Differential Pressure

[mech711]

I have a hot water system and pumps with VFD control. At the end of one of the loops I have a bypass with an automatic control valve and a pressure differential sensor before the bypass between the supply and return. both supply and return piping is 1-1/2" copper> How do I calculate the DP setpoint for that control valve to maintain 25 GPM. 25 GPM is the minimum flow through the boiler.

 

[pmover]

well, if flow is the primary process variable, then measuring flow instead of pressure is the best solution. otherwise, contact the control valve mfg and have them provide you the DP associated with 25 gpm. the mfg will likely need fluid properties and inlet pressures as well.

good luck!
-pmover

 

[dcasto]

because you have a VFD on the pump, the pressure going into the control valve will not be constant and therefore you cannot just use the valve curve. HOWEVER, the boiler manufacturer can tell you the differential pressure across the heater at 25 gpm. Have the control valve open to maintain at least that pressure drop across the heater.

 

[mech711]

As varous control valves on radiators and heating coils open and close on the loop to maintain space temps in various space the vfd will increase the speed of the pumps. How do you control the speed of the pumps? by the differential pressure setpoint right? how do I come up with that setpoint?

 

[BigInch]

If your object is to control flow to 25 gpm, you really should be controlling the speed of the VFD pump arrangement by a flow signal and a flow set point.

If your object is to control differential pressure, use the diff pressure signal.

If your object is to control temperature, use a temperature signal.

Controlling one variable via another variables's deviations require that some algorithm be implemented that can correlate the deviation of the signal variable to that of the controlled variable. It will probably require precise modelling in one form or another, or experimentation and tuning the response on the system after it is constructed.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[HEC]

mech711

If I read your post correctly and fill in the gaps, you have a hot water distribution system supplying into a header that in turn supplies to various users, heating coils, tank jackets, etc. These would all be connected to a return header that is connected to the boiler inlet.
The supply header pressure would be used to control the supply pump speeds, this is fairly standard. In the case where all consumers are off a by pass is needed. As consumers close off and the demand flow falls below the minimum boiler flow the bypass valve should open. The solution here is to control the bypass valve off header flow measurement.


Mark Hutton

 

[Ashereng]

mech711,

I am for simplicity. If you need 25 gpm, then I would put a flow measurement element in the line, and control the VFD to ensure there is 25 gpm.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[quark]

As far as hot water generators are concerned, I would not go for two birds in one shot.

VSD control by DP will ensure optimum power consumption and is used for chilled water system (primary secondary) when 2way control valves are used. My experience is with setting a DP of 0.5 bar (as a thumb rule) across the headers. Though, manufacturers don't put it on paper, they agree that safe minimum flow of pump reduces at lower speeds. So, I have no problems so far.

There are couple of options in your case.

1. Go for primary secondary kind of system with a decoupler. The primary loop maintains constant volume flowrate and secondary variable.

2. Go for DP control with an override using flow sensor, if the minimum flowrate condition is not met.

3. Select the bypass valve in such a way that it will allow 25gpm at the minimum possible DP of your system.

My preferrable choice would be option 2.

 

[bucs2047]

Not sure of the control system, but you maybe able to do the following:

1) Make a min opening for the c/v with regards to mfg min flow requirements
2) If you can obtain a set of Kv values for the valve and base the DP off this - would require some flow testing/commissioning, rent a UT flow meter for back-up info.

What type of DP sensor do you currently have installed - orifice or venturi, cause you could base a mass flow rate of this.