Questions that arise and decisions that must be made with respect to relief devices include the following: Where must a relief device be installed? Which of the several available devices should be chosen? How many should be installed at a given position? To where should the device be relieved? What pressure setting should be chosen? What is the required discharge capacity of the device? If outlet vent piping is called for, what size should it be?
It is convenient first to categorize several types of devices, such as those shown in Fig. 13.7. There is a distinction between a pressure-limiting and a pressure-relief device. An example of a pressure-limiting device is the highpressure cutout, Fig. 13.7a, which should be installed at the outlet of a reciprocating or screw compressor and before the discharge stop valve. If the pressure rises too high, the device shuts off the compressor but does not vent refrigerant.
A fusible plug, as shown in Fig. 13.7b, is a fitting whose port is sealed by an insert composed of an alloy with a low melting temperature. When installed on a vessel in contact with liquid refrigerant, the fusible material melts when the liquid temperature (which is the saturation temperature corresponding to the pressure) rises too high. Fusible plugs may be appropriate for small refrigeration appliances, but are not used in industrial systems, because the entire charge is vented when the fusible plug melts. Furthermore, if no liquid is in the vessel for some reason, the fusible plug cannot sense a high pressure.
The most widely used pressure-relief devices are spring-loaded valves for vapor, as shown in Fig. 13.7c, and the relief valves for liquid, as shown in Fig. 13.7d. The latter are usually small and no special attention is devoted to
specifying their capacity, because relief of even a minute amount of liquid is sufficient to reduce the pressure.
Relief valves for vapor must be installed on all vessels, which includes shelland-tube condensers and evaporators. The connection on the vessel should be above the liquid level, because the discharge of liquid would be especially dangerous and furthermore would have little effect on reducing the pressure. No stop valves may be installed between the vessel and relief valve, with the exception of a three-way valve serving a dual relief valve, Figure 13.8. The three-way valve should be positioned so that only one of the relief valves is subject to the vessel pressure, as is shown in Figure 13.8, and not in the midposition, which would allow both valves to relieve at the same time. The reason for this practice is that often if a valve opens once or twice to relieve pressure it does not reseat perfectly. If only one valve fails to reseat perfectly, it can be removed and replaced while the other provides the safety function. The sizes of both valves are selected so that either can handle the total refrigerant flow by itself.
From a liquid relief valve in a section of pipe where liquid could be trapped by valves, the discharge is directed to a section of the system where another relief valve is located. Vapor relief valves often vent to the atmosphere, although it is also possible to relieve from a high-pressure vessel to a low-pressure vessel that vents to the atmosphere. One section of the plant eventually must be vented to the atmosphere, and the valve controlling this relief must be of a size that it can accommodate the total flow rate of the vessels served. The flow capacity of the valve is influenced by the size and length of the discharge line from the valve. The minimum diameter of the discharge pipe is that of the outlet fitting of the valve. If the discharge line is unusually long, it is advisable to choose a pipe diameter one size larger than the outlet fitting.
Each pressure vessel with a volume larger than 0.085 m3 (3 ft3) must be provided with a pressure-relief valve, and vessels larger than 0.283 m3 (10 ft3) must be equipped with two parallel valves, as in Fig. 13.8. Some designers and plant operators always use double and never single relief valves. With a single valve there is no way to remove it for servicing. Each of these valves must have a venting capacity that prevents the pressure from rising more than 10% above the setting of the relief valve.