Pump Trip with Accumulator (English Units)

Pump Trip with Accumulator (Metric Units)

Summary

This example shows the effect of adding a gas accumulator to reduce the down-surge after a pump trip.

Topics Covered

• Using accumulator junctions

Required Knowledge

This example assumes the user has already worked through the Beginner - Valve Closure example, or has a level of knowledge consistent with that topic. You can also watch the AFT Impulse Quick Start Video (English Units) on the AFT website, as it covers the majority of the topics discussed in the Valve Closure example.

This model is based off of the Pump Trip with Check Valve example and model, and that example should be worked before this one.

Model File

• US - Pump Trip with Check Valve.imp

• US - Pump Trip with Accumulator.imp

Step 1. Start AFT Impulse

From the Start Menu choose the AFT Impulse 10 folder and select AFT Impulse 10.

To ensure that your results are the same as those presented in this documentation, this example should be run using all default AFT Impulse settings, unless you are specifically instructed to do otherwise.

Step 2. Open the Model

In reviewing the Pump Trip with Check Valve example it is observed that the pressure drops below atmospheric pressure along about 400 feet of pipe. This example addresses the case when this is not acceptable. To try to limit the low pressures, an accumulator will be sized and located.

From the AFT Impulse examples folder make a copy of the US - Pump Trip with Check Valve.imp model file and save it to a new location. This model will be used as a starting point.

Step 3. Define the Pipes and Junctions Group

The Pipes and Junctions group is already defined, but some modifications are warranted. The model will be altered to resemble Figure 1.

Note: Instead of adding a new pipe to the Pump Trip model in order to add the accumulator, you can instead hold down Shift, then drag and drop the accumulator junction on the pipe to Split the pipe. This will allow you to add the junction, define the new length for each of the sections of pipe, and define which section of pipe the Fittings & Losses should be placed in.

Split pipe P2 into two pipes with a Gas Accumulator junction. For Pipe A, set the length to 50. Select Pipe B under Move Fittings & Losses to. Rearrange the junctions on the workspace until is resembles to figure below.

Sizing and locating accumulators is very much a trial and error method. The typical assumption is that the accumulator should be located as near the source of the transient as possible. In developing this example, the accumulator was first placed 10 feet from the pump suction. Initial gas volumes of 0.25 feet3 up to 10 feet3 were used. The accumulator was effective in keeping the pressures in the discharge pipe above atmospheric, but the suction pipe, which previously had all positive pressures, now had sub-atmospheric pressure.

Therefore the accumulator was moved further away from the pump to see if this could be avoided. It was therefore first located at 700 feet along pipe P2 because that was the point of minimum pressure in the Pump Trip With Check Valve example. Thus pipe P2 was 700 feet long and P3 was 290 feet long and the pump yielded acceptable positive pressures. The accumulator was then moved closer to the pump, and ultimately it was discovered that placing the accumulator 50 feet downstream of the pump gave acceptable pressures on both the pump suction and discharge piping. If this were a real system, this would have the added benefit of locating the accumulator near the pump, supposedly an accessible location rather than halfway up the side of a hill.

The system is in place but now we need to enter the input data for the Gas Accumulator junction. Double-click the junction and enter the following data in the properties window.

Junction Properties

1. Gas Accumulator J4

   1. Tank Geometry = Unchecked

   2. Initial Gas Pressure = Calculate From Steady State

   3. Initial Gas Volume = Known Volume

   4. Value = 0.5 feet3

   5. Polytropic Constant in Transient = 1.2

   6. Max/Min Gas Volume = Unchecked

   7. Flow Restrictor = Unchecked

   8. Short Connector Pipe = Checked

   9. Friction Factor = 0.018

   10. Diameter = 4 inches

   11. Pipe Length = 2 feet

   12. Specify Elevation Change = Checked

   13. Elevation Change = 2 feet

   14. Junction Elevation = 10 feet

ØTurn on the Show Object Status from the View menu to verify if all data is entered. If there are objects that are not defined, the uncompleted pipes or junctions will have their number shown in red on the workspace. If this happens, go back to the uncompleted pipes or junctions and enter the missing data. If all objects are defined, the Pipes and Junctions group in Analysis Setup will have a check mark.

Step 4. Define the Pipe Sectioning and Output Group

ØOpen Analysis Setup and open the Sectioning panel. When the panel is first opened it will automatically search for the best option for one to five sections in the controlling pipe. The results will be displayed in the table at the top. Select the row to use one section in the controlling pipe. 

Step 5. Run the Model

Click Run Model from the toolbar or from the Analysis menu. This will open the Solution Progress window. This window allows you to watch the progress of the Steady-State and Transient Solvers. When complete, click the Graph Results button at the bottom of the Solution Progress window.

Step 6. Examine the Output

Graph the static pressure profile for all three pipes. This is shown in Figure 3. The pressure transient at the pump suction and discharge are shown in Figure 4. Note that the maximum pump discharge pressure has increased slightly above 167 psig, the value without the accumulator. For interest, it should be mentioned that an initial accumulator volume of 0.5 feet3 was also used. It successfully kept all minimum pressures above atmospheric, but caused the pump discharge pressure to rise up to about 180 psig. Although accumulators are often thought to be pressure reduction devices, they can also increase peak pressures. The pump speed decay is shown in Figure 5. Note how the pump speed does not decrease as quickly as in the Pump Trip with Check Valve example. The gas volume in the accumulator is shown in Figure 6.