Repads on Nozzles 1

[Tankman650]

API650 has repad sizes for nozzles. It also says that repads can be designed per API620. In API620, the size of the OD of the pad is limited to 2d of the nozzle.

Anyone know why the OD of the repad for API650 is called out as larger than permitted per API620???

 

[JoeTank]

I don't have my API-620 with me, but I suspect that it says that the limits of reinforcement (not the repad) are limited to 2D. If true, this would mean that any cross section area added beyond the 2D limit would not be counted as reinforcement for the opening.

The API-650 rules are based on area replacement in which the reinforcement area is based upon the hole cut in the shell. If you take a close look at each of the standard API-650 repads you will noted that they are typically slightly larger than 2D. This is simply the result of using the old repad dimensions from the days when we riveted tanks together.

Using a 6 NPS nozzle for example...
If the shell is 3/16" thick... the hole is 7.25" diameter...a 15.75" diameter repad is specified.

If the shell is 1-3/4" thick... the hole is 7.375" diameter...a 15.75" diameter repad is specified.

It makes no sense technically to limit the max size of a repad. Repads are often made largeer than needed for area repalacement for other reasons, such as for nozzle-mounted mixers.

API-650 does not limit repad sizes to 2D. Ditto for ASME VIII nozzles. I suspect that it is true for API-620 also.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[Tankman650]

SteveBraune,

You are correct in that 2d is the limit of reinforcement in API620.

I was wondering why the repad OD in API650 is always specified larger than the limits of reinforcement. It is reasonable to assume that this is due to the use of rivets in the past. I am surprised that fabricators have not complained about excess metal used in repads.

Thanks for the insite.

 

[JoeTank]

In the overall scheme of things, slightly oversized repads is a trivial cost and technical issues. The real charm comes from a standardized set of detaills that give repad size and elevation. This eliminates most of the piping input requirements from the owner. Often the piping details are still being worked out by the piping engineers while the tank design proceeds without much interference or holdup.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com