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How acid is formed in dry 14% SO2 gas line ?

How acid is formed in dry 14% SO2 gas line ?

How acid is formed in dry 14% SO2 gas line ?

(OP)
Molten Sulfur is burnt with dry ( Dew Point below Minus 70 Deg C ) air to form 14 % SO2 gas in a furnace. When this 1350 to 1400 Deg C gas is cooled to 200 Deg C, liquid acid accumulates in the pipelines, gas coolers etc.
The quantity of acid relased is about 20 kgs per ton of sulfur burnt.
How Hydrogen based acid ( H2SO4) gets condensed with bone dry sulfur + almost bone dry air. From where this Hydrogen comes?

RE: How acid is formed in dry 14% SO2 gas line ?

6748, What happens when a vapor phase is cooled below it's saturation temp? Right you are! Condensation! Now what happens when this comes into contact with the likes of Sulfur Dioxide? Right you are again, the production of Sulfurous Acid (H2SO3), also know as Sulfur Dioxide solution. To the Environmentalists and the world at large this is the evil substance know as "acid rain".

Just remember, any significantly advanced science/technology is indistingishable from "MAGIC"!

saxon

RE: How acid is formed in dry 14% SO2 gas line ?

(OP)
*The saturation temp of 14% dry SO2 gas will be even less/lower than Minus 30 or 40 Deg C when the gas is at atmospheric pressure.
*Water vapor picking up SO2 gas to form acid rain is not possible, because the gas is in closed pipeline till it is consumed in the process.
* The question is how acid is formed in a closed system which does not have access to any Hydrogen atom, except the moisture content feasible in a Minus 70 Deg dew point air.

RE: How acid is formed in dry 14% SO2 gas line ?

6748,
What is purity of sulphur?  Does it contain significant levels of H2S?  Where does the sulphur come from?

RE: How acid is formed in dry 14% SO2 gas line ?

If all you have is dry air (N2, O2, Ar etc) with no appreciable water where does the Hydrogen come from?

Are there sources of leakage into the system that contain hydrogen sources?  Perhaps a leaking steam generator or superheater in the flue gas line.

I've seen this type of problem before in a refinery sulfur recovery unit before with a heat recovery system on the flue gas stack.

RE: How acid is formed in dry 14% SO2 gas line ?

(OP)
The quality of as bought Sulfur, as analysed by an outside testing lab  is as below:
Assay as purity(Dry basis) ---> 99.9%
Moisture ---> 0.07%.  Ash -- > 0.04%.
Acidity ( H2SO4) ---> 0.002%
Once the sulfur is melted, the residual moisture in molten sulfur will be ZERO.
 There is no leakage of water anywhere. The pipeline from the sulfur storage tank to the burner is just 6 mts length. This jacketted line has been checked thoroughly for leakge of steam.
Then the burner. then furnace. then waste heat boiler. The gas is at 250 deg c. And then it starts releasing muddy acid mixed with ash in the pipeline, economiser as it comes down to 200 Deg c.
From where this acidity or Hydrogen atoms come: This is my question.

RE: How acid is formed in dry 14% SO2 gas line ?

6748, Let me see if I have this correct. You purchase sulfer in bulk then store it. After this is stored,it is then transported to furnaces where it is burned with natural gas/air or someother hydrocarbon to produce SO2 rich gas. This SO2 gas is then cooled, transported to the various units where it is consumed. In the cooling and transport phase the H2SO3 drops out as condensate.

The certified moisture content of the bought sulfur is one thing, what is the moisture content after storage/tansport/ storage? What  are the by-products of the combustion of any hydrocarbon gas? H20 and CO2. Are you absolutely positve there are no other sources of water vapor into the system? Where is the hydrogen and water vapor coming from?

saxon   

RE: How acid is formed in dry 14% SO2 gas line ?

(OP)
Saxon,
The solid sulfur is melted and stored in molten sulfur feed tank at 120 Deg C. This sulfur at 120 Deg C cannot hold water. Molten sulfur is burnt with dry air to form SO2.
The only source of water is the moisture that can accompany the Minus 70 Deg c Dew point air.
Still that water in air is  very low to cause condensing of SO2 in the form of H2SO3 at 200 Deg c.
Let me rephrase  the problem.
How much water can go with the air having Minus 70 Deg C dew point? Let us assume 1000 M3/hr of air flow. At 14% SO2 volume/volume how much SO2 can mix with that water? Presuming that all water that accompanied the air is getting condensed as H2SO3 even at 200 Deg c, let us quantify the acid that can form in the pipeline. Let the quantity of acid be  say X kgs.
The acid that gets drained is much much higher than this calculated quantity. ( multiple times X )


RE: How acid is formed in dry 14% SO2 gas line ?

What color is your sulfur?

RE: How acid is formed in dry 14% SO2 gas line ?

(OP)
Dear Hacksaw,
Are u asking a serious question ?
The sulfur used is from a reputed refinery. It is not a directly mined powdery sulfur.
It is as yellow as any sulfur could be. It is not a powder sulfur. It is granular in shape ( spherical ) having an average diameter 3 to 5 mm.

RE: How acid is formed in dry 14% SO2 gas line ?



It is a serious question.

The color is an indication of purity and indirectly, water content.

Assuming that you are not experiencing equipment problems, I suspect that you've got an incomplete assay.

If it is equipment related, then you've got some testing to do.

Good luck.


RE: How acid is formed in dry 14% SO2 gas line ?

After careful re-reading of your process description, even with chemically pure sulfur you can have acid formation under the conditions you describe.

Unfortunately those conditions are associated with incorrect equipment line-up, you process design, or failure of certain process equipment in your gas train.

It appears that you are dealing with a process design / warranty issue.

I can put u in contact with a fellow who has desifned many such plants and has the resources available to sort out the details, but i'll need your contact details.

RE: How acid is formed in dry 14% SO2 gas line ?

Dear 6748,
I presume it is a sulfuric acid plant.

How sure are you about the moisture content of air from dryer?

Is air supply through a drying tower, using 98 % sulfuric acid for drying the air???

If you are sure about moisture content.... >>> then ... is there acid mist carry over???

Last chance is leak from a boiler or steam superheater... Not the type of of heavy leak but a small tiny pinhole


Last (again assuming sulfuric acid plant) .. why are you cooling SO2 to 200 C.  Or is it a mixture of SO2 & SO3 which is being cooled?? in which case it may be SO3 condensed containing dissolved SO2.

Anyway,if you are sure about Hydrogen, source of hydrogen is either acid mist carry-over or a leak from Boiler or superheater or economiser ( I can go on assuming)
Do some moisture tests on the gas leaving each of the equipment... You'll know
By the way. when your problem is solved just say HellO

RE: How acid is formed in dry 14% SO2 gas line ?

Dear 6748,
I forgot one more area:
How about you sulfur gun / nozzle assembly?? Isn't is steam jacketted? you can have a leak there??
Please RUN to the plant & find the real culprit
All the Best

RE: How acid is formed in dry 14% SO2 gas line ?

To 6748,

You assume -apparently off-hand- that molten sulfur at 120 celsius becomes bone dry. I beg to disagree.

The drying mechanism of any molten "wet" liquid is based on the fact that the air on top of the liquid is less humid than the liquid itself and depends on residence time, the surface of the pool exposed to air and the depth of the "drying" sulfur. It might, just might, happen that molten sulfur absorbs water from humid air up to a critical concentration.
  
In order to be sure that the acid or its precursors are not already present in the sulfur being burnt, lab checking of sulfur samples taken just ahead of the burner might shed some light to your problem.

The amount of water stoichiometrically needed to produce 20 kg of H2SO4 per ton of sulfur burnt is just 3.6 kg or 0.36%.

RE: How acid is formed in dry 14% SO2 gas line ?

25362,

You are right on.

Even slight hydrocaron impurities decompose over time and release water.

The originator of the first question, appears to have dropped out.  

His use of the term "bone dry" sulfur does not address chemically bound water and is hardly an true indication of water content as you indicate. Melting the sulfur does not remove the chemically bound water- so it is never truely "dry".

Also suspect that the claim of H2SO4 formation he reported, was mistaken for SO3 being formed and allowed to condense. At 14% gas strength, the equilibrium SO3 production is significant. This can occur without water in the system.

SO3 dropout (condensation) is indicative of equipment malfunction and or mis-operation of the plant. It quickly converts to H2SO4.

Drip acid that results from water leakage into the sysyem is usually an issue downstream.


Interesting issues,

 

RE: How acid is formed in dry 14% SO2 gas line ?

(OP)
There is no equipment problem. The sequence is simple and visible.

Atmospheric air is compressed to 6 psi, chilled to Plus 10 Deg c, the condensed water is drained off. This 10 Deg Saturated air is sent to activated alumina dryers to get MINUS 70 Deg C dew point air. This is the air used in furnace for burning sulfur.
The sulfur having rain water etc and atmospheric moisture is melted in melting pits to 120 Deg C. The moisture comes out as steam at the first pit. And then sulfur gets melted. This molten sulfur goes as underpass to the 2nd 3rd pits. After the first pit, the molten sulfur does not get exposed to air. It always has sulfur vapour above the liquid surface. Sulfur at 120 Deg C cannot retain water. The only thing that goes with the sulfur is the ash. No hydrogen sulfide.  

The Assay of Sulfur is correct. This has been confirmed by variuos reliable testing agencies. We pay premium price for this Sulfur.

The sulfur gun is a steam jacketted. And is compressed air atomised. This compressed air is sourced from the MINUS 70 Deg C dew point air.
so no water contamination in this atomisation air. Any leakage of steam in the burner will make the flame wobble and the flame color will change.

There is no leakage of steam in the pits, pump, pipeline, gun/burner, waste heat boiler, economiser.

This is not a sulfuric acid plant. There is no acid drying tower. So no acid mist carryover in the air.

There is no process design/warranty issue. It is only a technical puzzle. None of the acid plants could reply. In acid palnt the gas is not allowed to go below 500 Deg C. Hence nobody knows waht will happen when this 14% SO2 is cooled to below 200 Deg C. We are cooling to 35 Deg C.  
Yes, SO3 can form. But it will not condense to Liquid at this condition. If you have handled Oleum 65% or 24%, you will know that one drop of liquid SO3 will release at least 1 Cubic metre of white fumes in ambient air. The acid containing 24% SO3 is called fuming sulfuric acid.

The acid that I am getting condensed in the pipelines and the economiser is totaly fumeless. This economiser is not a part of the WH Boiler. It is a stand alone economiser connected to the waste heat boiler with a pipeline.

No maloperation is feasible. This is a very small plant burning 12 Tons of sulfur per day. No superheaters. Just a Waste Heat Boiler generating saturated steam at 10 Kg/Cm2 pressure.  

So we are back to same question. From where this Hydrogen Atom comes, when all the inputs are devoid of Hydrogen atoms.

RE: How acid is formed in dry 14% SO2 gas line ?

Since you are using air as your oxygen source to combust with sulphur, you need to realize that air contains 5 ppm of hydrogen and 2 ppm methane (From analysis of Handbook of Physics and Chemistry). This is probably enough hydrogen to form the water and hence the acid that is causing the corrosion.

G. Gordon Stewart, P.Eng.
Gas & Oil Process Engineering Consultant
http://www.ggordonstewart.com/
ggstewar@telusplanet.net

RE: How acid is formed in dry 14% SO2 gas line ?

6748,

We appreciate the thouroughness you have given the condensation problem and I agree that it is a remarkably challenging problem.

The traditional view is that if all sources of water have been eliminated, the only other source of moisture is due to the "bituminus" content of the sulfur.

Industrially pure sulfur contains sources of moisture. It is not uncommon to maintain gas temperatures above 150~200 Deg C to avoid condensation.


A dewpoint of 200 Deg C suggest that the water content exceeds 500-1000 ppm.

The bitumin content of sulfur is not removed by melting and the the true water content can only be determined by ignition and collection of the products of combustion.
Heating sulfur to "dryness" normally does not reveal the chemically bound water.

Good luck,


RE: How acid is formed in dry 14% SO2 gas line ?

Eppur si muove... Molten "dry" sulfur can absorb moisture from the atmosphere physically as well as chemically.

RE: How acid is formed in dry 14% SO2 gas line ?

To 6748, if the formed acids (~20 kg/ton sulfur) were, for example, of polythionic (catenated sulfur) character, the moisture in sulfur as given, ~0.07%, would suffice stoichiometrically to form them.

Polythionic acids have the following formula:

HO3SSnSO3H

However, these acids are considered unstable, decomposing into S, SO2, and sometimes H2SO4. Please comment on your findings for us to learn from your experience.

RE: How acid is formed in dry 14% SO2 gas line ?

(OP)
Hello 25362,
we are draining this acid from the economiser, gas cooler etc. But we have never seen the concentration of acid to be above 80% or so. It is a slightly ash colored free flowing liquid. We checked the acid for corrosion by dropping MS plates. No severe corrosion was noticed. We have sent the acid in MS tankers over a long distance -- (Spent acid). No noticeable corrosion at the tanker or at the purchasers end.
So we, till now presume that the acid is above 97% concentration. The fine ash mixed with the acid is adding weight to the acid, thereby yielding less concentration in our Lab analysis.
If it is an unstable polythionic acid, some yellow sulfur has to come out on exposure to air or if stored in open tank below the drain valve,
No SO2 smell, No sulfur precipitation. Still the puzzle is ON.
There could be some plants existing that do make SO2 gas or liquid by bringing down the furnace  14 % or less % SO2 gas to below 100 DEg C. Have they experienced condensation?
 

RE: How acid is formed in dry 14% SO2 gas line ?

To 6748,
   This thread seems to have run for some time, and yet the drip acid formation is continuing, so I will put my 2 cents in also for what it is worth.  Your SO2 generator is different from the large 1,000 - 3,000 ton per day sulfuric acid plants, but there are similar points to consider.  Most of those larger acid plants do daily drain a gallon or more drip acid at economizer.  The smaller size of your SO2 lines probably have more "cold" surfaces such as support attachment points, and economizer casing surfaces where there will be condensation occurring.  The inlet BFW temperature to economizer is probably less than 100 deg C, and some condensation would be expected there on tubes even with 200 deg C gas temperatures.   The 35 deg C cooled SO2 would be expected to generate more drip acid.
   That leaves the question of source of hydrogen or water to combine with the SO2.  I would suggest checking three places.  The alumina dessicant air dryer could be functioning well for most of the time, but does it have a purge or regenerating cycle? There could be intermittent high moisture levels in the air stream that would not be detected during spot tests.  Another source of moisture has been mentioned as the sulfur gun.  The air atomized spray nozzle is different from most larger sulfur guns.  I wonder if there is a leak between the steam jacket and the air to the nozzle?  I would like to see how well the steam and air sections are kept separate at the nozzle end of the sulfur gun.  The temperature at the end of the sulfur gun could approach the decomposition temperature of water, and a leak could then occur as hydrogen and oxygen.  The steam flow through the sulfur gun is actually cooling the sulfur gun during normal operation, and internal baffles in the sulfur gun must ensure cooling flow at the hot tip and nozzle of the sulfur gun.  The steam outlet should be exhausted to atmosphere to keep steam pressure below the sulfur burner air pressure so any steam leaks would not go into SO2.  The steam jacket pressure through out the molten sulfur system (jacketed piping and valves, sulfur filter jacket) should be lower pressure than the molten sulfur to ensure any leaks do not have moisture and steam entering the SO2 gas stream.    
   One more source of air into the process would be other air purge points such as the sulfur burner sight glass ports or at valve stem packing purges.  If the connected air lines are not to the same dry air supply, but instead ordinary plant air, then that would be the entry point for moisture into your process.
  The air compressor (6 psi) wouldn't be passing lube oil into air stream ?  It would be difficult to imagine much of any lube oil passing through the alumina dessicant.  Yet even compressed air systems with good filter / drier will  end up with water and oil at the low point drains.        

RE: How acid is formed in dry 14% SO2 gas line ?

(OP)
Dear ApC2kp
01.    Yes, the economizer does have cold surface. The Boiler feed water at the inlet of the Economiser is at around 90 Deg C.  We do collect liquid acid from the economizer.

02.    Source of Hydrogen:  Intermittent high and low moisture level in the inlet to the dessicant is very very rare. Because this incoming air is coming out of  freon chiller coils working at say +0 Deg C. The air coming to the dessicant bed attains a temp of +5 Deg C. Sometimes it may go to + 10 Deg C Max. All the physical water that gets condensed from the air is drained out/bleeded out thru drain valves.
03.    The sulfur gun has jacketed steam passage for making sure that molten sulfur temp will not raise causing high viscosity. The steam jacket prevents the radiation heat from raising the temp of sulfur. Only the sulfur pipe is jacketed. The burner tip is cooled by the compressed air. The steam will not reach the burner tip.
04.    The steam jacket pressure cannot be lower than the liquid sulfur. Liquid sulfur is pumped into the furnace volume which may be at 0.4 Kg/cm2g pressure. Whereas steam in the jacket has to be at at least 2 Kg/cm2 to ensure that molten sulfur temp does not go below 120 Deg C. This line is hardly 5 metres in length.
05.    The steam traps of the pipe jackets and sulfur gun jacket are letting out condensate into ambient.
06.    The sight glass ports are not cooled by air.
07.    The compressor for atomizing air  takes air from the combustion air that comes out after the dessicant. So no water or oil carryover.
This is not a sulfuric acid plant. The desired product from this unit is COLD sulfurdioxide gas.  

6748

RE: How acid is formed in dry 14% SO2 gas line ?

To 6748,
Yes, your SO2 process is somewhat different from a sulfuric acid plant.  Most of your process conditions have similar parameters in the sulfur handling side of the process.  There are some things that I have some doubts, as to whether the operation of the alumina dessicant beds is actually up to the required performance but that is not my area of expertise.  I went back over your description  (Feb 6) of your sulfur handling process.  It has one thing missing from typical sulfur melter systems.
 "The sulfur having rain water etc and atmospheric moisture is melted in melting pits to 120 Deg C."  
Most users have their sulfur pile sitting outdoors exposed to rain and sand storms.  The sand and dirt cause other problems for acid plants that would not be a problem for your SO2 plant.  The water in the granular sulfur leads to the formation of acid that causes corrosion in equipment.  While you believe that all the water is removed in the melters, that would eliminate water going into the process.  That may or may not be totally true, but there still remains the presence of acid.  Most melter systems include a lime dosing system to neutralize the acid.  Does your sulfur system have lime addition for neutralizing the acids (into  sulfates)?  Otherwise the acids would only go downstream,  condensing at the cold end of your process.       
 Some users have their sulfur pile in a wood or aluminum warehouse to keep it from rain.  One user with a dust problem from his open sulfur pile thought it was OK to spray water on the pile and conveyor to hold down the dust. The lime dosing system was overwhelmed, and there was severe corrosion of his sulfur melting pit covers.  There was also severe corrosion at the gas-gas heat exchangers from the drip acid.

RE: How acid is formed in dry 14% SO2 gas line ?


To 6748, it appears that ApC2Kp has got it right: the acid forms in the first pit exposed to the elements, and does not appear in the as bought sulfur analysed by a third party. Have you tried to run a similar check on the sulfur entering the reaction ?

RE: How acid is formed in dry 14% SO2 gas line ?

6748,
I know of a similar plant which generates SO2 (and acid as a byproduct).  From talking to people at the plant I have learned 2 things.
1.  They use liquid sulfur, stored above 135C but still "expect" to have some moisture in their sulphur.(the tank is open vented, I am not sure if they actually run a purchase spec on water content.
2. They too have had problems with acid in their SO2 stream (typically 7% v/v).  However in this case, the SO2 stream from the burner was stripped of SO3 in a 98% acid tower.  This acid carried over to some extent following corrosion of the tower.  On tower replacement, which included a demister pad, the problem was dramatically reduced,however minor acids carry-over in the down stream SO2 is not a major issue on this plant, and they vave always collected small volumes of acid in the SO" lined which they assumed was minor carry-over.

How do you remove SO3 from your stream?,
Is the 14% w/w or v/v?
Is your SO2 compressed and stored or fed directally to a downstream process?

RE: How acid is formed in dry 14% SO2 gas line ?

6748,
I know of a similar plant which generates SO2 (and acid as a byproduct).  From talking to people at the plant I have learned 2 things.
1.  They use liquid sulfur, stored above 135C but still "expect" to have some moisture in their sulphur.(the tank is open vented, I am not sure if they actually run a purchase spec on water content.
2. They too have had problems with acid in their SO2 stream (typically 7% v/v).  However in this case, the SO2 stream from the burner was stripped of SO3 in a 98% acid tower.  This acid carried over to some extent following corrosion of the tower.  On tower replacement, which included a demister pad, the problem was dramatically reduced,however minor acids carry-over in the down stream SO2 is not a major issue on this plant, and they vave always collected small volumes of acid in the SO2 lined which they assumed was minor carry-over.

How do you remove SO3 from your stream?,
Is the 14% w/w or v/v?
Is your SO2 compressed and stored or fed directally to a downstream process?
Does the acid cause problems or is it justan inconvenience?

RE: How acid is formed in dry 14% SO2 gas line ?

(OP)
Scorcher,
01.    Sulfur purchase specs do not have water content as a clause. Water usually gets into the system during storage and handling.
02.    You have stated that “””still expect to have some moisture in the sulfur “””” .Do they expect because of their past/present experience? OR do they have any technical inputs for their expectation?
03.    The SO3 is removed by acid wash. Acid mist is removed by mist eliminator that is installed at the top of the acid tower.
04.    My gas composition is normally 12% v/v. But the system is designed for 14% v/v.  
05.    We are not compressing the gas. The 12 to 14% SO2 is directly used in the downstream process.
06.    Acid formation is an inconvenience, because the pipeline, economizer body etc upto the acid tower is getting punctured now and then.  Then it becomes a problem that can stop the complete plant at anytime of the day. Everytime you stop the plant, you are bringing in new cold spots in those pipeline etc that will fail the next day.

Right now we are draining acid from wherever it is formed. We conveniently bypassed the investigation by telling ourselves that our dew point measurement of the combustion air may not be accurate. We have declared  “TRUCE” without knowing who the enemy is.
6748

RE: How acid is formed in dry 14% SO2 gas line ?

To 6748,
There may only be a "TRUCE" in the war on corrosion.  One has to live with a number of annoying intrusions on life.  However, several items could influence the generation of acid in your SO2 gas process and reduce the volume of acid.  The 80% acid concentration is like that of the old lead chamber acid manufacturing process, but there may be only few who know enough about it to relate to your process.  The operating temperature of a sulfur burner could influence the proportion of SO3 /SO2 gases. A burner could be operating at the lower end temperatures and result in more than 3% SO3, or much less SO3 at the upper end of temperature range but generating more NOx. Also, now that it is understood that there is an acid absorbing tower in the process, then commonly known parameters about absorbing tower performance could also be addressed.  The temperature in an absorbing tower will optimize absorbtion, or will result in carryover of acid fumes, or will generate sub-micron mist by shock cooling that will be difficult to remove even by fiber bed mist eliminators.  (A mesh pad has practically no capability to remove 1-2 micron sized mist particles.)  Some one knowledgable of mist eliminators and absorption towers (acid plants) could be of benefit to you.

RE: How acid is formed in dry 14% SO2 gas line ?

To 6748,

Have you thought about natural gas? Do you have a pilot on the burner? Is the alumina regenerated by natural gas combustion?

RE: How acid is formed in dry 14% SO2 gas line ?

(OP)
To lairliq,

Let me place on record that natural gas is not used in any of the operations in this process. Sulfur + Oxygen in the air gives out SO2. There is no pilot flame/burner.
The alumina is regenerated by ambient air that is indirectly heated first by steam and subsequently by electrical heaters upto a temp of 150 Deg C.

All the three issues of your query: a)Natural gas burning for generating 10 to 14% SO2: b) Using a pilot burner for burning Sulfur with air: c) Alumina regeneration with natural gas combustion---> Are totally new to me.

6748

RE: How acid is formed in dry 14% SO2 gas line ?

Three thoughts:
You state that the air is "dry" based on the dewpoint, but even air at that dewpoint will have SOME water vapor.  Have you calculated what the amount of water vapor actually is?  IE, it may be small, but then again, the amount of acid seems to be small.

You state that the molten sulfur can't contain water.  I am reminded of the carbon dioxide in water in soda pop.  The CO2 isn't liquid at room temperatures, and you wouldn't expect to find it mixed with water, yet it does dissolve in the water.  Could something similar happen with water vapor and sulfur?

Lastly, I have found, that when looking for something, when you exhaust all the places where you think it is, then you have to start looking in places where you think it isn't.  While you may be sure no water is in the sulfur, no water is in the air, there are no leaks, etc., it is obvious that hydrogen has to come from somewhere, and therefore one of your observations has to be wrong.

RE: How acid is formed in dry 14% SO2 gas line ?

if 80% acid is being collected, then a water leak is being dealt with. it is that simple...



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