Using Modulating PRV, PSV or FCV during a transient simulation

Applies To 
Product(s): HAMMER
Version(s): CONNECT Edition, V8i
Area:  Modeling
Original Author: Scott Kampa, Bentley Technical Support Group


This TechNote will explain how to model dynamic valves (PRV, PSV and FCV) that modulate (throttle to maintain a target) during a transient simulation

discuss the use of the Modulating Pressure Reducing Valve (PRV) available starting with HAMMER V8i SELECTseries 4.


Modulating PRVs are available for a transient simulation in HAMMER V8i SELECTseries 4 (08.11.04.XX) and greater.

Modulating PSVs and FCVs are available as of HAMMER CONNECT Edition Update 3 (build

Control valves, such as pressure reducing valves (PRVs), will throttle (open or close) to control pressure or flow in the system. For example, PRVs adjust valve opening position to meet a target outlet pressure. This happens automatically in a steady state or EPS simulation. In HAMMER V8i SELECTseries 3 and earlier, PRVs always maintained a constant valve position throughout a transient analysis, based on the position in the initial conditions. In many cases this assumption is sufficient, in part because most transient events happen so quickly that these types of valves cannot react quickly, or do not contribute significantly to the transient results that are typically of interest (maximum and minimum HGL). Also, with the very small time scale used in a transient simulation (typically on the order of hundredths of a second), the rate at which such a valve can react is a factor and would need to be known to be able to simulate the dynamic modulation.

However, there are instances where modulation of the valve position may need to be accounted for. There are also cases where valve modulation itself can contribute to transient events.

Starting with HAMMER V8i SELECTseries 4, a new PRV property field was included which enables HAMMER to adjust the valve opening dynamically during a transient analysis. This TechNote will cover the steps used in using the new modulating PRV feature.

Note: While it is possible that other types of control valves can also modulate during a transient simulation, the modulating behavior is currently only included for PRVs in HAMMER V8i SELECTseries 4, 5 and 6, and in the CONNECT Edition.

Note: Although the "Modulate Valve During Transient?" option may display for FCVs and PSVs in versions through, the feature currently only works for PRVs. FCV and PSV modulation is planned to be available in late 2020. In the meantime, you can use the workaround mentioned below under "Manual Modulation" and here.

Using Modulating PRVs in HAMMER

Starting with HAMMER V8i SELECTseries 4, the option "Modulate Valve during Transient?" is available in the PRV properties. If this is set to True, two new properties become active:  “Opening Rate Coefficient” and “Closure Rate Coefficient.” The units for these properties are percent change of the opening per second per foot of HGL difference between the control valve setting and the calculated pressure at the previous time step (“%/sec/ft” or “%/sec/m”, for example).

The values used for the opening and closure rate coefficients are highly valve-specific. The closing and opening rates may be different for any given valve. Values will tend to be lower for larger valves, and could be much higher for direct acting valves than they would be for pilot controlled valves. The values should be calibrated based on the use of high speed pressure loggers in the field. If field data or manufacturer documentation is not available, a reasonable initial estimate for the rate coefficient would be on the order of 0.1.

When “Modulate Valve during Transient” is set to True, the PRV will start to open when it senses the downstream pressure dropping below the initial pressure setting. It will start to close when it senses the downstream pressure is greater than the initial setting. These changes in pressure can happen as a result of any transient event in the system, such as a pump shutting down or turning on, a valve closing, etc. The rate at which the PRV opens or closes is dictated by the opening rate coefficient and closure rate coefficient.

Illustration of Results for Different Modeling Cases

As an example of how Modulating PRV results can be different for different modeling cases, consider the simple model layout below, which you can download from the link at the bottom of this article:

This example illustrates a valve closure on the downstream side of a PRV. The upstream reservoir is at an elevation of 200 ft, the downstream demands at 100 ft and the PRV pressure setting is 170 ft. In the initial conditions, it is partially closed to meet the setpoint pressure. The traditional, default behavior for the PRV is to not modulate during the transient event ( “Modulate Valve during Transient” set to False). In other words, it will maintain the opening position from the initial conditions, which correlates to a specific discharge coefficient.

The screenshots below show the minimum and maximum hydraulic grade along the profile and the trend graph of flow and hydraulic grade on the downstream side of the PRV, as a result of the downstream valve closing in 10 seconds. Several different variations are compared.

In the profile, the black line is the initial hydraulic grade line and the orange line is the hydraulic grade line at the end of the simulation (the final steady state).

No Modulation

In this case, the "Modulate Valve during transient" option is set to False.

Note how after the valve closes, the PRV's downstream HGL increases (to about 195 ft) due to the change in downstream flow. In the profile's orange line (final steady state HGL), you can see that the reduction in flow caused a reduction in headloss through the PRV since it stayed in a fixed position, yielding a downstream HGL higher than the PRV's hydraulic grade setting. Normally a PRV would react to the change in hydraulics and start to close to maintain the 170 ft. setpoint. Since the modulation option was not enabled, this does not happen.

Modulating - Coefficient of 0.1

In this case, the "Modulate Valve during transient" option is set to True and both the open and closure rate coefficients are set to 0.1 %/sec/ft

Note how after the valve closes, the HGL eventually settles back down to the setpoint of 170 ft, due to the modulation of the PRV. In profile you can see how the final steady state HGL (orange line) on the downstream side of the PRV is matching the initial HGL (black line) which is the setpoint of 170 ft. With the rate coefficient of 0.1, the modulation does not appear to cause any transient effects, either. (look at the max HGL red line in profile above the PRV)

Modulating - Coefficient of 0.5

In this case, the "Modulate Valve during transient" option is set to True and both the open and closure rate coefficients are set to a higher value: 0.5 %/sec/ft

As with the previous case with the modulating option enabled, the final steady state HGL (orange line on profile) settles down to the setpoint pressure of 170 ft due to the modulation. However in this case, the modulation may have occurred too quickly, resulting in a significant change in momentum and subsequent surge. This can be seen in the red max HGL line in the profile, above the PRV. The upstream side of the PRV experiences an "upsurge", and the downstream side experiences a "downsurge" (see drop in HGL in the graph).

The Opening Rate Coefficient and Closure Rate Coefficient are not the only variables that can impact the results. The reaction of the Modulating PRV also depends on the minor loss or discharge coefficient entered for the PRV, as well as the valve type. For instance, by changing the Valve Type from “Globe” to “Butterfly,” the model results will be different for the same rate coefficient value, since different valve types will see a difference in flow based on the opening of the valve. You can see the calculated relative closure in the PRV properties in the initial conditions.


The best way to view results of a modulating PRV is to graph the Hydraulic grade or pressure at the pipe endpoint downstream of the PRV, in the Time History tab of the Transient Results Viewer. For example if the pipe downstream of the PRV is P-4, then you would graph "P-4:PRV".

To view the percent closure for the valve over time, currently you can find this in a special output file stored in the Windows temporary folder. The default location is: C:\Users\<Username>\AppData\Local\Temp\Bentley\HAMMER\PRVCLOSURE.TXT

You can also try typing the following into the address bar of File Explorer: %temp%\Bentley\HAMMER\PRVCLOSURE.TXT

Tips When Using Modulating PRVs 

  • If the opening or closing rates are set too high, it is possible to see some numerical instability in HAMMER. This will most often be shown in the profile animation or the time history graph, both of which are found in the Transient Results Viewer. If you see indications of instability, you should try a smaller value for the Opening Rate Coefficient and Closing Rate Coefficient, unless you feel that the instability might be "real" and caused by a valve that modulates too quickly.

  • When using modulating control valves, it is necessary to specify either a non-zero fully open minor loss coefficient or discharge coefficient. This value is set in the property “Valve coefficient type.”

  • Inaccurate results may occur if the valve becomes fully open or fully closed during a run, or the pressure drops below vapor pressure at the valve. Per the Reporting section above, check the "PRVCLOSURE.TXT" file to confirm.

  • For information on the formula used to calculate the valve position in HAMMER, see the HAMMER Help topic “Modulating Control Valve.”

Manual Modulation

If you set "Modulate Valve during Transient" of False, or if you're using an older version of HAMMER that does not have this feature, it is still possible to adjust valve opening during a transient run by changing the default value for "Operating Rule" from Fixed to an Operational (Transient Valve) pattern created in the Patterns dialog. In these patterns, the relative closure is a function of time (See HAMMER Help topic “Pattern Manager” for more information). Note that a PRV with an operating rule will not dynamically throttle. In other words, if the transient conditions causes a change in pressure downstream of the PRV, it will not automatically throttle (i.e., change its headloss) accordingly. See the "background" above for more information on this.

If you use an Operating Rule to change the valve's position over time, first note the initial calculated relative closure in the PRV properties and use that for the start of your operating rule, so as not to cause an initial surge. If you have difficulties with an operating rule on the PRV itself, consider using a throttle control valve (TCV). For more information on this, please see the articles in the "See Also" section below, "Modeling Existing valves as throttle control valves in HAMMER".

Example Model

The below model file corresponds with the example given above, comparing the results between not using the modulation option, and several different variations of using the modulation option.

Click to Download

NOTE: you must be logged in first to download this file. It is saved in V8i SELECTseries 6 format and cannot be opened in earlier versions of HAMMER.


See Also

Protective Equipment FAQ

General HAMMER V8i FAQ

Modeling existing valves as Throttle Control Valves in HAMMER

Modeling Reference - Valves

For additional information on modulating valves in general, please see the following paper:

"Dynamic Modeling of Pressure Reducing Valves" by Simon L. Prescott and Bogumil Ulanicki (Journal of Hydraulic Engineering, October 2003)

  • Created by Bentley Colleague
  • When: Tue, May 7 2013 2:55 PM
  • Last revision by Bentley Colleague
  • When: Fri, Nov 13 2020 1:30 PM
  • Revisions: 18
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