There are so many different types of couplings available and each one has their own specific application. For example, when connecting drop pipes in a well application, you may consider using merchant couplings to connect them together.
In this blog post, we will look specifically at merchant couplings and discuss the difference between full, half and extra strong merchant couplings.
Full merchant couplings
Full merchant couplings are available in black, galvanized and stainless steel materials. Full merchant couplings are used when a stronger coupling fitting is needed. For example, you can use a full merchant coupling in a drop pipe that’s in a water well because it would be holding the weight of the pipe, water and the submersible pump. If an even stronger coupling is required, there are drop pipe couplings available that are specific for these applications. Black full merchant couplings can also be used in applications where welding is required.
Please note that full merchant couplings should never be used as a pipe fitting in a plumbing system where a watertight seal is critical. However, taper-tapped merchant couplings would be acceptable to use for this type of application.
Half merchant couplings
Half merchant couplings are half the size and are used to provide a threaded outlet onto a flat surface of a metal tank. The tank is drilled with a hole/port and the half coupling is welded to the tank centered around the port. Half couplings are threaded all the way through but one end is square to help with welding onto a tank.
Half merchant couplings are designed for welding so they are only available in black finish. This means that they are not available in a galvanized finish as welding galvanized fittings creates fumes that are harmful to one’s health.
Thread standards
Merchant couplings are manufactured per ASTM A865 and are intended for use on steel pipe joints in accordance with ANSI B1.20.1. Sizes 1⁄8" NPS through 2" NPS are straight-tapped. Sizes 2 1⁄2" NPS and larger are always taper-tapped.
What’s the difference between straight thread and tapered thread? First, it comes down to cost. Tapered threads cost more to manufacture while straight threads cost less to produce.
Second, tapered threads seal better. They seal better because the threads get tighter the further down the coupling, while straight threads stay the same width throughout.
All merchant couplings are chamfered before they are threaded. This means that before the threading is done, a chamfer is created. It can be tricky to tell where the threads start on the chamfer, but it helps start threading the pipe into the coupling.
Extra strong full and half merchant couplings
Extra strong full and half merchant couplings, also known as API (American Petroleum Institute) line pipe couplings, are manufactured in accordance with ASTM A865. This type of coupling is furnished for extra strong and double extra strong pipe. All are taper-tapped.
Hopefully, you’ve found this blog post helpful, and have learned something both interesting and useful about the different types of merchant couplings.