Overspeed condition

Check valves are required in the piping system at any point where backflow of gas after a shutdown has the ability to restart the compressor, running it backwards or, for that matter, even in the normal direction. Reverse rotation is totally bad, as many components of the various compressor types are not designed for reverse rotation, and there is some possibility, generally remote, that the compressor could reach a destructive over speed. Forward rotation is bad primarily because the intent was to stop the compressor, and it is now operating out of control. This is a problem, particularly if the shutdown was caused by a compressor failure indication, and the need to stop was to prevent further damage. In this mode, it is unlikely that the compressor can attain an overspeed condition. An application with a high potential for backflow is the parallel operation of two or more compressors. 

This condition can be detected by a variety of devices, such as centrifugal switches, electronic tachometers, or strain-detecting devices installed on or near such components of the machine that are affected by the overspeed condition. 

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). 

As previously mentioned in Chapter 5, one of the most severe disturbances for the power recovery train (PRT) is a generator breaker opening. This event often causes the PRT to trip on overspeed or other process or machine conditions. A control solution has been developed to keep the PRT and the process under control during breaker opening. 

C.3.3.1 An overspeed trip should be considered if the HPRT and other equipment in the train cannot tolerate the calculated runaway speed (the maximum speed reached by the HPRT when unloaded and subjected to the worst combination of specified inlet and outlet conditions). Typically, overspeed trips are set in the range of 115 % to 120 % of rated speed. It is important to realise that runaway speed 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 the runaway speed with water. With such liquids, the runaway speed cannot be accurately determined. 

ISO3046/IV Reciprocating internal combustion engines Performance. Part 1 Specifications for standard reference conditions and declarations of power, fuel consumption and lubricating oil consumption. Part 2 Test methods Part 3 Specification of test measurements. Part 4 Speed governing. Part 5 Torsional vibrations. Part 6 Specifications for overspeed protection. Part 7 Specifications for codes for engine power. 

If the bond between the copper tube and the A286 support tube is neglected, the centrifugal pressure of the copper on the overwrap is 2.52 MPa (366 psi) at 12,000 rpm, which produces a hoop stress of 298 MPa (43,200 psi) in the composite overwrap. The hoop stress in the overwrap itself, due to the 12,000-rpm rotation, is 150 MPa (21,700 psi). Hence, the combined hoop stress in the overwrap is 447 MPa (64,900 psi) at design speed and 541 MPa (78,500 psi) at the overspeed (13,200 rpm) condition. The observed tensile strength in the hoop direction for the composite at 77 K was —896 MPa (130,000 psi). Hence, sufficient structural protection under rotation should be provided by the overwrap. Warm tests on the complete generator were recently conducted up to 12,000 rpm with no apparent problems with the shield construction. 

Table 5.9 — Participating condition of each unit in automatic 4 -(jear (overspeed Gear)...

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. 

Thermal and Overspeed Cutouts The temperature of an electric motor will rise during use. If the temperature exceeds a certain value, a dangerous condition may exist. A temperature-sensitive switch with a preset temperature limit can interrupt power. Some electrical motors, equipment with resistance heaters (such as hair dryers), and other electrical equipment have thermal cutout switches. 

Overspeed switches sense when a motor or other device operates too fast. Excessive speed may create dangerous conditions or indicate failure in the equipment. If a motor reaches excessive speed, the switch interrupts power to the equipment. 

Thermal and overspeed cutout devices are coimnonly used to protect electrical equipment (and thus the operator). A thermal cutout is simply a temperature-sensitive switch with a preset limit designed to interrupt power when the temperature exceeds a certain value. As its name implies, an overspeed switch operates when it senses that a motor or other device is operating at too fast a speed. Obviously, excessive speed may create dangerous conditions and indicate failure of equipment. The overspeed switch operates to shut down an overspeeding device by interrupting power to it. 

As discussed earlier the ESD system normally operates in a normally energized condition. Therefore initiating Pb, limit switch contacts, etc. shall be a normally open (NO) contact, but a few contacts such as contact from the motor control center (MOO), overspeed, reset/stop push button, etc. are normally closed (NC)-type contacts (refer Fig. VIII/4.1.4-1). 

Overspeed of the pump motor creates a more flow and more pressure condition. This would be disastrous to the ready tank, causing overfill of the tank. The recommendations for 2.1 and 3.3 are identical because even though the hazard was identified through different guide words, the cause and effects are the same. 

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