Threading and Galvanized Pipes

1. You have to thread after galvanizing, because galvanizing after would interfere with proper sealing of the threads. Threading cuts off the metal and galvanizing at that spot, it's unavoidable. It's usually the inside of the pipe that the galvanizing is protecting.

2.Galvanizing is bonding at the molecular level; like chrome plating it is done electrically, or hot dip plating. Hot dip is a lot like soldering, requires cleaning and other dips to prepare the metal for bonding.

3.I don't quite understand how anybody expects straight threads to hold pressure, not familiar with BSP. Maybe the last thread bottoms, and the seal is there. In NPT, the thread is tapered, on both the male and female, for pressure pipe. When the tapers take up on each other, the threads literally crush into each other. On brass fittings I was told thread sealant wasn't even necessary, because of the crush. Of course, taking a part loose and re-tightening would have some affect, I'm sure.

4.Like the other poster said, welding zinc releases poisonous gasses. If you have to weld it, I use 6011 rod and a big fan to blow away the smoke. Paint the weld with a good rust preventer.

5.As far as the sheets, electroplating is fairly controllable, hot dip is done with molten metal, can get quite lumpy sometimes. Triple hot dipped bolts and the like are galvanized after threading, but the threads are cut very loose to accommodate for the buildup of the zinc. If you look in the nuts, you may see the galvanize cut away by a tap, to make sure they fit.

6.Wikipedia is a great source for data.

Point 3 Parallel threads are not designed to hold pressure. They are used in conjunction with a washer that is compressed between the pipe or pipe fitting and a machined seating face on whatever the pipe is screwed into. On hydraulic systems, which usually are at higher pressures, this washer is often made as metal annulus with a concentric rubber, nitrile or teflon sealing material annulus bonded to the inside edge. The joint is seated tight onto the faces of the outer metal ring but the seal is made by the inner one. The outer ring also prevents high pressures from extruding the soft seal material out of the joint. Tapered threads are problematic if they have to be remade several times so in situations where this is likely to happen ie. hoses that have a shorter service life than the equipment they are attached to, parallel threads are used instead. It is good practice to replace the sealing washer every time a parallel joint is remade.

Hello K.Muthu,

Can I suggest you do actually get a book for on-site work, as all these questions are relatively unconnected one with the next.

The cutting of the thread depending on what grade you are working to ASME, BS etc, each has a slightly different thread and thread depth, but, whatever the thread it will alway cut through the relatively 'surface layer only' of galvanising.

From my experience the pipe is usually threaded on the outer surface. But whatever the galvanising, when you cut a pipe to length you are left with bare black pipe. Check out the sites below and you may see a tape which can be applied to a joint after making. But note: The area of about an inch (~25 mm) from the end of the thread must be clean. So only ever clamp the pipe to cut and to thread it about 4 inches (~100 mm) from the end.

The type of thread I think you will find depends on the size, and the intended use of the pipe. What it carries and the type of 'atmosphere' the outer surface with be likely to have to live with.

You can read about galvanising on-line or get a book on it but, make sure you are judging like with like. Industrial galvanising and joint-making may be entirely different and a lot more 'substantial' than 'domestic' pipe.

Why are you so worried about this. Are you using an acid in the pipe, or is it for air, or water, or something else?

If you refer to a straight Galvanised coupling as below, the externally threaded pipe will be screwed into the internally threaded coupling, from both ways. So at that point the pipe after it is connected will have at least twice the thickness of the rest of the pipe at this joint.

Galvanized Pipe Fitting - Galvanized Coupling [2007-11-21]

Items Name: Pipe/tube fittings, like elbows(45&90), tees, corsses, nipples, sleeves, couplings, unions, bushings, plugs, caps, flanges, hex lock ...

Steelinking Galvanized Pipes Industry Ltd-Fitting Dept. [Province:Guangdong, China]

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1. Pipe: Information from Answers.comMany industrial and government standards exist for the production of pipe and ..... The most common process pipe joint is the butt weld. The ends of pipe to be ... The most common pipe thread in North America is the National Pipe Thread .... Can Black steel pipe be replaced with Galvanized piping for gas piping? ...
   www.answers.com/topic/pipe-material
2. galvanized pipe products, galvanized pipe manufacturers ...galvanized pipe supplier catalog - himfr.com provides what you want, ... We produce lead forged particular for automotive industrial, electric, ... Ball features a recessed rope attachment for protection from injury. ... Acid Regenerat... category: [Metallic Processing Machinery] (pipe accessory pipe joint ) ...
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Galvanized Steel Pipe On GlobalSpec15 May 2009 ... Find galvanized steel pipe related suppliers, manufacturers, ... Industrial pipe is a broad category of fluid and solid transport hardware ... Common thread types include national pipe thread (NPT) and British standard pipe (BSP). .... stocking difficulties associated with ground joint pipe unions. ...

flow-control.globalspec.com/Industrial.../galvanized_steel_pipe

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Has details of pipes, pipe joints, and welding of Galvanised pipe.

1. Steel Pipes - Large Stockwww.cleveland-steel.com Tubes for structural & construction Over 40,000 tonnes in Stock UK

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1. galvanized pipe Manufacturers - Verified galvanized pipe ...galvanized pipe Manufacturers directory - World's Largest Manufacturers Trade Platform with ... Security & Protection, Service Equipment, Shoes & Accessories .... part,steel elbow,composite pipe,construction pipe,union joint,pipe joint ...
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2. ERW hot dip Galvanized pipes tubes India,Galvanized pipes tubes ...SPEL India - Galvanized pipes tubes in India by ERW hot dip galvanized steel pipes tubes ... Zinc's natural corrosion resistance provides long-term protection, ... after applying a small amount of pipe joint compound on the threads. ...
   www.sparkelectrodes.com/erw-hot-dip-galvanized-steel-pipe.htm

Check out the various links below. The Manufacturing Process link is interesting.

I hope you will view all these sites and details and perhaps keep a written Document about this for future use?

Take care and I hope you find what you are looking for in my advice, good luck.

K;

I hope this helps some.

BST is also a tapered thread I believe the same 1/16th inch taper per inch of thread as NPT however it has a 55 degree included angle and different root and peek profiles. Both threads are available tapered or parallel (BSP / NPP) and both are about the same depth in either case. The 5 degrees difference is trivial if you consider the allowed variabilities in pipe diameter, thickness (respective of pipe schedule), and roundness (or should that be out of roundness?). Parallel threads are typically used for electrical or mechanical joints where you may wish to use more thread length. Tapered threads are used for fluid transfer as they compress some what during assembly and provide a more leak resistant joint. Straight threads should never be used with any gas applications (natural / propane) and typically are not advisable for the transfer of other fluids.

Welding of any pipe after it is threaded into a fitting as a means to prevent or stop leaks is a taboo. During the thread cutting process oils are used and typically during the thread connecting process some type of sealant is used. Thereby imparting impurities into the weld zone and creating an inferior weld. Rust and corrosion will do the same thing. Also even if everything were sufficiently clean you still have a compromised weld area due to the threads themselves. (Never weld on threads)

Typically welding is done with weld fittings. (look up "Weldbend" they are a major manufacturer of weld pipe fittings.) with either a butt weld, socket weld or slip joint. Threaded fittings are not suitable for welding.

Welding threads on galvanized pipe is a triple taboo. You have the above issues plus, zinc has a melting point of around 900 degrees Fahrenheit. When welded on, the zinc oxidizes (zinc oxide) and becomes poisonous in certain concentrations (trust me I have experience with this. and no it was not a mistake of my own). This is also self defeating as when you burn through the exterior coating to perform the weld you also burn off the internal coating behind the weld zone and compromise your corrosion resistance internally where it is needed the most.

The proper way of dealing with leaky galvanized joints unfortunately usually involves some disassembly followed by reassembly with fittings that are not corroded or simply making sure that during reassembly you use the proper amount, and type of sealant as well as tightening it sufficiently.

The galvanizing does not truly "penetrate" the surface of the pipe save for surface imperfections. The steel beneath has a melting point of around 2600 deg. Much higher than the zinc and therefor the two are in bond and not fused together. This means that deep gouges can also compromise corrosion resistance.

As far as the thickness of the zinc coating goes I do not believe I can be of assistance as what I learned about it some 30 years ago may no longer be applicable.