Additional Installation Considerations 

GROUND MOVEMENT AND SEISMIC RESISTANCE
A large number of water main breaks occur every year due to soil settlement, freeze/thaw cycles and shrinking or swelling of expansive soils, not to mention the occasional widespread damage that accompanies earthquakes. Polyethylene’s flexibility and its fusion joints make it considerably less susceptible to damage from ground movement. California gas utilities recognize polyethylene’s excellent record in enduring seismic events without damage.

MECHANICAL RESTRAINT AND THE POISSON EFFECT
While polyethylene pipe is self restrained through the butt fusion joint, when it connects to a gasket jointed pipeline, the polyethylene pipe must be anchored or the gasket joints upstream (or downstream) from the transition must be restrained to prevent pullout of the gasket joints. Additional information on how to design for this situation is covered in the Performance Pipe Technical Note "Mechanical Restraint and the Poisson Effect."

ABOVE GRADE AND AERIAL INSTALLATION
Performance Pipe black polyethylene pipe contains carbon black allowing indefinite above grade storage and use. Design considerations for above grade applications are available in the Performance Pipe Technical Notes "Thermal Effects" and "Above Grade Pipe Supports."

VACUUM RESISTANCE (EXTERNAL PRESSURE)
Many pipelines operate under full or partial vacuum or experience negative internal pressures when subject to pressure surges. External pressure exceeding the internal pressure (external differential pressure) creates the same effect. Pipelines may be subject to external pressure during installation, submergence, grouting of sliplined pipe or directional drilling. All pipes have a limit to the amount of external differential pressure (or vacuum) they can withstand. Exceeding that limit will cause the pipe to collapse. Table 10 gives the allowable external differential pressure based on Equation 3-39 in Chapter 6 of the Handbook of Polyethylene Pipe with a safety factor of two against collapse and with 3 percent ovality in the pipe assumed. Higher resistance to collapse can be achieved by embedding the pipe in soil, flowable fill, grout or concrete. For additional temperatures, see the Performance Pipe Field Handbook.

FITTINGS, CONNECTIONS AND REPAIR FITTINGS
Performance Pipe manufactures HDPE molded fittings including tees and elbows in sizes through 8” diameter. Flange adapters for flange connections are available through 24” diameter. MJ Adapters for both DriscoPlex® 4000 and 4100 pipe are available through 12” diameter. Larger fittings are available through third party fabricators.

TRANSITION TO NON-POLYETHYLENE PIPES
Polyethylene pipe can be conveniently connected to metallic valves, pumps and even pipe. Normally the connection is made using a polyethylene Van Stone style Flange Adapter with a metallic backup ring which mates to a metallic flange or using a polyethylene Mechanical Joint (MJ) Adapter which mates to a DI mechanical joint bell. The MJ Adapter works with both IPS and DIPS polyethylene pipe. Acceptable methods also include metallic transition couplings that slide on, seal and grip the polyethylene pipe or metallic transition couplings that slide on and seal but require additional external restraint rings. These types of couplings may require the use of an insert stiffener in the polyethylene pipe. DriscoPlex® 4000 pipe may be inserted directly into an MJ Bell. This requires placing an insert stiffener inside the end of the DriscoPlex® pipe and restraining the connection with an external ring or clamp on the pipe. When selecting mechanical couplings or components for use with DriscoPlex® pipe, make sure the mechanical coupling manufacturer recommends the particular part for HDPE pipe. For additional information on HDPE to non-HDPE pipe transitions, see:

  • Plastics Pipe Institute’s Technical Note 36 General Guidelines for Connecting Potable Water HDPE Pressure Pipes to DI and PVC Piping Systems, and
  • Polyethylene Piping Systems Field Manual for Municipal Water Applications.

TAPPING
A variety of heat fusion and mechanical fittings make hot or cold tapping a straightforward process. Heat fusion jointed products include saddle fusion tapping tees, electrofusion tapping tees and branch-saddles. A number of manufacturers produce metallic full body tapping saddles and sleeves for polyethylene pipe. Performance Pipe recommends that the manufacturer be contacted to make sure their saddles work with polyethylene pipes. Service saddles are available, as well. These may come with double or extra wide straps, spring washers or both.

REPAIR
Polyethylene pipe has an excellent field record. However, circumstances may arise where repair is necessary. The most likely form of damage is impact or an underground strike which is usually localized. A variety of repair clamps (both mechanical and electrofusion) and tapping saddles are available. If a section of pipe has to be removed, a new piece can be inserted using mechanical couplings, polyethylene flange adapters or electrofusion couplings.