UNDERGROUND PIPING

UNDERGROUND PIPING

Underground piping is installed early in the project concurrently with civil foundations, earth work, and electrical duct banks. Prior to beginning any excavation, existing buried commodities must be reviewed to identify any potential obstructions. In instances where underground obstructions cannot be avoided, excavation is performed manually or by the use of an air spade or hydrostatic excavation system. 

Excavation activities at operating facilities must be done only with the approval and close involvement of plant operations and maintenance personnel. Excavation permits documenting verification of existing commodity locations is normally required at all Bechtel construction sites.

Typical buried piping systems include:

• Fire protection
• Storm water sewers
• Potable water
• Natural gas
• Drainage piping

Fire protection is usually installed in a ring around the site to encompass the work area with a header provided for all branches. The system must meet National Fire Protection Codes (NFPA) and any local code requirements. The fire protection system equipment is supplied with Underwriters Laboratory (UL) or Factory Mutual
(FM) approval and must be so identified. 

Depending on the type of piping system used, thrust blocks are installed for fire protection piping at changes in direction, at tees, and at fire hydrants . Thrust blocks provide lateral restraint to waterhammer and loads imposed by water flow. They are usually constructed of poured concrete or concrete block constructed at desired locations after the piping system has been installed and tested.

Storm Water and Drainage systems are installed at the same time as the fire protection system, but require that the slope and elevation of the pipe be maintained and checked per the design drawings prior to the backfill of the trench. Specified slope tolerances ensure the completed system will maintain a self-cleaning fluid velocity and prevent the formation of traps and pockets. The pipe material used for drainage systems may include pre-cast concrete, carbon steel, PVC, or HDPE.

Coatings

Coating and lining systems provide excellent corrosion resistance properties and provide the required smoothness to maintain flow capacity in the line. External coating systems protect steel pipe by electrically insulating the coated pipe from the environment. When reinforced, the coatings provide additional resistance to physical damage.  

Common types of underground pipe coatings include:

• Coal-Tar Enamel
• Coal-Tar Epoxy
• Hot or Cold Epoxy Enamel Applied Tape

Coal-Tar enamel is typically applied in a shop environment. To apply the coating, hot enamel is placed on the pipe wall surface and then the exterior coating is covered with paint or kraft paper. Pipe shipped or stored at cold temperatures (typically encountered in the winter months) may require a different coating system since the coal-tar enamel is subject to cracking in cold weather.

The Cold-Applied Tape process uses a cold primer and cold-applied tape. This can be used on steel pipe for soil conditions. Tape with both polyvinyl chloride and polyethylene backings are used. Thickness of tapes vary depending on the overlap and performance condition requirements including where mechanical damage may
occur due to handling and construction.

Testing of underground piping coatings is done with high voltage, low amperage holiday detectors and is done immediately prior to backfilling.

Cathodic Protection

This method of protecting underground piping is typically provided when soil resistivity is low. Pipe protection can be provided by either sacrificial anodes or an impressed current system. This will help prevent accelerated corrosion of the underground system.

Cathodic protection systems are usually specialized installations and their design, installation, and inspection are best left to specialists.  

EMBEDDED PIPING

The installation of piping systems embedded in concrete should comply with the following installation guidelines:

• Verify top of concrete elevations for floor drains, equipment drains, and cleanouts to ensure embedded piping is properly positioned. Pay particular attention to floor slope requirements.

• Piping that penetrates slabs or walls must be extended beyond the concrete surface to allow sufficient space to make up the next joint.

• The embedded portion of piping systems must be hydrostatically tested prior to concrete placement.

• Do not embed mechanical joints in concrete unless specifically required by the design drawings.

• Check project requirements for cutting or modifying reinforcing bar and for added reinforcing at penetrations prior to embedded piping installation.

• As-built embedded piping systems prior to concrete placement.

• Do not secure embedded piping by welding to other piping or to reinforcing bar.Ensure that embedded piping is adequately secured so that it will not move during the concrete placement.