Closed relief system

Liquid Drainage from Closed Relief System - Accumulation of liquid in closed rehef systems can impose appreciable back pressure and reduce relieving capacity. The following design features must be included to avoid these problems ... 

Blowdown stacks have been recognized as potentially hazardous for this type of service, and the industry has moved more towards closed relief systems to flare. Opportunities to tie the Splitter relief lines into a flare system were not taken, and the site continued to use F-20 as part of the relief and venting system for the Raffinate Splitter. The use of a flare. system would have reduced the severity of the incident [20]. 

Often, the blowdown valve is routed to a closed flare system, which services other relief valves in the facility to ensure drat all the gas is vented or flared at a safe location. In such instances, a separate manual blow -down valve piped directly to atmosphere, with nothing else lied in, is also needed. After the compressor is shut down and safely blown down through the flare system, the normal blowdown valve must be closed to block any gas that may enter the flare system from other relief valves. The manual blowdown valve to atmosphere protects the operators from small leaks into the compressor during maintenance operations. 

Balanced bellows type valves are normally used where the relief valves are piped to a closed flare system and the back-pressure exceeds 10% of the set pressure, where conventional valves can t be used because back-pressure is too high. They are also used in flow lines, multiphase lines, or for ptu affinic or asphaltic crude, where pilot-operated valves can t be used due to possible plugging of the pilot line. An advantage of this type of relief valve is, for corrosive or dirty service, the bellows protects the spring from process fluid. A disadvantage is that the bellows can fatigue, which will allow process fluid to escape through the bonnet. For HjS service, the bonnet vent must be piped to a safe area. 

All waste hydrocarbon gases (vents, relief valves, and blowdowns) should be routed to a flare or returned to the process through a closed header system. Release of vapors to atmosphere may produce a vapor cloud, and even through the release may be remote from the facility it may drift or the effects of ignition (i.e., blast overpressure) of the cloud will be felt at the facility. 

Effect of the initial depressuring rate on the closed pressure relief system and flare... 

In sizing depressuring valves, it should be assumed that heater burners are shut-off, reboilers are shutdown, and normal flow in the vessel has ceased. Vapor depressuring valves should be designed such that the initial, instantaneous depressuring flow rates do not exceed the capacity of the closed pressure relief system and the flare. 

On the night of the accident, operators heard a screeching noise from the relief valve on one of the tanks. Unfortunately, the closed blowdown system had been taken out of service for maintenance. It was later established that while operators were on their shift change or on a break, someone disconnected a pressure gauge from the cover plate on one of the tanks and attached a water hose. A quantity of water estimated between 450 and 900 kg entered the tank and caused a severe upset and release of MIC vapor. With no means of notifying the public and evacuating the community, thousands were exposed to the vapor cloud, resulting in the deaths and injuries. 

Adiabatic Dewar calorimeters are usually used in the closed mode. However, it is possible to incorporate a vent line to either an external containment vessel or to a burette for measuring the permanent gas evolution rate. This vent line contains an automatic valve to simulate the operation of the pressure relief system. 

The flow inclination number should be evaluated from equation (A8.15) above, arid the Figure to be used to obtain the correction factor (from Figures A8.3 to A8.5) is selected on the basis of the flow inclination number. If the static head change is small in comparison with the total upstream pressure, then the flow inclination number may be close to zero and Figure A8.3 can be used. In other cases, values of the correction factor may be read from the two graphs which bound the actual value of the flow inclination number, andl.the correction factor found by linear interpolation between the values. Flow inclination numbers higher than about 0.2 (as in Figure A8.5) are unlikely in practical relief systems. 

The time for the pressure to rise in a closed adiabatic system from the relief pressure to the maximum accumulated pressure. 

In addition to the three temperature levels (TP, MTSR, and TD24), there is another important temperature that at which technical limits of the equipment are reached. This may be due to the resistance of construction materials, or to the reactor design parameter as pressure or temperature, and so on. In an open reacting system, operated at atmospheric pressure, the boiling point is often used. In a closed system, operated under pressure, it may be the temperature on reaching the set pressure of the pressure relief system. 

In a closed system, the technical limit may be the temperature at which the pressure in the reactor reaches the set pressure of the relief system. In such a case, it may be possible to depressurize the reactor in a controlled way before the set pressure is reached. This allows tempering a reaction at a temperature where it is still controllable. 

The pressure increase depends on the nature of the pressure source, that is, gas or vapor pressure. Further, the characteristics of the system, that is, if the reactor is closed or open to the atmosphere will determine the consequences. In an open system, the gas or vapor will be released from the reactor, whereas in a closed system, the result of a runaway will be a pressure increase. The resulting pressure can be compared to the set pressure of the pressure relief system (Pset) or to the maximum allowed working pressure (PJ, or also to the test pressure (PM) of the equipment... 

Many specialists in SRVs and systems agree that as a practical matter, conventional PHA methods are not always the most effective tools for evaluating pressure relief systems. They also conclude that the pressure relief system design process could be improved. Working closely with a lot of the design firms, I concluded that they merely try to comply with the codes at a minimum cost and care very little about LCC (life cycle cost) of the components. This puts pressure later on the end users maintenance departments usually, these employees have little or no input in the selection of design components whose problems ultimately end up in their laps. In my opinion, it is recommended that, in order to reduce this deficiency rate, the industry should adopt a more equipment-based approach to pressure relief system design and maintenance... 

An accelerating rate calorimeter (ARC) can be used to provide design values for emergency pressure-relief flow requirements of runaway systems. The ARC is a device used to obtain runaway history of chemical reactions in a closed system (DeHaven, 1983 Huff, 1982,1984a Townsend and Tou, 1980). The experimental technique is fairly straightforward, but considerable engineering expertise is required to do the calculations needed to design a relief system from the ARC data. 

Pressure relief valve Closed-vent systems and control devices Leak = >10,000 ppm Leak = > 500 ppm ... 

Any consistent set of units may be used for pressure as long as the absolute pressure is used, not the gauge pressure. The ratio PcfIPi is called the critical pressure ratio. Typical values of this ratio are given in Table 13.6. If the downstream pressure is less than the critical flow pressure, then critical flow will occur in the nozzle. It can be seen from the table that this will be the case whenever the upstream pressure is more than two times the downstream pressure. Since most relief systems are operated close to atmospheric pressure, critical flow is the usual case. 

Safety devices should be piped with a minimum of inlet pipe runs because excess pressure drop before a safety valve will affect its operation. For a minimum of discharge piping, relief valves are located high on a tower in open relief systems. In closed systems relief valves are located just above the relief header. 

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