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Protection of Steel Boilers

bruce morgans

Joined: 21-01-05

Topics: 6

Replies: 18

Posted: Thu 16th Jan 2014, 9:04pm
Protection of Steel Boilers

I expect like many members who have steel boilers I use water treatment and do not drain my boiler after every use. The quality of the water is monitored and blow downs are carried out to control the tds and suspended matter. However as we all know corrosion still occurs so I have been considering installing a sacrificial anode in the form of a zinc strip attached to one or more of the washout plugs. Has anyone tried this and if so was it effective in reducing corrosion? My only experience of using sacrificial anodes is with structures and plant in contact with sea water. Will the reduced conductivity levels in steam boilers prevent it working? Your thoughts and comments would be appreciated. Regards Bruce
 

Replies To This Post

palmcoast

Joined: 1-01-70

Topics: 1

Replies: 1

Posted: Tue 21st Jan 2014, 4:30pm

I have yet to be faced with this problem, but have been considering, after blowdown and cooling, purging the boiler with Argon gas and leaving lightly pressurised. Would be interested in others comments.. Regards, Tom.
 

bruce morgans

Joined: 21-01-05

Topics: 6

Replies: 18

Posted: Thu 30th Jan 2014, 1:39pm

Some further thoughts on this subject: The water treatment might leave a coating on the anode and prevent it functioning, similarly the tannin on the cathode i.e the steel could have the same effect.
What will be the effect on the steel above the water level i.e out of the electrolyte?
Still need to use water treatment to prevent scale formation. Your views would be appreciated. Regards Bruce
 

georgedavis

Joined: 1-01-89

Topics: 0

Replies: 5

Posted: Sun 2nd Feb 2014, 10:56pm

At the end of a run, I fill my steel boiler to the top of the water gauge glass, blow down to near the bottom of the glass then fill the boiler as much as possible using the injectors. The next day, if is not to be used for a while, I top it up using boiled water that has cooled.
Water that has boiled contains very little oxygen - and it is oxygen that causes corrosion. My boiler is now nearly 25 years old and is showing little sign of corrosion on the inside.
 

bruce morgans

Joined: 21-01-05

Topics: 6

Replies: 18

Posted: Mon 3rd Feb 2014, 8:09am

George, I could not agree more with your comments and apart from topping my boiler up the following day I do exactly as you say. I have however noticed some pitting of the top row of tubes and that prompted me to consider the use of sacrifical anodes to provide additional protection. It is not surprising that the top row are most vulnerable as the water at the top of the boiler will contain the highest oxygen content. It is possibly not helped by having top feeds. Regards Bruce
 

georgedavis

Joined: 1-01-89

Topics: 0

Replies: 5

Posted: Mon 3rd Feb 2014, 9:40am

Bruce. I still think that filling the boiler to the brim is the best way as leaving an air gap above the water level could well result in more oxygen being absorbed by the surface water. I think the worst scenario would be to drain it and leave it as it would be virtually impossible to completely dry the inside without some pretty elaborate de-moisturising kit..
As regards sacrificial anodes, I have not heard of their use in boilers, but as a steel narrow boat owner I know that zinc anodes are used in salt water and magnesium ones are used in fresh. They have to be welded to the hull. Bolting is considered most unsatisfactory so their use in a removable plug might not work out.
 

bruce morgans

Joined: 21-01-05

Topics: 6

Replies: 18

Posted: Mon 3rd Feb 2014, 7:27pm

George, I will follow your advice and top the boiler up the following day, this will leave only the dome above the water line and will increase the height of the water level above the top tubes. I will add some water treatment at the same time for good measure.
I don't think it matters how the anodes are attached so long as there is electrical continuity between them and the steel. Where anodes are used to protect stern drives/ outboards they are usually bolted on. In the case of a steel hulled boat it is probably preferable to weld them on rather than drill holes in the hull below the water line.
I am advised by one of the society members that the use of sacrificial anodes was tried back in the 1920's on full size locos but it never caught on. I don't know why.
I have read about hydrogen induced embrittlement which occurs when hydrogen ( which would be produced due to the galvanic reaction ) is absorbed by the steel but as I'm not a metallurgist I'm not sure how serious a risk this would be. After all a steel boiler with copper tubes is a galvanic cell with the steel being the anode! A case in favour of not using different materials unless you want something to corrode in preference to something else.
 

georgedavis

Joined: 1-01-89

Topics: 0

Replies: 5

Posted: Mon 3rd Feb 2014, 8:04pm

Bruce. My boiler has steel tubes expanded into the tube plates but I do have some non ferrous fitlittings. As these are all screwed in they are sealed with ptfe tape so any electrical conductivity has to be "iffy" at the very most. I would be interested to know how you intend to seal the anode carrying washout plug and still absolutely guarantee conductivity (heavy gauge wire welded across from plug to adjacent plate? It would need to be cut and rewelded each washout but that might be a small price to pay if the anodes do the job. Let me know how you get on.
 

johnnicholson

Joined: 1-01-77

Topics: 12

Replies: 68

Posted: Mon 3rd Feb 2014, 10:00pm

At the company I worked at for 25 years making trailer equipment nearly all the components parts were zinc electroplated and we had to watch for hydrogen embrittlement. The vat all ran at about 60 degrees C. It depended on the grade of steel tat was being plated and as a rule of thumb mild steel up and including EN8 (080M40 in new money) were not a problem. However like EN14, EN16 and EN19 were always de-embrittled. This was done by placing them in an oven at about 150 degrees C for a period of about 8 hours. I am only using the above as an illustration but I am sure there will be some parallel scenarios in boiler work.

John Nicholson
 

Thomas Adler

Joined: 16-10-05

Topics: 0

Replies: 2

Posted: Wed 26th Mar 2014, 5:27pm

Hello
My boiler has copper tubes expanded into the tube plates and two zinc anodes screwed in the smoke chamber plate. The screw are sealed with a copper ring.
The disadvantage of this sacrifical anode is, that little parts of zinc covering the bottom ring of the boiler. When i drain the boiler under pressure, small pieces of this zinc mud are blocking the blow down valve.
I am using only water where all the hardeners are removed.
After the weekend the water is drained . The boiler is then filled up with car antifreeze. The ethylene glycol does not conduct electricity and does not absorb oxygen. Before the next driving day the antifreeze is collected and replaced by fresh water.
Every two years the antifreeze needs to be replaced.

Thomas Adler
 

Old Baldy

Joined: 31-05-12

Topics: 0

Replies: 4

Posted: Thu 10th Apr 2014, 9:21pm

Hi There
I find the best thing if you are using a good water treatment, is to fill the boiler completely with the injectors until they stop. This means filling over the top of the sight glass until the water level comes into the manifold. The injectors will stop then! However, injecting water can add a bit of air to the water, depending on the injectors. So I tend to let a bit of watery steam out by shutting the water off to the injectors and just let the steam out of the overflow for a few seconds. When next lighting up, drain the excess water out of the blow down valve until you are down to half a glass.
Unfortunately when the boiler water cools down, a vacuum is created inside the boiler, and this tends to suck air into the boiler through the whistle valve and the clack valves. I have a screw down valve between the manifold and the whistle valve, and this is screwed down. I then put a little rubber cap over one injector overflow, and put a short pipe on the other injector overflow from a container of water. As the boiler vacuums, it then draws water up through that clack, and thus keeps the water level totally full without introducing any air.
I suspect that cathodic protection really is only of use with seawater or very impure water. The better the water used in the boiler, the less corrosion/battery effect there will be anyway.

Peter Griffiths
 

bruce morgans

Joined: 21-01-05

Topics: 6

Replies: 18

Posted: Sat 12th Apr 2014, 6:55pm

Hi Peter Thank you for your comments. I believe you and George and myself basically agree that filling the boiler with water is the best option. I have always used the injectors to fill the boiler to above the top nut but filling above that level with the boiler still in steam risks water carry over into the cylinders. I will now do as George suggests and top up the boiler when it is cold. I will also add some water treatment to help absorb any oxygen present. I have also installed one zinc pencil anode and will see how it is affected and reacts.
I suspect however that corrosion of the top tubes is oxygen related and hope the changes I will have made will provide better protection. If I don't get twenty years use out of my boiler I will be disappointed ( if I'm still alive!!!! ) Regards Bruce
 

bruce morgans

Joined: 21-01-05

Topics: 6

Replies: 18

Posted: Fri 15th Aug 2014, 7:44pm

Hi Guys At the beginning of the season I added one zinc anode by attaching it to the LHS foundation ring washout plug. A short time ago I drained the boiler for the purpose of carrying out a thorough washout and was surprised to discover there was nothing left of the anode other than a load of oxide lying on the foundation ring. This was easily washed out. I have not replaced the anode and I'm still wondering what has caused such a rapid change in state. Regards Bruce
 
 
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