Silica Iron Fouling How to clean tubes?

[PeterPiper]

We have a heat exchanger that is getting fouled by a silica / iron deposit. It is 10% Fe and 70% Silica and deposits on the tubes as a brown sludge.

The operating temperature is 180°C and ambient pressure on the proces side 13 bar.g steam on the heating side.

We have used water jetting to 10,000 psi which has some benefit but it is short lived. We have tried to dissolve it in nitric acid on a lab scale but it has little effect.

Can anyone advise on what methods of cleaning are available. The HX is 316 st/st, HCl or similar acids are not permitted due to various process reasons, other chemical cleaning may be consideded but I think the preference would be for mechanical cleaning of some kind.

Is stream cleaning an option?

Regds,

Pete.

 

[aspearin1]

I suggest cleaning it out again (whichever method), then polishing or plating to get the pitting out. Water jetting is nice, but it will actually increase roughness, and create crevices for the silica to deposit. Implement a cleanable filter upstream of your heat exchanger. Perform backwash of filter and heat exchanger on regular basis.

ChemE, M.E. EIT
"The only constant in life is change." -Bruce Lee

 

[BRT549]

What is the tube-side material you are heating up? A silica deposit is pretty hard to get into solution - only hydrofluoric acid will clean it with any speed.

I have similar problems cleaning tubes fouled with a gypsum compound up to 1/8" thick. 10K won't really clean it, so we go to 40,000 psi. Also have to clean with the HX vertical so the crud will fall out. Cleaning in the horizontal position takes too long and hangs up the cleaning tips.

 

[unclesyd]

Before you anything else comeback with the number of tubes, size, and the length.
Are any of the tubes plugged?

At 10,00 psig you are still in the mass effect range of water blasting, which is not effective for tight scale.

You should get a company that has the mechanical tubular cleaning equipment that can get above 13,500 psig. Better yet get someone with the proper equipment that can get above 23,000 psig, preferably in the 26,000 psig range. It generally takes mechanical lance handling equipment for this equipment.
I’ve lost all my contacts but will look around and see who is still in the business.

If the bundle isn't gigantic there is possible of chemical cleaning.

Just one of several.

http://www.aquadrillinternational.com/Company/Our_Company.htm

 

[PeterPiper]

A bit of further information.
The bundle consists of 900 tubes each 1.5" id, 13' long. It is a vertical falling film condenser type unit but the evaporation is done in a single pass.

The fluid is a melted salt that we add a metal salt to to change its properties. The metal salt contains the the silica and small amounts of other impurities that we believe ar e fouling the tubes.

Thanks for the feed back so far.

Pete.

 

[rmw]

The HCL is also most probably not permitted for corrosion issues. Stainless hates chlorides. Bad things result.

Have you ever tried sulfamic acid? Phosphoirc has already been suggested.

What has been your history with other than hydroblasting type cleaning? Do you have steam available while cleaning?

rmw

 

[cub3bead]

Ammonium bifluoride (NH4HF2) is an alternative to HF for removing silica scale. HCl, H2SO4, citric (C6H8O7) or sulfamic (NH2SO3H) won't attack the silica.

This is not a do it yourself project, time to hook up with an experienced chemical cleaning consultant/contractor.

 

[BRT549]

Another technique to use on your tubes is chill-shocking. This may loosen the scale so that your blasting might be more effective. The concept is to alternately heat and cool the HX - steam and water. Expand and contract the tubes.

 

[cub3bead]

I suspect there is a good chance that acmite (NaFe3+[Si2O6]) is present. If it is, then you have a very dense and adherent scale that is difficult to remove.

 

[bewdley]

To my knowledge, hydrojetting is widely used in the phosphoric acid industry, where tubes and Ca sulphate scales are bread and butter. I believe pressures are up to 500 bar (75,000 psi), but this requires a safe knowledge of the use of the correct nozzles, or else.
I recently witnessed a hydrojetting of a series of HEs, including a falling film, probably scaled with Fe carbonates, Ca sulphate and carbonate, and some organics (analysis under way). Worked well at some 30,000 psi on the (vertical) falling film, almost no effect on the horizontal FC evaporator, which remained with its 1 mm scale almost untouched, even at 70,000 psi. Since these tubes are Inconel 625, next step will be warm inhibited 5 % HCl. But 316, mmmmmh....

 

[unclesyd]

The application of hydroblasting especially in the UHP range is dependent on several factors not the least being the nozzles. Hydrochem dissolved a company called Valley Systems that had the equipment that could do your job. It is called the Water Lazer and is multi jet rotating head machine. We use a flexible version of this tool to clean 2" dia heating oil vaporizer tubes. About 95% of the tubes(id) were cleaned to white metal. I definitely would get in touch with Hydrochem to discuss your problem. One caveat of using UHP is the inherent hazards of the process. Make sure you review and understand the safety procedures. Hands and feet are especially vulnerable.

http://www.hydrochem.com/waterlazer/tube_cleaning.html

Another option is the chemical cleaning of the tubes as stated above with any or all of the ingredients mentioned.
To be effective with chemical cleaning I would recommend an oxidizing pretreatment using Alkaline Permanganate solution near boiling. I personally would use HNO3 + HF pickling solution as the solvent. You could use other acids or a remote possibility of an alkaline based solvent, but the addition of the necessary fluoride could be a problem.
Not knowing your system another possibility would be to make you a small quantity of the process salt oxidizing to convert the scale prior to shut down.

 

[BRT549]

Yup, Water Lazer is what we use in our phosphoric acid evaporator heat exchanger tubes.

 

[AsgIv]

As already mentioned acids like HCl, H2SO4, citric acid, sulfamic acid etc. won't attack the silica.

HF will work and also ammonium bifluoride. A mixture of HNO3 and ammonium bifluoride (2-3% ammonium bifluoride in 30% HNO3 aq.) is an effective alternative. A "milder" variant is 2-3% ammonium bifluoride with 10-15% organic acid like citric acid, lactic acid or glycolic acid is good for frequent cleaning preferrable with low foaming acid stalble wetting agent (0,5-1%). In think BASF has some polycarboxylates that dissolves silica, but I have not tryed them.

 

[PeterPiper]

Thanks for the input.

We tried UHP water 30K psi. It took x3 passes to give a reasonable clean using the Stoneage Gopher nozzle but we didn't have the time to do that on all the tubes. We have removed about 50% of the scale at best and so it looks like it is going to be a chemical clean.

Are there any further suggestions for solutions that may tackle this scale. I would like to soak the tubes for no more than 24 hrs, it is about 1mm think layer of scale. Does anyone have a feel for how long it will take?

 

[unclesyd]

The cleaning time for your scale will be determined by the oxidation state of the metallic components of the scale. That is the reason that I recommended a two step process that includes the oxidation step using alkaline permanganate prior to any acid wash. While I haven't had the same results using ammonium bifluoride/nitric that I've had using hydrofluoric acid/nitric it will work if you first use an oxidizing step. The additional step of oxidizing will shorten the overall time that the tubes would have to be exposed to acid and to lower the concentration needed. I might add that I've cleaned everything from real precision parts, furnace parts, to molten metal filters with this process.

The alkaline permanganate can and should be used up to 95?C for a period of three hours with very little circulation needed. In using a fluoride/nitric mixture again very little circulation is need. The best approach is too circulate a short time and allow to sit for about tens times the circulation times, repeated for several cycles. Keep the nitric concentration as high as possible and the Fluoride as low as possible. After the acid wash circulate rinse water with a wetting agent and you may want to add an alkaline cleaner in low concentration. After cleaning hit the tubes with a water blast if there they still look dirty to see if the film can be removed. I would passivate the bundle after cleaning if you have time.

I have never found anything that will easily remove silica scale except a material made by the Liquid Carbonics Co. call RD-16. I did a lot of work with the product but was never told the composition, secrets you know. Before the days of the right to know.

I would get in touch with Hydrochem and discuss the Valley rotary nozzle. Like I posted this was the only Hyper Pressure rotary nozzle that I’ve seen to consistently do the job. I’m familiar with the Stoneage Nozzle and I don’t think it will compare to the Valley rotary.