What if the time delay until shutdown is zero?

Question

Hello all, 
 In the transient analysis of the pumps, when we model the pump as a pump which shut down after a time delay, if we make the time delay until shut down to zero (default value), what does it means...? Is it not maintaining a steady state condition before shut down? or the Bentley HAMMER does not account for such scenarios ? 
 If the Bentley HAMMER accounts for such scenarios, pump shut down at time zero should be the most critical scenario... isn't it..? 
 Highly appreciate your response in this regard. 
 Regards, 
 Nalaka Rajapakse

Scott Kampa · Accepted Answer

Hello Nalaka, 
 
If you have a time delay for a pump shut down set to zero seconds, that simply means that the pump will down in the very first time step. There is nothing wrong with this setup. I usually recommend a time delay of something a little more, like 5 seconds, simply because it makes viewing the results a little easier. For instance, if you were to look at a time history graph of the flow and pressure on the downstream side of the pump, the graph would include a little bit of the initial conditions before the pump shuts down. It's a bit better of a visual reference for the results. However, there is nothing wrong with setting a time delay of zero seconds, and the results would not be unstable as a result of it. 
 
The following link has more information on modeling a pump shut down event: communities.bentley.com/.../25660.modeling-a-pump-shut-down-transient-event . 
 
You can find some additional tips for using HAMMER at the following link: communities.bentley.com/.../general-hammer-v8i-faq 
 
Regards, 
Scott

Jesse Dringoli · Answer

In other words, the "delay until shutdown" is the time from start when the applied electrical torque drops to zero. For example if you enter 5 seconds, it means that the pump will continue operating as it was in the initial conditions, until exactly 5 seconds into the simulation, at which point power is no longer applied to the pump and it starts to spin down based on its inertia and the hydraulic conditions. 
 
It's usually a good idea to include a few seconds of steady state before shutting down the pump, as a way to check to make sure that there are no "spurious surges". Meaning, if there are any problems with the model setup causing an unexpected change right at the beginning of the transient simulation, you'll be able to see it in those first few seconds, when you know essentially nothing should be happening.