DPTransmitter

[RRedmond]

In a typical primary/secondary chilled water system, where is the best place to locate the differential pressure transmitter that controls the secondary pump VFD?
I believe it is 3/4 of the way down the longest piping run. Is that true?

 

[MechEngNCPE]

How about at the most hydraulically demanding coil.

 

[DRWeig]

MechEngNCPE got it. Got to make sure that the worst-case coil has enough pressure to do its job.

Only real pitfall is if the worst-case valve actuator dies with the valve in a non-closed position. You might, in that case, be over-pumping until it's found and fixed -- but there are ways to monitor that (direct position feedback is a reliable one).

Good on ya,

Goober Dave

 

[KiwiMace]

Assuming the system is a balanced direct return, I would locate the DP sensor on the supply and return headers in the plant room.

If you locate your DP transmitter at the terminal end AHU, and only AHU-1 is operating? It has a balancing valve set to require say 12psi over the branch, and you are trying to control for 6psi? choke.

The problem with the DP in the plantroom location is that even during even AHU turndown, the higher DP setpoint almost prohibits effective wind down in the VFD due to the higher pressure drop over the closing two way valves.

Sooo... This is where you use a BMS to implement a valve position reset to adjust the setpoint. This would work on any DP location, but it is cheaper to install and easier to maintain in the plantroom.
Seeing as you need position feedback on the valves to implement this, you also may as well add valve diagnostics like DRWeig suggested and find stuck valves, hunting valves and all those other maintenance goodies.

 

[RRedmond]

Everyone, thank you for responding, however, it does not sound like we have concensus of opinions. Anyone else care to respond, please do so.

 

[Xera]

Typically, I have always seen them at the end of the run. If you have the correct pressure there then all should be good.

 

[ChasBean1]

My input: in the main piping, toward the end of the run, in a place that will not be readily isolated.