Insufficient CHW water flow in existing pipe

[MEP.Learner]

Hi Engineers,

I have a existing premise with Central Chilled water system, which is undergoing a major renovation / refurbishment work.
There is a portion where only existing CHW FCUs replacement work is carried out. Then here comes our problem, after the replacement work it seems that we can’t get the required CHW flowrate through this loop of CHW pipes, therefore affecting the cooling to this area.
This loop of CHW pipe is not the furtherest from Chilled water plant (in fact they are near). And other further area cooling is still working well.
So potential issues that might cause this:
-airlock in chilled water pipe
-faulty CHW valve that are stuck and not able to open fully.
-dirty Chilled water, debris in water due to the old building.
-other obstruction within the pipe.
However after many round of troubleshooting, flushing and purging of the water and air in the chilled water pipe, we still don’t manage the solve this issue.

Is there anyway to check the condition of the chilled water pipe or to pin point where is the exact location of blockage that restricted the chilled water flow?
Or please share your opinion on how to solve this issue.

Thanks.

 

[dbill74]

Did you replace the control valves?
Where / how are you testing flow thru the FCUs?

Many years ago when I worked for a T&B firm, we were testing flow through some coils. Spent most of the day getting unusual readings and couldn't get the coils balanced. Important note here is that the valves had an integral T&P port that we where using to attach our instruments. Problem turned out to be that the valves were installed backwards putting the T&P port on the wrong side of the valve to get accurate measurements.

To find an obstruction 3 ideas come to mind.
1) Using existing or intsall test ports and measure pressure drops on sections of pipe.
2) Scope the pipe. Remove a valve somewhere and run an endoscope down the pipe.
3) Sounds like you have an idea where there might be an obstruction, cut the pipe there and look inside.

 

[MintJulep]

Is the pressure v flow characteristic of the new coils the same as the old coils?

Have the cooling loads increased? (by design or accident)

Is the chilled water temperature correct?

Are pumps turning in the correct direction?

 

[Compositepro]

If the new piping allows greater flow, then the back pressure may not be adequate to get the desired flow through your old, existing loop.

 

[MEP.Learner]

Hi All,

Thanks and appreciate for all the replies.

@dbill74
yes the control valves are newly replaced.
There are two ports for differential pressure measurement which can then be derived to get CHW flowrate thru the control valve, which also gives the flow thru the FCU and cooling coil (as the control valve is on the return CHW loop, and the cooling coil is not stucked, or blocked)

1.
Install new test port is not an issue. But will this solve the problem?
What should be pressure drop across section of pipes, let's say section of 10m CHW pipe.
And i thought that since that is same loop of CHW pipe, the flowrate will be the same (which is too low in my case) and hence all the differential pressure will show the same reading. And we cant pin point the blockage. Am i wrong?


2.
is there any link of this endoscope for CHW pipe application? I will search further if there is any local supplier for this kind of system.
We only know the endoscope for sewer line due to the local authority requirement.

3.
unfortunately, we can't pin point where is the location.
and every cutting of of pipes and install with new joints and pipes involve $$


@MintJulep
Cooling load is remained as per existing capacity at this loop.

CHW temperature is fine as other area is working well.

CHW pump is at existing chiller pump and there is no work to it. And other area is still working fine. So we narrow down our issue to this loop of cHW pipes.

@Compositepro
Mind to elaborate more on this back pressure?

 

[moideen]

And other further area cooling is still working well:- have you replaced FCU in that area also? what is THE chilled water delta T for old fcu? have you sure the same design range is concerned for new FCU?

 

[MEP.Learner]

@moideen
partly replaced, partly still remained as existing. And all these other loops are working well.

Delta T in my case is 6 deg C.
This area with low CHW flow are replaced with the same FCUs as other area that CHW flow is just fine. (so the issue is not with the FCUs selection.)

The real issue is still insufficient CHW flow to this loop. But just can't figure out the way to pin point it and solve it.

 

[LittleInch]

Have you got a diagram of this system?

How many FCUs are on the circuit?

Can they be individually isolated?

Can you detach the inlet and outlet pipes and test this using a stand alone pump and tank?

Isolate all the FCUs
They just start at the nearest FCU/ branch in the loop and measure flow though each FCU in turn.
Slow but methodical.

Or just add as many PTs as you can to see where pressure drop is high where it shouldn't be.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SAK9]

I would try the following:

.Shut off and islolate all FCUs that are working fine
.The undersupplied FCUs are to be kept operational
.Operate the pump at maximum head
.See if the FCU s are receiving design flows.
.Install few pressure gauges along the pipe supplying these FCUs and measure pressure and check which leg is producing maximum pressure drop.
.Investigate pressure drop by ice plugging the section and cutting it open.
.If you do not observe any significant pressure drop you can at least eliminate possibility of a choke.

 

[MEP.Learner]

@LittleInch , @SAK9

Thanks for the input again.

Due to the confidentiality of the project, I might not be able to share the diagram.
But The CHW piping can be simplified / described as below:
There is 3 main CHW loop from the Chilled plant room.
Each of the main CHW loop furthest point is about 400+m (one way, not including return pipe length) from the Chilled plant.
My current problematic circuit, is only about 60m from the chiller plant, which is branching off one of the main CHW pipe loop.

For this problematic circuit, there is only 10nos. FCU
This premise has more than 400+ nos. of FCUs, with 30+ nos. AHUs.

Can they be individually isolated?
-Some of the FCUs isolation valve are not easily accessible and also some are operationally sensitive area. So may have some challenge.

Can you detach the inlet and outlet pipes and test this using a stand alone pump and tank?
-Great suggestion. There is isolation valve at the branch off CHWP from the main loop, and probably we can run a stand alone pump and verify the CHW flowrate or to determine if there is any blockage.

Or just add as many PTs as you can to see where pressure drop is high where it shouldn't be.
-We can do this. But just wondering theoretically, what would the value of delta P that is expected if the pipe is flowrate fine as per design or if there is blockage.

If you do not observe any significant pressure drop you can at least eliminate possibility of a choke.
-High delta P means blockage?

Other info:
The branch off pipe is 100mm dia., and we are expecting a max design flow of 18L/s, but we measure only 20% max from flowmeter at this branch pipe.
The delta P is also measured from each of the FCUs' balancing valve, that give the CHW flowrate through each FCU's coil, and CHW flowrate fall short.
Therefore the space cooling cannot be achieve.

My main suspect is still some unknown blockage in the branch off CHWP.
Is there anyway that an un-disruptive X-ray image scanning that be carried out to check if any blockage within the pipe?

 

[LowDeltaTSolver]

Were any temporary strainers installed and not removed for this circuit?

 

[MEP.Learner]

@LowDeltaTSolver
Thanks.
As this is rather old building with aged CHWP, the CHW water condition is rather bad with mud, debris, foreign matters.
The strainer at the FCU was removed as they will choke in short period. The cooling coil however is verified still not clogged with the strainer removed.

From the CHW balancing valve catalogue, the delta P required to get the full flow is about 22 ~ 28kPA. With some calculation of pressure loss through other Cooling coils, valves, pipes and fittings, I guess we can roughly assume an expected required delta P.
We will verify the delta P at the branch off pipe to verify the required flow.
Do comment if this technically incorrect.
Thanks.

 

[ABADI8891]

Hi;

I would suggest checking the following:

1- Clean the Strainers
2- The Delta P between Supply and return line to be checked.
3- the Motorize valves to be checked whether it working properly or not (Call the Vendor and ask him to provide a report from the third party)​

Several Questions may assist us to solve this issue...

1- what the CHWS Pressure?
2- is there any changing into the pipe sizes?
3- have you checked the size of the internal loop of the replaced units?
4- Do you have a DRV system into your chiller water system (Point between Supply and return)?
5- have you did a water Balancing/Pressure balancing for your system?​