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a. Explosion – Operation of the diffusion pump without prior and continuous evacuation (below 0.5
Torr (66.5 Pa)) or coolant and introducing a strong oxidizer (such as air) or explosive vapors or
powders or materials which may react with pumping fluids into a hot (above 300°F or 150°C)
pump can cause an explosion. Such an explosion can violently expel valves and other hardware,
slam open doors that are not designed for appropriate pressure relief, or burst other components
of the vacuum system. Serious injury or death may result from expelled parts, doors, shrapnel,
and/or shock waves.
b. All diffusion pumps are typically cleaned with acetone and alcohol. When combined with air, oxy-
gen, and other oxiders, alcohol and most other solvents are very flammable and explosive. Never
permit any trace of these cleaners to remain in or on the pump. Always remove all traces of alco-
hol and acetone and other cleaners with clean, dry, oil-free compressed air.
Three elements are required: fuel, oxidizer, and source of ignition. A combination of temperature and
pressure can be a source of ignition. Most diffusion pump fluids except mercury are fuels. Hydrocarbon
oils are more prone to oxidize and explode than synthetic silicone-based oil. Oxidizer can be air (a
strong oxidizer) from a leak or can be deliberately introduced in a process or can be inadvertently
admitted to the system by operator or process controller error. Oxygen and other strong oxidizers are
even more dangerous. Certain conditions of temperature and pressure can cause a combustible mix-
ture to explode. The larger the diffusion pump, the greater the risk of explosion and the greater the risk
of damage and/or injury. Never operate diffusion pumps larger than 10 inches in diameter with hydro-
carbon oils without a complete safety analysis for the complete system and application. Never operate
any diffusion pump in the following ways because they increase the probability of an explosion.
Prohibited Action Explosion-Causing Condition
Run pump with no cooling water Overtemperature
Run pump with low level of Overtemperature
pump fluid
Run pump without proper backing or Overpressure
holding pump
Run pump when not evacuated below Overpressure
0.5 Torr (66.5 Pa)
Admit air to, or rough through, Overpressure plus strong oxidizer
a pump with hot boiler
Open drain or fill plug while pump is Overpressure plus strong oxidizer
under vacuum especially when it is hot
Hazard Suggested Corrective Action