plating for pressure vessel

[engineeringguy981]

I am working on a design project for my engineering class and I need help on deciding the type of plating I need for my pressure vessel. Originally I had planned on constructing the entire pressure vessel out of stainless steel but it is too expensive and difficult to machine.
I want to switch to mild steel to save cost however since the vessel will contain water I do not want it to rust.
The vessel is constructed out of a 10.75 outer diameter tube with a 3/8 wall thickness and a lenght of 2 feet. The bottom end of the tube is closed off with a plate welded in the inside and top is a larger plate which is completely removable. The vessel will be filled with tap water and the pressure inside will brought up to 300 psi(max). The pressure inside the tank will reach 300 psi and will be held there for up to 1 hour and brought down to atmospheric. This cyclic process will be done 100,000 times.
My questions are;
what type of plating do you recomend? I want to achieve the lowest cost. If you can give me an rough estimate on the cost of this.
what type of considerations do I need to make?
thank you for your time.

 

[moltenmetal]

If the fluid is water, there are plenty of lining materials which will work better than plating for corrosion resistance. Have a look at

http://www.plasite.com

as an example.

 

[kenvlach]

Temperature?

Is the tapwater chlorinated?

Is the pressure adjusted hydrostatically or by gas? Any gas volume above the water? If so, use nitrogen, not air.

Probably cheapest to add a corrosion inhibitor to the water and/or raise the pH to 9.5-10.

A sacrificial zinc anode should also work.

 

[metengr]

I would hope that your vessel is being designed and fabricated in accordance with ASME Boiler and Pressure Vessel Code, Section VIII, Div 1 requirements or other recognized code of construction!?

 

[swall]

Following up on kenvlach's suggestion of adding an inhibitor to the water, you could make the system closed loop with a water reservoir filled with DI water with inhibitor added.

 

[engineeringguy981]

thank you for your responses.
kenvlach-
The temperature will be room temperature.
Yes, the tap water will be chlorinated.
The pressure will be developed hydrostatically. I am assuming there will be a small amount of air trapped.
metenger-
Yes, we design it using the ASME Pressure vessel code D1 with a fatigue analysis, it has not been fabricated but it will be constructed using the asme code.

Thank you again.

 

[engineeringguy981]

It suggested that we could simply paint the inside with a special paint to prevent it from rusting. I am thinking that if we use some type of paint it will crack because of the cyclic pressure.

 

[engineeringguy981]

Is there a simple do it yourself lining kit applable for our application?

 

[rconner]

This is probably a stupid question (and I could be misunderstanding what you mean), but if each cycle is held for an hour and there are 100,000 cycles it would appear that this project could take take over 11 years to complete. It would appear some students might be grown up and have kids/students of their own by the time it completes (or maybe there is some job security here)! In any case, one would think 300 psi pressure in a 10" steel vessel 3/8" thick should create very little strain in the steel or lining etc..

 

[EdStainless]

The other apporach is using clad plate for the vessel.
But as small as this is using solid stainless construction is the easiest and most reliable way to go.

If you use a coating of plating you will need a procedure for detecting coating failure. This will require periodic inspections and such.

You are going to use a piece fo sch80 pipe?

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[kenvlach]

All paints have some moisture permeability which will be increased by the pressure cycling. There is also the danger of bacteria under the paint causing MIC. A high-T baked-on paint may kill the bugs but is generally more brittle. Plating may flake, have pinholes or cause galvanic corrosion.

So, don't rely on painting or plating. There's probably a suitable lining & definitely cladding ($$$), but then you have direct & inspection costs. Better to 1) use an inherently corrosion resistant material, 2) make the environment non-corrosive or 3) provide a corrosion allowance (thicker wall). With uncertainty or critical applications, do 2 or even all 3 of these.

I like EdStainless's suggestion (?) of Sch. 80 pipe, especially if make it a closed, ambient T system.
Hot dippped galvanized Sch. 80 pipe (10” diameter has 0.593” wall) + some corrosion inhibitor (e.g., Prestone) should work. HDG is closer to a cladding than a plating. Pipe sizes at

http://www.pipingdesigners.com/downloads/pipechart.htm#10

Aluminum pipe is available in marine grade, Al-5086, in 10” Sch. 40 NPS (maybe also Sch. 80; do a search). Chromating per MIL-C-5541, Class 1A, + Prestone should work. Millions of Al air cylinders have been used to higher pressures (although some fiberlass wound) by divers.

ASME code switches from pressure piping to pressure vessel at 15” diameter (? please correct if my recollection is wrong). Some useful pipe info (pressure ratings by size & strength):

http://www.pveng.com/Charts/Charts.htm

Ken

 

[stanweld]

Assuming that you are using pipe to SA-53B or SA-106B, the applied stress is only 13% of the specified minimum yield strength and most likely less than 10% of the actual yield strength of the material. I'm not sure that you will see anything significant with such low applied stress/strain regardless of the number of cycles. I must admit that I am not aware of what the premise of your project is.

 

[sreid]

How about Hot Dipped Galvanizing?

http://www.galvanizeit.org/

 

[codeeng]

1/8" corrosion allowance.

 

[mshimko]

You can have the tank blasted and painted with two or three coats of epoxy, which will provide 10-20 years of service in salt water, and longer with fresh water.

Heck of a lot simpler (and cheaper) than platting.

As to chance of failure and pressure cycling, this is the system the Navy uses on submarine tanks, which see pressure cycling a heck of a lot more severe than this tank will ever see.