Relief Installation Practices

Figure 8-11 Relief installation practices. Adapted from Eric Jennett, Components of Pressure-Relieving Systems, Chemical Engineering (Aug. 19,1963), pp. 151-158.

The ASME code requires every pressure vessel that can be blocked in to have a relief valve to alleviate pressure build up due to thermal expan sion of trapped gases or liquids. In addition, the American Petroleum Institute Recommended Practice (API RP) 14C, Analysis, Design, Installation and Testing of Basic Surface Safety Systems on Offshore Production Platforms, recommends that relief valves be installed at vari ous locations in the production system and API RP 520, Design and Installation of Pressure Relieving Systems in Refineries, recommends various conditions for sizing relief valves. 

Regardless of how carefully the relief is sized, specified, and tested, a poor installation can result in completely unsatisfactory relief performance. Some installation guidelines are illustrated in Figure 8-11. During field construction, sometimes expediency or construction convenience leads to modifications and deviations from acceptable practice. The engineer must take the responsibility for adhering to standard practices, especially when installing relief systems. 

Figure 9-2 Viscosity correction factor Kv for conventional reliefs in liquid service. Source API RP 520, Recommended Practice for the Sizing, Selection, and Installation of Pressure-Relieving Systems in Refineries, 6th ed. (1993), p. 35. Used by permission of the American Petroleum Institute, Washington, DC.

API Recommended Practice 520 Part II, Installation This part covers methods of installation for pressure relief devices, including recommended piping practices, reaction force calculations and precautions on pre-installation, handling and inspection. 

The safety of an installation handling LNG is paramount for obvious reasons. While it has become common practice since the mid-1960s to protect low-pressure LNG storage tanks with pilot-operated safety relief valves (POSRVs), many cryogenic installations under higher pressures are still relying... 

Looking beyond the incoming complaints about the valve itself, a lot of installations are just not protected at all. A statistical analysis shows that the pressure relief systems on nearly half of the equipment in the oil, gas and chemical industries lacks adequate overpressure protection as defined by recognized and generally accepted good engineering practices and by the codes, hence the law. 

Berwanger for instance, states it has performed 2000+ audits and that they have never encountered a plant that did not have a pressure relief issue. From my personal experience, I can confirm that only a few of the plants I visited had no problems in complying with good safety relief practices and/or codes, but more importandy many had regularly installed the wrong valve for a particular application or simply installed the valve incorrecdy. This is of concern, to say the least. The extent of this risk merits the attention of those who are accountable to the company s stockholders and the safety of its personnel. 

American Petroleum Institute Washington www.api.org API Recommended Practice 520 Part 1 - Sizing and Selection API Recommended Practice 520 Part 2 - Installation API Recommended Practice 521 - Guide for Pressure Relief and Depressurizing Systems API Standard 526 - Flanged Steel Pressure Relief Valves API Standard 527 - Seat Tightness of Pressure Relief Valves API Recommended Practice 576- Inspection of Pressure Relieving Devices... 

Definitions of terms used in connection with overpressure relief systems. (Reproduced courtesy of the American Petroleum Institute from API Recommended Practice 520, Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries, Part I - Sizing and Selection,... 

Loss of electric power. This is another failure that frequently sets the column relief requirements. A common practice (9) is to study the installation to determine the effect of power failures and to set the required relief capacity for the worst condition that can occur. All electrically driven equipment, such as pumps, compressors, and fans (including those in the site cooling water system or steam supply system), may fail, and so will electronic controllers and computer control equipment. 

As stated above, the hazard and risk assessment and allocation may be concurrent activities or allocation may in some circumstances take place prior to hazard and risk assessment. Decisions on the allocation of safety functions to safety layers are often taken on the basis of what has been found to be practicable by the user organization. Established industry good practice should also be taken Into account. Decisions will then be taken on the safety instrumented systems, assuming credit for the other safety layers. For example, where relief valves have been installed and these have been designed and installed according to industry codes, it may then be decided that these are adequate on their own to achieve adequate risk reduction. Safety instrumented systems would then only limit pressure where size or performance of the relief valve(s) was insufficient for the application or release to the atmosphere is to be prevented. 

A recommended safe practice is to install hydrostatic pressure relief valves in all pipelines wherever the possibility exists that liquid fluorocarbons may become trapped (such as between two valves) to prevent piping rupture through thermal expansion of the fluorocarbon. 

Block and beam floor slab 0 It is good practice to install ventilation in all foundation systems to effect pressure relief as a minimum. Breaches in floor slabs such as joints have to be effectively sealed against gas ingress in order to maintain these performances. 

One potential problem with the use of rupture disks below relief valves concerns the possibility of pinhole leaks in the rupture disk. If such a leak occurs, then the pressure on the downstream side of the disk will be roughly equal to the process pressure. Therefore, the absolute process pressme required to rupture the disk may be almost twice as high as expected. Moreover, the existence of the pinhole leak may lead to the development of the problem that it was intended to avert, e.g., the relief valve could be plugged with polymer without anyone being aware of the situation. Because of the dangers associated with pinhole leaks, it is good practice to install a pressure gage in the spool piece between the rupture disk and the relief valve. 

Block valves should not be installed in any relief valve system of any boiler or steam generator constructed to the requirements of the ASME Boiler and Pressure Vessel Code, Power Boilers, Section I. Many state boiler laws also prohibit this practice. 

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