Pressure Control vs. Flow Control in Service Water System 1

[CuriousElectron]

Greetings all,

I know there is a recent thread discussing a similar system, but I wanted to create new post to ask a couple fundamental questions, since my system is setup differently.
I've got a distribution system in a pumping plant that will utilize a PLC to control two pumps supplying water to a number of users. The pumps will draw water from a reservoir(river) and supply the distribution ring.
Should the controls system be based on the characteristics of the system(load profile)? The way I see it, if there is a constant demand for water, it would make sense to maintain flow in order to level out the pressure in the system and avoid pressure drop, say when a valve is opened. If the demand is low and has a discrete (step function) characteristic thoughout the day, then perhaps it would be better to maintain pressure in the piping and not care about flow? This would minimize run time of the VFD as well, it there is no demand for 6 hours, the VFD would pressurize the system piping and shut off; check valve would prevent water from flowing back into the pit.
Currently there is no discharge back into the reservoir, but we may add a valve to "dump" excess flow essentially back into the reservoir if we elect to maintain a minimum flow in the system. To me, it would be wasteful to maintain a minimum flow for low demand system.
My background is not Mechanical, but am curious if I'm on the right path approaching the design criteria.

Thanks all.

 

[itsmoked]

There are lots of variables as you probably realize but it would help if you provide more info.

Is this a people town or some industrial complex?
If it's a town what kind of population?
What's the maximum elevation that needs to be serviced and is the high elevation a couple of users or lots.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[thebard3]

Based on the limited description you provided, if using a VFD, I would probably prefer to keep a single pump running, at low speed, during times of minimum usage to maintain system pressure. This should minimize pressure drops when water is eventually called for somewhere. However, as you noted, during extended times of NO usage, it would be necessary to return a portion of water to the source as the pump will eventually overheat if dead-headed. Also, it's always a good idea to include a lead/lag circuit to alternate which pump is run first and which one will start if the demand exceeds the capacity of the lead pump.

Brad Waybright

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

 

[djs]

If this is a potable water system, then pressure is THE priority. If the min flow is below the min capacity of the smallest pump, then perhaps a hydro-pneumatic tank is called for.