Pressure Safety
REQUIREMENTS AND POLICIES MANUAL
https://commons.lbl.gov/display/rpm2/Pressure+Safety
RPM | REQUIREMENTS AND POLICIES MANUAL
Work Process E. Vacuum Systems
(Edited from SLAC-I-720-0A29Z-001-R023.4, Chapter 14, “Pressure Systems”)
A vacuum system consists of a vessel, its associated piping, and related components operating below atmospheric pressure. A vacuum vessel can pose a potential hazard to personnel and equipment from collapse, rupture during backfill pressurization, or implosion due to vacuum window failure.
Vacuum systems must be designed, fabricated, and operated in accordance with applicable codes and sound engineering principles. The essential design requirements for positive pressure systems apply to vacuum systems.
Vacuum System Classification. Any new vacuum system, or any legacy system that must be brought into compliance as described above, must be categorized as follows:
Category I. Category I vacuum vessels include all vessels in which the differential operating pressure can never exceed 15 psig.
Category II. Category II vacuum vessels include all vessels that can be protected from pressurization exceeding 15 psig through such engineering controls as pressure relief devices.
Category III. Category III vacuum vessels include all vacuum vessels that are not or cannot be protected from pressurization exceeding 15 psig.
Shielding Requirements. The type of component most likely to fail catastrophically in a vacuum system is a brittle component such as a viewport (window), glass bell jar, glass ion gauges, glass or plastic vessel, or glass or brittle plastic tubing. Component failure (e.g., failures caused by an inadvertent blow or a scratch by a hard sharp object) can produce sharp-edged shrapnel.
Protective barriers may be used to reduce the likelihood of injury to personnel and damage to equipment. Some common shielding strategies include one or a combination of the following:
Placing mechanical protective shielding around components such as glass or brittle plastic tubing and glass ion gauges
Operating a system within a hood with the hood door down (size permitting)
Operating the system behind or within a polycarbonate or metal shield
Wearing personal protective equipment (PPE) such as safety glasses or a face shield
Glass viewports in Category III systems should be protected when not in use. A common strategy is to fasten a polycarbonate cover over the viewport. View ports in Category I and II systems may be protected with clip-on polycarbonate covers, if desired.
The view ports in vacuum vessels are to be treated as brittle materials and shielded from the operator or positioned away from the operator and others in the area. Any time the operator needs to look through an unshielded viewport under vacuum, safety glasses or a face shield must be worn. Operators must not look through unshielded viewports during backfilling or purging operations.
Protecting a Vacuum System from over pressurization. A common cause of catastrophic vacuum vessel or system failure, particularly if there are brittle components, is the inadvertent application of internal pressure beyond the design rating. Such pressure can occur as a result of one or more of the following:
Failure of a valve or regulator that is connected to the backfill source.
Pressure generated by a chemical reaction involving reactive gases.
Pressure realized by the accidental connection of the exhaust port of a fore pump to the inlet of the vacuum system. The vacuum pump inlet and outlet should be labelled to avoid switched connections.
Components that can protect a vacuum system (i.e., one that must be purged or backfilled with a high-pressure source capable of causing the system to exceed its MAWP) include the following:
A safety manifold
Relief valves
Rupture discs
A rupture disk (also referred to as a “burst disk”) may be incorporated into a vacuum system design to limit the internal pressure to less than 15 psig following any equipment failure. A rupture disk must be adequately sized for the credible identified failure mode and must be rated to fail at internal pressures of less than 15 psig. The rupture disk must be connected to the vacuum system and must not be isolated from the system by a valve.
"Vacuum-only" systems are not required to be equipped with relief devices as long as it can be demonstrated that it is impossible to over-pressurize the system by a credible failure scenario.
Testing and Inspection. A vacuum vessel can pose a potential hazard to personnel and equipment from collapse, rupture due to backfill pressurization, or implosion due to a vacuum window failure. Prior to use, a visual inspection should be conducted to ensure that brittle components are in safe working condition. Refer to Appendix D, Table D-3: Pressure Relief Device Inspection Frequency.
Most vacuum systems are designed to tolerate limited external pressure and contain components that render an overpressure internal proof test inappropriate. Other means are necessary to document the safety of these systems, and the appropriate method must consider system type, system size (stored energy), system complexity, and associated hazards.