Typical relief valves

Figure 40.20 shows a typical relief valve. System pressure simply acts under the valve disk at the inlet of the valve. When the system pressure exceeds the pre-load force exerted by the valve spring, the valve disk will lift off of its seat. This will allow some of the system fluid to escape through the valve outlet. Flow will continue until the system pressure is reduced to a level below the spring force. 

Safety Relief Valves Conventional safety relier valves (Fig. 26-14) are used in systems where built-up backpressures typically do not exceed 10 percent of the set pressure. The spring setting or the valve is reduced by the amount of superimposed backpressure expecied. Higher built-up backpressures can result in a complete loss of continuous valve capacity. The designer must examine the effects of other relieving devices connected to a common header on the performance of each valve. Some mechanical considerations of conventional relief valves are presented in the ASME code however, the manufacturer should be consulted for specific details. 

FIG. 26 13 Typical pressure relief system configurations (a) rupture disk system (h) pressure relief valve system. 

Figure 2. Typical conventional safety relief valve.

As normally designed, vapor flow through a typical high-lift safety reliefs valve is characterized by limiting sonic velocity and critical flow pressure conditions at the orifice (nozzle throat), and for a given orifice size and gas composition, mass flow is directly proportional to the absolute upstream pressure. 

Figure 4. Typical balanced bellows safety relief valve.

The soft-seated spring-loaded pilot valve is so constructed as to have a long built-in blowdown. For a flowing type pilot, at the point where the pilot supply line feeds the system pressure to the pilot relief valve, it passes through a variable orifice, which is also the main valve blowdown adjustment. When the pilot opens, the flow through the supply line causes an immediate pressure drop across the orifice. By adjusting the size of the orifice and thus the amount of pressure drop across it, one can obtain any desired system blowdown (5 to 7% is typical). 

Table 4 provides an example of a typical Pressure Relief Valve specification sheet. The following notes indicate the basis for the times which are required in the Design Specification. 

A distinction must be made regarding the length of service of the pressure reducing systems. Fatigue failure of any mechanical system depends on time, i.e., the number of cycles to failure. Therefore, the treatment required for a continuous service may not be justified for a short term service. A System in short term service is defined as one which operates a total of 12 hours or less during the life of the plant. Pressure relief valves typically meet this limit. Systems in short term service exceeding the screening criteria indicated above should be evaluated. 

Pilot-operated relief valves use the pressure in the vessel rather than a spring to seal the valve and a pilot to activate the mechanism. Figure 13-6 is a schematic of a typical pilot-operated valve. A piece of tubing communicates pressure between the relief valve inlet and pilot. When this pressure is below the set pressure of the pilot, the pilot valve is in the position... 

Often a system (a group of vessels not capable of being isolated from each other by block valves, or containing restriction to flow and release of pressure) may need a relief valve set reasonably close, sat +15% to 20% when system is below 1000 psig above, typically use 7% to 15% above as set criteria related to normal operating pressure to catch any pressure upswing. Then this may have a backup valve set higher (but within code) to handle further pressure increase. Or, the second device may be a rupture disk. It is not unusual to have two relief de ices on the same equipment set at different pressures. 

Typical pressure relief valve without a stop valve... 

Typical pressure relief valve mounted on process line... 

Typical pilot-operated pressure relief valve installation... 

Typical rupture disk assembly installed in combination with a pressure relief valve... 

Typical installation avoiding process laterals connected to pressure relief valve inlet piping... 

Typical installation avoiding excessive turbulence at pressure relief valve inlet. 

For flow tested combinations, see a few typical data in Table 7-12. Note, for example, that using a Continental disk reverse acting knife blade rupture disc with a Crosby JOS/JBS pressure relief valve that the combined effect is to multiply the rated capacity of the Crosby valve by a multiplier of 0.985 for a set pressure in the 60-74 psig range... 

Data regarding relief valves, feed and expansion cisterns, etc. are given in Tables 27.11 and 27.12. Cistern sizes shown in Table 27.12 are based on typical system designs and are approximate only. An estimate of the water content of the particular system should always be made where there is any doubt regarding these typical data, to ensure that the cistern capacity is adequate to contain the expansion volume. 

As with the case of mass, there are several approaches to metrics for this aspect. One can simply sum numbers and/or mass of chemicals possessing hazards in different areas for example, process safety, occupational exposure, or environmental hazard. Typically, most companies will use a banding approach for materials that allows a quick identification of the hazard category, and usually marries hazard with a suggested control approach for example, layers of protection, pressure relief valves, and so on. One is then able to rapidly identify issues and potential opportunities for elimination, substitution, or control. 

The networks that interconnect various process units and vessels to the discharge zones or flares occur widely in refineries and chemical plants. Figure 11 shows a typical configuration in which the root represents the flare, the terminal vertices represent the relief valves, and the edge (each labeled with an arabic numeral) represents a pipe section between two physical junctions (valves, flare, or pipe joints). The configuration of such a network is dictated by the layout of the process unit. In this discussion both the lengths of the pipe sections and the interconnections will be treated as specified variables. 

Rupture discs are available in much larger sizes than spring-operated relief valves, with commercial sizes available up to several feet in diameter. Rupture discs typically cost less than equivalently sized spring-operated relief valves. 

Carbon dioxide is usually purchased in a tank, inside the tank the mobile phase exists as a liquid. Typically, the tank does not come with a pressure gauge but is hooked up to a pressure relief valve and rupture disk, which are set above the tank pressure should a tank leak occur. 

Air emissions include point and nonpoint sources (Chapter 4). Point sources are emissions that exit stacks and flares and thus can be monitored and treated. Nonpoint sources are fugitive emissions that are difficult to locate and capture. Fugitive emissions occur throughout refineries and arise from the thousands of valves, pumps, tanks, pressure relief valves, flanges, and so on. Although individual leaks are typically small, the sum of all fugitive leaks at a refinery can be one of its largest emission sources. 

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