overspeed trip in generator

Over Speed Trip in Diesel Engines & Types of Over Speed Trips

An over speed trip is a safety feature provided on the diesel engine of the ship to restrict uncontrolled acceleration of the engine, leading to mechanical failure or untoward accidents. In order to prevent the speed of a diesel engine to go beyond the pre-set speed range, an over speed trip is used in the diesel engines.

How Over Speeding can be Harmful?

A Diesel engine is designed for the mechanical stress associated with the centripetal and centrifugal forces of the moving parts inside it in a specified operational range. Centripetal force is directly proportional to the square of the rotational speed, stress increases rapidly with increase in speed. Mechanical connection strength can be overcome by the exceeding stresses due to the increase in operational speed. These can result in breaking of rotating parts or damage to the machinery itself. Over speed is thus a serious safety hazard and can lead to a fatal situation.

What does Over Speed Trip Does?

Due to sudden changes in the load on the diesel engine, the speed of the engine may vary. Though a governor is provided to control the speed of the diesel engine, the speed might go out of control, damaging the engine. Thus, for this reason over speed trips are used.

No matter what type of the over speed trip the engine uses, the main aim of the over speed trip is to cut the fuel supply to the engine cylinders in case the engine speed rises above a specific level.

Preventing Over Speeding of Engine

Reducing the likelihood of an uncontrolled and catastrophic over speed is essential and can be done by two methods:

a)      Mechanical over speed trip

b)      Electronic over speed trip

In this article we will have a look at the electronic over speed trip.

Electronic Over Speed Trip

To understand the electronic over speed trip, a normal lay out of the system is described below. The electronic over speed trip consists of

a) Fly wheel mounted speed sensor

Magnetic speed sensor is preferred in generator engines. Due to the discontinuity of actuator surface (gear tooth of flywheel) voltage is excited in the pick off coil of sensor, producing an electric analog wave. This cyclic wave created by the flywheel is read by the sensor.

b) Signal condition unit

This unit act as a receiver to the speed sensor. Basic function of the signal conditioner is to convert one type of electronic signal which may be difficult to read into another type into a more easily read format. This can be achieved by amplification, excitation and linearization of an electrical signal.

c) Detection and Comparison unit

There is a set value which is normally 10 % above the rated speed and acts as base value for this unit. Signal condition unit output is continuously detected and compared with the set value.

d) Trip signal unit

If the difference between the set value and detected value is above the limit, then this unit gives a trip signal which in turn shuts down the generator.

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About Author

An ardent sailor and a techie, Anish Wankhede has voyaged on a number of ships as a marine engineer officer. He loves multitasking, networking, and troubleshooting. He is the one behind the unique creativity and aesthetics at Marine Insight.

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24 comments.

can you send to me mechanical over speed tripper journals and drawbacks…

hello sir, i want to know , how main engine overspeed trip is tested & what is the time duration of testing

Dear Pundari,

It is better to test the functioning of the over speed trip rather testing it at its real set speed. The set point is lowered then the required speed. once the generator reaches this lowered set point, it will trip. To run the generator in the actual overspeed condition is dangerous and long run in such speed can lead to problem in the bottom end bolts of the con rod.

anish, good day sir. im working in a ship, our aux generator is running but its not getting a load. the generator is running fast in 5sec then it will go slowly, what is the problem of this. tnx

The problem seems to be with governor of the AE. Check the droop setting of the same.

With thanks for all above information , kindly note recently we faced with aux engine over speed on one o our fleet vessel an led to total failure , explosion and fire in engine room.

Same aux engine was repaired/overhauled just 20 days back of accident in repair yards. Over speed trip were not activated and cause explosion.

Kindly advise possible factors which to be consider for such big accident.

With many thanks in advance.

i am working on CAT engine its many time show over speed alarm i change relay but the problem still there let me know what is the main problem and i can diagnose i

THIS SIGHT IS GIVING VERY VALUABLE INFORMATION

Its good to finally find a site like this. THUMBS UP!!!

Please assist me with this issue, my 5 cylinder Sulzer Harbor Generator is starting, only to be tripped by the over speed device when attempting to attain its rated RPM of 760. I suggested that the UG 8 governor might be faulty. Please what seems to b the problem?

rly a good rply by this site

How i can test overspeed alarm in generator vatepillar c-18. Answer please.

Please can you kindly help me, there is a cummins gen, and it has been tripping on over speed. Please can you kindly assist/give me a better idear to sort out this problem..

Please Sir, I need possible remedy or solutions to overspeed shutdown of Cummins power generator.What are the curses of that over speed?

Reasons have already been listed in the article. Overspeeding of engine is not healthy for rotational part of the engine (bearing housing bolts, crankshaft and its bolts etc.)

A nice site, I’m working on CT6 with overspeed issue… Have adjusted the Woodward the problem still persist, what really the possible causes?

We have created online forums for such technical discussions. Please visit: https://forums.marineinsight.com

You can contact any time related with all type of Woodward governing system and caterpillar engines related Quires, My contact Number is +91 9987396573

Good day Please is it possible test over speed from PLC

@Alan: in Many engine circuit, a test knob is provided. You can use that to test the overspeed trip.

How to reset mechanical overspeed trip

8 DK28 daihatsu generator engine .how to try out overspeed trip.i have a sensor in in flywheel so it may be a electronic type .how to carry outtests can somebody help sharing information？

@Benasher: Usually, the value of the speed sensor controller is reduced so that as the generator starts and reeatrip will operatoe ched the speed, the

Please comment on what to check in alternator…..

@Ankit: Please check this article- https://www.marineinsight.com/tech/generator/important-points-while-carrying-out-alternator-maintenance-of-ships-generator/

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What is Overspeed Trip, its types and how it works

• ·        Sudden removal engine load such as propeller coming out of water in rough weather, failure of clutch, fracture in a drive shaft of engine.
• ·        Stuck fuel control shaft.

Types of Overspeed Trips

Mechanical type overspeed trip.

Electro Pneumatic Overspeed Trip

Electronic Type Overspeed Trip

So this was all about the over speed trip that are used in the engines. If you want to add more info in to this topic ‘overspeed trips’, please feel free to comment below.

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5 Replies to “ What is Overspeed Trip, its types and how it works ”

Hello, so the difference between the mechanical type overspeed trip and the electronical type overspeed trip is not the way that the turbine trips but taccuracy of speed sensing? in both systems the trip is achieved with the bolt ejecting out?

HI there all of the topics here are very important to us, how much it cost to subscribe or to buy a book from you.? GOD BLESS hope to hear from you soon.

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Hi everyone Please tell me how to do the main engine fuel pump plunger stuck during main engine operation in rough sea so can not stop ME Thanks in advance.

I think this is one of the most significant information for me. And i’m glad reading your article. But should remark on some general things, The web site style is perfect, the articles is really great : D. Good job, cheers

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The Cause Of Overspeed In A Diesel Generator

Diesel generator sets are mainly used in power supply of various prime, standby and emergency power sources, also can be used in electric power supply for electric propulsion system. Its importance is self-evident. When we use diesel generator set, it maybe a overspeed problem, this is a special fault, it will not only cause serious damage to spare parts, but also may endanger personal safety. What is the cause of overspeed and how to resolve the problem?

1. Fault phenomenon of diesel generator overspeed

Under normal operation, the engine speed suddenly rises, so that the engine speeds up beyond the rated speed and loses control, and emits a huge roar, the exhaust pipe emits a lot of black smoke or blue smoke.

2. Cause of failure

The fundamental cause of diesel engine overspeed is that diesel engine loses or changes its speed regulation characteristics. Faults of fuel injection pump and governor, such as stagnation or looseness of relevant components, will cause excessive oil supply and loss of normal speed regulation characteristics, while the combustion chamber entering additional diesel or oil will change the speed regulation characteristics of diesel engine.

1) Faults of injection pump and speed regulator

A. When assembling and repairing, impact the plunger, oil outlet valve seat of fuel injection pump, too much tightening torque will cause the deformation of plunger sleeve, impurities entering into the clearance of plunger pair, too thin gasket on the positioning bolt of plunger sleeve or too long bolt, which will cause the plunger of fuel injection pump to rotate inadequately or be stuck in the position of large oil supply;

B. Loosening of fixing screw for adjusting gear ring of fuel injection pump plunger which causes injection pump plunger loss control;

C. The ball head of the plunger adjusting arm or the gear rod adjusting arm of the fuel injection pump does not enter the adjusting fork groove, so that the plunger is in the position of large oil supply;

D. Accelerating pedal pull rod or oil supply adjusting gear rod and the movable part of governor are stuck, and oil quantity adjusting gear rod and speed adjusting pull rod are out of joint;

E. The excessive lubrication oil, too viscous or too dirty in the governor cause the weight of the governor can not be thrown out in time when the speed rises, which causes the pull rod or the gear rod to lose control of the governor;

F. Full speed governor can not be thrown away because of skew of flying ball seat or abrasion of inclined sliding groove of thrust plate transmission plate, which increases the sliding resistance of flying ball;

G. The springs of injectors are too weak or broken, which results in too low injection pressure and excessive fuel consumption in cylinders;

H. Flying ball Pin Shaft of governor broken off and flying ball flying off.

The above reasons make the speed of diesel engine increase, while the "speed characteristic" of plunger fuel injection pump increases the fuel supply with the increase of the speed, which leads to the vicious circle between the speed and the fuel supply, resulting in the occurrence of overspeed accidents.

2) Additional fuel enters the combustion chamber

A. Engine oil level too high causes excessive oil to rush into the cylinder then enter the combustion chamber to burn;

B. Serious wear of piston rings and excessive clearance of cylinder wall lead to a large amount of oil entering combustion chamber to burn;

C. Excessive gap between air door and its catheter coordination is too large, which cases oil for valve distribution chamber enter into combustion chamber to burn;

D. After cleaned air cleaner, the excessive residual gasoline makes the higher concentration gasoline vapor enter the combustion chamber to burn;

E. A diesel engine with a supercharger enters the combustion chamber to burn because of the damage to the oil seal of the supercharger;

F. The solenoid valve of the cryogenic starter leaks oil, which causes excess diesel oil to enter the combustion chamber to burn;

G. Horizontal diesel engine inclines seriously, causing oil to flow into the valve chamber and be sucked into the combustion chamber to burn.

3. Solution to overspeed fault of diesel generator set

1) Forced stoppage

Once the diesel engine has overspeed, the following measures should be taken promptly to force the diesel engine to stop immediately

A. Quickly retrieve the accelerating pedal to the stop position;

B. For fuel injection pumps with exposed rods or toothed rods, the rods should be quickly pulled back to the stop position;

C. For diesel engines equipped with decompression devices, the decompression handle should be quickly pulled to the decompression position, so that the diesel engine shuts down because of the uncompressed gas;

D. If a closing device is installed on the intake pipe, the extinguishing device shall be closed immediately or the intake port of the air filter shall be blocked quickly with cloth, and the fuel tank switch shall be closed to cut off the gas and oil circuit, stop the oil supply and gas supply, and cause the cylinder to be suffocated and shut down;

E. Put the transmission into the high gear in time, while trampling the brake pedal to the bottom, and slowly lift the clutch pedal, so that the engine is forced to extinguish.

2)After the generator set stop working, to make a definite diagnose

A. If the acceleration pedal is raised rapidly, the engine will not continue to rise after reaching the maximum speed, the fault is the acceleration pedal rod or arm is jammed;

B. If the acceleration pedal is raised rapidly, the engine speed can not be reduced or extinguished immediately, usually the governor is out of order;

C. If the acceleration pedal is picked up, the engine speed continues to rise, indicating that the fuel injection pump has a fault;

D. If the fuel supply system is good, it means that there is extra fuel or oil entering the cylinder.

All above problems should be maintained by professional personnel. 

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Understanding Overspeed Trips

• Categories : Marine machinery, engines & controls
• Tags : Marine engineering

Introduction

The main aim of any diesel engine governor is to keep the speed of the engine within reasonable limits without getting affected by the changing load on the engine. In case the governor is not able to keep the speed of the engine within specific limits, the engine might speed up to such a level that it can damage the engine itself. In engines wherein the governor does not fail safe, over speed trips are used.

Overspeed Trip

The main function of an over speed trip is to cut the fuel supply to the cylinders in case the engine speed rises above the pre-set level. Generally fitted on steam turbo alternators or generators, over speed trip prevents the engine from getting damaged because of extremely high speeds.

How Does An Over Speed Trip Work?

The design of an over speed trip consists of a heavy bolt with a comparatively heavy head. The bolt is fitted at the forward end of the engine shaft where no engine torque is transmitted and is bored throughout the diameter of the shaft. Just like any other conventional bolt, the over speed trip bolt is held in place by using a nut and a spring.

At normal speeds, the bolt stays at the same position; however if the speed of the engine rises beyond the pre-set range, the centrifugal force generated by the heavy head of the bolt makes it fly out of the spring and nut’s compression pressure. The bolt eventually comes out of the bore, getting rid of the spring and washers.

Moreover, the arrangement is made in such a manner that the bolt head that comes out of the bore hits a trip lever which shuts the supply of fuel to the engine cylinders, eventually stopping the engine. The mechanism for stopping the fuel supply varies according to the type of the ship and engine. For e.g. in motor ships having steam turbo alternator, the over speed trip is controlled with the lubricating oil low pressure cut-out and the steam stop valve; whereas in few other ships the over speed trip is controlled using a system of levers and linkages that are connected by the turbine steam stop valve.

The steam stop valves actually controls the supply of fuel going to the cylinders. When the over speed trip operates, the pin holding the steam stop valve in the open position is released, closing the valve and stopping the flow of both, the steam and the fuel. The set speed for operating the over speed trip can be adjusted by adding washers below the spring and then adjusting their compression pressure.

Apart from over speed trips, permanent magnet alternators from engine camshafts along with rectifiers can also be used to prevent engine from racing at high speeds. However, this technique can only be used for electronic governors. As the speed of the engine crosses the pre-set speed, the voltage automatically increases, making the governor to reduce the fuel supply to the engine cylinder and lowering its speed.

Lambs Question and Answers - Stanley G. Christensen

Main Engine

Over speed trip in diesel engines & types of over speed trips.

• July 11, 2019

An over speed trip is a safety feature provided on the diesel engine of the ship to restrict uncontrolled acceleration of the engine, leading to mechanical failure or untoward accidents. In order to prevent the speed of a diesel engine to go beyond the pre-set speed range, an over speed trip is used in the diesel engines.

How Over Speeding can be Harmful?

A Diesel engine is designed for the mechanical stress associated with the centripetal and centrifugal forces of the moving parts inside it in a specified operational range. Centripetal force is directly proportional to the square of the rotational speed, stress increases rapidly with increase in speed. Mechanical connection strength can be overcome by the exceeding stresses due to the increase in operational speed. These can result in breaking of rotating parts or damage to the machinery itself. Over speed is thus a serious safety hazard and can lead to a fatal situation.

What does Over Speed Trip Does?

Due to sudden changes in the load on the diesel engine, the speed of the engine may vary. Though a governor is provided to control the speed of the diesel engine, the speed might go out of control, damaging the engine. Thus, for this reason over speed trips are used.

No matter what type of the over speed trip the engine uses, the main aim of the over speed trip is to cut the fuel supply to the engine cylinders in case the engine speed rises above a specific level.

Preventing Over Speeding of Engine

Reducing the likelihood of an uncontrolled and catastrophic over speed is essential and can be done by two methods:

a)      Mechanical over speed trip

b)      Electronic over speed trip

In this article we will have a look at the electronic over speed trip.

Electronic Over Speed Trip

To understand the electronic over speed trip, a normal lay out of the system is described below. The electronic over speed trip consists of

a) Fly wheel mounted speed sensor

Magnetic speed sensor is preferred in generator engines. Due to the discontinuity of actuator surface (gear tooth of flywheel) voltage is excited in the pick off coil of sensor, producing an electric analog wave. This cyclic wave created by the flywheel is read by the sensor.

b) Signal condition unit

This unit act as a receiver to the speed sensor. Basic function of the signal conditioner is to convert one type of electronic signal which may be difficult to read into another type into a more easily read format. This can be achieved by amplification, excitation and linearization of an electrical signal.

c) Detection and Comparison unit

There is a set value which is normally 10 % above the rated speed and acts as base value for this unit. Signal condition unit output is continuously detected and compared with the set value.

d) Trip signal unit

If the difference between the set value and detected value is above the limit, then this unit gives a trip signal which in turn shuts down the generator.

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Seconds From Disaster – Overspeed Devices

Basically, the generator load is a gigantic magnetic braking force.  In normal turbine operation, the driving force (steam or gas pressure) is equal to the braking force of the generator and the turbine-generator speed is constant. When the braking force is suddenly removed, the turbine force must also be removed before the turbine and the generator components rapidly spin out of control, potentially causing millions of dollars in damage. Your overspeed protection system must be ready to avert this disaster.

Overspeed protection devices can be mechanical (trip weight), electrical or a combination of both.  Units can have a mechanical governor or EHC controls.  Regardless of the mechanism, all overspeed protective devices are designed to stop the steam or fuel from entering the turbine(s) upon an increase in normal speed.  Most overspeed trip mechanisms are set to trip the unit at 110% of rated speed, but most turbine-generators are designed to temporarily operate at up to 120% of rated speed. Lower overspeed settings may be required for certain reheat units and nuclear applications.

Your turbine should have at least two trip devices – electronic and mechanical. Both systems must be inspected and tested regularly – please check your operating manual or call us with any questions.

Failure to remove the driving force (steam or combustion) can unleash torque forces which can destroy your turbine-generator. For example, a modest-sized steam turbine can have a start-up flow of 40,000 lb/hour to synchronize the generator to the grid and close the breaker. As the generator takes on load, steam flow is increased from the initial 40,000 lb/hour to a much greater flow (let’s say 2,000,000 lb/hour) and the turbine goes no faster! That additional 1,960,000 lb/hour steam flow has created torque to drive the generator.

Under a full load rejection, the generator armature reaction that opposes those additional pounds per hour of steam flow is suddenly removed. Now there is an unopposed 1,960,000 lb/hour of steam flow accelerating the turbine. We should not be concerned so much with how fast the turbine will go, because the speed will be way too much. We are more concerned with how fast the turbine-generator will accelerate and how quickly the turbine inlet steam valves will have to close. If the steam is not shut off, the turbine achieves the 120% rating value in seconds. Seconds later, bearings are failing, blades are failing, disks are failing or being pushed into the diaphragms, generator retaining rings are failing, and the rotor is being pushed into the core.

Combustion turbine operators may feel safer because the compressor load acts as an additional brake on acceleration. However, this retarding force will only afford you a few extra seconds. Whether steam or combustion driven, your turbine-generator needs to be ready and personnel need to be ready. There is no warning and you have so little time to act!

Minimize overspeed risk by:

* Having at least two trip devices – electronic and mechanical.

* Properly calibrating your overspeed devices.

* Routine testing to exercise those devices that must operate in an emergency.

* Reporting unusual events that could be indicators of increased risk (a sticking valve that fails to respond to load demand is a good example).

* Not performing tests (exercises) in severe weather conditions where the risk of load rejection is much higher.

Follow the OEM’s recommendation’s for the recommended testing frequency of your over-speed trip and record it’s trip speed.  A good time to test is when you are required to shut down the unit and load is removed from the generator.

The bottom line is that your turbine-generator needs to be ready; personnel need to be ready, as there is no warning and you have so little time to act!

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Mechanical overspeed protection or electronic overspeed protection?

10 April 2020 Theme: Machine protection

In the past, overspeed turbine protection systems were often designed by the turbine OEMs themselves as part of the mechanical system. These mechanical protection systems are still in use, especially on medium-sized machines. However, electronic systems offer significant benefits and are becoming increasingly accessible, in both their economical and technical impact. This article explains the benefits of electronic protection over mechanical protection.

The principle of mechanical overspeed protection

To understand the difference between the two types of protection, it is important to understand how mechanical overspeed protection works. Mechanical overspeed protection systems use a weight on a spring of which the spring force is known. Due to the centrifugal force as a result of the shaft rotation speed, the spring is stretched further and further and the weight moves out. As long as the rotational speed is still within the design specifications, the spring will not stretch too far and the weight will not protrude. However, if it exceeds its design specifications, the weight will run out and hit the trip bolt. This will release the pin and the trip will be activated.

The operation of a mechanical overspeed monitoring system.

The principle of electronic overspeed protection

Electronic overspeed protection consists of a gear or other measuring surface, speed sensors and a measuring system with trip logic and relay outputs. The speed sensors look at the measuring surface and give a sine or square wave signal, which the measuring system sees as pulses. The measuring system then counts the time between pulses to determine the rotation speed. The relay outputs are controlled by trip logic. In order to increase safety and availability, these protection systems are often built with redundant voting structures.

Why switch to electronic overspeed protection?

Switching from mechanical overspeed protection to electronic overspeed protection has significant advantages. Verifiable security is the primary argument. To illustrate: where mechanical overspeed protection systems have to be tested by actually overspeeding the machine (with all the risks involved!), an electronic overspeed protection system can be tested by simulating overspeed. In addition, a simulated test is quicker and easier to perform.

In this overview we compare both forms of machine overspeed protection:

Switching to electronic overspeed protection is not complex. Firstly, the mechanical trip lever and all associated system components are removed. Secondly, a measuring surface (gear) is mounted on the shaft if this is not already available, after which one or more sensors are placed around this measuring surface. The monitoring system can then be placed near the machine or in a cabinet.

Learn more about overspeed:

What does the API Standard 670 state about overspeed detection systems? »

SpeedSys 300 – Overspeed Protection System

The SpeedSys 300 is a SIL 3 rated electronic overspeed protection system for rotating machinery. It delivers the core layer of protection with a compact architecture. Its small technical footprint enables low-impact retrofitting and extends the range of machinery which can be protected.

Learn more about SpeedSys 300 »

What does the API Standard 670 state about overspeed detection systems?

5 speed-related failures with rotating machinery, sensors for speed measurements, learn more:.

12 July 2022 Theme: Machine protection

The API Standard 670 is an industry standard and describes the minimum requirements for a machine protection system (MPS). This includes, measuring radial shaft vibration, casing vibration, axial shaft position,…

22 November 2019 Theme: Machine protection

Rotating machinery such as turbines, pumps and compressors are continuously subjected to major mechanical forces. As most of these machines are critical to the process, systems are implemented with protection…

10 October 2019 Theme: Machine protection

Choosing the right speed sensor for an application is of crucial importance for an accurate and reliable measurement. After all, the signal of the sensor is the input for an…

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Turbine overspeed testing

Turbine overspeed events lead to costly and dangerous catastrophic failures. Therefore the overspeed system has to be periodically tested to ensure the system has the adequate response time to protect the turbines, plants and personnel.

Below are excerpts from the paper, "Turbine overspeed systems and required response times" by Scott L Taylor of Woodward and Sheldon S Smith of American Electric Power at the 2009 Turbomachinery Symposium in Houston.

Regular, periodic testing is the only way to ensure a protection system is working. In some cases, without exercising the system, mechanical components may become stuck rendering the protection system inoperable. Regular testing of stop or trip valves is generally recommended on a weekly or monthly basis where possible. Additionally it is recommended to shut down with the trip valve(s) whenever possible.

One of the largest owner/operators of power generation equipment requires annual testing on large machines. For mechanical bolts a minimum of two tests are required. If the trip speed of the second test is higher than the first, then a third test is required. Electronic systems can be tested at lower speed. Where there are exceptions to this rule the specific test requirements would have to be recommended by the turbine OEM.

Another very large owner/operator allows low speed trips for testing but requires a “full stress” (full speed) test after any front standard work that could affect speed sensors, wiring, and gap.

Specific test recommendations will come from OEMs and insurance carriers and each owner/operator will establish test requirements for a specific unit. There are a number of studies that discuss testing frequency based on recorded failure probability either across the industry or for a given site. These procedures should not be considered static. As more experience is gained with the unit or as the unit is modified, these procedures, test scope, and test intervals, should be reexamined. Also, as root causes of incidents of all units are examined and shared, new best practices may be recognized.

Low Speed T esting

On generator applications, the full overspeed test point exerts tremendous forces on the rotating components of the turbine and generator. This can lead to increased maintenance costs in the long run so there is some incentive to perform the overspeed system test at a lower rpm.

On compressor or pump applications, it is often (but not always) physically impossible to achieve the overspeed setpoint with the compressors or pumps connected. So users perform overspeed tests with the units uncoupled. Uncoupling and recoupling is a labor intensive process. When uncoupled, the rotational inertia of the system is greatly reduced. This means the control dynamics for the normal system and the uncoupled system are different. This may require entering a different set of dynamics or retuning the governor system for testing (and making sure the original governing system dynamics are restored when the test is complete). In some cases special functions have been added to the governing system to support additional modes for uncoupled testing. Again, there is a risk that the modes of operation be specified or selected correctly.

In both of the above cases there are compelling reasons to perform at least some of the tests of the overspeed protection system at a lower speed. But, insurance carriers and corporate testing requirements must also be met. Some sites require validation of the trip system functionality at full speed but then allow lower speed tests provided no changes are made in the system, particularly with the speed pickup, brackets, or sensing gear that could affect the ability to properly sense speed. There are some specific case examples that demonstrate the value of full speed testing.

T rip System T esting While Running

Many units run for long periods of time, multiple years, between outages. So there may be extended periods of time where actual trip testing cannot be performed. To ensure reliability and availability of the overspeed protection system, periodic partial system testing is performed. This may include confirming mechanical/hydraulic operation of a part of the trip system or performing some simulated test on part of the electronic system while the trip circuit is partially blocked. Ideally the systems should be designed with enough redundancy that protection is not compromised during the test process (but this is not always the case).

In some systems it is possible to do some level of valve testing either at rated or reduced load conditions. While this is valuable, and in some cases necessary to ensure the operation of the valve, it does not guarantee a proper seating of the valve. So there is still a case for a full functional trip test (although this need not be from full trip speed).

Risks of T esting

An overspeed event requires three components:

• Loss of load,
• Failure of the control, and
• Failure of the overspeed protections system.

During an overspeed test the unit is not loaded and the control may be partially compromised by operating above normal operational speeds and, where driving equipment is uncoupled, with a significantly reduced inertia. The overspeed test requires putting the protection system into an abnormal state—either by changing setpoints, overriding normal governor limits, and possibly operating with a different set of system dynamics. So the last line of defense may be the very protection system that is being tested to confirm its functionality. A significant number of overspeed incidents, by some estimates nearly half, occur during testing. Obviously some preliminary testing is required prior to performing a full overspeed test. This may include performing a low speed test prior to full speed testing.

After a successful test is performed it is equally important that the system be restored to the normal operating state. A second set of tests, such as simulated speed testing if the setpoint was changed, should be performed to confirm the system is correctly restored to its normal operating conditions.

INCIDENTS AND ROOT CAUSE ANALYSIS

In one case a boiler feedpump turbine (BFPT) was tested at less than full overspeed trip for years. The setpoint was dialed down for the test and then reset. A safety audit required a full speed test. At 8000 rpm the unit did not trip and an emergency trip was selected. Next year the trip failed again. The third year the controls group got involved. The trip system electronics and program appeared to be correct. When the turbine was tested, above 6200 rpm the overspeed device started to miss pulses and the sensed speed dropped but not enough to trigger loss of speed sensed logic. Work was done by the OEM on the front standard and on the sensing wheel and the unit worked. Although the exact cause of the problem was not identified, one thought is that the rotor shifted slightly at high speeds and that only in those conditions would the speed signal be partially lost. Tests with a frequency generator to simulate speed would not have exposed this problem.

In another case a BFPT was tripped for no apparent reason. Just prior to this, another BFPT had tripped and the distributed control systems (DCS) increased the demand to the second BFPT. It picked up speed and tripped. It was not immediately obvious that an overspeed had occurred. On analysis of the data it was recognized that it tripped at the mechanical overspeed trip setpoint. The electronic governor setpoint was scaled as 0 to 10,000 rpm when the operating range was only around 8000 rpm. The controllers protected overspeed were set to 105 percent of this total range (not 105 percent of the normal maximum operating point). On this unit additional instrumentation was added to annunciate the overspeed bolt trip and the controller was rescaled to the proper operating range.

It is critical that the industry look to and share incidents and root cause analysis to determine how to prevent the repetition of mistakes.

RESPONSE TIME TESTING

API 612 (2005) 16.3.4.6 states, “The response time of the overspeed shutdown system shall be recorded to confirm compliance with the requirements of 12.3.1.1.” But this is in a list of optional tests that may be performed as shop tests and not a regular test requirement.

A great deal of emphasis is placed on overspeed testing but this is done under controlled conditions. Speed is raised slowly to the trip point. This confirms the overspeed trip setpoint but does nothing to confirm the dynamic response of the trip system. This factor is as critical as the setpoint.

The authors have shown that the response time of the entire trip system is critical to the protection of the turbine and that relatively small variations (undetectable to an operator) are very significant. Since the timing of these electrohydraulic circuits is so important and the systems so complex, the need to periodically record the trip time and then trend those response times to identify any progressive degradation in the system seems obvious. In the past, achieving a resolution of 1mS to evaluate the response time of a trip system would require specialized equipment but today this is readily available from sequence of event (SOE) cards in DCS systems and programmable logic controller (PLC) systems.

Periodic trip system response time testing, as with the overspeed setpoint testing, is needed to ensure the overspeed prevention system is operating as designed and that the required level of equipment and personnel protection is provided.

Related Content:

What Causes A Generator Overspeed Fault?

• September 17, 2023

If your generator has an overspeed fault, you may wonder about the cause and how to fix it. Fortunately, we have done some research for you, and here is what we found.

Generator overspeed fault could be a result of the following:

• Excess fuel in the combustion chamber
• Changes in speed regulator
• Faulty components

It is best to tackle the problem right away so it doesn't worsen. Keep reading to get detailed information on how to deal with generator overspeed faults.

The phenomenon known as overspeed occurs when an engine is compelled to turn faster than it should.

Overspeed in a generator can be seen as any speed above the manufacturer's recommended operating range. Overspeed frequently causes severe harm to the engines of generators.

The following are common causes of  generator overspeed :

Excess Fuel In The Combustion Chamber

Usually, you want your generator to be full to guarantee steady power, but a high engine oil level also increases the risk of a generator overspeeding.

This is because the extra oil will burn in the combustion chamber, speeding up the pistons and perhaps resulting in overspeed.

Changes In Speed Regulator

Most generators, if not all, are built with correct speed limits, which are built into the system, although this does not guarantee that they work effectively.

There might be significant damage if the regulator makes a mistake and changes or modifies its speed regulations.

When power is delivered without or with a light load, most uncontrolled engines will overspeed.

Faulty Components

Like any machinery, practically every component is made to perform and function together. Due to this, any potential component failure in any part might affect and cause overspeed.

Issues associated with the fuel injection pump and the accelerator pull rod are a few more prominent problems.

What Should I Do If My Generator Overspeeds?

When an engine overspeeds , the following actions should be taken right away to tackle the problem:

Check The Speed Regulator

If your generator is overspeeding, you should check the speed regulator. If it is already faulty, make sure to fix it, as this may stop the overspeeding.

Examine The Governor's Adjustment

The generator may overspeed if the governor is not correctly set. Look for any air intake or exhaust limits if the generator is still running too quickly.

These limitations may make the engine work harder and operate more quickly. Clean the air intake and exhaust, and remove any obstructions. If the generator is still running too fast, the engine might have a problem.

Check The Generator's Load

Another thing you can do is check the load your generator is carrying. If it is too much, reduce and power designated appliances. If the generator is overloaded, it may overspeed.

Use Mechanical Overspeed Trip

Mechanical trips usually function like a raking system, where several parts cooperate to slow down and prevent the pistons or turbines of the generator from operating. This will aid in tackling an overspeeding generator.

Use Electronic Overspeed Trip

Since they operate automatically without human intervention, electrical trips are likely the most often used for most modern diesel generators.

It does this by comparing the set value , which is frequently 10% over the rated speed, versus the detected value.

Then, it will register if an overspeed has already taken place. So, the generator will automatically stop running at this point and start slowing down. You can also contact a professional to help you resolve the issue.

How Do You Test An Overspeed Generator?

To test for overspeed on your Generator, kindly adhere to the following steps :

Step 1: Change The Generator's power Switch To On

Set the choke of the generator to Start. After that, try turning the generator's key to the On position. If your generator has a recoil cord, pull it so the engine will come on.

Change the voltmeter's dial from the Off position to the "AC Voltage Test" position. If you are unsure which setting is the AC voltage testing setting, refer to the voltmeter's manufacturer's instructions.

Step 2: Connect The Voltmeter To The Generator

Use the alligator clip on the cable's other end to attach it to any part of the generator's frame after inserting the black cable with the voltmeter into the black socket .

This will ground it to prevent electrical wires and other components from being harmed by sudden power surges.

Connect the voltmeter's red lead to the generator's output plug outlet. Any power cord should be plugged into the output plug socket of the generator.

To read the voltage, insert the metal tip at the end of the red cable into the output plug of the generator after inserting the red cable's red socket into the voltmeter.

Depending on the voltage, the generator may have different output plug outlets. The outlets can appear different but will be named. Both of them can be tested in the same manner.

Step 3: Carry Out The Test

Turn up the engine speed when there is no load on it. At 115% of the speed limit, an overspeeding journey will shut off the fuel supply.

Step 4: Turn Off The Generator

To turn off your generator, flip the power switch to the OFF or ignition switch to the OFF position.

How Does Overspeed Affect A Generator?

An engine's speed is typically expressed in revolutions per minute [rpm].

The effects of operating an engine too quickly vary depending on the kind and model of the engine and are influenced by several variables; the length of the overspeed and the speed reached seem to be the most vital.

Momentary overspeed with some engines can lead to significantly shorter engine life or catastrophic failure.

  Since every machine component is made to function together, any potential component failure in any part might affect the other parts.

Leaving an overspeed fault unresolved for an extended period can damage the engine block, conn rod, crankshaft, bearings, and the generator at large.

Can You Test The Overspeed Trip Of A Generator?

Yes, it is possible to test an overspeed trip on a generator. Depending on the generator you are testing, you can use either a combination of mechanical, electronic, or any. For a generator set with electronic control, it is the easiest to check.

Testing The Overspeed Trip Of A Generator

Usually, you can enter the controller menu and look for a test mode.

Usually, this involves decreasing the overspeed set point so that when you start the generator, its average operating speed is higher than the overspeed test trip set point, causing the unit to alarm and immediately shut down.

After that, you can revert the overspeed set point to its default value for everyday use by returning to the menu. Some systems offer a straightforward "over speed shutdown test" key switch or button that can be pressed.

The typical method for systems with mechanical trips is to gradually increase the speed setpoint until the engine speed exceeds the designated overspeed setpoint. At this point, the engine shuts down.

In each of these scenarios, you should be aware of the exact overspeed setting, which is often around 5 percent over the average operating speed, and take care not to overspeed the generator to an unsafe level.

Generally, the generator is not filled throughout the test in each situation.

To ensure a safe, harm-free test, you should always be able to locate the equipment's pertinent documentation and follow the manufacturer's instructions.

Generator overspeed fault can be caused by different problems, including e xcess fuel in the combustion chamber, change in speed regulator, and defective components.

To deal with the issue, you must check the speed regulator and load of the generator. You can also implement an overspeed strip.

If you enjoyed reading this post, here are similar articles you may like:

What Size Generator To Run A Heat Pump?

What Size Generator Do I Need To Run A Space Heater?

How Long To Run An Ozone Generator In A Bedroom?

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Lo purifier low flow alarm comes and then the purifier back pressure does not come back up and even feed pump pressure drops.This happens after 3 days what could be the reason

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MEP-804A random overspeed shutdown

• Thread starter micochico
• Start date Jun 24, 2020

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I am helping a friend troubleshoot a recently purchased MEP-804A that randomly shuts down for an overspeed trip. Sometimes it trips almost immediately after startup and other times will run for 5+ hours. Sometimes it will actually trip immediately when cranking before the engine starts. I have never witnessed the generator actually speeding up when the trip occurs. It will even trip with the load contact open and the trip doesn't even seem related to actual slight increases in frequency that occur when the electrical load is reduced. The generator seems to do an excellent job regulating frequency and voltage with varying loads. I have confirmed the mag pickup to be ~2.4V while cranking. I believe it increase to about 9.8V with the genset running at 60 hz. I ran the genset up to 66Hz by adjusting the freq trimpot on the woodward governor controller and it did not overspeed trip. I adjusted the overspeed trimpot down on the woodward governor and it did not trip until I had gone several turns downs. When it finally did trip the generator would not crank or restart (overspeed as soon as cranking was attempted). I adjusted the overspeed trimpot back up a couple turns and the generator would start and run but still randomly overspeed trips. TM makes it sound like overspeed trimpot should be about 1Hz per turn. My only thought is the governor controller is bad or the overspeed trimpot is broken. Can anyone advise anything else to check or additional testing to do to the mag pickup which I have concluded (maybe incorrectly) is the only speed reference for the overspeed trip circuit? Thank you for any assistance Tom  

Get a chair, with a nice seat on it. Start the set up, and let it run. Flip up the S7 Battleshort switch. Let the set run. Dont need a load. Watch it. Will the set at some time show an over speed fault on the A2 malfunction indicator? Will the set keep running when this happens? If the set should go into over speed, that's the reason you are sitting in the nice chair. To be there, and turn off the set, should it happen.  

Thank you for the reply. Based on my interpreation of the TM 9-6115-643-24 section 1-12.1.7 the battle short switch will NOT override the engine overspeed circuit. Can anyone confirm this is true? It is definitely the overspeed circuit tripping and the A2 panel indicator illuminates overspeed when the genset shuts down. As soon as the light comes on the set shuts down or vice versa as soons as the set shuts down the light is on. Should the light come on first before the set shuts down or will the battle short let the light come on and the set keep running? It is stranger to me that it sometimes trips overspeed when first trying to crank when the engine is definitely not overspeed. I feel like the Mag pickup side of the woodward controller must be working because the governor does a good job of keeping the genset at the set frequency and I don't know how it would be able to do that if it was getting an inconsistent or bad signal from the MPU. I assumed my MPU voltage of 2.4 VAC while cranking is ok because I seek the spec of 2-3V listed. Does the range mean any value between 2 and 3V will work or mean some units like 2 volts and some like 3 volts and you must find what works on each particular unit? Does this need to be adjusted down by increasing the MPU air gap which would lower the voltage of the MPU signal at the governor? My assumption is that the governor actually reads the frequency of the MPU signal to determine engine speed and not the voltage. I never see the genset output Hz spike or hear the engine speed up when the unit goes over speed, it just calmly shuts down for no reason. Thank Tom  

Tom, Indeed, the MPU only reads RPM. The closer to 3 volts AC that you get from the MPU, the better. If you want to pull the MPU and clean/readjust it, by all means do so. But it sounds like it works. The reason I wanted to see if the S7 was engaged, was to see if the light came on before the fault, or after it. Next thing I would check is K14. Attached is the test procedures. Ignore the K20 shown below. I used this to help someone else, with a different problem. Same type relay, just different wires. Anyway, test the relay, AND look for loose wires, broken connectors or any other thing wrong with the relay, and socket. COULD be just loose, never know. Are you good with schematics? Have you down loaded them? You are on line now, so maybe we can do some real time troubleshooting.  

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Mechanical overspeed protection or electronic overspeed protection?

10 April 2020 | Theme: Machine protection

In the past, overspeed turbine protection systems were often designed by the turbine OEMs themselves as part of the mechanical system. These mechanical protection systems are still in use, especially on medium-sized machines. However, electronic systems offer significant benefits and are becoming increasingly accessible, in both their economical and technical impact. This article explains the benefits of electronic protection over mechanical protection.

The principle of mechanical overspeed protection

To understand the difference between the two types of protection, it is important to understand how mechanical overspeed protection works. Mechanical overspeed protection systems use a weight on a spring of which the spring force is known. Due to the centrifugal force as a result of the shaft rotation speed, the spring is stretched further and further and the weight moves out. As long as the rotational speed is still within the design specifications, the spring will not stretch too far and the weight will not protrude. However, if it exceeds its design specifications, the weight will run out and hit the trip bolt. This will release the pin and the trip will be activated.

The operation of a mechanical overspeed protection system.

The principle of electronic overspeed protection

Electronic overspeed protection consists of a gear or other measuring surface, speed sensors and a measuring system with trip logic and relay outputs. The speed sensors look at the measuring surface and give a sine or square wave signal, which the measuring system sees as pulses. The measuring system then counts the time between pulses to determine the rotation speed. The relay outputs are controlled by trip logic. In order to increase safety and availability, these protection systems are often built with redundant voting structures.

Why switch to electronic overspeed protection?

Switching from mechanical overspeed protection to electronic overspeed protection has significant advantages. Verifiable security is the primary argument. To illustrate: where mechanical overspeed protection systems have to be tested by actually overspeeding the machine (with all the risks involved!), an electronic overspeed protection system can be tested by simulating overspeed. In addition, a simulated test is quicker and easier to perform.

In this overview we compare both forms of machine overspeed protection:

Switching to electronic overspeed protection is not complex. Firstly, the mechanical trip lever and all associated system components are removed. Secondly, a measuring surface (gear) is mounted on the shaft if this is not already available, after which one or more sensors are placed around this measuring surface. The protection system can then be placed near the machine or in a cabinet.

Learn more about overspeed:

What does the API Standard 670 state about overspeed detection systems? »

The SpeedSys is a SIL-rated overspeed protection system for rotating machinery. It delivers the core layer of protection with a compact architecture. Its small technical footprint and low-impact installation enables advanced protection to a wide range of applications. SpeedSys is available in two versions. SpeedSys 200; for less demanding SIL 2 applications and SpeedSys 300 for demanding and critical SIL 3 applications.

SpeedSys 200 »

SpeedSys 300 »

What does the API Standard 670 state about overspeed detection systems?

5 speed-related failures with rotating machinery, sensors for speed measurements, knowledge & cases.

Learn more about speed measurements and overspeed protection on our blog.

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Marine Engineering Study Materials

Information for marine engineers, browsed by tag: test g/e over speed trip, meo oral examination questions on engine governors, october 30, 2015 jaz comments 2 comments, what is governor .

• Governor is a device which controls the speed of engine automatically in the prescribed  limits.
• The governor does its job in two steps.
• By measuring the speed and
• By controlling the a amount of fuel supply to the engine.

What is the function of governors ?

• To adjust the rate of fuel supply in such a way as to keep the engine running at a steady  speed regardless of the load.
• To control the engine running at a steady speed under all conditions of load.

What is governor droop?

• When considering the engine and governor combination,  the difference between the no load  speed and full load speed  is called governor droop.
• Small droop results in rapid swing.
• Large drop results in slower response to change in speed.

What are the types of governors ?

• Mechanical governor.
• Hydraulic governor.
• Inertia governor [fitted on older slow speed engine]
• Electronic governor.

What is compensation ?

• The use of temporary speed droop to prevent over correction of the fuel supply is called  compensation.

It require two actions:

• Droop application – as the fuel supply is changed
• Droop removal – as the engine response to the fuel change and returns to original  speed .

What types of governor used in G/E and M/E ?

• G/E governor is a droop speed governor with over speed trip.
• M/E governor is a constant speed governor with over speed trip.

Why over speed trip is fitted in main engine ?

• It is fitted to control the sudden load change and sudden increase in speed. e.g severe load  change when loss of propeller in heavy sea .
• Thus, to prevent engine parts damage, over speed trip  is provided.
• Set the speed 15% more than MCR.
• It is attached at camshaft through gearing.
• If engine speed rise more than 10 to 15% above the rated speed,  It shut of fuel & stops the  engine.
• The mechanism has to be manually reset before engine start again.

Why hydraulic servo governor is widely used ?

• Hydraulic governor is widely used because of their:
• sensitivity
• isochronous and
• having grater power to move the fuel control mechanism of the engine .

What is hunting of governor ?

• It is a fluctuation of engine speed due to the over correction of the fuel supply. (too much increase or  decrease fuel supply)

What is the cause of engine hunting ?

• It is unavoidable time lag between the movement of the governor act and the movement of  engine response.

What is isochronous governor ?

• It is a constant speed governor.
• It is able to maintain exactly constant speed without hunting.
• This type of governor that has proportional and reset is called isochronous governor.

What will happen when over speed occurred ?

• Engine revolving and reciprocating parts may damage due to inertia effects.

Why will you test G/E over speed trip ?

• It can be tested by increasing the engine speed by speed adjuster at no load condition.
• Over  speed trip will cut-out the fuel supply at 115% of normal speed.

Why speed droop governor is used at G/E ?

• It is fitted to get load sharing ability since isochronous governor gives constant speed, thus can not share the load.
• AVR is fitted at alternator.

Why over speed trip provided although governor fitted ?

• Governor only control the engine speed in prescribed limits.
• When accidental sudden load  change from full load to no load happens, engine speed become too high above 15% of rated speed.
• Due to time delay of governor control, engine parts may become damage.
• Thus to prevent  this effect over speed trip must be provided to shut down the engine by cutting fuel.

Note : It need to reset before restart the engine.

How to operate/act isochronous governor ?

• It maintains constant speed without hunting.
• Speed droop for it is temporary

It has 2 actions:

• Droop application (as fuel supply is changed)
• Droop removal (as engine responses to fuel change and returns to original speed)

What is sensitivity ?

• Ability to control the engine speed, within narrow limits

Why Inertia type Governor is not used nowadays ?

• Although very simple type, it requires an engine speed increase of 5% or more to make it  operate.
• In some cases, increase of engine speed will bring into or near to critical speed that can cause  severe vibration.

What are the governor  maintenance  carried out on ships ?

• Periodically check oil level
• Change the governor oil
• Grease the linkage and joint

Oral Guide by – MIN ZAR TAR