Compressor and Pump Curves

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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

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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,

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Installing a Stream

1. 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 [i-C4] 0.08

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Adding the Compressor

1. Add a Compressor to the PFD.

2. Enter the following information on the Connections page:

3. On the Parameters page, ensure that the Polytropic and Adiabatic efficiency boxes both read <empty>.

These values must read <empty> 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.

Inlet Stream Feed

Outlet Stream Outlet

Energy Stream Comp Duty

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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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6. 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.

Optional Exercise

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

What is the Outlet Pressure of the compressor? __________ What is the Adiabatic Efficiency? __________

The Polytropic Efficiency?___________

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)? __________

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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:

3. Add a Compressor to the PFD with this data:

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

Name LP Gas

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

Inlet LP Gas

Outlet HP Gas

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5. 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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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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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.

Optional Exercise

1. Delete the specified compressor speed of 11 000 per minute. 2. 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 pressure of the HP Gas stream? __________

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.

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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 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:

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.

Name LP Mixture

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

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Adding the Pump

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

2. On the Curves page, enter the following data:

Inlet LP Mixture

Outlet HP Mixture

Energy Pump Duty

Efficiency (Parameters Page) 75 % Pump button

Coefficient A 3000

Coefficient B -2.0

Coefficient C -0.005

All Other Coefficients 0

Units for Head m

Flow Basis Act. Vol. Flow

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3. 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? __________