Cast Iron Boiler. Removing Sludge 1

[PegSneggle]

I'll set the scene and then ask my questions. Feel free to give short sharp answers.

I have a domestic central heating system which is quite old and is based on a good quality boiler with a cast iron heat exchanger.

The boiler knocks and bangs due to the presence of sludge, I guess. I want to clean it out and my researches tell me that phosphoric acid is the ideal chemical, as it attacks the rust sludge with enthusiasm and is much kinder to the cast iron.

Questions
1. How much should I dilute the acid before putting it inside the boiler? (The acid I buy will be supplied at maximum industrial strength. The salesman mentionned 85%, but I don't know what he was talking about)

2. Should I run the boiler whilst it has the acid inside to improve or speed up the action?

3. I will clean and flush the boiler a number of times, to be sure of removing all the sludge. Is there an easy way to tell from the emerging acid, whether the sludge has all gone or whether I should repeat the cleaning process.

4. Any advice as to how long the acid should be left circulating within the boiler?

5. Will the acid harm the copper pipework held together by lead solder.

6. Do I need to do something to neutralise the system, once the process has been completed.

7. Apart from the obvious safety precautions of goggles, gloves are there any major pitfalls.

Please reply even if you can only answer some of these questions.

Many thanks in anticipation of a reply

 

[Metalguy]

I once used phos. acid to descale my steam locomotive boiler. Did it at room temp. for a few hours, until the white-scale covered tubes turned a nice black. Then I flushed and flushed, while checking with pH strips. Couldn't get it down under ~8 or 9 until I dumped in a few lb. of trisodium phosphate (TSP). Even had to do that twice. But the boiler transferred heat better after that, and I could once again spin the drivers at any speed!

 

[Metalguy]

Following up, I don't remember exactly what conc. I used-probably 5-10% or so. I also did some corrosion measurements BEFORE I started, and there was no measureable loss on a test piece of carbon steel. But I did notice that prolonged acid exposure dissolved the non-metallic inclusions out of the surfaces, so the small test block had tiny holes on each end (ends were perpen. to the rolling/grain/inclusion axis direction), while the sides and faces were roughed up and a little uneven.

See if you can borrow/rent a flexible borescope and take a look inside to see if your sludge is gone.

 

[moltenmetal]

Are you sure your knocks and bangs are not due to the presence of air in the system? And is this a true "boiler" system or is it a thermosyphon hydronic system? My house has the latter- it knocks and bangs like crazy unless all the rads are thoroughly vented so the system is completely water-full (except for the expansion tank, of course!)

 

[unclesyd]

Here a product that you might look at. I've had very good results on various water jacket cleaning jobs with this material.

http://www.rydlyme.com/