safety valve problem

[Chris ShewardParticipant @chrissheward84999]

I have recently completed a 5inch Stirling single which was a project which came with faulty safety valve. I have since purchased a new valve from Blackgates Engineering. this blows off at a factory set 80psi. the problem I have with it is that it uses most of the boiler pressure before it shuts off again. I did not fit superheaters . Someone has suggested that perhaps the water gauge reading is under stated and there is too much water in the boiler causing this affect. I would appreciate any members views on this. It is a tall pop type valve. thanks Chris.

[Chris ShewardParticipant @chrissheward84999]

[Former MemberParticipant @formermember19781]

[This posting has been removed]

[duncan webster 1Participant @duncanwebster1]

send it back and ask for a refund would be my first approach. Otherwise there was an article in one of the mags by Gordon Smith which went into pop type valves in detail. You can reduce the pop effect by judicious machining, but if you're going to start machining you might as well make it from scratch. I think Polly sell the springs and drawings, proper springs being the most difficult bit to find

Edited By duncan webster on 09/03/2020 18:01:10

[SillyOldDufferModerator @sillyoldduffer]

I recall reading about safety valves as developed for full-size that considerable effort had to be put into designing valves that couldn't be interfered with. Chaps were much inclined to add extra weights or tighten safety valves down to get extra power out of the engine, often causing the boiler to explode.

Once safety valves were idiot proofed, the view seems to have developed that the best way to manage a full size boiler was for the operator to keep it below safety pressure. That agreed, safety valves were designed purely with safety in mind and not dual-purposed as a pressure relief valve intended to economise on steam. This type of safety valve can assume excess pressure is a genuine emergency and immediately dump as much energy as quickly as possible to protect the boiler and bystanders. The objective is to prevent an accident, not regulate the boiler pressure.

If it's impossible for a model loco driver to keep the boiler at operating pressure himself, how about fitting a second valve, one that acts several psi before the safety, throttles the steam output, and is designed to close immediately the pressure drops rather than pop open. Probably too crude for a model loco but a pressure cooker type weighted valve has the right characteristic.

Dave

[Geoff RogersParticipant @geoffrogers81118]

as per Duncan, I am sure Polly will be able to provide a Gordon Smith design valve that will work.

Geoff

[Former MemberParticipant @formermember19781]

[This posting has been removed]

[Jeff DaymanParticipant @jeffdayman43397]

+1 on the Gordon Smith safety valve designs. None better that I have found. They have a soft lift / open action and usually close at 5% or so pressure drop from lifting pressure. You don't empty the boiler as happens with many other valves!

[Paul KempParticipant @paulkemp46892]

Chris,

As others have said Gordon's work is a very good reference on these valves. Also if you can get a copy an article written by Ross Bishop in Australia. I designed a pair of valves for my traction engine with guts as per their advise and they took a little tuning, but work just fine.

The problem you describe is unlikely to be a high water level especially if all you see is steam coming from the valve, if it was picking up the water you would A, get very wet. B the water level would drop in the glass very quickly!

Sounds like the hysteresis of your valve is wrong. Basically to get the valve to open at a predetermined pressure is quite a simple task of balancing the force on the 'valve' (ball) exposed to the boiler pressure by the seat area against the static load of the spring at that point. However as soon as the valve opens you get a larger area presented to the steam pressure and you now also have a dynamic component due to the flow / velocity of the steam past the seat. Thus to get it to reseat quickly with minimal pressure loss you need a good spring rate (load / extension) and the correct annular area around the valve for the steam to escape.

To fix your valve you may need either a higher rate Spring, an increase in annular area or a combination of both. Making and tuning your own valves is not difficult just fiddly on adjustments (no point tuning on air as it doesn't react the same with steam) and it can be a challange to get the correct / ideal springs.

Paul.

[Peter Simpson 1Participant @petersimpson1]

In my previous live as maintenance supervisor on an offshore oil platform we maintained 1" x 1" domestic hot water to 6" X 8" Hydrocarbon Pressure Safety Valves. I hated them but the larger valves were easier to maintain and recertify than the smaller ones. We used a contract valve company to overhaul and recertify them, They were brilliant at sorting them. Never want to see a PSV again.

[Nigel Graham 2Participant @nigelgraham2]

Dave (SOD) –

It's not a matter of being "too crude" that means a weighted safety-valve as a pressure-cooker won't work well on a loco, but stability when the locomotive is moving. I think they found that very early on, in full-size.

Otherwise that pattern, with a concentric column-type weight rather than steel-yard style, can work well, and was used on static boilers – locked of course to prevent anyone adding weights!

On a miniature though, the weight necessary may also be of too large diameter and height for scale, even if of lead (inside a brass sleeve).

I think many builders do set the valves at slightly different pressures, by about 5psi, or tolerance-differences give that effect; but I am not sure it achieves much. I don't understand your sentence about steam economy though. As you say, the safety-valve is exactly what its name says, and although steam released through it keeps the pressure to the safe limit it wastes water and fuel, (the latter less significantly in our sizes perhaps – except in an IMLEC run!).

For economy and efficiency the aim is to maintain pressure as near as possible to a tad below the red line, so safety-valves that close promptly as you suggest would be right – but we should still try, as on full size, to minimise the valves lifting at all.

It does depend on the individual engine and a larger boiler is easier to manage by reacting more slowly, but it's all still part of the fireman's art!

++++

A Digression but Related!

My club built a 7.25 "g. version of LBSC's Juliet, for portable track use. We found it possible to keep it simmering with a dull fire, under watch but little other attention, for even a fairly leisurely lunch-break. Once, at a (full-size) TE Rally with the portable railway, my mate Norman and I had stopped for lunch with Juliet happy on high water, pressure at about 50psi from 90, blower a very gentle breath with an occasional little wake-up to keep the deep fire a soft red – all set for easy recovery.

A young man parked the roller he was driving solo, between us and the beer-and-chips tent about twenty yards away; and disappeared into it. After perhaps ten minutes its valves lifted, and were soon blowing hard. Some twenty minutes later again, we were wondering if it would be polite to give its gauge-glass a look when His Nibs re-appeared anyway, and put its injector on. For an appreciable time, too.

He saw us watching him, came over and enthused about how he was driving for its owner absent that day, and that as a fireman on a certain preserved railway "you should see how I have 'em blowing off!". All right, he knew that roller's characteristics (we hoped), we didn't. Even so…. Norman and I just looked at each other in disbelief…..

[duncan webster 1Participant @duncanwebster1]

Some of the pre-grouping railway companies used to fine the crew if the loco blew off, wasting coal!

[Simon CollierParticipant @simoncollier74340]

Some, perhaps many, commercial fittings are not fit for purpose. I have been forced to make my own safety valves and check valves, for starters. The Gordon Smith articles were in EIM several years ago, from memory. I ordered springs from Polly and made safeties for my Simplex and Springbok. Kozo is a good resource on making these fittings too. It was a tip in his The New Shay that enabled me to make check valves that sealed completely.

[Author]

Posts