PVC, LDPE and HDPE Nonmetallic Piping

This blog gives you basic information about use of PVC, LDPE and HDPE Nonmetallic Piping, its design, fabrication, assembly, erection, inspection and testing.

Design and use of PVC, LDPE and HDPE Nonmetallic Piping:

Use of these plastic materials for piping is limited to 10 kg/cm² pressure and 80^oC temperature.

Usually the design of nonmetallic piping conform to the requirements of Petroleum Refinery Piping code ANSI B 31.3. When another code, rule or regulation is specified i.e. clients standards, specifications, the design to conform such specified requirements and to such portions of this other standards which do not conflict.

Most of the cases in any design, the design temperature and pressure are the most severe condition of coincidental pressure and temperature expected in normal operations. Hence same is applicable for nonmetallic piping design.

If piping configuration includes control valve, then on the upstream side of and including control valves to be designed for pressures resulting from minimum flow conditions (closed or throttled control valve) and piping circuits downstream of the control valve to be designed for the downstream terminal pressure plus friction and head losses. Increased pressures downstream of the control valve due to closed or throttled valves not to be considered except as short time conditions.

Solid nonmetallic flanges of plastic material normally not be used, unless otherwise mentioned or client requirement. Instead of this, stub ends and backing flanges with drilling according to ANSI 16.5 to be used.

For eccentric and concentric reducers, injection molded fittings to be used.

Plastic pipe supports need continuous piping or pipeline supports using structural steel material. Plastic pipes being flexible, the free span a stretch of pipe can stand unsupported is limited.

The continuous support must be designed for economy of structural steel. Strength of the material of the pipe, its thickness, its self weight and weight of the fluid handled are the other parameters to be taken into consideration while designing the continuous supports.

Flange joints to be provided on plastic piping systems to facilitate ease of construction and maintenance.

Mirrors used for welding the pipes are generally attached to jigs, which are heavy. So unlike metallic pipes, welding in difficult positions cannot be done in the case of plastic pipes. The number of flange joints in each system has to be decided keeping this fact in mind.

When plastic piping systems are designed to handle slurry lines, the possibility of chocking is an important criteria for deciding the number of flange joints. For such lines, the general practice is to provide flange joints in every six meters and at fittings like elbows and tees.

Fabrication, Assembly and Erection of Nonmetallic plastic piping:

Splicing of pipes, jointing of fittings to pipes, etc. to be done only using PTFE lined electric welding mirrors. If welding using acetylene torch is to be carried out, prior permission to be obtained from the Engineer-in-charge.

Before fusing the ends of pipe /fitting, the surface must be cleaned properly. Squareness of the ends, freedom from cracks, etc. to be ensured before the pipe / fitting is fixed on the jig.

The pressure applied to be uniform around the circumference. Heat sensitive crayons to be used to ensure that the temperature of the mirror surface is enough to fuse the plastic.

When the ends are molten, the pipes are detached from the mirror, mirror removed and the surfaces are pressed together. It has to be ensured that both the surfaces are fused equally. They are to be pressed together for a period of three minutes.

Fins formed during the process of fusing together the pieces must be less than 2 mm in dimension. Undercut also not to reach up to the surface of the pipe /fitting. If the fin formed is more than 2mm, it must cut and removed. In such an event, it is possible that the fin may be there in the inside of the pipe also. For smaller diameter pipes, this may obstruct flow. Where it is accessible, these fins may be cut and removed from the insides.

If the line size is small and the fin formed outside indicates that the fin in the inside can obstruct the flow through the pipe, the joint may be cut and redone.

Mitered elbows, equal tees, etc. may be fabricated at site using fusing mirror. The pipe to be cut and fused in such a sequence as to enable fusion in one plane.

For unequal tees, injection molded fittings only are generally used. Under specific conditions, fabricated equal tee with an injection molded reducer may be used for this purpose.

Inspection of nonmetallic piping:

Generally only visual inspection is carried out for non metallic piping systems. Visual examination of welding of HDPE /LDPE / PVC, filling for FRP and tightness of glass piping to be done for all joints.

Testing of nonmetallic piping systems:

Prior to commencement of pre-commissioning activities, each piping system to be hydro tested or pneumatically tested as per the project design specifications.

Normally pneumatic testing shall be at 1.1 times the design pressure and hydraulic testing at 1.5 times the design pressure.

For glass piping, pressure testing may be waived at the discretion of the engineer in charge.