Overheat protection

Small compressors will have motor overheat protection adjacent to the hermetic shell or built into the winding (see Section 4.8) and larger motors will have contactor-starters with overcurrent devices. Overheat protection is also fitted on many machines, to guard against high motor winding, cylinder head or oil temperatures. These usually take the form of thermistor detectors, connected to stop the motor. 

Temperature sensing Temperature indicating Temperature measurement Overheat protection for Power transistor, transformers... 

Task detail 1) Calculate the spurious trip rate and the fail to danger rate for a single channel trip loop on an overheat protection example. 2) Model and calculate the same for a 2oo3 sensor arrangement. ... 

When the motor overheats, the thermostat opens, interrupting motorline current. Pilot thermostats mounted on the windings of larger motors trip the motor starter rather than interrupt line current. This method gives good protection for sustained overloads, but because of the thermal time lag between the copper winding and the thermostat it may not provide adequate protec tion for stalled conditions or severe overloads. 

If the field excitation is also lost, the generator will run as an induction motor again driving the primer mover as above. As an induction motor, it will now operate at less than the synchronous speed and cause slip frequency current and slip losses in the rotor circuit, which may overheat the rotor and damage it, see also Section. 1.3 and equation (1.9). A reverse power relay under such a condition will disconnect the generator from the mains and protect the machine. 

While great public protection is provided by these barriers, accidents can happen. Regardless of the cause of an accident, the core cannot overheat while in contact with liquid water. Furthermore it cannot be critical in the absence of water (because of the low enrichment of the fuel) thus, any accident involves a subcritical core that is heal by decaying radionuclides with inadequate cooling. Figure 8.1-1 shows the rate of heat evolution as a function of time after shutting down a 3,0(K3-MW reactor (Cohen, 1982). Even after an hour, th leat production is about 40 MW. 

When all the solid has been added, the solution is gently heated until the evolution of hydrogen chloride has ceased care must be taken not to overheat it otherwise some product may be lost through volatilization. The material is then distilled, using an air condenser, until the temperature of the vapor reaches 165°C the receiver consists of a 250-ml round flask protected from moisture. As the acid attacks cork and rubber readily an all-glass apparatus must be used. The crude product is redistilled from the receiver and the colorless liquid is collected at 149-154°C pure chloro-... 

Features on the pump, which are important to the safe operation of the unit operation, is setting of the internal relief to protect the pump, column, valving or piping. The design pressure of the pump needs to exceed the maximum equipment design pressure. It is important not to undersize the pump and to calculate that the motor has sufficient running time compared to the ramp-up time to not overheat the motor. 

The DBA fire scenario was an electrical wiring short or overheats that caused an electrical fire. Then the fire spread to combustible packaging material and spread to involve all of the radioactive material in storage. Power isolation could prevent a fire of combustible material other than the electrical wiring if the short or overheat condition were detected promptly by an operator or isolated by an automatic circuit protection device (circuit breaker). To investigate fire termination by power isolation, the event tree analysis for this DBA considered three scenario variations ... 

The heating effect of an electric current is also the principle upon which a fuse gives protection to a circuit. The fuse element is made of a metal with a low melting point and forms a part of the electrical circuit. If an excessive cument flows, the fuse element overheats and melts, breaking the circuit. 

Do not overheat solutions and remember that solutions and containers can be very hot. Use protective face/eye equipment and protective gloves. Superheated water can explode when the container is moved, causing severe burns. 

Centrifugal pumps are subject to mechanical damage when they lose suction pressure or when the discharge flow is blocked. Furnace tubes may overheat and fail when liquid flow through the tubes is greatly reduced. Low-flow trips protect process equipment against these failures. This trip may be used to shut down a pump or block off fuel to a furnace. To test an orifice-type low-flow trip, proceed as follows ... 

The winding insulation in electric motors of the enclosed ventilated type will overheat and may break down if the ventilation is impaired, e.g. by foreign matter clogging the air vents and dncts. The resultant short circnit between turns or to earthed metal may cause an ignition, but may not canse a sufficient increase in the supply current to operate the excess current protection. 

Event 7 Temperature Control Failure during Heat-up Step—Overheats Batch This event leads to a runaway reaction similar to Event 1. The event impact and protection aspects are similar to Event 1, except that the temperature transmitter is not considered as suitable as risk reduction for this event. During heat-up the operator has time to add the Shortstop to prevent the mnaway. However, since the temperature transmitter is used for control, it may be part of the initiating cause of this hazard. Therefore, operator response to a high temperature alarm would not be an IPL for this event. 

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