Testing Safety Valves

Hello

What is the best way of setting the pressure of a safety valve

I have made a couple to use on a mamod type of

boiler so low pressures involved

Best Wishes Martin

Hi Martin,

Here is a description of testing of loco boilers, including safety valve. The subject is also discussed here. Hope that will be of help.

Thor

Edited By Thor on 17/06/2014 07:20:57

A simple way to test/set a safety valve is to connect it to an air line with a pressure regulator and gauge, then just slowly increase the pressure till the valve lifts. The small Mamod style boilers are low pressure (less than 20 psi) and limited volume so may or may not require a steam certificate for public events but it all seems a bit grey and open to interpretation of the rules.

PLEASE SEE COMMENTS BELOW THE 'BLUE BOOK' HAS BEEN SUPERCEDED!

The standard limit for not having a certificate is 3 bar-litres. One bar is 14.5 psi, so you can have a small boiler running at reasonable pressure without needing a test certificate (unless that's a special requirement of an organiser/host).

I am not sure this has ever been clearly explained in ME, but ironically our sister publication Model Boats has an excellent explanation here:

**LINK**

Bear in mind that boat boilers tend to be smaller and lower working pressure, so as he says some 90% of boat boilers are below the limit. The same isn't true of most model locos, but it does apply to many boilers for stationery engines, small traction engines and smaller gauge locos.

Example: my centre-flue boiler is 8cm diameter with a 2.5cm flue and 14 cm high (reason for using cm will become obvious). The cross section is 50 cm2 - 5 cm2 = 45 cm2, so that's volume (including steam space) of 630 cm3 or 0.63 litres. Lets say 0.65 litres to allow for the half-dozen small-bore cross tubes.

The working pressure is 30 psi or 2.1 bar, so the capacity in bar litres is 1.37. I could happily run it at 60 psi (it's designed to work at such a pressure, but there would be no point), or it could have been 4"/10 cm in diameter.

The test I applied to the safety valve of the (gas fired) boiler was to set the safety valve and turn the gas as high as it would go (any higher and the flame blew itself out). The safety valve passed all the steam the boiler could make without the pressure rising significantly (the official test says no more than 10%).

I can't say that any club or exhibition would allow me to steam the boiler on that basis, but it would be 'blue book' compliant and it should be possible to insure it without a test. I've seen a Tich boiler estimated at about 0.45 litres volume, but I'm not sure if any club would let one run without a pressure test.

Hope that helps.

Neil

Edited By Neil Wyatt on 17/06/2014 18:07:38

Very interesting, Neil

... and it reminded me to check this HSE publication

Slightly off-topic, but I wondered if the tank for my compressor would need an independent inspection. However, [subject to the usual HSE caveats] the threshold for this seems to be 250 bar-litres [which I do find surprisingly high] so the answer is "probably not".

MichaelG.

Neil,

The blue book is now obsolete ... it has been replaced by the 'Green' Book .... where the 3 Bar/L limit is no longer mentioned .... it is an area of great confusion in a lot of Clubs .... as the Federation's Insurance still quotes that loco's under 3 Bar/L are exempt!!!!

Dave

A 25 litre air receiver at ~100psi (i.e. 170 bar-litres- below the PSSR Regs' limit) as is typical of most home compressors [mine included], if it failed, contains sufficient stored energy to demolish a brick-built garage.... a sobering thought.

(PSSR Compliance is one of my work hats.)

Air compressor tanks seem to rot away quite quickly, at least in the small sizes, hobby use. Luckily, in my experiance, they just start leaking rather than blow PS had the pressure tank on my water pump blow, that did make a mess.

The green book does still mention small boilers, defining them as boilers below 3 bar litres in section 14 and gives details for examination and test. There is a requirement for a unique identification number. A written scheme of examination and test certificate is also available for use on clubs and societies sites. Colin

With an odd loophole for small boilers - don't fit a pressure gauge and all you have to do is show the safety valve can vent all the steam...

Serious question, does the requirement for testing apply to boilers made before 2012?

Neil

The green book supercedes the various previous books starting with the first in 2006 (the red book) It does not mean that boilers made before then are exempt from testing as they are clearly also a danger to anyone near them. A boiler of any age would need a certificate to be operated in a club or public environment and is required for insurance purposes. If the boiler cannot be tested to the code due to not requiring a pressure gauge it would probably not be insurable and would not be allowed . Colin

.

Thanks for the sanity-check, Andy

Definitely sobering !!

MichaelG.