Pump Controls 3

[flexiblycool]

The Sequence of Operation of the chilled water system pumps (depicted in the attached sketch) is that when there is demand for cooling, the Primary Pump comes on. Upon proof of flow from the flow switch, the chiller associated with this primary pump comes into operation. If the cooling demand is not met with one chiller, the second Primary pump and its chiller comes into operation and likewise upon further increase of cooling load, the third primary pump and its chiller comes into service.
Likewise when one primary pumps is automatically switched off based on returning chilled water temperature, the flow in the system reduces to 400 GPM from 600 GPM. The fan coil units that are still in service will now see less than design flow. If two primary pumps are switched off based on returning chilled water temperature, the flow in the system reduces to 200 GPM and the fan coil units that are still in service will get much less flow than design flow for their capacity.
The Secondary Pumps are continuous duty constant speed pumps.
This scheme of things therefore appears flawed. This system would be fine for the 2 way valves, but not for the 3 way bypass valves. So the proposal is that instead of removing the primary pumps from service, let all the primary pumps stay in duty regardless of demand and just switch the chillers on or off to match the load. Is this acceptable?

 

[flexiblycool]

Thanks again LittleInch (Petroleum):
Your retrofit solution will indeed require considerable money and expertise!
I wonder what would happen in the existing scenario in which only 600 GPM is available and the secondary pumps are battling to pump 840 GPM in the buildings. Could this cause secondary pump cavitations? No unusual sounds or vibrations have been observed however, and the secondary pumps appear to be plugging on fairly smoothly.

 

[flexiblycool]

I suspect the behavior and performance of secondary pumps is a little irregular due to insufficient available flow. Let me take some pressure and flow readings in the next couple of days and get back to the Forum with my findings. Thanks.

 

[BigInch]

The secondary pumps will pump the lower flowrate (70%), probably at an even higher pressure, just keep their suction pressures above NPSHR. What I don't understand is why the primaries are 200 gpm and the secondaries are 280 gpm.

If you want a VFD solution, try monitoring suction pressure and adjusting the 2nd pump speeds to keep them constantly above NPSHR, yet always maintaining discharge pressure below whatever is required for max flow while keeping >NPSHR at the primaries. Try combining that first with simple on/off control of 2 of the primaries, leaving one running constantly to keep minimum flow going to the chillers).

Independent events are seldomly independent.

 

[flexiblycool]

BigInch: The primary pumps are only 200 GPM compared to 280 GPM capacity of the secondary, because all the occupied spaces will not require cooling all the time. Some rooms that are facing the Sun will require the most cooling and then with the time of the day some others will face the sun. That is the only explanation that comes to the mind. Designing for such a diversity factor is also flawed because all the secondary pumps remain on all the time at constant speed, so each pump will try to get its 280 GPM which is simply not coming from the Primary pumps.
I am trying to arrange an Ultrasonic Flowmeter for the readings. So will get back to the forum ASAP with the data and findings.

 

[LittleInch]

HOw the system was supposed to work with a fixed speed booster pump sized higher than the pump(s) feeding it when the flow demand doesn't change due to the three way valve and bypass arrangment baffles me. The flow spread will be quite interesting - my guess is that the nearest building gets more flow than another one unless there is some "balancing" valve set-up which isn't shown on the diagram. Data will help on flows and pressures at the various points around the system. If you can get current going to each fixed speed pump tat will give you some idea of flow variance if you know the pressure drop from inlet to discharge.

I still don't understand if you have a problem you're tryiing to fix and what that problem is or are you trying to commision this mess??

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[BigInch]

That is the only explanation that comes to the mind. No matter how you divide up the sunny or cool rooms or shut one, two or three pumps on/off, that won't be the reason.

Pumps don't pump what "they" want, unless they are running on a test bench. Pumps don't just pump at BEP or rated flowrate and they don't even try to get there. There is a system curve that between the pump curve and the system curve, decides how much goes round and round. Pumps operate with whatever corresponding flow shows up at their suction flange.



Independent events are seldomly independent.

 

[flexiblycool]

LittleInch/BigInch: Your emphasizing the point that the flows of primary and secondary must be the same helped. The actual flows of both the primary and secondary have been investigated on site and they are indeed the same. In other words the secondary pumps are also 200 GPM each. Thanks for your kind persistence about this point that led us to confirm this on site.
We still have to get back to you with the field measurements of pressures to see if the secondary pumps could be removed from the system without even having to upgrade the primary pumps because there seems to be a 25% factor of safety built into the primary pump head calculations! Maybe the field measurements will unravel or cause more mysteries.
So please bear with us. Thanks.

 

[BigInch]

I think I'd try to run with primaries on/off and (perhaps) put VFDs on the secondaries, but only after you confirm that they are indeed necessary.

Independent events are seldomly independent.

 

[LittleInch]

In any multi system with a parallel, but uneven system, you normally find what I term control valves, but could be called balancing valves somewhere in the individual loops to be able to balance the flows equally between in this case buildings. VFDs can perform the same function, but you need to find those valves which become critical to balancing flow between each user. Whether the primary pumps are man enough for the job without your booster pumps will depend on the pressure drops accross the buildings and the friction losses in the loop and these valves may need some adjustment to make it work.

One thing your system didn't show was your pressurisation system to keep the closed loop at a set pressure. Do you have one?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[flexiblycool]

LittleInch & BigInch:
With close monitoring from you, a lot of progress has been made in unraveling the system and discovering some incorrect assumptions and mistakes (such as different flows of primary and secondary pumps).
Yes we have a expansion/pressure tank to maintain system pressure.
Getting the pressure readings will take another couple of days (or a few days at most), so your follow up at that time will be appreciated. We want to see the pressure at the entrance of the building, so the pressure gauges have to be arranged and installed, not only at secondary pump suction and discharge but also at the incoming headers to measure pressures with and without the secondary pumps working.
Thanks
Flexiblycool

 

[flexiblycool]

A CLARIFICATION
The system described in the previous schematic sketches was simplified hypothetical model in which some buildings and equipment were deleted to fit everything on an A4 sheet. Since the discussion has evolved into a very technical one, I have reinstated the components of the system that I previously thought could be omitted.
In any case please refer to the new sketch which together with the last schematic describes the system more closely.
The flow readings could not be taken, but I think, the pressure readings are of substance. In any case the primary pumps are 700 GPM each. The combined GPM of all four primary pumps is 2800 that is divided equally in all the 3 branches.
The pressure readings of building no. 6 are anomalous but we cannot pin down the reason.
The pressure 5 meters upstream of the secondary pump of building 6 is 60 psi and that of building 5 is 57 psi.
Thanks in advance for your anticipated feedback about whether the secondary pumps can be deleted.

 

[BigInch]

57-60 may have something to do with the change in velocity near the dead leg. (If it works, don't fix it.)

Independent events are seldomly independent.

 

[LittleInch]

Flexibly cool,

This is a bit of a different scenario, but the same advice applies. Your pressure anomoly I can't understand, but in both instance the booster pumps appear to be adding about 15 psi. If the system is running at a third of the load as branch 1 and 2 are shut off, my advice is limited to say try it, but somewhere that extra pressure drop needs to be accomadated. You may need to run two main pumps to maintain flow, but would need to bypass the secondary booster pumps, not just flow through them.

Is the schematic correct in terms of the actual flow path? i.e. does the return line run FROM bldg 1 past all the others before coming back to the main pumps as you show in the diagram or does it in reality run from blg 6 in the other direction??

Without a full set of data (pump curves, elvations of buildings, lengts and sizes of pipes, location of valves, pressure drops and flow rates accross fan units etc etc) it is difficult to do much more.

Hope this helps.

LI

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[flexiblycool]

Hello LittleInch: Yes your feedback was of great help. Thanks a lot. The Autocad operator was not available to draw the sketch with all the buildings. I did this lash up job on the Xcel!, that's why the return line loop is not running side by side to the supply.
Today, I collected more data with 1 pump running only and water going only in the branch with the largest pressure drop, and also have data with 2 pumps running with 2 branches active. The pressure readings are a bit doubtful. Even the new pressure gauges are giving conflicting readings!
I have probably tired out those who followed and responded to this thread and have certainly become exhausted myself. It is not easy to get the readings on site.
Instead of speculative work, we are calling for water balancing of the entire system. If the water flows from each FCU at its design GPM without the secondary pump, we will advise the removal of these pumps.
I hope to be able to get back to the Forum with any observations of interest when we reach that point within a month or so.
Thanks to all the members of the Forum and particularly to BigInch & LittleInch.

 

[LittleInch]

I think you're right to call it a day on this thread. Essentially its the users that matter and if they're getting the right flow rate of water, then everything else is academic. With this many buildings it will take some time to balance the system in my view - good luck!

LI

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[flexiblycool]

LittleInch: Thanks for the insights.
By the way, the Main Contractor probably knew very well what it will take to water balance the system (which I modeled for simplification as consisting of 60 FCUs, actually has over 400 fan coil units and about 30 AHUs). So guess what, the shrewd Contractor has contracted the work to a SUB CONTRACTOR!!!!!!!!!!!!
Maybe will meet you in another thread on this Forum.

 

[cloa]

fleaiblycool clicking "Like this post? Star it!" to a helpful post- is the standard appreciation method.

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[flexiblycool]

cloa (petroleum)
Sorry I did not understand your message. Please elaborate

 

[cloa]

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