How to increase the efficiency of a lock?
An important economic aspect to measure the efficiency of navigation locks is their throughput capacity. In other words, the less time lost by ships passing through the lock, the better for economy. The design of the filling and emptying system of the lock is crucial in this regard. This blog item focusses on how to reduce the locking cycle time.
There’s a large array of different filling and emptying systems for navigation locks. These systems include a.o.:
- bottom filling systems: the water enters the lock chamber through uniformly distributed openings in the lock floor;
- side filling systems: when the water is diverted through longitudinal sewers along the lock walls and introduced through openings in these walls;
- short sewer systems: when the lock heads are bypassed by short sewers and water is introduced near the lock head;
- filling & emptying through the lock gate: in this case openings are made inside the lock gates that can be closed off during lockage. Similar to this system, in case of vertical lift lock gates, it is in principle also possible to fill/empty the lock chamber by raising the lock gate.
There are obviously many aspects that affect the final design choice for a given filling and emptying system in a navigation lock. One very important aspect concerns water availability and measures that can be taken to reduce water consumption. Other aspects include head difference, available space, cost, saltwater intrusion (if the lock complex is situated on the border between a fresh and saltwater system), etc. None of these design criteria are the subject of this blogpost, however. In this post, we shed some light on the speed of a filling and emptying system and how simple design changes can reduce the locking cycle time significantly.
Increasing the number of valves at the Pierre Vandamme lock
A fine example of the economic value of faster locking cycles is the renovation of the Pierre Vandamme lock in Zeebrugge, Belgium. During a thorough renovation of the most seeward lock gate earlier this year, for which SBE has composed the tender technical specifications, this lock gate has received an update of its filling and emptying system. The existing lock gates have five so-called ‘butterfly’ valves that allow the water to migrate through the lock gate. Over time it has been notified that it takes too much time to pass from the open sea into the inner harbour. Therefore, it was decided to increase the number of valves by four per lock gate.
The choice to increase the number (and/or size) of these filling and emptying valves, however, is not made easily. There are many restraints and boundary conditions that have to be taken into account for the design. The most important aspect that was formerly often overlooked is the influence of the water jets entering the lock chamber at high speeds when the valves in the lock gate are opened. These water jets induce forces on vessels moored within the lock chamber. To avoid any damage to the ship and/or the quay walls, these forces must be strictly limited and to do that, quick calculations can be made to get an idea of the locking time.
Several quick tools are available on the market that allow a designer to compute the longitudinal forces on design vessels in the lock chamber based on a one-dimensional approach. These forces may be induced by translatory waves, momentum variation, skin friction, the filling jet and a water density difference. Because these models are based on a one-dimensional approach, they allow a quick assessment of the design of a filling and emptying system. Especially in early design stages and for decision making, these quick tools are very useful.
Different scenarios can be calculated in no time by adjusting the size of these valves, by increasing the number of valves, by changing their location above the lock floor and by adjusting the opening speed of the valves. SBE is fully equipped to make these quick design calculations to advise its clients on the best possible design possibilities.