Liquid Pumping
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Liquid Pumping
© 2000 AEA Technology plc - All Rights Reserved Nat Gas WS 7_3.pdf
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Liquid Pumping
Workshop The liquids generated in the Refrigerated Gas Plant LTS are to be sent to storage facilities. It has been determined that a pump will be required to overcome frictional losses in piping. To flow into the storage vessel, the pressure of the LTS liquids must be increased to at least 8000 kPa.
Learning Objectives In this workshop you will learn how to:
• Model pumps in HYSYS • Use pump curves in HYSYS
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Liquid Pumping
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Building the Simulation In this workshop, a Pump is added after the Low Temperature Separator that was modelled in the Refrigerated Gas Plant in Module 4.
The Pump Operation The Pump operation is used to increase the pressure of an inlet liquid stream. Depending on the information supplied, the Pump calculates either an unknown pressure, temperature or pump efficiency.
Calculations The Pump will calculate the energy required to raise the pressure of a stream. Multiphase streams (including streams with an sizable vapour fraction) are acceptable. Calculations are based on the standard pump equation for power, which uses the pressure rise, the liquid flow rate and density: Calculations are based on the standard pump equation for power, which uses the pressure rise, the liquid flow rate and density: ( P ou t – P in ) * Flow rate -----Power Re qui re d ideal = --------------------------------------------------Liquid Density
This equation defines the ideal power needed to raise the liquid pressure. The actual power requirement of the Pump is defined in terms of the Pump Efficiency: Power Re quir ed ideal Ef fici ency (%) = --------------------------------------------------------* 100 Power Re quir ed actual
When the efficiency is less than 100%, the excess energy goes into raising the temperature of the outlet stream. Combining the above equations leads to the following expression for the actual power requirement of the Pump.This equation defines the ideal power needed to raise the liquid pressure. The actual power requirement of the Pump is defined in terms of the Pump Efficiency: ( P ou t – P in ) *Flow rate*100 Power Re qui re d actual = -------------------------------------------------------------------------Liquid Density*Efficiency (%)
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Liquid Pumping
Finally, the actual power is equal to the difference in heat flow between the outlet and inlet streams: Power Re qui re d actual = ( Heat Flow outlet – Heat Flow inlet )
If the feed is fully defined, you need only supply two of the following variables for the pump to calculate all unknowns:
• Outlet Pressure or Pressure Rise • Efficiency • Pump Energy HYSYS can also back-calculate the i nlet Pressure.
Note that for a Pump, an efficiency of 100% does not correspond to a true isentropic compression of the liquid. Pump calculations are performed by HYSYS with the assumption that the liquid is incompressible; that is, the density is constant (liquid volume is independent of pressure). This is the usual assumption for liquids well removed from the critical point, and the standard pump equation is generally accepted for calculating the power requirement. However, if you wish to perform a more rigorous calculation for pumping a compressible liquid (i.e. one near the critical point), you should instead install a Compressor to represent the pump.
If you choose to represent a pump by installing a Compressor in HYSYS, the power requirement and temperature rise of the compressor will always be greater than those of the pump (for the same fluid stream), because the compressor treats the liquid as a compressible fluid. When the pressure of a compressible fluid increases, the temperature also increases, as does the specific volume. More work is required to move the fluid than if it were incompressible, exhibiting little temperature rise, as is the case with a HYSYS Pump.In addition to specify a pressure change across the pump, you can select to have the pressure change calculated via a Pump Curve, which is a quadratic expression which calculates pressure rise as a function of flow.
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Liquid Pumping
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Installing the Pump 1.
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Open the Refrigerated GAs Plant case built in Module 4. If you have not saved the case you can open it from the diskette, a:\Module4.hsc Add a Pump operation to the simulation.
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On the Connections page, supply the Inlet, Outlet and Energy streams.
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Move to the Parameters page. Use the default Efficiency of 75%.
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On the Work Sheet page, supply a discharge pressure of 8000 kPa (1160 psia) .
Pump button
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Liquid Pumping
What is the discharge temperature?___________ What is the energy requirement?__________
Pump Curves Actual pump operating characteristics can be supplied in the form of coefficients for a quadratic equation which will determine the discharge pressure from the pump. These coefficients are supplied on the Curve page tab of the Pump unit operation. Note: If using curves to calculate the pump discharge pressure, ensure that the discharge pressure is not specified.
You can supply your actual Pump curve in EXCEL and then regress the points to fit a Quadratic Equation which can be put into HYSYS.
If you wish to supply a pump curve, move to the Curve page and provide the coefficients for the quadratic pump equation, as well as the units for pressure and flow. Then select Enable Curves, and HYSYS will determine the pressure rise across the Pump for the given flowrate. This functions in dynamic mode as well, with the outlet pressure varying as a function of flow.
Exercise 1 You meet with a pump salesman who insists that his top-of-the line pump, which is very expensive, will meet your minimum discharge pressure (8000 kPa (1160 psia)). He provides you with the following coefficients for the pump’s performance curve. Should you start preparing a purchase order? A=450 B=-2 C=-0.05 The units are:
• Head, m • Flow Basis, Act Vol Flow • Units for Flow, m3 /h
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What is the discharge pressure of the pump when the curve is used? ________________ Will this pump meet the pressure requirements of the system? ________________ What is the maximum flowrate that this pump can handle and still meet the pressure requirements? __________
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