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Process Connections

Often we see pressure instruments coming into our laboratory for calibration with damaged threads.  In some cases it’s clear that there has been a mismatch in process connection to instrument.
We felt it might be helpful to summarize the thread types that are available.
Thread1 Thread2
We’ve noted that many people find the choice of thread types confusing, so here we de-mystify the process.
Threads may be parallel or tapered
As the name suggests, a tapered fitting tapers from a large diameter to a smaller one.
Straight-V-Taper 300
Helpful Tips
  1. Tapered threads seal by metal-to-metal wedging
  2. Parallel or ‘straight’ threads seal on an ‘O-ring’ or washer at their base
  3. Taper threads ALWAYS require thread sealing (tape or liquid)
Thread Tape Anaerobic app 1 2 P
dyk Liquid Thread-seal is a special adhesive which is ‘Anaerobic’.  Anaerobic adhesives require 2 things to cure;
  1. No air
  2. Metal surface (chemical reaction with carbon in the seal) 
One of the common errors we come across are when tapered connections are connected to parallel processes and vice versa. When this occurs, there may be leakage and/or thread damage.
Note:  A parallel thread often has a ‘spigot’ at the end of the process thread where it seals.  This makes for easy identification.


While there are several others, the most likely process types one will come across on pressure gauges in NZ are
BSPT                      BSP Taper
BSPP                      BSP Parallel
NPT                        NPT
BSP stands for British Standard Pipe, and NPT, for National Pipe Thread. The pitch (spacing) and angle of the threads vary between these types in things like ‘threads-per-inch’ and is beyond the scope of this article, but feel free to contact us if you wish us to send you more detail.
The most important thing to be aware of is that most products from New Zealand or the United Kingdom will be BSP types, while most from the United States will be NPT.
If a process connection is described as a “G” type, this is BSPP.  The “G” refers to “Gas” and most gas installations are BSPP.
Manufacturers produce gauges in different process connections.  The following example is from Teltherm Instruments Ltd, our NZ manufacturer.
Dial Size
Thread Options
3025 63 mm ¼" BSPP  
    ¼" NPT  
3063 63 mm ¼" BSPT Standard
    ¼" Flare  
3100 100 mm ¼" BSPP  
    ¼" NPT  
    3/8" BSPT Standard
    ½" BSPP  
    ½" NPT  
3400 100 mm 3/8" BSPP Standard

An example of an adaptor kit for converting thread sizes and types including quick connectors is shown below. Contact Homershams if you require any further information.

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