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The HEW gas engines have run without damage for 25 years

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Electricity, gas, water, long-distance heating – HEW HofEnergie+Wasser GmbH supplies the citizens of Hof, a city in the north of Bavaria, with the four classic utilities and many additional services. HEW is a subsidiary company of Stadtwerke Hof GmbH and supplies district heating directly to the home. To use the environmentally friendly district heating, thermostats in the home need only be set to the desired temperature.

District heating is economically and ecologically sound. With better efficiency levels than decentralised heat production, this form of heating can be adjusted to any changing need, is stable in terms of price and does not require lots of maintenance. Water which reaches the customers via a pipe system generally acts as the heat carrier. There, the hot water delivers heat in the heat exchangers to heat or warm water. The cooled water then flows back to the heating station where it is heated again. The district heating supply is therefore no different from a large central heating system. However, this way allows whole city districts to be supplied with ready-to-use heat, rather than just a single building.

HEW operates seven combined heat and power stations (CHP stations) in different city districts, each with a manageable district heating supply network. There are 11 natural gas modules and one microgas turbine being used in total. The power of the natural gas powered engines is simultaneously used to run the generators to produce electricity. This power/heat coupling allows up to 90% of the natural gas to be used and at the same time 160 million kWh electricity to be generated a year at the same time.

The first CHP station was commissioned in Hof in 1989 with an electrical output of 920 kW. And from then on, the first customers were supplied with district heating. The further six CHP stations were installed one after the other until 2005. HEW is celebrating the 25th anniversary of district heating in 2014!

HEW is in the lucky position of having a highly motivated team who „think outside the box“ which is very important when operating combined heat and power stations. Its employees also service the nine GE Jenbacher modules each with an average electrical output of approx. 530 kW, the MTU onsite energy modules with 230 kW and the Vaillant modules with 5 kW, as well as a microgas turbine with 30 kW. A modernisation programme has been running since 2009 due to the age of the systems. So far six of the nine GE Jenbacher and the MTU aggregate have been swapped for new modules from the respective manufacturers with an almost equal power rating. The rest of the aggregate are to be modernised in 2014.

Generally, a large inspection is due every 30,000 to 34,000 operating hours and the motor must be swapped after approximately 60,000 operating hours. However, the modernisations of past few months meant that these works could be avoided recently. Nevertheless, oil changes of course could not be dispensed with. Each module requires approximately 230 litres of SAE 40 low-ash gas engine oil. Due to the constant operating conditions when using natural gas, it is changed at fixed intervals and the oil service life is not completely exhausted.

Dipl.-Ing. (FH) Gaby Siliaz, group leader for construction and operation of the heat generation plant and CHP system: „Our philosophy has proven to be successful over the years. We have had no engine failures in 25 years! Being better able to plan maintenance during fixed oil change periods also means that personnel can be deployed in the best way possible. However, we also take samples that we send to OELCHECK to be analysed during every oil change. This means that should there be a claim we can prove, for example to the insurer, that we have completely monitored operation – the entire OELCHECK system is one unit. From sampling with tubing, vials, labels and mailers through to the lab report, which summarises the values in a manner that is easy to understand. The operator, who is an oil expert in the rarest cases, can quickly get an idea of the condition of the engine. We usually receive the results on the second working day after the sample is taken.“

The values for wear, oil oxidation and base number in the OELCHECK lab reports are particularly important for the HEW employees. The latter indicates the content of alkaline additives in the oil and provides information on how many acidic components can still be neutralised and rendered harmless by the oil. The parameters acid number and i-pH value, which are necessary due to the greatly fluctuating gas composition for landfill gas engines, only then play a role if biomethane is added to the natural gas – as is often common practice today.

Ultimately, however, the experienced tribologist‘s comment is crucial. Each OELCHECK lab report is issued with one. These substantiated comments were also very important criterion for the HEW employees when choosing OELCHECK as the analysis laboratory.

Gaby Siliax: „Our oldest analysis by OELCHECK dates back to 1993, by the way! We unfortunately searched for it in vain in our records, but we struck it rich with the customer portal www.laborberichte.com! Back then the gas engine oil was checked at 5 week intervals and Dipl.-Ing. Peter Weismann gave the oil the go-ahead for recycling in his comment.“

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