Fri. Sep 24th, 2021

City gas distribution

Transporting future fuel

#73 Polyethylene Pipelines

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Polyethylene network starts from DRS & goes up to meter regulator. Usually two pressure levels are considered for “Gas distribution PE pipeline networks” & the following standard is adopted:

  1. Medium pressure network : 0.5 bar to 5 bar
  2. Low pressure network : <= 0.11 bar

The interface between MP & LP system is assured by additional pressure reducing station, i.e. Service Regulator.

  1. PE pipeline must be used only for underground pipes with proper anti-corrosive coating on it.
  2. PE pipes cannot be used in water & railway crossing.
  3. The MP PE network starts from the downstream of DRS & distributes the natural gas till the different service regulator located nearby a cluster of domestic customers or a main commercial customer or industrial customers.
  4. The LP PE network starts from the downstream of Service Regulator & will bring the natural gas to domestic customers.
  5. When two pipeline (MP & LP) are laid in the same trench then minimum 300 mm distance is maintained between two pipes.
  6. Also, MP pipeline is laid at road side & LP pipeline is laid at extreme side of the road.
  7. Some of the design criteria for PE pipelines are: Gas demand, Source Pressure, Distribution velocity & Pipe material.

Gas Demand:

  1. Gas demand will depend upon season; day of the week; time of the day & nature of gas use (heating or cooking).
  2. It must take into account “diversity of demand”.
  3. Diversity of demand, D is defined as, D = Maximum potential gas demand / Maximum actual gas demand

Source Pressure:

  1. Consideration should be given to designing an intermediate/medium pressure supply system, where suitable source of pressure is available.
  2. For discrete, new PE network, the source pressure can be 75 mbar, however, where new PE network are interconnected with older, leaking metallic pipes; the source pressure is normally limited to around 50 mbar or less.

Distribution velocity:

  1. Gas velocity should be limited to 20 m/sec where there is a dust problem; otherwise gas velocities up to 40 m/s are acceptable.

Pipe Materials:

  1. Pipe material is polyethylene for new pipelines & may be PE 80 or PE 100 grades of pipe.
  2. Long term testing has established that PE pipes will have a lifetime of at least 50 years when subject to constant hoop stress of 80 bar for PE 80 & 100 bar for PE 100.

Design:

Nominal wall thickness of plastic pipe is calculated as per the following formula;

P = 2S * (t / D-t ) * 0.32 , where, D= Specified outside diameter in mm, P= design pressure in psig, t = Nominal wall thickness in mm, S = long term hydrostatic strength in psig

Weymouth formula can be used for calculating the diameter of PE pipelines, or for calculating the flow rate through a pipeline for a given geometry.

PE Pipeline Laying:

  1. Polyethylene pipe conforming to IS 14885 or ISO 4437 are acceptable according to PNGRB regulation.
  2. Colors of pipe used for gas service are yellow for PE 80 grade & Orange for PE 100 grade.
  3. A trench of at least 300 m width is excavated & the bed of the trench is made free of sharp objects, stone etc. In rocky areas, trench is padded with soft soil or sand to minimum depth of 150 mm below the pipe. Then the pipe is placed in the trench.
  4. Plastic pipe joints are made by electro fusion fitting only. Branch connections are made only by socket type electro fusion tees or electro fused saddle connections.
  5. When open cut techniques are used for pipe laying, a warning tape of yellow color is laid 200 mm above pipe.

PE pipe joining procedures:

  1. PE pipes or fittings are joined to each other by heat fusion or with mechanical fittings.
  2. PE pipe may be joined to other pipe materials by means of compression fittings, flanges, or transition fitting.
  3. The joints can be classified as permanent connections or detachable connection. Connection by thermal heat fusion methods & electro fusion fall under permanent connections & connection by flanges & unions fall under the category of detachable connections.
  4. In the conventional thermal heat fusion method, three types of joints can be considered viz. Butt, Saddle & Socket fusion.
  5. Two pipe surfaces are heated to a designated temperature, & are then fused together by application of a sufficient force. This force cause the melted materials to flow & mix, thereby resulting in fusion. After fusion the joint area has a higher tensile strength & properly fused joints are absolutely leak proof.
  6. Butt fusion is the most widely used method for joining individual lengths of PE pipe & pipe to PE fittings. It produces a permanent, economical & flow-efficient connection.
  7. In the saddle type fitting, both the external surface of the pipe & the matching surface of the saddle type fitting are simultaneously heated with concave & convex shaped heating tools until both surfaces reach proper fusion temperature.
  8. In socket fitting, both the external surface of the pipe end and the internal surface of the socket fitting are simultaneously heated until the material reaches fusion temperature. The pipe end is inserted into the socket & is held in place until the joint cools.
  1. In electro fusion, joint is heated internally either by a conductor at the interface of the joint or by a conductive polymer. Heat is created as an electric current & is applied to the conductive material in the fitting. Electro fusion joining is done with special electro fusion couplings. The pipe pieces to be joined are cleaned & inserted into the electro fusion fitting with a temporary clamp. A voltage of around 40 V is applied for a fixed time depending on the fitting in use. The inside of the fitting & the outside of the pipe wall is then melted by heat from built-in heater coils, which weld together producing a very strong homogeneous joint. The joint is then left to cool for a specified time.
  2. Flanges & Union threads are used for detachable connections. Flanges made with gaskets & appropriate fasteners are very suitable for assembly under difficult circumstances, connection to valves & pumps & connection from PE to other pipe material. They are used in conjunction with metal backing plates, and rubber sealing gaskets in order to provide a demountable joint.
  3. Union thread couplings are used to make simple detachable screw joints & are generally used in transitions from PE to other materials. They are useful for connecting small pipe sizes, but are not advisable for lines with high temperature & flow Fluctuations.

Testing of PE pipeline:

  1. After construction, PE pipe is tested with air or nitrogen for test pressure up to 100 psig. Above 100 psig, water is used as a medium.
  2. Pipe is kept under test pressure for 24 hours for length greater than 1 km & minimum 4 hours for length shorter than 1 km.
  3. After testing, pipe is commissioned & loaded with natural gas.
  4. When plastic pipeline is crossing road/railways, it is provided with casing of RCC pipe of min. NP3 class.

GI & Copper Pipelines:

  1. These are low pressure pipeline; Pressure for GI & Copper pipeline is 75 mbar & 21 mbar respectively.
  2. GI pipe is fitted on the walls of building & connected to the PE network of medium pressure.
  3. The pipe size is reduced inside an apartment & copper pipe option becomes feasible.
  4. Code of standard for copper pipe is IS-1239 & that of GI is BSEN 1057.

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