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Fowler Z7S Steel Boiler

Plate thickness for a Fowler Z7S steel boiler

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Steve Bright 212/09/2021 15:55:56
17 forum posts
14 photos


As a newbe here I hope this is in the right place.

A question for boiler builders/testers. I am hoping to build a 2" Folwer Z7S Ploughing Engine as designed by John Haining in 1988 which has a steel boiler..

As it was designed in the days when we still used Imperial measurements and materials John designed it accordingly.The boiler barrel is 24 1/2" x 6" dia hot rolled steel with a wall thickness of 5swg .212" (5.4 mm). The closest available is 6" dia hot rolled 1/4" (6.6 mm) seems to be the closest size available, if so I'll have to use it. I don't mind it being a little thicker and am aware filling the backhead will be interesting.

The biggest problem is the sides, throatplate and backhead. John specified 5 swg (.212" here and the firebox of 6 swg .192" (4.9 mm).

Herein lies the problem. swg is no longer available, these days its all metric. For 6 swg the ,1 mm I can accept. Some people have said that I must go to the next size above for boiler testing and others say it will be fine building it using 5 mm plate in place od 5 swg. If I have to use 6 mm then there will be considerable changes to the firebox size to keep the foundation ring original size. 5 mm throughout will mean almost no firebox changes.

So help please. I would like to use 5mm plate throughout even with a 6 mm boiler tube wall rather than 6 and 5 mm, But if I build it thus there is a risk the boiler testers will say it was built below original specification and refuse to test it.

I know whatever the answer I must get a coded pressure vessel welder to do the welding and have a couple available

So can I build the boiler in 5 mm or must I use 6 mm and 5mm please?


Steve Bright

JasonB12/09/2021 16:09:00
22751 forum posts
2653 photos
1 articles

You probably need to show your boiler inspector some calculations that show your thinner 5mm material is upto the job. You may be able to show the 5mm is OK or by reducing stay spacing show that there is sufficient strength to at least equal the original design. As it's only 100psi you could use the Australian code as a guide to what is upto the job

Not looked at my Superba drawings for a while but at the time I was going to build one I would have gone for the thicker material but did hope to have a higher WP.

have a word with Steam Technology, they used to have a photo of one of these boilers in their adverts so should be able to say what they used as well as give you a quote

Edited By JasonB on 12/09/2021 16:30:58

Bob Worsley12/09/2021 18:28:05
118 forum posts

Use copper?

Copper is at a historic high price, £6,500 tonne, so will be expensive, but will also last much longer than a steel boiler IF you only use it a few times per year. Yes, there are lots of articles in ME about how to lay up a steel boiler, but it only takes one bubble in one corner to start the rust.

Up to you of course, but for such an expensive engine saving £2k on the boiler seems silly. Talk to Station Road Steam and the other model buyers if they would buy the engine with a steel boiler.

I think copper is wonderful stuff after reading up about its strengths and weaknesses.

JasonB12/09/2021 19:40:54
22751 forum posts
2653 photos
1 articles

I think Haining's main reason for using steel on this one unlike the copper boiler of the BB1 was that it's larger and longer so would have to be a substantial thickness to have the required strength of steel as it's under a lot more stress than the average traction engine when there is a load on the drum.

fizzy12/09/2021 20:30:27
1843 forum posts
120 photos

The steel boiler will be a lot stronger than a copper one, the problem is that whilst the cu boiler will not lose its strength over time the steel one will corrode and evenyually fail, or rather be deemed as a test fail. Tensile strength of generic carbon steel is between 1.5 and 2 x that of copper, so 5mm of steel far exceeds the strength of 6mm copper. Although my business now only produces small copper boilers there was a time when we made 7 inch gauge steel boilers. These were a doddle to make. Depending on your boiler tester an alternative test is to complete the boiler welding and present it to a certified NDT professional - if they issue you with a cert (ultrasonic test cert) you are safe knowing that every inch of every weld has been tested.

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