Pressure relief valve atmospheric discharge

Mak has developed an improved method of relief valve manifold design. The APT has adopted this method, which starts at the flare tip (atmospheric pressure) and calculates backwards to the relief valves, thus avoiding the trial and error of other methods. This is especially helpful when a large number of relief valves may discharge simultaneously to the same manifold. 

Set Pressure - The set pressure (expressed as kPa gage or other increment above atmospheric pressure) is the inlet pressure at which the pressure relief valve is adjusted to open under service conditions. For a relief or safety relief valve in liquid service, the set pressure is to be considered the inlet pressure at which the valve starts to discharge under service conditions. For a safety or safety relief valve in gas or vapor service, the set pressure is to be considered the inlet pressure at which the valve pops under service conditions. 

For balanced bellows pressure relief valves, the maximum superimposed baek pressure should be specified. (For balanced bellows valves discharging to a safety valve header which is at atmospheric pressure under non-flowing conditions, the maximum back pressure is zero.)... 

Pressure relief valves may be discharged to atmosphere only, if the following conditions are satisfied ... 

Venting. Vent lines provided to exhaust the hydrogen gas from the pressure relief valves to atmosphere shall be extended to a location where the hydrogen gas may be discharged without undue hazard. Vent lines shall have sufficient capacity so that they will not inhibit the performance of the relief valve. Any pressure relief shall apply to unconfined space and be remote from any hazards. See CGA G-5.5-2004. 

Pumps connected to variable speed drivers should be equipped with an approved relief valve. The discharge of the relief valve should be directed back to the source where possible or to drains. Piping of a relief valve back to pump suction piping should be avoided. Whenever the relief valve does not discharge to the atmosphere, a sightglass flow indicator should be installed. Constant speed pumps also require relief valves when the shut-in pump discharge pressure exceeds the rated pressure for the system components. 

The cold water feed can be provided by direct coimec-tion to the mains (in which case all components must be able to withstand mains pressure) or from a feeder tank (which is in an elevated position and filled from the mains through a float valve similar to the water closet-cistern float valve). For mains pressure systems most utilities or water supply authorities require the installation of a nonreturn valve. Whereas low pressure (feeder tank) systems are open to the atmosphere through a vent pipe, mains pressure systems must be protected against overpressure (due to thermal expansion of water as heated) by a pressure relief valve, as well as against collapse (implosion), which could be caused by a suction effect (vacuum formation) in case of an aceidental water discharge, by an air intake (or breather) valve. 

Pressure Relief Devices The most common method of overpressure protection is through the use of safety rehef valves and/or rupture disks which discharge into a containment vessel, a disposal system, or directly to the atmosphere (Fig. 26-13). Table 26-8 summarizes some of the device characteristics and the advantages. 

Conventional relief valves should only be used where the discharge is routed independently to atmosphere, or if installed in a header system, the back-pressure build-up when the device is relieving must be kept below 10% of the set pressure so the set point is not significantly affected. The set point increases directly with back-pressure. 

The immediate cause of the disaster was the contamination of an MIC storage tank by several tons of water and chloroform. A runaway reaction occurred, and the temperature and pressure rose. The relief valve lifted, and MIC vapor was discharged to atmosphere. The protective equipment, which should have prevented or minimized the release, was out of order or not in full working order the refrigeration system that should have cooled the storage tank was shut down, the scrubbing system that should have absorbed the vapor was not immediately available, and the flare system that should have burned any vapor that got past the scrubbing system was out of use. 

If the plant safety shutdown is not rapid enough and an overpressure situation develops, then the pressure relief system is activated. Pressure vessel design codes such as the ASME Boiler and Pressure Vessel Code require relief devices to be fitted on all pressure vessels (see Section 13.17). If the relief system has been properly designed and maintained, then in the event of an overpressure incident, the plant contents will be vented via relief valves or bursting disks into the relief system, where liquids are recovered for treatment and vapors are sent to flare stacks or discharged to the atmosphere if it is safe to do so. The pressure relief system should allow the plant to be relieved of any source of overpressure before damage to process equipment (leaks, bursting, or explosion) can occur. 

In general, vapour vent lines must be led to a height and location where they are remote from access by personnel and from where any vapour discharge is carried clear of all buildings and populated areas. Equally, since the vessel pressure can fluctuate below as well as above atmospheric pressure, a vacuum relief valve should be fitted. 

Pressure relief equipment includes relief valves, safety valves, rupture discs, piping, drums, vent stacks, pressure indicators, pressure alarms, pressure control loops, and flare systems. Pressure relief devices can be placed on pumps, compressors, tanks, piping, reactors, distillation columns, refrigeration systems, and many other kinds of equipment. Materials that cannot be released to the atmosphere are recycled back to the system, or sent to a scrubber or flare system. The discharge from pressure relief equipment is collected in a closed piping system and sent to a flare stack. Harmless gases are discharged at a safe distance from plant operations areas. 

Each main steam line goes from its steam generator and passes through the containment boundary. The first valve encountered in each line is the main steam isolation valve, which is outside the containment boundary each isolation valve has a bypass valve in parallel with it. Beyond the main steam isolation valve in each of the two steam lines are the six steam safety valves every one of these safety valves vents to atmosphere through a discharge pipe and vent stack. Downstream of the safety valves is the power-operated atmospheric relief valve, which vents to atmosphere through a vent pipe and silencer. The operation of the power-operated relief valves is automatically controlled by steam line pressure during plant operations the power-operated relief... 

Assume that the back pressure on the safety, valve will be limited to 10% of the gauge set pressure. (The back pressure required is the sum of the constant and built-up back pressure. If the relief system discharges direct to the atmosphere, then the constant back pressure will be zero.) If the relief pressure is 4.4 barg, the set pressure is 4.4/1.1 = 4.0 barg, and the maximum back pressure is 0.1 x 4.0 = 0.4 barg. Thus 4... 

---