How to test a monotube boiler

[Mike Cole]

Hi
My brother in Law, is thinking of buying Grampus. for full details she is in the SBA register. He is coming from the sailing side of boats, so at the moment knows nothing of steam and I have never even been in a boat with a monotube boiler.
It is very doutfull if we can have the boiler in steam before he makes a offer, so what should he be looking at inspection. I expect that he can do a cold water pressure test as per a normal boiler, is this correct with a monotube.


boiler and engine as below
Boiler
Monotube Built 1994 by Colin Groves at Chichester
Fuel: paraffin, Pressure: 200 psi, Heating area: 30 sqft, Output: 300 lbs/hr,
All SS, SS casing, Lune Valley burner.

Engine
Single. 3 1/2" X 3"
Built 1934 by Drysdale & Co. at Glasgow
pv valve. Non-reversing valve gear.
On 4 turned columns, gear driven 5:1 reduction air pump and fp, 500 rpm 6½ IHP.

She has not been used for many years and we would welcome any advice.

Mike

[gondolier88]

Mike,

Yes, hydro test to see if the tube is ok if you felt the need, but if it is a true monotube with no water content then it doesn't legally require a test, just get it on some water somewhere, light the burner and away you go- if it works it's fine, if it doesn't it isn't.

Greg

[cyberbadger]

Depends on the price for Grampus...

If it's on the cheap and your brother in law can afford a replacement boiler later I'd just go for it.

But if it's a big purchase for your brother in law - I'd ask if the seller or you/your brother in law could carefully remove the casing and see the boiler properly with someone experienced with monotube boilers and water tube boilers. Nearby SBA members or steamers?

-CB

[Mike Cole]

Thank you. In the end he decided not to go ahead with it.

[fredrosse]

"if it is a true monotube with no water content then it doesn't legally require a test........"

I am surprised that a boiler of any type would be exempt from a pressure test. Is this written in some relevant rule boiler/pressure vessel codes or rule book?

It seems that a full hydrostatic pressure test would not be difficult with a monotube type boiler, and I am curious as to the reasoning that allows this type of boiler to be exempt for pressure testing.

[dhic001]

Very simple, it isn't a boiler. A monotube isn't a boiler, but a steam generator. A true monotube has next to no volume, so can be 'blown' up without any danger. Does your boiler inspector insist on hydraulicing all your pipework, your economiser, your condenser? A true monotube holds less steam than each of those things, and what volume of it does have is basically too cold to flash into steam anyway. I've seen the rubber feed hose come off a small monotube, there was a small puff of steam and the engine stopped. The feed was refitted, the hand pump pumped, and away went the engine again, all done with the fire still going.

If monotubes had to have pressure tests by the regulators, the steam cleaner industry would be in real trouble.

Daniel

[cyberbadger]

A true monotube boiler is intrinsically safer then a water tube or fire tube boiler, but I have to agree with Fred. Commercial/Industrial steam generators are under more rigid standards, that just doesn't apply for a steamboat boiler built 5, 10, 15 years ago by whomever with materiels unknown. Much better to err on the side of caution with any boiler/steam generator and have it inspected and tested regularly.

-CB

[dhic001]

And who is going to inspect it? A friend had the boiler inspector in to do the survey on his watertube boiler, and asked the inspector about the monotubes. The inspector took one look, said "its not a boiler, its a steam generator, I'm not interested in it" and walked away. May be different in other places, but here its up to us to maintain it and test it. As I said earlier, we "blew one up" and had it running again in less than a minute. I guess its up to the operator, but I wouldn't bother after the initial hydro test when built, you'll know when it gives out as your engine will stop, and it won't show much sign of failure until then.
Daniel

Much better to err on the side of caution with any boiler/steam generator and have it inspected and tested regularly.

-CB

[barts]

Keep in mind that the while the danger of an explosion is basically gone with a monotube, there's still the chance that it might stop working, and leave you w/o power in an awkward place; remember, you can not always pull over and wait for help. A hydro test at 50% over safety valve setting seems simple and prudent to catch incipient failures.

- Bart'

[fredrosse]

Some clarifications here with respect to pressure tests required for monotube boilers in the USA jurisdiction, governed by the ASME Boiler and Pressure Vessel Code. If there are any code rules that differ significantly for other countries/jurisdictions, I would very much like to be informed about this.

It is noted that a monotube steam generator is indeed a boiler, if boiling occurs within the device. The only occasions where this type of steam generator is not a "boiler" is where steam is generated at operating pressures above the critical pressure of 3206 PSIA (221 BARA). Many large utility plants operate at these supercritical pressures, but these machines are far removed from the technology used in our steam launches, with our steam pressures ranging to a few hundred PSI at most.

The ASME Code requires pressure testing the same for water level boilers, both watertube type or firetube type, as well as "Steam generators having no fixed steam/water level". Generally the pressure test requirement is 1-1/2 times maximum allowable working pressure.

On more than one occasion I have seen boiler inspectors shirk off testing requirements where they are not familiar with the specific rules applicable to the pressure vessels in question. I would think that if the inspector who stated that a monotube steam generator would not require a pressure test because it is not a "boiler", he or she would almost certainly be unwilling to put this in writing.

Logic holds that a ruptured tube in a monotube boiler, while generally less dangerous than ruptures within a watertube or firetube type boiler, can cause injury to personnel, and that is why there are rules governing the design, construction, operation and maintenance of these devices.