API 12F Testing requirements, why no hydrostatic?

[stanfea]

It is clear to me that API 12F is a pre-engineered standard. It is unclear to me however what the testing requirements are. The standard says to test to 1.5x the design pressure. The design pressure is listed as 16 oz/in^2 or 1 psi. The tank will clearly see 6.5 psi just due to static fluid pressure, as the fill height is 15 feet. Why does the testing procedure not call for at least a hydrostatic test? All comments welcome.

Thanks
Stan

 

[dcasto]

Not all tests are mechanical strenght tests. ANSI B31.3 includes a leak test at 1.5 times MAOP. At that pressure you will not test the strenght.

All the componets in the API tank and ANSI B 31.3 are tested before final assembly. The material is qualified and tested, the weld procedure likewise, and finally the human or machine welding it.

If we had to test every assembled item as a final check, how would that work. My SPC teacher put it this way, he was from Oakridge Tenn. If we assembled a nuclear device and we had to verify the completed project, how would even make the final project because you would have to trigger the device and explode it.

 

[GregLamberson]

As dcasto points out, not all tests are required or carried out.

For example, API STD 650 waives radiography for small tanks if built to API STD 650 Appendix A, Appendix J (ship fabricated) or Appendix S (stainless steel) if a low shell weld joint efficiency factor (0.7) is used. The rewsults are lower allowable stresses and greater shell thickness than for radiographed tanks. However, there are reasons not to waive this requirement, while the cost savings to waive radiography is significant, owner insopections cossts will be increased consioderably as radiography is the main method for shell quality control. Trade offs.

There are a few technical reasons why you would not want to carry out a hydrotest, but in your case, why would you not?

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[JStephen]

Suppose you take a 15' diameter by 20' high tank. Per API-650, App. A, with 70% joint efficiency, minimum thickness calculates out to 0.050", or about 1/5 of the actual thickness. So on tanks of that size range, unless the welding is really bad (ie, no visual inspection at all), the hydrotest is just going to be a leak test, and won't accomplish much structurally.

 

[GregLamberson]

Jstephen

You are correct, it would technically be a hydrostatic leak test.

I've had leaks have occur in new tank floors even after a full vacuum box test of floor welds. In some cases, leaks can occur from an arc strike.

The cost of cleaning a tank after a product leak has been detected exceeds the cost of a test.

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[JStephen]

Greg, I was assuming that they were doing some sort of pneumatic leak test in lieu of the hydrotest. Yes, I agree that the tank should be leak tested. I don't have a copy of that particular standard handy to see how it reads.

 

[chicopee]

I dont know anything about the application of API12F, however a hydrostatic test at 1.5 x design pressure suggest a pressure vessel testing protocol.