The bulk of the electricity we use across the globe is produced by steam turning turbines connected to electrical generators. According to the Steam-electric power station in Wikipedia:
In a Power Engineering International article, How to prevent boiler damage in power plants, Emerson’s Robin Hudson shares ways to apply technology to avoid damage to turbines and boilers.
Almost all coal, nuclear, geothermal, solar thermal electric power plants, waste incineration plants as well as many natural gas power plants are steam-electric. Natural gas is frequently combusted in gas turbines as well as boilers. The waste heat from a gas turbine can be used to raise steam, in a combined cycle plant that improves overall efficiency.
He opens highlighting the repercussions of not managing water levels in steam production correctly:
Should the boiler on a power plant run dry, or the turbine blades be damaged by water induction, the outcome will be dramatic and highly expensive.
Repairing such damage to a power plant can take months, if not years, to complete and can cause plant downtime costing millions in lost production and income.
And from a safety perspective:
If the [steam drum] level is too low, the boiler tubes can overheat, with the very real danger of a plant explosion. This may sound like dramatic stuff, but it highlights the need for safety devices in this type of application.
Traditionally, manually checking on site tubes on the boiler drum was performed on a periodic basis. In addition to being manually intensive, ongoing maintenance is another issue:
Given the critical importance of these water level measurements, electrical power producers required reliable dual-redundant technology for electronic steam/water gauging systems. Robin pointed to the Hydrastep system as one to meet these requirements. He describes the technology:
Mechanical/optical methods are regularly used for measuring levels within vessels, but the drawback with such devices is that they are susceptible to wear, and therefore also need regular checking and maintenance.
This increases maintenance costs and with 24/365 operation, downtime for such maintenance is not always possible, leading to insufficient checks and subsequent reliability issues. Sight glasses are still in use today, but only as the final verification method of drum level if all other methods are unavailable.
These systems consist of a number of electrodes installed within a water column attached to the boiler. The electrodes act as the ‘seeing eye’ above and below the normal water level.
Electrodes are arranged on each side of the column and connected to an electronics unit by separate specialist cables.
This arrangement provides redundancy against failure in any part of the system. The principal of measurement is fairly simple. The electronics are constantly looking for a change in resistance with respect to ground. A step change in resistivity between two adjacent electrodes identifies the water level.
Water and turbines don’t mix:
Water ingress into steam turbines may have catastrophic consequences and it is essential that automatic protection devices are in place to prevent this occurring. Even a small amount of water can cause enormous damage to the turbine blades, the cylinders and the housing. There is very little chance of an operator assessing a deteriorating situation quickly enough to judge whether or not water, water droplets or flash steam are present in bled steam lines.
Ways water can get in include high-pressure and low-pressure water level caused by tube leaks, de-aerator water level, undrained bled steam lines, and condensation during startup. Also:
An electronic water detection system, such as the Hydratect system, uses electrodes in steam lines to measure the change in resistance to find the unwanted presence of water.
Reverse steam flow in the bled steam lines can potentially carry water from heaters or un-drained low points to the turbine with consequential damage. Water ingression is not only a problem when the turbine is at operating speed; water flowing onto hot cylinders while the turbine is on turning gear can cause severe chilling with distortion or cracking of the cylinders.
Protection is critical to plant safety, but false trips are also a major issue, so any system must not just be completely reliable when it comes to detection and prevention of water in turbines, it must also prevent nuisance trips that reduce plant efficiency and throughput. The probability of Emerson’s Hydrastep missing an actual trip is less than 1 in 300 million. The probability of creating a nuisance trip is less than 1 in 10 million.
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