Differential Pressure
Differential Pressure
(OP)
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.





RE: Differential Pressure
good luck!
-pmover
RE: Differential Pressure
RE: Differential Pressure
RE: Differential Pressure
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
RE: Differential Pressure
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
RE: Differential Pressure
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
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RE: Differential Pressure
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.
RE: Differential Pressure
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.