News 2007

Munich, February 27, 2007

High pressure - a key medium in hydroelectric power stations

Generally speaking it is quite understandable to pose the following question: "What is the medium high pressure used for in such power stations?" Normally everybody knows that in these kind of plants electrical energy is generated from water by means of turbo-engines with generators and hydraulic oil is used for the control system of these machines. The following article will help to explain to the technically interested reader why and how high pressure is used there.

BAUER KOMPRESSOREN GmbH, known all over the world for more than 50 years as leader in the field of high-pressure system technology for air and noble gases and world market leader in the diving sport sector and industrial breathing, has been delivering custom made and reliable compressor units to Voith AG for decades. These units are employed exclusively in hydroelectric power stations.

History - who is Voith?

When the J.M. Voith company was founded in 1867 it started off with the product lines paper-making machines and material technology and as the years went by it expanded further to include the sectors of water power and mechanical and hydrodynamic driving and breaking systems. It is in these sectors that Voith has won world-wide reputation.

It all began in a small locksmith’s shop in Württemberg when in 1825 J.M. Voith took over his father’s workshop with 5 workers in Heidenheim. The headquarters of the Voith business-group is still in Heidenheim today.

The holding company Voith AG is mainly supported by 3 columns: Voith Paper Technology, Voith Turbo and Voith Siemens Hydro Power Generation. The group has over 24,000 employees world-wide, of which approx. 2,400 work for Voith Siemens Hydro. In 2002 this joint business’ received orders for the year reached 652 Million €.

The air receiver

Hydroelectric power stations use various kinds of water turbines according to the performance and height of fall:

• Pelton turbines
• Francis and Kaplan water turbines and pump turbines

In order to adjust the power output of the generator and turbines due to changed flow conditions of the medium water, the turbine wheels have to be regulated.
These regulating devices are usually activated via oil-hydraulics.
For safety reasons the actuating power needed for a water turbine regulating device is stored in a so called „air receiver“. This air receiver is filled with approx. one third hydraulic oil and two thirds air and is therefore the system’s energy storage. For physical reasons the maximum allowable operating overpressure for these receivers is 64 bar. It’s geometrical volume depends on the size of the turbine. The receivers built by the Voith company have a water volume of 2,000 to 15,000 litres. Because these receivers don’t have a separating element between the mediums air and oil, the air is dissolved in the oil. Therefore every time oil is taken out of the system, air is taken out too. A high-pressure compressor automatically replaces at short notice the air that is then no longer available.

Depending on the requirements and conditions in the power station the compressed air supply can be realised in 2 different ways:

• a high pressure compressor system is installed in each air receiver or
• a central compressed air supply unit consisting of two high pressure compressors and an intermediate pressure container feeds all air receivers.

Flow scheme for hydraulic oil and hp air with compressor unit and air receiver

Selection of the HP compressor

VERTICUS 5 – Open Unit

Voith Siemens Hydro Power Generation has been purchasing high pressure units with delivery rates between 85 and 350 l/min free air from BAUER KOMPRESSOREN for more than 20 years. Over the years the flow capacities have been adjusted to suit the higher turbine efficiency, consequently the missing high pressure air in the air receiver can be replaced within 15 to 20 minutes of the compressor running. After maintenance work however the whole receiver is to be filled above the oil level. During this process the air cooled compressors and those equipped with a pressure oil supply have to run constantly, depending on the size of the receiver and number of containers, for 2-3 days on average. One alternative would be to employ more powerful industrial compressors when the whole air volume of all air receivers has to be filled within a short time after maintenance work. This is not advantageous however and cannot be recommended, because the compressor that would refill the air receiver would only run a few minutes and therefore its durability would be considerably reduced and would consequently prove to be inefficient. An exception to this however, would be if the compressor was also used to supply other pneumatic installations in the power station.

The models Voith has used over the years range from the small industrial version K13 with crash frame, include the Verticus I, III and take us to the present-day’s powerful and compact larger version Verticus 5. This new line covers the whole delivery volume range from 85 l/min to over 610 l/min free air.

The compressor unit is monitored by BAUER B-CONTROL, a programmable electronic compressor control with LCD. It is standard for the oil pressure and the final pressure to be monitored in these models. In addition the temperature and pressure of each single stage can also be monitored.

Compressed air assures advantages

2 sets high pressure compressor unit model I 23.0-30 are supplying 5 Francis waterturbines, each 250 kW. Project- hydroelectric power station in Ghazi Barotha, Pakistan

In older power stations there are still compressed air systems installed in the hydraulic supply control unit with operating pressures, for example of 40 bar. Because over period of time the residual water that enters the control circuit via the compressed air can cause corrosive damage to the valves and air receiver, the compressed air should contain as little humidity as possible.

Employing a refrigerative dryer the air pressure can be put through a final drying process to a pressure dew point of approx. 5-7 °C, which is equivalent to an atmospheric dew point of approx. -35 °C. This procedure is very expensive.

The same drying effect can be achieved with a 4-stage or 3-stage compressor without a refrigerative dryer by setting the compressor’s pressure maintaining non-return valve higher than the air receiver’s supply pressure. The supply pressure is approx. 80 bar. By setting the valve at approx. 150 bar, all stages of the block (the last stage at max. 150 bar) are drained. It was this consideration that lead Voith already years ago to integrate the BAUER KOMPRESSOREN 4-stage high pressure compressor model into their plans for new power stations or the modernization of existing ones. In practise 4-stage compressor units have proven to be fully up to the mark in this field.

Where is the trend leading to?

The construction of hydroelectric power stations is on the increase world-wide in order to supply the population and their industries with electrical power. This can be observed especially in Asian countries like for example China and India and most particularly in South America. In Europe increasingly more already existing plants are being modernized; this is happening less in the previously mentioned countries.

Conclusion

BAUER KOMPRESSOREN; reliable partners in the high pressure sector, apply the appropriate know-how to achieve their main priority; high quality products. This quality feature has been crucial in running a long-lasting successful mutual business partnership with their customer Voith Siemens Hydro Power Generation.

Author:
Dipl. Ing. Ludwig Kühlwein
Project Engineer