Steam and Water Analysis System (SWAS) is a system dedicated to the analysis of steam and/or water. In power stations, it is usually used to analyze boiler steam and water to ensure the water used to generate electricity is clean from impurities which can cause corrosion to any metallic surface, such as in boiler and turbine.
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Steam and Water Analysis System (SWAS)
Corrosion and erosion are major concerns in thermal power plant operating on steam. The steam reaching the turbines need to be ultra-pure and hence needs to be monitored for its quality. A well designed Steam and Water Analysis system (SWAS) can help in monitoring the critical parameters in the steam. These parameters include pH, Conductivity, Silica, Sodium, Dissolved Oxygen, Phosphate and Chlorides. A well designed SWAS must ensure that the sample is representative till the point of analysis. To achieve this, it is important to take care of the following aspects of the sample:
- Extraction
- Transport
- Conditioning
- Analysis
- Hygiene
Sample Extraction
To ensure that the sample that is going to be extracted for analysis represents the process conditions exactly, it is important to choose the correct sample extraction probe. The validity of the analysis is largely dependent on the sample being truly representative. As the probe is going to be directly attached to the process pipe work, it may have to withstand severe conditions.For most applications, the sample probe is manufactured to the stringent codes applicable to high-pressure,high-temperature pipework.
The selection of right type of probe is a challenge. Its use depends on the process stream parameter to be measured, the required sample flow rate and the location of the sampling point (which is also called the 'tapping point'). An important aspect of the sample extraction probe design is that the steam must enter the probe at the same velocity as the steam flowing in the pipeline from where sample (it can be steam or water) is extracted.
While transporting the sample, it is important that the sample meets least resistance. Hence joints and bends in the pipeline need to be minimal.
Sample Conditioning System in some countries is also called Sampling System, Wet Panel or Wet Rack. This is intended to house various components for sample conditioning. This may be an open rack and/or a closed enclosure with a corridor in between. The system contains sample conditioning equipment and a grab sampling sink. In this system stage, sample is first cooled in Sample Coolers, depressurized in Pressure Regulator and then fed to various analyzers while the flow characteristics is kept constant by means of Back Pressure Regulator.
The need to condition the sample exists, because the sensors used for online analysis are not able to handle the water/steam sample at high temperatures or pressures. To maintain a common reference of analysis, the sample analysis should be done at 25 °C. However, due to temperature compensation logic being available in most of the analyzers today, it is a practice to cool the sample to 25-40 °C. with the help of a well engineered sample conditioning system and then feed the conditioned sample to the analyzers.
However, if an uncompensated sample is to be analyzed, it becomes essential to cool the sample to 25 °C +/- 1 °C. This can be achieved by two stage cooling. In the first stage cooling (also known as 'Primary cooling'), the sample is cooled by using available cooling water. In most of the countries, cooling water is available in the range of 30 - 32 °C. This cooling water can cool the sample unto 35 °C(considering an approach temperature of 3 to 5 °C). A Sample Cooler is used to achieve this. Sample Cooler is heat exchanger specially designed for SWAS applications. Preferred Sample Cooler for Primary Cooling is Double Helix Coil in Shell type design providing contraflow heat exchange.
The remaining part of cooling (i.e. from 35 to 25 °C) is achieved by using chilled water in the secondary cooling circuit. A chilled water supply is required from the plant or else an independent Chiller package can be considered for this purpose along with SWAS.
The sampling system can be an 'Open Frame Free Standing' type design or a fully or partially closed design, depending on the choice of the user, the environment it is supposed to operate in & the criticality of operation.
In the sampling system, Sample Coolers play a major role in bringing down the temperature of hot steam (or water) to a temperature acceptable to the sensors of the on-line analysers. Some of the important design aspects of Sample Coolers are :
1. Preferably a sample cooler design should be double helix, coil in shell type, so designed as to provide contra-flow heat exchange. This makes the sample cooler more compact, yet highly effective in terms of heat exchange.
2. Sample coils made of stainless steel SS-316 are suitable for normal cooling water conditions. However, if the chloride content in the cooling water is high (more than 35ppm), then other suitable coil materials such as Monel or Inconnel need to be used depending upon the quality of cooling water.
3. A “built-in” safety relief valve on shell side of the cooler is a must, so as to prevent explosion of the shell in event of sample coil failure.
4. The sample cooler design must be meeting ASME PTC 19.11 standard requirements
After sample is cooled, the pressure of the sample must be reduced to meet the requirement of the sensors that receive this sample. Usually, the sensors like pH, Conductivity, Silica, Sodium, Hydrazine etc. require low pressure sample for healthy operation.
A Rod-In-Tube type pressure reducer is the most effective method of pressure reduction. As per the latest technology, a variable rod-in-tube pressure reducer with thermal and safety relief valve device (VRTS) is considered to be the most reliable and safe device. Variable Rod in Tube System (VRTS) is a system in itself that takes care of some important aspects of sample conditioning. The pressure reducer in the VRTS is rated for high temperature up to 550 Deg C and high pressure. There is no need of filters before the VRTS, as cleaning is on-line, without using any tools. For maintenance, no-shut-down is required. Cleaning can be done simply by rotating knob for 20 turns and re-applying the pressure. During high temperature cut-off, the operator can collect the sample through separate port. It's an energy saving device as it operates without any external electrical power. There is built-in safety for the operator, as a safety valve is built-in with the system.
Sample Analysis System in some countries is also called Analyser Panel, Dry Panel or Dry Rack. It is usually a Free-standing enclosed panel. The system contains the transmitter electronics, usually it is mounted on panels. In this system stage, sample is analyzed on its pH, conductivity, silica, phosphate, chloride, dissolved oxygen, hydrazine, sodium etc.
When it comes to Safety and/or Efficiency of Steam Turbine and Boiler in a power plant, Silica becomes one of the most critical factor to be monitored. Deposition of various impurities on turbine blades has been identified as one of the most common problems. Various compounds deposit on the turbine blades. Of all theses compounds, Silica (SiO2) deposits can occur at lower operating pressures also. Therefore Silica deposition is quite common in turbines than other types of deposits. Silica usually deposits in the intermediate-pressure and low-pressure sections of the turbine. These deposits are hard to remove, disturb the geometry of turbine blades and ultimately result in vibrations causing imbalance and loss of output from turbine.
Other important area of concern as far as Silica deposition is concerned is boiler tube. Silica scale is one of the hardest scale to remove. Because of its low thermal conductivity, a very thin silica deposit can reduce heat transfer considerably, reducing efficiency, leading to hot spots and ultimately ruptures.
Because of all these issues, it is extremely important to closely monitor Silica levels by using on-line Silica analyzers that can measure Silica levels to a ppb (parts per billion) level.