Corrosion Allowance

Corrosion allowence is mostly used for equipement being used in conjuction of accidic properties. Most the time if a tank is to be used in an acid environment, it is good to use stainless tanks which are rated for certain types of chemicals.

Corrosion allowence is calculated for time wear and properties of the materials in use. Such as a line down the middle of a road and the paint used to differentiate the seperation of the lanes. The actual wear of that line peing exposed to the road traffic and weather and oil residue from traffic. So, if you have a road that will handle 250K vehicals per day, you would want the lines to last more then 5 years of daily use. So you will need to calculate a usable line width, that after 5 years of 250k daily traffic, to actual be visable and usable in a given stretch of time. So you will also need to know the endurance of your paint being applied for that line as well.

Hence, the corrosion allowence of a line down the middle of the road.

Maximo

<Most the time if a tank is to be used in an acid environment, it is good to use stainless tanks which are rated for certain types of chemicals.>

There is a growing trend towards using plastics for some applications, for which a corrosion allowance is not required; polypropylene is a particularly attractive material in this respect for both acids and alkalis. Do not let hydrochloric acid anywhere near nylon, though!

It is common to place highly concentrated sulphuric acid in mild steel tanks beacuse, curiously, mild steel is not attacked by this aggressive material. The key thing is keeping water away from it. Polyvinylidene diflouride (PVDF) is a good material for pipework on sulphuric acid, and is reflected in its value.

Acrylonitrile butadiene styrene (ABS) and polyvinyl chloride (PVC) have their uses in piping systems for acids as well.

http://www.coleparmer.com/techinfo/chemcomp.asp is a useful link in any Engineer's 'favourites'.

we primarily use PFA in bulk chemical distribution systems and PVDF in AWS.

Check this out:

http://www.hghouston.com/Technical%20FAQs/TFAQ116.html

Corrosion allowances are most commonly used for process vessels rather than piping. For piping, you select the pipe schedule (i.e., Schedule 40, Schedule 60, and so forth). The idea is to select a pipe schedule whose pipe thickness includes your desired corrosion allowance.

As for the amount of allowance itself (on vessels or piping), it depends on what will be flowing through the pipe or vessel, the temperature and pressure of the flowing fluid, the piping material (carbon steel, stainless steel, aluminum, etc.), and the preferences of the pertinent industry.

Point 1.

a. Corrosion allowance is required to be taken under consideration especially at the 1st stage of design of any process equipment, pressure vessel, heat exchanger, pipeline, ..etc.

b. The corrosion is considered to be occurred at the surfaces subjected to corrosive fluids, i.e in piping, corrosion is considered to occurred at the inside pipe wall surface.

c. Also, in piping systems, an erosion can be occurred due to relatively higher velocities of fluids, and must be taken under consideration. Pipe threads shall be treated as corrosion, especially in determining the pipe wall thickness.

d. In piping work, a tolerance of ± 12.5% of pipe wall thickness shall be taken under consideration as a code requirement to compensate any reduction of thickness of new piping materials.

Point 2.

a. In sizing of a pipeline, it is required to define the surface roughness of the interior pipe wall surface, which have a great effect on calculating the friction loss. If you have a relatively high friction loss, the designer is requested to increase the pipe diameter to minimize the pressure drop.

b. Pipe surface roughness may be affected by type of corrosion, scaling, pitting, smooth and uniform corrosion.

c. For corrosion -as a reduction in thickness of pipe wall material, and in turn increasing the inside diameter- I think there is no significant effect on hydraulic calculation.

I would say corrosion is a process of "degradative byproduct formation by reacting to the medium that is in contact". Most of the time the byproducts are oxides and salts.

Corrosion allowance is an additional thickness you provide to piping, pipelines and equipment to manage uniform corrosion anticipated in that environment. But this is not a magic bullet that will take care of all types of corrosion as it is only based on uniform corrosion models and will not cater for localised corrosion. In other words it is insurance for the system so you could maintain them with reasonable level of safety.

Thank You very much Mr. Raghvan. This is what I was looking.

We are currently overhauling some old single stage steam turbine pump drivers from a refinery that was shut down 10 years ago.

The machines we not preserved and rusted away in seaside conditions

The turbines seem to have lost several mm in wall thikness on the pressure casings

What is the corrosion allowance used in the design of this kind of turbines?

How can we be sure the remaining wall thickness is sufficient for operating (ie steam) conditions?

Thanks for some help

Oh my God, 10 years ago?

Nothing to do either:

1. Ask the manufacturer,

2. Repair to its original thicknesses and finish, or

3. Take it out.

the corrosion allowance is a function of the corrosivity of the fluid, so you've to look at the original design documents. Regarding the remaining wall thk you've to make some calculation based on pressure and maybe stress analysis ad see if it's enough.

S.

corrosion control & protection

Now after reading all these comments I have a doubt,

If I calculate a vessel and get 20mm of minimum thickness and add no corrosion allowance.

after 4 years, I do an inspection, and probably the thickness has decreased. do this means the vessel is no good for use?

Also, I've seen a design of a vessel with C.A. of 6mm to CS parts and 0mm to 304L parts. The product is sour water at 10bar @ 100ºC. Is this reasonable? or it is expected to have problems with the SS parts?

cheers

Is not so simple. What you've calculated is the thickness to resist the pressure. What you have to do is to assess the corrosivity of the fluid and then decide if an extra thickness(corrosion allowance) is needed considering the corrosion rate and design life.

if after 4 years you find corrosion then you've to assess how much is the corrosion rate and then what is the minimum thickness for your vessel. it's possible the the corrosion rate is so low that you can use your vessel until the end of its design life.. if the corrosion rate is too high then you've a problem....

regarding the SS 304, on SS is never used a corrosion allowacne since SS is never, usually, affected by general corrosion..

hope this help

S

corrosion prevention