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Leaky safety valve
Posted by: Old timer (86.112.62.---)
Date: September 23, 2012 08:58AM

Hi all,
Our Stanley's safety valve whisps steam a bit, although I don't know exactly at what pressure it starts to. I'm guessing it will stop the car from siphoning full of water overnight, so how do you cure this? Is it just the case of replacing the ball inside or is it more complicated than that? The car runs at 550-575PSI and the safety is set at 600PSI. Thanks. George.

Edited 1 times. Last edit at 09/23/12 01:11PM by Old timer.

Re: Leaky safety valve
Posted by: (
Date: September 25, 2012 03:32PM

Hope you get a good answer, George. I've always heard that once these lift, they never properly seat again, and that has been my experience with my freshly rebuilt safety valve. It now behaves just as you describe yours. My boiler does siphon, though. Usually.


Re: Leaky safety valve
Posted by: barry herbert (Moderator)
Date: September 26, 2012 05:35AM

George - I do not think you have your safety valve set at the right level. It should be set at 750 psi for a car with a working pressure of 600 psi.

Your valve is leaking because your working pressure of 575 psi is too close to the present set level of 600 psi. It is starting to do its job.

I do not know how your boiler inspector has missed this.


Re: Leaky safety valve
Posted by: Mark Drake (62.189.28.---)
Date: September 26, 2012 08:21AM

Hi George,

I've gone through this with both of my condensers.

Both cars had a safety valve spindle turned to a conical 90 degree point - no ball. The 735 spindle is made from a decent grade of bronze, but started to leak after a couple of years use. I have since re-machined the cone and lapped it in with Brasso; it now does not leak after 4-5 years use.

The 740 had a spindle made of a rather inferior brass, and this started to leak substantially after 1 years use. On inspection under a microscope, there was a multitude of microscopic radial lines 'etched' into the seating surface of the cone. I suspect that very low level leaks actually eroded tiny paths in the surface of the brass, which got bigger with time; the sum of which was a large leak. I have since replaced the spindle with stainless 304 and lapped it in with Brasso - all's well so far.

These safety valves do tend to leak after they've been lifted for whatever reason (debris probably), so my view is that they should be overhauled if allowed to lift.

It's not a bad thing to take the valve off and test it on a hydraulic rig. I set both my SV's to 750psi and tested them with the inspector present, just to be sure. I then re-fitted to the cars while we drank tea and waited for the pilots to warm up!

I hope this helps - happy steaming all.


Re: Leaky safety valve
Posted by: Mike L Clark (
Date: September 28, 2012 05:47AM

No doubt steam of hot gases can erode brass. I tried a commercially available brass gas jet in my pilot - the bore very soon became much oversize - probably made worse by the heat of course so not just gas erosion.


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