Pressure Relief Piping

The most important design factor about pressure-relieving devices is the underlying principle of intrinsic safety. They must fail safe or not at all. 

Discharge Pipe Size to 1 Discharge Pipe Size to 

Closed system (to vent stack, burning stack or scrubber) Open system (to atmosphere) 

Flammable or toxic fluids are considered hazardous. Discharge pipe not required if outlet over 7 feet above walkway and/or directed away from personnel. 

Metal covers in some sizes are also available commercially. 

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The relief piping should be segregated into pressure relief and atmospheric relief systems. If there is a wide range of set pressures on relief valves, the pressure relief piping is sometimes divided into high pressure and low pre.ssure systems. 

A-8 SPECIFIC INDUSTRIAL PIPING SYSTEMS -STOP VALVES IN PRESSURE RELIEF PIPING (SEE PARA. IP-7.2.1)... 

If stop valves are located in pressure relief piping, and if any of these stop valves are to be closed while the equipment is in operation, an authorized person should be present. The authorized person should remain in attendance at a location where the operating pressure can be observed and should have access to means for... 

The chlorine absorber is enclosed by a jacket through which a hot or cold fluid can be pumped to maintain the absorber contents at any desired temperature. A pressure relief pipe emerging from a point near the top of the absorber is equipped with a rupture disk, a thin metal membrane designed to break if the absorber pressure rises above a certain value. If this happens, gas flows from the absorber through the relief pipe to a surge tank, lowering the pressure in the absorber while containing the released gas. 

The water leak rate increases as the tubes break, but is limited by the feedwater pump capacity. After the water leak rate reaches the peak value, the main steam pressure decreases because the turbine trips due to the reduction of the main steam flow rate, and the feed water pump trips due to the reduction of its rotational firequency. Due to these quasipassive features, the water leak rate decreases and adverse consequences can be avoided by providing two pressure relief pipings with rupture disks at Ae bottom of the steam generator which operate at a pressure of 11 kg/cm. ... 

The discharge of vents and pressure relief pipes should be to areas safe from both the fire and toxic hazards. It should not be within a building. Since the vent of the feedstock storage tank must discharge a similar vapour in a safe place, the feedstock tank is often a suitable catchpot for the disengagement of any liquid droplets that may be carried by vent discharges, provided that sufficient ullage in the tank is maintained at aU times. 

To avoid this situation, some regulators are designed with a built-in over-pressure relief mechanism. Over-pressure relief circuits usually are composed of a spring-opposed diaphragm and valve assembly that vents the downstream piping when the control pressure rises above the set point pressure. 

Particularly reasonably priced explosion isolation of systems involves the use of a relief pipe with which the flow direction can be diverted by 180°. It prevents flame jet ignition with precompression in constructionally protected equipment. If suction is present, explosion propagation can occur. To prevent this, the use of an additional extin-gui ing barrier or a rapid-action gate valve is necessary. If a diverter is installed where positive pressure feed is given, then the diverter is, in general, safe against an explosion propagation. 

Provide automatic isolation of associated equipment via quick closing valves Provide weak sections (for pressure relief) in piping and duct work... 

Pressure relief is not required when the heat exchanger, including upstream and downstream piping and equipment, is designed for two-thirds of the high pressure side design pressure (section 3.18.2). 

While the above permitted variations are not used for selecting the pressure setting of pressure relief devices, these variations may be used for piping considerations such as startup, shutdown and emergency conditions, provided that the pressure vessel limits are not exceeded and that such variations are permitted by local codes. 

The basis for design overpressure described in this section is related to the ASME Boiler and Pressure Vessel Codes and ANSI B31.3, Code for Petroleum Refinery Piping. Compliance with these codes is a requirement, or is recognized as the equivalent of a requirement in many locations. Where more stringent codes apply, the local requirements must be met. Therefore, local codes must be checked to determine their requirements. For example, some countries do not permit the use of block valves underneath pressure relief valves, unless dual valves with interlocks are installed. Also, in some cases, 20% accumulation under fire exposure conditions is not permitted, and accumulation allowed may be lower than the ASME Codes. In the United States, the ASME Code is mandatory, since it is a requirement under the Occupational Safety and Health... 

Except for special sihiations, pressure relief devices are not provided for fire exposure of heat exchangers, air fins, or piping, nor are the exposed surfaces of such items included for calculating the fire exposure heat input. Special situations may be congestion and substandard spacing, or unusually large equipment with normal liquid holdup over about 4 m and/or which represents over 15% of the total wetted surface of the system to which it is directly connected for pressure relief. 

This section describes the requirements for the design and installation of pressure relief valve inlet and outlet piping manifolds and valving, including safety valve and flare headers. 

Some method of pressure relief is required on all pressure vessels and for other proeess equipment where inereasing pressure might rupture the vessel. Mueh of the piping used in modern ehemieal operations also requires overpressure proteetion. Safety relief valves or rupture dises are employed for pressure relief. In many eases, either a rupture dise or a safety relief valve ean be used. Safety relief valves are usually used for proeess proteetion and rupture dises are used for vessel proteetion. The safety relief valve or rupture dise must be designed to operate at a known pressure and prevent the pressure within the system from inereasing. Therefore, it is important to eon-sider the flowrate the valve ean handle. 

Water flowed from the relief pipe (20 ft elevation) to tank E610. It first flowed through the jumper to the process pipe to the normally closed blow-down valve which was open (possibly inadvertently left open or failed to seat in the attempt to pressurize the tank). About 119 gallons of water flowed through this open blow-down valve, through another, normally open, isolation valve to enter tank E610, where it reacted with MIC. 

Reactor Cooling System - The coolant and fission products are confined within the piping and vessels of the reactor cooling system. This boundary may fail by high pressure relief,... 

The most important safety devices in a production facility are the pressure relief valves, which ensure that pipes, valves, fittings, and pressure vessels can never be subjected to pressures higher than their design pressures. Relief valves must be designed to open rapidly and fully, and be adequately sized to handle the total flow of gas and liquids that could potentially cause an overpressure situation. They relieve the pressure by routing this stream to a safe location where it can be vented to atmosphere or burned. 

For gases with specific heat ratios of approximately 1.4, the critical pressure ratio is approximately 0.5. For hydrocarbon service, this means that if the back-pressure on the relief valve is greater than 50% of the set pressure, then the capacity of the valve will be reduced. In other words, if the pressure in the relief piping at the valve outlet is greater than half (he set pressure, then a larger relief valve will be required to handle the same amount of fluid. 

The pressure in the relief piping is usually equal to atmospheric pressure as long as no relief valve is relieving. There is a common miscon-... 

When the relieving scenarios are defined, assume line sizes, and calculate pressure drop from the vent tip back to each relief valve to assure that the back-pressure is less than or equal to allowable for each scenario. The velocities in the relief piping should be limited to 500 ft/sec, on the high pressure system and 200 ft/sec on the low pressure system. Avoid sonic flow in the relief header because small calculation errors can lead to large pressure drop errors. Velocity at the vent or flare outlet should be between 500 ft/sec and MACH 1 to ensure good dispersion. Sonic velocity is acceptable at the vent tip and may be chosen to impose back-pressure on (he vent scrubber. 

The process involved in the incident is concerned with the separation of crude into three phases. The crude is pumped into a two stage separation process where it is divided into three phases oil, gas, and water. The water is cleaned up and dumped to drain. The remaining mixture of oil and gas is then pumped into the main oil line where it is metered and sent on for further processing. A simplified process diagram is shown in Figure 7.1. The case study described here is centered on a flange leak in one of the oil pipeline pumps (pump A) and its associated pressure relief valve piping. 

Started. Since pump A and its associated pipework was off-line, the supervisor took the opportunity to carry out scheduled maintenance on the pressure relief valve (PRV) downstream of pump A. The valve had been malfunctioning, and although the work was not scheduled to be done for some weeks, the specialist contractor team who maintain the PRVs had a team available to carry out the work immediately. The supervisor therefore now had two teams working on the pump A systems the shift maintenance team working on the pump itself, and a two-man contractor team working on the PRV and its associated pipework. The PRV for pump A is not located immediately adjacent to the pump, and is above floor level, close to a number of other pipe runs. The following description represents a hypothetical sequence of events based on the inquiry findings, but embellished for the purposes of the case study. 

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