This TechNote describes the process by which you can model a pump start-up transient event in HAMMER CONNECT Edition and HAMMER V8i. It also provides an example model file for demonstration. This model can also be found in the Samples folder in the HAMMER installation folder: C:\Program Files (x86)\Bentley\HAMMER\Samples.
In older version of HAMMER (V8 XM), a lengthy procedure was required to model a pump startup: (See Modeling a Pump start-up transient event in Bentley HAMMER V8 XM) The user needed to first obtain the rated operating point of the pump by turning it on, then turn the pump off, copy the initial conditions to the user defined fields and enter the rated head/flow, along with valve initial status. This can be cumbersome in some situations. For this reason, HAMMER V8i introduced a new transient pump type called "Pump Start - variable speed/torque." This allows you to specify the rated head and flow of the pump without having to specify initial conditions. This can greatly simplify the steps need to model a pump start-up event.
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. If you would like to see the transient effects of the pump turning on during high demands and low tank levels, ensure that the demands and tank settings are adjusted as such. Ensure that the efficiency and transient rotational speed in your pump definition represent the "nominal" conditions. The speed should be set to the speed at which the pump produces the flow and head seen when the pump is on (usually full speed), and the efficiency should be the efficiency at that nominal flow and head.
This walk-through 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. If there is no other source of flow and your pump is off, you may receive a "Disconnected demand nodes" user notification, which could keep you from getting accurate transient results.
1. First, turn the pump on by selecting "On" as the "Status (Initial)" under the Initial settings section of its properties.
2. Go to Analysis > Compute Initial conditions. This computes pressure engine to allow you to see the pump flow and head when the pump is on. to see the point where it would operate on its characteristic curve, when it turns on.
3. Double-click the pump in question to view the properties. Under the "Results" section, you will see the pump operating point. Note the values for "Flow (Total)" and "Pump Head," as you will need them in the next step.
If you have any active valves in the model (TCV, GPV, PRV, PSV, FCV, PBV), in places where the flow is zero or near zero in the initial conditions, you will need to find the correct discharge coefficient during this step. To do so, either check for the computed discharge coefficient in the results section of the properties, or temporarily select "true" for "specify initial conditions?" in the transient calculation options, click the valve, then go to Tools > Copy Initial Conditions, choose "selection", then OK - you will now see the discharge coefficient in the "transient (initial)" section of the properties. Record this value and set the "specify initial conditions?" calculation option back to "false". For any valves that you need to do this, morph them into a TCV, choosing "discharge coefficient" as the type, "Active" as the initial status, then enter the discharge coefficient that you recorded. If you do not do this, then HAMMER may use a discharge coefficient that will be inaccurate for higher flow rates once the pump turns back on. (since it will be based on zero or near-zero flow).
4. Under the "Transient (Operational)" section of the pump properties, select "Pump Start - Variable Speed/Torque" 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)". Alternatively, you can enter the time that it takes for the built-in valve to open (5 sec, 10 sec, 30 sec, etc...). To simulate a pump with no check valve enter a very small number like 0.1 seconds, so the valve opens immediately. Most likely you will enter zero for this. This is an important consideration - please read this article for more.
5. Enter the pump flow and pump head found in step 3, in the "Flow (nominal)" and "Head (nominal)" fields.
6. Change the pump's Status (Initial) under the initial settings to "Off" and re-compute initial conditions.
7. 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 speed multiplier should be zero (meaning the pump is off). In the bottom-right table, define the pattern by entering time values and the corresponding speed multiplier.
IMPORTANT NOTE: The multipliers you enter here multiply against either the speed or the electrical torque, depending on the selection you make for the pump's "Control Variable." If you choose Speed (the default), the multipliers will multiply against the full speed entered in the pump definition, so 1.0 means full speed. This means that you cannot simply "flip the switch" in the pattern and have it go instantly from zero to 1.0 (or within a very small time frame) as it would normally take some time for the pump to ramp up to full speed. In the example below, the speed jumps from zero to 1.00 (full speed) between 5 and 10 seconds, and then stays on for the duration of the simulation. If the pump takes short or longer to ramp up, then the pattern needs to be adjusted accordingly
8. Close the pattern manager and select the pattern that you just created, from the "Operating Rule" drop down in the pump properties. At this point, the pump properties should look similar to this:
9. The model is now correctly set up and you can compute the transient simulation (Analysis > Compute).Note: If your pump is a variable speed pump ("Is variable speed pump?" = "True"), then you may encounter a notification stating that the rotational speed must be greater than zero. If you encounter this problem, you will need to re-run the initial conditions with the VSP turned on, note the computed relative speed factor, enter this as the "Relative Speed Factor (Initial)" and set the initial status back to "Off". Then, set "Is variable speed pump?" to "False", re-compute initial conditions, and then compute the transient simulation.
Note: If your pump is a variable speed pump ("Is variable speed pump?" = "True"), then you may encounter a notification stating that the rotational speed must be greater than zero. If you encounter this problem, you will need to re-run the initial conditions with the VSP turned on, note the computed relative speed factor, enter this as the "Relative Speed Factor (Initial)" and set the initial status back to "Off". Then, set "Is variable speed pump?" to "False", re-compute initial conditions, and then compute the transient simulation.
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, go to the Time History for the pipe end adjacent to the pump:
You can also view a profile through the pump to some downstream point to see how the system reacts to the pump starting up.
You can also select a transient profile and click the Animate button at the top of the profile. This will show you the impact of the pump starting up along the profile over time. This can be especially useful in cases where a transient event may occur because of the pump starting up. In doing this, you can see that the maximum pressure occurs at around 12 seconds. As the animation moves forward in time, it settles on a lower value.
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 the pump properties. For example, entering a value of 10 would mean that extended data will be reported every 10 time steps. To view this, open the Transient Results Viewer and go to the Extended Node Data tab. Certain element types have available extended data, including pumps. Select the pump and the available attribute (in this case Speed), and you can see how the pump speed will change with time.
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 and downstream head:
To model a pump start up followed by a shut down, follow the same steps above, but configure your transient operating rule to drop the multiplier back down to zero at the time when the pump shuts down (after some delay). Details on the steps for this can be found here:
Modeling a pump startup and shutdown transient event in the same simulation
In some cases, once the pumps turn on in the transient simulation, they may not settle exactly on the nominal head/flow that you saw when you ran a steady state with the pumps on. This can be due to many reasons. Here are some of the more common reasons:
If all else fails, you could consider starting your transient simulation with the pumps on, then use the variable speed transient pump type to have them turn off then turn back on again. You can use the "Report History After" transient calculation option to have the transient reports begin after the pumps have settled in their off position.
This may be due to the "Time (For Valve to Operate)". See this link for details:
Operating Rule not being followed after computing pump shutdown or start up
If the pumps is initially off, but turning on when you compute the initial conditions calculations, you likely have a control applied to the pump. By default, the initial conditions calculation will take into account simple controls in the Controls manager. To help with this, you can set the Steady State/EPS calculation option setting "Use simple controls during steady state run?" to False. The controls will then be ignored if you are computing a steady state run.
It typically takes some time for the pump to overcome the discharge hydraulic grade before is can pass flow. See this link for details:
Flow from pump is delayed after pump startup
If the pump appears to start up too quickly and the initial positive pressure spike appears to be higher than expected, it could be related to the Control Variable and the Operating Rule. See this link for details:
Pump Startup occurs too quickly / initial upsurge too severe
If there are high points in the system, the initial conditions may not reflect the true system conditions with the pump off. See this link for details:
"Initial pressure less than vapor pressure. At the pipe end(s), the elevation(s) or head(s) are incorrect"
The below model is an example of a pump startup in HAMMER. Note:
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Modeling a pump shut down event in Bentley HAMMER
Modeling a Pump start-up transient event in Bentley HAMMER V8 XM