Plastic pipe classifications

There are three main systems if industrial plastic pressure pipe classification. These are broadly categorized as follows: The USA follows an imperial system with pressure ratings that conform to an ASTM (American Society for Testing and Materials) standard, the UK also have an imperial system, but this one conforms to a British Standard and most other countries, unless they have adopted the UK or USA systems in the past, conform to a metric ISO (international standards organisation) standard.  Some countries such as the UK are a mix between two systems – in this case the imperial (BS) and the metric.

These standards are common across ranges of pvc pipe and abs pipe.

The ASTM and BS systems are mostly compatible except for two sizes of plastic pipe; 2 ½”, where the UK system measures 75mm and the USA systems measures 73mm, and 5” where the UK system measures 140mm and the USA system 141.3mm> the other critical difference in the USA and UK systems is for threaded fittings. In the USA, threaded plastic fittings generally have an NPT (National Pipe Thread) thread and those from the UK generally as BSP (British Standard Pipe) thread. Oddly enough the metric system has adopted the BSP system for its threads. NPT and BSP plastic pipe threads are generally not compatible, and efforts to marry them together will result in stripped threads and leaking joints in the pipe system. Plain fittings from the metric classification are not compatible with either system (with the exception of 125 and 140mm sizes for the UK system). Most manufacturers supply adaptor fittings to take metric on one side and UK imperial on the other.

Within each pipework classification there are subcategories which are mostly indicative of working pressure ratings. The USA system has two different ASTM standards, the SDR system and the Schedule system. In the SDR system, subcategories have largely the same pressure ratings, no matter what the pipe size, whereas in the Schedule systems (such as schedule 40, 80, 120) the pressure rating decreases with an increase in pipe diameter. Nevertheless, the schedule system is the more common of the two systems and that which is most often specified for pipe fitters.

The UK plastic pipe classification system operates much like the USA ASTM/SDR system, and used a series of letters, most commonly B,C,D,E and a number, 7 to indicate a pressure rating that is consistent throughout the size range. For example 2” class C pvc pipe has a pressure rating of 9 bar, as does every other size of class C pipe.

The metric system of plastic pipe classification uses a more straight forward set of abbreviations. For example PN10 pipe has a pressure rating of 10bar, PN6 pipe, a pressure rating of 6bar and so on.

Pipe fittings, generally only come in one available pressure rating, with metric being 16 bar up to 160mm and then 10 bar to 450mm, UK imperial pipe fittings being 15bar up to 6” and then 9 bar for fittings between 6” and 12”. 

In the ASTM system, the classification of pipe fittings is rather more convoluted; the pressure rating of the fitting is associated to its schedule and so, as with the pipe, changes with size. An SDR classification is not available for plastic pipe fittings.