|Credit: Emerson Process Management|
Boiler damage can be hugely costly, both in money and time. Employing a suite of protection systems can prevent unnecessary downtime, writes Robin Hudson
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.
Modern boilers used in power generation are designed to provide clean dry steam.
On top of the boiler sits a vessel known as a steam drum. It is here that the steam/water level is monitored. If the level is too high it can generate wet steam, which may in turn lead to turbine blade erosion.
If the 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.
All national legislatures require monitoring and indication of feed water levels in steam generation plants. Drum level indication is normally indicated both locally to the drum and remotely in the control room where the plant engineers will carry out an emergency shutdown should the conditions require it.
Very accurate monitoring of these points needs to take place to avoid false alarms that could force a plant to shut down with a potential significant loss of revenue.
Traditionally, sight glasses (water gauges) were fitted to the boiler drum. However, these can suffer from reliability issues, requiring intensive maintenance programmes.
The drum is normally well away from the control room, meaning that a secondary form of surveillance equipment is needed to report the level back to the engineering team. For these devices to be used effectively they must be monitored 24 hours a day, 365 days a year.
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.
The power industry therefore demanded reliable dual redundant technology and customer-led development saw the birth of electronic steam/water gauging systems.
Emerson was at the forefront of this technology. Its Hydrastep system is seen by many as an industry standard and is used widely to monitor and control water levels within boilers.
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.
The important thing for the end user is to be able to set alarms and trips to aid the operations teams.
Alarm relay outputs deliver high- and low-level alarm indications or trips. Local and remote displays provide operators with high visibility of boiler levels.
An alarm/trip also occurs should a fault occur within the system or its associated wiring or power supply. This fault-tolerant operation eliminates the need for routine testing. Fault conditions are also shown on the displays.
The reliability of these systems can be dependent upon the quality of the water in the system. Whilst this is usually clean, the presence of dirty water can foul the electrodes. However, in more sophisticated systems this will not cause a fault or a trip.
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. This is complicated by the fact that a manually initiated trip of the turbine may further aggravate the situation, as the decay of pressure in the turbine stages to vacuum can potentially cause reverse flow.
Water can also reach the turbine from various feedwater plant sources and under a number of operating conditions.
For example, if the water level is too high in either the high-pressure or low-pressure feedheater, which would be caused by tube leaks or the failure of the drainage system.
A high water level in the de-aerator is another potential source. If there is a mismatch between the inflow and outflow the vessel can flood. In each of these cases the water may flow, via the bled steam lines and against the steam flow, towards the turbine.
Un-drained bled steam lines are yet another source. Wet steam can deposit water on the pipework walls, and condensation can occur at bends in the pipework and at valves. Condensation is also a problem during startup when the steam lines are being warmed.
If a unit trip occurs or there is a sudden load reduction, this can result in a pressure reversal. During a trip, the high pressure (HP) turbine pressure decays rapidly and the intermediate/low pressure (IP/LP) falls to condenser vacuum almost immediately. In contrast, the pressures in the feed system change relatively slowly. Large pressure differentials are created, which will tend to stimulate flow towards the turbine from the feed system.
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.
|Hydrastep monitors and controls water levels
Credit: Emerson Process Management
Turbine water induction protection
The different resistive properties of steam and water may also be exploited in turbine water induction prevention (TWIP) systems.
By installing electrodes in steam lines and measuring the resistance, the unwanted presence of water can be detected, allowing the appropriate safety measures to be taken. An electronic water detection system, such as Emerson’s Hydratect, provides high levels of reliability for detection of water or steam in lines.
Each electrode is specified as being either normally in steam or normally in water. An alarm/trip output is given should the electrode detect a ‘not normal’ condition. ‘Steam normal’ is used for high water level detection in steam drums, feedheaters and in turbine water induction prevention systems on steam lines. ‘Water normal’ is used for low water level detection. A level switch installed on the drain pot in the super-heated steam line will detect the level of condensed water and operate a drain valve, protecting the turbine.
|The Hydrastep system
Credit: Emerson Process Management
For applications requiring the detection of either steam or water, resistivity measurement is a proven technique. Using an electronic method to indicate water level or differentiate between the presence of steam or water offers a very high level of system self-checking and integrity compared to mechanical methods since there are no moving parts. This greatly reduces the requirement for routine maintenance.
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.
Robin Hudson is Applications Support Engineer at Emerson Process Management
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