Hot vapour cleaning: Addition of fresh solvent
This OC gives details of hot vapour cleaning processes using chlorinated solvents eg trichloroethylene which are very widely used, particularly in the engineering industry.
1 Gassing accidents may occur when plant attendants replenish tanks by pouring significant amounts of cold solvent from open vessels onto the hot surface of the hot liquid already in the tank. The exchange of heat between the hot and cold liquids can cause the temperature of the added liquid to rise very rapidly and cause it to vaporise violently in the face of the attendant.
Guidance note EH5
2 Advice about this dangerous practice is given in Guidance Note EH5 Trichloroethylene: health and safety precautions(file 294) para 46, the text of which is:
"Addition of fresh solvent
Fresh solvent should only be added when the bath has cooled down. Under no circumstances should cold trichloroethylene be added to hot sump liquor since sudden and violent vaporisation can occur. This maloperation has been known to result in the gassing of the operator and to have caused fires."
3 The Engineering NIG note that ICI Ltd is offering customers the option of a pumped system by which cold solvent can be added to a tank safely below the hot surface of the solvent already in the tank. This system can offer significant improvements in safety by removing the need for manual filling, transport and manipulation of solvent. It should also help employers to comply with COSHH reg.7 by preventing or controlling exposure to solvent vapour. While GN EH5 para 46 might be interpreted as forbidding the use of the pumped system described above under normal operating conditions it was not intended to do so. In due course GN EH5 para 46 will be amended to make this clear.
4 It is of course important that the user ensures that the pumped system is well designed and properly constructed from appropriate materials, and that an adequately considered safe system of work is devised and employed. With open-topped degreasing plant it is not possible to provide physical safeguards to ensure that cold solvent is not poured onto hot liquid in the sump of the plant. The prevention of such a dangerous procedure must depend upon the correct training and management of plant operators. Useful advice about the adoption of safe systems of work is contained in IND(G)76L Safe systems of work(file 200). The system employed should include familiarity by the operator with the operating instructions provided by the supplier of the plant. This should incorporate a warning about the danger of adding cold solvent to hot liquid, (ICI Technical Service Note No TS/B/2227/4 Operation of ICI Cleaning Plants).
5 In pumped systems where the solvent is introduced through spigots included in the plant design, the spigots should be positioned at the sump level. This ensures that the spigot is below the safety cut-out in the case of under-sump heated equipment, and below the heating coils in the case of immersion heated plants. In both of these cases before the plant oil level falls to the position of the spigot the heating of the plant will cease, either by low level shut-down or by the liquid falling below the heating coil. The plant is effectively empty if the liquid level falls to the position of the transfer spigot.
6 The transfer of hot solvent from solvent degreasing equipment except by distillation is not a recommended practice. Again the control of this aspect of the work depends upon the training and discipline of the personnel involved, but it is only mentioned rather indirectly in the guidance given to all customers by ICI Ltd and in their COSHH aid package which is on sale. In these publications customers are advised to allow plant to cool before solvent is removed. Compliance with HSW Act s.6 is a matter to be considered in appropriate cases.
7 The distilling of solvent is one situation where the transfer of warm solvent is an accepted practice. In this case the condensate produced by the plant operation is collected by gravity flow into solvent drums. The process design is such that a dip pipe from the distillate equipment enters into a solvent drum through the normal drum opening. This leaves a relatively small open area around the dip pipe where it passes through the opening which allows air to pass from the drum. The process of filling the container will have a cooling effect on the liquid being collected. The emission of solvent air mixtures from within the drum is kept to a minimum by the size of the opening. An added precaution is the use of portable ventilation which can be positioned near to the drum opening.
8 Solvent drums are suitable containers for collection of distillate. The design of the container is standard for liquid drums. Hence, most commonly available clean liquid drums can be suitable for this operation. However, appropriate labelling is necessary. Local ventilation systems are normally constructed from proprietary equipment to suit the particular site in question.
20 November 1991
Disc No: FOD1C\EDITORS\J108\10.91\DH\CP
(NEW DISC REF: J:\EDITORS\CA1\J108OC91.SAM)
Chlorinated hydrocarbons: degreasing: safe systems of work: solvents: trichloroethylene.