DriscoPlex® 4000 / 4100 pipes are characterized as hydraulically smooth and typically have an absolute surface roughness (e) of 0.000005 ft. The Hazen-Williams Friction Factor (C) equals 150 to 155 for polyethylene pipes. Even though the inside diameter of polyethylene pipe may be smaller for the same nominal size as metallic or concrete pipes, flow is often equal or greater through polyethylene pipe. For example, an 8” DR17 DriscoPlex® 4000 pipe has a lower pressure drop per given flow rate than an 8” CL350 concrete lined DI pipe (C equals 120). For gravity flow, the n-factor in the Manning equation is typically taken as 0.009 for clear water and 0.010 for sanitary sewer. For design information, see the PPI Handbook of Polyethylene Pipe, Chapter 6.

When it comes to surges, polyethylene has two advantages over most piping materials. First, as Table 3 shows, it has the capacity to handle surge pressures significantly in excess of its pressure rating. 

Additionally, it has the lowest surge pressure of all common water pipes. For example, a 5 ft/sec velocity change in a DR17 Polyethylene pipe will produce a 56 psi surge, a DR18 PVC pipe will have one of 88 psi and a Class 50 DI pipe will create a surge of 268 psi. 

The lower polyethylene pipe surge pressures means less wear and tear on valves, hydrants and other system components. Additionally, when surges occur, HDPE pipes may be quite capable of handling them with a lower Pressure Class (PC) than required for other materials.

Repeated surges will cause fatigue stress in pipelines. This is particularly significant in certain thermoplastic pipes, excluding polyethylene. Fortunately, polyethylene has an excellent resistance to fatigue. The projected design life for DriscoPlex® 4000 / 4100 pipes exceeds 100 years for pipe operating at a velocity of 4 fps with a surge frequency of 4 times per hour continuously. See Bulletin PP-402, Working Pressure Rating and Fatigue Life.

Polyethylene pipe is routinely used in mining applications above the Arctic Circle and can withstand water freezing internally. A product that can be handled in these extreme conditions has to have excellent impact resistance. The Izod Impact Strength of high density polyethylene using ASTM D256 Method A is 4 to 5 ft-lbs/in at 73°F, again a value significantly greater than other plastic pipe materials.

Impact damage, fatigue or joint failure in metal or thermoplastic pipes under certain operating conditions can lead to long, running cracks that will propagate through fused joints and can travel hundreds of feet. This cracking is referred to as Rapid Crack Propagation (RCP).

Polyethylene pipe has excellent resistance to RCP. In fact, laboratory testing has shown that RCP cannot occur in a water filled polyethylene pipe. PP838, Preventing RCP in Fused Water Pipelines indicates that the best way to avoid this type of cracking is to specify the use of polyethylene pipe as opposed to other thermoplastic pipes.

Polyethylene’s superior performance is due to its fused joint, toughness and flexibility. Comparisons of polyethylene to other piping materials based on PC alone can lead to costly over-designs, since the definition of “Pressure Class” varies from material to material (see AWWA C906, C905, etc). When correctly incorporating HDPE’s lower surge magnitudes, higher surge allowances and greater fatigue strength into the design, the PC required for HDPE may be much lower than the PC required for other pipe materials.