Relief valves defined

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. 

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. 

When the relieving scenarios are defined, assume line sizes, and calculate pressure drop from the vent tip back to each relief valve to assure that the back-pressure is less than or equal to allowable for each scenario. The velocities in the relief piping should be limited to 500 ft/sec, on the high pressure system and 200 ft/sec on the low pressure system. Avoid sonic flow in the relief header because small calculation errors can lead to large pressure drop errors. Velocity at the vent or flare outlet should be between 500 ft/sec and MACH 1 to ensure good dispersion. Sonic velocity is acceptable at the vent tip and may be chosen to impose back-pressure on (he vent scrubber. 

In this chapter, we will define what is considered a potential overpressure scenario in process systems and where the safety relief valves (SRVs) are needed. 

API Standard 527, Seat Tightness of Pressure Relief Valves This standard describes tests with air, steam and water to determine the seat tightness of metal- and soft-seated PRVs. Valves of conventional, bellows- and pilot-operated designs are covered. Acceptable leakage rates are defined for gas, steam and liquid. 

Typical solutions to meet the 10% rule include installation of a bellows kit or a pilot-operated safety relief valve. Alternatively, piping modifications can be considered, or a more rigorous analysis can be performed to further define the risk of chattering. 

A control valve consists of a valve, an actuator, and usually one or more valve control devices. The valves discussed in this section are applicable to throttling control (i.e., where flow through the valve is regulated to any desired amount between maximum and minimum limits). Other valves such as check, isolation, and relief valves are addressed in the next subsection. As defined, control valves are automatic control devices that modify the fluid flow rate as specified by the controller. 

The American Society of Mechanical Engineers (ASME) understands the need for a sufficient volume of gas to properly test a safety relief valve. Until the ASME provides some well-defined guides, a concerned chemical plant may wish to develop its own criteria for... 

Occasionally, there may be business pressures or maintenance scheduling problems that would encourage the delay of prooftesting of safety critical alarms and shutdown systems. Such situations can also delay of vessel inspections and safety relief valve testing. Some type of variance procedure or review policy should be defined to handle this occasional need. Such a policy ought to require the review of all of the inspection and test records on the specific equipment involved as well as an approval of the superintendent of the area. 

The analyst must then define the various equipment failures and human failures that could lead up to that event. For the release of toxic gas from a reactor safety relief valve, the analyst may consider loss of cooling and the operator ignoring the high pressure alarm. Another path to this leak of toxics may be a double charge of one of the energetic reactants. 

The top event of an event tree could be a release of a toxic gas from a relief valve. The team leader writes the top event on a flip chart. Each member of the multi-disciplined team to focus that event. Each of the second level events are defined and added to the detailed tree by repeatedly asking, Why did this event occur Because the system pressure rose. The question Why Because the alarm was ignored during a busy time. This continues until the true system causes are identified, such as the culture is to use shortcuts or not enough operators available for a difficult startup. [4]... 

The analyst must then define the various equipment failures and human failures that could lead up to that event. For the release of toxic gas from a reactor safety relief valve. 

Flow-through relief valves have traditionally been analyzed by treating the valve as a convergent noz-2ie [i 29.30] discharge coefficient (kactual mass flow rate to the theoretical ideal mass flow rate through a one-dimensional convergent nozzle of exit area equal to the nominal bore of the tested valve. Measured discharge coefficients are derated by 10% (Aidr) to specify valve capacity. 

For air entering a valve at room temperature R = 284kJ/kg/K, 7 = 1.4, and iJ/ = 0.04042. However, real relief valves suffer from area contraction due to separated flow patterns, and the flow is neither one-dimensional nor isentropic. Consequently, the flow rate (m) will differ from that in an ideal duct having the same minimum section. A general form of discharge coefficient can then be defined by... 

Some coimtries have taken precautionary measures to avoid the fired BLEVE introducing specific rules of transport regulation. Canada and the USA allow the transport of flammable liquefied gases only in tank wagons with a thermal insulation and a pressure relief valve (PRV) (CGSB 2005, CFR 49). However, such protective measures are not compulsory in Europe, where no passive fire protection of LPG tankers is presently required by ADR and RID regulations (Directive 2006/89/EC, Directive 2006/90/EC) that define the standards required respectively for the road and rail LPG tankers. Moreover, the extent of risk reduction due to passive fire protections is rather imcertain, and a cost-benefit analysis is still lacking. 

In the present case, uncertainty about the failure rates of the pumps, block valves, check valves and relief valves is represented using the lognormal probability distributions defined in the probabilistic approach, whereas the possibihly distributions introduced in the possibihstic approach are used to represent the uncertainty about the failure rates for the explosive valves and the tank. 

As a part of control measure assessment it is necessary to demonstrate all control measures such as physical control (say barrier), engineering control (say process control, relief valve), and administrative control such as defined procedure, etc. During demonstration the operator needs to challenge the control measure to improvise the process. In this way, alternative controls could be taken into account, especially in those cases where all risks are not in SFARP. Helpful guidance toward both additional control measures and past disaster issues are highlighted in the following ... 

The first, 1914 edition of the ASME Code did basic things like defining the term maximum allowable working pressure, and defining the minimum allowable distance between rows of rivets. The 1914 Code stipulated that the maximum allowable pressure on cast iron boiler headers should not exceed 160 psig (about 10 atmospheres or 10 bar). Minimum capacities for boiler pressure relief valves were defined. 

Release zone an area in and immediately surrounding a hazardous substance release assumed to pose an immediate health risk to all persons, including first responders Reliability block diagrams diagrams that define the series dependence, or independence, of all functions of a system or functional group for each life-cycle event Relief valve a valve designed to release excess pressure within a system without damaging the system... 

As indicated by its name, it involves assessing layers of protection other than just the instrument protective functions. For instance, a contribution toward risk reduction by independent protective layers (IPLs) such as alarms and operators or basic process control is explicitly defined as a risk reduction factor. The combination of the risk reduction factors for all IPLs provides the total risk reduction possible. It is fundamentally a simplified quantitative method that considers the risk reduction contributed from each IPL typically by order of magnitude risk reduction (i.e., say 0.1 for a DCS, or 0.01 for a relief valve, etc.). 

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