Merchant Coupling
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      1. Boshart Knowledge Base
      2. Merchant Coupling

      Why are steel merchant couplings up to 2” threaded NPS vs. NPT?

      Why NPS?

      This is a very good question. The quick answer is that they are inexpensive! This is the ONLY benefit.

      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 unless otherwise specified. Larger sizes, like 2 1⁄2" NPS and larger, are always taper tapped.

      The only reason for straight (NPS) threads is the low cost! Machining straight through NPS threads on a parallel inside diameter is much less costly than threading tapered pipe threads (NPT) as they must be tapped from each end. This increases the production cost significantly.

      Sealing NPS Threads:

      The question as to whether a proper seal will form between a tapered (NPT) pipe threaded into a straight tapped (NPS) threaded merchant coupling is often asked. There is no question, NPS couplings are less likely to form a watertight seal. Tapered threads seal better! The reason they seal better is because the threads seal on a much greater contact area compared to straight threads which stay the same width throughout, reducing the contact area between the mating parts.

      Many consider merchant couplings to be little more than thread protectors. Merchant coupling are most often used to join pipes together in non-pressure related applications such as structural railings. While some choose to use them in low-pressure piping, they are less than ideal and typically they will not stand up well in situations where high pressure is involved. For this reason, merchant couplings have chamfered ends which not only help start the threading, but also are helpful for the welding of the connection when required. Depending on the application they may be required to be welded together after they are fastened to a pipe to ensure a positive seal.

      Why offer NPS merchant couplings?

      The reason we offer NPS merchant couplings is due to the demand for low cost merchant couplings which are the industry standard.

      Does Boshart have NPT threaded merchant couplings?

      Yes and no!

      In steel up to the 2” size the answer is no!

      In 2-1/2” and larger sizes – yes.

      We do supply our ½” through 2” 304 and 316 stainless steel merchant couplings with NPT tapered threads.

      https://boshart.com/product/merchant-coupling-3/?attribute_pa_size_size=2_size

      ADDITIONAL RESOURCES:

      https://support.boshart.com/are-merchant-couplings-the-same-as-wrought-steel-couplings

      https://support.boshart.com/what-is-the-pressure-temperature-rating-for-your-series-gmc-blmc-blhmc-merchant-couplings

      https://support.boshart.com/making-npt-connections