Setting Safety Valves

This is question for the boilersmiths! I'm displaying my ignorance here but when an engine completes its overhaul one of the jobs is to check and set the safety valves. Is this done simply by one person in the cab calling the pressure from the pressure gauge while the boiler dept chap sits on top of the boiler with a spanner adjusting the valve until it blows at the required pressure? It strikes me that this must be a most unpleasant and deafening task, especially on a boiler with 250psi!! Not for the faint hearted I imagine - and of course you have to hope that valve is a good 'un.

 

I remember setting the valves 0n 8F 8233 like that in the very early days of the SVR, but usually you do it on a test rig on the bench.

 

The way you describe is correct for the type of spring safety valve as were fitted to industrial loco's, but they were usually 160.lb per square ins BP.The blower would be turned well up when setting.At resetting, say after a boiler inspection ,they would start blowing off at about 145 then reach 160. As the year went by one valve would start sizzling sometimes as low as 130. A bit annoying if piece and quite was required when standing . A sharp tap with a hammer would cure this for a short while. It wasn't a skilled mans job ,done it several times myself .I believe pop valves were a different mater.

 

Seen old footage of a man with a large spanner setting the salter spring valves of a double Fairlie at ffestiniog. Fascinating to watch

 

Presumably the process described above makes the assumption that the gauge reads accurately. Is calibrating the BP gauge generally done prior to setting the safety valves in this way, or do they tend to be inherently accurate?

 

Steam pressure gauges have to be calibrated, this is normally done on a test rig, and the valves are set using balance weights, to a pre determined psi, which then has to be tested live, that's when you see the boilersmith sat atop of the boiler with ear defenders and a large spanner to increase/ decrease the spring pressure, on the valve because the variables of steam means that the setting can only ever be a close to the red line, not exactly right, so some fine tuning needs to be done to ensure the safety valve feathers just below the red and lifts fully on the red, but closes again with in I think its 10 psi of the red line

 

There is an EK video of the safety valves on an 01.5 being set at Meiningen Works. Ear defenders a vital accessory when setting German Ackermann valves!

 

Presumably you are talking about pop safety vales. In the ten years I worked on steam loco's with the spring type safety valves ,not one pressure gauge was ever calibrated . The company's boiler inspector was quite happy using the loco' pressure gauge .This was the same for hydraulic test where the pressure was brought up, with water pressure ,to half as much again ,eg 240 for a 160 lb boiler.
Pressure gauges could be damaged by frost if a loco was stored out-side in the winter, it was normal to cover the gauge with an old coat or better still take it off .

 

That's the way most safety valves are set. Whilst you can set them using a hydraulic pump it is not the same as setting them with steam as they are cold and you have to make an allowance for this. In times past, railway workshops would set the valves using a works boiler so that they would be put on a loco already set. However, with our cottage industry I don't think any railway or supplier has the facility to set them off the loco.
As regards using the locos pressure gauge, this is quite normal. However, the gauge should have been checked by the inspector or otherwise independently calibrated as part of the annual inspection. In earlier times, the inspector would put his own calibrated gauge on the boiler as a check and all boilers were provided with a plug test cock specifically for this purpose, certainly so with industrial boilers.

 

Done this a few times on Flying Scotsman took two of us to do it on top of the boiler one in the cab and quite hairy to say at 250 pounds when you are about 2 feet way then blows off and there is a gape of fresh air between the steam and safty valve i have some vidio of this and very interesting to watch at close range especially the gap in between steam and safty valve.

 

It's not actually a gap - it's just that you can't see anything until the steam that has escaped via the safety valve starts to condense, at which point you can see the 'white stuff'. That's a massive danger with superheated steam - you can't see it, but it's definitely there. One of the first things taught to me as a Junior Engineer on a ship with a high pressure and superheated boiler - never look for steam leaks with your bare hands - use a cloth on a long stick. A jet of even slightly superbeated steam can cut through skin and bone - the good news would be that it would cauterize as it cut... And for the record, even after 30 years of setting boiler Safety Valves during Surveys, as part of my responsibilities as Chief Engineer, I still hate doing it

 

Never heard that before, but from experience they do seem to lift at different pressures, I've only experienced 35005 at close hand, and I never got to the point where I had all three up at once in normal service the only time ive seen it was when she had her yearly boiler test, I was on the footplate and as you say, one lifted, then the second, just before the line then all three were up just past the line ,and it was deafening .

 

In our country only pop safety valves are allowed on steam locos.
To my experience they are more reliable than the locos own pressure gauge, the pressure gauges tend to weaken over time and gradually show a higher pressure that the real pressure. Thus this is on the safe side and the boiler inspectors allow a larger misreading in this direction than in the other.
My conclusion is that the safety valve should never be set after the loco pressure gauge. We have a few old precision pressure gauges, previously used by boiler inspectors but now been supplanted by electronic sensors. We use one of these (recently calibrated) to set the safety valves.
As you say, a very noisy and not really pleasant job. Also very coal consuming with a pop valve where you loose a significant amount of steam at every blow off.
At the yearly steam test, the boiler inspector notes the misreading of the loco pressure gauge and urge us to do something about it if it tends to be too big.
Pictures from last Sunday, preparing our first loco for our 60 year anniversary coming Saturday. The loco passed the steam test on Monday morning.
More on our blog: http://museijarnvagenimariefred.se/

 

All safety valves should be reliable. However, in the UK we are usually required to dismantle them for inspection and there is no guarantee that the valve will blow at the correct pressure when re-assembled, even if the setting hasn't been disturbed. Hence the need to check and, if necessary, set them. Pressure gauges essentially have to be checked annually, as well. These are not a specific legal requirements but the inspector generally requires this and it is usually done as part of your maintenance regime.
Your first photo shows the plug test cock with a test gauge fitted, as I mentioned in my earlier post.