CONTROL VALVE HANDBOOK Episode 2 Part 1 pdf

147

Chapter 6

Special Control Valves

As discussed in previous chapters,

standard control valves can handle a

wide range of control applications.

The range of standard applications

can be defined as being encom￾passed by: atmospheric pressure and

6000 psig (414 bar), −150
F (−101
C)

and 450
F (232
C), flow coefficient

Cv values of 1.0 and 25000, and the

limits imposed by common industrial

standards. Certainly, corrosiveness

and viscosity of the fluid, leakage

rates, and many other factors demand

consideration even for standard ap￾plications. Perhaps the need for care￾ful consideration of valve selection be￾comes more critical for applications

outside the standard limits mentioned

above.

This chapter discusses some special

applications and control valve modifi￾cations useful in controlling them, de￾signs and materials for severe ser￾vice, and test requirements useful for

control valves used in nuclear power

plant service.

High Capacity Control

Valves

Generally, globe-style valves larger

than 12-inch, ball valves over 24-inch,

and high performance butterfly valves

larger than 48-inch fall in the special

valve category. As valve sizes in￾crease arithmetically, static pressure

loads at shutoff increase geometrical￾ly. Consequently, shaft strength, bear￾ing loads, unbalance forces, and

available actuator thrust all become

more significant with increasing valve

size. Normally maximum allowable

pressure drop is reduced on large

valves to keep design and actuator

requirements within reasonable limits.

Even with lowered working pressure

ratings, the flow capacity of some

Chapter 6. Special Control Valves

148

Figure 6-1. Large Flow Valve Body for

Noise Attenuation Service

W6119/IL

large-flow valves remains tremen￾dous.

Noise levels must be carefully consid￾ered in all large-flow installations be￾cause sound pressure levels increase

in direct proportion to flow magnitude.

To keep valve-originated noise within

tolerable limits, large cast or fabri￾cated valve body designs (figure 6-1)

have been developed. These bodies,

normally cage-style construction, use

unusually long valve plug travel, a

great number of small flow openings

through the wall of the cage and an

expanded outlet line connection to

minimize noise output and reduce

fluid velocity.

Naturally, actuator requirements are

severe, and long-stroke, double acting

pneumatic pistons are typically speci￾fied for large-flow applications. The

physical size and weight of the valve

and actuator components complicate

installation and maintenance proce￾dures. Installation of the valve body

assembly into the pipeline and remov￾al and replacement of major trim parts

require heavy-duty hoists. Mainte￾nance personnel must follow the

manufacturers’ instruction manuals

closely to minimize risk of injury.

Low Flow Control Valves

Many applications exist in laboratories

and pilot plants in addition to the gen￾eral processing industries where con￾trol of extremely low flow rates is re￾quired. These applications are

commonly handled in one of two

ways. First, special trims are often

available in standard control valve

bodies. The special trim is typically

made up of a seat ring and valve plug

that have been designed and ma￾chined to very close tolerances to al￾low accurate control of very small

flows. These types of constructions

can often handle Cv’s as low as 0.03.

Using these special trims in standard

control valves provides economy by

reducing the need for spare parts in￾ventory for special valves and actua￾tors. Using this approach also makes

future flow expansions easy by simply

replacing the trim components in the

standard control valve body.

Control valves specifically designed

for very low flow rates (figure 6-2) also

handle these applications. These

valves often handle Cv’s as low as

0.000001. In addition to the very low

flows, these specialty control valves

are compact and light weight because

they are often used in laboratory envi￾ronments where very light schedule

piping/tubing is used. These types of

control valves are specially designed

for the accurate control of very low

flowing liquid or gaseous fluid applica￾tions.

High-Temperature Control

Valves

Control valves for service at tempera￾tures above 450°F (232°C) must be

designed and specified with the tem￾perature conditions in mind. At ele-