Pump VSD during transient

Question

Hi, 
 
 I am modelling a pumping system, with one pump on a &lsquo;fixed head&rsquo; VSD controlling the pressure of a header pipe which distributes to six drains via orifice plates. I am trying to model a transient case where one of six isolation valves closes rapidly (using a TCV element). However during a transient run, when the valve closes, the pressure in the header rises, but the VSD does not respond to slow down the pump and maintain the target pressure in the header pipe. 
 
 Does the Hammer variable speed drive function during a transient? Do you have any suggestions as to how I could model this case (e.g. do I need to use a variable speed pump battery element, or do I need to manually enter a pump transient operation rule to match the closure of the valve)? 
 
 Also is there a way of setting a delay or deadband on the vsd during a transient? 
 
 Thank you

Sushma Choure · Accepted Answer

Hello Luke, 
 Hammer assumes that the pumps are controlled by an operating rule or, in the case of the "Shut down after time delay" transient pump type, a time value at which the pumps will stop operating. 
 You will need to setup the operating rule to model this transient event of valve closure. 
 The below article contains information about variable speed of pump in Hammer, which you may find useful. 
 http://communities.bentley.com/products/hydraulics___hydrology/w/hydraulics_and_hydrology__wiki/25660.modeling-a-pump-shut-down-transient-event

Jesse Dringoli · Answer

Just to add to Sushma's answer - typically a transient event happens so quickly that an action such as a variable speed pump speeding up or down does not happen quickly enough, or generally does not impact the transient results that an engineer would normally be interested in. It would also be difficult to determine how fast the VSP and related components would react to the change in pressure, which can be a factor when looking at a transient simulation where the timestep is on the order of a hundredth of a second. (with a steady state or EPS, this lag is not a factor) 
 
So, although you could use the variable speed option that Sushma mentioned (whereby the speed is controlled manually with a pattern), you may want to consider if this is necessary. The minimum and maximum HGL (transient envelope) may be influenced only by the transient/surge that occurs when your valve first closes. A change in speed in the VSP to maintain the target after this event 
 
If in doubt, you could check the sensitivity; run the model with the transient pump type as Constant Speed, then run another scenario with the pump set to variable speed (with at least an estimated pattern configured) and compare the results.