Diferential pressure to low

[fwildt]

All the building where I work are on a central chilled water loop. At the building in question the pressure controler is set to maintain a differential pressure of 8 psi. This is not happening. My Varable speed drive for the building is running at 100% but my pressure fluxuates between 1 and 4 psi depending on the degree of openness of the actuator valve controling flow through the chilled water coils. I installed a guage first on the supply and then on the return chilled water piping where they enter and leave the building to insure the differential pressure control is working properly and it is, my reading match the controlers. I did an over ride on all air handler chilled water valves at the same time stoping flow. I should mention all the valves are 2 way, not 3 way ( no bypass ). When I stoped flow my D.P. went down to about 1 psi, when I comand all the valves open 100% my pressure goes up to about 4 psi. I don't understand, I thought less flow would mean more pressure?! If I open drain piping between the supply and return creating a bypass my pressure goes whay up and my motor drive backs off then I can maintain the chosen differential pressure. However now I am taking flow away from my chilled water coils creating temperature problems. What should I do? Thank's Fred

P.S. Hope my spelling isn't to bad.

[mechdorn]

No 3-way valve, Diff_pressure is the wrong control scenario. The pump's drive should be using the supply pressure for setpoint. Override all valves open, confirm total GPM at the main circuit setter, adjust the rpm to proper system GPM, then record the supply pressure. The pressure recording is the new setpoint.

[chiller rob]

fred sounds like you have been caught outside of your pump curve... is there one airhandler that you could cheat open a little...

I am guessing that with all the system valves at 100% the pump will only supply a 4 delta... pump will pull fla...

can you afford to increase the chilled water temp a little to increase the valve postions?

good luck

[flyrfan]

All the building where I work are on a central chilled water loop. At the building in question the pressure controler is set to maintain a differential pressure of 8 psi. This is not happening. My Varable speed drive for the building is running at 100% but my pressure fluxuates between 1 and 4 psi depending on the degree of openness of the actuator valve controling flow through the chilled water coils. I installed a guage first on the supply and then on the return chilled water piping where they enter and leave the building to insure the differential pressure control is working properly and it is, my reading match the controlers. I did an over ride on all air handler chilled water valves at the same time stoping flow. I should mention all the valves are 2 way, not 3 way ( no bypass ). When I stoped flow my D.P. went down to about 1 psi, when I comand all the valves open 100% my pressure goes up to about 4 psi. I don't understand, I thought less flow would mean more pressure?! If I open drain piping between the supply and return creating a bypass my pressure goes whay up and my motor drive backs off then I can maintain the chosen differential pressure. However now I am taking flow away from my chilled water coils creating temperature problems. What should I do?

It sounds like you have a variable flow system that uses a remote DP transmitter to control flow...2-way valves at the coils are the correct!

Is this a campus CHW system serving multiple buildings with building booster pumps?

For me to help you, you need to describe the locations where the DP readings are...i.e. across the pump, at the coils, where?

Where is the DP transmitter/controller located (see above)?

BTW, when you close the valves, you're dead heading the pump and you should see the full pump head....see if you have a curve for that pump and look at the head at 0 GPM.

[bunnyruff57]

where is this building in relation to the location of the master s.p. for the main chilled water loop pumps?your building can only supply what the main loop allows

[fwildt]

I work at the Ringling estate in Sarasota Florida. There are 6 buildings on the chilled water loop. The chiller plant has of 2, 650 ton trane sentrifical chillers, 2 250hp.varable speed drive pumps circulate the water throught the loop, they maintain a constent 18 dp. All of the buildings have v.s.d. booster pumps, in three of the building I don't use the building pumps the system pumps do the job well enough that I don't need them. The building with the problem is second closest to the chiller plant. It is not starving for water. The Differential pressure transducer that controls (ha) the building is located at the supply and return piping where it enters the building ( all the buildings have the control about the same location. This time of year my demand for chilled water is small so the buildings using booster pumps the pumps are running at about 25% of there capacity. The problem building has a psi of 55 entering the building and the pump brings it up to 85 at 100%. the more I restrict water flow the lower my DP is with everything closed I see a -4 to -5 DP if I open everything I get a slight positive DP +2 or 3.

[mechdorn]

Let's look at this again, (I should mention all the valves are 2-ways, not 3-ways, no bypass. When I stopped flow my D.P. went down to about 1 PSI), the system is not exposed to the pump. Why would there be any pressure drop? (When I command all the valves open 100% my (DP?) pressure goes up to about 4 PSI). The system is now exposed to the pump; the pump now has something to work against, creating a pressure drop. (I don't understand, I thought less flow would mean more pressure)?! If you want more pressure with your garden hose, do you open or close the valve? A decrease in system exposure, valves closed, would create a higher supply pressure on a constant flow system; unfortunately differential pump pressure is the measurement, not pump supply pressure. Currently, as the process variable (Diff_pressure) decreases (valves are closing) the controller responds with a increase to pump speed, reverse acting, it's backwards. Measuring pump supply pressure with a reverse acting PID (valves open, pressure reduces, pump increases speed). output is the way to go.

[Tech Rob]

Sounds like you need to get Bentzel back out there :p

[mechdorn]

Who's Bentzel?

[Tech Rob]

Bentzel Mechanical did some work at Ca D'zan and installed a couple of CVH's at the Ringling Center for Cultural Arts, didn't they? Is that a different property altogether?

Look anything like this?
http://i93.photobucket.com/albums/l74/BirthOfDisease/rChill1.jpg

http://i93.photobucket.com/albums/l74/BirthOfDisease/rChill2.jpg

[flyrfan]

IThe Differential pressure transducer that controls (ha) the building is located at the supply and return piping where it enters the building ( all the buildings have the control about the same location.

So the high side of Booster Pump's DP trans is located on the suction side of the pump? If so...that's not right! The booster pump's DP variable should be measured closed to the hydrauically farthest coil....not at the inlet to the building. The plants pumps should measure at the building.


The problem building has a psi of 55 entering the building and the pump brings it up to 85 at 100%.

You're loosing me here....is the CHW supply pressure 55 psig or the supply/return differential is 55 psid?

Same for the 85 PSI...supply pressure or psid?



the more I restrict water flow the lower my DP is with everything closed I see a -4 to -5 DP if I open everything I get a slight positive DP +2 or 3.

what DP are you talking about?

[fwildt]

I moved the high side of DP transducer from suction side of boster pump to discharge side of pump and guess what, the drive started controling my DP! I will have to move the control close to the furthest coil as suggested to get it to work at its best potential. I want to thank all you guys for you input, I didn't do to good expressing the problem but you managed to solve the problem anyway! Well done and thanks Fred