By Leif M. E. Svensen III, PE, and Jeff R. Zeigler, PE, Santee Cooper; and Frank D. Todd and Greg C. Alder, True North Consulting
With the combination of increased demand for electric power and the advancing age of operating equipment, it is more important than ever to monitor and improve the thermal efficiency of plant operation.
There are many factors to monitor in power plants, but one that is frequently overlooked is cycle isolation. Often this is an area where plant personnel can find “low hanging fruit” with great return on investment, especially high energy valve leakage. This type of leakage leads to increased heat rate, potential valve damage and lost generation. The fundamental question to ask is, “What is 100 Btu/kW-hr of heat rate worth to your plant?” On a 600 MW coal-fired power plant, a 1 percent leakage can lead to an 81 Btu/kW-hr impact on the main steam cycle and a 64 Btu/kW-hr impact on the hot reheat cycle.
Leakages can occur from tank-level control problems, air-operated valve setting issues, motor-operated valve thermal expansion, relief valve drifting, steam cutting of valves, improper valve alignment, foreign material in a valve, maintenance on the system and other problems.
High energy valves to monitor typically include, but are not limited to, the following: feedwater heater dump valves, main steam line drain valves, gland seal unloader valves, turbine bypass valves, feedwater heater vent valves, reheater drain tank dump valves (nuclear plants), condenser sparger valves, gland steam isolation valves, extraction steam line drain valves, moisture separator dump valves (nuclear plants), heater bypass valves, feed pump recirc valves, before and after seat drain valves, steam traps and steam drain line orifices (and orifice bypass valves).
There are several methods to assist in detecting leaking valves and to monitor cycle isolation. Valve location, type and characteristics play a part in what method to use. Some of these methods are used by turbine vendors for before- and after-testing to identify leaking valves to ensure corrections are made for guarantee purposes.
Acoustic equipment can be used to determine changes in sound that indicate potential valve leakage. Additionally, manual inspection of valves and “listening” for steam leakages can also be of value. For valves located below reservoirs or tanks, monitoring levels can be valuable in detecting a valve issue. Feedwater heater level indicators and drain cooler approach value changes can be helpful in finding an open or leaking valve.
Monitoring the changes to normal level valve position and controlling air pressure and changes to high level dump valve air pressure can assist in finding potential valve issues. Measuring downstream temperatures with handheld infrared guns, contact pyrometers and thermocouples is the method often used to monitor potential leakage.
Finding a Leak
Measure the temperature immediately upstream and downstream of the valve. If both temperatures are near the ambient temperature, the valve is not leaking. If the upstream temperature is significantly above but the downstream temperature is near the ambient temperature, this valve has a very small leak. If both upstream and downstream temperatures are significantly above the ambient temperature, this valve has a significant leak.
Establishing or modernizing a valve monitoring program will reduce the time between when a leakage begins and when it is solved. In addition, raising plant personnel awareness of the impact of leaking high energy valves on plant performance has value. Personnel need to understand that high energy water or steam leaking past a valve seat will erode the seat, disc and the downstream piping causing the leak to increase over time.
To establish a program, the following steps may be helpful.
Determine the Valve List: Use plant diagrams (P&IDs and isometrics) in conjunction with heat balance diagrams or performance monitoring software to determine the valves to monitor. There are hundreds of valves in a power plant but only those with significant impact should be tracked. Typically, this will range from 40 to 80 valves in a fossil plant and up to 120 valves in a nuclear plant.
Determine the Measurement Location: If possible, the measurement location should be at least 10 diameters downstream of the valve. However, temperature can be affected by other plant equipment, pipe diameter changes, headers, measuring too close to a condenser and so on. Often the plant geometry will not allow measurement at the optimum location. Therefore, the most important factor is consistency of the measurement location.
Determine the Measurement Method: For fossil, combined cycle and pressurized water reactor nuclear plants use infrared, contact pyrometer or thermocouple temperature measurement devices. For boiling water reactor nuclear plants, only thermocouples with wires running to remote locations can be used because of the radioactive environment. These are often fed into separate data acquisition systems. Some plants are starting to look into using wireless measurement equipment to monitor temperatures. In addition, if using an infrared device, ensure the material’s emissivity is accounted for and that the hole in the insulation is adequate for a good reading based on the distance between the infrared device and the piping. Typically, a 2-inch hole is adequate. Make the measurement directly on the pipe and not at an angle. Again, consistency of measurement is the key.
Establish Frequency: As part of the program, a frequency for walkdown measurements should be determined to monitor valve changes over time. An efficient walkdown order should be determined to reduce the time to complete the walkdown measurements. Make certain those performing the valve measurement walkdowns understand how to use the measurement equipment and the locations are well-identified. Many plants place numbers at the measurement locations to correspond with walkdown collection forms. Plant operators are often involved with cycle-isolation walkdowns, which can be a benefit by increasing the operators’ awareness of the importance of monitoring potentially leaking valves.
Quantify Leakage Impact: Determine a method to quantify the leakage and assess its impact on cycle efficiency. Establish an action plan for leaking valves that are found and for documenting results. This will help equate the severity of a leak with the downstream temperature so that adjustments can be made to leakage calculations based on empirical data.
What Does the Data Mean?
Just because it is hot does not mean there is a significant leak. Some valves have bypass orifices or discharge into headers or flash tanks, which result in normally higher downstream temperatures. In addition, it is important to be careful of conduction across the valve and piping; this is generally taken care of by measuring a sufficient number of diameters downstream of the valve. If the valve is close to the source of the heat, the upstream temperature can be higher than the downstream temperature and the valve may not be leaking.
It is critical to know the drawings. Look for other paths that could introduce heating or cooling at the location being measured. Value also exists in keeping a good history of valve damage versus temperature data. Some valves can be repeat offenders.
Prioritizing leaks is a function of thermodynamic impact on the cycle (heat rate) found from the energy level of the liquid or vapor upstream of the valve and the amount of the valve leakage. Ranking valves is important because large valves are expensive to repair, so make sure the benefit is there. Identify and track leakage on all valves in the defined list.
Santee Cooper Case Study
For some time, Santee Cooper has been proactive in tracking cycle isolation issues. Frequent valve walkdowns have been performed for years, collecting downstream measurements using handheld equipment.
With performance software, Santee Cooper has been able to obtain the thermodynamic (heat rate) impact in the plant cycle for leaking valves per pound of leakage. This ratio provides a loss factor to rank the valve for efficiency impact. However, Santee Cooper wanted a tool or method to estimate valve leakage flows. The estimated leakage flow combined with the existing loss factor can then be used to quantify the impact on heat rate and knowing the fuel cost, the loss in dollars for leaking valves can be estimated. With this information, the leaking valves can be ranked from worst to least impact and a return on investment for valve maintenance established.
Cross Unit 1 was selected for implementation of a new software product, TP-Plus-CIM, designed to estimate the flow rates of potentially leaking valves. This software also provides the heat rate and power impact for each valve. Cross Unit 1 is a 1,000 F/1,000 F/2,400 psig reheat coal unit rated at 605 MWe gross.
TP-Plus compliments the existing program in monitoring and tracking valves and estimating the impact on plant cycle performance. TP-Plus automatically prioritizes the leaking valves to assist in justification for repair or replacement.
Santee Cooper performs frequent valve walkdowns at Cross 1, taking downstream temperature measurements using handheld equipment. Some TP-Plus users install thermocouples to record temperature measurements that can be helpful in automating part of the data collection process.
Each measurement is recorded using TP-Plus walkdown worksheet forms or can be uploaded from spreadsheets or handheld recording devices or measurements recorded in the plant historian. At the time this article was written, Santee Cooper had imported and recorded 13 valve walkdowns into TP-Plus, providing a long-term history of each valves’ behavior.
TP-Plus was designed and developed for advanced cycle isolation and cycle leakage predictions using first principal calculations and actual plant valve leakage data. The software contains up to five different methods to predict valve leakages for saturated and superheated conditions. If a method is not applicable to the valve type and condition, it is excluded from the averaging calculation. Input forms for each valve are available for easy access to add or edit valve information. Specific characteristic data for each valve are entered into TP-Plus. Once the valve characteristic information is entered, the user enters the measured temperature readings into the valve walkdown sheet forms and the results are computed and placed in tables and graphs. This information is then used to prioritize maintenance and repair of valves.
Santee Cooper has tracked and repaired several leaking drain valves on Cross 1, worth approximately 147 Btu/kW-hr. In addition, they have recently implemented this tool at the remaining three Cross Station units with similar results. A valve walkdown is performed on each unit at least once a month. Operation, maintenance and performance personnel then work together to make sure leaking valves are repaired as soon as possible, often at the next outage opportunity.
Establishing a valve monitoring program for tracking valves in all types of power plants on a regular basis can be rewarding. Using the techniques described in this article, significant cost savings from repair and replacement of high-energy valves are achievable. Remember that the size of the leak today will not be the same as the size of the leak tomorrow. Finding leaking valves in a timely manner and using tools to quantify the impact on cycle performance to justify repair is critical in today’s power industry.
Authors: Leif M. E. Svensen III, PE, and Jeff R. Zeigler, PE, are with Santee Cooper based in Moncks Corner, S.C. Frank D. Todd and Greg C. Alder are with True North Consulting LLC, based in Montrose, Colo.