View Full Version : Location of differential pressure sensor
04-28-2011, 05:31 PM
Have a Kele ,differential pressure transmitter,two acurate gauges are right next to the sensor, outputing an accurate signal
to three Condenser water pump V.F.D. 's
The sensor and pumps are located on the sixth floor,one floor below the rooftop Cooling towers.
The tenant's P.E. has informed us that the sensor should be re-located .
I would say the the taps,located on the supply and return risers, are about 11 feet from the condenser water pumps.As far as I can see everything has been working good for three cooling seasons, including the A.C. units in the basement.The P.E was not two specific about why.
What are the general rules of thumb for locating a differential pressure sensor?
04-28-2011, 06:00 PM
Depending on the piping layout with automatic valves and other pumps. I normally install the hi press. tap at the beginning of the supply line and the low press. tap at the end of the return line.
04-28-2011, 06:40 PM
I take that to mean the supply line starts at the pumps and the return line ends at the pumps.
The P.E. is filling me in they are doing this for energy conservation.
it looks as if they want to run the pumps with about a ten pound differential
as opposed to the current 25 P.S.I.
It looks like I will be reinstalling it closer to the basement, end of the run,that is where the DElta P is always about 8 P.S.I.
Will be running extra wire from the existing penthouse location down to the Basement or first floor.the condenser water demand does always changing rapidly as units go on/off line,or condenser water reg. valves are always opening closing.I guess their logic is that if the Delta P is sufficient towards the end of the run, then it will be sufficient closer to the top of the run where the pumps are.
04-28-2011, 07:34 PM
04-28-2011, 07:41 PM
At these pressures, the water is incompressible and their is no significant lag in sensing differential pressure changes.
It does not matter where the sensors are located as long as the setpoint gets the water through the piping, chiller bundles, and tower nozzles.
I think the chillers want constant flow, they most likely have an internal evaporator pressure regulating valve for head pressure control. You do want to make sure that your system is not reacting to this control.
Move the sensor it he wants, but the real trick is establishing the setpoint and speed/reaction of the control loop.
04-28-2011, 07:43 PM
On a open or closed loop system the sensor is usual located 2/3 way out on the system piping to get a actual system Delta P. Here it sound like they actual want to control to Pump Delta P. I wonder if they realize it is just a number and that the number can be adjusted?
04-28-2011, 09:35 PM
Bill and retube are right. Move it if you have to, but you could just change the setpoint until you get what you need where you need it. If it is that critical, them someone ought to be putting some balancing equipment on it where it is critical. Other that that you are just satisfying a whim.
04-28-2011, 10:29 PM
Doesn't matter, system pressure is system pressure. The closer you are to the pumps, the higher the differential pressure is. Balancing 101 if this is a closed loop system. The only thing that changes is the setpoint. 3' from the pump high.... 2/3rd's down the system low.
This is a variable flow application.
The PE is instructing you to move the differential transducer location to take advantage of the systems lower resistance to flow at reduced load conditions. In otherwords, as the water flow lowers, the resistance of the pipe lowers. This affects required hp to move the water.
If the transducer is located directly in the pump discharge and return, the differential has to be set to provide proper waterflow at full water flow. Basically, the vfd can not efficiently take advantage of the reduced friction of the pipe caused by reduced flow, because it is set to maintain system design perameters, in your case 25 psi diff or somewhere around 58 hdft. This is the system resistance at full water flow.
If it is located at the furthest load, the differential required is constant and only enough differential has to be supplied to satisfy that unit. This allows the vfd to take more efficient advantage of the reduced required hp of the lower friction of the pipe.
Relocate it where he said, and set it to 8 or 10 psi diff. I thing you will find proper water flow and reduced hp consumption.
I like to install taps directly in the risor near the furthest load instead of installing "T"'s in the piping to the unit. This is especially true if the water piping to the unit is marginally sized.
I used to not believe that placement did not matter until I saw the results with my own eyes. more constant water flow to all units will occur and the pumps do not hunt as much either.
Yaskawa has a 4 page easy read on this on their web site.
On a water booster (pressure only)application such as a potable booster pump, location does not matter. Adjustment to the setpoint will compensate for location.
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