Pressure Relief System Considerations

In the revamp project, the maximum allowable working pressure (MAWP) of existing equipment may have to be either increased or decreased. Similarly, some new pressure 

During retrofit, it is often required to increase the existing centrifugal pump capacity and/or head. Pump capacity can be increased by impeller replacement (if the pump casing has allowance for a bigger impeller), pump replacement or installation of more pumps in parallel. Pump head can be increased by impeller, pump replacement or installation of more pumps in series. If the pump head and/or capacity are increased, its downstream piping and equipment shall be reviewed for over-pressure protection. 

New on-off valves closing speeds shall be reviewed and adjusted to prevent hydraulic shock waves or water hammer. Also, piping velocity shall be checked so that it is well within erosion velocity. The possibility of multi-phase mixtures inside pipes shall be evaluated carefully to apply proper hydraulic equations and estimate adequate pipe sizing. 

New process equipment installed within 7.6 meters from the grade/ground level will require pressure relief devices due to fire scenario. For revamp cases, it may be advantageous to place vessels on platforms at 7.6 meters if fire case relief load is most credible and bottlenecks the existing flare system. Fire circle or zone is defined as the maximum affected area during any equipment fire in the facility. API 521 (2014) defines its area as 230 to 460 m. Addition of process equipment inside an existing fire circle may increase the fire circle size. Hence, care shall be taken to review the fire circle size with each equipment addition. It will impact the peak relief load during the fire scenario. 

In some process units, loss of process control can result in a significant change in temperature and/or pressure, which can result in exceeding the design limits of the materials 

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A-2.11.1 Storage Vessel Failure. The release of GH2 or LH2 may result in ignition and combustion, causing fires and explosions. Damage may extend over considerably wider areas than the storage locations because of hydrogen cloud movement. Vessel failure may be started by material failure, excessive pressure caused by heat leak, or failure of the pressure-relief system. 

Besides the safety aspect another consideration is that about 30% of process industry losses can be at least partially attributed to deficient pressure relief systems. These losses alone could subsidize a closer look at the pressure relief systems in a process plant or a better upfront selection of the safety systems. 

A pressure relief system is normally required on all bioreactors and pressurised tanks as a safety feature to comply with pressure vessel design regulations. In a few cases, some companies appear to have overcome the need to use a pressure relief system on the vessel either by ensuring pressure relief is provided on relevant pipework to the vessel and/or ensuring there are no pumps transferring material which could lead to a build up of pressure if the outlet pipework were closed. The absence of pressure relief on the vessels can considerably simplify the process plant. The safety and regulatory requirements, as well as insurance inspection requirements, for pressure relief is currently a confused area. 

Discharge of pressure relief systems and leaks . Although strictly not a category of waste distinct from those above, the design of a containment and treatment system for effluent must take into account the likelihood of the unexpected. Further consideration of these aspects is given later in this chapter. 

A cargo tank or portable tank for liquid oxygen under pressure must be equipped with a primary system of one or more pressure relief valves and a secondary system of one or more rupture disks or pressure relief valves. Considerations for settings and capacity requirements are to be found in Title 49 of the U.S. Code of Federal Regulations and equivalent Canadian regulations. [15] and [16]. Also see CGA S-1.2, Pressure Re-... 

Relief systems are expensive and introduce considerable environmental problems. Sometimes it is possibly to dispense with relief valves and all that comes after them by using stronger vessels, strong enough to withstand the highest pressures that can be reached. For example, if the vessel can withstand the pump delivery pressure, then a relief valve for overpressurization by the pump may not be needed. However, there may still be a need for a small relief device to guard against overpressurization in the event of a fire. It may be possible to avoid the need for a relief valve on a distillation column... 

New systems or processes may also need to be qualified from an operational safety perspective. This is particularly relevant in the case of chemical synthesis involving exothermic reactions. Critical safety aspects are usually identified using hazard operability or HAZOP assessments and studies. For example, a HAZOP analysis of an exothermic reaction vessel would involve consideration of the consequence of failure of the motors for mixers or circulation pumps for cooling water. Thus, the qualification of such a system would involve checks and assessment to ensure that the system/process can be operated safely and that pressure relief valves or other emergency measures are adequate and functional. 

If it is not possible to be confident that the relief system will operate without blockage, then, consideration should be given to installing further measures to prevent runaway occurring and/ or the use of alternative measures to mitigate its effects (see Annex 1). Unfortunately, the same mechanisms by which a relief system may be expected to block may also cause blockage of pressure or temperature measurement points within an instrumented protective system, so care should be taken, in such cases. 

The required relief rate will first be calculated using equation (A6.2). It can be seen from Table A6.2, that both dT/dt and QG are considerably lower at the relief pressure than at the maximum accumulated pressure. The calculation will therefore be performed at the relief pressure since this should give the smallest relief size. Operation of the relief system will then prevent the pressure rising to the. maximum accumulated pressure, for which a larger relief system would be needed. 

Examples of common safe practices are pressure relief valves, vent systems, flare stacks, snuffing steam and fire water, escape hatches in explosive areas, dikes around tanks storing hazardous materials, turbine drives as spares for electrical motors in case of power failure, and others. Safety considerations are paramount in the layout of the plant, particularly isolation of especially hazardous operations and accessibility for corrective action when necessary. 

An accelerating rate calorimeter (ARC) can be used to provide design values for emergency pressure-relief flow requirements of runaway systems. The ARC is a device used to obtain runaway history of chemical reactions in a closed system (DeHaven, 1983 Huff, 1982,1984a Townsend and Tou, 1980). The experimental technique is fairly straightforward, but considerable engineering expertise is required to do the calculations needed to design a relief system from the ARC data. 

In the Three Mile Island core meltdown, it is known that there was also a Zircaloy-water chemical reaction leading to a considerable release of hydrogen. However, the reactor circuit pressure relief and containment system there prevented damage with no significant fission product release to the atmosphere. There was no free oxygen in the vessel so that hydrogen did not react chemically. 

The maximum pressure that the reactor can withstand must be known this is usually a design constraint that will depend on the material used to build the reactor and the thickness of the wall, among other technical considerations. If a relief system has been incorporated, the pressure at which the reactor will be vented will determine the maximum pressure. 

Pressure-relief valves are offered in many material combinations. Manufacturers recommend materials for specific applications. Proper selection of materials requires that service conditions be specified temperature, pressure, media contained within the system, and any external environmental conditions that need consideration. 

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