Safety valves malfunctioning

Please note that, some of the identified precursors (re-occurring deviations) are manifestly affected safety barriers. That is to say the precursor itself is a safety barrier and malfunctions repeatedly e.g. the pressure relief valve defect (number 4 in Table 16). Moreover, the precursor may be a process control measure (often tripping of a pressure relief (safety) valve). The presence of these kinds of precursors illustrates what is stated in Chapter 3, that if actors in the operational process don t perceive a deviation as possessing direct safety related consequences, they permit these deviations to exist in the operational process. 

Deviations from the set points, and possible malfunctions, must be guarded against by proper precautions such as controls, shut-off valves, shut-down devices, safety valves, rupture discs, etc. 

The main steam safety valves will be designed to withstand the following transients without failure or malfunction. 

Malfunctioning may occur if the safety valve is connected to the vessel via a long pipeline or if pressure drops suddenly or if it gets choked dne to deposition of solids... 

Malfunctioning of Safety Valves Pmchaser should keep the following in mind and discuss the following possibilities with the vendor of pressure vessels (who may provide the vessel with bought out safety valves). 

The second hazard is the tank s location close to ultra-high compressed air lines and equipment. A high pressure pipeline explosion could result Ifom a malfunctioning safety valve, a human error in operating the equipment, damage to a pipeline, or from other causes. Blast or flying debris could conceivably strike the propane tank, rupture it and cause it to explode with the same consequences as for a run-away vehicle. 

Weeping safety valves not only cause undesirable watermarks and steam losses but may indicate valve malfunction. A check for excessive amounts of system makeup water should be made by means of a water meter on the inlet line. 

The other type of safety device commonly used on the inner vessel is a safety head or rupture disk. In case the relief valve malfunctions, these disks are set to burst at approximately 1.2 times the working pressure. Typical bursting disks are of the simple dome type, which burst due to tensile stresses. Recently, reverse buckling disks, which have the dome oriented toward the pressure, have proven themselves to be a highly cost-effective alternative. An advantage of the reverse buckling disks, in contrast to conventional disks, is that they are ruptured as a result of compressive load and thus are not subject to creep failure. 

As long as pressure, level, and temperature control devices are operating correctly, the safety system is not needed. If the control system malfunctions, then pressure, level, and temperature safety switches sense the problem so the inflow can be shut off. If the control system fails and the safety switches don t work, then relief valves are needed to protect against overpressure. Relief valves are essential because safety switches do fail or can be bypassed for operational reasons. Also, even when safety switches operate correctly, shutdown valves take time to operate, and there may be pressure stored in upstream vessels that can overpressure downstream equipment while the system is shutting down. Relief valves are an essential element in the facility safety system. 

Sulfate scaling poses a special problem in oil fields of the North Sea (e.g., Todd and Yuan, 1990, 1992 Yuan et al., 1994), where formation fluids are notably rich in barium and strontium. The scale can reduce permeability in the formation, clog the wellbore and production tubing, and cause safety equipment (such as pressure release valves) to malfunction. To try to prevent scale from forming, reservoir engineers use chemical inhibitors such as phosphonate (a family of organic phosphorus compounds) in squeeze treatments, as described in the introduction to this chapter. 

Relief valves are preferred for use on clean materials, because automatic closure prevents excessive discharge once excessive pressure is relieved. Rupture disks are less susceptible to plugging or other malfunctions but may allow complete emptying of the vessel, thus creating a safety or environmental hazard. Where fluctuating pressures or very corrosive conditions exist, or where polymerizable materials could prevent proper operation of a relief valve, some designers install two safety devices in series, ie, either two rupture disks or an upstream rupture disk followed by a relief valve. With either arrangement, it is imperative that the space between the two relief devices be monitored so that perforation or failure of the relief device closest to the vessel may be detected (86). 

It is common on chemical plant to install safety devices such as trips and relief valves which protect the plant in the event of a malfunction of control systems or human error. Unfortunately, these devices can (and do) fail occasionally. The problem is that the failures cannot be seen until they are tested or until they are called upon to act (a plant may operate perfectly normally even though, say, a pressure relief valve is faulty, because under normal conditions the valve is never activated). It is thus necessary to test safety devices periodically to ensure they are functioning. 

Critical safety systems are usually considered to be those that are necessary for safe plant operation during processing and also those required for the safe shutdown of the plant in the event of an unanticipated malfunction. Within an incineration plant, examples of critical safety systems inclnde agent monitoring systems, testing or inspection of pressure rehef valves, ventilation flow and control systems, fire protection systems, emergency alarm and shutdown systems, process interlocks, and furnace temperature control systems. 

Can feed, auxiliary, intermediate and final products enter pipes or parts of the plant where they should not be, due to operator mistakes or malfunctions (e.g. cooling water or products in drinking water pipes, oxygen or air in inert gas pipes etc.) What safety measures are implemented to prevent this (e.g. check, valves, double shut-off valves with a vent, interlocks of valves etc.) ... 

Failure modes and effects analysis (FMEA) A systematic, methodical analysis performed to identify and document all identifiable failure modes at a prescribed level and to specify the resultant effect of the failure mode at various levels of assembly (NSTS 22254) the failure or malfunction of each system component is identified, along with the mode of failure (e.g., switch jammed in the on position). The effects of the failure are traced through the system and the ultimate effect on task performance is evaluated. Also called failure mode and effect criticality analysis (ASSE) a basic system safety technique wherein the kinds of failures that might occur and their effect on the overall product or system are considered. Example The effect on a system by the failure of a single component, such as a register or a hydraulic valve (SSDC). 

The outlet air temperature and pressure from the compressors gives a good indication of the state of downstream process plant, where additional pressure drop may be experienced due to the entrainment of dust in the drying bed, breakdown of the catalyst or malfunction of the control valves. The gas discharge pressure and preferably also the temperature should be recorded. In addition, for safety reasons and to protect the compressor, a high-pressure switch/alarm combination should be used to stop the compressor in the event of overpressure. The sulphur pumping system must be interlocked with compressor operation, to stop the pump when no process air is available. 

Malfunctioning of safety related check valves could create unacceptable situations during accidents and contributes to the risk associated with postulated core-melt accident sequences. 

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