Hi Jack
The main pitfall with three way valves is the usual application -
trying to do the work of two valves with only one. It is difficult to
generalise for three way valves because there can be so many
configurations. However, given a "simple" type of valve you want as
high an authority as practicable, for both streams, if you want
reasonable control. Similar system head loss for each branch helps a
bit if the valve ports are the same size.
In general I try to avoid the use of three way valves unless it is a
special situation with a purpose made valve, such as a boiler LCV.
3-way valves are often used in HVAC and are often seen hunting, if
they havent been locked in position. hunting can normally be overcome
by slowing down the response (increase P band, I time) but this
reduces controllability. A smaller valve trim can often help sharpen
response, but at the expense of maximum flow rate, unless the
available differential head is increased.
Cheers
Steve
- In PipingDesign@yahoogroups.com, Jack Osborn <jhosborn@...> wrote:
>
> Steve.....
>
> thanks for the description of control valve authority. What would
you say about similar requirements regarding three way control valves
in a circuit? Should there be similar head loss (or authority)
sizing? Any pitfalls to sizing a three way valve in a circuit?
>
> Jack
>
> *******
> Jack Osborn, P.E.
> Mechanical Engineer / Project Manager
> Lee & Ro, Inc
> 925 937 4050 ext 402
>
>
> Hi there
> For accurate control of simple systems it is necessary for the
> control valve to have a fairly high head loss across it compared to
> the entire system pressure loss. The ratio of valve loss to total
> system (including valve) loss is called authority, and for good
tight
> control it should be around 0.5 (1:2) or better. A system with all
> the head loss across the valve will have an authority of 1. An
> authority of 0.5 means 50% of the head is lost across the valve,
and
> the remaining 50% is lost across the rest of the system. So for
> control we want the valve to have a high head loss which means its
> trim will be small, and the rest of the system to have a low head
> loss which means big pipes.
> So thats the reason why you see smaller that line-size control
> valves, apart from cost of course.
>
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Received on Sun Nov 04 15:49:00 2007