Maintenance Tip: Measuring Condenser Air In-Leakage
by Tony Smith, Nash Field Service Technician
Several movies have used the premise that if you steal fractions of a cent from financial transactions, you can become rich without getting caught. In a thermal power plant, excess air in-leakage, if not caught, can stealthily reduce efficiency, costing the plant thousands of dollars over the course of a year.
High air in-leakage increases the back pressure on the steam turbine, resulting in less power output for a given heat input. A more insidious loss occurs when the amount of air in-leakage is not large enough to noticeably reduce power output: it can still be large enough to allow dissolved oxygen levels to increase to the point where chemicals must be used to control oxygen levels in the boiler water. The first step in stopping this theft is to show that it is happening.
If you are using a Nash condenser exhauster vacuum package, there is a simple method to measure air in-leakage on your condenser. All of our condenser exhauster packages are equipped with a tool that assists with this procedure – the rotameter.
A rotameter is a variable-area type flow meter. Nash mounts it on the discharge separator, with an isolation valve upstream of the device. If the rotameter has been lowered below the connection to the separator to make it easier to read, there may also be a drain valve. On top of the separator, there is a manual discharge valve. These items can all be seen in the drawing. During normal operation, the isolation valve will probably be closed.
When you are ready to take an air meter reading, while the condenser exhauster is on line and running, open the isolation valve to allow air to flow to the rotameter (and close the drain valve if there is one). Next, manually close the discharge valve on top of the separator, using the handle. This will force all of the air to go through the rotameter. There is a small piece inside the rotameter, the float, which will be pushed upward by the force of the air.
Observe and document the reading at the edge of the float, using the striations and numbers on the rotameter to find your scfm (standard cubic feet per minute). You may even consider using tape or a zip tie to mark the reading to easily compare it to future test results. This reading is the amount of air in-leakage being removed from the condenser. If the float is bouncing, slightly close the isolation valve until it settles at a stable position.
Ideally, the air leakage should be as low as possible (the float should stay near the bottom of the rotameter). The “HEI Standards for Steam Surface Condensers” recommends rates based on the condenser size, roughly one quarter of the rated capacity of the condenser exhauster. For instance, on a 10 SCFM condenser exhauster, the air in-leakage should be kept at less than 2.5 SCFM. Another rule of thumb is 1 SCFM per 100 MW of power generation.
A very high level of air in-leakage would signal that the system has excessive leakage. Use this information during planning for the next system shut down and plan to inspect and repair any piping, cracking, etc. within the system in order to keep the back pressure on the turbine as low as possible.
I recommend that this simple inspection be performed on a monthly basis and that it be documented and charted. The level should not change drastically but if, for example, the leakage has doubled compared to the previous month’s level, you will learn that there is a significant change in the system and further inspection is required.
After you have measured your air in-leakage, be sure that the manual discharge valve on top of the separator returns to the open position. There is a spring on that valve which should allow the valve to open once it is released, but occasionally that spring becomes fatigued and may need to be opened with the handle. You also need to close the isolation valve on the rotameter piping and open the drain valve if you have one.
Dynamic pipe strain is much more difficult to check since it occurs while the system is in operation. It is commonly due to thermal expansion of the piping, the weight of the system fluid or both of these along with inadequate piping or piping support. Dynamic pipe strain is most often detected by using a measurement device to check for differences in machine movement between static and dynamic conditions.
Pipe strain causes distortion of the machine frame. As the frame or casing is distorted, so are the bearing housings, the shaft and other components. These problems can lead to excessive bearing temperatures. The bearing problems can be detected through vibration analysis, which will have bearing fault frequencies as the dominant peaks. Packing and mechanical seal performance will also be affected by pipe strain because of the additional forces caused by the distortion to the bearing bore. Anything that leads to shaft deflection or reduced bearing clearances can quickly lead to catastrophic failure, particularly of bearings and seals.