Munich’s main airport electrical power requirement is covered about 44% through the grid. The remaining 56%, about 122 000 MWh p.a., is produced at the airport by nine power generators. As combined power/heating/cooling machines, the seven duel fuel engines and two spark ignition gas engines used for this purpose, operate not only for power generation: the released heat is also used for the central heating during the winter period, whereas in summertime the air-conditioning benefits from the lost heat due to the employment of absorption cooling machines.
During the warm-up phase, which lasts for a few minutes, duel fuel engines operate solely on diesel. After this, a natural gas/air mixture is fed into the engine which is ignited by the diesel fuel element, its proportion now reduced to 10%. As a result, the power generators can be operated more efficiently. Sliding gate valves with digital positioners have recently been installed on the first two power generators to regulate the natural gas/air mixture.
Inherent control performance
The maximum stroke of a sliding gate valve is a mere 8 mm. This facilitates extremely short opening and closing times (optionally 10 msec) for the complete stroke. Through a resolution of 0,1% of the stroke position, a dynamic response is obtained which not only improves the control quality but forms the basis for control circuits with very short reaction times. This is the key to highly efficient and precise control operations.
With its special design based on two slotted discs sliding against each other and forming a seal, sliding gate valves combine high control accuracy with low leakage rates. Also the throttling element – the slotted discs that slide against each other – suffers scarcely any wear so that long service lives can be achieved under extreme conditions, an inherent feature of the system design.
Schubert & Salzer Control Systems offer a positioned range in 11 different versions for the sliding gate valves. All versions can be configured via a PC interface by means of the DeviceConfig graphic configuration software and are therefore easily adaptable to the particular application case.
Sliding gate valves provide significant economic advantages
As a result of retrofitting to sliding gate valves with digital positioners, the engines in the generating plant at the airport can be operated directly from the control room at maximum efficiency. The exhaust gas temperatures are measured continuously to optimise the combustion control of the natural gas/diesel/air mixture and the sliding gate valves accurately adjust the quantity of natural gas needed. Thus, the high control precision of the sliding gate valves minimises the consumption of natural gas as well as the ammonia used to clean the exhaust gas.
Alongside optimisation of consumption and less stress in the operation of the power generators, the substantially tighter control and operating performance represent a further advantage of this retrofit. Before, the gas supply was controlled by butterfly valves with a mechanical stop. These could only be adjusted in the engine room. Thanks to the sliding gate valves with positioners, the amount of gas/exhaust temperature of the power generators can be precisely controlled and corrected if necessary from the control centre.
Retrofitting made easy
In each power generator, two butterfly valves were replaced by sliding gate valves. Since the dimensions of both valves are identical, only marginal mechanical adaptations were necessary.
Also, if operating under emergency power which is on diesel fuel only, the control valves can be closed very quickly to ensure a smooth switch-over to diesel only operation.
The excellent experience in the use of sliding gate valves to control the gas flow in the first two duel fuel engines is providing the basis for the retrofitting of the five remaining power generators.
For more information contact Keith Hedges, Macsteel Fluid Control, +27 (0)82 575 9354, [email protected], www.macsteel.co.za
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