Descaling with hydrochloric acid 1

[andief]

I have to descale 3 hot water tanks (350 litres each, in series) prior to installing a water softener.

To do this I've ordered an IBC of 5% hydrochloric acid which I was planning to recirculate for 3-4 hours to react away the scale and then drain and flush the system.

However, I've just realised that the impeller of the pump in the system is made from stainless steel (grade 304). Obviously stainless steel and HCl don't mix but will this cause problems in this situation given that the acid is quite weak and won't be in the system very long?

Your thoughts would be much appreciated!

 

[GarySCWSVI]

Descaling with HCl in those tanks could create problems for you. The scale is likely not an even coating and there is likely to be holidays in the scale exposing metal surfaces. Go ahead and install the water softener. Soft water will start to descale the tanks. It will take as long to remove the scale as it took to put it there. The upshot of that is that you won't corrode the tanks with the acid.

Gary Schreiber, CWS VI
The Purolite Co.

 

[andief]

Hi Gary

Unfortunately leaving it for a year (which is how long it's taken to build up) to get rid of the scale isn't an option as we're already experiencing problems further down the system with blockages and have lost a few days work as a result. I'm not worried about the tanks as they're made of copper, or the fittings which are brass. It's only the pump impeller and possibly a couple of ball valves which are SS304 and are therefore a concern.

 

[bimr]

The common stainless steel types, 304 and 316 should be considered non-resistant to hydrochloric acid at any concentration and temperature. Of course, you intend to only descale for a very limited time and all of the corrosion charts are based on continuous service.

http://www.bssa.org.uk/topics.php?article=30

You have presented very little information on the materials of construction and the operating pressures, so it is difficult to make any type of recommendation. However, it appears that you are operating at low pressures with relatively inexpensive equipment.

It seems like you could just replace the tanks if they only have a capacity of 350 liters for a small cost. The pump can not be that expensive either. Also, if you clean the tanks, you probably will not be able to inspect the tank interiors to see how much material has been removed by the descaling anyway.

I am aware of industrial facilities that do periodically descale using the methods that you have proposed. But these applications involve non potable water.

So it doesn’t look like you have a lot to risk, as long as you observe safety precautions when you work with the acid.

 

[andief]

Apologies for lack of detail:

These are 3 x 350litre copper tanks with immersion heaters (copper probe+temp reg and HCl resistant heaters) used to contain and maintain water at 70C for large scale QC and prototype testing of (domestic) water systems downstream. Pressures are pretty much ambient. Piping is copper, connectors are brass, some ball valves may be SS304. Pump housing, casing and impeller is SS304.

We are in a hard water area (~300mg/l CaCO3) and do a LOT of testing as part of an on-going project (3-5,000litres/day - average: sometimes more). When the tanks were installed this quantity of throughput wasn't foreseen (client thinking: make things in china because it's cheaper; current situation: spend ages after sorting out the lack of QC and then test and fix in the UK as it's quicker and will work.... all down to client: and yes, we told them so.... i could go on, and on, but it's futile: we are where we are).

Anyway, I did a corrosion test and put plate of SS304 in 20%HCl for 4 hours: no visible effect; 8 hours: some discolouration on the metal (dulling), but no pitting under a microscope; 24 hours: dulling and visible pitting, pale green HCl solution: i.e. leeching of FeClx complexes.

The pumps aren't expensive (but the hassle - and time (6 weeks lead time) - of replacing them is though) but given our corrosion test we're going to do the HCl wash to react out the scale - if 8hrs of 20% doesn't have much effect 4 hrs@5% shouldn't be a problem. It arrives tomorrow so please shout now if you think this is a big no, no!

In the meantime thanks for the helpful insights - this forum is a fantastic place for practical help and I'm very, very grateful for it.

I'll let you know how we get on.

Best wishes
Andrea

 

[dfmorvan]

Hi Andrea

For descaling, you could use a Citric Acid solution @ 20 g/l. That way you will get a ph around 4 and no corrosion or pitting problems. Also Citric acid poses no security problems as will HCl (fumes when diluting, etc...).
We use this type of solutions to descale RO membranes. Also you can find food grade citric acid very easily.
To monitor the descaling process, follow pH. If pH begins to get higher, just add some more citric acid.
You could use also sulfamic acid, but the pH will get to 1 so I fear for copper.
Please pay attention, descaling with acid will produce CO2 that will need to be degassed from the water. Best way to descale is to use a circulating pump and a tank so that CO2 is vented to atmosphere. Once descaling is done, flush with water until pH is equal to that of inlet water. 4 hours should do the job except if scaling is heavy, in tha case it could take longer.

 

[kenvlach]

andief,
To ~safely descale metals using HCl solution, add a pickling inhibitor. This is what steel mills & metal finishers add to acids used for removing rust. Some brands available in the UK are
RODINE from Henkel Surface Technologies

http://www.henkel.com

PICKLE AID from MacDermid plc

http://www.macdermid.co.uk

GARDACID from Chemetall plc

http://www.chemetall.co.uk

You may be able to buy some from a nearby metal plater. For 350 Liters of 5 wt% HCl, you only need about 0.2 L of inhibitor. Most ingredients (benzotriazole is one that protects copper) aren't commonly available.

Using an inhibitor enables less acid to do the descaling. It also avoids dissolving heavy metals thereby avoiding the creation of hazardous waste.

 

[ProcessWaterChemist]

Using HCl is not a good idea!
A deposit analysis needs to be done. Once the chemical make-up of the scale determined you can decide what chemical to use in removing scale!

The 2 most common chemicals used to remove scale are citric and sulfamic acid!

I have used both many times. The best method is to use a big plastic garbage can. Hook up a cheap recirc pump by the can. Suck water out of can and push into apparatus to be descaled. Hook up a return line from an overflow or another pipe and return it back to the garbage can.