Article from Spirax Sarco’s Newsletter
Advances in microturbine technology promise a new way to tackle pressure reducing duties in steam systems ... and deliver substantial energy savings.
Pressure-reducing valves convert high-pressure steam from the boiler into steam suitable for use in other areas of the plant, but they can be something of a let-down when it comes to energy savings.
Most steam systems raise high-pressure steam in the boiler, then reduce the pressure before the steam is actually used. That's because high-pressure steam is more energy-dense, so it's more efficient to move from the boiler to the point of use than piping low pressure steam over long distances.
Conventionally, high-pressure steam from the boiler is lowered to a working pressure by using a pressure-reducing station comprising a valve and associated controls and ancillaries. However, with today's turbine technology, a new way to reduce pressure becoming available to many steam-using organisations, not just those in the power generation industry that use very high pressure steam to drive large electricity-generating turbines.
Passing plant steam through a microturbine enables operators to use the energy released by that pressure drop to supplement their electricity supply, rather than losing the extra energy as waste heat. A steam microturbine producing 300 kW of electrical power can generate typical cost savings of over £150,000 per year.
How does it work?
Just like the big turbines in power stations, a microturbine is coupled to a matching electrical generator that produces electricity as steam turns the turbine blades. While some turbines convert enough energy to condense the steam, microturbines designed for use in steam systems only convert enough energy to lower the pressure, so the steam emerging on the other side can still be used by the process. This is known as a back-pressure turbine.
Save money of course, the electricity generated isn't 'free' – fuel in the boiler is still burned to generate the steam in the first place. But energy that would otherwise be lost is now recoverable, reduces the need for electricity from the grid, cutting electricity bills and overall carbon emissions. Better still, users can save even more money when the cost of electricity is more than the cost of the fuel used to generate the steam. In the UK, for example, electricity from the grid typically costs around 9p/kWh, compared with 3p/kWh for natural gas.
The arrival of microturbines won't mean the end of conventional pressure reducing stations altogether, because the two technologies are normally used in parallel. A microturbine will often be sized for a baseline load (for example, in the summer), so a pressure reducing station will still be needed to cope with the extra steam when the load is higher (for example, in the winter). A pressure reducing station can also provide a useful bypass for the steam when the microturbine is undergoing maintenance, for example, although turbines are generally a low-maintenance technology.
Download original Steam Turbine PDF.