Acronyms are inevitable as they are ambivalent. So, let’s first get the symbols right on BSP and NPT threads before we explore steel fittings in South Africa.
- BSPT stands for British Standard Pipe Thread ( often just a BSP acronym)
- NPT stands for National Pipe Tapered
Interestingly, steel pipes and parts are still threaded and categorised according to their geographical origin. By and large, U.S. manufacturers use NPT threads, while Britain and most other countries use BSP threads. At a push, some manufacturers will accept an order for BSPT thread on a made-to-order basis if preferred in a country where NPT is the standard.
You’ll likely need no assistance in appreciating the value of standards for thread in pipes and parts. Couplings, elbows, and tees are threaded ends and fittings that are usually made of steel, brass, bronze, cast iron, nylon, or plastic and are only as effective as their thread. In order to create secure joint and bend connections, these parts must lock together to form a tight seal.
Added Clarity on BSP vs NPT Standards
The NPT standard is divided into two categories:
- National Pipe Tapered (NPT)
- National Pipe Straight (NPS).
Likewise, the BSP standard is differentiated by:
- tapered (BSPT)
- parallel (BSPP).
The critical factor in either standard is how well the design interlocks to fulfil its function, as follows:
- Tapered pipe thread creates a tight seal by perfectly fitting male and female parts.
- Straight pipe threads usually require a gasket or an O-ring to seal effectively.
You may, for instance, have a tapered MNPT (Male Fitting National Pipe Tapered) that locks into an FNPT (Female National Pipe Tapered). The former (male part) microscopically ‘stretches’ the latter (female part) before the force between the two can be expected to withstand pressure from whatever media is intended to travel through the pipes.
But you’ll need your O-ring or gasket to secure an MNPS (Male Fitting National Pipe Straight) to an FNPS (Female National Pipe Straight) - are you still with us?
Thread Spread Differentiation
Providing it’s not too small, you may be able to see the difference between threading that is tapered or straight. But not so much when it comes to the difference between the NPT thread and BPS thread.
- NPT is cut to a 60-Degree Spread
- BSP is cut to a 55-Degree Spread
Whether the thread on a steel part or fitting is tapered or straight can be ascertained using a calliper or pitch gauge, which can measure the degree of spread and the angle of the pitch. All very technical.
For us, in SA, ignoring the line showing which country favours BSP or NPT, America’s Gemini Valve has a helpful comparison chart: