Laying of HDPE Pipes
Laying of HDPE Pipes ( As per Code IS 7634 (Part 2)):
The procedure is applicable for water supplies upto and including 45 deg. C.
Commonly used joints are as follows:
a) Fusion Welding :
- Butt Fusion Welding
- Socket fusion welding
- Electro Fusion Welding
b) Insert Type Joints
c) Compression Fittings
d) Flanged Joints
e) Spigot and Socket Joints
a) Fusion Welded Joints:
The principle of fusion welding is to heat the two pipe surfaces to a designated temperature and then fuse them together by application of sufficient force. This force causes the melted materials to flow and mix there by resulting in fusion.
I. General :
- The pipeline shall be laid inside the trench with a slack of upto 2m/100m.
- The pipe prior to connection with sub surface structure (such as preset valve etc) should be allowed to cool at ambient temperature else excessive pull force could develop.
- HDPE piping system joined by butt welding, electro fusion and flanges do not require external joint restraints or thrust joint anchors.
- Metallic locating tapes or copper wire can be placed along the pipe slightly above the crown of the same before the final backfill.
Earth work and Pipe Support ( Trenching )
Trench Bedding :
HDPE pipe requires no special bed preparation for laying the pipe underground except that there shall be no sharp objects around the pipe. However, while laying in rocky areas suitable sand bedding should be provided around the pipe and compacted.
Trench Depth :
The trench depth shall be as per table given below. The initial back fill up to 150 mm above the crown of the pipe should be compacted mechanically or by manual means with screened excavated material free of sharp stones or objects or with fine sand where no such material is available. Also wherever road crossing with heavy traffic is encountered – a concrete pipe encasing is recommended.
Water in the Trench
The pipe shall be laid on a stable foundation. Where water is present, the excess water should be removed before laying the pipe. In case there is a chance of floatation because of likely flood, the pipe shall be encased with concrete weights as per buoyancy calculations.
Under Water Installations
HDPE pipes can be aligned along the recommended route over the water surface and then submerged with suitable weights. Submerged installations require permanent concrete ballast rings attached around the pipe to ensure submergence and stable system once it is submerged.
II. Installation :
III. Lowering of HDPE pipes
When jointed outside of the trench, the jointed pipeline shall be lowered into the trench (for underground installations) carefully (preferably with mechanical handling equipment for sizes greater than 160 mm ) without causing undue bending that can cause kinking.
The coefficient of expansion for HDPE pipes is about 14 times than that experienced by metal pipes. Hence for exposed PE pipes , provisions shall be made for the effects of thermal movement. The support anchor for exposed PE pipes should not grip or distort the pipe but should allow free movement of the pipe due to temperature variation.
Valve Anchoring and Support Spacing
All types of manual controls and valves in particular should be anchored firmly so as to avoid the turning torque imparted by the operation of the hand wheel of the valve onto the pipe. In short the valve should not be ‘hung’ on the pipe, as is normally done for metallic pipes.
Suitable supports as agreed to between the buyer and the supplier for horizontal over ground run of PE pipe with pipe clamps/brackets should be used. At >40ºC, continuous support is recommended, if the pipe is carrying heavier liquids.
Polyethylene pipes may be encased in concrete, wherever necessary. Compressible padding material at least 3 mm thick and at approximately 150 mm from the face of concrete shall be provided around pipes at the entry and exit points to eliminate any potential sharp edges from rubbing against the pipe wall. Pipeline shall not be filled with water until the concrete has developed sufficient strength.
On completion of the pipe laying operations up to a length of about 1 000 m while further laying work is still in progress, refilling of trenches of this stretch shall be carried out up to 300 mm above pipeline. Pipe laying shall follow closely the progress of trench excavation. Only soft earth and gravel of good quality free from boulders, roots vegetable matter, etc, shall be used first. If sufficient quantity of suitable (sharp edge stone free) excavated earth is not available, the trench shall be filled by borrowed gravel or material up to 300 mm above top of the pipe.
Care shall be taken during back filling for not to damage the pipe or joints. Filling has to be carried out simultaneously on the both sides of the pipes so that unequal pressure does not occur. Load on the buried pipeline shall not be permitted unless the trench has been filled to the height of at least 300 mm over the top of the pipe. Filling shall be done in layers of 150 mm, with the first layer watered and compacted by stamping or by mechanical means. The trench shall be refilled so as to build up the original ground level keeping due allowance for subsequent settlement likely to take place.
Field Testing of Pipeline
The pipeline to be tested shall be filled with water slowly allowing for splurging the entrapped air. Air valves at high points should be open to allow air to escape while water is being filled. Before pressure is applied, the pipeline section under test shall be restrained against movement.
The following procedure is recommended for HDPE pipe testing:
- HDPE pipelines shall be pressure tested at ambient temperature. After filling with water the pipeline shall be left to stabilize for a period of 1 h.
- Fusion joints may be covered during testing. Flanged joints shall be kept open for visual inspection. The pipeline shall be filled with water and pressure tested from the lowest point.
- During the test period, make-up water is continuously added to maintain the pressure.
- The test pressure shall be 1.5 times the rated pressure of pipes or of the proposed maximum design pressure of the section. Apply the pressure by continuously pumping at a constant rate.
- Under no circumstance, air is to be used instead of water for testing.
- Tests should be performed on reasonable lengths of pipelines. Long lengths more than 2 000 m may make leak detection more difficult.
- Acceptance criteria — If the pressure remains steady (within 5 percent of the target value) for 1½ h, leakage is not indicated. Flanged connections shall be visually inspected.
- If the test is not complete because of leakage or equipment failures, the test section shall be depressurized and allowed to relax for at least 8 h, before starting the next testing sequence.
- Testing outside the trench is to be avoided, as pipe rupture may involve safety issues.