window patching for pressure vessel

[280474]

Dear All,

What is exactly mean by word "window patching" for pressure vessel. Are there any calculation/references to perform this window patching for both of vertical and horizontal cylindrical pressure vessel..?

regards,
Nasrul

 

[samuelliu]

Refer to API 510, para. 7.2.7, please.

 

[280474]

Thank you Samuelliu,

I am currently online from my house and the API book is in my office. Cannot wait until tommorow to check the API, could you please give any response to my following problem :

I have my horizontal vessel(actualy LNG sump tank), that have some deformation on the shell parts. Our Inspection recommendation is to repair the vessel by applying window patching to replace the deformed part of shell. To ensure the vessel will not fail when being cut during window patching process, we need to calculate the maximum dimension (L x W) for the rectangular window patch. Is there any calculation example of this "window patching" available on the net, especially for horizontal cylindrical vessel..?

regards,

 

[LSThill]

Hi 280474 (Mechanical)


National Board Inspection Code (NBIC),ANSI/NB-23 2002 NBIC ADDENDUM Date of Issue: December 31, 2002.

Page 174:
APPENDIX 6-EXAMPLE OF REPAIRS AND ALTERATIONAS


B. EXAMPLES OF REPAIRS:
B. 11 the installtion of a flush pach to a pressure retaining item;

and B.12 the replacement of a shell course in cylinderical pressure vessels;

Leonard@thill.biz

http://www.thill.biz

 

[prex]

280474, don't understand your point.
If you're afraid of vessel weight because you are cutting the window somewhere between the saddles, then you should use temporary supports.
The dimension of the replaced plate should be dictated by the dimension of the damaged part, certainly not by problems of support during the operation.
Anyway the repair of a vessel is a serious matter: please consult a local specialist. prex

http://www.xcalcs.com

Online tools for structural design

 

[deanc]

Agree with prex. Why did this occur to begin with? Perhaps this needs to be investigated prior to any repair. The use of temporary supports is common practice.

 

[280474]

Agree with Prex & deanc,

Temporary support is common practice. But for my reference, let's say I don't want to use any temporary support, is there any calculation standard/paper regarding the stability of horizontal vessel during window patching process..?

I have a reference only for vertical vessel that had carried out calculation to find maximum dimension of window patch at any location of vessel shell. This calculation is to ensure the stability of vertical vessel during window patching process without any temporary support required.

thanks

 

[Tankman650]

280474,

This is an interesting problem. Where on the cylinder do you need to cut out the shell??? Is it on the top, side or bottom and is it between or near the saddles???

The difference between vertical tanks and horizontal tanks is that the vertical tank is mainly in axial compression when empty. I have repaired sections of the shell by boxing around the cut out with stiffeners to transmitt the compression around the hole and maintain the strength and shape of the cylinder during repair.

The horizontal tank has compression stresses on the top and tension on the bottom. In addition , there are high shear stresses especially near the saddles. These shear stresses will cause out of plane distortion if a hole is cut in the shell. I can imagine that a stiffening system could be attached around the hole to transmit the shear. I have not seen any calculations concerning this.

It could be analyized using a FEA program. In any case, expect the see local distortions from weld shrinkage due to the welding of the insert plate.

I agree with the earlier post, you should know what caused the problem in the first place before trying to repair which could cause a greater.

 

[280474]

Thanks Tankman,

There are a number of local deformation as form of dents were formed on the shell of LNG sump tank. The defects are located on both sides of both shell circumferential welds. The assessment for roundness based on ASME Sec. VII Div. I para UG-80(a) shows that the ratio of nominal tank diameter and minimum diameter reduction is 1,09% which exceed the code permissible out of roundness.
Currently Investigation to determine the reason of shell deformation is being carried out. As I mentioned before that is nice to know if there is any calculation reference to ensure the horizontal tank stability during window patching process.
Refer to vertical vessel stability calculation during window patching, I think the methode can be applied. But unfortunately I have a problem to determine the cross section properties for half-cut horizontal cylindrical shell of the tank(vessel). Can anybody help me for this..?

thanks,

 

[280474]

Additional Information for my previous thread,
How to find the cross section properties such as moment of inertia,center of gravity of the following shape (half-cut of horizontal cyilinder) :

window patch
I---------------I I------------I
I---------------I I------------I
I I
-I-.--.--.--.--.--.--.--.--.--.---.-I-.--center line
I I
I----------------------------------I
I----------------------------------I

Thanks,

 

[chicopee]

Before you embark on repairing this LPG tank, better get the manufacturer's data sheet from the National Board. Afterward you can follow the guideline for window patching by getting copies of the National Board Inspection Codes detailing repair procedures and the necessary forms to repair and restamp the tank, ASME Unfired Presure Vessel Code VIII(two volumes), and their welding code.
You want to legally protect yourself in the event of a catastrophic blow up following repairs.
If your insurance company provides boiler and machinery coverage to your firm get them to help you before proceeding.