MECHANICAL OVERSPEED TESTING OF NUCLEAR SAFETY-RELATED TURBINES WITHOUT DRIVING STEAM Little J. 1 1 ILD, Inc. Baton Rouge, Louisiana, USA 1. Background Rotating equipment, particularly steam turbines, generally employ control systems that perform a variety of functions, including tripping. Tripping is the shutting down of a turbine when certain abnormal situations occur, for example, low bearing oil pressure, high bearing temperature, and rotor overspeed. Rotor overspeed, if unchecked, could cause a rotor to fly apart, resulting in substantial damage, and in some instances, catastrophic results including loss of human life. Consequently, most steam turbines are equipped with electro-hydraulic or electro-mechanical control systems and backup mechanical overspeed trip devices to prevent rotor overspeed. These devices must be periodically tested to ensure proper functioning. This periodic testing is typically required by the entity insuring the equipment against loss. In most instances, testing turbine overspeed trip systems requires driving the turbine rotor to trip set-points, typically set at 103-120 % of the normal operating speed. See, e.g.,united States Patents No. 5,133,189 and 5,292,225 covering modern over-speed protection devices. 2. Typical historical over-speed testing methods In commercial nuclear power plants, small to medium horsepower turbines are routinely used as prime movers (source of rotation), and, as discussed above, are periodically tested to ensure proper functioning. Generally, less risk is involved when overspeed trip testing is performed at a time when the turbine is not required to be operational, for example, during refueling outages when the nuclear reactor is not critical. During refueling outages, maintenance and testing activities which, if delayed, would delay the return to service (productivity) of the power plant are identified as being on critical path. By contrast, maintenance and testing activities that do not increase the outage duration are identified as off critical path. Nuclear power plant management typically prefers that all maintenance and testing activities, including overspeed testing, be performed off critical path where possible to minimize outage duration and lost production. However, the costs associated with conducting these tests can be significant because an alternate source of steam has typically been required to spin the turbine since the reactor can no longer produce steam. These costs can include the rental of an alternative steam source capable of spinning the turbine rotor beyond its normal trip set-points, in addition to manpower costs for engineering, maintenance, and operations support. Furthermore, the logistics of installation, operation, and removal of the required equipment can add complexity to an already complex refueling outage schedule. Alternatively, overspeed trip testing could be conducted using steam provided by the reactor once it is again operational. However, this testing method is generally not preferred because of the losses in productivity that result. More specifically, when testing a turbine using steam provided by the reactor, the tests are performed during the plant start-up from the refueling outage. This testing method is generally considered on critical path because the testing activity becomes a series activity in the start-up sequence and plant return to service cannot proceed until a successful over-speed test has been accomplished. -1-
3. Alternative over-speed testing method An unfilled need therefore existed for a method and apparatus that allows overspeed testing to be performed off critical path, and without the need for driving steam. Additionally, an ideal method and apparatus should allow testing without subjecting the tested turbine to any unacceptable stresses. One option for such a method is to connect the turbine shaft to a source of rotational power and accelerate the turbine rotor to its mechanical over-speed set-point with only air in the turbine casing. To rotate a turbine rotor beyond normal trip set-point in this manner requires a high power motive drive systems that is capable of overcoming windage effects. Windage generally refers to a power loss due to fluid drag on a rotating body. Windage increases are directly proportional to the cube of the speed of a turbine rotor. Windage effects for rotors spinning in air at high speeds are significant. Such a method was developed in the United States in 1998, and has been successfully used for testing of small turbines of less than 1,000 horse power using only ambient air to surround the rotor during testing. This test device comprises an operator control system and a drive motive power assembly utilizing a standard alternating current induction motor, variable speed drive electronics and a belt driven power transmission. Once installed, this device is used to accelerate the turbine rotor to its test velocity without the use of steam. Rotor speed and acceleration are controlled with a high level of precision, virtually eliminating the likelihood that, in the event an overspeed mechanism malfunction occurs, the turbine will be damaged. Another, more complex device was developed later and patented, also in the United States [1], which enables much larger turbines to be tested in much the same way. For turbines with rotors approaching 1 meter in diameter, and over-speed set-points approaching 6,000 revolutions per minute (RPM) two new problems arise which are of no consequence when testing the smaller turbines; 1. Windage losses become very large, necessitating delivery of much power to the turbine shaft. This problem can result in bearing side load issues, as well as very large electrical power requirements. 2. Turbine rotor tip speed can approach sonic velocity relative to the turbine casing. In order to avoid these problems, a purge gas assembly is added to the device design. This assembly provides a purge gas for which sonic velocity is substantially higher than air, thereby eliminating sonic velocity concerns. Windage losses and power requirements are both minimized by selecting a purge gas with a low atomic/molecular weight. Since its development in 1998, this method has been used in over 25% of the US nuclear power units, has been endorsed by the Electric Power Research Institute (EPRI) and has won acceptance by the US Nuclear Regulatory Commission (USNRC) [2, 3, and 4]. 4. Mechanical details of alternate over-speed testing device While basic theory is the same for testing of turbines of any size, there are significant differences in both mechanical and installation details for small, versus large turbine implementations. Typical test implementations for small turbines, less than 1,000 HP, have been temporary, as they can be set up and restored in just a few hours. These small turbines can be tested using power transmissions which utilize synchronous gear belt drives due to the low windage, power requirements and resulting minimal bearing side loads. Larger turbines require the introduction of purge gas to the turbine casing, as described earlier. Additionally, rigid power transmissions are also required to prevent unacceptable asymmetrical bearing loading due to the application of motor torque during rotor acceleration. -2-
4.1 Testing of small turbines, less than 1,000 HP A typical temporary installation for the alternate testing device can be seen below, in Figure 1. The pictured test set-up is for a model number GS-2N Terry Turbine used for Reactor Core Isolation Cooling (RCIC) in a US Boiling Water Reactor (BWR) nuclear plant. Note the synchronous gear belt drive and small drive motor size which can be used for testing of such small turbines. (turbine under test is to the left and the driven pump, currently uncoupled, is on the right) Figure 1 Installation of drive motor assembly on a RCIC turbine in USA 4.2 Testing of larger turbines A typical permanent installation for the alternate testing device can be seen below, in Figure 2. The pictured test set-up is for a model number CCS Terry Turbine used for High Pressure Coolant Injection (HPCI) in a US Boiling Water Reactor (BWR) nuclear plant. Note the rigid gear-box type power transmission and large, permanently mounted motor required for testing of the larger turbine models. (turbine under test is to the left and the driven pump, currently uncoupled, is on the right) -3-
Figure 2 Installation of drive motor assembly on a HPCI turbine in USA As discussed earlier, testing of larger turbines requires the addition of purge gas to the turbine casing to mitigate the effects of windage loss and eliminate rotor tip sonic velocity as a concern. The purge gas is typically added to the turbine case, and then measured at the turbine shaft glands, as can be seen in Figure 4. Figures 3 and 4 show schematically how the test device is installed on the HPCI turbine which appears in Figure 2. Stop Valve Gland Leakoff HPCI Booster Pump HPCI Pump G L A N D HPCI Turbine G L A N D R P M Trip Local Cabinet P Gland Pressure P Figure 3 Schematic representation of HPCI turbine /pump set, with coupling spacer removed -4-
Coupling Coupling Figure 4 Schematic representation of HPCI turbine /pump set, with testing equipment installed 5. Example testing results Figure 5 shows typical results for an over-speed test performed on a HPCI drive turbine. Due to the permanent nature of the drive motor installation the turbine can be uncoupled from its driven pump set, all testing performed and the system restored in less than one 12 hour shift. As can be seen in the figure below, actual over-speed testing of the turbine takes less than 35 minutes. The turbine control test device (TCTD) used to perform the test for which the results are shown in Figure 5 utilizes a micro-processor controlled variable speed drive. This computer control results in very reproducible rotor acceleration and is responsible for the narrow band of over-speed test results for sequential tests. HPCI Turbine Over-Speed Test Results 6000 5000 Turbine Speed, RPM 4000 3000 2000 Turbine RPM Minimum Acceptable Maximum Acceptable 1000 0 0 20 40 60 80 100 120 Time, in minutes Figure 5 Typical over-speed test results for a HPCI drive turbine -5-
6. Conclusion In conclusion, the described alternate method for over-speed testing of nuclear Safety-Related drive turbines does not require steam to test the mechanical over-speed trip set-point. It allows testing to be performed quickly and safely, with far more consistent results than were previously possible. The method has been used in over 25% of the US nuclear power units, has been endorsed by the Electric Power Research Institute (EPRI) and has won acceptance by the US Nuclear Regulatory Commission (USNRC). 7. References [1] Turbine Control Testing Device. United States Patent 6,582,184B2. Date 7-17-01. Taiwan Patent 515861. Issue Date 7-17-01. International Patent Cooperation Treaty, Application No. PCT/US02/015312. Date 7-17-01. [2] Terry Turbine Maintenance Guide, HPCI Application: Replaces TR-105874 and TR-016909-R1, EPRI, Palo Alto, CA: 2002. 1007459. [3] Terry Turbine Maintenance Guide, RCIC Application: Replaces TR-105874 and TR-016909-R1, EPRI, Palo Alto, CA: 2002. 1007460. [4] Terry Turbine Maintenance Guide, AFW Application: Replaces TR-105874 and TR-016909-R1, EPRI, Palo Alto, CA: 2002. 1007461. -6-