Pressure-relief devices

Compressed gas cylinders should be secured firmly at all times. A clamp and belt or chain, securing the cylinder between waist and shoulder to a wall, are generally suitable for this purpose. In areas of seismic activity, gas cylinders should be secured both toward the top and toward the bottom. Cylinders should be individually secured using a single restraint strap or chain around a number of cylinders is often not effective. Pressure-relief devices protecting equipment that is attached to cylinders of flammable, toxic, or otherwise hazardous gases should be vented to a safe place. 

Cylinders should be placed so that the rotary cylinder valve handle at the top is accessible at all times. Cylinder valves should be opened slowly, and only when a proper regulator is firmly in place and the attachment has been shown to be leak-proof by an appropriate test (see Chapter 5, section 5.H). The cylinder valve should be closed as soon as the necessary amount of gas has been released. Valves should be either completely open or completely closed. Flow restrictors should be installed on gas cylinders to minimize the chance of excessive flows. The cylinder valve should never be left open when the equipment is not in use. This precaution is necessary not only for safety when the cylinder is under pressure, but also to prevent the corrosion and contamination that would result from diffusion of air and moisture into the cylinder after it has been emptied. 

Most cylinders are equipped with hand-wheel valves. Those that are not should have a spindle key on the valve spindle or stem while the cylinda- is in service. Only wrenches or other tools provided by the cylinder supplier should be used to remove a cylinder cap or to open a valve. In no case should a screwdriver be used to pry off a stuck cap or should pliers be used to open a cylinder valve. Some valve fittings require washers or gaskets, and the materials of construction should be checked before the regulator is fitted. 

The general procedures discussed in Chapter 5, section 5.C, can be used for relatively minor leaks, when 

If a leak at the cylinder valve handle cannot be remedied by tightening a valve gland or a packing nut, emergency action should be taken and the supplier should be notified. Laboratory workers should never attempt to repair a leak at the junction of the cylinder valve and the cylinder or at the safety device rather, they should consult with the supplier 

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For most processes, the optimum operating point is determined by a constraint. The constraint might be a product specification (a product stream can contain no more than 2 percent ethane) violation of this constraint causes off-specification product. The constraint might be an equipment hmit (vessel pressure rating is 300 psig) violation of this constraint causes the equipment protection mechanism (pressure relief device) to activate. As the penalties are serious, violation of such constraints must be very infrequent. 

Pressure-relief-device requirements are defined in Subsec. A. Set point and maximum pressure during relief are defined according to the service, the cause of overpressure, and the number of relief devices. Safety, safety relief, relief valves, rupture disk, breaking pin, and rules on tolerances for the reheving point are given. 

Testing, certification, and installation rules for reheving devices are extensive. Eveiy chemical engineer responsible for the design or operation of process units should become veiy familiar with these rules. The pressure-relief-device paragraphs are the only parts of Sec. TII, Division I, that are concerned with the installation and ongoing operation of the facility all other rules apply only to the design and manufacture of the vessel. 

Safety Devices Pressure relief devices, flame arresters, and methods for handhng effluent from controlled releases provide control of accidental undesirable events. Special equipment should be considered for highly toxic chemical service. The following matters are considered ... 

Pressure Relief Devices The most common method of overpressure protection is through the use of safety rehef valves and/or rupture disks which discharge into a containment vessel, a disposal system, or directly to the atmosphere (Fig. 26-13). Table 26-8 summarizes some of the device characteristics and the advantages. 

When the pressure relief device is set to open at greater than 15 psig (critical flow will result), it is normally not uecessaiy to be concerned about the pressure drop in the separator. If the hquid is to be drained from the separator during the emergency blowdown, a vortex breaker and false bottom should be used (Fig. 26-18, view BB). 

Provide deflagration pressure relief device/system... 

Provide pressure control regulator and pressure relief device... 

Emergency Relief Device A device that is designed to open during emergency or abnormal conditions to prevent rise of internal fluid pressure in excess of a specified value. The device also may be designed to prevent excessive internal vacuum. The device may be a pressure relief valve, a nonreclosing pressure relief device, or a vacuum relief valve. 

While the above permitted variations are not used for selecting the pressure setting of pressure relief devices, these variations may be used for piping considerations such as startup, shutdown and emergency conditions, provided that the pressure vessel limits are not exceeded and that such variations are permitted by local codes. 

Selection of Pressure Relief Device - From the range of available pressure rehef valves and other devices, selection is made of the appropriate type for each item of equipment subject to overpressure. Instrumentation, check valves, and similar devices are generally not acceptable as means of overpressure protection. 

The ASTM fired pressure vessels code requires pressure-relief devices to prevent pressures from rising more than 6% above the maximum allowable working pressure. 

Except for special sihiations, pressure relief devices are not provided for fire exposure of heat exchangers, air fins, or piping, nor are the exposed surfaces of such items included for calculating the fire exposure heat input. Special situations may be congestion and substandard spacing, or unusually large equipment with normal liquid holdup over about 4 m and/or which represents over 15% of the total wetted surface of the system to which it is directly connected for pressure relief. 

This section describes the various pressure relief devices that are commonly used, with their characteristics and criteria for selection. Basic calculation procedures for sizing PR valves are covered in subsequent discussions. 

The pressure relief device used most often in refinery and chemical plant equipment is the spring-loaded, top-guided, high-lift, nozzle-type safety relief valve, which is illustrated in Figure 2. The spring is usually external and enclosed by a bonnet for weather protection, and the bonnet chamber is vented through an internal passage to the valve outlet. 

A rupture disc device is a non-reclosing pressure relief device actuated by inlet static pressure and designed to function by the bursting of a pressure containing disc. 

Superimposed back pressure The static pressure existing at the outlet of a pressure relief device at the time the device is required to operate. It is the result of pressure in the discharge system from other sources. 

In accidental releases, pressure within a vessel at time of failure is not always known. However, depending on the cause of vessel failure, an estimate of its pressure can be made. If failure is initiated by a rise in initial pressure in combination with a malfunctioning or inadequately designed pressure-relief device, the pressure at rupture will equal the vessel s failure pressure, which is usually the maximum allowable working pressure times a safety factor. For initial calculations, a usual safety factor of four can be applied for vessels made of carbon steel, although higher values are possible. (The higher the failure pressure, the more severe the effects.)... 

When pressure relief devices are intended primarily for protection against overpressure due to external fire or heat, have no permanent supply connection, and are used for storage at ambient temperature of non-refrigerated liquefied compressed gases, they are excluded from requirements of Par. UG-125c (1) and C (2), with specific provisions. See ASME code [1] for detailed references and conditions. 

The capacity of the combination of the rupture disk device and the spring loaded safety or safety relief v alve may be established in accordance with the appropriate paragraphs of UG-132, Certification of Capacity of Safety Relief Valves in Combination with Non-reclosing Pressure Relief Devices. 

Never place a block valve on the discharge side of a pressure relief device of any kind, except see [1] Par. U-135(e). 

Donat, C., Selecting and Dimensioning of Pressure Relief Devices for Dust Explosions, Staub-Reinhaltung der Luft, 31, No. 4, 1971, pp. 17-25. 

Terminolog of Pressure Relief Devices, /American National Standards Institute (ANSI) No. B95, 1 (latest ed.). 

Where, of necessity, the vent pipe rises to a considerable height, excessive internal pressure on the tank may result, due to the pressure head of oil should an overflow occur. To prevent any possible tank failure due to such an occurrence a vent pipe pressure-relief device must be provided. It should be self-draining to reduce the risk of blockage, particularly when using heavy oil fuel. Codes require that these devices should not place any restriction on oil flow and must discharge within the bund area. 

The compressor intake air must be clean and free from solid and gaseous impurities abrasive dust and corrosive gases are particularly harmful. Exhaust fumes present a hazard if compressed air is required for breathing purposes. The possibility of contamination of the intake by discharge from pressure-relief devices of other plant must be taken into consideration and changes of wind direction must not be overlooked. 

Installation requirements for rotary positive-displacement compressors are similar to any rotating machine. Review the installation requirements for centrifugal pumps and compressors for foundation, pressure-relief, and other requirements. As with centrifugal compressors, rotary positive-displacement compressors must be fitted with pressure-relief devices to limit the discharge or inter-stage pressures to a safe maximum for the equipment served. 

All reciprocating compressors must be fitted with pressure relief devices to limit the discharge or inter-stage pressures to a safe maximum for the equipment served. 

Two types of pressure relief devices are available, safety valves and relief valves. Although these terms are often used interchangeably, there is a difference between the two. Safety valves are used with gases. The disk overhangs the seat to offer additional thrust area after the initial opening. This fully opens the valve immediately, giving maximum relief capacity. These are often called pop-off safety valves. 

Receivers are pressure vessels covered by the provisions of BS.4434 1980 and require safety pressure relief devices as outlined in Section 6.10. In cases where there is no shut-off valve between the condenser and receiver, such protection may be fitted to one or the other, providing the total volume is considered. 

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