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Chapter 13 : Pneumatic control element - Coggle Diagram
Chapter 13 : Pneumatic control element
Pneumatic control elements
Industries used it to control of fluid process
Flow rate is most important of control parameter for fluid operations
Principle of control valve.
Flow rate expressed as volume perunit time
If mass flow rate is desired , it can be calculated from the particular fluid density
Purpose of control valve is to regulate the flow rate of fluids through pipes in the system by placing variable size restriction in the flow path.
Size of opening can change as the stem and plug move up and down thus the flow rate will change
Flow rate : Q = K√(∆p)
Types of control valves
Quick opening
For FULL ON or FULL OFF control applications
The valve characteristic shows that a relative small motion of the valve stem results in maximum possible flow rate through the valve
Linear Opening Valve
Flow rate varies linearly with the stem position
Represents ideal situation where the valve alone determines the pressure drop
Q / Qmax = S / Smax
Equal percentage opening valve
A given percentage in stem position produces an equivalent change in flow
Doest not shut off the flow completely it its limit of stem travel
R = Qmax / Qmin
Q min: minimum flow when the stem is at one limit of its travel
Qmax: maximum flow
Increase in flow rate for a given change in valve opening depends on the extent to which the valve is already open.
Q = Qmin R^(S/Smax)
Control valve sizing
Correction factors due to the non ideal characteristic of the material that flow.
These correction factors allow selection of the proper size of the valve to accomodate the rate of flow that a system must support.
Using this factor, a liquid flow rate is : Q = Cv √(∆p / SG)