throttle/balance valves and pump cavitation

Mark J Strawcutter

Earlier this fall I posted an inquiry here about locating throttle/balance valves on the supply vs return of a zone and if locating on the zone return would it tend to cause circ cavitation problems.

As I've been thinking about this, it occurs to me that in a proper "pumping away" configuration, the zone return is still on the circ's supply side. The only place where a throttle/balance valve could cause problems would be between the PNPC (expansion tank) and the circ inlet.

If this is correct, then I'm thinking I like the idea of balance valves on zone returns to keep pressure higher in the zone and in turn keep air in solution.

Thoughts/comments welcome as always.

Mark

Jerry Boulanger

Air binding in hydronic systems..

is a balance problem. If each branch has the correct flow, it should also have adequate water velocity (about 2 fps) to carry the air with it out of the branch and to the air separator. While it is true that locating the balance valves on the return will maintain higher pressure in the branch, and water can hold more air in solution at higher pressure, the difference will not be significant in most systems.

A throttling valve will have no impact on whether or not the pump cavitates unless, as you say, it is located between the pump suction and the expansion tank connection point. In fact, reducing the flow through a pump will reduce the net pump suction head (NPSH) required by reducing the velocity at the inlet of the pump impeller, and help to prevent cavitation.

hr

Not a good idea Mark

Check this link for the reasons not to put flow restricting devices on the inlet side of circs. Y strainers on the inlet side can also cause problems with only a small amount of debris in the basket.

There are some good drawings with this article but they don't always show up online!

hot rod

http://www.pmmag.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2379,4270,00.html

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Mark J Strawcutter

I agree, but

A circulator, well, circulates :-) So where does the supply/outlet side end and the return/inlet side start as you travel around the circuit? I'm proposing/supposing that it's the point of no pressure change that marks the boundary.

Consider a pumping-away setup zoned with zone valves. You leave the boiler, next is the expansion tank, then the circ, then zone valves off a manifold. The zones return to another manifold then back into the boiler.

Since the expansion tank is the PNPC, the "inlet side" of the circs would be anywhere between the expansion tank and the circ inlet connection (usually a very small distance).

Anywhere else can be considered part of the "outlet side" and a balancing valve there should not cause a problem (?)

So, once could place them on either side of the zone valve at the zone supply manifold, or at the end of the zone near the zone return manifold. Neither should cause a cavitation problem with the circ since both are on its "outlet side" but the latter has the advantage of keeping higher pressure in the zone.

hr

I would say

half way around the loop from the PONPC would be where it could be said the change from supply to return occurs?

I have worked on dozen of old solar roll systems, 1/4" diameter tube. These generally ran 26-99 or 26-96, sometimes 43-75'S. I'm here to tell you the high temperature (staple up) applications were cavitation fiascos! Coupled with the fact they all seemed to lose pressure over the summer, you have a perfect "climate" for cavitation. High temperature, low static fill pressure, and often 160-180- degree operating temperatures, coupled to high head pumps.

Opening up one of these pumps will show you an impeller that looks like it was "ice picked" or sand blasted. Proof enough for me

I suspect what you propose may in fact work fine. But from my prospective why run the risk? I get the call backs

This is the major drawback to the small diameter above floor products currently on the market. The margin for error is pretty tight when using high head pumps, in my opinion. They sure have gotten pricey (high head pumps) Time will tell.

hot rod

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