primary/secondary - Secondary pump location with respect to the bridge.
primary/secondary - Secondary pump location with respect to the bridge.
(OP)
I normally put my secondary pump, air separator, and expansion tank downstream of the primary/secondary de-coupler bridge. For reasons of significant piping simplification in the field, I want to put them upstream on a job. I know that will put the bridge (and chillers) on the discharge side of the secondary pump and raise the pressure in that part of the piping, but is that a problem? The closely spaced tees with both have the elevated pressure.
Any reason why I should not do it that way? See attached. Thank you.
Any reason why I should not do it that way? See attached. Thank you.





RE: primary/secondary - Secondary pump location with respect to the bridge.
Looks like your primary system is setup with coupled pumps. Coupled systems should only be used when all the equipment in the system is the same capacity (chillers, cooling towers, pumps, etc.) If this is not the case, consider a headered system with control valves.
Also, air separators and expansion tanks are typically placed in the primary loop on the suction side of the primary pumps in a primary/secondary system. An alternate location in primary/secondary systems with a single secondary circuit may be the suction side of the secondary pumps; as you have indicated "where you normally put it".
EDIT: Attaching URL for BELL & Gossetts Primary / Secondary Pumping Applications Guide:
http://documentlibrary.xylemappliedwater.com/wp-co...
See page 3.
RE: primary/secondary - Secondary pump location with respect to the bridge.
RE: primary/secondary - Secondary pump location with respect to the bridge.
Static pressure by definition is the one not dependent on pump. General point, though, stays - if you put pump as you showed, total pressure in most of secondary will be lower than total pressure in common point. as secondary has much more joints than primary, and they are spread all over the building, and those joints are generally not well-suited for underpressure - you will have many points of possible air suction.
RE: primary/secondary - Secondary pump location with respect to the bridge.
RE: primary/secondary - Secondary pump location with respect to the bridge.
I am certainly not advocating that putting the secondary pumps upstream is better, but I just want to rule out problems.
In either situation for this situation, my expansion tank will be on the suction side of the secondary pumps. So pressure in the secondary system will be almost the same either way, won't it? The B&G article does not takes this into account when they indicate that it is wrong to pump toward the common piping. In the B&G article, the expansion tank is on the primary side of the common pipe, so you would be pumping toward it. So I agree that it would be wrong in that case.
So back to my question, provided the expansion tank is on the suction side of the secondary pumps (wherever they a placed), is there any reason why I should not pump toward the common pipe? It will substantially raise the pressure on the common pipe and the overall pressure in the chiller loop, but why is that a problem? Or will the primary pumps add head that must be overcome by the secondary?
I'm confusing myself the more I think about it.
RE: primary/secondary - Secondary pump location with respect to the bridge.
As you say, the key issue is that it raises the pressure in the common pipework and the chillers compared to your normal scheme. That becomes an issue if the pressure rating of the chiller loop is not as high as the secondary loop discharge pressure. In general the lower the pressure the reduction in stress and less chance of a leak or failure in the chillers.
The primary pumps shouldn't add any head, The end connections essentially see the same pressure, with the purpose of the pump simply to overcome frictional losses in the pipework from the common header and losses in the chiller.
Depending on flows from primary or secondary pumps, the flow in the connecting bridge could be in either direction or no flow if the flows are exactly matched, but the pressure comes from the secondary pump
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RE: primary/secondary - Secondary pump location with respect to the bridge.
RE: primary/secondary - Secondary pump location with respect to the bridge.
RE: primary/secondary - Secondary pump location with respect to the bridge.
The typical pumping arrangement resolves this issue by placing the load and chillers (read: inertia) between the pumps which dampens down the pressure fluctuations seen at the sensors as the system adjusts it's operating point.
RE: primary/secondary - Secondary pump location with respect to the bridge.
pressure level is determined by expansion vessel only, not by pumps in such system, so if you put it in the mentioned position, you have to raise expansion tank setting by calculated loop pressure drop, plus contingency, to make sure underpressure is avoided.