Excessive pressure can promote premature system failure so using the right pipes for the job is an important part of design and installation.

Pressure pipe design is based around the maximum pressure that the system is expected to see in service so it is important to account for the important loads and pressures. Although ignoring loads can result in failures, being over-cautious can result in unnecessary costs for more expensive pipes and fittings. 

For convenience, pressure pipe and fittings are given a pressure rating which shows the maximum working pressure the pipe is designed to handle. The pressure rating of the pipe is based on the ability of the pipe material to resist the internal pressure, with a safety factor to allow for higher test pressures or for pressure surges and for other loads and properties that are difficult to predict – soil loads, minor ground movement, manufacturing variations in individual pipes and joints.

In a working pipeline system, however, the useful working pressure can be limited by the sealing ability of the joints, the pressure rating of the fittings or by the ability to the resist the end-loads that result from pressure. 

The maximum system pressure should normally be lower than the maximum pressure rating of the pipe and fittings used. The main exception is that site commissioning tests (leak tests) are often carried out above the rated pressure but this is usually only for a short time and the effects of the excess test pressure should be covered by the safety factor. The recommended test conditions vary for different pipeline materials, and may vary from job to job so it is important to check. 

Pressure rating is described in a number of ways – for example by a pressure Class “A”, “B”, ”C”, etc or by nominal pressure as PN10, PN16 etc (PN comes from the French for Nominal Pressure, which is the official pressure rating rather than the actual pressure capability of an individual pipe). 

Rather than explain them all in detail, the table below shows how the different descriptions of pressure class compare with each other. The conversions aren’t exact, but are close enough for everyday purposes.



Approximate conversions between common pressure units.

Pressure class	PN	Bar	Metres head	MPa	kPa	Psi
A	3	3	30	0.3	300	45
B	6	6	60	0.6	600	90
C	9	9	90	0.9	900	135
D	12	12	120	1.2	1,200	180
E	15	15	150	1.5	1,500	225
F	18	18	180	1.8	1,800	270
No Class defined	10	10	100	1	1,000	150
No Class defined	16	16	160	1.6	1,600	240
No Class defined	20	20	200	2	2,000	300
No Class defined	25	25	250	2.5	2,500	375

Until recently, the pressure class of ductile iron pipes (and cast iron pipes before that) was described through the wall thickness K9, K12, K14 etc, which was used to work out the pressure rating. Since 2004, these K classes have been replaced by PN ratings but K classes will be around for a while as old stocks are used up and, of course, older fitting and pipelines with these ratings will remain in service for many years to come.