On the Parameters page, ensure that the Polytropic and Adiabatic efficiency boxes both read .

These values must read because the efficiencies will be calculated from the compressor curves, and defining the same value in two places will always result in a consistency error.

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Compressor and Pump Curves

4.

On the Curve page, select the Adiabatic radio button in the Efficiency group. Press the Add Curve button, and enter the data as shown here:

5.

Close the above view and activate Curve-1 on the Curves page. Ensure that the Enable Curves box on the Curves page is checked.

Be sure to use the correct units for the curve.

We do not need to enter a compressor speed because we are only entering one curve. However, multiple curves with different speeds can be used.

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Compressor and Pump Curves

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The pressure of the outlet stream can be seen on the Work Sheet page, or you can choose Show Table when object-inspecting the outlet stream on the PFD. The compressor’s efficiencies can be found on the Results page. What is the Outlet Pressure of the compressor? __________ What is the Adiabatic Efficiency? __________ The Polytropic Efficiency?___________

Optional Exercise It is desired to have an outlet pressure of 300 kPa (44 psia). Use an Adjust operation with the following information: In This Cell...

Enter...

Adjusted Variable

Feed - Molar Flow

Target Variable

Outlet - Pressure

Specified Target Value

300 kPa (44 psia)

Step Size

5 kgmole/hr (10 lbmole/hr)

Iterations

50

What is the Molar Flow rate if the Outlet Pressure is set at 300 kPa (44 psia)? __________ Can you think of an easier way of doing this? __________ How? __________

Save your case!

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Compressor and Pump Curves

Multiple Curves Typically, industrial compressors are able to run at multiple speeds depending on the current demand. HYSYS allows users to enter multiple compressor curves that each represent a specified speed. Once the curves are entered, any compressor speed can be specified and the head and efficiency are calculated automatically. In this exercise, a Natural Gas compressor will be examined to determine the outlet pressure of a multi-speed compressor. 1.

Begin a new case and import the fluid package Nat-Gas.fpk from the diskette provide with this course.

2.

Add a new stream to the PFD with the following data:

In This Cell...

Enter...

Name

LP Gas

Temperature

10 oC (50 oF)

Pressure

1700 kPa (245 psia)

Molar Flow Rate

1500 kgmole/hr (3300 lbmole/hr)

Comp. Mole Fraction - C1

0.99

Comp. Mole Fraction - C2

0.002

Comp. Mole Fraction - C3

0.0005

Comp. Mole Fraction - N2

0.005

Comp. Mole Fraction - CO2

0.0025

3.

In This Cell...

Enter...

Inlet

LP Gas

Outlet

HP Gas

Energy

Comp Duty

4.

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Add a Compressor to the PFD with this data:

Delete the default Adiabatic Efficiency value on the Parameters page. Again, the efficiency will be calculated from the compressor curves.

Compressor and Pump Curves

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Add the four curves shown below to the compressor. Note that the curves have been supplied in Field units. (If you are using SI units, you must change the Flow and Head units to those shown here before you enter the curve data).

Note that compressor speeds must be entered here as multiple curves are being used.

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Compressor and Pump Curves

The plots for efficiency and head versus flow can be seen by pressing the Plot Curves button on the Curves page.

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Compressor and Pump Curves

6.

Ensure that all of the curves are activated, and the Enable Curves box is checked. These curves are polytropic curves, therefore the Polytropic radio button must be checked in the Efficiency group on the Curves page.

7.

On the Curves page, enter a speed of 11 000 per min.

Instead of entering all of the curve data, open the HYSYS case Comp_Shortcut.hsc on the disk supplied with this module, and begin on step 6.

What is the pressure of the HP Gas stream? __________

Optional Exercise 1. 2.

Delete the specified compressor speed of 11 000 per minute. Enter a pressure of 5000 kPa (725 psia) for the HP Gas stream.

3.

HYSYS will automatically calculate the compressor speed needed to meet this outlet pressure. What is the compressor speed needed to achieve the specified outlet pressure? __________ What are the Adiabatic and Polytropic efficiencies of the compressor under these conditions? __________ What is the temperature of the HP Gas? __________

Save your case! 11

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Compressor and Pump Curves

Pump Curves As with compressor curves, pump curves are used to allow HYSYS to accurately model existing pumps. Pump curves allow the pressure rise across the pump to be dependent on the flow rate of liquid.

The coefficients can be obtained from a spreadsheet program capable of nonlinear regression, such as EXCEL, or may be supplied by the pump’s manufacturer.

The pump curves are entered into HYSYS using a form different than the form used for compressor curves. With pump curves the coefficients of an expression, up to the fifth order, are entered into HYSYS rather than the actual data points.

Defining the Fluid Package 1.

Begin a new case and select the Peng Robinson EOS package.

2.

Add the components n-Hexane, n-Heptane, and n-Octane.

Installing a Stream Add a new stream to the PFD and enter the following information:

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In This Cell...

Enter...

Name

LP Mixture

Temperature

25 oC (77 oF)

Pressure

120 kPa (18 psia)

Liquid Volume Flow

500 m3/hr (76,000 BPD)

Comp. Mass Fraction [Hexane]

0.60

Comp. Mass Fraction [Heptane]

0.30

Comp. Mass Fraction [Octane]

0.10

Compressor and Pump Curves

Adding the Pump 1.

Pump button

Add a Pump to the PFD and enter the following information:

In This Cell...

Enter...

Inlet

LP Mixture

Outlet

HP Mixture

Energy

Pump Duty

Efficiency (Parameters Page)

75 %

2.

On the Curves page, enter the following data:

In This Cell...

Enter...

Coefficient A

3000

Coefficient B

-2.0

Coefficient C

-0.005

All Other Coefficients

0

Units for Head

m

Flow Basis

Act. Vol. Flow

Units for Flow

m3/hr

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Compressor and Pump Curves

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Ensure that the Activate Curves box is checked.

4.

The pressure of the product stream can be seen on the Worksheet tab.

What is the outlet pressure of the pump? __________ The pump sales representative, who supplied the curve data, guaranteed an outlet pressure of 5000 kPa (725 psia) at the specified flow rate. Should you fill out the purchase order? __________

Save your case!

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