Question from a customer

[nigel jones 5Participant @nigeljones5]

A customer of mine has the set up below, two 5" boilers set up running a Victoria powering a dynamo. He asks: as the boilers cool they suck in water via the clacks – if there were no clacks could they implode? Now we know they dont, and blowing down isnt mandatory so how much negative pressure is there? I have never experienced this but its worth asking you folks.

[nigel jones 5Participant @nigeljones5]

[nigel jones 5Participant @nigeljones5]

anyone?

[SillyOldDufferModerator @sillyoldduffer]

Crush rather than implode maybe. A completely sealed boiler left to cool would create a partial vacuum inside potentially squeezing the outside at full atmospheric pressure – 100kPA, or 14psi. However a complete seal is unlikely and even a small leak would tend to equalise the pressure. Also, any water inside the boiler would reduce the effect to less than 14psi.

Putting a cupful of boiling water into a tin container and screwing the cap tight before swirling will crush the can effectively. But tin cans are weak compared with a model sized copper boiler. I don't know if anyone's tested one. As pressure vessels are much stronger in tension than compression, I suppose it's possible a model boiler might buckle, but it's far more likely to happen to a full-size because they have large surface areas.

Dave

[JasonBModerator @jasonb]

A lot will depend on how much water is in the boiler when the heat is turned off as the water itself won't contract much but any space taken up by steam will. Certainly remember at school the teacher boiling a small amount of water in a can and then sealing it up and watching it collapse as things cooled down.

[EdHParticipant @edh]

Why not fit each boiler with a snifting valve.

If there isn’t a spare bush I’m sure you could come up with one to fit in the steam supply line.

[Robert Atkinson 2Participant @robertatkinson2]

As Dave says, maximum differential is less than 15 PSI (higer at sealevel on a nice day)

Tanks can implode see https://www.youtube.com/watch?v=Zz95_VvTxZM Anyone wo has see an old, non-protected TV tube smashed will testify to the forces involved with larker volumes.

Robert G8RPI.

[DMBParticipant @dmb]

Always blowdown to try and get rid of muck inside then leave bd valve open til next steam up. Do it every time and it will become a habit to (a) blowdown regularly and (b)close bd b4 filling for a steam up.

Last week saw a 5"G loco blown down over a bucket. Hell of a lot of what looked just like horticultural silver sand left in bucket, maybe 2/3 dessert spoons sort of quantity, far too much to leave in there solidifying on top of the foundation ring, the lowest point.

[EdHParticipant @edh]

If you used a tee (T) with the steam line going across the top and the snifting valve hanging down, in the closed position it should seal at low pressure but any vacuum would lift the ball off it's seat.

[JasonBModerator @jasonb]

Can I ask why the differential will only be 14-15psi. If the heat were removed when there was say 100psi in the boiler would there not be a drop of 85psi on the volume of steam?

 

Also why add a snifting valve to the setup as the existing feed water clack is doing the same thing.

 

Edited By JasonB on 06/10/2019 13:46:59

[Nick Clarke 3Participant @nickclarke3]

Posted by JasonB on 06/10/2019 13:45:26:

Can I ask why the differential will only be 14-15psi. If the heat were removed when there was say 100psi in the boiler would there not be a drop of 85psi on the volume of steam?

If a valve were fitted to the feed line and the boiler allowed to cool completely then there would be low pressure in the boiler. The minimum this could ever be would be an absolute vacuum – and in real life it won't be that low.

if the valve is now opened the pressure of the atmosphere in would be 14.7 lb/sq in hence this would be the biggest differential pressure. The 85psi pressure would be pushing out, not sucking in.

[JasonBModerator @jasonb]

The 85psi would reduce as the boiler cools so sucking in.

Edited By JasonB on 06/10/2019 14:02:16

[Trevor Drabble 1Participant @trevordrabble1]

Fizzy , In principle , l agree with you , assuming the boilers are designed and tested for a working pressure above 30 psi . Such pressure reversals is how both our porous load and instrument sterilisers worked every day . I would also wish to offer the following constructive comments ,

1) The end caps on the boilers are wrong . They should at least be flat ( depending on design calcs ) , or preferably convex , but certainly not concave as shown .

2) The steam header is wrong for effective balance . Rather than having a single central take off , there should be two teed take offs , each preferably spaced one third of the length along the header with each tee feeding a smaller short header , and then a single centrally placed teed feed to the engines .

Trevor

[Robert Atkinson 2Participant @robertatkinson2]

Posted by JasonB on 06/10/2019 13:45:26:

Can I ask why the differential will only be 14-15psi. If the heat were removed when there was say 100psi in the boiler would there not be a drop of 85psi on the volume of steam?

Also why add a snifting valve to the setup as the existing feed water clack is doing the same thing.

Edited By JasonB on 06/10/2019 13:46:59

There would be a relative drop of 100 PSI but even with a full vacuum inside the boiler there is only atmospheric pressure, 1 Bar, outside. The pressures quoted for steam work are "Gauge" or relative to local air pressure. If you connected an absolute pressure gauge (which is referenced to zero or a vacuum) to the same point it would read 14-15 PSI higher depending on local air pressure.

Robert G8RPI.

[Nick Clarke 3Participant @nickclarke3]

Jason's video –

"Pressure on the outside of the can is over 3000 lb" – a bit high and by quoting the force on the can as a whole as pressure misleading to say the least!!

[not done it yetParticipant @notdoneityet]

The answer of 14.7psi or 1 Bar is correct and can be ‘verified’ by checking any ‘vacuum gauge – they don’t make them with dials greater than that!

The only way to increase the pressure beyond one atmosphere on the surface is to place the vessel in a pressurised container or sink it in a fluid. Think here of submarines – they have a maximum dive depth after which they are simply crushed by the outside pressure.

[not done it yetParticipant @notdoneityet]

Posted by Nick Clarke 3 on 06/10/2019 14:23:52:

Jason's video –

"Pressure on the outside of the can is over 3000 lb" – a bit high and by quoting the force on the can as a whole as pressure misleading to say the least!!

Lbs is the imperial unit for weight, which is a force, not a pressure (S.I. unit of weight is the Newton). Pressure is defined as force per unit area. So that 3000 lbs is likely spread over a couple hundred square inches – at 14.7 lbs per square inch

( not seen the video as it did not play)

[nigel jones 5Participant @nigeljones5]

The way I thought about it is….if the boiler were just a strong sealed container (which it effectively is) with water in it and heat applied, the water would turn to steam, the pressure would go up. Let it cool down and the pressure comes down to exactly where it was when you started. Nothing can have changed. The only difference when using it for real is that there will be less water in it when it cools down, assuming no more has been added. Ive tested this with a smaller boiler with no clack, running the water level down whilst maintaing 40 psi driving a D10. Closing the steam valve and after cooling there is absolutely no negative pressure that I can detect.

[Paul KempParticipant @paulkemp46892]

Posted by fizzy on 06/10/2019 15:22:29:

The way I thought about it is….if the boiler were just a strong sealed container (which it effectively is) with water in it and heat applied, the water would turn to steam, the pressure would go up. Let it cool down and the pressure comes down to exactly where it was when you started. Nothing can have changed. The only difference when using it for real is that there will be less water in it when it cools down, assuming no more has been added. Ive tested this with a smaller boiler with no clack, running the water level down whilst maintaing 40 psi driving a D10. Closing the steam valve and after cooling there is absolutely no negative pressure that I can detect.

Depends how you try to detect "negative" pressure. It's differential pressure between outside and inside you need to see and that would really need a manometer to determine.

As to vacuum gauges not being made for more than 1 bar, there are plenty, evey vacuum braked train has at least two and they used to run at 21". Brake cylinders also work on differential pressure between the vacuum developed in the train pipe to hold the brakes off and the atmospheric on the other side trying to put the brakes on!

Paul.

[Howard LewisParticipant @howardlewis46836]

If the boiler is capable of withstanding an internal pressure of, say, 100 psi, it will be subjected to 15 psi during steam raising. Therefore a pressure differential in the opposite direction should cause no problems.

The stays and inner and outer shells will be subject to compression rather than tension, but if the material can withstand 100 psi, – 15 should cause no problems.

Howard

[Robert Atkinson 2Participant @robertatkinson2]

As to vacuum gauges not being made for more than 1 bar, there are plenty, evey vacuum braked train has at least two and they used to run at 21". Brake cylinders also work on differential pressure between the vacuum developed in the train pipe to hold the brakes off and the atmospheric on the other side trying to put the brakes on!

 

Paul.

Not on this planet. The maximum differential you can get with a vacuum is dictated by the local air pressure. This is variable but at sea level is about 1Bar, 14.7 PSI, or 30" of mercury. (Hg) The brake gauges are 21" of Hg not 21 PSI.
30" hg is 407" of water so that is the absolute maximum you can suck fresh water UP with a pump about 30 foot in practice.

 

Robert G8RPI

Edited By Robert Atkinson 2 on 06/10/2019 16:50:18

[Howard LewisParticipant @howardlewis46836]

As an aside, re vacuum brakes on British railways.

The Great Western ran at 25" Hg. This caused problems when coupling stock from another railway. It meant that the strings had to be pulled on every vehicle, destroying the 21" Hg vacuum, so that the GWR loco could then "make a brake". And the same procedure had to be carried out when coupling GWR stock to any of the other railway companies in Britain.

Howard

[not done it yetParticipant @notdoneityet]

..Higher and reduced pressure vessels require different approaches. Long ago a bulk powder lorry tank ( filled by gravity from a silo, unloaded by aeration and blower into a silo) was converted to operate as a large vacuum cleaner to pick up spillages. It was not a simple job, as I recall, due to the risk of implosion.

The classic school lab experiment was that of reducing the pressure in an aerosol can. When evacuated, if it did not collapse as air was removed, it would implode with a slight tap with a suitable edge (a ruler, usually).

Submarines are under increased pressure of one Bar for about every 10m they are submerged.

[Paul KempParticipant @paulkemp46892]

Posted by Robert Atkinson 2 on 06/10/2019 16:34:46:

As to vacuum gauges not being made for more than 1 bar, there are plenty, evey vacuum braked train has at least two and they used to run at 21". Brake cylinders also work on differential pressure between the vacuum developed in the train pipe to hold the brakes off and the atmospheric on the other side trying to put the brakes on!

Paul.

Not on this planet. The maximum differential you can get with a vacuum is dictated by the local air pressure. This is variable but at sea level is about 1Bar, 14.7 PSI, or 30" of mercury. (Hg) The brake gauges are 21" of Hg not 21 PSI.
30" hg is 407" of water so that is the absolute maximum you can suck fresh water UP with a pump about 30 foot in practice.

Robert G8RPI

Edited By Robert Atkinson 2 on 06/10/2019 16:50:18

Robert, good point well made! I was thinking inches (as I put the symbol) just not enough for an atmosphere! Did cause me to do a quick bit of research though and stumbled across an article from an American University regarding absolute maximum vacuum / minimum pressure where he was claiming -20 atmospheres? This was linked to raising water up trees and another reference to a round board being towed behind a space shuttle that made atomic spacing 10 times smaller giving a 10 times better vacuum. All too much for me!

Paul.

[Bill Davies 2Participant @billdavies2]

The submarine has increasing external pressure due to depth, but the internal pressure won't increase very much, as it depends on the internal volume, which will reduce slightly. The original poster's question regarded a reduction in internal pressure, caused by drawing in water and the condensing of steam – the principle of atmospheric engines, such as Newcomen's.

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