Capacitance Discharge Welding?

Discussion in 'Technical Discussions' started by Clifford Bryan, Apr 3, 2008.

  1. Clifford Bryan

    Clifford Bryan Industry Observer

    I have been part of several projects in the past where capacitance discharge welding was the method of attachment of antenna mounts to the top of the water tank. I was not very fond of this process several years ago when we participated in these projects, and am wondering if anyone had done this recently? I was convinced then that it was not effective, but maybe new developments in the industry have made this more successful?

    Just curious-
  2. Mike Brant

    Mike Brant Guest

    Good question. I would be interested to hear some more recent news on this as well. This was purported as the superior technique for avoiding damage to the tank liner caused by conventional welding or for not dealing with the hassle of using epoxy, which presents its own problems in attaching to cold steel surfaces.

    I had the same experience you did in that it seemed a high number of the welds would fail. As the project engineer on a project where we did 15 or so of these, I had the A&E firms' structural engineers specify redundant weld attachments due to the problems we were seeing with this. I think it was also important that the erection crew really have someone who was experienced in doing the welding in a wide variety of conditions. In talking to a few of the welders, the problems seemed to be related to the fact that the thickness of the steel to which the weld attachments were being made is typically much thicker on the tanks than in the standard application for such methods (steel form decking). We ending up getting this to work, but not without a significant amount of rework and associated change orders.

    I wonder how those welds have held up over time...

  3. Tim Kearns

    Tim Kearns Guest

    There are several contributing factors involved in this process which many people overlook. The length and gauge of your extension cord used to power the unit, the choice between copper and stainless studs, metal preparation, ground placement, method of ground attachment, and polarity used for welding. We always try to determine from the tank owner, the exact grade and size of steel used to build the tank, and obtain a piece of similar grade for test purposes. Be sure that you are using the proper collet in the CD gun for the size and type of stud chosen, and it is best to start with fresh parts. We have also used an epoxy based construction adhesive as a back up on critical attachment points such as antenna and boom mounts.

    The key to trusting your welds is educating your operator, a quick visual inspection of each stud will tell you most all you need to know, if the operator knows what to look for. There are books available from most all of the CDW manufacturers that will give you a basic setup for steel sizes and types, along with what to look for in a good weld, and reasons for failure on bad ones, based on visual inspection.

    Good Luck,
    Tim Kearns
    T2K Companies, LLC
  4. Patrick  Barr

    Patrick Barr Friend of the Community

    I have read the above post and have a few comments. First of all, CD welding is used to try to prevent the tank interior linings from burn damage and If you have ever been inside a tank that had CD welding performed on it, and the tank lining has been in service for a year or two after the CD welding has been performed, you will see rust and total coating failure at these locations in most cases. The fact that CD welding does not leave a huge black burned area the moment you are finished installing a mount, such as welding does, does not mean that the CD welding process has not damaged the coating - and in several months the lining will fail causing the same amount of damage as if it had been full seal welded in the first place.

    The next concern is the above mentioned questions of structural integrity and I would also like to mention that all exteriors of water towers are seal welded to prevent corrosion and rust staining due to moisture becoming trapped between two steel surfaces and rusting from the back side as well as rusting the uncoated stud. I have seen several installations that try to prevent this problem by caulking the perimeter bolt plate. It?s a nice idea, but if it was such a good idea, why don?t the tank manufacturers skip weld all their exterior seams above the water level and then caulk them and save tons of money? Probably because they have warranties and are aware as I have noticed that it never holds up for more than a few years.

    The next problem is the maintenance crews that will later come along and work on this structure and might accidentally tie off, or rig the suspended staging or fall protection from one of these questionable mounts. The loads created from this type of use are very dynamic and much greater than most engineers have compensated for while doing these types of designs.

    The epoxy or glue process has been widely used and is a disaster. If anyone thinks this statement is incorrect, please give me the name of the epoxy you are using and I will call the manufacturer and I bet they will not warranty it for this type of application. (I have called several already) Most A and E firms for water tower/wireless designs are not familiar with the dynamics of an elevated water tower and this is where the problem lies.

    The exterior of a water tower will experience shape change or oil canning due to expansion and contraction of the steel. Many times a steel surface of a water tank will be 135 degrees and during the fill process will quickly change to water temp which may be near 50 degrees on some structures. This thermal swing will cause buckles and depressions on the roof of many structures and later they will go bang and the dent or depression will be gone. We call these temporary bird baths.

    Where full seal welding is specified, provisions for future rigging is designed into the antenna supports, and regards to the repair of the tank's lining is considered usually results in a happy ending. It may not be the least expensive solution but it is the correct solution. There are a few reputable water tower installation firms that refuse to perform glue-on or stud weld designed antenna supports. I am sure there are a bunch that swear by them as well. The few and reputable are also the old and profitable and will be around when these more "cost effective" disasters begin falling off.
  5. Tim Kearns

    Tim Kearns Guest

    Pat you bring up some very valid points, I too prefer using a solid seal weld on the tank, combined with a series of plug welds to insure the structure moves with the tank as uniformly as possible.

    I have installations that are 10 years strong on H20 tanks that are stud welded, again, I don't claim the process to be any less harmful to the tank; however, it can be an effective process if performed properly.

    I do believe that a modification in the process of tank attachment needs to be brought about from the engineering and tank owners side of the fence. The proper application of products such as TENEMEC, and the requirement of third party inspection companies to supervise and record the process, constitutes the acceptance of the tank owner, and the engineering firm who has to stamp and certify the process, and are normally required to consult the manufacturer for preferred methods of attachment. If the owner is advised of the potential for liner damage, or the deficiencies associated with the different application methods, perhaps this change will come about, until the liner manufacturers set forth a standardized attachment procedure and declare this method unacceptable, CM and engineering firms will continue to take the cheapest route they can.

    I agree with your thought process Mr. Barr, I just don't believe that the installation crews should be held accountable for a poor design, only an improper installation.
  6. Patrick  Barr

    Patrick Barr Friend of the Community


    Thanks for your comments. This is a great thread because you and others on it are sharing information that we can all learn from ? and even if we might disagree (I refuse to believe that one person has a lock on knowledge), it?s being done civilly which is rare in forums and Yahoo boards today.

    The installation crew, unfortunately, has no power over the design and if it is a poor design it will not result in a good installation. I elected to no bid many times in the past for poor designs and would contact the A and E firm, as well as the carriers and express my concerns. If I was to submit a bid then our crews would be forced to install as per designed. I?m not sure if it was the best business decision, but I am still in the wireless industry after several decades.

    The coating manufacturer you mentioned, Tnemec, is a widely used coating supplier that most tank manufacturers use.

    Their name comes from the word cement spelled backward. At the turn of the century an old farmer back East mixed cement with oil based paint to coat the inside of his basement water collection tank and the mixture lasted many years longer than any other product he had ever used in the past. He spelled cement backwards, started a company? ?and now we all know the rest of the story".

    The AWWA (American Water works Association) was working on an outline that addressed the above mentioned concerns as well as several others (Antennas and Related Communication equipment).

    I believe the trouble today is that most A and E firms designing water tower/wireless sites are from the tower side and are not aware of tank standards, and are not even aware that there is information published to help them make a good design.
  7. Tim Kearns

    Tim Kearns Guest

    I agree with your last statement 100% Pat, thanks for keeping the thread going, as you appear very knowledgeable in this area, and I am always open to learning.

    I believe that the owners and manufacturers should have to certify installation crews and procedures, as it pertains to the attachment to the tanks, and then let RF crews route and terminate their coax. By regulating who can work on the tanks, I think we have a better shot at eliminating sub-standard or short cut installation methods.

  8. Clifford Bryan

    Clifford Bryan Industry Observer

    Wow! Thanks for the replies guys! I had not been on here in a while and was surprised to see such informative feedback!

    I too agree that the CD method is not a suitable substitute for standard structural welding techniques, and am surprised to find out that this method does result in the failure of the tank coating opposite the welded area after all. That fact by itself is enough ammunition to put CD welding in the "not a viable option" category when brought up in water tank installation discussions.

    Pat - Thank you also for the Tnemec info! I've been working with Tnemec products on local tanks for almost a decade now and never knew the story behind the name.


    Clifford E. Bryan
    Communication Builders, Inc.

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