Steam Turbine Power Plants Can Reap Big Benefits from Small Investment in Vigilant Instrument

While US power generation providers, industry wide, constantly strive to maximize the efficiency and reliability of existing facilities to meet customer demand and reap the premium prices of selling to the grid, most might consider their equipment already optimized. Steam turbine operators of, however, are quickly discovering that a lack of closely monitored process control on the crucial turbine condenser systems can quietly cost their plant millions of dollars per year.

The condenser system is largely perceived as a very reliable workhorse that collects turbine exhaust steam under vacuum and recycles this steam as condensed high purity water. The condensate is sent back through the boiler where steam is regenerated. The vacuum pressure in the condenser alone is critical to plant efficiency since it controls the turbine back pressure which limits the amount of energy drawn from the steam.

Ironically, condenser air in-leakage that is infrequently measured or only scrutinized when serious problems exist, can quite readily, be continuously monitored. This capability can provide instant system feedback that allows personnel to stay ahead of this constant specter threatening optimum plant performance. This type of in-leakage is notorious for stealing power plant capacity and efficiency, and also contributes to heat exchanger and boiler problems - all of which can result in decreased power, availability and fuel losses.

It is well recognized that an increase of just one inch of mercury (in. HgA) in condenser back pressure can limit unit capacity as much as 2 percent. The heat rate penalty can be just as severe and is approximately 16 Btu/kW-Hr for every 0.1 in. HgA increase in turbine back pressure. Air-in leakage that gives rise to excess back pressure, is also a major cause of dissolved oxygen in condensate. Often ignored is the consequential increase in other dissolved noncondensibles, ammonia and carbon dioxide, all causing corrosion that eventually results in forced outages. Over time these losses quickly become staggering.

When a turbine condenser is optimized for performance and closely monitored, the impact on the bottom line can be so dramatic that plants implementing this monitoring approach have realized a return on investment (ROI) for system cost in a matter days or weeks. Process innovator and patent holder INTEK, Inc., of Westerville, OH, manufactures the only line of flow instruments that provide the complex four-component measurement system needed for this type of vacuum monitoring

According to Brian Harpster, INTEK Product Specialist, it's not unusual for condenser air in-leaks to cost between $3,000 and $5,000 a day, plus a loss of potential revenues in MW hours of $10,000 to $100,000 daily.

"Most engineers and executives are surprised to discover how expensive air-in leakage can be, particularly when we define the potential losses they may face over the long haul" says Harpster. "Plus utilities never like to miss the opportunity to sell excess power back to the grid, especially at peak periods."

"Not only does air in-leakage contribute to back pressure on the turbine, which reduces system efficiency," Harpster explains. "But back pressure may also be caused by vacuum pump partial failure which may not be easily identified by a search for in-leakage. The ‘symptoms' may go unnoticed for several months under conventional industry maintenance programs." With failing exhausters, dissolved oxygen and other noncondensibles quietly build up in the condensate."

Excessive back pressure or correctable back pressure is essentially a pressure increase above the design pressure which is a function of load and inlet circulation water flow rate and temperature. This excess back pressure is generally caused by air in-leakage into condenser, degraded exhauster capacity or unclean (fouled) condenser tubes, all of which can also affect hotwell temperature and its equilibrium vapor pressure.

As much of a problem as excessive back pressure may become, it's correctable by proper monitoring and maintenance. However, identifying and then quantifying which of the aforementioned problems as the causative responsible for excessive back pressure is not an easy task. A real-time, effective vacuum monitoring setup such as the INTEK RheoVac Sentry system can solve this problem through the use of multiple sensor monitoring probes, which are installed in the vent lines upstream of the turbine venting equipment. They provide a continuous flow of data used to evaluate the performance of the vacuum system equipment.

Until recent years, the most common method to measure the flow of removed noncondensibles has been the rotameter. However, since the rotameter can be calibrated to function on 100% water vapor concentration, but is intended for use in mist-free air, false high flow rates are often indicated, due to water droplets in the exhauster flow stream, making the rotameter seldom reliable. The measurement requires a physical inspection and is generally performed during each shift or only once a day; if it is scheduled. A noted rise in condenser back pressure under familiar operating conditions is typically the signal that triggers air-in leak measurement and inspection.

Continuous vacuum system monitoring also helps to identify back pressure problems that are not due to condenser air in-leakage or exhauster performance. Tube fouling, which frequently occurs from debris congesting the cooling tubes, will impede proper heat exchange, thereby causing hotwell temperature rises, which in turn increases turbine back pressure. Turbine back pressure is also affected by seasonal and daily variations in cooling water inlet temperature. The back pressure problems resulting from these symptoms can be differentiated by an effective vacuum monitoring system.

Systems such as the RheoVac Sentry will measure the volumetric flow of air in-leak regardless of where it occurs. For example, at a recent installation on one customer site the system monitor indicated an abrupt air in-leak change of between 40 and 50 SCFM following turbine start-up. Under the circumstances, immediately following scheduled maintenance where repairs were made, such an air in-leak did not seem possible. Yet, inspection of the system validated that a large leak was indeed present, the equivalent of a one-inch hole, which was costing over $4,500 per day in excess fuel. It was finally determined that a new butterfly valve, installed on the suction line of an out service exhauster during maintenance had not been completely closed. If this had gone unnoticed between standard service intervals, the cost could have been astronomical.

Gains in plant efficiently generate both fuel savings and the potential for added power generation. With problems such as excess back pressure a turbine would need to burn more fuel to produce the same amount of power or maintain fuel input with a consequential loss in load. This loss in available power to sell is actually more costly to a utility than fuel losses.

While an effective monitoring system will identify and help solve back pressure problems, INTEK's Harpster says the major gains made through continuous monitoring are in the prevention of these problems.

"It's not just solving the problem, but preventing your equipment from getting to that point," Harpster says. "That's where the big savings really come in. Without a monitoring system, if you check your system with a pressure measurement only and perform an air in leakage survey once a year, you could have a back pressure problem for 11 months without knowing about it. That could easily amount to many hundreds of thousands of dollars in excess costs, lost load and silent corrosion between survey periods."

As many power providers struggle to squeeze incremental improvements out of existing highly optimized equipment, steam turbine operators can realize a significant opportunity to harness a resource already in place. This will not only benefit the industry, but will make an impact for generation utilities where it matters' most - the bottom-line.

Exclusive RheoVac technology, developed by INTEK, Inc., has been in use since 1994. INTEK has also been manufacturing the Rheotherm® line of flow instruments since 1978. These instruments are used worldwide for precise, reliable liquid and gas flow monitoring. For more information contact INTEK, Inc. at 751 Intek Way, Westerville, OH 43082-9057; phone (614) 895-0301; fax (614) 895-0319; e-mail sales@intekflow.com; or visit the web site: www.intekflow.com.

Intek, Inc., 751 Intek Way, Westerville, OH 43082. Tel: 614-895-0301; Fax: 614-895-0319.