CONTROL VALVE HANDBOOK Episode 1 Part 4 potx

Chapter 3. Valve and Actuator Types

47

Figure 3-11. Eccentric-Disk

Rotary-Shaft Control Valve

W8380

They use standard pneumatic

diaphragm or piston rotary actuators.

Standard flow direction is depen￾dent on seal design; reverse flow re￾sults in reduced capacity.

Eccentric disk rotary shaft control

valves are intended for general ser￾vice applications not requiring preci￾sion throttling control. They are fre￾quently applied in applications

requiring large sizes and high temper￾atures due to their lower cost relative

to other styles of control valves. The

control range for this style of valve is

approximately one third as large as a

ball or globe style valves. Conse￾quently, additional care is required in

sizing and applying this style of valve

to eliminate control problems associ￾ated with process load changes. They

work quite well for constant process

load applications.

Eccentric-Plug Control Valve

Bodies

Valve assembly combats ero￾sion. The rugged body and trim de￾sign handle temperatures to 800F

(427C) and shutoff pressure drops to

1500 psi (103 bar).

Figure 3-12. Sectional of Eccentric￾Plug Control Valve Body

W4170/IL

Path of eccentric plug minimizes

contact with the seat ring when open￾ing, reducing seat wear and friction,

prolonging seat life, and improving

throttling performance (figure 3-12)..

Self-centering seat ring and

rugged plug allow forward or reverse

flow with tight shutoff in either direc￾tion. Plug, seat ring and retainer are

available in hardened materials, in￾cluding ceramics, for selection of ero￾sion resistance.

Designs offering a segmented

V-notch ball in place of the plug for

higher capacity requirements are

available.

This style of rotary control valve suits

erosive, coking and other hard-to-han￾dle fluids, providing either throttling or

on-off operation. The flanged or

flangeless valves feature streamlined

flow passages and rugged metal-trim

components for dependable service in

slurry applications. Mining, petroleum

refining, power, and pulp and paper

industries use these valves.

Control Valve End

Connections

The three common methods of instal￾ling control valves in pipelines are by

means of screwed pipe threads,

bolted gasketed flanges, and welded

end connections.

Screwed Pipe Threads

Screwed end connections, popular in

small control valves, offer more econ-

Chapter 3. Valve and Actuator Types

48

Figure 3-13. Popular Varieties of

Bolted Flange Connections

A7098/IL

omy than flanged ends. The threads

usually specified are tapered female

NPT (National Pipe Thread) on the

valve body. They form a metal-to-met￾al seal by wedging over the mating

male threads on the pipeline ends.

This connection style, usually limited

to valves not larger than 2-inch, is not

recommended for elevated tempera￾ture service. Valve maintenance might

be complicated by screwed end con￾nections if it is necessary to take the

body out of the pipeline because the

valve cannot be removed without

breaking a flanged joint or union con￾nection to permit unscrewing the valve

body from the pipeline.

Bolted Gasketed Flanges

Flanged end valves are easily re￾moved from the piping and are suit￾able for use through the range of

working pressures for which most

control valves are manufactured (fig￾ure 3-13). Flanged end connections

can be used in a temperature range

from absolute zero to approximately

1500F (815C). They are used on all

valve sizes. The most common

flanged end connections include flat

face, raised face, and ring type joint.

The flat face variety allows the match￾ing flanges to be in full face contact

with the gasket clamped between

them. This construction is commonly

used in low pressure, cast iron and

brass valves and minimizes flange

stresses caused by initial bolting-up

force.

The raised face flange features a cir￾cular raised face with inside diameter

the same as the valve opening and

with the outside diameter something

less than the bolt circle diameter. The

raised face is finished with concentric

circular grooves for good sealing and

resistance to gasket blowout. This

kind of flange is used with a variety of

gasket materials and flange materials

for pressures through the 6000 psig

(414 bar) pressure range and for tem￾peratures through 1500F (815C).

This style of flanging is normally stan￾dard on Class 250 cast iron bodies

and all steel and alloy steel bodies.

The ring-type joint flange looks like

the raised-face flange except that a

U-shaped groove is cut in the raised

face concentric with the valve open￾ing. The gasket consists of a metal

ring with either an elliptical or octago￾nal cross section. When the flange

bolts are tightened, the gasket is

wedged into the groove of the mating

flange and a tight seal is made. The

gasket is generally soft iron or Monel

(Trademark of Inco Alloys Internation￾al) but is available in almost any met￾al. This makes an excellent joint at

high pressure and is used up to

15,000 psig (1034 bar), but is general￾ly not used at high temperatures. It is

furnished only on steel and alloy valve

bodies when specified.

Welding End Connections

Welding ends on control valves are

leak tight at all pressures and temper￾atures and are economical in first cost

(figure 3-14). Welding end valves are

more difficult to take from the line and

are obviously limited to weldable ma￾terials. Welding ends come in two