Need Advice to place Pressure Sensor

**azka** · 01-16-2012, 12:09 PM

Hi expert forumer,

I'm beginner in control automation. I'm need some advice from all of you regarding to this

1) which is best location to place water pressure sensor to control Variable Speed Drive Pump? - refer picture below

2) Which parameter (delta pressure or delta Temperature) to control bypass valve?

Please refer to this picture for more info.

Thank you

**xarralu** · 01-16-2012, 01:33 PM

I would put it almost to the furthest end of the piping run. Also, control the bypass to pressure not Delta T. If your running from a chiller, it will take care of the temperature for you.

**crab master** · 01-16-2012, 02:41 PM

This tends to be argued but I'd basically agree w/Xarralu. 2/3 or further from the pump so you get more of a system pressure and can sense the further zones better. Sensing your 'furthest' zone helps ensure you have proper pressure out to those zones. Granted one many times can tune due to poor locations, but it's better just to have it right from the start, especially since sometimes it takes a whole lot of time to tune...

**azka** · 01-18-2012, 03:46 AM

Originally Posted by xarralu

I would put it almost to the furthest end of the piping run. Also, control the bypass to pressure not Delta T. If your running from a chiller, it will take care of the temperature for you.

Thank you for suggestion, xarralu and crab master.
my mistake for Delta T.
So, Delta T for control bypass and Delta P for control Variable Speed Drive Pump. May I know why need to place the sensor furthest end ? Is it will effect the delta P ?

**crab master** · 01-18-2012, 12:44 PM

Originally Posted by azka

So, Delta T for control bypass and Delta P for control Variable Speed Drive Pump. May I know why need to place the sensor furthest end ? Is it will effect the delta P ?

Originally Posted by crab master

2/3 or further from the pump so you get more of a system pressure and can sense the further zones better. Sensing your 'furthest' zone helps ensure you have proper pressure out to those zones.

Proper pressure out to those far zones ensures your valves to have the proper pressure so they will have their rated flow.

**azka** · 01-18-2012, 07:51 PM

Originally Posted by crab master

Proper pressure out to those far zones ensures your valves to have the proper pressure so they will have their rated flow.

Thank you for clear explanation, I think that take a lot of time to tuning the PID for VSD and valve

**exwtk** · 01-21-2012, 11:59 PM

The differential pressure should be at the most remote AHU. Find out the minimum required Diff pressure for the coil of this AHU and that should be your setpoint. This setpoint should be maintained by modulating the pump speed. If you maintain this pressure then all AHU's will have sufficient water flow

The Bypass control is a different story. Your system consists in essence of a primary and a secondary loop but without the use of primary and secondary pumps. I am seeing this setup more and more and this will probably be the future for chilled water control.
The reason for the Bypass is to ensure minimum flow through the chillers. To make this work you will need differential pressure sensors for each chiller or flow meters for each chiller.

When the load in the building decreases, the AHU chilled water valves will close and pump speed will decrease. Until you get to the point that there is not enough water flow through the chiller. At this point you will start to modulate the bypass valve open. Because the bypass begins to open, the diff pressure at your most remote AHU will drop causing the pump speed to increase. And system will find a new balance point at which Pump Speed maintains the most remote AHU Diff Pressure at setpoint and the Bypass Valve is opened to the point that the chillers have there minimum required flow.

By the way in your picture, I believe your Chilled water labeling "To AHU" and "From AHU" and your 2 pressure sensors are reversed

**exwtk** · 01-22-2012, 12:24 AM

Actually ... your labeling might be correct but then the discharge and suction of the pump is backwards. Typically with centrifugal pump the water line in line with the pump shaft is the suction and the one coming out of the pump housing is the discharge.

**Greend88** · 01-22-2012, 09:14 PM

Most Sensors should be put 10 Pipe Diameters from the closest fitting, as a general rule of thumb too. Flow, Pressure... anything that can be affected by turbulence.

**azka** · 01-02-2013, 09:15 PM

Thank you Exwtk, I understand it the function to control it.

**mechdorn** · 01-02-2013, 10:33 PM

You need a flow meter. http://www.trane.com/commercial/libr...31_4/index.asp

**wolfdog** · 01-02-2013, 10:41 PM

.

**air1** · 01-02-2013, 11:23 PM

Originally Posted by exwtk

The differential pressure should be at the most remote AHU. Find out the minimum required Diff pressure for the coil of this AHU and that should be your setpoint. This setpoint should be maintained by modulating the pump speed. If you maintain this pressure then all AHU's will have sufficient water flow

The Bypass control is a different story. Your system consists in essence of a primary and a secondary loop but without the use of primary and secondary pumps. I am seeing this setup more and more and this will probably be the future for chilled water control.
The reason for the Bypass is to ensure minimum flow through the chillers. To make this work you will need differential pressure sensors for each chiller or flow meters for each chiller.

When the load in the building decreases, the AHU chilled water valves will close and pump speed will decrease. Until you get to the point that there is not enough water flow through the chiller. At this point you will start to modulate the bypass valve open. Because the bypass begins to open, the diff pressure at your most remote AHU will drop causing the pump speed to increase. And system will find a new balance point at which Pump Speed maintains the most remote AHU Diff Pressure at setpoint and the Bypass Valve is opened to the point that the chillers have there minimum required flow.

By the way in your picture, I believe your Chilled water labeling "To AHU" and "From AHU" and your 2 pressure sensors are reversed

Good answer.

**Questionz** · 01-02-2013, 11:43 PM

While I understand this concept, have been on a site where I have seen a pressure sensor installed at the ground floor, while the pumps are in the PH level (42 floors up). I couldn't get the differential pressure to vary between the supply and return. Both supply and return pressure sensors are at the Ground Floor, and the differential pressure is essentially 0 KPa. Could you clarify where the supply and return pressure sensors should be installed?

Assuming 5 PSI for each floor, that's 210 PSI head pressure to the ground floor. Both the Supply and Return read 210 PSI roughly, meaning 0 differential pressure, resulting in the VFD Pumps running at max speed.

**orion242** · 01-03-2013, 07:18 PM

>Both supply and return pressure sensors are at the Ground Floor, and the differential pressure is essentially 0 KPa. Could you clarify where the supply and return pressure sensors should be installed?

I would bet the static head pressure far exceeded your pressure sensor's max pressure rating and that's why it read 0. 210psi is a lot for a dP sensor to handle.