Steam & Water Analysis System
editROLE OF STEAM & WATER ANALYSIS SYSTEM (SWAS) IN POWER PLANT AUTOMATION
In any power plant using steam, there are a lot of hidden culprits that slowly eat into the Boiler tubes/Super-heaters or more importantly, the Turbines. These are impurities associated with parameters such as Dissolved Oxygen, Silica, Phosphate, Sodium, Chlorides & many others. These impurities ultimately result in erosion & corrosion which can finally lead to turbine failures / boiler shutdowns or in extreme cases, permanent damage to your equipment. The cost of unplanned shutdown can be very high. As per EPRI, USA findings, 50% of the unplanned shutdowns are due to inadequate analysis/control on impurities & the erosion / corrosion resulting from it.
As per another finding, corrosion, if not monitored properly, can result in a loss of turbine efficiency by @ 10%. This means a recurring loss of 10% revenue! Further, this loss can increase out of control may there be damages to the turbine or in a worst scenario - even a failure of turbine. This is not just a acedemic warning ! It has actually happened to many power generating turbines in India & elsewhere – turbines have ACTUALLY failed / suffered irrecoverable damage.
How this happens? Perhaps it can be better understood with anology of our own body. Corrosion, Erosion & Deposition are similar to Cholesterol levels in our body. Each one of us knows, it is essential to control Cholesterol in our body to appropriate levels, but HOW ? Unless there is a proven, reliable & accurate method to measure Cholesterol levels, one doesn’t know what levels exist. And, if we don’t know what levels exist, how can we control it? Anything that can be measured accurately, can be controlled effectively.
What is needed to control effects of cholastoral is a good regular method of monitoring it, so that we can take appropriate actions. Same way, to control effects of erosion, deposition or corrosion, it is important to have reliable & accurate method of on-line analysis. Good on-line analyzers supported by a well engineered sample conditioning system is a real solution. The complete package that comprises of sample conditioning system & on-line analyzers is called Steam & Water Analysis System (short form - SWAS)
With the help of a well engineered Steam & Water Analysis System (SWAS), one can monitor various dissolved impurities that bother the turbines & other steam equipment to ppb (parts per billion) levels - for 24 hours a day - 365 days a year!
Forbes Marshall is perhaps the largest manufacturer of SWAS packages in India. Forbes Marshall (FM) can offer an integrated sample conditioning & analysis system that includes the conditioning equipment & world class on-line analyzers. Backed up by state of the art technology from their JV partners M/s ‘Hach Ultra Analytics, Europe’ for on-line analyzers, Forbes Marshall (FM) offers a total solution for steam & water chemistry.
Any good SWAS package needs expertise in Steam Engineering as well as Control Instrumentation'. Forbes Marshall is perhaps the only company in India today that has this unique combination.With an installed base of over 350 SWAS packages, Forbes Marshall (FM) today are in a position to offer solutions to most complex requirements in Steam & Water Analysis.
How does the concept work ?
In the power industry control of water purity is pre-requisite for the safe and efficient operation of the boiler plant. Typically up to 20 different samples per boiler unit are withdrawn from the various parts of the steam / water cycle, some of these serving several analytical instruments, others being for occasional manual sampling only. Sample conditions as severe as 200 bar at 560°C are now common and demand equipment of highest quality and integrity. Equipment used vary in complexity from a single sample probe with cooler, pipe work and valves for manual sampling to fully automatic multi stream sample conditioning consoles for centralized monitoring.
In today’s power industry, on-line monitoring of various boiler parameters has become quite common. In old days, people used to monitor these parameters in laboratories. Grab samples used to be taken and the sample tested in the laboratory. This method involved a lot of provision of errors. The errors could be from atmospheric contamination, from use of improper equipment for carrying the sample or the human errors due to repetition. The On-line analyzers when came in to vogue, most of these errors were eliminated.
Another aspect was the sample conditioning. After development of On-line analyzers, it became essential to condition the sample. This was required because the sensors used for On-line analysis were not capable of handling the water / steam sample at high temperature or pressure. To bring in uniformity it was made mandatory to cool the sample between 25°C to 40°C.
The next development in the analyzers was the temperature compensation concept. With this new concept it became easier to monitor the parameter at any temperature and interpret it as if it was at a particular temperature, say 25°C.
Today we have state of the art equipment to take care of all the requirements of sampling, sample conditioning and sample analysis. The most popular equipment for on-line analysis is called the steam and water analysis system (SWAS).
Any SWAS package necessarily consists of two parts, namely the conditioning unit and the analysis unit. Since majority of the sample is handled in the former, it is called as wet panel. The analysis of the samples is done in the latter, which handles mostly the signals and gives various outputs for control or alarm etc. This part is called the dry panel.
The parameters, which are most commonly monitored in power stations, are conductivity, pH, dissolved oxygen, silica, hydrazine, and sodium. Others include alkalinity, hardness, calcium, chloride, phosphate, dissolved Ozone and so on. Let us see the significance of each of these parameters.
pH :
The steam which is finally used for power generation, i.e. the steam which goes to the turbines has to be ultra pure. This implies that the water used for generating this steam should be in its purest form. Monitoring the pH value of the feed water gives direct indication of alkalinity or acidity of this water. The ultra pure water has pH value of 7. In steam circuit it is normal practice to keep the pH value of feed water at slightly alkaline levels. This helps in preventing the corrosion of pipe work and other equipment. Typical points in the steam circuit where pH should be monitored are : Drum water, High pressure heaters, Make-up condensate, Plant effluent, Condenser, Cooling water.
2) Conductivity :
Conductivity measurements give indication of contamination of water / steam with any kind of salts. These salts can get added to the water / steam from atmosphere or due to leakages in heat exchangers etc. The conductivity of ultra pure water is almost close to zero(say 1..2microsiemens/cm), while with addition of even 1ppm of any salt, the conductivity can shoot up to even more than 100 micro siemens/cm. Thus conductivity is a very good general purpose watch dog which can give a quick indication of plant malfunctioning or possible leakages. Typical points in the steam circuit where conductivity should be monitored are . Drum steam, Drum water, High pressure heaters, Low pressure heaters, Condenser, Plant effluent, D.M. plant, Make-up water to D.M. plant.
3) Silica :
The presence of silica in the steam and water circuits of power generation plant is associated with a number of problems both in the superheater and turbine sections.
The solubility of silica in stream increases with pressure. The presence of silica in the steam can lead to deposition in superheater tubes and on the turbine blades. Small deposits on the turbine blades can result in a loss of efficiency, whilst larger deposits can cause permanent mechanical damage.
In order that the turbines are operated at maximum performance, continuous monitoring of silica in steam, boiler water and feed water is highly recommended.
The monitoring of anion and mixed bed ion exchanges safeguards and optimizes the operation of demineralization plant. Silica analysis is required at this stage also.
Thus the typical points in steam circuits where silica analysis is required are High pressure and Low pressure turbines, Drum steam, Drum water, CEP discharge, Make-up water, D.M. plant, Supply water to D.M. plant.
4) Dissolved Oxygen :
Within a temperature range of 200...250°C (feed water), dissolved oxygen causes corrosion of components and piping, i.e., Condensers, Low pressure preheaters, Feed water tanks, High pressure preheaters and Economizers. The resulting pitting may eventually cause puncturing and failures.
Dissolved oxygen also promotes electrolytic action between dissimilar metals causing corrosion and leakage at joints and gaskets.
To minimize corrosion under alkaline operating conditions, mechanical deaeration and chemicals scavenger additives are used to remove the dissolved oxygen. An analytical check of process efficiency, therefore, is essential.
Dissolved oxygen monitoring is imperative in power stations using neutral or combined operating conditions (pH 7.0-7.5 or 8.0-8.5). The typical points in steam circuit where dissolved oxygen monitoring is required are . Condenser outlet, L.P. heaters, Economizer inlet.
5) Hydrazine :
The use of hydrazine as an oxygen scavenger and a source of feed water alkalinity has well known advantages e.g. : a) It prevents frothing in the boiler. b) It minimizes deposits on metal surfaces.
In addition to its oxygen-scavenging function, hydrazine helps to maintain a protective magnetite layer over steel surfaces, and maintain feed water alkalinity to prevent acidic corrosion. The nominal dosage rate for hydrazine in feed water is about three times its oxygen level. Under dosing of hydrazine leads to increased corrosion; overdosing represents a costly waste. Monitoring the dissolved oxygen levels is not sufficient to control the optimum concentration because its provides no measure of any excess hydrazine. The typical points in steam circuit where hydrazine monitoring is required are . Re-heaters, Economizer inlet, L.P. heaters.
6) Sodium :
The measurement of sodium is recognized - among other chemical parameters - as an effective telltale to reveal the condition of a high-purity water/steam circuit.
The presence of sodium signals contamination with potentially corrosive anions, e.g. chlorides, sulfates etc. Under conditions of high pressure and temperature, neutral sodium salts exhibit considerable steam solubility. NaCl and NaOH, in particular, are known to be associated with stress corrosion cracking of boiler and superheater tubes.
The measurement of sodium, acting as a carrier of potentially corrosive anions, is now recognized as an effective means to monitor steam purity.
The ubiquitous character of sodium in the environment makes it a useful indication to reveal possible leak conditions within the circuit, particularly in the condenser section where the measurement of sodium detects cooling-water leaks with a much higher sensitivity than conductivity measurements. Such controls assume an even greater importance with stations having no condensate polishing. The typical points in steam circuit where sodium monitoring is required are D.M. plant, Condensate pump, Condenser, Drum steam, D.M. plant output.
Monitoring of other parameters such as alkalinity, hardness, calcium, chloride, phosphate, dissolved Ozone is also required depending on the size of the plant and the quality of water / steam equipment.
THE NEED OF SAMPLING :
All the analyzers mentioned above work efficiently if the temperature, pressure and flow conditions of sample are maintained properly. Further, the sensors are capable of handling the water/steam samples at particular temperatures and pressures only. This necessitates the use of the sample conditioning system i.e. Wet panel.
Sample conditioning systems have several functions. The sample must be:
Withdrawn from the process, Transported, Conditioned, Introduced into the analyzer, and Disposed of …
All without changing it’s composition !
Probably the most common problem in sample-system design is the lack of realistic information concerning the properties of the process at the sampling point. Also, while the sample is being conditioned, utmost care has to be taken to see that the sample does not get contaminated and
it is truly representative. The condition of sample reaching at the end point i.e. the analyzer should match exactly with that at the tapping point. Thus the first important component in the sampling system is the sample extraction probe. The correct choice of probe should never be over looked, since the validity of analysis will be questionable, if representative samples are not withdrawn. Being directly attached to the process pipe work, the probe may be subject to severe service conditions, and for most applications, this item is manufactured to the stringent codes applicable to high pressure, high temperature pipe work. The type of probe to be used will depend on the process stream parameter to be measured, the required sample flow rate and the position of the sampling point in the system.
SAMPLE EXTRACTION PROBES :
As a general rule, when sampling from pipes for suspended solids, one of the various types of isokinetic probes is used. This class of probe is designed to ensure that the sample enters the port(s) at the same velocity as the main process stream, thus reducing kinetic segregation of suspended particles to a minimum. Recent studies by the CEGB (Central Electricity Generation Board, UK) suggest that a more important factor in obtaining representative particulate samples may be the maintenance of a sufficiently high transport velocity in the sample line to prevent hideout of the suspended species. However, this type of probe continues to be in demand for these applications. Isokinetic probes may be of single port, multiport or capillary types, and should be installed with the port(s) facing upstream into the oncoming flow.
SAMPLE PIPE WORK AND VALVES :
Sample lines should be kept as short as possible consistent with the convenient siting of equipment, and the use of unnecessarily large line diameters should be avoided, as undue size and length lead to a delayed response and the damping-out of transients. Sample pipe work must be of a material which will not react chemically with any species in the sample. For the majority of applications, Stainless steel AISI 316 is standard. The number of bends and joints should be kept to a minimum, and if possible sample lines should fall continuously from the sample point to the sample conditioning system. Care must be taken to ensure that the lines are not obstructed, particularly where joined by welding or where bent on site.
The important consideration in the choice of valve are :
1. Suitable pressure / temperature rating for inlet isolation, including the highest duties to be found in modern power plant.
2. Rugged mechanical design to withstand severe site conditions. Sensitivity and ease of operation.
3. Specific designs for throttling applications.
SAMPLE COOLERS :
The sample coolers form the heart of the sampling system. These should be preferably coil-in-shell type design. This should make use of counter flow type of heat exchange principle. Double helix coils are in vogue nowadays. The coolers with double helix type coil design give better approach temperature than conventional single coil designs, apart from being compact in size. A built-in shell relief valve takes care of possible mishap due to high pressures caused by coil rupture etc. These should be rated to severe temperature and pressure conditions existing in power plant and sized to take care of the heat loads. The design standards to be followed can be GDCD code 163 & 234.
PRESSURE REDUCTION :
An important aspect of sample conditioning lies in making suitable provision to ensure that the analyzers can never be subjected to a pressure higher than the safe limit. With source pressures as high as 250 bar, this warrants serious consideration, and the use of components of high integrity. A pressure regulator that can maintain the down stream pressure at constant set limit irrespective of up stream fluctuations becomes an ideal choice. On closing fully, the regulator should ensure zero flow condition and should withstand the total upstream pressure. A built-in safety valve to this pressure regulator makes the unit fool proof. Simple pressure reducing devices are not adequate, as regulation of pressure is equally important.
HIGH TEMPERATURE PROTECTION :
In the event of cooling water failure, on-line systems require a reliable means of immediate sample shut off and alarm, before high temperature samples can reach instrumentation. Temperature sensing can be done by means of electrical contacts on the outlet temperature gauge, or by a dedicated sample thermo switch. The electrical output of the above can be used for sample shut off, by means of solenoid valve.
In the same manner, a pressure switch also can be added for additional safety.
CHILLED WATER :
Use of chilled water becomes necessary when the cooling water available on site is not capable of cooling the sample to the temperatures required by the analyzers. If available cooling water is upto 40°C, a well designed sampling system can ensure sample outlet temperatures of 45°C. Most of modern analyzers can work comfortably with these sample temperatures. However, if cooling water temperature exceeds 40°C, use of chilled water becomes necessary. Normal practice is to make use of available cooling water for extracting as much as heat as possible from the sample and using the chilled water for the removing the remaining fraction of heat. Chiller with isothermal bath is a compact unit which saves a lot of space. In this unit, the chiller unit provides the chilled water to a container called isothermal bath. In this container, the sample coils are immersed. Thus one can save use of individual heat exchangers. The SWAS vendor who can manufacture chillers / isothermal baths is an ideal choice looking into the aspect of single point responsibility
To conclude, it is essential to control the parameters that are silent killers of your steam power plant. Anything that can be measured accurately, can be controlled properly. Hence a well designed Steam & Water Analysis System is must for every power plant using steam.