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Lecture 3 Control valvesPowerPoint Presentation

Lecture 3 Control valves

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Control valves

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

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.

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.

Three way valves

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

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

Safety consideration

- Air-to-open (AO) Failure close
- Air-to-close (AC) Failure open

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

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

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.

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”

Determine valve capacity to meet required flow rate--- Computing CV

For liquid service:

For steam:

Cf: critical flow factor

Tf: degree of superheat in oF

P1:upstream pressure

Gf: specific gravity at 14.7psi and 60oF

There are still many other formulas in

Provided by control valve manufacturers.

The formulas have different forms but give

similar results.

To meet working flexibility, CV, DPV and

DPt are to satisfy the following:

Given either three of the following five variables

( Q ,qo, CV, DPt , DPV ), the other two can be

Computed.

Rangeability of a control valve:

After being installed:

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.

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