1. Lecture 3
Control valves

2. Elements of the final control
Automatic
Valve actuator
I/P transducer
Valve body

4. Current Signal
4-20 ma
Flapper
Nozzle
Back
pressure
Pneumatic
Signal
3-15 psi

5. Diaphragm control valves

8. Globe valve

9. Rotary valve

10. Rotary valve

11. Globe valve
Most common control
valve style
Can be single- or double-
seated

12. Single-seated valves Usually are employed when
Tight shut-off is required,
In sizes of 1 inch or smaller where unbalance forces acting on the valve stem is unimportant.
Usually have a top guided construction

13. Double-seated valves Usually is top and bottom guided.
Practical leakage approaches 0.5% of the rated CV.
Advantage lies in reduction of required actuator forces.
Have upper and lower ports of different diameters---allow to withdraw smaller plug through the larger port.

17. Angle valve
Single seated valves with special body configuration to suit specific piping or flow measurements.
May be used in case where the piping layout does not allow installing a globe valve.

19. Three way valves A design extension of a typical double-seated valve.
Can be used for diverting service and for mixing service.

22. Actuators for control valves:
Pneumatically operated diaphragm actuators;
Piston actuators;
Electro-hydraulic actuators;
Electro-mechanical actuators;
More than 90% in use are pneumatically operated piston or diaphragm type

25. Safety consideration Air-to-open (AO) Failure close
Air-to-close (AC) Failure open

28. Loss

29. Defined at maximum
Valve opening
Related to valve inherent characteristics

32. Over sized plug to provide
Additional Cv
Special seat machined
Into the body

33. Equal percentage valves: A (X) = e k(x/xo-1) = a (X-1)
Linear valves:
A (X) =X= x/xo
Equal percentage valves:
A (X) = e k(x/xo-1)
= a (X-1)
Rangeability

34. Equal Percentage Valve

35. Equal Percentage Valve

37. Distortion after installation

42. Valve hysteresis

43. Valve positioner

48. What accomplishment a positioner can have?
Provide precise positioning of the valve
Provide adequate power on high-pressure applications
Increase control valve speed of response
Reverse valve action
Provide split range operation

49. Typical positioner performance
Pneumatic signal ranges: 3-9, 3-15, 9-15, 3-27, 6-30 psig
Air supply pressure: 20 to 100 psig
Repeatability: within 0.1% of stroke
Hysteresis: within 0.3% of stroke
Linearity: 0.5% of stroke

50. Split range control valves

52. Control valve sizing 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.

53. Allocating pressure Drop
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”

54. Determine valve capacity to meet required flow rate--- Computing CV
For liquid service:

55. For gas and vapor: For steam: Cf: critical flow factor
Tf: degree of superheat in oF
P1:upstream pressure
Gf: specific gravity at 14.7psi and 60oF

56. There are still many other formulas in
Provided by control valve manufacturers.
The formulas have different forms but give
similar results.

57. To meet working flexibility, CV, DPV and
DPt are to satisfy the following:

58. Given either three of the following five variables
( Q ,qo, CV, DPt , DPV ), the other two can be
Computed.

59. Rangeability of a control valve:
After being installed:

60. There are standard procedures to size a
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.

63. To select an equal percentage valve:
Calculate , the value yields at least 100% of qd with the available pressure drop.
Compute q as a function of valve position using the rated Cv and DPv. The valve characteristics should be reasonablly linear in the operation range of interest.
If not suitably linear, adjust the rated Cv

65. At design flow rate, qd :