Overpressure, equipment safety

Reactor A process hazards analysis identifies a hazard associated with a reactor dump valve. If the dump valve opens when the reactor is not depressurized, downstream equipment is overpressured. An SIF is implemented to prevent the reactor dump valve from opening when the pressure is high, using a solenoid-operated valve that controls the instrument air supply to the dump valve actuator. To support batch operation, the SIF energizes the solenoid-operated valve for each batch when the reactor pressure is low. However, from a safety perspective, the SIF prevents a hazardous event only when the control system fails and tries to open the control valve when the reactor is under pressure. This failure is estimated to be approximately 1 in 10 years. The SIF is operating in demand mode with regard to the hazard. 

Rupture disks or explosion doors may be used as venting devices. Safety valves are not suitable for this purpose. Obviously, the static-activation overpressures Ps at of he venting devices have to be equal to or smaller than the strength of the equipment to be protected (c-orre-sponding to the Pred.max)-... 

For any proposed suppression system design, it is necessary to ascribe with confidence an effective worst-case suppressed maximum explosion overpressure Pred.max- Provided that the suppressed explosion overpressure is less than the process equipment pressure shoclc resistance and provided further that this projected suppression is achieved with a sufficient margin of safety, explosion protection security is assured. These two criteria are mutually independent, but both must be satisfied if a suppression system is to be deployed to provide industrial explosion protection. 

This chapter describes the basic principles and procedures for the evaluation of overpressure potential in plant equipment, and for the selection, design and specification of appropriate pressure relieving facilities. The design of closed safety valves and flare headers is included in this chapter, but blowdown drums and flares are covered separately. To properly discuss this subject, the reader should become familiar with the following terminology. 

The header is rated the same as the highest pressure rated equipment connected to it or it is fitted with a safety valve if designed for a lower pressure rating. Sections of the header, separated by check valves, may be designed for different pressure ratings, but safety valve protection is still required for the lower-rated sections, unless the header cannot be overpressured to more than 1.5 times the design pressure. 

Some method of pressure relief is required on all pressure vessels and for other proeess equipment where inereasing pressure might rupture the vessel. Mueh of the piping used in modern ehemieal operations also requires overpressure proteetion. Safety relief valves or rupture dises are employed for pressure relief. In many eases, either a rupture dise or a safety relief valve ean be used. Safety relief valves are usually used for proeess proteetion and rupture dises are used for vessel proteetion. The safety relief valve or rupture dise must be designed to operate at a known pressure and prevent the pressure within the system from inereasing. Therefore, it is important to eon-sider the flowrate the valve ean handle. 

All pressure vessels should be equipped with one or more pressure safety valves (PSVs) to prevent overpressure. This is a requirement of both the ASME Code and API RP 14C (refer to Chapter 14). The PSV should be located upstream of the mist extractor. If the PSV is located downstream of the mist extractor, an overpressure situation could occur when the mist extractor becomes plugged isolating the PSV from the high pressure, or the mist extractor could be damaged when the relief... 

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. 

To reduce the risk of container failure, the pressure vessels are equipped with several safety features. These can include an effective self-venting system where unforeseen overpressure is released by a quick open-resealing step, or the use of safety disks which rupture when their pressure limit is reached. The small vials (0.2-20 mL) of some monomode reactors are protected by the pressure limit (20 bar) of the caps used, which is significantly lower than the operating limit of the vials themselves (40-50 bar). 

Commercial dewar are equipped with several safety features. Note that there are usually two overpressure safety valves, adjusted for about 1.5 atm and 6 atm respectively. The former safety valve can be usually excluded by a small lever valve which allows liquid transfer operations. If it is forgot closed, the pressure reaches 6 atm and, due to the higher conduction through the gas, the liquid consumption can become as high as 5% per day or more. 

Since discharges of vapors from highly hazardous materials cannot simply be released to the atmosphere, the use of a weak seam roof is not normally acceptable. It is best that tanks in low-pressure hazardous service be designed and stamped for 15 psig to provide maximum safety, and pressure relief systems must be provided to vent relieved overpressure to equipment tfiat can collect, contain, and treat the effluent. 

I-3.6.2 Electronic control, monitoring, and hydrogen gas measurement equipment shall be properly grounded and isolated from piping to help prevent overpressure/ accidental shutoff situations caused by equipment failure due to lightning strikes and electrical transients and to prevent safety hazards caused by fault currents. Electrical isolation equipment for corrosion control purposes should not be installed in buildings unless specifically designed to be used in combustible atmospheres. 

The above case study warns that hydrates can be hazardous to personnel and to equipment. Yet hydrate plugs can be safely dissociated with the use of CSMPlug and User s Manual on the CD in the endpapers of this book, with the User s Guide Examples in Appendix B. The safety option of this program and process should be considered first, so that the potential for overpressurization and eruption can be considered. 

Protecting personnel against dangers caused by overpressure in the equipment when all other safety equipment has failed... 

These creative individuals had set up a scheme to purge through the top of the air dryer and out of a partially opened ball valve on the lower piping. They, no doubt, failed to realize that the dryers were not individually equipped with safety relief valves. The overpressure protection was on the compressors. A new operator to the area did not open the exit valve sufficiently during the cool-down step. 

High-class Hydraulic HDP are generally equipped with a design approved PRV on the hydraulic side. This teprovides a safety device against overpressure in case the HDP is the only pressure increasing unit in the system. 

If the plant safety shutdown is not rapid enough and an overpressure situation develops, then the pressure relief system is activated. Pressure vessel design codes such as the ASME Boiler and Pressure Vessel Code require relief devices to be fitted on all pressure vessels (see Section 13.17). If the relief system has been properly designed and maintained, then in the event of an overpressure incident, the plant contents will be vented via relief valves or bursting disks into the relief system, where liquids are recovered for treatment and vapors are sent to flare stacks or discharged to the atmosphere if it is safe to do so. The pressure relief system should allow the plant to be relieved of any source of overpressure before damage to process equipment (leaks, bursting, or explosion) can occur. 

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