- An overspeed failure on a big steam or gas turbine is one of the most frightening industrial accidents.
- A turbine overspeed accident can be caused by a lightning-induced power surge, a fouled pilot valve, an electrical fault, operator failure, or any of a few dozen other problems.
- The trip mechanisms on most turbines are required by law to be tested periodically.
Most industrial jobs bring with them a few potential mishaps that can keep a conscientious technical person awake at night. For operations and technical support people who look after large steam or gas turbines, overspeed failures can provide frequent nightmares.
Figure 1. This open view of a turbine shows the close fit between the wheels and the housing on a smaller Elliot E-Line steam turbine. If the rotational speed of the turbine exceeds the safe operating limits of the unit, the main shaft and impeller wheels can be pulled apart by centrifugal force.
An overspeed failure on a big steam or gas turbine is one of the most frightening industrial accidents. A huge amount of thermal, chemical, and mechanical energy courses through a big turbine when it runs. If the rotational speed of the turbine exceeds the safe operating limits of the unit, the main shaft and impeller wheels can be pulled apart by centrifugal force. In the worst case, the disintegrating parts can break through the turbine housing, flinging hot, fast-moving shards of metal in all directions. The results of such a failure are always very costly and can be fatal to personnel in the area (Figure 1).
A turbine overspeed accident can be caused by a lightning-induced power surge, a fouled pilot valve, an electrical fault, operator failure, or any of a few dozen other problems. Turbines are equipped with shutoff systems that are designed to automatically stop them if they exceed the design speed. These systems are built to cut the energy supply to the turbine so that it will coast to an orderly stop. Such a stop is usually referred to as a “trip” by operating staff.
The trip mechanisms on most turbines are required by law to be tested periodically. Turbines up to 100 hp are typically tested annually, says Ron Reeves, manager/partner at Turbonetics Engineering & Services (testexas.com) in Corpus Christi, Texas. The largest units, which may run as long as five years between shutdowns, should be tested whenever they are taken out of service.
Operators face a quandary with regard to testing trip systems on large turbines. One way to perform the test is to use electrical or other mechanisms to mimic an overspeed situation. In this way, the turbine can go through the shutdown sequence without risking an actual overspeed event. This approach, while it seems safer, doesn’t test all parts of the trip mechanism. The other approach is to actually bring the turbine to an overspeed situation and let the actual trip mechanism activate and cause a shutdown. This is a more complete test, but it brings the turbine closer to a dangerous overspeed situation.
Turbine people are divided on the choice of test approach. One faction cites the importance of keeping the turbine out of an overspeed situation. The other points to the risk of not testing the entire trip mechanism.
A field service perspective
Turbonetics’ Reeves says he’s “pro-functional test,” meaning the wiser course is to test the whole trip mechanism. “The trip system may include mechanical, hydraulic, and electronic systems, all working as a unit,” he says. “I want to see them all function together.”
Figure 2. If a trip fails to happen, shut the turbine down and find out why it didn’t trip. Most of the components can be individually tested off the machine to help with the diagnosis.
As to the risk of actually running the turbine beyond its normal operating speed, the answer is planning and control, explains Reeves. “You don’t just pour the power to the turbine,” he says. “You have to sneak up on the overspeed. You uncouple the turbine from all loads to minimize the energy in the system and then accelerate gradually to normal operating range. Beyond that, you add steam or fuel a very little bit at a time, stabilizing the system each step of the way, even if you have to connect a smaller feed to provide more delicate control. Remember, an accelerating system has momentum. It won’t necessarily stop accelerating as soon as you stop adding power.”