Overpressure safety valve

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. 

Relief valves are designed to respond automatically to sudden increases in pressure. A relief valve opens at a predetermined pressure. In a relief valve, a disc is held in place by a spring that will not open until system pressure exceeds its operating limits. Tremendous pressures can be generated in process units. When a system overpressurizes, safety valves respond to allow excess pressure to be vented to the flare header or atmosphere. This prevents damage to equipment and personnel. Relief valves are designed to open slowly, and thus are best for pressurized liquid service. They do not respond well in gas service, where quicker pressure reduction is needed. 

This schematic diagram of the prototype fast reactor (PFR) illustrates the steam drum and its unique waterside overpressure safety valve. 

The designers then went one step further - in addition to the normal spring-loaded overpressure safety valves on the top (steam-side) of the steam drums, they also installed a mechanical overpressure safety valve on the underside (the waterside) of the steam drum. This was highly unusual - the overpressure safety valves on steam drums are normally only on the steam-side (the top). The waterside overpressure safety valve meant that, in extrane overpressure fault conditions, high-density water - instead of low-density steam - from the steam drum would be blown through the roof, so the many tonnes of water in the drum available to feed a steam-sodium leak accident could be removed in a few seconds. (By contrast, it would take very much longer to remove all the water if only the steam-side overpressure relief valves were used.)... 

This waterside overpressure safety valve was operated by a spring-loaded pilot valve which, for test purposes, could be isolated from the water pressure in the steam drum by a valve called the PLIV - the pilot line isolating valve. During normal operation the PLIV was locked open with a large padlock. 

So far, so good. The operational problem, however, was this the waterside overpressure safety valve had to be tested every three years to satisfy the boiler inspectors who insured the plant. Watching this operation being carried out was fascinating, and also one of the hairiest operations I have ever witnessed in my career, as I shall now describe. 

The waterside overpressure safety valve was set to operate at a pressure such that it was the last one on its steam drum to operate in a high-pressure fault - the steam-side Safety Valves would operate first. The normal steam operating pressure at PFR was about 160 atmospheres. The superheater overpressure safety valves operated... 

Eventually we had got to the point of being ready to test the waterside overpressure safety valves. The other safety valves had been tested and had been isolated, and the steam drums were at the highest pressure and temperature they had ever experienced. 

The only way to avoid the risk of hogging the drum was to have someone next to the Pilot Line Isolating Valve, the PLIV, to shut it immediately after the main waterside overpressure safety valve had opened. By closing the PLIV, the Pilot Valve would close, making the Safety Valve close also. The PLIV was a small handwheel-operated valve directly under the main steam drum, which was not normally readily... 

Whoever got the job of closing the PLIV was going to be extremely close to the drum, in a very hot and uncomfortable position he would have to wait until he heard the very loud noise of the waterside overpressure safety valve opening, and then immediately close the PLIV. Tonnes of scalding pressurized water were going to flow through the safety valve a few inches away from him. 

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)-... 

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. 

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. 

Note that the recommended value for p is not always conservative. In some cases, heat input may be so high that the safety valve cannot vent all the generated vapor. In such cases, the internal pressure will rise until the bursting overpressure is reached, which may be much higher than the vessel s design pressure. For example, Droste and Schoen (1988) describe an experiment in which an LPG tank failed at 39 bar, or 2.5 times the opening pressure of its safety valve. Note also that this method assumes that the fluid is in thermodynamic equilibrium yet, in practice, stratification of liquid and vapor will occur (Moodie et al. 1988). 

If failure is due to fire exposure, the vessel s overpressure results from external overheating and can reach a maximal value of 1.21 times the opening pressure of the safety valve. This maximal value is called the accumulated pressure. As overheating reduces the vessel s wall strength, failure occurs at the point at which its strength is reduced to a level at which the accumulated pressure can no longer be resisted. If vessel failure is due to corrosion or impact, it can be assumed that pressure at failure will be the operating pressure. 

The failure overpressure is assumed to be 1.21 times the opening pressure of the safety valve. Thus ... 

Usual practice is to use the terms safety valve or relief valve to indicate a relieving valve for system overpressure, and this will be generally followed here. ATien specific types of valves are significant, they will be emphasized. 

Safety Valve this is an automatic pressure-relieving detice actuated by the static pressure upstream of the valve and characterized by rapid full opening or pop action upon opening [1,10], but does not reseat. It is used for steam or air service (Figure 7-2). Rated capacity is reached at 3%, 10% or 20% overpressure, depending upon applicable code. 

Figure 7- l4. Safety valve design operational check sheet. Adapted and added to by permission, N. E. Syivander and D. L. Kiatz, Design and Construction of Pressure Relieving Systems, Univ. of Michigan Press, Ann Arbor (1948). Six items of overpressure list above by this author and from API Rec. Practice 521 (1982). 

Rupture disks are used for the same purpose as safety valves and, in addition, serve to relieve internal explosions in many applications. If the pressure rise can be anticipated, then the volume change corresponding to this change can be calculated by simple gas laws, and the capacity of the disk at the relieving pressure is knowm. The system must be examined and the possible causes of overpressure and their respective relief capacities identified before a reliable size can be determined. See Figure 7-14. 

Safety valves are often required on the shell side of exchangers and sometimes on the tube side. These valves may require sizing based upon process reaction, overpressure, etc., or on external fire. For details, see Chapter 7, Vol. 1 on safety-relieving devices. 

Industry literature typically cites concern with open air explosions when 4,536 kgs (10,000 lbs.) or more of flammable gas is released, however, open air explosions at lower amounts of materials are not unheard of. When the release quantity is less than 4,536 kgs (10,000 lbs.), a flash fire is usually the result. The resulting fire or explosion damage can cripple a hydrocarbon processing facility. Extreme care must be taken to prevent the release of hydrocarbon from vessels resulting in vapor releases and resultant blast overpressure. Measures such as hydrotesting, weld inspections, pressure control valves, adequate pressure safety valves, etc., should all be prudently applied. 

Within this Workbook, the maximum pressure required to fully open the pressure relief device will be referred to as the "relief pressure". (Caution some papers on relief sizing refer to "set pressure" but mean "relief pressure"). For a bursting disc, the relief pressure will be the maximum specified bursting pressure and for a safety valve, it will be the set pressure plus 10% overpressure (or whatever percentage overpressure the valve has been certified at). 

---