Overpressure device

Standards and codes have very speeific safety requirements for Uie design of tanks for flaimiiable liquids or gases and pressurized chemicals such as aimnonia. Overpressure devices and vents are included in Uie design. Tank and vessel supports should be built for the ma.ximum intended load. 

Internal Detonations or Explosions An internal detonation or explosion may occur due to several scenarios. Air leakage into the system may cause a combustible mixture to form, undesired chemical reactions may occur, and extremely rapid vapor expansion may occur. These almost instantaneous events have to be carefully protected against as many overpressure devices do not react quickly enough to prevent the vessel from rupturing. 

A small block valve is usually installed at a tank on any instrument impulse line just in case a leak develops on the small tubing. In this unit block valves are generally avoided on overpressure or vacuum relief devices. If a block valve that can defeat an overpressure device cannot be avoided, such a valve is chained and locked open or car-sealed opened and a protocol is developed to administratively control accidental closing of such a prime safety feature. 

Operations supervisors should provide procedures to ensure proper isolation of flammable, toxic, or environmentally sensitive fluids in pipelines. Typically these procedures must be backed up with the proper overpressure device. If the trapped fluid is highly flammable, has a high toxicity, or is otherwise very noxious it is not a candidate for a standard rupture disc or safety relief valve, which is routed to the atmosphere. Those highly hazardous materials could be protected with standard rupture disc or safety valve if the discharge is routed to a surge tank, flare, scrubber, or other safe place. 

This 12-ft. (3.7 m) diameter and 24-ft. (7.3 m) high tank was equipped with a fill line from the process, a 4-inch (10 cm) overflow line, a 3-inch (7.5 cm) vent line and a 6-inch (15 cm) vacuum relief device. Overpressure protection was intended to be supplied by the vent system piping or the overflow line. The designers did not include a high-maintenance pressure-relief device, because the 3-inch (7.5 cm) valve-free vent line was the overpressure device relieving into the scrubber. Neither the vent line nor the overflow line were equipped with block valves. The 4-inch (10 cm) overflow line was routed to a chemical collection/treatment sewer. 

With a newly installed precautionary blind in the overflow line, the hidden blind in the vent line, and no additional overpressure devices, the tank was destined to fail. As liquid was pumped into the vessel the inerts had no place to go and the tank was pressurized to destruction. To the best of everyone s knowledge and the evidence, this hard-to-detect blind may have been inserted over a year prior to the incident for a previous internal inspection of the acid tank. Obviously the mechanic(s) who removed the other blinds a year or so before and on this occasion did not notice this one. 

At the PPG Lake Charles facility, in 1990, about 30 percent of the overpressure devices are tested annually another 60 percent are tested every two years and the remainder are tested at other frequencies. Typically, SRVs on the following types of equipment are tested at the corresponding intervals. [8]... 

In the previous stage of the reconstruction programme for Units 1 to 4, several technical measures to improve nuclear safety during low power and shutdown conditions were implemented for example, installation of a cold overpressurization device in all units, development of a feed and bleed procedure, and installation of complementary emergency feedwater system located outside the turbine hall were completed. 

There are several overpressure devices, each with particular applications. Some devices are suitable for gas and some for steam, others for liquids. Some work for combinations of gases, vapors, or liquids. Some require more maintenance and testing than others to ensure they operate correctly and reliably. Some are subject to corrosion, scale buildup, and other problems that could render them inoperative or make them operate poorly. Valves are suitable for clean materials, because they must reseat after relieving pressure. Valves minimize loss of material. Rupture discs are better for... 

The American Society of Mechanical Engineers (ASME) has a standard for pressurized piping and containers. A well-established standard is the Boiler and Pressure Vessel Code. ASME also has standards for piping, flanges, fittings, and other elements under pressure. The standards give specifications for installation and certification testing of overpressure devices. 

Explain the operating principle for the following overpressure devices ... 

Pressurized Equipment Gas cylinders properly marked, stored and handled Overpressure devices in place and operational Training and safety procedures in place ... 

At the PPG Lake Charles facility, in 1990, about 30% of the overpressure devices... 

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