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Oh No … !  Your Tank Doesn't Have Nozzle Repads?

Have you ever had a tank inspector say that to you?  If you ever hear that, you might want to throw up a red flag in your mind.  We have inspected tanks over the years that had reinforcing pads (a.k.a. repads) on nozzles that didn't need them.  Upon inquiry, the tank owner told us that a tank inspector had previously looked at the tank and told them they were required to have repads on their tanks, so they put them on.  When we investigated the tank calculations, we found that the tank was designed to meet the code without the use of repads.  This is not an isolated case.  So, what does your tank really need?  API-650 doesn't say you are required to have reinforcing pads on your shell nozzles.  But, it does say the opening has to be reinforced.  Let's take a closer look.

When an opening is cut in a steel tank shell, the cross section of the plate in that areas is reduced and, therefore, the strength of the shell is also weakened to a certain extent.  In order to maintain the strength of the shell, additional material is added around the opening to replace the removed cross sectional area.  In the current edition of API Standard 650 "Welded Steel Tanks for Oil Storage", Section 3.7.2, it states "Openings in tank shells larger than required to accommodate a NPS 2 flanged or threaded nozzle shall be reinforced."  This section goes on to explain, in detail, the accepted forms of reinforcement and the required dimensions and locations.  Essentially, nozzle opening reinforcement can be achieved with the following procedures: 

  1. Reinforcing Pad:  This is the most common form of reinforcement.  A typical round or diamond shaped plate is welded to the tank shell surrounding the opening.  This is a very simple method to both design and install.  As an alternative to this method, however, this style reinforcement can also be installed on the interior of the tank.

 

  1. Excess Shell Thickness:  This method generally occurs on small diameter tanks.  The API-650 minimum shell thickness for tanks less than 50 foot diameter is 3/16".  If you run through the calculations for the required shell thickness on some tanks, it may be far less than 3/16".  In many cases the difference between the required thickness and the API-650 minimum thickness is enough to qualify as reinforcement.  In some cases the fabricator may be able to, simply, move up to the next nominal plate thickness and cover the reinforcement requirements.

 

  1. Nozzle Neck Material:  There are some instances where the thickness of the pipe penetrating the tank shell may be used as reinforcement.  API-650 requires a minimum pipe wall thickness for each size pipe.  It is acceptable to increase the pipe wall thickness and use a certain portion of that material as reinforcement.

The methods of reinforcement listed above require specific engineering calculations and all involve additional requirements not listed here.  This article is intended as general information to keep the tank owner/operator aware of possible designs.  If you have a question about a specific tank or you are working on a design of a new tank, give TEAM Consultants a call, we can help.

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Copyright © 2002 Tank Engineering And Management Consultants, Inc.
Last modified: June 19, 2007