Primary-Secondary Balancing Valve Location

[psulee]

I am investigating a low hot water flow issue for a single building served by a district heating plant. The pumping system is designed as a constant speed primary distribution with varible speed pumps serving the building loads. A three way mixing valve maintains the building loop temperature. The building loop is decoupled from the primary hot water system via an open flow path. The constant volume primary system is regulated with a balancing valve downstream of the decoupler/building return point of connection. This balancing valve maintains primary loop flow. However, during building use, it appears that the decoupler could have reverse flow even at flows less than system design. Since the balancing valve is downstream of the decoupler point of connection, if the pressure is higher in the building return than the supply line (possible with the pump energy added at the building), the flow is reversed. Is this a possible issue that I should study? My initial thought is that if the balancing valve was in the decoupler line, then this would essentially create the pressure drop that the system requires to work properly. Reverse flow would be less likely.

In reviewing the ASHRAE Systems and Equipment handbook, the balancing valves in Fig. 19 appear to be shown in the position that is currently installed in this building and not in the decoupler (common pipe). Is there a reason why it does not get installed in the decoupler?

Thank you.

 

[RossABQ]

The decoupler line needs to have as little pressure drop as possible in order to decouple completely. You sometimes see a check valve in these lines to prevent backflow; the disadvantage of that is the check valve's pressure drop.

 

[psulee]

Thanks for the reply. I guess the pressure drop issue is my whole misunderstanding with this issue. The distict system feeds multiple secondary systems off the same primary loop. The balancing valve location is effectively adding a pressure drop in the return leg. With the balancing valve in the decoupler, it would add pressure drop, but would result in lower probability of reverse flow. I am not concerned about the return flow as I am getting controlled return flow through the three-way. A check could be appropriate becuase I am restricting flow on the return leg anyway. I can just open the balancing valve slighlty to account for the additional pressure drop caused by the check.

Thanks again.

 

[ChasBean1]

It would definitely be helpful to see a visual, but you explain it well. The three-way is not on the decoupler line, correct? In other words, there are two crossover lines, one being the decoupler and one being the three-way line to the secondary pump inlet?

It sounds like this system is misapplied, causing the central plant to move much more water than it has to. If all buildings have decoupler lines, I can see why they put a circuit setter at each building. This setup will cause full (and excessive) plant flow all the time and probably not enough differential pressure to meet individual buildings' demands.

Primary/secondary decoupled flow is great for a single building with say, three boilers. Each boiler can have its fixed primary flow and the secondary pumps can vary speed to meet building demand.

But for multiple buildings, I would suggest that they look into getting rid of the decoupler philosophy and putting check valves on the decoupler lines (preventing direct supply to return flow at each building), so that the volume of primary hot water delivered can vary. This would help each building meet demand.

The three way line should also be done away with and replaced with a simple return deny valve at each building, where the balancing valve you mention is currently located. The return deny valve can be throttled down to ensure return temperature is below 150°F for example, if necessary, to make sure each building uses only the amount of primary hot water that it needs.

Also, if you have variable speed pumping in the building you're evaluating, have you confirmed that hot water control valves at coils have a two-way and not three-way piping configuration? Three-way control valves at each coil in a building with variable speed pumping should also be replaced with two-way.

I've run into similar issues a couple times. Hopefully some of this is helpful... Best, CB

 

[RossABQ]

I understood his situation to be multiple VFD-driven pumps, each aligned with a building. The three-way valve at the end-of-loop (most remote device) in each building is a typical ploy on a system with 2-way valves elsewhere, to maintain full temperature (or cold) throughout the system. It amounts to a small bypass flow and doesn't cause any issues.

If you have access to the ASHRAE books, they go into a fair amount of detail on the theory of primary/secondary pumping schemes. I've mostly seen problems on a P/S system when it is "improved" with the check valve or other devices, or when there are basic piping configuration problems (i.e., too small a decoupler, or too large, poor geometry, etc.). Geometry of the return piping connections to the loop is important; are there any bullheaded connections, or geometries that cause an increased pressure drop in one building's returns? Where are the pressure differential sensors for each loop? There may be an incorrectly calibrated dP set in one or more loops. Was the system thoroughly commissioned?