Where escape of the working fluid to atmosphere is undesirable for environmental, safety or economic reasons. The stem seal needs particular consideration in such cases and specially-engineered arrangements are frequently employed. For smaller sizes (up to 6” size) a bellow seal may be used to prevent any leakage of the fluid and a secondary stem seal in the form of a packed gland or o-ring is often provided in case of bellows failure. Bellow seals are most often used in rising stem design (gate, globe) but are also available for quarter turn valves (ball, butterfly , etc.).
Valves for cryogenic applications (below -50°C) should comply with BS 6364. They are normally equipped with extended bonnets, which allows a reasonable temperature gradient up to the gland and point of operation. It is also allows a liquefied gas to reach a vaporizing temperature. Normal leakages in other applications are not accepted for cryogenic service because leaking fluid could freeze and affect the plant. Seats and seals are often selected in KEL-F and soft seated valves require facilities for relieving overpressure caused by thermal expansions temperature rise.
Gate, Globe and Check valves shall be selected according to body and trim materials referred to the pressure class.
Ball valves shall be selected with more accuracy for materials and sealing.
And often are equipped with extended stem to dissipare the high temperature.
According to the temperature degree (from 200°C to 1.000°C) body and sealing will be proposed in accordance with process conditions.
Soft Seated valves may be specified, including high performance butterfly valves and for extremely high vacuum, metal-to-metal closing mechanism are likely to be required. Bellow stem seals may be specified and secondary packing should be required.
Valve packing must be suitable for 25 mm Hg absolute pressure.
They are used on firewater deluge service.
They open virtually instantaneously whereas a process control valve can require several seconds to operate. They fail safe despite damage to the pneumatic detection or actuation system.
Excess flow rate are designed to close automatically when the flow through them exceeds a specific rate. They must be installed where the fluid leakage through a ruptured line would cause serious damage and as close as possible to the equipment to be protected.
Standard types of valves intended for ESD service must achieve the highest degree of reliability and integrity.
The actuators are usually fail-safe hydraulic or pneumatic type and affect the valve’s ability to perform its shut-off function.
The main requirement is that the valve can be relied upon to close when called to do so under any likely conditions of operation.
To assure it, new generation of special "Partial Stroke" devices are applied on the actuator in order to test the valve functionality at specified periods.
Double Block and Bleed valves assure double isolation thanks to two obturators inserted in a single body and they provide the bleed function by means of a bleed or vent system located between the two obturators (ball, gate or needle). Different configurations are available, floating or trunnion mounted, full or reduced bore, with main advantages as: