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Monitor transients and stay in control of over-pressure risk

Ruptures of penstock, manifold and turbine casing. Lack of monitoring of the transients causes poor control of internal pressure fluctuations. However, such fluctuations are involved in over 40% of the ruptures in hydro power plants. This impacts the penstock, manifold and turbine casing.

Causes are multiple but always associated with the hydraulic functioning. Drifts of the settings managing the turbine start-up and shut down operations, defaults of key components, are commonly involved in these events. In the absence of monitoring, these risky trends go unnoticed until they turn into causes of rupture.

Despite their mechanical harmfulness, transient behaviors are not monitored continuously. Only a few pressure transient measurements are made during the life of the equipment. But these tests only make it possible to reproduce one single hydraulic phenomenon: the hammering of mass. Thereby one forgets to quantify all the other types of hydraulic phenomena frequently involved in penstock failures. Although post-rupture structural analyses are achieved, these analyses are only carried out after the penstock’s damage. So, it is often too late to have a thorough understanding of the hydraulic phenomena involved and related loading on the damaged penstock.

Finally, it is often difficult to take from these ruptures a reusable experience to improve the operation safety of penstocks.

Allowing a continuous and easy transients monitoring, HYDROSURGE  is designed to automatically diagnose typical transient regimes

MOTRHYS field experience derives from over three decades of measurement and diagnose of faulty hydraulic behaviours.  This experience has been confirmed by the monitoring of transients of a fleet of penstocks. We thus showed that the use of a simple hydro acoustic model is relevant in order to analyze a pressure transient signal. Especially for early detecting and diagnosing transient problems affecting a penstock. This approach shows that any pressure overloads can be broken down into one or more of the following phenomena impacting the whole length of the piping system:

The four phenomena potentially responsible for penstock rupture

  • Over pressure (mass water hammer or wave water hammer) which is created when the intake flow drops abruptly. If the closure time is higher than the reflecting time of the penstock, a mass water hammer happens. If the closure time is below the reflecting time, a wave water hammer happens.
  • Water column separation which occurs during a quick opening of a valve or a fast turbine start-up. If the opening time is below the reflecting time of the penstock. This creates a vacuum cavity in the pipe.  This cavity then implodes when the pressure recovers causing extreme pressure.
  • Resonance or self-sustained pressure oscillations. This may occur during the no-load unit operation. Or during rough operating of Francis turbine. Or even when the plant is completely stopped due to guard valve leakage instability.

HYDROSURGE also improves low head plants safety 

Monitoring of transients is a key tool to control reverse pressure surges in the turbine draft tube.

Tripping of reaction hydraulic turbines may lead to a sudden increase of the draft tube rope vortex volume, as well as the separation of the draft tube water column. Reverse water hammers may thus happen when the pressure recovers, the turbine casing, and runner exhibiting high-pressure surges.

Kaplan turbine's draft tube water column separation

These well-known phenomena are taken into account during the turbine design stage. Mitigation measures are developed such as the adapted setting of the speed governor, venting valve implementation to prevent the occurrence of void cavities in the turbine casing, etc. However, drifts in speed governor settings or a malfunction of the venting device are common. These faults can slowly but surely worsen the transient behavior in the turbine draft tube. Without continuous monitoring, thorough understanding and estimation of these phenomena are not managed. Major damages such as blade failures of axial runners or shaft line lifting with thrust bearing damage are thus encountered in low head plants.

The improvement of penstock's safety by the monitoring of transients It is possible to detect, quantify and predict all the dangerous hydraulic phenomena affecting penstock and turbine structures.

That is what continuous monitoring over the entire pipe system length brings.

By embedding operation processes expert rules, advanced signal processing methods, HYDROSURGE provides key insights on the root cause of the phenomena. MOTRHYS suggests corrective or preventive actions. So allowing to improve equipment lifetime, reducing production downtime and ultimately improving the operator bottom line.

HYDROSURGE is a factor of safety and efficiency