Safetys

[farmerden]

the use of safety valves on our boilers .I was reading on another site about the newer safety valves having flat seated valves rather than tapered seats like the older varieties .One of the new valves came with the instructions "If Valve Is Ever Activated It Should Be Replaced" Hmmm! I've always checked my safety by manually lifting it when the boiler achieved about 75% of operating pressure.How do I know if the safety works if I don't manually check it? Any of you wiser than me [that's just about all of you! ] care to comment? Thanks Den

[Lopez Mike]

I don't see how a safety valve can be a 'pop' valve with a flat surface. Maybe a relief valve like on top of my electric hot water tank at home or on hydraulic systems but not on a gas pressure vessel like our boilers. I believe that the spec for a proper steam safety valve say that when it opens it must stay open until the pressure has dropped some certain percentage before closing abruptly.

I test mine regularly too. You would have to pop it every minute for a long time before it wore out.

I've been wrong before (a lot, actually) but this smells funny to me. Might as well throw it away from the beginning and use a burst diaphragm like a scuba tank!

Mike

[87gn@tahoe]

Kunkle still makes safeties with tapered seats

[fredrosse]

Steam Safety Valves are typically arranged with a spring forcing the valve disc against its seat, when the force of steam pressure acting on the valve disc area exceeds the spring force, the valve unseats and begins to relieve steam.

Steam Safety Valves usually have a “Huddling Chamber” concentric to the disc and seat, which collects extra upward force when the valve starts to lift off of its seat. The steam released into the huddling chamber acts on the additional area of the huddling chamber, and the valve “pops” open. Only after steam pressure is significantly reduced can the spring force close the valve, so when steam pressure is reduced to a value lower than the set pressure, the valve reseats and “pops” shut. The reduction of pressure before the valve re-seats is called “blowdown”, not to be confused with the blowdown process of intentionally releasing water from the boiler to reduce sludge and dissolved solids.

This action can (and does) work regardless of whether the seat is tapered or flat.

As to testing the relief valve, simply operating the manual valve stem lifting handle is not a proper test. Steam pressure should be brought up to the valve pressure setpoint, and the safety valve should lift and relieve pressure. This test should be conducted annually, although I usually test the safety valve a couple of times each season. The valve must be adjusted so that the Maximum Allowable Working Pressure (MAWP) is not exceeded before the valve lifts. These are requirements according to the boiler code, and are mandatory requirements.

Throwing away a valve after it has relieved pressure seems absurd, and if the manufacturer recommends this, then it is not a proper steam safety valve. Or possibly the marketing department has gotten over zealous in promoting extra sales! What make of valve had the "If Valve Is Ever Activated It Should Be Replaced" statement?

[fredrosse]

One thing I have experienced is the potential danger by attachment of a safety valve which is way too big for the boiler. I had a 150 PSI safety valve with a nameplate rating of 1260 Pounds Per Hour (PPH), and 10 PSI blowdown. Attaching this valve to a small boiler (12 inch diameter, 24 inch long VFT), when the valve lifted it let so much steam out that the water level swelled up enormously, and a two phase mixture of steam/water began to issue from the valve. The entire sight glass went white with a froth mixture of steam/water. This flashing mixture pressurized the huddling chamber in the safety valve enough to keep the valve open way longer than it should have, the valve did not “pop” shut until boiler pressure was down to about 80 PSI. When the valve reseated the swelling subsided, and I had no level showing on the sight glass, water level was dangerously low.

The rules require a safety valve big enough to relieve the maximum steam generating capability of the boiler, which for most of our small launches this is up to about a few hundred PPH. Oversizing by too big a margin is not good.

[Lopez Mike]

Our Northwest Steam Society (U.S.A.) has a handout on boiler safety and inspection that, among many other things, warns about too large or too small of a safety valve and recommends steaming to the safety valve release point after an annual hydro test.

As Fred says, this either not really a steam safety valve, is poorly or improperly designed, or the advertising department has run amok! I also smell lawyers under the porch.

"The first thing we do, let's kill all the lawyers."
Shakespeare. Henry The Sixth, Part 2 Act 4, scene 2, 71–78

[DetroiTug]

A one shot safety valve would be a bit pointless. This is probably wording for liability issues. Evidence of use in the vent etc. It's understandable; the manufacturers have no control on where or how these relief valves will be used, a very old boiler with lots of scale, one pop-off and the valve could be full of contaminants and will not function thereafter.

-Ron

[Lopez Mike]

Yeah, Ron, I suppose crud could get in there but I would think that then it wouldn't seat on closure.

More to the point, from my point of view, is this continuing 'nanny' stuff. I bought an iron years ago for my laundry and it had a warning not to iron clothing while I was wearing it. What sort of dolt would try that???? Sad.

Mike

[87gn@tahoe]

The same one who should have never been allowed to leave the... Oh nevermind

[DetroiTug]

One thing I have experienced is the potential danger by attachment of a safety valve which is way too big for the boiler. I had a 150 PSI safety valve with a nameplate rating of 1260 Pounds Per Hour (PPH), and 10 PSI blowdown. Attaching this valve to a small boiler (12 inch diameter, 24 inch long VFT), when the valve lifted it let so much steam out that the water level swelled up enormously, and a two phase mixture of steam/water began to issue from the valve. The entire sight glass went white with a froth mixture of steam/water. This flashing mixture pressurized the huddling chamber in the safety valve enough to keep the valve open way longer than it should have, the valve did not “pop” shut until boiler pressure was down to about 80 PSI. When the valve reseated the swelling subsided, and I had no level showing on the sight glass, water level was dangerously low.

The rules require a safety valve big enough to relieve the maximum steam generating capability of the boiler, which for most of our small launches this is up to about a few hundred PPH. Oversizing by too big a margin is not good.

This is good to know. To corral these points, someone should, if they haven't already, make a list of safe boiler operation "commandments", and this should be one of them.

-Ron