PWHT of storage tanks

[dharmeshajoshi]

I am designing a sulfur storage tank. It has a design tempetature of 160 deg. C.
The tank needs to be Post weld heat treated as per material requirements.
There will be an expansion of the tank shell and the bottom during PWHT process. There is a need to provide a slot in the anchor chair top plate to take care of the expansion of the tank during PWHT.
If the slot is not provided then the tank shell will expand and the anchor bolts will bend as they are fixed from the bottom.
How do I calculated the expansion of the shell?
How much will be the expansion of the bottom plate?

There are 3 layers of fire clay bricks 75 mm thk and one layer of 40 mm thk refractory castable provided below the bottom plate.
What will be the heat transferred from the bottom plate to the tank foundation through this layers?

If anyone has desgined or fabricated such tank with PWHT requirements please let me know regarding the procedures, precautions needed to be taken during the same.

 

[JStephen]

Radial expansion will be the temperature difference times the coefficient of thermal expansion times the radius.

For the heat transfer, you might try assuming the subgrade remains at ambient temperature and calculate heat loss through the fire brick- should give you the upper limit of heat loss through the bottom, assuming all is dry.

 

[JoeTank]

Radial expansion is just one of many problems. Collapse of the tank during PWHT is one of the grestest concerns. It comes down to a balance between holding the right internal pressure and tank uplift. Most issues are diamter dependent, so not much help can be offered unless you share more info. I did PWHT on a large tank many years ago. We calculated and measured over 14" of circumferential growth at 1200F.

Joe Tank

 

[MJCronin]

The choice of a flat-bottomed tank may not be the best for sulfur storage.

Many sulfur storage tanks I have encountered have been horizontal axis, supported by two saddles. One saddle has slotted anchor bolt holes for expansion.

The tank is designed, fabricated and tested to ASME VIII but not code stamped.

-MJC

 

[dharmeshajoshi]

Mr. Joe

The details of the tank are as follows:
Tank dia : 15 m
Tank ht : 15 m
Material : IS 2062 Gr B
Content : Sulfur ( Specific gravity = 1.8 )
Design Temp. : 160 deg C
Design Pressure : 300 mm of w/c
If you need some more information let me know.

You had done PWHT of a tank few years ago.
Was that tank anchored?
Was the PWHT done after the tank was kept on the foundation or before that?

During PWHT there will be thermal expansion of the tank bottom plate and the shell. We have calculated the expansion and it comes to around 60 mm.
What precaution should be taken so that during expansion the tank bottom plate and shell do not hit the anchor bolts.
We are thinking of increasing the Bolt circle diameter and provide slot in the bottom plate and anchor chair top plate to take of the expansion.
Is there any other method/modification that can be done?

Thanks & Regards,
Dharmesh Joshi

 

[JoeTank]

Provide expansion slts in the anchor chairs.
Avoid slotting the tank bottom.
Differential temperature gradients at the shell-to-bottom joint and the roof-to-shell joint must be checked. These areas areas created great concern on our project. We have to control the delta-T at these areas by slowing down to heat up rate.

Joe Tank