Metal cleaning can be performed through spraying/immersion in solvents, corrosive solutions and vapor degreasers, through mechanical cleaning, and, by heating treating in oil or molten salt baths (discussed in our February 2011 article). In this month’s article, we will focus on vapor degreasing.
How Does a Vapor Degreaser Work?
At the bottom of each degreasing unit is a sump where the cleaning solvent is heated into its vapor stage (boiled). The part to be cleaned is placed in a perforated basket within the vapor zone of the unit, which is above the boil sump. The solvent, in its vapor stage, encompasses the part, condenses (liquefies and removes the oils and soils from the part by dripping back into the boiling solvent. In some cases a manual spray wand is provided to allow the solvent to be directly applied to hard-to-remove materials from the part. Usually there is a second, unheated (rinse) sump; sometimes this sump is provided with an ultrasonic generator.
Near the top of the unit is a set of cooling coils that catches the cleaning vapor before it escapes from the unit (known as the vapor line). It cools and condenses it back to its liquid form and flows it back to a clean condensate tank and finally it goes back to the boil sump or the rinse sump.
The oil and grease that were removed from the part boil at a higher temperature than the cleaning solvents. If the temperature for the boil sump is set correctly, only the solvent is vaporized ensuring that only clean vapor is used to clean the part.
Degreasing solvents are mostly chlorine-, fluoride- and bromide-based. Chlorine-based solvents (trichloroethane, trichloroethylene and percholoroethylene) have fallen out of favor due to worker safety and environmental concerns.
The main concern for a vapor degreaser is to prevent overheating of the cleaning solvent to a temperature where the solvent decomposes into corrosive materials. There have been some cases where it was reported that the solvent caught fire.
NFPA does not provide a specific standard on vapor degreasers. FM Global has Property Loss Prevention Data Sheet 7-97, Metal Cleaning, parts of which describe vapor degreasers.
The first and most obvious thing is to avoid the use of flammable solvents in the degreasing unit. The operating range of the degreaser will put the flammable solvents into their vapor range; if the vapors escape the unit the chance of the vapor igniting well away from the unit exists.
Preferably, the unit should be heated using steam. If the unit is heated by fuel-fired equipment or electricity, low liquid-level interlocks are needed on the boil sump. Pressure regulators and relief valves, depending upon the type of solvent used can be used in lieu of some temperature interlocks if steam is used.
Proper equipment settings from a temperature standpoint need to be maintained. The unit manufacturer needs to be informed of the type of degreasing solvent used to ensure that the proper temperature control/interlock settings are done prior to delivery. If a change of solvent is contemplated, the manufacture needs to be contacted to determine what change of settings are needed, or if event physical changes are needed on the unit itself.
Sprinklers should be provided over the unit, but can be omitted if there are no other combustibles present and either: the roof is a noncombustible construction and the dirty solvent chamber does not exceed 25 sq. ft.; or, the roof is a fire resistive construction and the dirty solvent chamber does not exceed 40 sq. ft.
Regular testing of all controls and interlocks on the unit should be performed on a regular basis, as recommended by the unit manufacturer.
As the unit is operated, the oil and soil that drips into the boil sump increase the boiling point of the degreasing solvent. The solvent needs to be replaced when its boiling point increases to a certain point, as determined by the type of solvent. Precautions need to take place when replacing the solvent, especially is the unit is arranged to distill the solvent in the boil sump (reducing the amount to be replaced).
Procedures should be in place in the event that the runaway condition on the unit (boilover). The procedures should be geared to limit the extent of any corrosive or fire damage to the unit itself and the surrounding areas (building, other equipment and stock) through examination and cleaning.
Please contact Risk Logic Inc. if you have any further questions on vapor degreasing.