circut setters

RJ_4

A co worker ask me why the opening on a Taco 3/4 inch circuit setter was not a full port opening. Asking for different explanations
Thanks
RJ

Harvey Ramer

If you look inside, you should see a cone shaped piece that narrows the channel as it passes through the ball valve. On the other side will be this same shaped cone to bring the channel back out to full diameter. What the cone does, is increase the velocity of the fluid through the ball without created a huge pressure drop. With a higher velocity through the ball, the ball has greater authority and more precise control over flow. It's kind of like leverage in a sense.

RJ_4

Thanks Harvey does the smaller opening affect the gpm ?

Jamie Hall

RJ said:

Thanks Harvey does the smaller opening affect the gpm ?

No. Due to the design of the valve -- as @Harvey Ramer pointed out -- there is very little net head loss through the valve when it is wide open. Thus there is essentially no change in gpm -- when it is wide open. The whole idea, though, is to increase the head loss for a given gpm as it is closed, and thus reduce the flow.

RJ_4

Thanks Jamie The info from you and Harvey is greatly appreciated.
RJ

hot_rod

Valves, all valves, will have a Cv number. The Cv number is the gpm flow thru the valve with a 1 psi pressure drop . Ideally valves are sized based on this flow rate, not the pipe size.

The Cv number on a balance valve is the wide open position.

There are various types of balance valves, some have fixed, other variable office design. With a variable design the cv changes across the adjustment range.

The taper built in is the venturi designed into the valve, to create a pressure drop.

Gman66

As Hot Rod indicated that throat in the balance valve is a venturi which provides the differential pressure across the measurement ports. Contrary to what Jamie wrote, the pressure drop across these types of balance valves is not insignificant. The Taco 3/4" has a CV of 3.9; so at a rather typical 4 gpm flow in 3/4" pipe, the pressure drop across the wide open valve is 2.4 ft head. About equal to 40' of 3/4" copper.

hot_rod

For example, here is a common Apollo 3/4 ball valve and the Cv assigned to the valve.

Flowing 30 GPM thru a 3.9 Cv valve looks like this.

Jamie Hall

Gman66 said:

As Hot Rod indicated that throat in the balance valve is a venturi which provides the differential pressure across the measurement ports. Contrary to what Jamie wrote, the pressure drop across these types of balance valves is not insignificant. The Taco 3/4" has a CV of 3.9; so at a rather typical 4 gpm flow in 3/4" pipe, the pressure drop across the wide open valve is 2.4 ft head. About equal to 40' of 3/4" copper.

I was misunderstood; what I was referring to was the pressure drop difference due to the venturi/reduced port design vs. a full port design. So long as the venturi is smooth on both the contraction and expansion side, absolute pressure in the throat of the venturi is above the vapour pressure of the moving fluid and the pressure reduction in the throat of the venturi is the specific temperature, the pressure will be essentially fully recovered upon the following expansion.

If the reduced port is not preceded and followed by a smooth transition, then the pressure drop can be significant, and is better characterized by considering the valve to be an orifice, rather than a venturi.

The total pressure drop of the valve, to which @Gman66 refers, is influenced by a number of design factors.

Note that none of this discussion applies to saturated steam, which has entirely different characteristics.

RJ_4

good stuff thanks all