(35b) Size Control Valves for Lab-scale Laminar Flow

Control valves for low flows of liquids, even of low viscosity, must be sized to take into account laminar or transitional flow in the valve trim.

We have found that the equation below allows the accurate sizing of control valves with seat-guided type trims for laminar, transitional and turbulent flow:

A single new parameter  is used to adjust the sizing of families of valve trims of similar geometry, for viscosity dependence in the laminar and transitional regions. 

This new parameter has a value of approximately 0.14 for seat-guided annular needle-type trims such as those used in Badger Research Control Valves and for Jordan LowFlow control valves for trims of C_v. 0.06 and smaller.   A single Samson valve of similar trim type was tested and found to have an   of 0.18, which we suggest to use for valves of similar design for conservative trim sizing in the absence of other data.  We found  to be approximately constant with varying stem position for trims of this geometry. 

For seat-guided vee-groove control valve trims manufactured by Emerson (Baumann), the value of  is approximately 0.06 at full open, and tends to smaller values as the valve is closed.

This equation can be solved to be explicit in Q and tends to the normal turbulent flow Cv sizing equation when the product  is sufficiently small.