The lid of a solvent cleaning tank is not there to keep the solvent clean. It is held open by a copper link that melts at low temperature. If the contents ignite, the link gives way, the lid drops of its own accord and the fire smothers for want of air. A purely mechanical safety device that works without electricity and with nobody in the workshop.
The lid that closes itself
Three checks are enough, and they take ten seconds. Does the lid drop freely, with no stiff point and no seized hinge? Is the copper link present and undeformed? Does the sieve sit properly, so that parts do not soak at the bottom among the deposits?
The one move that cancels everything. A tank whose lid is propped open with a rag, a screwdriver or a workpiece is no longer a safety tank. It is an open container of flammable solvent sitting on a bench. Propping is the first thing an inspector looks for, because it is the most common.
Why 30 °C is the threshold that matters
The Swiss accident insurance fund, SUVA, draws a clear line: a flammable liquid becomes dangerous as soon as its flash point falls below 30 °C. Below that, a mixture of vapour and air forms above the liquid that can explode. A match flame, a hot surface or a single electrostatic discharge spark is then enough.
The same flash point then serves another purpose. Swiss fire protection uses it to classify liquids: up to 23 °C they are classes 1 and 2, from 23 to 60 °C class 3, and above 60 °C there is no classification at all. That class is what will determine how much you are allowed to keep on site.
Vapours flow along the floor
This is the point SUVA states most plainly, and the least intuitive one. Solvent vapours are heavier than air, invisible, and spread across the floor like liquids. They do not rise, they sink, spill over the edge of the bench and travel at floor level until they find an ignition source several metres away.
The rag trap. Cloth soaked in solvent or oil, piled into an open bucket, can heat itself by oxidation until it ignites, with no external source at all. It belongs in a metal container with a lid. Never in a cardboard box, never in a bag.
The three documents to have. The safety data sheet of the solvent you actually use, which gives the flash point in section 9 and the handling measures in section 7. The SUVA checklist 67071 on the storage of highly flammable liquids, which is free and designed to be filled in. And the contact details of your hazardous waste contractor, before you need them. Outside Switzerland, your national safety authority publishes equivalent documents.
So, is the copper link on your tank still in place? Tell us in the comments.
The 30 °C threshold and the behaviour of vapours follow SUVA guidance on flammable liquids. Quantities and liquid classes follow the Swiss fire protection code AEAI 26-15 "Dangerous substances". Disposal follows the Swiss Ordinance on the Movement of Waste, SR 814.610. Flash points are indicative. This article is for information and is not a compliance assessment: the applicable measures depend on the product used, the quantities and the room. SUVA and your cantonal inspectorate answer enquiries from companies.
