Boiler safety valve testing?

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mengwalton
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Boiler safety valve testing?

Postby mengwalton » Wed Nov 21, 2012 4:24 am

Hi all,

Tomorrow I will be sitting my EOOW oral exam at tyne dock, UK
Does anyone have a official way to test the boiler safety valves?

Thanks
J

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Big Pete
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Re: Boiler safety valve testing?

Postby Big Pete » Thu Nov 22, 2012 4:52 am

Looks like you left it too late..
Hope the exam went well
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Re: Boiler safety valve testing?

Postby JK » Thu Nov 22, 2012 7:15 am

yes, I hope it went well.

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Re: Boiler safety valve testing?

Postby markenzie » Sat Dec 01, 2012 11:00 am

I had the priviledge some years ago of volunteering aboard the restored steam tug SS Master ( Arthur Moscrop, Vancouver,1922, triple expansion engine one of a pair built in 1918 for a cancelled Royal Navy minesweeper) while she underwent her steamship inspection and to test the safety valves we simply fitted an inexpensive electric pressure washer to a fitting and monitored the pressure at which the valves lifted. That appeared to satisfy the inspector. I don't recall wether he fitted his own pressure gauge to monitor the performance or if we relied on the boiler's gauge. I'm curious to find out if that is the "official" thing to do as well. Incidentally The SS Master Society can always use your support. The are operating the only wooden steam tug still running on her original machinery in North America and they do it with almost nothing.

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JK
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Re: Boiler safety valve testing?

Postby JK » Sun Dec 02, 2012 9:03 am

To set the safeties on the scotch marine boilers I worked on, the boilers were flashed to working pressure with the safeties screwed down.
It was then fired to the pressure that the safety was set to lift, with someone relaying the pressures to the person setting the safety. It was a fine art to time the screwing out the safety to time the lift to the correct pressure. If you didn't get it set right, you had to do it over again.
Then it would be gagged and the second safety set. After the setting was demonstrated to TC, the easing gear was installed. Pretty straightforward, except for standing on the boiler and waiting for the safety to lift. You always jumped. And sweat..a lot!
The SS Master Assoc did it that way to avoid firing the boiler. It takes 30 hours to get a big scotch marine boiler up to pressure correctly and if they were just flashing it to set safeties, a waste because they would then have to shut it down, let the pressure die and water wedge it for storage.
The boiler on the Master is probably a smaller one, but there is still a lot involved to flashing a straight steam plant. All of the auxiliary pumps and fans that you need, would have the exhaust valves taken off and air connected to the supply. The FO pump, feed water supply, and FD fan are run on air until the boiler gets up to the minimum pressure that you could run the machinery from. We used to divide into teams with specific tasks. When the boiler was put on the range everyone ran, the air was disconnected and the pumps started. Then you had to get the exhaust valves reinstalled with exhaust steam blowing by your face. This was done as fast as you could, because if you lost the steam pressure before everything was online, you would have to start all over again on air supply.

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JollyJack
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Re: Boiler safety valve testing?

Postby JollyJack » Sun Dec 02, 2012 8:01 pm

All you need is a blank flange with a screwed-in fitting. Bolt the flange to the bottom of the safety valve (which has been removed from the boiler) and fit a tee to the flange fitting with a pressure gauge on one branch, the fitting on one and a source of pressure, either water or air, high enough to pop the valve at the correct pressure, on the other. Pressurise and pop, while monitoring the gauge.
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Big Pete
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Re: Boiler safety valve testing?

Postby Big Pete » Mon Dec 03, 2012 6:35 am

I have seen it done the way Jolly Jack describes, but many Surveyors insist it is done on the boiler using steam. On the boiler the safety valve and its components will all have expanded with the temperature, and different components have different co-efficients of thermal expansion, so a hot safety valve will not lift at the same pressure as a cold one.

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JollyJack
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Re: Boiler safety valve testing?

Postby JollyJack » Mon Dec 03, 2012 10:43 am

That the materiels of which the valve is made will expand at different rates is true, but the coefficient of expansion is so small that the difference in "popping pressure" is quite miniscule. For example, the coeffecient of linear expansion for steel, the materiel used in the valve spring, is 0.000012, (Engineering Formulae booklet, p 2, on the "Exams" page) which means it will expand 0.000012mm/mm length. Lets take, for example, a coil spring of 300 mm free length and steam at 8 bar, the temperature of which is around 170C. (Steam tables) Suppose the workshop where the valve is tested is 25C, then the temperature difference is 170 - 25 = 145.

To find the total expansion of the metal, we must multiply delta T by original length by coefficient of liner expansion

145 x 300 x 0.000012 = 0.522mm or, if you like, a 12" spring will grow 21 thou when a boiler is flashed up from cold to working pressure.

I doubt if that would make any discernable difference to popping pressure.
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Big Pete
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Re: Boiler safety valve testing?

Postby Big Pete » Fri Dec 07, 2012 11:34 am

and the valve body, valve spindle and valve seat will all have expanded as well, each at their own different rate of expansion.

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JollyJack
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Re: Boiler safety valve testing?

Postby JollyJack » Fri Dec 07, 2012 11:42 am

True Pete, but the amount of expansion is so small, it can be ignored in practice.
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markenzie
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Re: Boiler safety valve testing?

Postby markenzie » Tue Dec 11, 2012 3:48 pm

This is great stuff, gentlemen. I really appreciate the discussion. And yes, they did want to avoid having to force the boiler through a cycle to get steam up. They used the same little Karcher pressure-washer to wedge the boiler when the Master was laid up in her off season. Speaking of which- the Samson V's tripple-pass boiler was last steamed almost 20 years ago then blown down and drained with the hand-holes left open. For some years after her retirement the sternwheeler Samson V was run at the dock seasonally by a group of volunteers. She was never designed for this. When in DPW service she was kept steamed up all year around except for cleanings and inspections. Also her tripple pass/ induced-draught boiler had the typical single expansion sternwheeler arrangement of exhausting steam to atmosphere up the funnel for additional draught, which was badly needed. From what I understand blow-back was a real danger. She was generally run at full speed for part of every work day. Anyways- she was run as a demonstration for some time on weekends at low levels of draught during the late 1980s beginning of the '90s. I don't know what damage might have been done by rapidly cycling the boiler, but after getting to help punch the tubes on the Master, I got excited at the possibility of at least doing some maintenance on the Samson's boiler to at least conserve it and unbolted the front smoke box cover. I was dismayed to find the tubes clogged with soot and scale ( from being run at low draught, I guess and then left) and the smokebox was filled to about two feet with soot. Unfortunately the Samson was also allowed to sink at her dock back in 1995 and the soot and scale in the smoke box sat there ever since and the asbestos lining of the smolebox has crumbled into the soot. I realized I was monkeying around with something I couldn't fix so I bolted the smokebox back up and she's sat like that ever since. Previously I removed an enormous ammount of soot and scale from the induced draught fan housing and uptakes and treated the sometimes paper thin sheet-metal w/ rust stabilizer. I am worried about the tube plates where the soot is lying on them and how fast they may be corroding. The City of New Westminster has no plans to run the ship again and I am not directly involved at the present time, but in the long run the survival of this ship will likely depend on her being as close as possible to a working vessel. This is a big question, but are there some simple things I could suggest to the current maintenance/restoration people to limit deterioration?

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JK
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Re: Boiler safety valve testing?

Postby JK » Wed Dec 12, 2012 4:41 am

The soot is from not soot blowing the boiler regularly, more then running it at low fires. A good fireman can set the pressures and draught for little smoke at any steaming condition. It is frowned upon (to put it mildly) to blow soot at dock. The neighbours get a tad peeved.
The asbestos puts it into a whole different perspective because now you have to do an environmental assessment and it has to be disposed of in an approved manner. THAT is where the cost is and what would kill the project.
Normally I would look at having a professional company pressure wash the boiler to have a good look at the corrosion now present. Maybe a little UTM.
I am not overly optimistic that the boiler will have survived the conditions you describe without significant damage.
The boiler should have been cleaned and dried when it was decommisioned and a couple of bags of barbecue charcoal sliced open and put in the furnace and manholes.

Also, do not do any work on that plant with no precautions for asbestos. The boiler insulation, pipe insulation and gaskets are probably asbestos.

ETA, I was just looking at photos of the ship. It is in much better condition then I had pictured. That's a wee boiler!!

A picture of the engine :
Image

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carbob
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Re: Boiler safety valve testing?

Postby carbob » Mon Jul 22, 2013 8:50 pm

The boilers fitted here have 2 safety valves on each, and in order to test them; we put a gag (supplied by the manufacturer) on one of them, flash the boiler in manual until the one lifts, stop the fire, then do the same process for the other. They are removed during docking for testing, but the surveyors want to see them lift during annuals as well.


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