Vents and relief valves

Vessels should be provided with overpressure protection as required. Vents and relief valve vent piping should be so arranged Uiat Uie vented vapors will not constitute a liazard. Relief valves must be kept free from corrosion or fouling and should be operable at all Umes. 

The vapor outlet is usually a vertical nozzle located on the top head of the tower. It is usually a single nozzle, but in certain cases (e.g., towers with very large diameters), more than one nozzle is specified. On large-diameter vapor lines, the vessel connection could be butt-welded instead of flanged. In addition, the vent and relief valve could be located on the top head instead of attached to the overhead piping. Exhibit 10-32 shows a typical top head arrangement. 

Where vents and relief valves are piped to a closed system, the relief line should vent away from sources of ignition. 

Process Facilities 15 Area electrical classification 16 Accessibility for mechanical integrity (sampling, maintenance, repairs) 17 Protection of piping and vessels from vehicles and forklifts 18 Protection of small-bore lines, fittings from external impact, personnel 19 Routing of process piping, critical controls cable trays, critical utilities 20 Vent, drain, and relief valve discharge locations... 

Chlorine flow to environment Internal relief valve sticks open Both internal pressure valves fail open and relief valve opens Operation Potential low chlorine flow to Tower Water Basin -see above Chlorine released to environment - potential personnel injury due to exposure Distinctive odor Pressure check valve located outdoors -unlikely to accumulate significant concentration III Action Item Consider venting relief valve above ground level... 

When possible, point vents from relief valves and bursting disc so they will discharge in a direction away from critical areas. 

In many cases, corrosion under piping insulation is a substantial threat. Corrosion is found adjacent to pipe supports, on small bore piping for the vents or relief valves as the small diameter piping protrudes from the top of an insulated larger line or protrudes beneath the pipeline. Water can enter and accumulate in these locations. 

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. 

Miscellaneous drain and vent lines, relief valves, etc. 

When this combination-type pressure relief device is installed, some regulations require that the space between the rupture disk and relief valve be vented to the atmosphere or monitored to detect any pressure buildup on the downstream side of the rupture disk, which could prevent proper functioning of the rupture disk. 

The steam side of the affected SGU has to be depressurised to stop the flow of steam or water into the sodium. Fast-arting isolation valves disconnect it from the rest of the steam system, and relief valves open to vent the steam to the atmosphere. 

Man Cognitive function (observation) Observation missed Second operator did not observe closed status of isolation valve and closed block valve, isolating the ammonia pressure control valve from the heat exchange. Afterwards, he started to steam the process line to clean the pipeline. The closed isolation and block valves prevented the increasing ammonia pressure from safely venting through either the ammonia pressure control valve or the rupture disk and relief valve. 

In the fifth layer of protection, passive rehef devices, such as rupture disks and relief valves, provide physical protection by preventing excessive pressures from being generated within process vessels and pipelines. If over-pressurization occurs, the relief valve or rupture disk opens and the fluid is vented to an appropriate location, such as a combustion flare stack, incinerator, scrubber, waste treatment faciUty, or the atmosphere. Such passive devices operate independently of the SIS system. 

Rupture Disks A rupture disk is a device designed to function by the bursting of a pressure-retaining disk (Fig. 26-15). 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 are commonly used in series (upstream) with a relief valve to prevent corrosive fluids from contacting the metal parts of the valve. In addition, this combination is a reclosing system. 

Eliminate turned-down vents from safety relief valves, i.e., upside down U. Possible accidental ignition of releases from such vents wiU likely result in flame impingement on the top external surface of the tank, above the internal wetted surface. BLEXT Some means to handle rainwater from a desirable upward vertical vent have been listed by Bodurtha (ibid., April 1988). Moreover, a safety relief valve must function properly when required and must be sized properly to help prevent an explosion. 

Design criteria and materials of construction (for example, vents, relief valves)... 

Ah regulator, relief valve, and relay settings must be doeumented by the vendor. The expander-eompressor lube system must be test-run at design differential pressure with the reservoir vented to atmosphere. 

Most refinery process units and equipment are manifolded into a collection unit, called the blowdown system. Blowdown systems provide for the safe handling and disposal of liquids and gases that are either automatically vented from the process units through pressure relief valves, or that are manually drawn from units. Recirculated process streams and cooling water streams are often manually purged to prevent the continued buildup of contaminants in the stream. Part or all of the contents of equipment can also be purged to the blowdown system prior to shutdown before normal or emergency shutdowns. 

Conventional Safety Relief Valve - A conventional safety relief valve is a closed-bonnet pressure relief valve that has the bonnet vented to the discharge side of the valve and is therefore unbalanced. The performance characteristics, i.e., opening pressure, closing pressure, lift and relieving capacity, are directly affected by changes of the back pressure on the valve. 

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