Results / Simulation of particle movement in ribbed sand traps

Simulation of particle movement in ribbed sand traps

Optimum capture of sediments before they end up in the turbine can save power producers large amounts of maintenance costs.

HydroCen has developed a method to simulate how particles behave in a ribbed sand trap. It will be a useful tool in the transition to more peak power and the use of pumped stock.

Simulation of particle movement in ribbed sand traps

Optimum capture of sediments before they end up in the turbine can save power producers large amounts of maintenance costs.

HydroCen has developed a method to simulate how particles behave in a ribbed sand trap. It will be a useful tool in the transition to more peak power and the use of pumped stock.

Masses in front of the upstream intake hatch in the sand trap at Tonstad power plant. Photo: Kai-W. Nessler

Expected more sediments

In Norway, we have solid mountains which mean that there is not so much sand, gravel and stone that is pulled loose when the water flows through the water tunnel. This is very different from high mountain areas elsewhere in the world such as the Himalayas, the Andes and the Alps where sediments are a huge problem. However, wear and tear from sediments in the water has turned out to be a bigger problem than first thought - also here in Norway - partly due to an increase in production which means that larger quantities of water flow through the tunnel.

Particles that come with the water cause great damage to the turbine, which in turn leads to high costs in the form of downtime and maintenance. To prevent gravel and stones from wearing down the turbine, the power plants have sand traps to filter out these particles before they enter the turbine.

When hydropower is transitioning to more peaking power and pumping storage, good solutions are also needed to keep the sediment particles in place in the sand trap when the currents in the water are expected to vary more.

FACTS ABOUT SAND TRAPS

The sand trap is often the last part of the inlet tunnel before the water flows down the penstock and on to the turbine. It is a chamber that has a larger volume than the rest of the tunnel, which makes that the water slow down when it enters the sand trap.

The figure illustrates the structure of a typical Norwegian hydropower plant, with the location of the sand trap between the surge chamber (left) and the penstock (right).

The reduced speed of the water causes particles in the water (pebbles and gravel) to fall to the bottom of the sand trap and sediment there. The sediment-reduced water will flow on to the penstock and the turbine.

In Norway, two types of sand trap are mainly used:

Open sand traps

Just an open empty space, where the increased volume in the sand trap causes the water to lose speed and the particles fall to the bottom of the room (as sediments). The sediments are exposed to turbulence in the water, resulting to the distribution shown in the figure below (a & c).

(+) cheap to create/empty the sand traps, (-) less effective at separating and protecting the sediments.

Ribbed sand trap

The sand trap has ribs that lie above a deeper chamber. When the water moves with reduced speed over the ribs, the particles will fall between the ribs and settle on the bottom of the chamber. The ribs lead to the sediments being better protected from the turbulence in the water, see the figure below (b & d).

(+) effective at separating and protecting the sediments, (-) more expensive to create/empty the sand traps.

The figure shows an open sand trap seen from the side (a) and from above (c), and a ribbed sand trap seen from the side (b) and from above (d).

Looking at how particles in the water behave

As a follow-up to the project "Fleksible sandfang" - which concluded that rib constructions were both a cheap and effective method in the long term for capturing sediments, Ola Haugen Havrevoll has in his PhD gone further to understand how the particles in the water move around the ribs, precisely to find the most optimal design of the ribbed sand trap. In his work, he developed a method for simulating the movement of the particles in the water, and has looked at how these would be captured by differently designed rib sand traps.

The simulations showed that different distances between the ribs and the size of the ribs had a lot to say about how effectively the sand trap was able to capture the particles.

The results from the project showed that for the used prototype (Sandfang 3, Tonstad hydropower station) 1-meter ribs with 1-meter intervals were the most efficient design (see figure below).

The figure shows the capture efficiency for ribs with 1-meter width and 1-meter spacing, in model scaling (1:20), 0.05-0.06 mm particles. Many particles are still not captured.

Extends the lifetime of the turbine

The developed method will be a useful tool for consultants who work with the design of new or existing hydropower plants in Norway, as it can be adjusted and invesigate how the conditions in other hydropower plants will affect the transport and sedimentation of particles in the sand trap. By improving the sand trap, less sediment will enter the turbine, and the turbine will therefore last longer before it needs to be maintained or replaced.