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Commissioning - B

Are there any special factors that affect the testing procedures when commissioning PE pipe?

The only special requirements for purging PE pipes is the need to take into account discharges due to static electricity. As with other materials care should be taken to protect the environment when purging or discharging following pressure testing.  

What precautions do customers and installers generally advise against discharge of static electricity?

PE pipe has a high electrical resistivity and static charge can accumulate on the surface of the PE pipe. Normal handling from pipe slings, cloths, etc generates the static charge, particularly in dry conditions.

Static occurs inside PE pipe in gas distribution systems because gas is electrically resistive. That means that electrons that have been scraped off gas molecules by friction as they flow against the PE pipe surface are resisted from reinserting themselves into gas molecules that have lost electrons. Instead, the free electrons build up on the interior PE pipe surface creating a negative charge with the ever-present possibility of arcing and igniting the gas any time the pipe is breached. Dust particles in gas also generate tension which needs to be discharged.

High static electric charges can develop on PE pipes during squeeze-off, when repairing a leak, purging, making a connection, etc. Safety procedures have been developed by the major gas utilities to prevent static electricity igniting the flammable gas-air mixture.

Ensuring good contact with the ground can easily dissipate the static charge, effectively ‘earthing' the PE pipe. This is achieved during normal backfilling operations or by wrapping a damp cotton cloth around the pipe and ensuring it is in contact with the ground.

The following precautions may be taken to ensure safe working where there is a risk of static discharge:

• Use an earthed wet tape conductor wound around, or laid in contact with, the entire section of the exposed piping.
• If gas is already present, wet the pipe starting from the ground end with a very dilute water and detergent solution. Apply tape immediately and leave it in place.
• Wet the tape occasionally with water. Where temperatures are below 0°C add glycol to the water to maintain tape flexibility. Earth the tape with a metal pin driven into the ground.
• Do not vent gas using an unearthed plastic pipe or tubing. Even with earthed metal piping, venting gas with high scale or dust content could generate an electric charge in the gas resulting in an arc from the dusty gas cloud back to the pipe which could ignite the gas. Vent gas only at a downwind location remote from people or flammable material.
• Dissipating the static charge build-up with wet rags, a bare copper wire, or other similar techniques may not be as effective as the above procedure.

In all cases, use appropriate safety equipment such as flame resistant and static free clothing, breathing apparatus, etc    

Is PE resistant to the chemicals commonly used for disinfection, e.g. chlorine?

PE pipe is resistant to the chemicals commonly used for water treatment and disinfection.

Disinfection of water mains is a frequent operation. Disinfection takes place when commissioning newly constructed potable water mains; mains that have
been removed from service for planned repairs or for maintenance that exposes them to contamination; mains that have undergone emergency repairs due to physical failure; and mains that, under normal operation, continue to show the presence of coliform organisms. The chlorine disinfection process puts pipe in contact with a strong oxidising agent.

Several studies have been conducted to verify the effects of chlorine disinfection on the performance of PE pipes. A study by the US Plastic Pipe Institute included pre- and post-exposure testing of several characteristics of PE pipes, including resistance to slow crack growth. The testing performed in this study indicated that chlorine disinfection, when conducted within the guidelines of AWWA-C651, did not have a significant adverse affect on the subsequent performance of PE pipe.

This is supported by many years of extensive trouble free use in potable water applications. However due to the variety of chemicals, concentrations and practices in various countries if there is any doubt, advice can be sought from the polymer or pipe manufacturer.

For pipe applications where chlorinated water is to be carried over a long term period, rather than the short term disinfection process, further advice should be sought.

Chemicals commonly used in the disinfection and water treatment process include; chlorine, chlorine dioxide, monochloramine and ozone.