Does buried pipe need temperature derating?

Soil temperature at burial depth is more stable than air and usually stays near the local annual mean, so on most buried water mains the design temperature is close to the reference. That is not an automatic exemption: a shallow line, a hot climate, or a line carrying warm fluid has to be checked at its real temperature.

How do I calculate MOP?

MOP = fT × PN. Take PN from the pipe specification and fT from the derating table for that material grade at the line's operating temperature. If you are working toward a required working pressure, invert it: the PN you need is working pressure divided by fT.

What happens to pipe at high temperature?

It usually does not burst immediately. What happens is that design life shortens: slow crack growth proceeds faster and failure appears years earlier than intended, without prior warning. That is what makes a temperature error so hard to detect in service.

Does pipe colour affect its temperature?

Black pipe runs hotter in direct sun, but the carbon black responsible is also what protects it against UV. The right answer for a surface line is not to avoid black; it is to apply the temperature factor in selecting the pressure class, and to shade or bury the line where possible.

Can I apply more pressure in cold weather?

In long-term strength terms the factor below 20 °C is greater than one, but that gain is usually not taken in design. In cold conditions polyethylene becomes stiffer and more impact-sensitive, and that is the governing constraint for handling, installation and welding.

BUKAN PIPE

Temperature Effects on PE Pipe Working Pressure

Operating temperature and allowable pressure.

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Prepared by the Bukan Pipe technical department — polyethylene pipe manufacturer since 1997, INSO 14427-2 standard mark licence holder

Published 3 September 2026

The number printed on polyethylene pipe — PN10, PN16 and so on — is not an absolute. It is the allowable working pressure for water at 20 °C over a 50-year design life. If your line runs warmer, the real allowable pressure is lower than the number on the pipe and has to be calculated with a temperature derating factor.

MOP

Maximum operating pressure at the line's actual working temperature, from MOP = fT × PN, where fT is the derating factor for that temperature and that material grade. MOP, not the printed PN, is the design basis.

Why temperature reduces allowable pressure

Polyethylene is viscoelastic: its behaviour depends on time and temperature. The PN figure derives from the material's long-term strength (MRS) at the reference temperature, and that strength is resistance to slow crack growth over 50 years, not instantaneous burst resistance. As temperature rises, molecular chains slide past one another more readily and the same stress produces more strain in less time. The result is that a hotter line reaches the end of its design life sooner — not that it necessarily bursts that day. Which is exactly why temperature-related failure usually appears years later and without warning.

A worked example

Take a line laid on the surface in direct sun, where the water inside reaches 40 °C in summer. The derating factor at that temperature is around 0.74. So PN10 pipe has an allowable working pressure of roughly 7.4 bar under those conditions. Run it at 10 bar and it is being loaded beyond its design capacity.

The practical consequence is to invert the order of selection: instead of choosing PN and then worrying about temperature, start from the required working pressure and the line's real temperature, and derive the PN you need from PN = working pressure ÷ fT. In the example above, delivering an effective 10 bar at 40 °C requires a class above PN10.

Where to get the factor

The derating table depends on the material grade (PE80 or PE100) and on the fluid, and is published in the standard governing your project. Do not take it from a generic table found online, and do not apply one grade's factor to another. For water supply lines the basis is the national standard for that product; on projects with a client specification, the client's approved table.

Transient and continuous temperature are not the same

What counts is the temperature the line operates at, not an instantaneous maximum. A short peak lasting a few hours a year is not the same as continuous operation at that temperature. Equally, if the line runs hot for months every summer, that is no longer transient and should be the design basis. For lines that cycle hot and cold, the designer has to weight the time spent in each condition.

Buried versus above ground

Buried line: soil temperature at depth is stable and usually close to the local annual mean — daily swings do not reach it
Surface line: wall temperature in direct sun runs above air temperature, especially when flow stops and water stands in the pipe
Line in an unventilated duct or culvert: can run hotter than a surface line
Warm process fluid: the fluid temperature governs, not the ambient

Below 20 °C

Below the reference temperature the factor exceeds one, so in long-term strength terms a cold pipe is better off. In practice that gain is usually not taken in design, because cold introduces a different problem: polyethylene becomes stiffer and more notch-sensitive at low temperature. Unloading, handling and welding pipe in cold weather calls for more care, not less.

FAQ

Does buried pipe need temperature derating?: Soil temperature at burial depth is more stable than air and usually stays near the local annual mean, so on most buried water mains the design temperature is close to the reference. That is not an automatic exemption: a shallow line, a hot climate, or a line carrying warm fluid has to be checked at its real temperature.
How do I calculate MOP?: MOP = fT × PN. Take PN from the pipe specification and fT from the derating table for that material grade at the line's operating temperature. If you are working toward a required working pressure, invert it: the PN you need is working pressure divided by fT.
What happens to pipe at high temperature?: It usually does not burst immediately. What happens is that design life shortens: slow crack growth proceeds faster and failure appears years earlier than intended, without prior warning. That is what makes a temperature error so hard to detect in service.
Does pipe colour affect its temperature?: Black pipe runs hotter in direct sun, but the carbon black responsible is also what protects it against UV. The right answer for a surface line is not to avoid black; it is to apply the temperature factor in selecting the pressure class, and to shade or bury the line where possible.
Can I apply more pressure in cold weather?: In long-term strength terms the factor below 20 °C is greater than one, but that gain is usually not taken in design. In cold conditions polyethylene becomes stiffer and more impact-sensitive, and that is the governing constraint for handling, installation and welding.