Boiler test question

Howdy all - I was pondering something... Obviously boilers have both hydraulic and steam tests - but what is the reason for needingboth? Is it that a hot boiler may show up leaks that a cold one won't? Assuming it's not to test the fittings as often it takes place with things blanked off.

No reason for asking other than a general wondering - be interested to see what the answer is!


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The hydraulic test is normally carried out at a higher pressure than the operating pressure of the boiler, so it provides evidence that the boiler is sound at these higher pressures so should be safe at working pressure. As water is essentially incompressible, if the boiler does fail during the hydraulic test there is little risk to anybody nearby as the pressure rapidly falls in the event of a leak. A boiler failing under steam pressure is another matter entirely as there is a lot of stored energy to be dissipated.

Andy

 

The two tests are looking at slightly different things with the same common objective of having safe boiler. The hydraulic test is a proof test carried out at well above working pressure to safely check the integrity of the structure. Water is an incompressible fluid so that when pressure is released (through a leak or failure) the energy stored is little and for a failure you are likely to get wet clothes and a bit of a fright.. Steam on the other hand is highly compressible (hence why we use it's expansive properties when notching up). Having that amount of energy coming out in a steam leak or failure could take you, the boiler and half of the neighbourhood with it if it is a major failure. Also the hydraulic is usually carried out on the boiler shell whereas steam test usually also check the boiler fittings etc for operation and it's generally carried out at (just above) working pressure.

 

Thanks both, that makes sense... Aside from the fittings though, is it likely that a steam test would show up problems a hydraulic wouldn't?


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You can get some slight movement with thermal expansion and it is not unknown for a boiler which showed no weeps with water to have some slight weeps under steam, correction of these is known as caulking and you can find many ways of rectifying this. Traditional methods include special closing punches or bags of potatoes and corn flour, thesedays such traditional methods are not so widely used... But sometimes you can get a whiff of a boiler that has been treated in this way (not I hasten to add on any railway).

 

I'm sure I read something about a loco on the Isle of Man I think it was way back in the day was known to have a leak so the cure was Porridge oats being stirred in somewhere to gum it up and make it steam tight! Couldn't get away with something like that now!

 

.... there was some truth in that! Other popular cures were 'cowdung 'n porridge' or horse manure. A recently retired IMR employee who started in the early 70's was told by an 'old boy' to make sure it was a "golden horse" i.e. one fed on straw rather than grass - apparently it was the tiny bits of chewed up straw that stopped up the leaks. If you want to see some 'traditional' boiler repairs have a look at No.1's old boiler on one of our forthcoming workshop tours or No.3's boiler in MOSI - 'wet' patches and all.
Ray.

 

Such traditional 'cures' work very well in a temporary fashion to help bung up small seeps and drips enough to let natural corrosion do it's thing and create a permanent seal along the overlapping plates, along with silt/scale etc. They're useless on serious leaks, but frustrating recurring fizzes that defy constant attention with a chisel can be eased with them just enough to stay shut permanently. It's the rivets (and friction, and obviously stays etc) that provide the structural integrity, caulking the joint creates a pressure tight seal. In an ideal world leaving a boiler sat with water in it without pressure for as long as possible before being put to use can help to get it fully sealed up as well. I dunno about railway manufacturers, but certainly with road steam engines it's not total coincidence that many manufacturers had boilers 'sat on the shelf' for a number of months before they got built up into engines.

I wouldn't know about railways that run on silky smooth rails with the boiler balanced on nice firm frames, but with road steam vehicles where the boiler is the main structural chassis, and they're bouncing along hard rough pot holed roads, sometimes without any springs, getting a newly worked on boiler to stay totally drip and seep free can be a constant battle waged with a caulking chisel never far from reach until they'll stay totally dry. Wacking insulation and cladding on straight away can hide a multitude of fizzes, and if they're not too bad can also help cure those as well - the dampness from the trapped moisture under the insulation can be enough to just get enough surface rust to form around the fizz to totally seal it up such that it's no longer a problem. The danger is if it's more serious and the trapped moisture encourages serious corrosion on new platework....

...oh, and in response to the OP, a hydraulic test will generally occur every 10 years, whereas a visual inspection and steam test will happen annually every 14 months (special years! ), a steam test will also demonstrate that things such as injectors work properly that can't be demonstrated hydraulically.

 

I seem to recall reading a rather catty comment from an observer at the Rainhill Trials that Sans Pareil had been "given more oats than would feed a mare" - or did I imagine that?

Tom

 

Are the fittngs hydraulic tested individually? I regard them as the weakest link in a pressure system.

 

Hydraulic test.

This is to test the integrity of joints, seams and tube condition. It is not to prove the integrity of the boiler shell or firebox, most boilers have a safety factor in excess of 4 when newly manufactured and should withstand 4 times their normal operating pressure when cold, hydraulic tests are undertaken at 1.3 or 1.5 of working pressure depending upon design and boiler pressure. For boilers with copper fireboxes the tensile strength of the copper drops off as temperature rises, particularly in the 225 to 250 psi range and for these boilers the copper is more highly stressed during the steam test and in normal operation than it is when subjected to the hydraulic test. (one very good reason why it is very important to avoid a build up of scale on the waterside of the copper as this will insulate the copper from the cooling affects of the water and raise it's temperature higher than necessary.)

Steam test.

This is to further check for the integrity of joints etc and to prove the functionality of other fittings such as water gauges and safety valves. The safety valves must be able to release all of the steam however hard the boiler is forced, without undue accumulation.

For further information see HRA Boiler Guidance note HGR B9160 Examination in Service on the HRA web site for details of what the Competent Person inspects.

http://www.hra.uk.com/mem_docsdb.php

The HRA guidance notes cover all aspects of boiler maintenance, repair and operation and should be read by all associated with locomotive boilers, some are even written for the guidance of footplate crews.

Andy.

 

Steam fittings are generally " blued in " in other words a metal to ,metal face that has been filed to create a leak free face and that will withstand the steam pressure, most other fittings are overhauled on the bench, so there will always a degree of tightening up nuts and refitting, often a boiler will be tested at a lower pressure first to test things like injectors and then the safeties will be set and the boiler pressure increaced slowly and valves adjusted to open at the boiler pressure before the boiler inspectors official inspection

 

It's usually much more convenient to hydraulic test a boiler that's had a bit of work done than it is to steam test it - you can let the pressure off and on a matter of minutes if you need to fiddle with something (e.g. a boiler we did at work a couple of weeks ago we had to take the stuffing box off to deal with a particularly annoying weep from one of its studs).
You can also go in the firebox and nip tubes up or knock up a side stay head a tad(neither of which are usually practical when it's in steam ).

I quite often hydraulic boilers on their sides - it makes caulking the foundation ring (usually one of the favorite bits to try to leak) very easy.

Although a hydraulic text isn't intended as a structural integrity test, I can think of several boilers I know of which have suffered mechanical failures under hydraulic testing, mainly in the form of stays breaking. Nasty modern all welded boilers with corner welds everywhere rather than flanged plates and stays poked through and welded in seem particularly prone to such failures.

Most railway boilers get three formal tests at the end of an overhaul - a hydraulic test, then a steam test sat on the ground, then a steam test with the engine assembled.

 

As a side question. I remember reading in the early 80s there were people who wanted to acid wash a boiler but others who wouldn't sign the boiler off if it had this treatment. Can people explain why this would be?

 

The final job when re-building Jaguar V12 engines was to add a bottle of Bars Leaks. I've even seen it marketed in Jaguar branded bottles.

 

If a boiler component fails under hydraulic test it is only because the usual inspections and surveys prior to the inspection have failed to identify the defect. In normal circumstances the Competent Person should have requested a range of tests and inspections throughout the boiler. If a boiler has deteriorated to such an extent that part of it fails at hydraulic (or is very close to failing) that boiler is likely to have other similar components that are also close to failing, but the user will expect it to last potentially for a further 7 years before it is subjected to another hydraulic test. Yes the Competent Person will undertake inspections at 14 month intervals but if they have not picked up defects at overhaul they are unlikely to do so at the periodic inspections when access is more difficult. This is a worrying situation.

Andy.

 

If the loco was regularly used in a hard water area the dissolved salts act as a very good way of stopping seeps and weeps, they effectively chemically bung-up the hole. In soft water areas this doesn't happen as there are very few dissolved salts in the water.
Hard water is slightly alkaline, soft water is slightly acid. If loco has been used for some time in a hard water area and then migrates to a soft water area, it is not usual for weeps to start occurring. I seem to remember Britannia falling foul of this and being failed at Hellifield just prior to setting off up the S&C some years ago.
The same situation exists in your central heating system. Hard water areas have less corrosion and almost no leaks, soft water areas just the opposite. That's why you need inhibitors.
Flush outs in soft water use alkaline chemicals to move the crud, hard water areas use acidic chemicals to do the same. This is what often opens up weeps and seeps.
I used to manufacture radiator sealer for automotive use, a dispersion of fine wood flour in an alkaline aqueous solution of shellac resin. We sold tons of it for putting in brand new mini's radiators, because the build quality was so low in those days they leaked from day one!!