Freelance 3/4” traction engine

[Team ricky]

Ah ok …. An incorrect wife’s tale I’ve remembered!

So onto the most sensible approach of dropping the pressure I think….

I would at a guess think that during the pressure test you would get signs of issues as the pressure rises, I.e visible distortion of the flat surfaces and or cracking sounds both of which are a fail even if the boilers in tact it would show it’s been pushed too high !

I can’t think of any real need to go over 40psi WP unless I’m missing something here !?

[Jasonb]

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[Team ricky]

That’s a interesting read

Thanks for sharing that !

I guess the 80’s were just built different ! Accepting bulging seems a bit wild !

[propforward]

I would at a guess think that during the pressure test you would get signs of issues as the pressure rises, I.e visible distortion of the flat surfaces and or cracking sounds both of which are a fail even if the boilers in tact it would show it’s been pushed too high !

Also, if you're pumping away and the pressure won't go up - that's a problem. Or if the pressure drops during the hold periods. There should not be any pressure drop. Not all failures are accompanied by a sudden sound on pressure vessels. Keep an eye on all your joints for little bubbles coming out. It doesn't always take much of a leak before the pressure suddenly won't hold, even on a great big vessel. I'm more familiar with those than small model ones.

As for bulging it's a question of how much - I think the article mentions "slightly" which is a bit ambiguous. Everything will deflect some amount - it's just how much is allowable. If the material yields that is a problem. But the material can still permanently deform and be sound. I can't tell if the author means for the material to bulge permanently or temporarily (elastically - i.e. goes back where it started when the pressure is relieved). I think I'd prefer that myself, but I am the cautious sort.

So, there I go complicating it again. Interesting though, isn't it?

[Michael S.]

I can still remember testing my self-made Kupfee boiler with a pressure of 4.8 bar. The normal operating pressure is 3.0 bar. Operating pressure x 1.6 = 4.8.
The copper, which is still soft after soldering, will deform slightly during the pressure test. It should not be visible to the eye.
It could be that the test pressure is calculated differently in each country. For us it is a factor of 1.6.
(If it hasn't changed again)
My copper boiler was an 89mm diameter tube.


Greetings Michael

[Jasonb]

Usually twice working pressure here.

Best to look for water not bubbles as you should really be hydraulically testing the boiler as should something let go the water inside won't go bang like it would if the boiler was being tested with 150psi of air. Usual trick is to put a small boiler on a paper towel and see if it darkens where it gets wet. Even if using compressed air to provide the pressure for testing the boiler should be filled with water and air only in the eairline and  a bit of pipe work.

You should also be able to isolate the pump so that any drop is not due to leak back through the pump

[crueby]

What Jason said!  With water filled boiler, any minor seep is visible as drops forming on the outside if tiny, anything larger will throw a visible stream like a squirtgun. Even full size boilers are tested with water.

[propforward]

Even full size boilers are tested with water.

Yes - it tends not to go "bang" as badly if something hasn't worked out. 

[propforward]

Just for the sake of interest - not wanting to go too far off topic - in the states the Boiler and Pressure Vessel code is used to govern just about everything. The hydro test in Section VIII Division I is based on the ratio of material stress at test temperature divided by the material stress at design temperature - all multiplied by 1.3. So not a fixed factor so to speak. That said every model pressure vessel I have seen simply goes to 2.0 X the working pressure, because the actual code requirement lands between 1.5 and 2.0 typically, so 2.0 is easier to remember and adds a bit of safety factor.

The piping codes have a different value. The BPV code also makes allowance for pneumatic testing, and in that case it's the stress ratios multiplied by 1.1.

But again, this is all for interest value only. Everyone needs to go with their local laws and codes and be guided by their local clubs / inspectors.

[Team ricky]

Thanks for all the tips chaps

I now know what to look out for !

I have a garden spray pump which I’ll use for the test ….. should get 100psi at its upper limits….

A bit more done today, I drilled the steam ports into the cylinder and machined up the slide valve which turned out very good 😊