A steam and water analysis system (SWAS) conditions, analyzes and monitors the chemical properties of the steam and water used to generate electricity. One of the primary factors that affect the results from a SWAS is sample temperature. Controlling temperature in a SWAS requires the correct equipment to eliminate analysis compensation and sources of error in measurements to improve chemical feed accuracy and cost savings.
Control temperatures with the right equipment
Sample temperatures are controlled by cooling water temperature, cooling water flow and sample flow rate. However, cooling water temperatures vary wildly between seasons, regions and sources. While analyzers have standard temperature compensation factors defaulted in the software, solution temperature compensation (a separate factor from standard temperature compensation due to the disassociation of ammonia in pure water) is difficult to predict with varying temperature and chemical feed concentrations.
These factors can be controlled with properly sized cooling water isolation skids and secondary cooling by a closed loop from a temperature control unit (TCU), eliminating analysis compensation and sources of error in measurements. Eliminating error in critical analysis improves chemical feed accuracy, often resulting in significant cost savings.
Close Temperature control
While temperature compensation equations have gotten more sophisticated for various boiler chemistry treatment options, the inaccuracy of both the temperature measurement and the solution compensation (which depends on the exact chemical composition of the water) can introduce errors. When monitoring high purity water, for example, the correction can be as great as 3% for every 1°F away from 77°F.
Many sample conditioning systems are designed with close temperature control. Close temperature control of samples in power plants is considered most serious in the following general applications:
- Nuclear power plant secondary, turbine and feedwater systems
- Fossil plants with high boiler water purity, typically 1500 psig-2000 psig or higher
- High cooling water temperature, such as 95°F or higher
- Varying cooling water temperature over the daily cycle, such as when cooling tower water is used without temperature control
Primary sample cooling
A sample cooler is a compact, high-efficiency heat exchanger used to condense steam samples and to cool steam and water samples.
Primary cooling is the first step in the sample conditioning process. A primary cooler’s goal is to reduce the steam or water sample to a safe temperature, generally between 100°F-122°F (30°C–50°C). Primary sample coolers are designed to remove the majority of the heat from incoming samples and can reduce the incoming sample temperature to within 5-10°F (3-6°C) of the cooling water supply temperature.
To minimize deposition and pressure drop, it’s important to place primary coolers as close to the sample extraction source as possible. According to EPRI, primary coolers must be located upstream of the pressure reducing valves. Temperature reduction of the sample must always be completed prior to pressure reduction to prevent flashing of the sample to steam. Sample lines leading to the inlet of a primary sample cooler should be insulated to protect operators and sample coolers should be sized properly to prepare the sample for secondary cooling.
ASTM and EPRI guidelines recommend cooler coils with a corrosion resistance at least as good as 18% chromium and 8% nickel. 316 stainless steel is a common material selected for this application as well as sample line and the cooler’s coil. Cooler shell material should be selected to limit exterior corrosion and pitting.
Alternate coil and shell materials should be considered for applications with cooling water constituents such as chlorides and dissolved oxygen, as in potable water systems and cooling towers treated with biocides. This is of concern for high-temperature samples, such as boiler water or super-heated steam. Alternate materials for the coil include Inconel 625 and cupronickel shells.
A widely accepted, high-efficiency cooler design includes a coil in shell arrangement with internal baffles. This design provides efficient heat transfer, compact size, cleanability and relatively low cooling water flow requirements.
Secondary sample cooling
Secondary coolers are used to adjust the final sample temperature from as high as 140°F (60°C) down to 77°F ± 1°F (25°C ± 0.5°C). They can reduce the sample temperature to within 1°F (0.5°C) of the cooling water supply temperature, despite variations in flow rate and incoming temperature of each line. In general, individual high-efficiency sample coolers provide much better control of final sample temperature than other secondary cooling methods.
One widely used method for secondary cooling is individual sample coolers. These provide individual, high-efficiency sample coolers for each sample stream. The secondary cooler bank is supplied with chilled water typically controlled to about 75-76 °F +/- 1°F (23.9-24.4 °C). For best results, a chiller with ultra-precise temperature control (+/- 0.5°F) should be used. Special chillers with hot gas bypass help maintain very close temperature control and can add heat into the system in the case of sub-cooled samples, such as in the winter months.
In general, individual high-efficiency sample coolers provide much better control of final sample temperature than other secondary cooling methods.
Sentry has extensive field experience applying primary and secondary cooling samplers to SWAS solutions around the world to remove temperature bias and give sampling systems more flexibility to handle changing conditions.
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