Heating Hot Water Differential Pressure Bypass Valve

[BronYrAur]

I have a heating hot water system with 2,500 GPM centrifugal pumps at a 90' head design condition. They are constant volume pumps serving a variable volume load of air handler coils. All coils have 2-way valves. At one time, a pneumatically controlled bypass valve was at the end of the line, but it has been removed.

I have been asked to install a new bypass valve at the end of the line to prevent dead-heading of the pumps. According to the manufacturer, i will need to size the valve for the pump minimum flow of 725 GPM. The customer wants a "self-controlled" valve with no pneumatics or electrical controls.

I am not sure what differential pressure i will normally see at the end of the line. Maximum would of course be dictated by the dead-head condition of the pump. Also, pump curve is relatively flat as it backed down toward the minimum flow (red line on the curve. see attached). Might be difficult to control.

Any thoughts or recommendations on how to handle this?

 

[Orsiz]

Can you provide a PFD/P&ID of the system?

You can use a self-acting backpressure regulator at the end of the line which will prevent the pumps from dead heading. The valve can be sized for the full pump flow.

 

[EdStainless]

You need to know the conditions when the least flow possible is going through a single coil.
You might want to returning some flow then, after all you want the pump running as near BEP as possible.
So when the smallest furthest coil is the only one with demand what is the pressure and flow?
That is likely when you need the valve to start opening.
Backpressure sensing is how they do this.


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[georgeverghese]

Agreed, flat profile curve may pose some difficulty in sensing 112ft diff head. See if you can find a self op dPCV with a narrow operating spring range for this service. Else, given the constraints imposed, it may be necessary to operate this dPCV at some where around 1750gpm, at a power draw penalty of approx 10hp.
If the plant owner is keen on avoiding this incremental power draw, another option would be a min flow FIC - FCV if current constraints are waived. You could have this control loop as an all electric loop if instrument air requirement is impractical (ie electric operated FCV). A simpler alternate to a min FIC loop would be a dPIC-dPCV.