Addressing pressure loss and oil leakage in Kaplan turbines and the impact on efficiency

Performance

The figure below is a graph for a Kaplan turbine reference test. This graph indicates performance curves for several different blade positions (dashed curves, called partial helices) as well as a solid curve (the correlation curve) representing the optimal blade position for best turbine performance, depending on the guide vane position. For optimal performance, the blades must constantly follow the correlation curve, which requires the angle of the turbine blades to be adjusted by the hydraulic system according to the position of the guide vanes.

Profitability

An oil leak in the system that regulates blade position can result in an incorrect angular position of the blades, generating, for example, an efficiency loss of 1% per deflected degree of the angular position. The blade position no longer coincides with the gate/blade angle correlation curve. This misalignment can also lead to increased cavitation, more intense vibration and accelerated aging of turbine parts. Decreased efficiency is also visible in Figure 1, above, since for a given relative power output increase, efficiency will decrease, because it would then follow a partial helix curve. Knowing that a Kaplan turbine can produce up to 200 MW, we can evaluate the financial loss for an average 50 MW turbine with 92% efficiency at maximum output. Let us assume an electricity cost of $0.08/kWh and a turbine utilization factor of 0.75, given the water that is available and the needs of the network. In this case, the financial loss would amount to $262,800 per deflected degree per year:

Other maintenance issues

Efficiency issues are one of the many results of problems associated with oil leakage. In some cases, oil leakage does not result in problems with runner blade positioning. However, it can lead to other types of malfunctions, such as:

• Oil escaping into the environment.
• Mechanical parts and mechanisms deteriorating.
• Cavitation of turbine blade surfaces.
• Protection devices activating and so preventing start-up and operation of the turbine/alternator group.
• A dangerous drop in oil pressure in the wicket gate.
• Part wear caused by the constant movement of the servo valves, servomotors and runner blades to compensate for oil leakage.

Over time, some issues can lead to a complete system failure, resulting in costly unplanned major overhauls, while others may demand unplanned maintenance.

Low-cost diagnostics can lead to major savings

Low-cost diagnostics are possible and can quickly yield a return on investment. However, operators often hesitate to invest in corrective maintenance, since it is not always easy to quantify the benefits when the problem is identified. On the other hand, once the problem worsens, it becomes easy to determine its impact on turbine performance and to decide whether a repair is economically viable or when it will become so. Such diagnostics can also optimize the service life of the high-pressure oil circuit and other components that require planned overhauls by helping to develop a mitigation plan and ensure that replacement parts are readily available.

Finding the location of a leak is often challenging. Many types of leaks can occur in a hydraulic system. It is important to take an iterative approach in order to gradually rule out each possibility:

• The speed governor
• Pump and hydraulic unit components (HPU)
• Gasket seals and oil head components
• Gasket seals and runner blades
• Locating the problem using instruments added to the hydraulic system
• Sampling oil at different points in the system
• Checking for possible leakage inside the turbine
• Noise and heating of components and pipe work
• Reviewing maintenance reports and most recent modifications made to the system
• Reviewing data recorded by the PI/SCADA system, to help identify the source of an issue.

The BBA team has gained solid expertise in this area, having participated in diagnostic testing both in power plants and remotely, to identify various types of oil leakage issues in Kaplan turbine runner-blade actuating systems. Our proven methodology makes it possible for clients to maximize the use and profitability of existing equipment.