Suviana disaster, the possible technical explanation of the accident at the Bargi power plant

After about a month and a half after the Bargi hydroelectric power plant disaster on 9 April 2024, in which an explosion and fire hit the power plant located on Lake Suviana causing 7 deaths and 5 injuries, the situation has not changed much: the the well containing the damaged systems is still flooded and the leak has not yet been dammed. This would imply that probably, by today, the water level inside the power plant would have equaled that of the lake, completely flooding the power station. Let's start with the cause that triggered the accident is not yet known and will be the subject of investigations, what we can limit ourselves to doing now is to hypothesize what could have happened immediately after the accident started.

The situation of the power plant before the disaster

The plant was in the testing after i extraordinary maintenance work. Specifically, maintenance included replacement of rotary valves installed at the end of the penstock. To be clear, the “taps” That they close the flow of water coming from the upper lake of Brasimone. These valves are installed in the well of the power plant immediately upstream of the hydraulic turbine-pump group.

Most likely the testing operations were concentrated precisely at check the correct functioning of these valves, both in ordinary and emergency (simulated) conditions. These valves are very heavy to move and in fact they are operated by hydraulic systems and given their strategic importance in the event of a hydraulic failure (they must interrupt the flow of water coming from Lake Brasimone) they must be able to close even in the absence of electricity or breakage of the hydraulic system, closing by gravity, or thanks to the pressure of the water itself which “pushes” on the valve parts.

What we can venture to say is that in all probability the correct intervention of these maneuvering bodies occurred, averting a much more devastating scenario, namely the uncontrolled discharge of water from Lake Brasimone directly into the power plant well. If this had happened, the power plant would have flooded within a few days a few secondswith the consequent leakage of a imposing column of water from the roof. The death toll would have been much worse. An accident of similar magnitude occurred on August 17, 2009 in the Russian hydroelectric plant of Sajano-Šhušenskaja, in that case 75 workers lost their lives and the plant was completely destroyed.


The possible dynamics of the accident

What can be assumed to have happened in Bargi probably presents a dynamic closer to another, little-known accident which occurred in Pont-Saint-Martin in Valle d'Aosta in 1958, caused byuncontrolled acceleration of the hydraulic unit which led to the subsequent destruction of the alternator. In technical jargon this condition is called “leak”.

Most likely at the moment of the accident the second group under testing (the first group had already been positively tested) was working in generationor the turbine drove the alternator to produce electricity moved under the flow of water coming from the Brasimone basin. For reasons related to the testing procedures (or due to a fault), at a certain point the alternator was instantly disconnected from the electricity grid.

By suddenly disconnecting the alternator from the mains, the balance of the mechanical forces acting on the shaft disappeared: the torque imparted by the turbine was no longer counterbalanced by the reaction torque produced by the alternator, with a consequent increase in the number of revolutions group rotation. Obviously, to avoid their escape, there are control and emergency systems which, in a completely automatic manner, intervene in the shortest time possible to slow down or at least divert the flow of water entering the turbine. Here, in this case, in all likelihood these defense systems did not intervene, and if this were the case the investigators will have to investigate the reasons why this protection failed.

The possible scenarios

Following this fact, two different scenarios could have occurred with the consequent and inevitable destruction of the group:

  1. Thrust bearing failure. the entire rotating group is supported by special bearings whose task is twofold: to support the hundreds of tons of weight of the group and to guarantee its rotation on its axis with minimum friction and minimum unbalance. Under the excessive rotation speed, one or more of these parts would have failed, unbalancing the shaft and causing the destruction of the alternator rotor with the high-speed projection of the various pieces that compose it into the engine room where it is contained. The fire that resulted from the accident could have occurred due to the spillage of the pressurized oil contained in the bearings which would have atomised when it came out violently, but the start of the violent fire was instead produced directly by the alternator which, when destroyed, generated sparks or electric arcs.
  2. Destruction of the alternator rotor. Instead, it could have been the alternator that failed before the bearings, and consequently the bearings, which released the pressurized oil and caused the fire.

Following the uncontrolled and violent release of mechanical energy, in conjunction with the fire, the attic between the floors –7 and –8 it collapsed, collapsing towards the lower floors. Both dynamics presented are also compatible with the witnesses' accounts and the images recorded immediately after the accident. The loud noise, the roar, the explosion as well as the smoke recorded in the images shot by the fire brigade helicopter escaped from the well cover. There was also a release of oil, in the images recorded from above by the Fire Brigade you can clearly see the patch of hydrocarbons floating on the free surface of the water of Lake Suviana.


The entry of water into the power plant

The entry of water into the power station following the accident may have occurred for two main reasons:

  • destruction of cooling system of the alternator which used an air-water heat exchanger; water taken directly, in an open circuit, from Lake Suviana.
  • In addition to the destruction of the alternator, the metal case containing the turbine-pump filled with water could also be damaged. This means that part of the water coming from both the upstream pipes and Lake Suviana flowed into the well, but only for a few moments, during the time needed for the rotary valve and the downstream discharge gate to close in an emergency.

Presumably the water could have entered precisely due to a combination of these two points.