Combined steam and power facilities, also known as cogeneration plants, use heat recovery steam generators or traditional boilers to provide both high-quality steam and electric power from one fuel source. With natural gas achieving about 30-40% efficiency for power only, creating and sampling your process steam enables combined cycle cogeneration plants to operate at efficiencies between 65-90%.
High-pressure process steam carries the heat from cogeneration plants. This steam delivers the latent heat to the manufacturing process, leaving liquid condensate. Theoretically, the condensate should be clean water. In practice, though, the processing system introduces contaminants into the liquid stream. A list of common process impurities include:
- Iron, copper, and ammonia: These can indicate corrosion in the boiler as the piping deteriorates due to ionic attack of the base material
- Magnesium, calcium: Leaks from city water-carrying these components can cause scaling and hard material buildup
- Organic carbon: These can induce solids throughout the system or coat the piping walls, leading to soot formation at elevated temperatures
All of these contaminants negatively affect the feedwater systems, boilers and turbines in the plant potentially causing unnecessary downtime and equipment damage.
Real-time condensate measurement
Monitoring condensate supplies a real-time picture of the conditions inside the processing system. Total Organic Carbon (TOC), for example, indicates the cleanliness level of wastewater.
Operators and chemistry technicians also measure cation conductivity on the condensate sample and cycle chemistry. Cation conductivity indicates potential contamination. Power plant condensate is different than cogeneration condensate as cogeneration condensate can pick up contaminates from the industrial process. Significant chemistry “events” can occur if the system designers improperly specify the chemistry of the process steam as well as fail to measure for key quality standards.
monitoring vs. representative sampling
Sampling the condensate stream is the best way to confirm what the contaminant is and what might be causing it. For example, corrosion products and a condenser leak both increase conductivity but if you add a sodium measurement to the condensate stream in a unit with saline cooling water, you can more positively identify a condenser leak at low levels before it creates major damage.
Measuring cation conductivity of condensate, typically at the condensate pump discharge, is common to all systems and other measurements are useful depending on plant design. For example, condensers cooled with saline water should consider adding sodium measurements and dissolved oxygen measurements can also indicate air ingress.
Proper sample system design
Condensate return system monitoring offers numerous benefits to cogeneration plants. Checking for TOC, iron, and other contaminants help to identify chemistry excursions that can cause failure. There are a wide range of corrective measures to adjust the chemistry, improve system performance and avoid equipment damage and down time.
The most basic filtration that captures particulates down to a 5 µm maximum dimension would remove a majority of the contaminants. A well-designed system includes a condensate polishing system, usually a mixed bed of cation and anion resin to remove contaminates and ensure that high quality condensate returns to the boiler.
Practical limitations exist that can prevent cogeneration systems from incorporating a condensate polisher. These include poor system design, designers’ lack of familiarity with mixed systems containing process steam and power generation steam, and cost.
Proper sampling and minimum key instrumentation in cogeneration plants has many benefits. Checking the value of TOC, corrosion-causing materials and scale-causing materials during operation alerts process engineers of an issue. This step confirms the source of the issue and allows the engineers time to remedy the failure and keep the plant running.
Learn more about sampling's impact on your cogeneration plant here.