Hydraulic design of surge tank and pressure tunnel

Hydraulic design considerations of the surge tank

• Location: as close to the powerhouse as possible, on stable ground at a level high enough that the maximum surge level stays below the tank top, and the minimum level stays above the tunnel crown.
• Type: simple, restricted orifice, differential (Johnson), or with an upper/lower chamber, chosen by the head, topography and the required reduction of the surge.
• Size: the cross-section must be larger than the Thoma area for stability under small load changes:

$$A_s\ge A_{th}=\frac{L\,A_t}{2g\,\alpha\,(H_0-h_f)}$$

• Surge levels: maximum upsurge on full load rejection must not overflow, and minimum downsurge on full-load demand must not expose the tunnel crown (air entry) or the tank bottom; the oscillation period $T=2\pi\sqrt{LA_s/(gA_t)}$ should be checked with the governor.
• Connection: the area of the connecting orifice and the shaft should be sized to give small head loss, but enough damping.
• Provide an overflow, air vents, and access.

Hydraulic design considerations of a pressurised tunnel

• Diameter and velocity: from the economic diameter; velocity usually 2.5-4 m/s.
• Friction and other losses: Manning/Darcy, with $n$ of lining; plus entry, bend and transition losses.
• Internal pressure: the maximum internal pressure includes the static head and the surge/water-hammer rise.
• Minimum rock cover: the cover should resist the internal pressure (about $C_{rm}\ge\dfrac{h_s\gamma_w F}{\gamma_r\cos\beta}$); otherwise the section is steel-lined.
• Hydraulic grade line should stay above the crown at all times, in the transient too, so as to avoid vacuum/negative pressure.
• Lining and grouting: concrete lining for smoothness, steel lining where the rock is weak or the cover is low; contact and consolidation grouting; drainage.
• Protection: air vents and an emergency gate at the intake, and a surge tank at the downstream end to reduce the water hammer in the tunnel.