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Note: This is a sub-section of Richard Trevithick
1803 September 8th. One of Trevithick's stationary pumping engines in use at Greenwich exploded, killing 4 men. A contemporary account:-
'Tuesday se'nnight, a dreadful accident happened at a steam engine, employed to assist in clearing the works from water at the Tide Mills, now erecting in the marsh between Greenwich aud Woolwich. The engine was on Mr Trevitheck's plan, and worked by the expansive force of the steam only, without employing corndensation. Unfortunately the high pressure of the steam burst the boiler, and, literally blew the engine to pieces; by which accident three men were killed on the spot, and three others, all that were there at the time, so much hurt, that two of them are not expected to recover. It was a fortunate circumstance that the accident happened at a time when the workmen were at dinner, or a much greater number might have lost their lives. The engines on Mr Trevitheck's plan require a boiler of immense strength, for they work with a power sometimes equal to 60 pounds on the square inch. (Common engines, even Mr Watt's, seldom work more than 5lb.) After such an accident as this, it would be no easy matter to induce manufacturers to employ engines furnished with boilers similar to the one that thus exploded; but another engineer has just invented a boiler, consisting of a combination of cylinders, which unites the double advantage of exposing a much larger surface to the actopn of the fire, than boilers on the common construction, with a degree of strength equal to any power of steam.'
Although Trevithick considered the explosion was caused by another case of careless operation rather than design error, the incident was exploited relentlessly by his competitors and promoters of the low-pressure engine, Boulton and Watt, who highlighted the perceived risks of using high pressure steam.
Trevithick's response was to incorporate two safety valves into future designs, only one of which could be adjusted by the operator. The adjustable valve comprised a disk covering a small hole at the top of the boiler above the water level in the steam chest. The force exerted by the steam pressure was equalised by an opposite force created by a weight attached to a pivoted lever. The position of the weight on the lever was adjustable thus allowing the operator to set the maximum steam pressure.
The second valve was in fact a lead plug critically positioned in the boiler just below the minimum safe water level. Under normal operation the water temperature could not exceed that of boiling water and therefore kept the lead below its melting point. In the event of the water running low, once it had exposed the lead plug the cooling effect of the water was lost and the temperature could rise sufficiently to melt the lead. This would release steam into the atmosphere, reduce the boiler pressure and provide an audible alarm in sufficient time for the operator to damp down the fire and let the boiler cool naturally before any permanent damage could occur