Caustic Storage Tank - Steel or FRP 2

[amjp]

When selecting a storage tank material for Caustic, it is said that steel tanks are cheaper on CAPEX but the FRP tanks work out to be cheaper on OPEX over its life cycle.

Does any Life Cycle Cost comparison (capital and maintenance) exist between both steel and GRP tanks and if yes what is the payback period for FRP.

 

[JoeTank]

What is CAPEX and OPEX? What is tank size?

Joe Tank

 

[metengr]

Joe;
I believe CAPEX is Capital Expenditure and OPEX is Operating Expenditure. Different ways for project justification.

 

[metengr]

When selecting a storage tank material for Caustic, it is said that steel tanks are cheaper on CAPEX but the FRP tanks work out to be cheaper on OPEX over its life cycle.

Not sure I would agree with this statement. We follow specific accounting rules for capital project justification - cost and new design (not an in-kind replacement). If a project falls under CAPEX, an NPV spreadsheet is established to deal with carrying charges and payback.

For an OPEX, it is one time cost that goes against your Op budget. If a project is deemed capital because of your accounting rules, you have no choice but to treat it under your capital budget.

 

[amjp]

The tank is 50m3

 

[amjp]

I am looking at the option of either going the steel or the GRP way depending upon the NPV cost for a life cycle of 25 years.

This means that the total estimated cost for both the tanks are the capital cost of constructing tanks with either material and the maintenance cost associated with the steel or GRP for 25 years at NPV.

 

[MJCronin]

amjip...

You may require the services of an engineer...

What is the strength of the caustic and what at temperature do you intend to maintain the liquid..?

Higher strength solutions cannot be stored in GRP....

-MJC

 

[moltenmetal]

You can store 50% NaOH in a GRP tank to 180-200 F if the tank is constructed correctly.

Concentration matters, though, to both options. As does temperature.

Do you care how much iron is in your caustic?

 

[amjp]

Caustic strength is 50% at Ambient temperature. Iron does not matter.

Process and capacity wise, both materials are acceptable. However the choice would depend on the Life Cycle Cost (say 25 years)of using either a heat treated steel tank or a GRP tank.

If the higher GRP capital cost offsets the capital and maintenance (external repainting + replacement due to caustic corrosion etc) cost of the steel tank, we would like to pursue the GRP alternative.

 

[DefenderJ]

We have 50% caustic tanks made from steel.
There is no need to heat treat if caustic is not above 35°C

Corrosion has been a problem for us, mainly due to corrosion under insulation.

Getting the insulaation materials and detailing correct as well as the painting correct during construction will save expensive bills later. Also important to ensure base design prevents water ingress that woudl lead to underfloor corrosion.

 

[amjp]

I am looking at Ambient of 50 degrees Celsius, desert/dry environment, winters of around 5 degrees celsius min., dry weather with very little rain. Forgot to mention that.

What is your annualised maintenance cost per meter cube per year. Did you have to go for tank or sectional replacement due to excessive corrosion. What is the average life of these steel tanks.

It is said that tank maintenance is about one dollar for each cubic meter (size of tank) per year of service. This can not be applied to small tanks but do you monitor the tank maintenance cost and is it somewhere in the range as above.

If the tank maintenance costs are high, have you looked into replacing the steel tank with a FRP tank.

 

[DefenderJ]

The NACE curve would indicate that for 50% caustic you can use un-stress relieved carbon steel up to 46°C - looks a bit marginal for you.
I don't have any maintenance costs per cubic metre.
Two of our 50% tanks are 8,300 m3 (vertical ø27m and insulated), the other two are 27m3 (horizontal & heated).
We have not considered using FRP, but have not needed to replace or add another tank.
We have had the tanks 27 years.
When we had corrosion on the insulated tanks we did not have to replace steel sections, but did put lap patch plates on and paint. (original spec was bare steel under rockwool insulation! and we have plenty of rain here)

 

[amjp]

Thanks defenderJ, that was pretty useful information.

Considering the ambients described above and cost as the driving factor, steel would be the material of choice for the service.

Your maintenance cost for lap patch plates probably was due to external corrosion (heavy rains....) and not due to the product itself. Since rains are practically negligible in this case, external corrosion would not be an influencing factor in this case.

Our concern was about Corrosion and Caustic Stress Corrosion Cracking (CSCC) in caustic service, but your post addresses this well.

 

[TomEun]

In the area A & B of NACE caustic chart, the carbon steel may be a common material for long term reliability and effective maintenance unless metal (iron) contamination must be avoided (e.g., potable water).

Thomas Eun