SURGE TANK AND PENSTOCK

Surge tank:
• Transition between the low pressure tunnel and reservoir and high pressure penstock and power house.
• No surge tank required if the distance between the power house and headwork is short.
• No surge tank required if the headrace is open channel flow.
Why surge tank?
• To protect low pressure conduit system from the high internal pressures.
• Helps to prevent/intercept/dampen the high internal pressure waves created due to the sudden closure of valve after penstock during load rejection by turbines (also called water hammer effect) for entering into the low pressure system.
• Thus it gives a facility to design entire headrace pipe as a low pressure system.• Storage functions of surge tank.


Point to be noted:
• Surge tank doesn’t directly influence the magnitude of water hammer pressure.
• Since pressure being transient and oscillatory in character, frequency of water hammer oscillation would depend upon the location of surge tank.
• Nearer the surge tank to power house greater frequency of oscillation
• Though water hammer pressure oscillation is dampened by the surge tank it gives rise to frequency of mass oscillation of water in surge tank and reservoir connected by low pressure tunnel.
• Design of surge tank is done by taking account of mass oscillation


Penstock:
• It’s a conveyance from fore bay or surge tank to turbine.
• Usually steep ground profile is selected for its location.
• Design is based on the total pressure occurred inside it (i.e. static pressure created by the water and dynamic pressure due to the water hammer effect)
• Pipe(penstock) bursting is dangerous because its usually constructed at steep slope and the water emerging out of the bursted penstock has the potential of causing landslide.
• Optimization of penstock for the selection of particular diameter is required since its installation requires significant portion of construction cost.
• Its design means the selection of pipe material, economical diameter, and safe thickness.


LOCATION OF PENSTOCK:
• Steeper slope 1:1 or 1:2 V:H because flatter the ground longer the penstock and hence uneconomical
• If steeper than 1:1 additional cost on slope stabilization for construction of support piers and anchor blocks will be required.
• The profile starting from gentle slope to steeper slope is avoided as it gives rise to negative pressure causing sub-atomic pressure.• Minimum bends selected as more bends means more head loss and more requirements of anchor blocks.
INTERESTING: When the velocity of the water inside the penstock is increased then the diameter designed will be decreased. If we see it through the general point of view, it would be see that for passing out the higher velocity thicker diameter of penstock should be used but the mathematical formulation shows somewhat different which is the actual case. The reason is defined by the conservation of total energy.
For any system, TOTAL HEAD (ENERGY HEAD) IS CONSTANT.i.e. VELOCITY HEAD + PRESSURE HEAD = CONSTANT
This shows if the velocity is increased then the pressure head is decreased. As the penstock thickness is designed considering the pressure head hence the thickness is decreased ultimately.