Safety pressure relief devices

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. 

C-14 Procedures for Testing of DOT Cylinder Pressure Relief Device Systems. Describes a new set of test procedures and apparatus for fire testing compressed gas cylinder safety (pressure) relief devices as required by DOT regulation, 49 CFR, Section 173.34(d). The procedures are applicable for cylinders which are less than 500 lbs. internal water volume and are designed to provide a means of testing to DOT requirements anywhere, with reliable test data and repeatable test results. Previous edition cited in 49 CFR (13 pages). 

There are four basic types of safety pressure relief devices in use. 

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. 

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. 

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. 

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.)... 

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. 

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. 

The term engineered safety covers the provision in the design of control systems, alarms, trips, pressure-relief devices, automatic shut-down systems, duplication of key equipment services and fire-fighting equipment, sprinkler systems and blast walls, to contain any fire or explosion. 

Pressure relief valve (PRV) A pressure relief device designed to open and relieve excess pressure and to reclose after normal conditions have been restored. PRV is a generic term applied to relief valve (set up for liquid flow), safety valve (set up for gas or vapor flow), and safety relief valve (set up for either liquid or compressible flow). 

Gravity Separator Safety Considerations and Features A pressure relief device may be required on vapor-liquid gravity separators, based on the following assumptions ... 

Within this Workbook, the maximum pressure required to fully open the pressure relief device will be referred to as the "relief pressure". (Caution some papers on relief sizing refer to "set pressure" but mean "relief pressure"). For a bursting disc, the relief pressure will be the maximum specified bursting pressure and for a safety valve, it will be the set pressure plus 10% overpressure (or whatever percentage overpressure the valve has been certified at). 

The relief pressure is that at which the pressure relief device is fully open. Safety valves typically require 10% overpressure to achieve this (N.B. this should be checked for each specific application). Thus ... 

Although many pressure relief devices are called SRVs, not every SRV has the same characteristics or operational precision. Only the choice of the correct pressure safety device for the right application will assure the safety of the system and allow the user to maximize process output and minimize downtime for maintenance purposes. Making the correct choice also means avoiding interference between the process instrumentation set points in the control loop and the pressure relief device limits selected. These SRV operational limits can vary greatly even when all are complying with the codes. 

Generally spoken, the use of non-redosing pressure rdief devices will offer, in most cases, the lower cost solution but requires that the process is shut down or redirected through alternative safety systems to allow for replacement of the burst device. Subsequendy non-redosing pressure relief devices will only be selected as primary relief solutions in cases where loss of process media or shutdown for repair is tolerated or possible (see Section 5.4). 

Reclosing pressure relief devices have a variety of names, although there used to be a clear definition based on the US market and API. However, when the Pressure Equipment Directive (PED) came into effect in 1997, this somewhat added to the confusion, as PED uses the overall term safety valve for every pressure-relieving device subject to the PED code. Originally the following were the definitions for the different terms per API and are still in use today. 

Power actuated/assisted safety valves (CSPRS - controlled safety pressure relief system) A spring-operated safety valve actuated or assisted by an externally powered control device which can be hydraulic, pneumatic or electric (Figure 3.15). 

CEN - European Committee for Standardization Brussels wvwv.cenorm.be EN/ISO 4126- Safety Devices for Protection Against Excessive Pressure Part 1 - Safety Valves Part 2 - Bursting Disc Safety Devices Part 3 - Safety Valves and Bursting Disc Safety Devices in Combination Part 4 - Pilot Operated Safety Valves Part 5 - Controlled Safety Pressure Relief Systems Part 6 - Application, Selection and Installation of Bursting Disc Safety Devices Part 7 - Common Data... 

Where hazardous conditions can develop within a process, a protective system of some type must be provided. Sometimes this is in the form of process hardware such as pressure relief devices. However, sometimes logic must be provided for the specific purpose of taking the process to a state where the hazardous condition cannot exist. The term safety interlock system is normally used to designate such logic. 

Alternative energy processes, just like any others, need to be protected from excessively high or low pressures. The methods of protection include pressure regulation, alarm, or safety interlock actuation when preset pressure limits are violated, and providing pressure relief devices, which need to be replaced after each operation (rupture disks) or can automatically reclose (relief valves). The features and characteristics of these devices are discussed in the following subsections. 

The District Engineering Inspectors Report confirmed that the machine was completely isolated and the ammonia system was not protected by a safety valve or any type of pressure-relief device. The cause of the accident was the presence of the steam hose underneath the equipment to melt the ice on the drain valve or to help evacuate the system. The inspectors report could not determine if these preparations were intentional or accidental. [5]... 

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