Safety relief valves installation

Figure 7. Pressure conditions for safety relief valve installed on a pressure vessel (vapor phase) supplemental valve used for fire exposure only.

Figure 7-19 illustrates a newer approach at simplifying the dual safety relief valve installation, ASME Sect. VIII, Div. 1, UG-135(b) [1] and API RP-520, Part II Conformance [33]. Note that the SRV valves are mounted on top of each of one dual vertical connections and are bubble tight. Also see cross section view. The flow C, valves for each size device are available from the manufacturer. 

Figure 7-19. Safety selector valve for dual safety relief valve installation with switching. By permission, Anderson, Greenwood and Co. AGGO.

Figure 7-20. Recommended API-520 piping for safety relief valve installations. Reprinted by permission, American Petroleum Institute, Sizing, Selection and Installation of Pressure Relieving Devices in Refineries, Part ll-lnstallation, API RP-520, 3rd Ed., Nov. 1988.

Note To emphasize the difference between overpressure (safety relief valve characteristic) and accu-mulation (code limitation on the pressure vessel), safety relief valves installed for fire cases will have an overpressure of 10% like most safety relief valves, even if the allowed accumulation on the pressure vessel is 21% in the case of ASME VIII. ... 

Balanced Bellows Safety Relief Valve - A balanced safety relief valve incorporates means for minimizing the effect of back pressure on the performance characteristics opening pressure, closing pressure, lift and relieving capacity. This is usually achieved by the installation of a bellows. 

Figure 7-11. Safety valve and rupture disk installation using pressure rupturing disk on inlet to safety relief valve, and low pressure disk on valve discharge to protect against back flow/con-osion of fluid on valve discharge side, possibly discharge manifold. By permission, Fike Metal Products Div., Fike Corporation, Inc.

The stamped capacity of a spring loaded safety or safety relief valve (nozzle type) when installed with a rupture disk device between the inlet of the valve and the vessel shall be multiplied by a factor of 0.80 of the rated relieving capacity of the valve alone, or alternatively, the capacity of such a combination shall be established in accordance with Par. 3 below ... 

Note that in lieu of testing, Par (b) 2 and (b) 3 above allows the use of a capacity factor of 0.80 as a multiplier on the stamped capacity of the spring loaded safety relief valve (nozzle bpe). Some manufacturers test specific valve/rupture disk combinadons and determine the actual capacity factor for the combination, and then use this for the net capacity determination. See Figures 7-10, 7-11, 7-12, 7-13A and 7-13B. c. A rupture disk de ice may be installed on the outlet side of a spring loaded safety relief valve... 

Safety (relief, or safety-relief) valves are used for set pressures from 10 psig to 10,000 psig and even higher. At the low pressures the sensitivity to relieving pressure is not always as good as is required for some processes, and for this reason most valve installations start at 15 to 20 psig. 

Conventional safety relief valves, as usually installed, produce unsatisfactory performance w hen variable back pressure exists [10, 33]. See Figure 7-6. The same variable back pressure forces affect the set pressure release also. At low back pressures, the valve flow falls rapidly as compared with the flow for a theoretical nozzle. See Figures 19 and 20 in Ref. [33a]. 

A rupture disk device may be installed on the outlet side30 of a spring loaded safety relief valve... 

Rupture Disks A rupture disk is a non-reclosing device designed to function by the bursting of a pressure-retaining disk. This assembly consists of a thin, circular membrane usually made of metal, plastic, or graphite that is firmly clamped in a disk holder. When the process reaches the bursting pressure of the disk, the disk ruptures and releases the pressure. Rupture disks can be installed alone or in combination with other types of devices. Once blown, rupture disks do not reseat thus, the entire contents of the upstream process equipment will be vented. Rupture disks of nonfragmented type are commonly used in series (upstream) with a safety relief valve to prevent corrosive... 

Are shut-off valves not installed between safety relief valve and container ... 

Other of such standards which are useful are the EN 764-7 and EN 12952-10 Water-tube boilers and auxiliary installations - Requirements for safeguards against excessive pressure. Although its Annex ZA shows that it supports the PED clauses 2.11 and 7.3, this standard does not give any indication on the set pressure and the overpressure of the Safety Relief Valves, in which case one can refer back to the above. 

Codes and standards relevant to safety relief valves (SRVs) can vary quite considerably in format around the world, and many are sections within codes relevant to boilers or pressure-containing vessels. Some will only oudine performance requirements, tolerances and essential constructional detail, but give no guidance on dimensions, orifice sizes and so forth. Others will be related to installation and application. It is quite common within many markets to use several codes in conjunction with one another and it is not uncommon that specifications call for sections taken from several codes, which makes compliance by manufacturers complex and uneconomical. An overview of most common worldwide codes and standards is given in Appendix M. 

Some specialized literature regarding the maintenance of safety relief valves (SRVs) is available, but it is best to always consult the manufacturer s detailed installation and maintenance manuals on the subject, as some type of valves might require special attention. 

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... 

In this chapter, we will provide just one safe example of how end users can specify their requirements for safety relief valves (SRVs) so that the risk for having wrong valves installed is minimized. This example is, of course, not exhaustive but provides guidance on what sort of information could be provided to the SRV manufacturer in order to ensure a safe situation on the site. 

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. 

Chlorine tank car manway, four angle valves, safety relief valve, packing gland leaks, installing valve hoods and outlet valve nuts, yoke assemblies, and checking tank car pressure. 

If two or more safety relief valves are required and are installed on one connection, the cross-sectional area of the section should be equal to the combined inlet of the safety devices connected to it. 

---