By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by Allendale Jan 24th

Safety Valve

LBSC 5" Safety Valve

All Topics | Latest Posts

Search for:  in Thread Title in  
Speedy Builder501/02/2021 15:34:55
2498 forum posts
196 photos

Having problems in making an LBSC safety valve.

Have made the valve for my 5" SPEEDY but can't get it to seal properly to retain pressure up to the required 90psi.

The valve has the conical seat as per drawing, but I don't seem to be able to get the seat perfectly steam tight much over 20psi. At 90 psi, It far bubbles away when testing with air.

In desperation, I have tried lapping with "Brasso" to no effect. What other tricks do you use ?



Jeff Dayman01/02/2021 15:51:27
2200 forum posts
45 photos

I see one potential issue in your pictured section of the LBSC design. The point at the end of the 1/8" dia spring rod applies force above the seating line of contact. This will likely cause the valve poppet to oscillate on the seat due to unbalanced forces. drilling into the poppet and making the rod longer to apply the spring force below the valve seating may help.

Or you could change to a Gordon Smith safety valve design and have no more trouble ever. A 5" gauge mild safety valve by Gordon was presented in ME 31 May 2002 page 300 as part of the "Loch" and "big goods" locomotive design series by Neville Evans. I just happened to have a clipping of that page in my Gordon Smith file. I have built several safety valves to Gordon's designs of various sizes and they all work really well.

Jeff Dayman01/02/2021 16:00:49
2200 forum posts
45 photos

sorry it was page 399 not pg 300

SillyOldDuffer01/02/2021 16:12:34
7904 forum posts
1725 photos

Pondering this in complete ignorance, I'm more confident of making suggestions because Jeff has spotted the same thing, and I rate his opinion.

LBSC's design is very simple and I can only see 3 ways it could go wrong:

  • The spring isn't strong enough
  • The valve and seat aren't perfectly round, smooth and aligned.
  • The plunger tilts the valve sideways on the seat. As Jeff suggests this could well be because the plunger point sits too high on the valve; ie LBSC's cone shaped hole isn't quite deep enough. Or maybe the plunger is too loose a fit in the nicked cap so the rod can tilt sideways, or the cone hole and rod point are slightly off centre causing the valve to rock on the lip rather than seal evenly. Of these I like Jeff's suggestion most.


Edited By SillyOldDuffer on 01/02/2021 16:13:18

Dave Wootton01/02/2021 16:19:18
228 forum posts
56 photos

Hi speedy builder

Jeff is exactly right, I made these to the drawing for a friend, same problem, if you extend the pointy bit down past the contact line they do work, but we did find the re closing was hit and miss. I made some pop valves to replace them, and some for my Simplex to Jim Ewins design which work perfectly, but the pop action is so sudden it makes everyone jump and small children cry!

The Gordon Smith valves are really excellent, Polly models sells the drawings and springs and they are easy to make.

I must admit we didn't persevere or experiment much with the LBSC valve, so could probably have been made to work better if we tried harder.


duncan webster01/02/2021 16:21:28
3702 forum posts
69 photos

+1 for Gordon Smith design. If you contact Polly models they will sell you the right spring and send you a drawing. Having the right spring is important.

Speedy Builder501/02/2021 16:30:47
2498 forum posts
196 photos

The spring holds back the main flow of air (steam not possible yet) up to the set pressure of 90psi, its the weeping which is the problem. I will lower the point where the "spring rod" makes contact with the valve and see what different that makes.

Can someone paste a picture of a G. Smith design SV please ?

Durhambuilder01/02/2021 17:09:51
70 forum posts
5 photos

try this

Dave Wootton01/02/2021 17:31:57
228 forum posts
56 photos


I made a mistake in my earlier post re Speedy safety valves, it was my Simplex valves to Martin Evans drawings that wouldn't re close consistently, The problem with the Speedy valve, after modification was that it wouldn't clear the steam on the steam test, the pressure rose above the limit and the boiler tester wasn't happy. I think Speedy only has the one safety valve, sorry for any confusion, it was in 1985!.

As I said we didn't experiment further, it was quicker to make new valves to the proven, Jim Ewins design.


All Topics | Latest Posts

Please login to post a reply.

Magazine Locator

Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!

Find Model Engineer & Model Engineers' Workshop

Support Our Partners
JD Metals
rapid Direct
Eccentric July 5 2018
walker midge
Eccentric Engineering
Subscription Offer

Latest "For Sale" Ads
Latest "Wanted" Ads
Get In Touch!

Do you want to contact the Model Engineer and Model Engineers' Workshop team?

You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.

Click THIS LINK for full contact details.

For subscription issues please see THIS LINK.

Digital Back Issues

Social Media online

'Like' us on Facebook
Follow us on Facebook

Follow us on Twitter
 Twitter Logo

Pin us on Pinterest