One of the factors that plays a major role in destabilising the steam boiler and turbine is corrosion and deposition. They not only reduce efficiency of the boiler and turbine resulting in reduced output, but also reduce the effective life of the equipment.
Why Steam and Water Analysis?
In any plant running on steam, the purity of boiler feed water and steam is absolutely crucial; especially to steam turbine, steam boiler, super heater, condenser and other steam equipment.
Impurities associated with parameters such as pH, conductivity, dissolved oxygen, silica, sodium, phosphate, etc. gradually erode this critical steam equipment and lead to turbine failures / boiler shutdowns or in extreme cases, permanent damage to the equipment.
The cost of unplanned shutdown can be very high. Studies have shown that over 50% of the unplanned shutdowns take place due to inadequate analysis/control on impurities and the resulting corrosion and deposition. To prevent damage to this equipment due to scaling and corrosion, on line steam and water analysis is a must.
What is SWAS?
SWAS is nothing but a Steam and Water Analysis System that measures the quality of steam used by plants. A good SWAS effectively monitors contaminants like pH, conductivity, silica, sodium, phosphates, dissolved oxygen, etc. in process steam parameters and helps in maintaining healthy operation of the plant. Plants equipped with a well engineered, well maintained SWAS work with higher efficiencies, registering over 90% uptime.
Where is it used?
On-line monitoring of various boiler parameters like pH, conductivity, silica, sodium and dissolved oxygen has become quite common. These parameters are monitored for different services such as DM plant, CEP discharge, dearator, feed water to economiser, boiler drum, superheater, condenser etc. A scientifically designed SWAS package provides proper conditioning of samples before they are passed on to the analysers. Sample conditioning is done in the sampling system, while analysis is done in the analyser panel.
How Do Steam and Water Analysis Systems Work ?
A Steam and Water Analysis System (SWAS) contains two main sections – the wet portion also known as wet panel or wet rack, and the dry section which is also called dry panel or dry rack. Sample conditioning for samples to be fed to the analyser takes place in the wet section. The samples at tapping points are very hot and at very high pressures, and cannot be transferred directly to the analyzer as the sensor cannot handle samples at high temperature and pressure. Samples are therefore conditioned as per ASME PTC 19.11 – 2008 standard.
Sample Conditioning
- The process begins at the initial isolation valves and then to the DHx series sample cooler where the sample is cooled using the plant cooling water. The sample cooler coil metallurgy needs to be selected taking into consideration the quality of the cooling water.
- Once the temperature of the sample is reduced, the high pressure sample is then transferred to a pressure reducer (rod-in-tube type pressure reducer for pressure above 35 kg/cm2; needle valve reducer for lower pressures).
- After pressure reduction, the sample is transferred to a blow down valve for blowdown during initial startup and startup after long shut down to ensure clean samples are fed to the analyser sensor.
- The sample is then transferred to a secondary DHx series sample cooler to further cool the sample with the help of a chiller package which supplies chilled water as cooling water.
- The sample flow is shut off after the sample is transferred to a temperature protection device to protect the analyser sensor. The shut device should be equipped with a manual reset feature as per ASME PTC 19.11 standard. The contact output can also be connected to a distributed control system.
- The sample is then transferred to a rotameter with an inbuilt needle valve so the user can set the flow to suit requirements of the analyser sensor. Used in conjuction with a back pressure regulator, this ensures a constant pressure zone across the sensor.
- After all these steps, the sample is transferred to the analyzer for analysis.
The Forbes Marshall Solution
Forbes Marshall offers complete integrated sample conditioning and analysis systems that include the conditioning equipment and on-line analyzers. These systems are backed by a unique dual expertise in steam engineering and control instrumentation and cater to the most complex process requirements.
All the critical components of the Forbes Marshall SWAS such as Isolation Valve, Sample cooler, Pressure Reducer, High temperature Protection Valve, Back pressure regulators, High pressure stop valves & even the Chillers pH and Conductivity Analyzer are company manufactured / supplied.
Sample Isolation & Blowdown Valve
Forbes Marshall sample isolation valves and blowdown valves are designed for high pressure and temperature conditions in ultra-supercritical pressures in power boilers. The meet the design requirements of ASME B16.34 / EN 12516. for guaranteed low emissions to meet the 100 ppm maximum fugitive emission levels.
Sample Cooler
Forbes Marshall sample coolers are double helical coil-in-shell-type contra-flow heat exchangers. They are available with various options for coil material like stainless steel, inconel and other material, to meet application requirements, and, on request, with PTFE coated shell. The counter flow technique ensures that the sample and the cooling water flow in opposite directions, thus resulting in effective cooling. DHx series sample coolers are designed in line with ASME PTC 19.11-2008 and ASME Section VIII standards. Both mechanical and thermal designs for these coolers are validated by a third party. The design of these coolers is validated by Indian Institute of Technology, Bombay for required approach temperatures as per ASME PTC 19.11 standards.
Sample Pressure Reducing Valve
Forbes Marshall SWAS packages are equipped with Relisafe TM- PR sample pressure reducing valve (SPRV). This is a rod-in-tube type pressure reducing device which serves the combined function of pressure reduction and flow modulation. As the high pressure sample is forced to pass through the narrow annular region between the rod and tube, the frictional resistance causes the pressure to drop, while at the same time modulating the flow.
High Temperature Safety Shut-Off Valve with Manual Reset
Forbes Marshall's RelisafeTM-TP is a mechanical device which ensures protection against temperature overshoot during operation of the SWAS. It blocks off high temperature samples which could otherwise damage the delicate and expensive analyser sensors. It is designed as per ASME PTC 19.11 – 2008 standard.
Back Pressure Regulator
The Forbes Marshall back pressure regulator (BPR) ensures priority sample flow to analysers.
It works in a manner reverse to that of a pressure regulator, i.e. it provides constant upstream pressure conditions irrespective of fluctuations. The BPR ensures extremely stable conditions, resulting in accurate, reliable and stable performance for the analyzers.
4 Wire Dual Channel/Multi parameter Transmitter
The Forbes Marshall Aqua4Trans, 4 wire transmitter series is a versatile platform for various analytical parameters and provides state-of-the-art online water and process monitoring. It offers single as well as dual channel measurement for pH, conductivity, ORP, TDS and temperature, with flexible combinations and is suitable for field as well as panel mounting requirements. It is equipped with several unique features, covering the needs of almost every industrial process under water chemistry.