Description and Operation
The lock has two identical ship caissons (like bathtubs) in which vessels ascend and descend. Both caissons are enclosed at each end by pivoting gates, and there are pivoting gates at the upper and lower reaches of the canal at the junctions with the caissons. The gates on the caissons fit into slots on the gates on the reaches, so that they open in unison.
Each caisson sits on a ram, the shafts for which are sunk into the ground, are filled with water, and are connected with a pipe that has a crossover control valve. The caissons are guided up and down on either side by rails affixed to concrete towers. The caissons with water (1040 m3 or 228,093 imperial gallons) weigh 1,700 tons (1,542 tonnes) and are 140 ft (42.7 m) long, 33 ft (10 m) wide and 7 ft (2.1 m) deep.
No external power is needed: the lift lock functions by gravity alone using the counterweight principle. One caisson always ascends and the other always descends during each locking cycle. When one caisson reaches the top position, it stops 30 cm (12 inches) below the water level of the upper reach, and the control valve is closed; Siemens ultrasonic sensors are used to help determine the 30 cm differential. The upper reach and top caisson gates open, and water flows into the top caisson until the level equalizes. The weight of the extra 30 cm of water is 144 tons (130.6 tonnes), making the total weight of the upper caisson 1,844 tons (1,672.6 tonnes). Any vessels that just ascended in the top caisson exit into the upper reach, and any new vessels making a transit of the lock then enter the bottom or top caisson from the lower or upper reach respectively. Once the vessels are secured, all gates are closed and the crossover valve in the connecting pipe between the ram shafts is opened. Since the upper caisson weighs more than the lower caisson (1,844 vs 1,700 tons), it pushes down on its ram, forcing out water from its shaft via the connecting pipe into the shaft of the bottom caisson. The force pushes up on the bottom caisson's ram, raising the caisson up to the top position. When the gate of the newly descended top caisson and lower reach gates open at the bottom, the extra 30 cm of water flows out and equalizes with the water level in the lower reach of the canal, and any descended vessels exit, allowing the cycle to start over again.
Read more about this topic: Peterborough Lift Lock
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