Overspeed trips

Safety features such as overspeed trip, low-od trip, remote-solenoid trip, vibration monitor, or other special monitoring of temperature, temperature changes, and casing and rotor expansion... 

In contrast to steam turbines, in which runaway overspeediug is always a problem, pump-turbines operating at design head go to zero torque at about 130 to 140 percent of design speed. Thus, overspeed protection may not be necessary if the pump-turbine can withstand 140 to 150 percent of design speed and it is the sole driver. When a steam-turbine helper is used, it should be provided with the usual overspeed trip-out mechanism. 

The rotor speed must then be inereased to trip speed to eheek the overspeed trip setting, then redueed to maximum eontinuous speed and held for a minimum of 1 hr. Extended testing time should be alloeated if neeessary to reaeh satisfaetory equilibrium eonditions. Tlie lube oil must be held near design inlet temperamre during tlie meehanieal test. Data logging should be done in 15 min intervals for all measuring points. 

The expander shall be equipped with an eleetronie overspeed trip. The piekups shall sense the rotation of a 30-tooth wheel and be eonneeted to a 2-out-of-3 voting system. 

Mechanical protective equipment, such as overspeed trips. 

Overspeed trip Releases trip valve when speed exceeds 115% of maximum rating Stainless steel... 

E.3.3.1 An overspeed trip should be considered when the HPRT and other equipment in the train cannot tolerate the calculated overspeed (runaway speed). It is important to realize that overspeed with inlet liquids rich in absorbed gas or with liquids that partially flash as they flow through the HPRT can be several times higher than overspeed with water. With such liquids, the overspeed cannot be accurately determined. 

E.4.4 It may be useful to verify the overspeed trip setting for the HPRT at the manufactnrer s test facility. Determining the runaway speed during a water test may be considered, but this speed can be accurately calculated once performance with water is known. Rnnaway speed for gas rich-steams cannot be determined by water tests. 

An ordinary American-type turbine is designed to run at 3600 rpm. Its overspeed trip will cut off the motive steam flow at about 3750 rpm. But the turbine can be run at any lower speed. There is usually a small knob on the left side of the governor-valve assembly, that is used to... 

The pump speed rises to 3800 rpm. The trip speed has been set at 3750 rpm. The turbine s overspeed trip is unlatched, and the machine shuts down. The operator relatches the trip, but every time the flow is throttled back, the turbine overspeeds, and trips off. 

Turbines, both gas and steam, also have overspeed trips. These consist of a little flywheel, constructed from three balls. The little balls are spread apart by centrifugal force. The greater the rpm, the greater the centrifugal force. If the balls spread apart too far, they activate the trip. We have James Watt to thank for this neat innovation, still used in its original form. 

Vibration monitor and overspeed trip on large-scale rotating equipment... 

Normal Consequence—Class 3. All other containment equipment or instrumentation serving the operation whose failure could result in minor environmental releases, property or production losses, or injury to personnel or reduced economic life. It may include less critical refrigeration systems, turbine overspeed trips, lubrication system alarms and similar important, but not crucial items. 

Turbine overspeed trip can resuit in loss of cooling flow to reboiler. Turbine speed must be maintained beiow 3500 RPM to prevent tripping. 

OVERSPEED TRIP - On steam turbines, a mechanism that provides absolute reliable overspeed protection by shutting off the steam supply. 

All safety devices and safety interlocks pertaining to a plant item should be made operational, calibrated, and proved before starting it. If this is not fully possible, say in the case of a turbine overspeed trip, then proving the device under controlled no-load conditions should be the first operation after starting. A witnessed checklist should be utilized for recording the proof of safety device and interlock operation. 

Steam enters a turbine through the steam chest. The steam chest typically has a strainer on the inlet side to remove solids. Inside the steam chest is a device called the governor valve. The governor valve opens and closes to admit steam into the turbine. A governor system controls the position of the governor valve. An overspeed trip mechanism is attached to the rotor and will shut off the flow of steam into the turbine when it reaches 115% of its design limit. The shutoff valve is typically located in front of the governor valve. 

Overspeed trip set low Ammeter trip (fuse) set low Motor running hot... 

A routine shutdown of a piece of equipment offers an excellent opportunity to test alarms and trips. For example, if a turbine is to be shut down for maintenance, its speed can be carefully brought up to see if the overspeed trip is operative. It is no exaggeration to state that if an overspeed trip is inoperative, the turbine should not be run. 

When a compressor is running at its maximum rated speed, it is speed-limited. The maximum rated speed is stamped on the compressor nameplate. There is no physical restraint to prevent operation of the compressor above its nameplate speed. However, compressors are equipped with overspeed trips, which automatically shut the compressor driver down when the compressor reaches a preset speed. Certainly, compressors that run well above their nameplate speed can self-destruct in a dramatic and tragic fashion. 

When a compressor is reported to be speed-limited, one should take a tachometer and compare the actual speed of the machine against the nameplate speed. The machinist who set the overspeed trip may have set it too low. The operators are careful not to let the compressor trip off. They avoid this by running at speeds 100 rpm to 200 rpm below the set trip speed. 

USNRC Generic Letter 90-04, Request for information on the status of licensee implementation of Generic Safety Issues resolved with imposition of requirements or corrective actions, April 25, 1990. USNRC Information Notice 93-05, Repetitive overspeed tripping of turbine driven auxiliary feedwater pumps, July 9, 1993. 

In addition to the various turbine trips, two independent and redundant overspeed trip systems, the mechanical overspeed trip (OST) system and the electronic overspeed system (EOS), are provided to trip turbines at 110% and 111% of rated speed, respectively. Therefore, this system is considered to meet the regulatory requirements. 

Turbine over speed could result in a maj or turbine feilure that produces missiles these would pose an internal hazards threat (see APIOOO Internal Hazards Topic Report, Reference 6.3). The turbine stop valves and the reheat stop valves are all closed by actuation of the overspeed trip system, which is completely independent of the turbine control system (Sections 10.2.2.4.3 and 10.2.2.5.1 of Reference 6.1). The probability of destructive overspeed condition and missile generation, assuming the recommended inspection and test frequencies, is less than 1x10 per year. In addition, the orientation of the turbine-generator is such that a high-energy missile would be directed away from the nuclear island (Section 10.2.2 of Reference 6.1). 

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