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, chief amongst them the duration of the overspeed and by the speed attained. With some engines even a momentary overspeed can result in greatly reduced engine life or even catastrophic failure. The speed of an engine is ordinarily measured in revolutions per minute (RPM).
- 1 Examples of overspeed
- 2 Overspeed protection
- 3 Different overspeed occurrences and overview of prevention
- 4 Mechanical vs. electrical governors on turbines
- 5 Overview of overspeed detection system
- 6 See also
- 7 References
Examples of overspeedEdit
- In propeller aircraft an overspeed will occur if the propeller, ordinarily 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 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 complete destruction of the engine.
- In 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 (caused by it inhaling an unwanted external source of fuel), a diesel engine will overspeed if the condition is not quickly rectified.
Sometimes a regulator or governor is fitted to make engine overspeed impossible or less likely. For example:
- Many steam engines use a centrifugal governor, which centrifugally closes a throttle 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.
Large diesel engines are sometimes fitted with a secondary protection device that actuates if the governor fails. This consists of a flap valve in the air intake. If the engine overspeeds, the air flow through the intake will rise to an abnormal level. This causes the flap valve to snap shut, starving the engine of air and shutting it down.
Different overspeed occurrences and overview of preventionEdit
For aeronautics, overspeed occurs due to its jet engine design. A simple way to describe the way operates jet engine is made up of four stages: air is drawn through an inlet, compressed, mixed with fuel and combusted, and then fired out as exhaust out the back. These four steps contain turbines that are finely tuned to perform each specific task. To make sure each are up to regulation for safety, emissions, and other important points The Federal Aviation Administration put rules in place on the date of 7/18/2011. It states the overspeed margin has increased to 120 percent for one engine under no load while for operating conditions it is only 105 percent. After the overspeed requirements put forth by the FAA they 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 details on pitches, yawing, climb outs, and other maneuvering. The details include at the speed the pilot is performing the turn and the angle they use to prevent overspeed from occurring.
Internal combustion enginesEdit
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 type. Centrifugal meaning it depends on the revolving force created by its own weight. Hydraulic uses the centrifugal force but drives 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.
Overspeed for plant turbines can be catastrophic resulting in failure due to the turbines' shaft and blades, will be off balance and fling its blades and any other metal parts at very high speeds. This is cause for different array of protection and 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 loose pressure due to draining. This loss of oil affects the pressure and move a trip mechanism to then trip the system off.
For electrical overspeed detection system it involves a gear with teeth and probes. These probes detect how fast the teeth are moving and if it is moving beyond designated rpm it relays that to the logic solver (overspeed detection) then 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 turbinesEdit
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 (i.e. compressor, generator, pump, etc..) can suffer catastrophic damage and put people at risk.
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 (i.e. 0.015 sec). More so 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, and ensures response latency be reduced by half.
Electrical overspeed systems on turbines rely on a multitude of probes that sense the passages of the teeth of a spur gear to sense speed. Then using a digital logic solver, it 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 manufacturers specification to set the overspeed trip time accordingly. Typically, though unless specified otherwise, the response time to change the output relay will be 40 mS (i.e. 0.04 sec). 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 systemEdit
The biggest factor when configuring, testing, and running any overspeed systems on turbines or diesel engines is timing. Because overspeed response is so incredibly fast it's hardly perceptible to human reflexes, making detecting a fault in the system near impossible.
There is a strong argument to instrument the trip systems in such a way that the total system response can be measured. This way during a test a change in the response could indicate a degradation that might compromise system protection or point out a failing component.— Scott, 2009, p.161
All in all the burden of calibrating the correct overspeed response for a specific system falls on the manufacturer, however variability are always present and it’s important to for the owner/operator to understand the system in the event of needed 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 equalize the pressure difference between turbines and if are warped, the whole rotor is in need of replacement.
- AMOT Products.
- "Airworthiness Standards; Rotor Overspeed Requirements". Federal Register. 2011-07-18. Retrieved 2019-04-02.
- McLaughlin, Milton D. (1967). Simulator investigation of maneuver speed increases of an SST configuration in relation to speed margins. National Aeronautics and Space Administration. OCLC 762061730.
- "Submarine Main Propulsion Diesels - Chapter 10". maritime.org. Retrieved 2019-04-02.
- Perez, R. X. (2016). Operators guide to general purpose steam turbines: An overview of operating principles, construction, best practices, and troubleshooting. Hoboken, NJ: John Wiley & Sons.
- Taylor, Scott (June 2009). "Turbine Overspeed Systems and Required Response" (PDF). Semantic scholat. Retrieved March 14, 2019.
- Smith, Sheldon S.; Taylor, Scott L. (2009). "Turbine Overspeed Systems And Required Response Times". doi:10.21423/R19W7P. Cite journal requires
- National Marine Engineers' Beneficial Association (U.S.). District 1. ([1953?-]). Modern marine engineering. MEBA. OCLC 28049257. Check date values in:
- Lukáš Mrózek, Ladislav Tajč, Michal Hoznedl and Martin Miczán (28 March 2016). Application of the balancing holes on the turbine stage discs with higher root reaction (PDF). EFM15 – Experimental Fluid Mechanics 2015. EPJ Web of Conferences. 114. doi:10.1051/epjconf/201611402080.CS1 maint: uses authors parameter (link)