Specific measures for fire prevention of regulating valves
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2014-09-23
1. Fire prevention of valve body Although there are different opinions on the fire resistance of valves, different specifications, and different testing methods, there should be a relatively consistent view on how valves can be fireproof. For the valve body, it should have the following three performance points, and various specific measures should be taken to ensure these three performance points.
(1) Minimal internal leakage. In order to ensure this performance, the metal surface contact between the valve core and valve seat should be considered first. During or after a fire, the valve body is at high temperature. Regardless of the sealing structure, spring force, and external pressure changes, this should be ensured, which should be considered the key to ensuring precise closure.
(2) Minimal external leakage. To minimize external leakage, methods to consider include using fire-resistant valve stem sealing materials and avoiding the use of larger gasket type valve body connections.
(3) Having continuous operability. Valves that can still function normally after combustion naturally have the ability to resist deformation and damage.
In order to ensure the fireproof performance of the valve body, many manufacturers have made various attempts. For example, they wrapped multiple layers of felt covers on the valve and built a box with refractory materials to isolate the valve from the outside world. But the above methods are not very satisfactory because the valve needs to be dismantled and built every time it is repaired. Similarly, due to the limitations of the valve installation position, this method may not be feasible.
The current satisfactory method is to use a fireproof bag, which can be put on within a few minutes and is easy to remove during maintenance. The material of the bag contains multiple layers of ceramic fiber or fiberglass, which are tightly wrapped in nylon and tied to the device with stainless steel wire coated with vinyl resin. In general, there is no need to change the position of the pipeline, and the installation space is small. After burning in a flame at 2000 ℉ for 30 minutes through testing, the valve remains undamaged and the performance is satisfactory.
2. Fire prevention of the actuator Due to the fact that the actuator directly controls the position of the valve, it is even more necessary to study it carefully. In order to keep the valve closed in a timely manner, spring type pneumatic diaphragm steel (iron) actuators are often used in situations prone to ignition, mainly taking advantage of the low melting point of the diaphragm. When on fire, due to the low melting point of the diaphragm, it quickly damages, causing the spring to move and placing the valve in the closed position. The use of heat sensitive fusible hole plugs to reduce the pressure of pneumatic systems is particularly suitable for piston type reciprocating spring actuators.
Severe fires can alter the properties of metal parts, causing them to soften and lose their tempering properties, with some metals actually melting. The magnitude of spring force is directly related to the tempering characteristics. Whether the remaining spring force after combustion is sufficient to maintain the valve position requires combustion testing of the actuator to prove.
For butterfly valves, residual spring torque is often required after combustion to maintain the tightly closed position of the valve, and the force required to close seems to be much smaller than the original output force of the actuator. On the actuator of a spherical valve, residual spring force is important. Under the same pipeline pressure, balanced ball valves require less spring force than unbalanced ball valves. Therefore, when applied in situations prone to ignition, people are willing to use balanced ball valves.
To ensure that the actuator can still operate after combustion, it is necessary to protect the spring from the influence of flame annealing. There are three ways to protect springs: using insulation materials, using sprinklers, or using fire-resistant coatings. Including multiple layers of insulation, use insulation covers or fire resistant bags to ensure that the actuator operates normally for half an hour after a fire. However, this unique technique and sealing method are cumbersome and inconvenient, taking up a lot of space. Installing a sprinkler on the top of the actuator increases installation and maintenance costs; Moreover, due to frequent water shutdowns during fires, it is not possible to supply water to the sprinklers. The actuator can be coated with an expanded thin layer containing epoxy based substance to ensure normal operation of the actuator. In a combustion test, due to the protection of the coating, although the fire temperature reached as high as 1400-1700 ℉, the actuator was still able to operate for 42 minutes. The experiment used a 1600 pound spring cast iron actuator, approximately 7 feet long, with a cylinder diameter of 12 inches, and burned using 9 propane torches. During the entire combustion process, the spring of the actuator reciprocates once per minute. After combustion, the output torque of the spring only decreases by 6%, mainly due to a decrease in bearing accuracy and an increase in friction. The experiment shows that the spring is not affected by combustion flames, and the sealing between the piston and connecting rod remains as old, with good sealing. After completing the experiment, immerse the actuator in cold water. Simulate the sudden cooling effect of a high-temperature shell. Experiments have shown that the shell and internal parts of the actuator are not adversely affected by sudden cooling.
The expansion coating is applied, just like cement, and can be applied on-site. Be careful not to cover the sealing area, as it may cause inconvenience in maintenance. When the coating solidifies, a hard and impermeable sealing layer is formed. Jamesbury company has also manufactured plates and boxes in the main parts, which are coated separately, so that the plates can be removed during maintenance. If the actuator is an aluminum shell, even with paint, it is difficult to prevent fire because the melting point of aluminum is low (1033-1150 ℉). In a hydrocarbon mixed flame above this melting point, the parts will fail and the valve cannot be closed.
Finally, a method of connecting rod protection is also introduced. Reciprocating spring devices are supplied by various regulating valve manufacturers. These spring devices are in a "ready to strike" position and can only be triggered in case of fire. When triggered, the valve moves to an automatic safe position. There will also be holding components that can use fusible links, brittle links, or electric heating links. The electric heating connecting rod and the fusible connecting rod both have the same function as being able to start when heated. At a dangerous moment of fire, these connecting rods are immediately disconnected by the smoke follow-up device, placing the valve in the safe position
