circulator cavitation

ALH_4

Is the TV valve working properly?

Could you post a photo or a diagram of the installation?

radioconnection_2

circulator cavitation

Can circulator (or throttle valve) cavitation be an issue in a hot water system if the flow is restricted on the suction side of the circulator? The only cavitation references I've seen relate to steam systems. I also wonder how the Circuit Setters handle throttling applications since throttling isn't usually recommended by ball valve manufacturers.

Brad White

Yes it can

Any time you reduce the inlet pressure on a circulator (either by isolating it, heating it above the fluid's vapor pressure or both) you will cause cavitation.

Throttling or restricting the discharge side does not affect this if your expansion tank is on the suction side of the circulator as it should be. Isolating the suction side of any circulator -especially isolating from even a properly placed expansion tank- will trigger this effect.

Whenever you lower the fluid pressure it essentially "boils" (at whatever temperature it happens to be). The bubbles collapsing and reforming will chew your impeller like so many hungry mice.

ALH_4

Vaporization Pressure

Google "Net Positive Suction Head". The pump manufacturer will give you a curve for NPSHr (Net Positive Suction Head Required)on the pump curve diagram for a particular fluid and temperature. You must compare this NPSHr to the NPSHa (Net Positive Suction Head Available), which is the pressure available at the suction side of the pump while in operation. This is dependent on your system design. If NPSHA < NPSHR the pump will cavitate and destroy itself.

As Brad mentioned, in a heating system the problem is magnified because of the elevated fluid temperature. This is likely the reason boiler manufactures generally locate circulators on the boiler return and one of the reasons zone valves should be located on the zone returns.

Hydraulics and Energy are the most interesting topics on the planet, in my opinion. A textbook I like that covers quite a bit about hydraulics in a fairly clear way is Fundamentals of Fluid Mechanics by Munson, Young, & Okiishi, any edition but the 4th edition where they left out the 12th Chapter on "Turbomachines". I also have a book called Cameron Hydraulic Data that has been useful.

radioconnection_2

vaporization

Thanks for the replies. The suspected problem is a pump that is located between the return on a Biasi B14 and a Danfoss TV valve. I'm finding that the throttle valve setting needs to be about 80 percent closed to get flow in the return loop once the boiler is warmed up. That is much, much than I had expected would be needed.

Pete

radioconnection_2

Danfoss

Hi Andrew

The TV valve is apparently working--once the return hits 155 degrees or so the supply temp remains constant at 175 degrees and all of the heat is going to the heating loop. It takes a bit more bypass throttling than I had expected, though. Opening the throttle more causes the supply to creep upwards until the boiler cycles on the high limit. Would a Fluke clamp-on ampmeter show a change in current when (and if) cavitation begins as the valve is throttled?
I'm really sure that it is an issue since the restriction pretty much goes away once the boiler reaches operating temperature.

The heating loop is piped P/S on the return to the Danfoss.
Those Tees are just out of view. Airscoop and tank on the top of the loop. The second jpg shows the supply side above the LWCO, the return and bypass legs to the TV valve are after the airscoop and tank.
Pete

radioconnection_2

FIXED IT!

A piece of floatsam got into the valve and must have been binding it. Took it apart and cleaned it, and now the TV valve is working like a dream. No throttle needed, and the boiler circulator is fine on the lowest setting. I feel much better. (Yeah, solder drop from the copper fitting above.) Thanks to everyone for the advice and information.

Pete