Specific measures for regulating valve fire prevention
The control valve is the final control element that changes the process parameters such as medium flow, pressure, temperature, liquid level by means of power operation by accepting the control signal output by the adjustment control unit, also known as the control valve. The fire prevention measures of the regulating valve are described below through the fire prevention of the actuator and the fire prevention of the valve body:
Actuator fire protection
Since the actuator directly controls the position of the valve, more careful study is required. In order to keep the valve closed in time, steel (iron) actuators with spring-type pneumatic diaphragms are often used in situations that are prone to fire, mainly because of the low melting point of the diaphragm. In the event of a fire, the diaphragm is quickly damaged due to its low melting point, so the spring moves to hold the valve in the closed position. Use heat-sensitive fusible hole plugs to reduce pressure in pneumatic systems, especially for piston-type reciprocating spring actuators.
Severe fires can alter the properties of metal parts, causing the metal to soften and lose its tempering properties, and some metals actually melt. The size of the spring force is directly related to the tempering characteristics. Whether the residual spring force after combustion is sufficient to maintain the valve position needs to be proved by the combustion test of the actuator.
For butterfly valves, residual spring torque is often required to maintain the valve’s tightly closed position after combustion, and the force required for closing seems to be far less than the original output force of the actuator. On ball valve actuators, the residual spring force is important. Under the same pipeline pressure, the spring force required by the balanced ball valve is smaller than that of the unbalanced ball valve, so people are willing to use the balanced ball valve when it is used in the occasion where it is easy to catch fire.
For the actuator to operate after burning, the spring must be protected from flame annealing. The springs can be protected in three ways: with thermal insulation, with sprinklers or with a fire retardant coating. Including the upper multi-layer insulation belt, use the insulation cover or use the fireproof bag to make the actuator operate normally for half an hour after the fire. But this kind of trick and sealing method is cumbersome and inconvenient, and takes up a lot of space. Putting a sprinkler on top of the actuator increases installation and maintenance costs; and because of frequent water outages during fires, water cannot be supplied to the sprinkler. The actuator can be coated with a thin, intumescent layer of an epoxy-based substance that allows the actuator to function properly. In a fire test, the actuator was able to work for 42 minutes despite the fire temperature as high as 1400-1700°F due to the protection of the paint. The test used a 1600-pound spring-loaded cast iron actuator, approximately 7 feet long, with a 12-inch cylinder bore, fired with nine propane torches. During the entire combustion process, the spring of the actuator reciprocates once per minute, and the output torque of the spring is only reduced by 6% after combustion, which is mainly caused by the decrease in bearing accuracy and the increase in friction. The test shows that the spring is not affected by the combustion flame, and the seal between the piston and the connecting rod is still the same as before, and the seal is good. After completing the test, immerse the actuator in cold water. Simulate the sudden cooling effect of a high temperature enclosure. Tests have shown that the housing and internal parts of the actuator are not adversely affected by sudden cooling.
The expansion coating is smeared on, just like plastering cement. It can be done on site when plastering. Be careful not to wipe the sealing part, which will make maintenance inconvenient. When the coating dries, it forms a hard, impermeable seal. Jamesbury also made the main parts of the plate and the box, and painted them separately, so that the plate could be removed for maintenance. If the actuator is an aluminum housing, it is difficult to fire even with paint, because aluminum has a low melting point (1033 to 1150 °F), and in a hydrocarbon flame above this melting point, the part will fail and the valve will not close.
A method of connecting rod protection is also introduced. Reciprocating spring assemblies are supplied by various regulator valve manufacturers. These spring assemblies are in a “standby” position that fires only when on fire, and the valve moves to a self-assured position when fired. It will also be possible to use fusible links, brittle links or electrically heated links for the holding element. The electric heating connecting rod and the fusible connecting rod have the same function, and can be started by heat. At the risk of fire, these connecting rods are immediately disconnected by the flue gas follow-up device, so that the valve is in a safe position.Fire protection of valve body
Although there are different views on the fire resistance of valves, different specifications and different test methods, there should be a more consistent view on how valves can be fireproof. For the valve body, it should have the following three performances, and various specific measures should be taken to ensure these three performances.
(1) The amount of internal leakage is small. In order to ensure this performance, the contact between the metal surface of the valve core and the valve seat should be considered first. During or after the fire, the valve body is at a high temperature, no matter what its sealing structure is, how the spring force and the applied pressure change. This should be guaranteed and should be considered the key to ensuring its precise closure.
(2) The amount of external leakage is small. In order to minimize external leakage, the methods considered are: use a fireproof valve stem sealing material and avoid connecting with a larger gasket type valve body.
(3) There is continuous operability. A valve that can still work normally after combustion naturally has the ability to resist deformation and damage.
In order to ensure that the valve body has fire resistance, many manufacturers have made various attempts. For example, they wrapped a multi-layer felt cover on the valve and built the box with refractory materials to isolate the valve from the outside world. However, the above methods are not satisfactory, because the valve must be disassembled and built every time it is repaired. At the same time, due to the limitation of valve installation position, this method may not be available.
A more satisfactory method at present is to use a fireproof bag, which can be put on within a few minutes and is easy to open for maintenance. The bag material contains multiple layers of ceramic fiber or fiberglass wrapped tightly with nylon and tied to the device with vinyl-coated stainless steel wire. In general questions, there is no need to change the position of the pipeline, the installation space is small, and the valve is still not damaged and the performance is satisfactory after burning in a flame of 2000°F for 30 minutes.
What I have shared with you above are the specific measures for regulating valve fire prevention. I have shared with you the “Standards of Fire Dampers” andThe Importance of Fire Protection for Control Valves.