Lecture 3 Control valves. Elements of the final control. Automatic Valve actuator. I/P transducer. Valve body. Current Signal 4-20 ma. Flapper. Nozzle. Back pressure. Pneumatic Signal 3-15 psi. Diaphragm control valves. Globe valve. Rotary valve. Rotary valve. Globe valve.
Most common control
Can be single- or double-
More than 90% in use are pneumatically operated piston or diaphragm type
Related to valve inherent characteristics
Special seat machined
Into the body
A (X) =X= x/xo
Equal percentage valves:
A (X) = e -k(x/xo-1)
Given expected pressure conditions, select
throttling control valve to pass the required
flow rate. It is a key step In ensuring that the
process can be properly Controlled.
Basic sizing practices have been standardized
Upon (e.g., ISA S75.01) and are implemented as
PC-based program by manufactures.
The value of b is important to the installed valve characteristic curve
The pressure drop is an economic loss to the process operation
Low pressure drop result in larger valve sizes and in decrease in a range of control
Rules of thumb: “20% to 50% of total dynamic pressure drop”, or, “25% or 10 psi”
For liquid service:
Cf: critical flow factor
Tf: degree of superheat in oF
Gf: specific gravity at 14.7psi and 60oF
Provided by control valve manufacturers.
The formulas have different forms but give
To meet working flexibility, CV, DPV and
DPt are to satisfy the following:
( Q ,qo, CV, DPt , DPV ), the other two can be
After being installed:
control valves, besides computing CV. In
most sizing problem, the size of pipe is
Known. It is usually to solve for CV and
reducers combined. Thus, a geometric
factor, Fp, is required in the flow formula:
The details are out of the scope of this lecture.