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07071975 (Mechanical) (OP)
22 Feb 06 7:31
Dear all,

I need to procure a sulphuric acid( Strength 98%) centrifugal pump. But the problem is that as per API 610 8th edition Table - G.1 and Table H-1, for sulphuric acid strength > 85% , the pump material class is S-1,(i.e. CS pressuer casing, CI impeller, CI inner case parts etc) and for acid consentration 85%-<1% the pump material class is A-8( AUS 316 base parts).
 In short, I want to point out that for higher acid concentration pump materlals class suggested in API 610 is of lower metallurgy, while for lesser acid consentration pump material class is of higher metallurgy.

Please guide me in selecting proper Acid pump MOC.

With Regards
07071975

 




 
Helpful Member!(2)  Chris0164 (Mechanical)
22 Feb 06 8:24
For 98 % Sulpheric Acid I would suggest to use a teflon lined pump and leave API for what it is.

Lower concentrations demand hastalloy or Alloy 225 alloys.

Kind regards,

Chris

Kind Regard,

Chris

Helpful Member!(2)  ApC2Kp (Mechanical)
22 Feb 06 14:39
07071975,

The size of the pump (flow required) will influence the type of pump selected, as well as the acid strength and temperature influencing the selection of material of construction. Chris0164 is correct in pointing out the need for alloy material for lower concentrations.  I would reserve the use of PTFE lined pumps for weak acid services.

The 98% sulphuric acid could be handled in carbon steel tanks and piping, if it is ambient temperature.  Once the acid temperature gets above 100 deg F the corrosion rate is increasing to a point where other materials are used.

The Charles Lewis Pump Co. (WEIR Group) has a range of vertical pumps for large flows of sulphuric acid and molten sulphur.  Large flows would be in the acid plants of 3500 tons/day and more.  Their standard pump casing is a process iron.  Pump impellors are hardened alloy.

In Europe the vertical pumps from Friatec Rheinhutte and from Ensival are commonly used in those large acid plants.  

Small flow pumps for neutralization / pH control service could be mag-drive pumps of Alloy 20 casing and impellor.  The mag-drive pump and the vertical pump both avoid the leak problems of packing or mechanical seals.

The website www.sulphuric-acid.com has a technical manual section describing pumps for acid service.  
Helpful Member!  JJPellin (Mechanical)
22 Feb 06 21:36
We have had severe problems in concentrated acid service when the project engineer used the tables to select material.  The most notable example was in 98% sulfuric acid.  We installed a pump in all 316 SS construction and it corroded through the case (3/4 inch) in about three weeks.  The corrosion tables predicted a low corrosion rate.  The problem is that higher temperature and higher velocity have a great influence on the corrosion rate.  we had to replace the pump will solid Alloy 20 construction and have had no problem since.  It seems backwards, but it is definately true that concentrated acid is much less of a corrosion problem that weak acid.  But even highly concentrated acid can be extremely corrosive at high temperature and high velocity.  I would not go with Teflon lined, but I would go with a more resistant alloy that the tables suggest.  Alloy 20 should be a minimum in 98% sulfuric.
Helpful Member!  pumphead (Petroleum)
23 Feb 06 14:44
You really need a reliable number on the concentration & temperature, that will help decide materials.  Sulfuric is more corrosive at lower concentrations as mentioned by others here.  Common concentrations I have come across in the US are 98% and 93% .  

Depending on your flowrate and pressure and unique application, you may not need to follow the API610 standard for centrifugal pumps in refineries.  You should be open to an ANSI standard OR for low flows & medium to high heads a diaphragm pump may also be suitable (saving horsepower vs. a centrifugal).  

I have seen both PTFE lined and Alloy 20 in sulfuric service. I have not seen centrifugal pumps out of steel or stainless.  I have seen piping and tanks out of carbon steel, but with corrosion allowances.

 If I may suggest, contact the company who supplies you with sulfuric acid & see what type of pump they use.

I am curious about the process iron and hardened alloy mentioned by ApC2Kp.  I'm not saying it's impossible, I just had not heard of it. I wonder if:
The casing may be a special recipe casting;  
& were special steps taken on the hardened alloy impeller?

Hope this helps a bit.
ApC2Kp (Mechanical)
24 Feb 06 15:32
To pumphead,

 The Lewis Pump casing material is a somewhat proprietary cast iron recipe.  The grain structure has a compacted graphite structure that is between that of grey iron's flake graphite, that of ductile iron's spheroidal graphite, and that of the pearlitic grains of tempered(?) steel.  Their line of Lewmet alloys are used for impellors, flow orifices, and valves where the higher velocities result in severe errosion even for Alloy 20.  The alloys range up in hardness to where the parts are not weldable, and shaping is done by grinding, not by conventional machine tools.  If you check the Friatec Rheinhutte pump materials, they seem to have a couple of dozen alloys of optional materials.
 The use of carbon steel for tanks and piping is possible where flow velocity does not remove the suphate film that provides protection to the metal. If the velocity goes higher than 2 ft/sec or 0.6 m/sec, then erosion does increase.  Stainless steel can tolerate some higher velocity, but temperature usually is limiting parameter and the problems of packing / mechanical seal temperatures with severe weak acid corrosion at the stuffing box / mechanical seal face will force the pump material selection to Alloy 20 or higher.

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