TEAM
Consultants

TANK DIVING – A RELIABLE INSPECTION OR UNDERWATER EXERCISE?

The scenario goes something like this.  You, the tank owner, really don't want to take your tank out of service to have it inspected.  Someone calls and offers to put a diver in your tank and inspect it without draining out the water.  Sounds like a real good deal!  But, hold on.  What do you know about this inspection process?  Is it reliable information?  What are other owner's experiences with this process?  Hopefully this article will address these questions and give you some insight into inspecting your water tank.

 Let's begin by looking at the American Water Works Association standard M42 – Appendix C – "Inspecting And Repairing Steel Water Tank, Standpipes, Reservoirs, And Elevated Tanks For Water Storage".  Section A-1.4 of this standard states "The owner … shall have the tank emptied for the inspector in order that the inside of the roof, sides, and bottom of the tank will be properly exposed for inspection.  The inside surfaces shall be washed down by the owner to remove slime from wall surfaces and loose deposits and dirt from the tank bottom before the inspector arrives.  This is essential."  Section A-2.1 outlines the inspection of the coating and asks for detailed descriptions of any corrosion spots.  Section A-2.2 outlines inspecting the steel for pitting and asks for details of any pitting found and possibly ultrasonic thickness testing if the remaining thickness is questionable.

As you can see by reading through the AWWA standard, very detailed information is required to properly analyze the condition of a water tank.  Unless adequate provisions are made, I believe that an underwater tank inspection is little more than an expensive diving exercise.  What are some of these provisions?  Consider the following:

1. Removal of slime and deposits from the tank walls and floor.  Most tanks may appear clean until the water is disturbed and then the surfaces may be barely visible.  Technology is available to clean tanks that are in service, however, the cost can be prohibitive. 
   
2. If the tank is a reservoir or standpipe, it is necessary to examine the floor for remaining thickness, using ultrasonics, to determine if there is evidence of underside corrosion.  If the tank is empty this process can take several hours to perform.  A 100 ft. diameter tank bottom can be expected to take up to 4 hours to examine.  This process will be even more time consuming and costly if performed by a diver.

There are safety issues to consider as well.  Before performing an inspection by diving the tank, it is mandatory that all valves into and out of the tank be closed and locked out.  Therefore, the tank is not really "in-service" during the inspection.  Another issue is the time a diver is able to be underwater.  If a problem area is found, time limitations could hinder the performance of a proper inspection.  The time issue is also tied to the depth of the tank.  Some standpipes are over 100 feet deep.  A dive of this depth must be performed with extra caution.  If a problem is found on the tank floor, a proper inspection could be very cumbersome.  In relation to an elevated tank, most divers never go down the riser pipe, so that area is never examined.  On any type of tank, the diver is required to guide a large bundle of cables used for the camera, lighting, microphone, etc.  Care must be taken not to get this bundle tangled with the structure.  Therefore, some areas of the tank are never viewed by the diver because they cannot by accessed.

I have heard several reasons for not draining a tank to inspect it.  One typical argument is from tank owners that don't want to be without their tank for several days.  My answer is this: a typical inspection and the following sterilization process generally takes three days.  On the other hand, a coating failure and subsequent corrosion damage will take weeks to repair.  If you don't have time for three days, when are you going to find time for a few weeks?  Another situation is with a fire protection tank.  These tanks do not contain potable water and therefore do not have to be sterilized when returned service.  In this case a tank only has to be out of service for four to six hours while the interior is cleaned and inspected.  This inspection can even be scheduled on a weekend or after business hours.  In both of these instances remember one of the safety issues – all connected valves must be closed before a diver enters the tank, so, the tank is essentially out-of-service anyways.

There is one instance where diving a tank is recommended and even required.  That instance is when there is a leak in a concrete tank.  A full head of water is usually necessary to open any stress cracks in the tank shell.  This type of inspection rarely addresses the tank floor or the coating system.

The bottom line is that taking the risk of an incomplete inspection from a diver's video is not only not necessary, but can cost you thousands of dollars in unexpected repair bills when a failure occurs.  So, why hire an inspector that's all wet?  Do it the right way the first time.

Send mail to webmaster@tankteam.com with questions or comments about this web site.
Copyright © 2002 Tank Engineering And Management Consultants, Inc.
Last modified: June 19, 2007