Safety relief valves problems with

IE Bulletin 80-25, Operating problems with target rock safety relief valves at BWRs, issued December 19,1980. 

On November 26, 1993, the USNRC issued Information Notice 93-89, "Potential Problems witii BWR Level Instrumentation Backfill Modifications," to alert licensees to potential problems that have been identified involving hardware modifications to the reactor vessel water level instrumentation system. This information involved the potential to pressurize the reference legs of the water level instrumentation if a backfill system is installed with the injection point on the instrumentation side of the manual isolation valve in the reference leg. If that valve is closed inadvertently during backfill system operation, the closure could result in a severe plant transient. At some plants, valve closure would cause all safety relief valves to open and potentially impact ECCS response. Licensees were advised to review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems. 

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. 

In 1984, one of the most notable process incidents occurred in Bhopal, India, at a plant owned by Union Carbide. On December 3, 1984, methyl isocyanate (MIC) was vented to the atmosphere from a vent gas scrubber after a run away reaction overwhelmed the scrubber. It is reported that more than 2500 people died and 20,000 people were injured as a result of exposure to MIC. The subsequent investigation showed that the run away reaction occurred in the MIC tanks after water was reportedly intentionally added to the tank. The safety systems, a pressure relief valve, a vent gas scrubber, and a flare were reportedly poorly maintained. The relief valve worked to vent the pressure, but the scrubber was overwhelmed, and the flare was down for maintenance. This incident highlighted several loss prevention areas that needed improvement. The first was mechanical integrity. No strong preventive maintenance effort appeared to be in place. The second was emergency preparedness and response. It appears that while the plant was built 1.5 miles away from the community, zoning problems allowed the community to expand to the plant limits. It does not appear that much effort was made to work with the community on commimication/notification or evacuation needs. ... 

The new Bayview Terminal had a design operating pressure of 740 pounds per square inch (50 atmospheres or 50 bar) and the accident pipeline was designed for much higher pressures, so it was necessary to install pressure-reduction equipment where the pipeline joined the new terminal. There were three layers of control and safety devices (i) a control valve to throttle the incoming flow, (ii) a pilot-operated spring-loaded relief valve, and (iii) three motor-operated isolation valves. This triple-barrier approach is sound design - a control system, backed-up by two protection systems (the relief valve and the isolation valves). In the event of a problem with the control valve, the relief valve should operate quickly, and only in the event of a fault with the relief valve should the isolation valves ever have been required to operate. 

Relief valves located on liquid-filled reflux drums can be a major safety problem depending on the design and size of the flare knockout (K.O.) drum. Some plants have giant flare K.O. drums with large capacity... 

This section evaluates the same example problem described in section 6.5. A reactor has a volume of 2 m3. The worst case runaway reaction has been identified and the data from a suitable adiabatic, low thermal inertia test, with a thermal inertia of 1.05, is given in Figure 6.4. Under these conditions, the reactor would contain 793 kg of reactants. The reacting system is a vapour pressure system. It is desired to relieve the runaway via a safety valve, if possible, with a set pressure of 0.91 barg (relief pressure of 1.0 barg = 2.0 bara). Evaluate the required relief size for an overpressure of 30% of the absolute relief pressure, which gives a maximum accumulated pressure of 1.6 barg = 2.6 bara. 

In the fine chemicals and pharmaceutical industries, reactors are often used for diverse processes. In such a case, it is difficult to define a scenario for the design of the pressure relief system. Nevertheless, this is required by law in many countries. Thus, a specific approach must be found to solve the problem. One possibility, that is applicable for tempered systems, consists of reversing the approach. Instead of dimensioning the safety valve or bursting disk, one can choose a practicable size and calculate its relief capacity for two-phase flow with commonly-used solvents. This relief capacity will impose a maximum heat release rate for the reaction at the temperature corresponding to the relief pressure. 

This upset initiates a runaway reaction that can catastrophically rupture the reactor. The impact of this event was judged to be extensive, which, as discussed in Table 6 Note 1, leads to a tolerable frequency of 10 /year for a single scenario. Several failures in the control system could cause this upset, with operating experience indicating that this type of upset occurs about once every 10 years. Protection per Table 5 was the Shortstop addition, but the runaway reaction may be too fast for the operator to respond to an alarm. This protection layer is not included for risk reduction. The area is normally occupied, so it was assumed that personnel could be impacted by the event. The pressure safety valves (PSVs) are only estimated to be 90% effective, since plugging is a common problem in this service. Since the PSVs share a common relief line, they are conservatively considered to be a single Independent Protection Layer. This led to an intermediate event likelihood of a 10 per year. Per the conservative assumptions used in this example, only the PSVs qualified as an IPL. The PHA team reviewed all the process safety risk issues and decided that a SIF was appropriate. As shown in Table 7, this requires a SIL 3 SIF. 

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