Overspeed
Overspeed is a condition in which an engine is allowed or forced to turn beyond its design limit. The consequences of running an engine too fast vary by engine type and model and depend upon several factors, the most important of which are the duration of the overspeed and the speed attained. With some engines, a momentary overspeed can result in greatly reduced engine life or catastrophic failure. The speed of an engine is typically measured in revolutions per minute.
Examples of overspeed
- In propeller aircraft, an overspeed will occur if the propeller, usually connected directly to the engine, is forced to turn too fast by high-speed airflow while the aircraft is in a dive, moves to a flat blade pitch in cruising flight due to a governor failure or feathering failure, or becomes decoupled from the engine.
- In jet aircraft, an overspeed results when the axial compressor exceeds its maximal operating rotational speed. This often leads to the mechanical failure of turbine blades, flameout and total destruction of the engine.
- In ground vehicles, an engine can be forced to turn too quickly by changing to an inappropriately low gear.
- Most unregulated engines will overspeed if power is applied with no or little load.
- In the event of diesel engine runaway, a diesel engine will overspeed if the condition is not quickly rectified.
Overspeed protection
- Many steam engines use a centrifugal governor, which closes a throttle at high rpm to restrict steam flow as engine speed increases.
- In motor vehicles, automatic transmissions will change gear to prevent the engine from turning too quickly. Additionally, almost all modern vehicles are fitted with an electronic rev limiter device that will cut fuel supply or sparks to the engine to prevent overspeed.
- Some aircraft have constant-speed units that automatically change propeller pitch to keep the engine running at the optimal speed.
Different overspeed occurrences and prevention
Aeronautics
In aeronautics, overspeed arises from jet engine design. In the simplest terms, jet engine operation can be broken down into four stages: 1. air is drawn through an inlet, 2. the air is compressed, 3. the compressed air is mixed with fuel and combusted, 4. finally, the result is fired out as exhaust out the back of the engine. These four steps contain turbines that are finely tuned to perform each specific task. To make sure each is up to regulation for safety, emissions, and other important points, the US Federal Aviation Administration put rules in place on July 18, 2011. The rules state that the overspeed margin has increased to 120 percent for one engine under no load, while for operating conditions, the margin is 105 percent. In addition to the overspeed requirements put forth by the FAA, they have also stated new rotor design criteria.Along with overspeed protection by automation controls, there are ways to prevent overspeed by maneuvering controls. Milton D. McLaughlin goes into detail on pitches, yawing, climb outs, and other maneuvering techniques. The details specify the turn and angle used at the speed the pilot is flying to prevent overspeed from occurring.
Internal combustion engines
An excerpt presented by the San Francisco Maritime National Park Association illustrates the types of overspeed systems with governor and engine control. Overspeed governors are either centrifugal or hydraulic. Centrifugal governors depend on the revolving force created by its own weight. Hydraulic governors use the centrifugal force but drive a medium to accomplish the same task. The overspeed governor is implemented on most marine diesel engines. The governor is a safety measure that acts when the engine is approaching overspeed and will trip the engine off if the regulator governor fails. It trips off the engine by cutting off fuel injection by having the centrifugal force act on levers linked to the governor collar.Turbines
Overspeed for power plant turbines can be catastrophic, resulting in failure due to the turbines' shafts and blades being off balance and potentially throwing their blades and other metal parts at very high speeds. Different safeguards exist, which include a mechanical and electrical protection system.Mechanical overspeed protection is in the form of sensors. The system relies on the centripetal force of the shaft, a spring, and a weight. At the designed point of overspeed, the balance point of the weight is shifted, causing the lever to release a valve that makes the trip oil header to lose pressure due to draining. This loss of oil affects the pressure, and moves a trip mechanism to then trip the system off.
An electrical overspeed detection system involves a gear with teeth and probes. These probes detect how fast the teeth are moving, and if they are moving beyond the designated rpm, it relays that to the logic solver. The logic solver trips the system by sending the overspeed to the trip relay, which is connected to a solenoid-operated valve.
Mechanical vs. electrical governors on turbines
In turbines and many other mechanical devices used for power generation, it is critical that the response times for overspeed prevention systems be as precise as possible. If the response is off by even a fraction of a second, it can lead to turbines and its driven load suffering catastrophic damage and put people at risk.Mechanical
Mechanical overspeed systems on turbines rely on an equilibrium between the centripetal force of the rotating shaft imparted on a weight attached to the end of a turbine blade. At the specified trip point, this weight makes physical contact with a lever that releases the trip oil header, which directly moves a trip bolt and/or a hydraulic circuit to activate stop valves to close. Because the contact with the lever occurs over a relatively limited angle, there is a maximum trip response time of 15 ms. The issue with these devices has less to do with response time as it does with response latency and variability in the trip point due to systems sticking. Some systems add two trip bolts for redundancy, which enables response latency to be reduced by half.Electrical
Electrical overspeed systems on turbines rely on a multitude of probes that sense speed through measuring the passages of the teeth of a spur gear. Using a digital logic solver, the overspeed system determines the propeller shaft rpm given the ratio of the gear to the shaft. If the shaft rpm is too high, it outputs a trip command which de-energizes a trip relay. Overspeed response varies from system to system, so it is key to check the original equipment manufacturer's specification to set the Overspeed trip time accordingly. Typically, unless specified otherwise, the response time to change the output relay will be 40 ms. This time includes the time required for the probes to detect speed, compare it to an overspeed set-point, calculate results, and finally output the trip command.Overview of overspeed detection system
When configuring, testing, and running any overspeed systems on turbines or diesel engines, one factor considered is timing. This is because the response to overspeed is usually too fast for people to notice.The responsibility of calibrating the correct overspeed response for a specific system falls on the manufacturer. However, variability is always present, and it is important for the owner/operator to understand the system in the event of maintenance, replacement, or retrofitting of outdated or worn out parts. After overspeed has occurred, it is essential to check all machinery parts for stress. The first place to start for impulse turbines is the rotor. At the rotor, there are balance holes that equalise the pressure difference between turbines, and if warped, would require the replacement of the entire rotor.