Identifying British Standard Pipe (BSP) threads, which are widely used globally, involves understanding their types and unique characteristics. BSP threads are available in two forms: Parallel (BSPP) and Tapered (BSPT). Both types share a common thread flank angle of 55°. This angle is crucial in distinguishing BSP threads from NPT threads, which have a 60° flank angle. This difference can be verified using a thread gauge.
BSP threads are measured in imperial units, such as 1/8, 1/4, 1/2, 3/4, etc. For sealing, BSPP threads often use a 30° chamfer on the male thread that fits into a 30° recessed cone on the female thread, specifically in swivel applications. In port applications, an o-ring and washer or bonded washer are required for a proper seal with parallel threads. On the other hand, tapered BSP threads achieve a seal through thread wedging, often supplemented with thread dope or Teflon tape.
Interestingly, a male BSPT thread can fit into a female BSPP thread if the female thread is fixed and non-swivel, due to the recessed cone seat.
To identify BSP parallel thread size fittings, you can perform a few calculations:
- Measure the outer diameter (O.D) of the BSP thread.
- Subtract 1/4 inch from the O.D measurement to determine the thread size. For instance, an O.D of 1 inch would result in a thread size of 3/4 inch, also referred to as “dash 12”.
Next, determine the number of threads per inch:
- Count the thread crests over a 1/4 inch length, then multiply by 4. For example, if you count 3.5 crests in a 1/4 inch, multiplying it by 4 gives 14 threads per inch.
- Combine this thread size with the number of threads per inch, and consult a chart to find the specific size and specifications you need.
Understanding these details is key to correctly identifying and using BSP threads in various applications.
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