Hey Guys:
This summer I worked for the Conway Scenic RR on their former CN 0-6-0 no. 7470. We this summer experienced a problem (popping low-- 85 p.s.i.) with the modern "code compliant" industrial safety valves which the engine has been fitted with. This seems to be a persistent problem in this industry, these industrial safeties seem not to like being blown on a day to day basis. I noticed a while ago that Strasburg RR was selling replicas of older railroad type safety valves to alleviate the problem.

I'm taking a class right now where I need a design project (I am a mechanical engineering student). I'm considering this as a potential project-- perhaps trying to design a valve that is modern code compliant and better suited to railroad use? Does anyone know what the principal design differences are between the modern valves and the ones originally used? Could this potentially be a fruitful (or at least meaningul) investigation or are there already solutions available on the market?

Regards,
Trevor Hartford

Reading between the lines from Kelly's post, it is not the design that is at fault, it is the lack of enough customers to justify the certification and insurance to make these valves compliant with boiler law. I believe the valves have to be laboratory tested to receive certification from the insurance industry, ASME, etc., and these costs are too high for the small number of valves that could be sold (unless someone really wants expensive valves).

I suspect these tests came into force after the valves ceased production. Even if the valve designs were grandfathered, I suspect the certification applies as much to the manufacturing facility as to the design. So even if the design is "certified", the manufacturing plant would have to send samples for testing to assure authorities that their quality control was adequate. Speaking of quality control, the certification process for safety items like this today would frequently include inspection of the quality control measures of the manufacturing facility as well, such as requiring ISO 9000 management procedures. So, I don't think a new design would solve the problem.

Modern code safety valves are kind of a one-use valve - unlike the old style of valve. They are designed to relieve pressure when the automatic boiler for some reason glitches and exceeds the limit, than blow down a large quantity of steam, hopefully shutting off the plant in the process. Ever try to limit the new valves to just a 3 or 4 pound blowoff? I don't think its possible....at least not in the ones I have tried. After the first use, the popping off pressure seems to decline also.

Lots of people use one noncode locomotive style valve set at a few pounds lower than the code valve - it does the real work, whole the code valve satisfies the inspectors.

I wonder if there may be a source for locomotive-usable valves, from a European source perhaps, with an international certification, that we could try?

dave

Try marine suppliers, marine boilers' safeties work the same way and RR steam engines do. Three or four safeties with them set to open sequentially as pressure rises and reseat when the pressure is 10 to 15 lbs below the lifting pressure. Biggest problem will be finding safeties that are small enough. Most of the safeties I worked with were about three or four feet tall from the mounting flange.

Locomotive safety valves were designed for continuous operation. The keys to successful operation are precise tolerances and proper spring size. The valves are very simple, but therein lies their complexity.

Today’s pressure vessel world is almost completely unfamiliar with operational requirements for steam locomotives. The new steam rule in 49 CFR requires that a valve be set to pop at MAWP. This was never a requirement in the old rules because the allowable operating tolerance of these valves for pressures between 71 and 300 psi is plus or minus 3%. Even the ASME code of 1952 for locomotives states that no valve shall be set higher than 6 lb above MAWP, but there is no requirement for setting one at MAWP. The railroads could not afford to lose 3% of steam pressure by setting a valve 3% below MAWP to ensure that the valve never pops above MAWP.

Typically we set our valves at around 197 psi for a 200 psi boiler. The first pop of the day is often higher than 197 but not over 200. This avoids a defective condition according to the CFR. If the valve popped at 203 psi, it would still be within the allowable tolerance for a new valve (circa 1952).

As we have come to expect precision in our digital world, we sometimes lose sight of the realities of some of the devices used on our locomotives. Blowback is another issue. While an old valve can be made to operate with 2 – 3 lbs of blowback, the old requirements stated that 2 – 4% blowback was acceptable. The valves work best with 3 – 4% ( 6 – 8 psi @ 200 psi).

Comments elsewhere regarding certification of these things are correct. If we sold a brand new valve to replace every valve in service today, we still wouldn’t have enough money to go through the certification process. The FRA allows the use of locomotive style valves while most states don’t. The practice of setting a non-code valve below MAWP as an “operating valve” and using the code valves for backup is usually accepted and works pretty well without too much loss of power.

The valves we make, based upon the Consolidated design are not perfect. With proper care they will work within the parameters required by the 1952 locomotive code. Maybe a better valve could be designed, but if it has no historical precedent, I would suspect that it would have to be certified for use in any jurisdiction. Designing one as an exercise would be a fascinating project.

As Linn stated locomotive safety valves are designed for continious operation. Stationary boiler safety valves are not. As someone who has made his career in boilers I can tell you that stationary boilers do not "pop off" on a regular basis. Most stationary boilers have a high limit switch that shuts down the boiler prior to ever reaching the pressure required to pop the safety. Thus most safety valves never pop. In fact many boiler users send out safety valves to be bench tested on a routine basis and never allow the valves to pop in regular service. Many really large safety valves will not close with a tight seal after just one pop.

Dear Trevor:
You didn't state what brand safety valve you experience difficulty with. I would be curious to know.
At The Valley Railroad, we have been using KUNKLE ASME top outlet safety valves assembled by North American Safety Valve Co. with stainless steel seats and disks for about 15 years with good results.
That being said, we have used only this one safety valve assembly firm to achieve these results. We have had trouble with other assemblers.
You see, KUNKLE no longer assembles it's own valves. They sell parts to various outfits which assemble, set, test, sell and service the valves.
An 1-1/4 inch top outlet valve with stainless steel trim costs about $450.00 and a simple servicing (lapping and resetting) costs about $225.00. Turn around time is usually less than a week.
Typically we get over two seasons use out of each primary valve before it needs to go back for servicing. Usually we discard the valve after two service cycles. It is more cost effective than a total rebuild. The secondary valve on our No. 97 is over ten years old.
We keep a spare set of valves on hand in case of problems which cannot be cured by a small adjustment.
Are these valves as good as a "real" locomotive safety valve? No. They are lightly made, not really rugged and designed for long life like an Ashton or Coale. But they are readily available and cheap.
Good luck with your project.
J.David

Are "Safety Valves" and "Reliefs Valves" the same? We had relief valves on the piping as well as the pressure vessels of the plant I worked at. On occasion, the steam consumption would get out of wack with the amount of steam being produced, and the relief valve on the main steam line would lift. I hated being on the nearby road when it did; you could hear it all across the plant.

On a side note, I worked with an engineer who used to work at PP&L's Eddystone plant. It had a supercritical boiler that once had the highest main steam pressure in the world (it has since been derated); he remarked that as you drove in, you could tell from miles away that the unit had tripped, the noise of the steam escaping from the relief valves was so loud.

-James Hefner
Hebrews 10:20a

Surviving World Steam Project

I'm sure I'll get corrected quickly if I'm wrong, but as I recall,

A Safety Valve lifts at one pressure, and resets at a lower pressure.

A Relief Valve lifts and resets at the same pressure.

Greg

The reprints of Crosby and Ashton manuals are available at

http://www.crosby-steam.com