Modeling A Pump Start-Up Transient Event In Bentley HAMMER V8 XM [TN]

  Applies To 
  Product(s): Bentley HAMMER V8 XM
  Version(s): 08.09.400.34
  Area:  Modeling
  Original Author: Jesse Dringoli, Bentley Technical Support Group

Overview

This TechNote describes the step-by-step process used to model a pump starting up (as opposed to shutting down) in Bentley HAMMER V8 XM Edition.

Note that Bentley HAMMER V8i offers an automated way of modeling this. The steps below are only for users of Bentley HAMMER V8 XM. For V8i, see this article.

Introduction

In most cases, the transient-inducing effects of a pump are modeled by having that pump shut down. In Bentley HAMMER, the "shut after time delay" transient pump type is used to do this. However, in some cases, an engineer may want to model the effects of a pump turning on (a pump start-up event.) In this case, HAMMER does not offer a direct way to do this; there is currently no "start after time delay" transient pump type. This is primarily due to the fact that the nominal operating point of the pump after it turns on is not known by the transient simulation, if the initial conditions describe the pump being off. So, a few extra steps are required to model this case.

 

Setting up the Model for a Pump Start-up

 

Before performing these steps, ensure that the demands, physical properties, and other settings in the model describe the condition that you would like to represent. Meaning, if you would like to see the transient effects of the pump turning on during high demands and low tank level, ensure that the demands and tank level are adjusted as such.

This walkthrough also assumes a steady-state analysis for the initial conditions and that you have storage downstream of the pump in question, or other pumps, either of which could supply the demands you have entered when the pump in question is off.

Note that if you do not have an active valve in your model, you can skip steps 6, 7 and 11.

  •  First, turn the pump on by selecting "On" as the "Status (Initial)" under the Initial Settings section of the pump properties.
  • Go to Analysis > Compute Initial Conditions. This invokes the pressure engine to allow us to see the point where it would operate on its characteristic curve when the pump turns on.
  • Double-click the pump in question to view the properties. Under the "Results" section, you will see the pump operating point. Write down the values seen for "Flow (Total)" and "Pump Head". We will use these later on.

  • Go to Analysis > Calculation Options. Double-click on the Transient Calculation Option that is assigned to your current scenario (designated by the red check mark.)

  • In the properties of the transient calculation options, ensure that the "Specify Iinitial Conditions" attribute is set to "True". This allows the user to manually specify the initial conditions that the transient simulation uses for its initial conditions.
  • By default, the user-defined initial conditions will be empty (zeroes). Go to Tools > Copy Initial Conditions. This tool will copy the computed initial conditions (from step 2 above) to the user-defined fields. This way, changes can be made to specific values, instead of having to type in all of them.
  • The first thing you will need to do is select a timestep to copy the conditions from. The initial conditions should be steady state, so select the default 0.000 hours. Make sure "All" is selected for "Features" and then click "OK".

  • At this point, if you had any valves (such as a TCV) in the model that were active during the initial conditions, the discharge coefficient will be copied over for you to view. So, if you have any valves, open the properties and note the value for the "Discharge Coefficient (Transient)".



  • In the properties of the pump that you would like to start up, change the "Status (Initial)" to "Off".
  • Compute initial conditions again, from the Analysis menu. This will compute the conditions in the system when the pump is off. This will be the true starting conditions of the transient simulation.
  • Go to Tools > Copy Initial Condition and copy the computed conditions again, just like in step 6 above

Now that the transient simulation has been provided the correct initial conditions, we need to specify a few extra key pieces of information: the nominal operating point and discharge coefficient that describe the conditions of the pump and valve(s) when the pump is on. This is why the above steps were performed.

  • In the properties of your valve(s), enter the discharge coefficient that you wrote down in step 7 above, under the "Transient (Physical)" section.
  • In the properties of the pump that will be shutting down, enter the flow and head values (that you wrote down in step 3 above), in the "Flow (Nominal)" and "Head (Nominal)" attributes, under the "Transient (Initial)" section.
  • Under the "Transient (Operational)" section of the pump properties, select "Variable Speed" as the "Pump Type (Transient)" and enter the appropriate diameter. If the pump has a built-in check valve, enter "0" for the "Time (for valve to operate)", or else enter the time that it takes for the built in valve to open. Most likely you will enter zero for this.
  • Now we must define when and how fast the pump starts up. 
  • Go to Components > Patterns to open the Pattern manager. Right click on "Operational (Transient, Pump)" select "New" and enter a name. On the right side of this window, enter zero for the starting multiplier, since the starting relative speed factor should be zero (meaning the pump is off). In the bottom-right table, define the pattern of time versus relative speed multiplier. In the example below, the relative speed factor jumps from zero to 1.00 (full speed) between 6 and 7 seconds, and then stays on for the duration of the simulation.



  • Close the pattern manager and select the pattern that you just created from the "Operating Rule" dropdown in the pump properties. At this point, the pump properties should look similar to this:



At this point, the model is correctly configured for a pump startup event and you can compute the transient simulation via Analysis > Compute.

 

Viewing the Results 

The results of this model can be viewed just like any other transient simulation. Go to Analysis > Transient Results Viewer. To view a graph of head and flow for the pump, you may want to select a time history for the pipe end adjacent to the pump:

 

As you can see, when the pump starts up, a transient occurs. After about 40 seconds, the head/flow stabilize to the nominal conditions.

Note that in many cases, you may not see flow start to occur through the pump until the speed has increased enough so that the downstream head can be overcome.

You can also select a transient profile and click the "Animation" button:

 

As you can see, there are some problems with vapor pockets forming upstream of the pump when it starts up. A surge tank may be required in this case.

Note that you can also view extended data specific to the pump by entering a number for the "Report Period" attribute of its properties. For example, "10" would mean that extended data will be reported every 10 timesteps. You can view this report by going to Report > Transient Analysis Reports > Transient Analysis Detailed Report. At the very bottom of this text report, you will see the table of flow, speed, upstream head, and downstream head:

 

 

Is There An Easier Way To Do This??

Yes. HAMMER V8i introduced a new transient pump type called "Pump start". This exposes the rated head/flow values without having to specify initial conditions. So, all you would need to do is turn the pump on first to obtain the rated head/flow and then turn it off and enter those values for in the "pump start" pump.

 

 

See Also

Product TechNotes and FAQs

OpenFlows Product Tech Notes And FAQs

External Links

Hydraulics and Hydrology Forum

Bentley LEARN Server

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