Sizing Needle-type control valves in laminar and transitional flow

[moltenmetal]

In case this topic interests you, here's a reference to an excellent paper on the subject. Regrettably it's in AIChE's Chemical Engineering Progress, so it's not public content- but if you use needle-type globe valves as control valves or manual flow regulation valves for liquids, you'll want a copy of this paper for sure. Most places which use a lot of these, particularly Badger Research Control Valves or Fisher Baumann 51000s or similar valves, have a drawer or box full of expensive replacement trimsets from valves which were mis-sized on the assumption that flow in the trim was turbulent, or because they used the garbage viscosity correction factors published by some vendors or relied on the vendor's sizing entirely. In our experience, even the most reputable vendors get this wrong, sometimes by a factor of ten in Cv or more.

http://www.aiche.org/resources/publ...ch/size-control-valves-lab-scale-laminar-flow

 

[moltenmetal]

To summarize, in case you don't have the AIChE membership:

Cv, required, = Q*sqrt(s.g/dP)*sqrt(1+(mu*av^2)/(Q*s.g.))

where Q = liquid flow, USGPM
mu = viscosity, centipoise
s.g. = specific gravity of liquid relative to water
av = laminar flow sizing factor
= 0.14 for a circular cross-section needle in a circular seat (Badger Research Control Valve, Swagelok fine metering valves, Samson, Jordan LoFlow etc.)- most valves which are "seat guided" meaning that the stem never completely leaves the seat
= 0.065 for a circular trim with triangular vee notches (Fisher Baumann 51000 etc.)

The paper also has an explicit solution for Q.

As mu*av^2/Q*s.g. approaches zero, the equation becomes the ordinary turbulent flow sizing equation.

Fits the data very well, according to the paper, fitting smoothly between laminar, transitional and turbulent flow.

Interesting to note that once flow slips into laminar, a "linear" trim takes on a characteristic more like an equal percentage valve. An equal percentage valve will waste more than 50% of its travel at the bottom end.