Page 217 - Valve Selection Handbook

204 Valve Selection Handbook

Modulating pilots are designed to open and close in increments
according to demand so that mass flow through the main valve is the
same as at the pressure source into the system. Such pilots introduce the
full system pressure to the dome until the dome pressure commences to
decay at a system pressure of about 95% of the set pressure. When the set
pressure has been reached, the valve commences to open and can be fully
open at an overpressure of about 105% of the set pressure. On receding
overpressure, the closing motion is in reverse.

Balanced and unbalanced pilots. Like spring-loaded pressure relief
valve, pilots may be back pressure balanced or unbalanced.
In the case of balanced pilots, the dome volume may be vented direct-
ly to the main valve outlet as long as the back pressure does not exceed
the system pressure.
In the case of unbalanced pilots, however, the dome must be vented to
the atmosphere at the pilot or to a preferred location to ensure proper
functioning of the pilot. Within this one constraint, the set pressure of
pilot-operated pressure relief valves is unaffected by back pressure in the
main valve outlet unless the situation arises in which the back pressure
exceeds the inlet pressure. To prevent reverse flow occuring in this case,
manufacturers provide the pilot on request with an appropriate check
valve arrangement, referred to as backflow preventer.

Types of main valves. Main valves are commonly classified by type as
diaphragm type, bellows type or piston type, depending on the type of
moving member that opens and closes the valve.
Figure 5-37 depicts a diaphragm type and Figure 5-39 a piston type
main valve in conjunction with a pilot. Diaphragm type main valves are
used on low pressure services only. Should the valve discharge into a
vessel in which unforeseen unwanted vacuum could develop, the
diaphragm would lift off its seat and protect the vessel from collapsing.

Direction of flow through main valves. The direction of flow through
main valves is commonly with the fluid flow acting on the underside of
the closure member, as shown in Figure 5-35.
The valve shown in Figure 5-41 deviates from this practice by revers-
ing the direction of flow through the valve. By this flow reversal, piston
and guide are located in the pressure zone of the valve. This location of
piston and guide has specific advantages in high pressure and high tem-
perature applications. First, piston and guide are at the temperature of the