Hydrotest of repairs to API653

[KiwiTanks]

API653 Section 12.3 relates to the hydrotesting of a tank after major repiars and states "A full hydrostatic test, held for 24 hours, shall be performed...."

What is the definition of a "Full Hydrostatic test". I have a client who just wants to go for the minimum possible. The only references I can find relate to the maximum. My understanding is that a hydrotest should be a full as possible, stopping where limitations such as floating suction, floating blanket, over-fill, or structural limitations prevent further filling.

For example, is it acceptable to fill to the same height of water that would give the same stress in the lower strake as if it were filled with product to the normal fill level - even when there is approx 3.5m of shell height above this level?

!!??!!Help!!??!!

 

[JStephen]

In normal use, the hydrotest is understood to be an overstress condition, not just stressing the same as product does. Of course, with products same or heavier than water, this isn't generally feasible.

 

[JoeTank]

A full hydrotest is understood to be filling the tank with water to the design liquid level or any other height limited by physical resticteions (as you have noted in your post). The only exception given is for a tank that cannot be filled without overstressing it, such as due to shell thinningcorrosion. There are waiver of hydrotest provisions also, but the owner assumes a greater risk in doing so.

Joe Tank

 

[786392]

It is true that the recommendation is a benchmark w.r.t. safety and withstanding strengths of storage tank(s).

But this is also true, that unless proper thickness scanning as due is made and overall structure's strength especially the lower shell tiers and bottom/ annular plates ensured
and the hydrotest liquid Sp.Gravity is not more than normal(soft) water.
Risks/Hazards involved be thoroughly reviewed by all concerned.

Since once a Mogas Tank gave way; while nearing top-up during hydrotest after repairs and nearby structure & equipment(s) were damaged at one of Shoreside terminal(s) around 20~22 years back.

Best Regards
Qalander(Chem)

 

[JoeTank]

Can you share more information about the failure?

Joe Tank

 

[KiwiTanks]

Thanks for your input guys.
We have now filled the tank beyond the safe fill level for product, and as high as the floating suction arm will allow us. The arm is actually tied down to the bottom of the floor, but no one is certain why. (Can anyone shed some light on this?) The arm is long enough that it won't 'flip over' at any fill level.

We had performed extensive NDT and calculated the structural (and seismic) fill heights, so we were not concerned with over stressing the tank at all.

I have seen photos of several tanks which have given out under hydro and really didn't want it to happen to us. I think the NZ Refinery had one many years back.

 

[IFRs]

If there is a floating roof, the floating suction needs to be either tied down or have additional weights attached to it so it won't damage the floating roof due to excessive buoyancy.

 

[786392]

Dear Joe Tank Since
1)this was not my working place rather a nearby terminal the info may take time to fetch and disseminate.
2)I am out of place for around 28 days w.e.f.15th Nov 2008 thus obtaining the info would not be possible untill I resume.

However from the recalled memory;this was around 50ft dia and 13.5 M height Fixed conical roof tank built as per API 650.(Contructed around 18 years prior to this repair.)

Contained Motor Gasoline and Blends upto Mid 1980's
Repairs made on Shell/Bottom lower peripheri& Scatterred shell plates,Bottom Replaced and Roof Repairs.
Usual Product Sp.Gravity ranged 0.65~0.75

It was under Hydro static testing with Sea Water;Gave way from Bottom/ shell peripheral portion at Level about 93% filled.

Best Regards
Qalander(Chem)