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Safety Valves
Posted by: (
Date: May 9, 2006 03:48PM

Some thoughts on Stanley Safety Valves

I have just (successfully!) completed the 7 – 10 year inspection of the wire wound boilers on my two Stanleys. Amongst all the other fettling and annealing tasks, I had a chance to have a really good go at the safety valves. Although these have been dismantled and cleaned each year whilst I have had the cars, I re-seated and re-balled them both prior to the hydraulic testing.

The first realisation was that in the state that they were after only a year’s steaming made it most unlikely that they would have opened at anything like the pressure to which they were set at the beginning of the year. A valve that is not in regular use should not in my view be considered a category 1 protection device within the meaning of the current Pressure System Safety Regulations. Every Steam Car owner should persuade his ‘Competent Person’ to consider the ‘Steam Automatic’ to fulfil this role. As a safety device it has the overwhelming merit of being in continuous use and thus more likely to work than an intermittent device.

So! Why were the safety valves fitted by Stanley?

We have had learned articles that demonstrate that on the small boilers it may be possible to dissipate the energy produced by the burner through a valve fed by a tube with a nominal diameter of only 1/8th inch. Such can surely not be the case for the larger boilers.
Perhaps the main reason why they were fitted was to protect the boilers from a hydraulic overload such as would occur if a full boiler is steamed without draining sufficient water for the steam space or, if the driver inadvertently left the pumps working into a full boiler. In a hydraulic situation the valve could save the day.

During my tests it was also apparent that while it may be possible to set the valve to open consistently at near a specific pressure, it will not shut until the pressure has dropped to less than a half. An examination of the valve explains this. The ball sits over a hole of about ¼ inch. This equates to a loading on the ball of about 30 lbs. at 600 psi. However, when the 7/16ths ball lifts, it is within a chamber that so restricts the steam evacuation that the whole diameter of the ball is presented. At this point steam at 600 psi presents a lifting force of about 90 lbs.

Was this what the Stanleys had in mind?

Perhaps some of our more knowledgeable owners could add their thoughts to mine.

Peter Williams

Re: Safety Valves
Posted by: Jeff Theobald (Moderator)
Date: May 12, 2006 07:56AM

Hi Peter,

I thought we had already established the operation of the Stanley safety valve in previous correspondence.

Your description of the operation of the Stanley safety valve is correct, which is why it is neccassery to have it set about 100psi above maximum working pressure. As I said before it is not uncommon under certain circumstances to see the steam pressure 50psi above maximum working pressure.

The only time I have know safety valves to open, is as you have said, when firing with too high a water level, and when not paying attention and leaving the hand bypass in. This soon wakes up an operator of a car, to the fact that something is wrong, and as you say the safety once lifted will not shut until steam pressure has fallen far below working pressure.

The Stanley safety valve as you have said, would be unable to release the steam being generated by the larger boilers, but here is where the capability of being able to instantly remove the fire comes in, if the safety valve has operated because the steam auto has stuck or has a small bit of dirt in it, allowing the burner to continue burning, the manual override will save the day.

Golden rules never leave a steam car during steaming; never leave a steam car, parked with the main fire held off by the automatics.

Hope this adds to your thoughts, all the best, Jeff.

Re: Safety Valves
Posted by: (
Date: May 12, 2006 10:07AM


I understand how we think these things should be set for optimum performance but I still don't know how we can persuade all 'Competent Chaps' to see it our way.

How do you persuade your inspector to accept a safety valve opening 100 psi higher than the Safe operating limit' unless he has conceded that the steam automatic is a 'pressure relieving device' as defined in the PSSR? With so many of the independent inspectors dealing with the 1800+ traction engines they inspect, I suspect that most still tend to depend upon a conventional safety valve.

My only concern is that some day an insurer may refuse a claim if it is found, after an accident, that the safety valve was set above the safe working pressure. It may be too late to argue the merits of the steam automatic at that point.

Enough for now!


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