Steam turbines have been used in the production of electricity since the 1880s. Despite steam turbines being very effective in their operations, they are often prone to over speed failure. It occurs when the rotational speed exceeds the recommended operating limits causing the impeller wheels and the main shaft to pull apart due to centrifugal force. Normally, the turbines have shutoff systems (trip mechanisms) that automatically stop the systems when they exceed the required speed. Periodic turbine inspection is critical to ensuring the system has enough response time and averts costly and catastrophic failure. While OEMs and insurance carriers outline specific test recommendations for gas turbines, many operators still face the dilemma of whether to perform a low-speed test or test the system while it’s operational. Here’s a detailed overview of each method.
What’s the Difference Between Mechanical and Electronic Over Speed Protection Systems?
A gas turbine is equipped with either of the two trip systems. Understanding how they operate is critical to performing routine borescope inspection. Mechanical trip protection systems are used on old steam turbines and comprise a spring-loaded piston that has been mounted on the shaft and screwed on the front of the turbine rotor. Therefore, when the turbine over speeds, the bolt extends further away from the rotor and comes into contact with the mechanical level causing it to open the valve. The result is rapid depressurization of oil supply to the steam valves which causes all the valves to close immediately.
An electronic over speed protection system, on the other hand, consists of electronic and mechanical components replacing the functionality of the traditional mechanical bolt. The idea is to exploit various mounted trip valves, a combination of speed probes, and control logic to perform trip tests. This means a turbine inspection can be performed off or online, i.e., the mechanical bolt can be removed or disabled as the current protection system is retrofitted.
Low Speed Testing
Full over speed tests exert excessive pressure on the rotating components of the gas turbine, which can lead to increased maintenance costs in the long run. When performing tests on pumps or compressors, however, it is impossible to achieve the over speed limit with the pumps connected. This means the units need to be uncoupled to minimize the inertia of the system.
As such, the control dynamics of the uncoupled system and the normal ones are different. Both scenarios present compelling reasons to perform turbine inspection and testing at low speed, especially on mechanical over speed protection systems. At the same time, operators should ensure that corporate and insurance carrier testing requirements are met. Sometimes the operators allow borescope inspection of gas turbines at low speeds as long as there are no significant changes in the sensing gear, speed pickup, or the brackets.
Trip Testing When the Machine is Operational
Some power equipment are operational for extended periods without outages. As such, there may be long periods when the over protection system has not been tested. Periodic inspections are imperative to ensure the reliability and functionality of the system. The technicians may decide to perform simulated tests on some parts of the electronic system because the system is designed with adequate redundancy to prevent damage during the test. An over speed signal is sent to the system electronically to test the electronic components, not the main steam stop valve or the throttle valve. Some systems allow valve testing at a reduced or rated load conditions.
If the power system is using a mechanical over speed protection system, then a low-speed test is befitting. However, if using an electronic system, technicians offering turbine inspection services can test the system while it is still running. Also, power operators should consider replacing mechanical over speed protection systems with electronic ones as they are less prone to mechanical failure. The systems are also more accurate, measuring the rotational speed to within 0.1%.