Knowing and understanding a system’s
operating conditions is the key to the development of a safe, long lasting piping
PRESSURE/TEMPERATURE RELATIONSHIP MAXIMUM
SUGGESTED DESIGN PRESSURE
Thermoplastic fittings do not carry the
same pressure ratings as equivalent sized pipe. In fact, fittings have never been pressure rated. Fitting standards (ASTM D-2464, D-2467, D-2466, F-439 and F-437) only require a 25% thicker wall (in the body) than equivalent-sized pipe.
When a system is pressurized, fittings have areas that contain higher stresses than pipe despite the increase in wall thickness. Testing and field experience suggest that systems with socket fittings be reduced in pressure by 40% of the equivalent pipe size pressure rating.
For example, if 2” sch 40 pipe is pressure rated at 280 psi then 2” sch 40 socket fittings should therefore never be subjected to pressures higher than 168 psi. This 40% reduction still does not give fittings a pressure rating but it does give the system designer a maximum pressure not to exceed.
This reduction from pipe pressure rating
is called maximum suggested design pressure and is the highest pressure the system should experience. Note that
threaded fittings have a 50% reduction because threads are formed by the removal of material. Table 2 (below)
shows the maximum suggested design pressures for fittings of various sizes at 73°F. When a thermoplastic piping system is to be operated at temperatures above
73°F it must be done at reduced pressure.
To determine the maximum system pressure for temperatures above 73°F, multiply the maximum suggested design
pressure from Table 2 by the appropriate temperature correction factor in Table 3.
A schedule 40 PVC 2” Tee will be subjected to temperatures of 100°F. What is the maximum suggested design pressure at this temperature?
- Step 1: From Table 2, the maximum suggested design pressure at 73°F is 168 psi.
- Step 2: This value is then multiplied by the temperature correction factor from Table 3 (168 X .62 = 104 psi).
The maximum suggested design pressure for a sch 40 PVC 2” Tee at 100°F is 104 psi.