Fire Safe Treatment

The concern for valve and actuator performance during a fire has brought about many interesting valve designs. The interest for fire safe valves is highest in petroleum and chemical plants. However, power plants, gas or oil fired, must also be concerned with fire safe treatment of valves.

Everywhere fire-caused valve failure could cause increased damage or jeopardize life, the engineer must analyze valve action and specify safe shutdown or opening of process lines.

Fire safety extends beyond the valve itself; the actuator may have to resist fire damage for at least one cycle even if it cannot be made as durable as the valve. Then to, a single actuation may not be sufficient. It is conceivable that a valve may have to be actuated several times during or directly after a fire.

Actuators have been designed to survive a certain amount of fire resistance. Of course, this assumes that the external control system is also fire resistant, which involves air lines and coincident wiring, small valves and relays. Routine inspection is necessary if the enclosure is to be ready at all times.

Fire safe valves are designed to service hazardous or flammable fluids, gases and other critical media.

One fire safe butterfly design (Figure 10-44) features triple duty sealing. There is a resilient insert for bubble tight sealing under normal conditions. This metal-to-metal sealing provides positive shutoff during and after a fire, even with partial soft-seal destruction. The temperature range is -100F to 450F with pressures to 1480 psi; shutoff bubble tight per MSS SP-61.

Spring-driven actuators, triggered by fusible-link arrangements at the first indication of a fire, are available to actuate quarter turn valves. Floating-ball valves with TFE seat rings are now common in fire safe designs.

Valves to be qualified as "fire-safe" must be subjected to rigorous tests. These tests vary in methodology and intensity. They take into account such matters as: how a fire begins and progresses in a particular service, what the particular process fluid is, what the temperature is during the critical part of the fire, how tight the valve should seal and what the post-fire capability of the valve must be.

Fire Safe Pneumatic Actuators with Fusible Elements

In high temperature or fire safe applications standard construction spring-return actuators may be used to provide one spring stroke if the following are considered:

1. Maximum operating temperature for one stroke.
2. Maximum operating temperature of actuator.
3. Duration of the presence of maximum temperature before spring return action must occur.

In applications where a standard pneumatic actuator is adequate to meet the above requirements, the actuator may be fitted with a fusible plug to respond to a fire. The fusible plug, like the fusible link, is a zinc and lead blend that melts at a specific temperature. A tee fitting is installed on the actuator pressure inlet port and the branch of the tee is fitted with a fusible plug. When exposed to a fire, the plug melts and represents a break in the air-supply line which exhausts air pressure from the actuator and provides for the necessary spring action. If the temperature at which the air-supply line break is to occur is not critical, some users elect to install the air system using polypropylene tubing which melts when exposed to fire.

For simple, fail safe action, each of the previously discussed systems can meet some or all of the requirements for some fire-safe systems.

In some applications, where it may be necessary to cycle a valve during or after exposure to a catastrophic fire condition Actuator manufacturers offer two different systems for use:

1. Pneumatic actuators constructed of materials and seals that can withstand a fire.
2. A protection system that prevents the actuator from reaching a temperature where seal failure or component distortion may occur.