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A uPVC pressure class is rated at 20 to 25 °C. As water and pipe warm, allowable pressure falls by a derating factor. Around 45 °C, published tables give roughly 0.5 to 0.65, so a 10 bar pipe may be limited to about 5 to 6.5 bar. The factor depends on the standard, so use the table for your pipe.
Saudi summers take water and pipe temperatures well beyond the 20 °C at which a uPVC pressure class is rated. In July 2026, Arab News reported that the National Center for Meteorology had confirmed 50 °C in Al-Ahsa and warned that 49 to 50 °C would affect much of the Eastern Province. Air temperature is not pipe temperature, but it caps what exposed pipe and standing water can reach.
Plastic pipe loses strength as it warms. The class on the pipe, Class 4 at 10 bar for example, is a rating at a reference temperature. Above that temperature, a derating factor (a number below 1) scales the allowable working pressure down. The difficulty is that the factor is not universal. The standard a pipe is made to, and sometimes the manufacturer, changes the number.
Derating factors as published in each source (see the table below). The tables use different reference temperatures, so the lines start at slightly different points.
Allowable pressure equals rated pressure multiplied by the derating factor. A Class 4 pipe (10 bar at 20 °C) with a factor of 0.63 is good for 6.3 bar in continuous service at that temperature. A Class 5 pipe (16 bar) with the same factor is good for about 10.1 bar.
| Derating factor | Class 4 (10 bar) | Class 5 (16 bar) |
|---|---|---|
| 0.87 | 8.7 bar | 13.9 bar |
| 0.70 | 7.0 bar | 11.2 bar |
| 0.63 | 6.3 bar | 10.1 bar |
| 0.58 | 5.8 bar | 9.3 bar |
| 0.45 | 4.5 bar | 7.2 bar |
Illustration only: rated pressure multiplied by a published factor. Surge allowance, and the factor for your own pipe’s standard, still apply.
We compared four tables that engineers commonly meet in the Gulf. All four start at 1.0 at their own reference temperature and fall from there, but they do not fall at the same rate.
| Source | Reference | Published factors | Upper limit |
|---|---|---|---|
| ISO 4422 (PVC-U water pipe), as published in PIPA technical note TN003 (Australia, 2005) | 20 °C | 30 °C: 0.87 · 40 °C: 0.70 · 50 °C: 0.58 | 50 °C |
| EN ISO 1452, as printed in an Italian manufacturer data sheet (M/TEC-1452, rev. 17) | 25 °C | 35 °C: 0.80 · 45 °C: 0.63 | 45 °C |
| ASTM-type Schedule pipe, Harvel Plastics bulletin “The Effects of Temperature on PVC Pipe” | 73 °F (23 °C) | 80 °F: 0.88 · 90 °F: 0.75 · 100 °F: 0.62 · 110 °F: 0.50 · 120 °F: 0.40 · 130 °F: 0.30 · 140 °F: 0.22 | 140 °F (60 °C) |
| Saudi manufacturer data sheet, SAPPCO TD-Q1035 (September 2024, no standard cited) | 20 °C | 30 °C: 0.90 · 35 °C: 0.80 · 40 °C: 0.70 · 45 °C: 0.60 · 50 °C: 0.45 · 55 °C: 0.30 · 60 °C: 0.15 | 60 °C |
Figures reproduced as published in each source. Celsius conversions of the ASTM-type table are rounded.
Take 50 °C. The ISO 4422 table gives 0.58, which leaves 5.8 bar on a 10 bar pipe. The Saudi manufacturer sheet gives 0.45, which leaves 4.5 bar. Both figures are published and both are credible. The first allows about 29 per cent more pressure than the second.
Three reasons are visible. First, the reference temperature differs: 20 °C, 25 °C or about 23 °C. A table that starts at 25 °C is already one step further down the curve. Second, the pipe differs. ISO 4422 and EN ISO 1452 describe water-pressure pipe in metric pressure classes, while the ASTM-type table belongs to Schedule 40 and 80 pipe, whose wall thickness is set by diameter rather than by a pressure class. Third, each publisher chooses how cautiously to draw the curve.
We have not seen the derivation behind any of these tables, and you do not need it. Use the table that belongs to the standard your pipe is made to, and ask the supplier to name it.
If a quotation or datasheet gives one derating factor with no standard and no reference temperature, ask where it comes from. Two tables can both be correct and still give answers a quarter apart at the same temperature.
Derating follows the temperature of the pipe wall in continuous service, which sits between the water temperature and the temperature around the pipe. For a buried pipeline carrying flowing water, a rule of thumb in Australian PVC technical guides (Vinidex) estimates the mean material temperature as Tm = (2 × Tw + Ts) ÷ 3, where Tw is the water temperature and Ts is the soil temperature. With 35 °C water in 30 °C soil that gives 33 °C. The 30 to 40 °C rows of a table matter in that case, not the 50 °C air reading.
Exposed pipe is a different case. Sunlight heats the wall above the air temperature, and the International Center for Biosaline Agriculture (ICBA) warns that PVC starts to deteriorate in direct sunlight and recommends HDPE for surface installation. If a line has to run above ground, bury it, shade it, or choose a material built for it. Our uPVC vs HDPE vs ductile iron comparison covers the trade-offs.
PVC expands by about 7 mm per 10 m of pipe for each 10 °C change in temperature (Vinidex gives the coefficient as 7 × 10−5 per °C). A 12 m exposed run that swings 30 °C between a cool night and a hot afternoon grows by about 25 mm. Buried, restrained pipe sees a much smaller swing. Exposed pipe needs joints or offsets that can take the movement.
4B Factory states its pressure ratings at 20 °C: Class 4 is 10 bar and Class 5 is 16 bar, and the watermain pressure pipe page notes that ratings de-rate per the standard at higher temperatures. For farm networks, where mains are buried but pump discharge lines often run warm, see PVC pipes for agriculture projects.
Send the diameters, design pressure, expected water and ground temperature, and whether the line is buried or exposed. The 4B Factory technical team will advise on class selection and quote factory-direct from Al-Kharj.
Request a Factory-Direct QuoteIt depends on the standard. The ISO 4422 table in the PIPA note runs to 50 °C, the EN ISO 1452 data sheet limits service to 45 °C, and the ASTM-type bulletin allows up to 140 °F (60 °C). Use the limit published for your pipe’s standard, and treat the top of any range as a heavily derated duty.
Yes, whenever the water or pipe wall is warm. Derating follows the pipe’s own temperature, not the air. Buried pipe is usually cooler than exposed pipe, but warm water still raises the wall temperature, so use site ground and water temperatures for critical lines rather than assuming.
Often it is a sensible choice for pressure mains, because Class 5 (16 bar at 20 °C) leaves more allowable pressure after derating than Class 4 (10 bar). It is not automatic: the heavier wall reduces the bore and raises cost, so confirm head loss and the surge case first. Our Class 4 vs Class 5 guide sets out when each one earns its place.
Derating tables cover temperature only. Sunlight adds a separate problem: the ICBA drip irrigation guide reports that PVC deteriorates in direct sunlight and advises against using it where it is exposed. Bury the line, shade it, or choose another material for surface runs.
Strength falls quickly. In the Harvel table the factor drops from 0.40 at 120 °F (49 °C) to 0.30 at 130 °F (54 °C) and 0.22 at 140 °F (60 °C). The Saudi manufacturer sheet falls from 0.45 at 50 °C to 0.15 at 60 °C. Above the limit for your standard, do not rely on the pipe at pressure.
