Pump time lag.

[Indy]

We are planning on installing a pump skid that had a pressure transmitter on the outlet for control. The outlet pipe will be chsrdged to a set point then the pump will stop and the system will remain charged to this pressure. When a valve is opened further downstream the pressure will drop and be sensed by the pressure transmitter which will start the vsd on the pump.

My question is how to work out the time lag between the downstream valve being opened and the vsd kicking in?

Thanks

 

[itsmoked]

Fast. The speed of sound in the media is how long it will take the PT to notice something changing.

How long it takes to get down to your system's go-back-on point is driven more by flow and the on/off differential you've set.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[thebard3]

Is the valve you mention a proportional valve, or simply open/close? I have similar operations where we control a pressure level using pump speed, but this would be used on a continuous, but variable flow system. I am trying to determine if this is analogous to a pressure switch and motor starter, or something a little more sophisticated?

Brad Waybright

It's all okay as long as it's okay.

 

[Compositepro]

Systems that operate as described in the OP always have a pressure tank to prevent short-cycling.

 

[waross]

With a pressure tank the time depends on how much flow through the valve, the initial tank pressure, the cut-in setting of the pressure switch, and the volume of water the the tank can supply before the pressure drops to the cut-in pressure.
Without a pressure switch, it is fortunate that you have a VFD because a DOL motor will burn out in probably less than an hour due to short cycling.
By the way, I have never had a well functioning application where it was at all important how long it took for the pump to start as long as it started.
In a poorly designed or failing system, the frequency of starts is important. As the system is failing, often due to pressure tank water logging, the frequency of starts increases until the motor fails due to short cycling.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

use a PID loop controller with a "sleep" function for when the system is satisfied. Many of the better quality VFDs now come with that built in. A PID loop will make the VFD respond to the change AND the rate of change.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Indy]

Thanks for all the replies. The down stream valve is open/close only and the system will come with an accumulator tank. I also think it is odd we are being asked about the time lag.

 

[thebard3]

That sounds to me like someone had a bad experience, figured it might be an issue so they turn it over to you. Just sayin'.

Brad Waybright

It's all okay as long as it's okay.