First and foremost, lets address what hot-dipped galvanized pipe is and how it is applied - especially for sprinkler systems.
Most, if not all, downstream sprinkler applications are screwed fitting joints. Galvanization, by common sense, is thoroughly removed when the pipe is threaded! There is no way to protect this area again with additional galvanization. It is forever doomed to a short, corroded life in the application it finds itself in (if the static water it contains is corrosive in the first place). Many young engineers don't analyze what they are applying or spend time studying the skills and application at hand. This leads to false dreams or anticipations about what results will be. Practical, experienced engineers will testify that galvanized pipe is for PLUMBERS - not for serious pipefitters in a process plant application. Plumbers LOVE galvanized pipe; the more they install, the more they will come back to replace. It's called job security.
In other words, galvanized pipe inherently has the weakness of having it's threaded section totally exposed to corrosion and thereby is weakened by this. Black iron or cast iron pipe many times is more effective - depending on the fluid and the application. I would not apply galvanized piping to serious Fire Sprinkler applications where I was not going to routinely inspect and monitor pipe integrity - and this costs money and time.
Think about it. If you are trying to protect a steel pipe by galvanizing it, isn't this a statement that you know you have inherently weak steel pipe that you must protect? And if you have an "Achille's Heel" (a breech in the galvanized coating) because you have forcefully removed the protective coating due to the installation need, what can you expect? I don't trust such a scenario.
What we have here in this scenario is a normal application in a retail center. The contractor has sent sch. 30 galv. pipe to the job site and there is no real corrosive aspect to this installation. What we are trying to decide is do we reject the galv. or not. To me it seems that in this application the galv. would be o.k. even if it is threaded. I do not know why the contractor has choosen to provide galvanized and of course we have never recieved any submittals on this and are now trying to answer the question with fitters standing at the truck. To reject or not to reject that is the question?
Static water is more corrosive that flowing water.
The galvanized pipe will work. That's not a problem; the problem is that future corrosion which will take place in the unprotected joints - that's a fact; no one can tell or predict when this will come to a head.
You originally stated that this was "hot-dipped" galvanized piping. This is expensive stuff because of the intensive manual input and the environmental restrictions. You may only have electrolytic or "sprayed" galvanization - which is another, much cheaper and ineffective protection.
As I said before, galvanized piping is the product of choice if we are talking about plumbers or commercial contractors. It's a logical decision on their part. We engineers, however, have to look at the long-term effects and costs. But, to go directly to your present situation, the galvanized pipe will work. My decision would be not to accept it unless it formed part of the original job specifications - but that's easy for me to say because I'm not in the hot seat like you are now. I suggest you make a documented memo,noting the pipe specifications and request a reason for its selection from the contractor while advising the owner/operator of what you have found and reported. This way, you have documented the suspicious spec and responsibly advised the owner and the contractor of your opinion on the spec.
Galvanising does not provide protection by surface coating. It is not like paint. You can damage the galvanising without great detrimental effect to the level of surface protection. However you do establish a corrosive cell as the bare metal and zinc have different potentials
the biggest problem with steel, galvanised or not , is the life expectancy in water service. In many installations this is no greater than 8 years. I have seen it fail in 9 months. If the corrosion inhibitors are used correctly and your system is well maintained it will work. Sadly this is a thing of the past.
Do NFPA codes allow the use of galvanised steel? Will the insurer sign off on it? have you considered cPVC? Sharing knowledge is a way to immortality
I have seen hot dipped galvanived piping used extensively in the petrochemical industry in dry fire protection systems. These systems remain dry until fire water is needed and then are flooded. Screwed, hot dipped galvanized pipe is a cheaper system to install than epoxy coated, welded piping.
We always specify hot dipped, if you spray on a coating that later flakes off and plugs up any spray nozzles, you could have a problem...
