bypass decoupler pipe in primary/secondary chilled water system
bypass decoupler pipe in primary/secondary chilled water system
(OP)
Hi everybody,
In the attached typical primary / secondary chilled water system, you will notice that there are two (2) bypass pipes. This is the first time I come across such arrangement. i.e. 2 bypass. Also the location is wrong. In addition, having visited the job site, one of the bypass has long and smaller pipe size.
Question: Is it necessary to have a bypass? Note: the system consist of 3 chillers, 3 primary constant speed pumps, and 3 secondary pumps with VFD.
If it is necessary, then where is the best location noting the strange current piping layout.
Thank you for your assistance.
In the attached typical primary / secondary chilled water system, you will notice that there are two (2) bypass pipes. This is the first time I come across such arrangement. i.e. 2 bypass. Also the location is wrong. In addition, having visited the job site, one of the bypass has long and smaller pipe size.
Question: Is it necessary to have a bypass? Note: the system consist of 3 chillers, 3 primary constant speed pumps, and 3 secondary pumps with VFD.
If it is necessary, then where is the best location noting the strange current piping layout.
Thank you for your assistance.





RE: bypass decoupler pipe in primary/secondary chilled water system
decoupling lines present points of zero differential pressure and you must not alter it if you don't plan to resize pumps.
in the marked schematics, green, blue and red lines are covered by secondary pumps, while primary pumps fight resistance of yellow lines only.
the arrow on upper bypass can change direction depending on flow variations in secondary system.
so the answer is - it is absolutely necessary to instal bypass lines per design.
RE: bypass decoupler pipe in primary/secondary chilled water system
It is a bit odd, I can't tell why they would need all those bypasses without speaking to the designer (something you may want to do). The bypass in the lower right-center of the drawing appears to allow the primary pumps to bypass the chillers if needed. The one in the upper part of the drawing is probably the main bypass. Like Drazen said, it would work fine so I would install it per the drawings, you never know why they might need one of them.
RE: bypass decoupler pipe in primary/secondary chilled water system
The color lines I add it to make it easier to identify the flow diagram of supply and return.
I have attached actual photo of the bypass from job site.
DRAZEN: I am not quit sure the bypass on the top-lift corner is at points of zero differential pressure!
I am interested about your statement that the primary pumps fight resistance of yellow lines only. Please elaborate. If true then I must recalculate both primary & secondary pumps heads. Finally, I agree about the necessity of bypass but not sure of necessity of TWO (2) or THREE (3) bypass and/or its location.
TYS90: I am not sure what is shown on the lower left hand corner on the drawings but I shall check it on next job site visit and report back.
I have been called on to balance/optimize the water/hydronic system.
The situation is a bit complected at the moment. The owner selected, provided and installed the Chillers. The contractor provided & installed the pumps and piping network. There seem to be a dispute between them over why the return HIGH water temperature. I have been asked to optimize the system irrespective whose fault it is. i.e. balance the system to make the most out of the current chillers.
The first thing I thought to do is to check the system for any wrong installation (piping, valves, vents, bypass, etc), then take temperature, pressure, and flow measurement. Then adjust the primary pumps valves to fulfill the chiller requirement. The secondary pumps are on VFD controlled by Pressure Differential sensor located next to the pumps on the supply & return pipes. Both pumps and chillers controlled by the BMS.
Following are main system data:-
Required (based on HAP software calculated load): 750 ton at design ambient 106 Deg. F
Delta-T: 44 / 60 F (16F).
Chillers: 3 x 387 TR at design ambient 115 Deg. F (580 GPM water flow rate).
Primary Pumps: 3 x 15hp 570 GPM @ 50 Feet - constant speed.
Secondary Pumps: 3 x 75hp 570 GPM @ 180 Feet - variable speed.
System is 2 way valve with preset dynamic balancing valve.
I would very much appreciate your views.
Thank you
RE: bypass decoupler pipe in primary/secondary chilled water system
RE: bypass decoupler pipe in primary/secondary chilled water system
The bypass near the primary pumps is the decoupling bypass, but personally I think it is misplaced and undersized. It should be an 10 or 12" line connecting from the (red) Secondary CHWR before the primary pumps to the (green) primary CHWS downstream of all the chillers near the 45deg bend along the wall.
RE: bypass decoupler pipe in primary/secondary chilled water system
The question then, can I get away without the bypass?
If I have to use it what would be the maximum acceptable PD ?
RE: bypass decoupler pipe in primary/secondary chilled water system
pressure drop through bypass should be made equal to the pressure drop of the whole network being bypassed, so you would likely need to throttle it even if pipe diameter looks too small to you.
it is crucial element of your hydraulic scheme, so you cannot remove it (if you decide so nevertheless, let us know what happened).
rectifying your problem with new chillers is not an easy task.
you should first check supply temperature - did it rise as well, or only return temperature rose? that could be sign of too low flow in secondary network.
RE: bypass decoupler pipe in primary/secondary chilled water system
RE: bypass decoupler pipe in primary/secondary chilled water system
RE: bypass decoupler pipe in primary/secondary chilled water system
Engineering Report Variable Pumping CHW
Chillers - Variable Flow Systems
Best to you,
Goober Dave
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RE: bypass decoupler pipe in primary/secondary chilled water system