Identifying the Right Pressure Release Valve Thread Type

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Onsite – size identification of BSP and NPT fittings

Pressure Release Valve Thread Types

Pressure Relief Valves (PRV’s) are devices used to release pressurised fluid from a wide variety of Pressure Vessels. PRV’s are used to maintain optimal pressure within a system, thus protecting the vessel from becoming over pressurized which can potentially cause damage or failure of a vessel. Pressure build-up related damage can be costly as well as a health and safety risk.  PRV’s act as the main safety mechanism of a pressurized system (see Australian Standard 1210 – 2010, 8 for more details).

Proper sizing, manufacture, installation, testing and selection is crucial for optimal protection of a system.  In this article, we will focus on common misconceptions when dealing with the actual and nominal size of PRV’s.  Along with the difficulties regarding the measurement of PRV’s, we’ll also look at different sealing thread types, including British Standard Pipe (BSP) fittings and National Pipe Thread (NPT).

BSP fittings are some of the most common thread fittings in the world and come in two different classes. These are known as British Standard Pipe Tapered (BSP) and British Standard Pipe Parallel (BSPP – often referred to as BSP).

Diagrams show the difference between BSP and BSPP

Figure 1: The above diagrams show the difference between BSP and BSPP


From the above images, the parallel sides of the BSPP type fitting and the tapered thread sides of the BSP type fitting can clearly be seen.  A common mistake, however, is to identify a BSP as a National Pipe Thread (NPT), since these two fittings are very similar in appearance.  When dealing with NPT and BSP one must remember that although they may seem similar, these two fittings have different thread flank angles and should therefore always be kept separate.  The threads are not compatible due to not only differences in thread angles, but also in thread forms.  NPT have a 60 angle and have flattened peaks and valleys, whereas BSP threads have an angle of 55  with rounded peaks and valleys (also known as Whitworth thread form).

Diagram shows the difference in shape and angle between NPT (left) and BSP (right)

Figure 2 shows the difference in shape and angle between NPT (left) and BSP (right)


The shape of the thread in BSP and NPT, as shown above, results in these two types of thread sealing in a similar fashion to one another.  The seal is acquired through the extremely high force of the male thread stretching the fixed female fitting into a connection that can hold pressure.

Pressure Release Valve Thread Sizing

BSP thread form is commonly used in Australia and is based on trade size rather than the actual physical diameter size. This means that the physical measurement of the threaded component in question will not be the same as the trade size required.  Naturally this leads to confusion regarding how to measure these components.

BSPP and BSP fittings and their specifications can be determined through the following calculations.  To work out the trade thread size of a BSP threaded PRV, measure the outside diameter of the male BSP thread, convert this measurement to inches and subtract ¼”.

Once the thread size has been determined, the number of threads per inch should be calculated.  This can be done by physically counting the number of thread crests over an inch or alternatively counting the crests over a ¼” length and multiplying this value by 4 to get the number of threads per inch.

Numerous tables have been produced to assist with determining nominal thread sizes such as the below table.

Table of BSP threads that relates Major (outer diameter) to Nominal Size

Figure 3: Table of BSP threads that relates Major (outer diameter) to Nominal Size.

For example, calculating the size for an ¾” BSP Type PRV:

After measuring the outer diameter of the threaded part of the PRV it is determined to have a size of approximately 26mm at the tapered end (end tip of PRV thread).  The measurement could be more accurately done using calipers and measuring the threaded area closest to the base, as the outer diameter tapers down closer to the tip of the PRV (due to BSP Type PRV). Taking the measured value of 26mm, cross reference on the above table and find the related Nominal size (trade size).  The T.P.I. value next to the nominal size reading is known as the Thread Per Inch value.

Therefore 26mm – 6.4mm = 19.6mm or ¾”.  So, if you measure a 1” thread you need to order a nominal ¾” PRV.  Conversely, if you order a nominal ¾” PRV, the thread size will actually be 1” in diameter.

Screwing the wrong thread type into an outlet may result in a leak and ultimately an incorrectly functioning PRV.  Furthermore, it is important to read through all the separate Manufacturer’s Data Reports for each individual PRV as there is the possibility that some PRV’s are different from others.  This is especially important when dealing with a combination of BSP and NPT Type PRV’s, as these thread types are difficult to visually identify.  If these two thread types were to be interchanged, the thread will not seal correctly and thus could compromise the vessel.  Overall, always remember to be diligent when checking, inspecting and changing PRV’s.  Check and double check visually and consult the Manufacturer’s Data Report whenever possible.



Difference between NPT, BSPP and BSP seals:

Table for BSP and BSPP thread dimensions:

How to measure BSP threads:

Table used in paper: