Railway Preservation News :: View topic - Tool for disabling safety valves during hydro test.

It was my understanding that these tools were necessary when setting safety valves on a large modern locomotive.

When there are multiple pops on a high-pressure locomotive boiler, they are set some increment apart so more than one doesn't open simultaneously and waste mass flow; I believe on N&W these are 2psi apart. In order to test those valves that open at higher pressure, the ones at lower pressure have to be gagged, otherwise by the time you get to #4 you'll have quite an adventure with fat violet MHD sparks crawling over everything, great clouds of steam and noise, and the need to fire the boiler dramatically enough to sustain pressure.
It was my opinion 'back in the day' that the gag used for this purpose could itself contain proportional spring pressure, calibrated roughly, in a way clearly indicated from a distance, on the device, so that should it be forgotten and left in place the associated pop would still work, just at a somewhat higher pressure well within the working factor of safety but high enough there wouldn't be an 'incident' while someone is tinkering with one of the higher safeties.

Perhaps this whole procedure has been replaced with hydrostatic pop testing with sufficient volumetric efficiency on the hydro pump to make up the mass loss when the valve opens. Seems to me this would be more difficult and much more expensive than setting the safeties under steam -- if you're reasonably diligent.

Author:	Frisco1522 [ Sun Aug 04, 2019 6:33 pm ]
Post subject:	Re: Tool for disabling safety valves during hydro test.
We just pulled them off and capped the pipes they were mounted on.

Author:	Bob Milhaupt steamy [ Sun Aug 04, 2019 8:55 pm ]
Post subject:	Re: Tool for disabling safety valves during hydro test.
Such devices are often called a "gag", as in "safety valve testing gag". A quick search for "safety valve gag" returned several hits, including Patent 1290520, assigned in 1919 to Ashcroft, shows a safety valve gag invention. It can be found by searching for that patent number at your favorite patent site, or, for example, at https://patents.google.com/patent/US1290520A/en . Follow the link for the PDF file which shows a diagram of the invention. In principle, the "gag" applies downward pressure on a "spindle", and is retained by "jaws" which grab the underside of a nut or ring which is a native part of the safety valve. The downward pressure is applied via a threaded rod in such a way as to force the far-end of the "jaws" apart, so that the retaining end of the "jaws" is forced positively against the sides of the nut or ring, guaranteeing that the "finger" on the jaw end stays under the nut or ring. This is a "feature" that is needed, so that the gag does not fly away when undergoing the hydro. While I have not seen a PRR gag, and haven't seen any safety valve gag in the last 20 years or so, I would expect in your PRR drawing that the horizontal rod at the pivot-points of the jaws is there for convenient handling while installing and removing the gag, and is nothing more than a rod. I would expect that the perpendicular view would not expose any new features. The VERY FIRST RULE when using a safety valve testing gag is to BE SURE TO REMOVE THE GAG AFTER THE TEST BUT BEFORE THE PRESSURE VESSEL IS PUT INTO SERVICE! Regards, Bob Milhaupt

Author:	David Johnston [ Sun Aug 04, 2019 10:51 pm ]
Post subject:	Re: Tool for disabling safety valves during hydro test.
For a few years I worked in an oil refinery. There were probably more than a 1000 pressure vessels that had to be hydro tested each year. Prior to a hydro test the boilermaker foreman would order a set of pressure relief valves to use during the hydro test. These were set to protect the vessel from over pressure during the test. As I recall they had large yellow metal tags wired to them with the test pressure marked on it. I always founding it interesting that the railway industry did not have a similar practice to protect locomotive boilers from overpressure during hydro tests.

Author:	Tails [ Sun Aug 04, 2019 11:51 pm ]
Post subject:	Re: Tool for disabling safety valves during hydro test.
Gear pullers make good inexpensive gags. Just don't torque down on them.....just a light snug

Author:	Dick_Morris [ Mon Aug 05, 2019 4:33 am ]
Post subject:	Re: Tool for disabling safety valves during hydro test.
Thanks the responses and the correct name. The drawing gives almost enough detail to make one but I was curious what it looked like from the side.

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Author:	tom moungovan [ Mon Aug 05, 2019 7:14 am ]
Post subject:	Re: Tool for disabling safety valves during hydro test.
I remember as an operator helping a machinist set safeties on our 800 lb. boilers. Six of our seven boilers were inside and under a roof which made for a noisy time. We'd coordinate the exercise with other steam producers in the refinery. The machinist would put on a big heavy pair of gloves, then carefully gag two of the other safeties, leaving the one that was to be tested. Then I'd slowly start pinching down on the non-return valve bottling up pressure in the boiler. This was at a reduced steaming rate. When it raised to the expected point, it would nearly always cause the safety to lift and I'd slowly reopen the non-return valve. This would be repeated with the other 2 safeties. Once in awhile, you'd get a safety that would get so hot that it wouldn't seat properly and would chatter. It was a real struggle to get the claws under and screw down on the gag to get it stopped. I recall one inspector who would frequently call in sick when it came time to set safeties, it bothered him that much. The machinist & I would always both confirm that all gags were removed and stowed before leaving the top of the boiler. That and asserting that the non-return valve was again fully open.

Author:	Mike Tillger [ Mon Aug 05, 2019 9:04 am ]
Post subject:	Re: Tool for disabling safety valves during hydro test.
Interesting article and photo of modern gag on pages 9 & 10 https://www.nationalboard.org/SiteDocuments/Bulletins/SU2014.pdf Mike Tillger

Author:	Overmod [ Mon Aug 05, 2019 9:53 am ]
Post subject:	Re: Tool for disabling safety valves during hydro test.
It was my understanding that these tools were necessary when setting safety valves on a large modern locomotive. When there are multiple pops on a high-pressure locomotive boiler, they are set some increment apart so more than one doesn't open simultaneously and waste mass flow; I believe on N&W these are 2psi apart. In order to test those valves that open at higher pressure, the ones at lower pressure have to be gagged, otherwise by the time you get to #4 you'll have quite an adventure with fat violet MHD sparks crawling over everything, great clouds of steam and noise, and the need to fire the boiler dramatically enough to sustain pressure. It was my opinion 'back in the day' that the gag used for this purpose could itself contain proportional spring pressure, calibrated roughly, in a way clearly indicated from a distance, on the device, so that should it be forgotten and left in place the associated pop would still work, just at a somewhat higher pressure well within the working factor of safety but high enough there wouldn't be an 'incident' while someone is tinkering with one of the higher safeties. Perhaps this whole procedure has been replaced with hydrostatic pop testing with sufficient volumetric efficiency on the hydro pump to make up the mass loss when the valve opens. Seems to me this would be more difficult and much more expensive than setting the safeties under steam -- if you're reasonably diligent.

Author:	Kelly Anderson [ Mon Aug 05, 2019 10:37 am ]
Post subject:	Re: Tool for disabling safety valves during hydro test.
.

Author:	jasonsobczynski [ Mon Aug 05, 2019 10:53 pm ]
Post subject:	Re: Tool for disabling safety valves during hydro test.
Overmod wrote: It was my understanding that these tools were necessary when setting safety valves on a large modern locomotive. When there are multiple pops on a high-pressure locomotive boiler, they are set some increment apart so more than one doesn't open simultaneously and waste mass flow; I believe on N&W these are 2psi apart. In order to test those valves that open at higher pressure, the ones at lower pressure have to be gagged, otherwise by the time you get to #4 you'll have quite an adventure with fat violet MHD sparks crawling over everything, great clouds of steam and noise, and the need to fire the boiler dramatically enough to sustain pressure. It was my opinion 'back in the day' that the gag used for this purpose could itself contain proportional spring pressure, calibrated roughly, in a way clearly indicated from a distance, on the device, so that should it be forgotten and left in place the associated pop would still work, just at a somewhat higher pressure well within the working factor of safety but high enough there wouldn't be an 'incident' while someone is tinkering with one of the higher safeties. Perhaps this whole procedure has been replaced with hydrostatic pop testing with sufficient volumetric efficiency on the hydro pump to make up the mass loss when the valve opens. Seems to me this would be more difficult and much more expensive than setting the safeties under steam -- if you're reasonably diligent. Start with the high pressure and work your way down, just run the adjusting screw down. As an example, on a 245lb boiler it requires only about 1 full turn (or less) to bypass the low and mid pops and achieve 249lbs on the high pop. Cheers.

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