Primary/secondard decoupling Line

[remp]

I have 4 nr. large medium temperature hot water boilers in parallel with the pumps retrofitted to run in primary/secondary configuration . Each boiler has a pump and all boilers are connected on a main header. Each pump is served from a return header. At the end of the header we go into the secondary pumps. All pretty normaly arrangement.
I now need to add the decoupling line. Do I need the tie-in points for the decoupling line at the end of the header nearest the secondard pumps and the last boiler (just like in all the text books) or can I put it up the other end of the header (far away from the secondard pumps) at the start of the first boiler...this is my prefered option due to the availablilty of valved tie in points left vacent from a redundant and stripped out boiler....

You help would be much appriciated...

 

[ChrisConley]

Possible to upload a sketch of what you're talking about?

 

[remp]

Hi

You can see what I am talking about in the attached
Also please note the primary header changes in size from 400dia to 200dia to 300dia but it is never undersized for the flow rate. It is largest at Boiler 1 and 2. Then down to 200 for about 20meters then up again at boiler 3 and 4 to 300dia.

 

[ChasBean1]

I think it would work, but you're still dragging secondary water through the boilers. The odd diameter changes would cause pressure differences and flows through idle boilers also, and not sure what you're doing with the flow meter(?) See attached. That would be a more typical type of decoupled loop with multiple boilers (note the close proximity of each boiler's taps).

 

[KiwiMace]

I think you should stick to having the decoupler btw the P and S loops.

The purpose of this is to balance the constant and variable flows, but also as the name conspicuously suggests, to remove the influence of the secondary pumps on the primary loop and vice versa.

The secondary loop will be trying to maintain an equilibrium with the closest boiler/s in your suggested alternative.

 

[atlas06]

Forget about option 2. you are doing a series pumping system with that, no more, defeats the purpose of P/S ssytem in my view. Your primary pump will need to be sized for half the system head instead of just primary loop. Alos, you will have two incompatbile system, one with PD and VFD, the other cosntant flow and yet on the same loop, your pumping systems will be fighting each other (each having a different flow).

Chass, i like the meter in the decoupler for one thing: It can be used for staing boilers, need to make the flow meter bi-directional. Combined with delta T measuring, depending on how water floow is thru the decoupler, one can tell when it is time to energize boiler staging (up or down one boiler).

 

[ChasBean1]

I've seen decoupler meter for staging before but would leave that for indication only. Tend to lean toward keeping things simple, staging boilers to maintain temperature and temp. set point on OA, but just my personal preference..