boiler hydraulic pressure testing

[billj pump]

hi gents, im stuck for a means of doing a pressure test after my boiler has been silver soldered together. has anyone any ideas of how to pressurise the boiler upto 100 psi with water please,,,


thanks,,,,bill

[havoc]

I tested mine with a bicycle pump. Those pumps can generate a lot of pressure.

Make an attachment for a bicycle valve. Then fill the boiler with water and the hose as close to the manometer as you can. Leave as little air between the manometer and the bicycle valve. So you will have only a little air to pressurise.

First use of my bicycle pump in 15 years or so...

[gwrfan]

Hi Bill,

Firstly, you must completely fill your boiler with water, ensuring all air is excluded, blanking off all except two of the bushes. To one bush, fit a large pressure gauge (usually advised to be minimum of 3 inch diameter), and having a range of about 4 times the desired working pressure of your boiler. To the other bush fit a suitable hand pump, which is immersed in a tray of cold water. For a one-off boiler most builders will use their own loco tender hand pump, but be aware that this can let water dribble back past the ballvalve seatings! Bring the pressure up slowly to twice the working pressure, examining your boiler all the time for weeps or leaks. The test pressure should be held for about 20 minutes, with no loss of pressure on the gauge.

Once this is satisfactory, the next test will be a steam test.

Best of luck to you.

Geoff

ps. You can try a bicycle pump if you like, but it depends on the size of your boiler! You still need a pressure gauge.

[billj pump]

hi and thanks for the quick replies, i have incorporated a bike inner tube valve into the boiler design to give me the pressure test reading anyway, this is how i was going to check the pressure,(with a tyre pressure guage) then the central spring loaded valve bit was going to be removed and then the main body of the valve was going to be used as the boiler water filler, but if i use a bike pump to pressure the boiler then surely im inserting air instead of water into the boiler so what is the point of removing the air out of the boiler in the first instance? i thought the boiler had to be filled with 100% water for the test? or am i been to strict with myself? is it ok to pump air into the boiler on this test?.

[havoc]

i thought the boiler had to be filled with 100% water for the test? or am i been to strict with myself? is it ok to pump air into the boiler on this test?.

The boiler has to be completely filled. I attached the pump to a short piece of hose, and only that piece of hose was filled with air.

[gwrfan]

Hi Guys,

I have to say that there are obviously boilers and boilers! I believe that if you intend to have your boiler working in a public place, or a club track, then your boiler hydraulic test will have to be carried out by the club's 'qualified' tester, who will issue a suitable test certificate if it passes. Your boiler tester will not use a bicycle pump, but test it in accordance with the details I've already quoted.

If, however, the boiler is for home use, for your own amusement, then it's up to you to decide if what you do is up to the required safety standards.

Just my own thoughts on the subject.

Geoff

[Malcolm]

Just fill your boiler COMPLETELY with water, and play your blowlamp gently on to the barrel. It will soon pressurise itself. Go gently, and be ready to release the pressure if it should try to overshoot.

[paul]

I'm a total novice so I don't know how much use this is but on my first tiny model I filled the boiler with water then connected a car type pump and pumped it up to about 3 times working pressure using the pumps guage.

[alanstepney]

In fact, that ISNT a lot of use. You should pump up to 2 x working pressure, and no more.

More than that can strain the structure and do more harm than good.

If you read through some of the Model Engineers, preferably old ones, testing of bolers has been covered in detail many times.
Worth reading.

[paul]

The kit instructions said working pressure was 1.5 bar and it should be tested to 4.5 bar so that's what I did - as a newcomer I thought it wise to follow the instructions for safety's sake.

In reality I doubt that I got anywhere near 4.5 for any sustained period as the connection wasn't brilliant. Anyway the boiler works fine - it's powered my little engine 40-50 times already

Thanks for the advice though, next time I'll just test to 2x.

[the_viffer]

alanstepney said:

You should pump up to 2 x working pressure, and no more.

More than that can strain the structure and do more harm than good.

I always have a problem with that. Way back in the early days of engineering Naysmith (I think) took a piece of anchor chain and loaded it until it broke. Then he took the bits and loaded them until they broke and so on until all he had was a pile of scrap iron. Conventional wisdom of the time was that the load to failure would decrease each time as they metal was strained. In fact it did not. What happened was that the weakest link broke (goodbye) and then the next weakest.

Leaving aside questions of cyclic loads I have a bit of a problem seeing how if a structure designed to burst at no less than 10x can withstand load 3x but will be weakened in withstanding load x later. I'm lead to believe by people who know that it does but I just don't follow it.

I might also say that for me it, if true, implies a safety factor of less than 3 times working pressure when working cold let alone hot which I find a bit worrying in a boiler.

Can someone explain it to me in simple but accurate terms please?

Someone, a propos of nothing, who should have known better once tried to tell me that a piece of metal was scrap because it had been stressed beyond its elastic limit. He never did quite take the point that in order to bend a piece of metal permanently you have to exceed its elastic limit.

[paul]

Stupid novice question alert:

How do I determine 'working pressure' for a simple small boiler (maybe 100 -150 ml capacity)? I guess I'd need to get the calorific value of the fuel or maybe max attainable temperature and plug it into P=vT if I remember that right from school?

[havoc]

Normally, it is the engine that determines what pressure is needed to do the work demanded from it. Likewise will the engine set the steam consumption. The pressure will give you the temperature of the steam (unless you overheat). Those factors and the fuel used will determine the needed heating surface. Thus the boiler is build to suit the engine.

But you can let any boiler work at any presure provided it is build to withstand it of course.

So if you just have a boiler, then the pressure will be derived from its constructional strenght.

[the_viffer]

paul said:

Stupid novice question alert:

How do I determine 'working pressure' for a simple small boiler (maybe 100 -150 ml capacity)?

There are no stupid novice questions.

Working pressure is determined by the strength of the boiler. So you calculate at what pressure you think the boiler will go pop. Then you divide by a suitable safety factor. 10x or 12x are not unreasonable for a boiler et voila your working pressure.

KN Harris' book on boilers will give you some good starts on calculating working pressures. It is simple enough to do but not intellectually that rigourous to my mind. I think however that the large safety factors used save us from any embarrassment.

I'm rather keen on performing the calculations before making or steaming the boiler.

As for what pressure your boiler will generate: there are two approaches to this one.

You can determine experimentally how fast your fuel burns and its calorific value. You can make a guess at how efficient the boiler is. You can then calculate using specific heat and latent heats of vaporisation the expected pressure for any given amount of steam take off.

Or you could steam the boiler and see.

Personally I know which I go for.

Make sure you have a properly working safety valve.

KN Harris' book also gives some ideas about the kind of steam production rate you might expect for various boiler types.

It is a book well worth buying but not for a couple of days until I've listed my copy on ebay.

[paul]

Thank you both. So, if I've tested the boiler to say 50psi and I have a working safety valve that I know will open at say 30 psi (i'm just plucking numbers out of the air here) I can pretty much ignore the 'working pressure' for practical purposes? I suppose if the steam generated doesn't power the engine satisfactorily it's a case of test higher (and maybe up-rate the valve) or try and improve the efficiency of the engine.

the_viffer:
"It is a book well worth buying but not for a couple of days until I've listed my copy on ebay."
Very droll. You could message/email me some details about that book and a price - if you think it's relevant for a newbie like me.

[waggy]

Isn't the means of pressurising the boiler Malcolm suggests a little dangerous?
Bad enough if the boiler were to fail when filled with cold water, you might get wet! If it failed when filled with heated water, you'd get wet and possibly scalded!!
People have died from having their bath water too hot.

Waggy.

[havoc]

Isn't the means of pressurising the boiler Malcolm suggests a little dangerous?

Not really. But I would have a go at it with a simple candle, not a blowtorch. The idea behind it is that you heat a boiler full of water. So it is the expansion of a liquid that builds the pressure, not a pressurised gas. Even if you need a lot of pressure, this way the water will be barely warm. And if you have a leak (or big failure) the pressure will be gone as fast as when puming extra water in it.

[billj pump]

thanks guys, i feel a lot more comfortable now carrying out the test by myself, ill use the bike pump method, many thanks for all your replies.

billj