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Back Mechanical Mechanical Engineering Dictionary Hydraulics Penstock, Wye-Bifurcation and Units Branches

Penstock, Wye-Bifurcation and Units Branches

Pen-stock is an important component of any hydro electric power plant and is the key member in producing the electricity.

Water from fore bay in case of an open channel or from tunnel in case of pressure is taken in to the turbine for power generation and after that the water is taken in to tailrace channel through proper arrangements.

Bifurcation is the branching of pen stock in to required numbers depending upon the

  • Capacity of the turbine
  • Type of the turbine
  • Size of the penstock
  • Material of the penstock
  • Topography of the area
  • Gradient of the penstock etc

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The steel chosen must be of high tensile quench-hardened HT80 steel of standard quality for bifurcation. The analysis and design of these bifurcations may be done using the ADINA finite element program. Plates for the bifurcation were rolled and fabricated in the on-site shop and then lowered down a shaft just upstream of the powerhouse into an enlargement in the high pressure tunnel. The steel plates for the two downstream bifurcations were then welded in this enlargement in the tunnel to form the bifurcations and when the fabrication of these bifurcations was completed, they were slid down the tunnels into their final positions. The main bifurcation was then assembled in the same enlarged portion of the high pressure tunnel and welded in situ. The enlarged portion of the high pressure tunnel and the lower portion of the access shaft were then back filled with concrete on completion.

A steel penstock (flanged type) with a required wall thickness is designed to pass a given maximum design discharge. The velocity within the penstock is kept a particular range and will be supported on concrete saddles and anchored with ring girders spaced at 5(say) m intervals. Expansion joints are installed to accommodate longitudinal movement caused by temperature. One at each end of these to straight reaches.

Using the criteria of USBR, using allowable stresses as hereunder, the hoop thickness has been worked as hereunder:-

  • Normal Condition

This condition includes:-

  1. Maximum static head plus pressure rise due to normal operation.
  2. Allowable stress equal to 2/3 of the minimum yield strength or 1/3 of minimum tensile  strength which ever is smaller.
  • Intermittent Condition

This may occur during filling, draining or earthquake events. An increase in static head of the order of 20% has been estimated. Allowable stress equal to 0.8 times of minimum yield strength or 0.44 times of minimum tensile strength whichever is smaller.

Wye-bifurcation and units branches shall be designed for full pressure of 4.785 bar (4785kN/m2) considering freely standing position All these structures shall be designed as per USBR Engineering Manual No. 3, AISI steel Plate Engineering Data- Volume 3&4, ASCE Engineering Practice No. 79 and ASCEs guidelines for waterways.

Wye-bifurcations and branches shall be designed for the following conditions:

  • Normal Condition

This condition includes:

  1. Maximum static head plus pressure rise due to normal operation.
  2. Allowable stresses equal to 2/3 of the minimum yield strength or 1/3 of minimum tensile strength which ever is smaller.
  • Intermittent Condition

This may occur during filling, draining or earthquake events. Allowable stress equals to 0.8 times of minimum yield strength or 0.44 times of minimum tensile strength whichever is smaller.

  • Emergency Condition

This includes following:

  1. Water hammer due to instantaneous shutdown
  2. Water hammer due to governor's cushioning stroke being inoperative

The recommended allowable stress is equal to 2/3 the minimum specified tensile strength, but in no case shall the allowable stress exceed the specified minimum yield stress.

  1. Exceptional Condition

This includes:

Test conditions

  1. Malfunction of control equipment of extreme nature Design Criteria
  2. A corrosion and wear allowance of 1 mm shall be added to the minimum required steel  thickness.
  3. Steel plates as per ASTM 516 Grade 70 or ASTM A537 Class 1 shall be used for the design of tunnel liner, wye-bifurcations and unit branches.
  4. Welding shall be carried out as per AWS D1.1 and electrodes type 7018 are used.
  5. A thickness of 300 pm of C-200 epoxy-coaltar paint shall be used for inside painting of the complete steel waterways, whereas the exposed branches shall be painted with vinyl paint as per SSPC-Paint 9 for total dry film thickness of 300 pm.

Rock load shall be explored in the detail design phase to economize the design.

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