Primary-Secondary Pumping Rule of Thumb

[carlosalberto]

A rule of thumb says that the location of the secondary pump in a primary-secondary piping arrangement shall always be in the supply piping of the secondary loop pumping away from the common pipe or decoupler, and the expansion tank shall be located in the return piping of the secondary loop upstream of the connection with the decoupler. What would happen if we connect the secondary pump in the return piping of the secondary loop, between the expansion tank and the connection to the decoupler, pumping into the decoupler?

 

[janners]

A contractor did this to me in a congested boiler room, where the actual position of the pump ( although the construction drawings showed it correctly ) went unnoticed until certification. The system like yours was un pressurised and upon start up, a low pressure system was set up in the suction line of the secondary pump, causing noise due to air being released from the freshly treated water and no doubt in the long term cavitational problems with the pump impellor. I would also imagine that in long term there would also have been excessive air pockets formed in the radiation devices leading to circulation problems and a generally noisy system. Hope that answers your question.

 

[ChasBean1]

Carlos, it would defeat the purpose of having a 1° - 2° setup. The main purpose, as I understand it, is to allow the primary loop to pump known volumes to allow proper heat exchange, e.g., volume X with only B1 firing, volume Y with B1 and B2 firing, and volume z with B1, B2, and B3 firing.

If we place the secondary pump where you mention, what's to stop the secondary flow volume from driving through the primary loop? With that setup, why have a primary loop that maintains constant flow through heat exchange devices?

Basic application - a secondary loop pump often has a VSD to maintain a fixed loop differential (that way, the right flow and Cv can be anticipated through controlling devices). How will a 2° loop pump maintain that 2° loop DP when a disproportionate amount of its discharge volume might be driven through idle primary devices??

Chew on that a minute, then feed back. -CB

p.s., 1° = primary, 2° = secondary…