Vacuum relief devices

Maintenance and testing. It is not a good idea to apply vacuum on a vessel during maintenance or testing without full knowledge of the external pressure rating, unless a suitable vacuum relief device is in place and operable. 

Location of Vacuum Relief Device (Carl Schiappa, Michigan Engineering, The Dow Chemical Company, Midland, Mich., personal communication, March 20, 1992.) If a vacuum relief device is used, locate the device at the highest point on the top of the tank. If the vacuum relief device is not installed in this location and the tank is overfilled with liquid, the relief device will be sealed in liquid and will be ineffec tive in protecting the tank. This is especially true for the part of the tank above the vacuum relief device if it is sealed in liquid, tne liquid level is lowered, and the tank goes into a partial vacuum. 

Examples of Vacuum-Related Accidents Figure 26-47 shows a jacketed tank, where the jacket was designed for low-pressure steam. When the steam was turned off and the drain valve and trap were closed, the steam condensed, causing the jacket to collapse. The jacket should have been designed for full vacuum, or a suit le vacuum relief device should have been installed on the jacket. 

Provide vacuum relief device/system (can be a source of oxygen in vapor space resulting in flammable atmosphere)... 

Failure of compo- Ensure all system components, including flexible nents in connectors are rated for maximum feasible subatmospheric vacuum conditions pressure convey-, Ensure adequate pressure control system and ing operations. back-up (e.g., vacuum relief devices) API 2000 CCPS G-3 CCPS G-11 CCPS G-22 CCPS G-29 CCPS G-3 9... 

As a general rule, vacuum relief devices are permitted on offsite storage vessels handling clean finished products, since there is essentially no possibility of an internal ignition source. However, vacuum relief devices which permit breaking of a vacuum with inerts or flammable vapors are not permitted on process equipment, since they are not judged to be sufficiently rehable to provide adequate protection under all circumstances. Vacuum devices which permit air to enter may be considered, however, in cases where the equipment does not or cannot contain flammables e.g., some steam systems. 

There is an adequately sized atmospheric vacuum relief device to prevent vacuum. 

There are none or inadequate vacuum relief devices on the equipment or system being evacuated. 

API (Venting Atmospheric and Low-Pressure Storage Tanks, Standard 2000, Washington, 1998) provides guidance for vacuum protection of low-pressure storage tanks. Where vacuum relief devices are provided, they should communicate directly with the vapor space in the vessel and should be installed so that they cannot be sealed off by the liquid contents in the vessel. Valves should be avoided in the inlets or outlets of vacuum relief devices unless the valves are reliably car-sealed or locked open, or excess relief capacity is provided (e.g., via multiple-way valves). 

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. 

The vent line from the stripper was designed to connect to the reabsorber feed line and the reabsorber had an atmospheric vent. When the reabsorber is flooded, the vent line from the stripper could not provide vacuum relief because it was not in the normal gas service. Vacuum relief devices were not originally provided on the stripper. See Figure 3—6, Reabsorber-Stripper sketch. 

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. 

Lack of Respect for an Open Vent as a Vacuum-Relief Device Results in a Partial Tank Collapse... 

The possibility of equipment, which normally runs at or near atmospheric pressure, going into a vacuum condition should also be considered. Vacuum relief systems and vacuum breakers don t always work. On a hot summer day, a sudden rainstorm can cool a large tank or hopper very rapidly. This can cause rapid cooling and contraction of air inside the tank and condensation of vapors, if they are present. This can cause very rapid lowering of pressure inside the tank, which can implode the tank if insufficient provisions have been made for air to enter the tank. These conditions are common in northern climates where the rate at which a vacuum can be produced may be increased in cold weather. In many instances, vacuum devices that are supposed to work are frozen and inoperative and the tank implodes. It may be inherently safer to design tanks to handle a vacuum than to depend on vacuum relief devices alone. 

Lack of respect for an open vent as a vacuum-relief device results in a partial tank collapse... 

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