Trips, safety

A safety trip can be incorporated in a control loop as shown in Figure 5.24a. In this system the high-temperature alarm operates a solenoid valve, releasing the air on the pneumatic activator, closing the valve on high temperature. However, the safe operation of such a system will be dependent on the reliability of the control equipment, and for potentially hazardous situations it is better practice to specify a separate trip system such as that shown in Figure 5.24b. Provision must be made for the periodic checking of the trip system to ensure that the system operates when needed. 

A common approach for reactors has been the provision of preventive measures, including control systems and safety trips, backed up by an emergency relief system. The addition of a relief,system is seen as having a number of advantages ... 

One of the most common safety trips is the automatic fuel-gas shutoff. We have this at home on our furnaces. We ignite the pilot light manually. The pilot light heats a thermocouple. The milliamp output from this thermocouple opens the fuel-gas valve to the main burner. The gas in the main burner is ignited from the pilot-light flame. Should the heat from the pilot light diminish below a certain point, the fuel gas to the main burner, as well as to the pilot light, will be shut off. 

Safety shutdown valves, which are normally wide open and operate infrequently, are expected to respond to a safety trip command reliably and without fault. To achieve the level of reliability required in this application, the safety valve must be periodically tested to ensure positive operation under safety trip conditions. To test the operation of the shutdown system without disturbing the process, the traditional method is to physically lock the valve stem in the wide-open position and then to electrically operate the pneumatic shutdown solenoid valve. Observing that the pneumatic solenoid valve has properly vented the actuator pressure to zero, the actuator is seen as capable of applying sufficient spring force to close the valve, and a positive safety valve test is indicated. The... 

This example illustrates how the techniques used in an operability study can be used to decide the instrumentation required for safe operation. Figure 9.6a shows the basic instrumentation and control systems required for the steady-state operation of the reactor section of the nitric acid process introduced in Figure 4.2. Figure 9.6b shows the additional instrumentation and safety trips added after making the operability study set out in this section. The instrument symbols used are explained in Chapter 5. 

The lower safety trip bar located at the front of the platen is spring-loaded. After it is pushed downward to stop the press, it returns to its original position. Resetting the safety trip is not necessary Pushing the start button can restart the press. 

Developments on simulation and staff training in the UK and elsewhere in the West appear to be superior to those available in the USSR and the Eastern Bloc. Although there may be problems, there is scope for interchanges and perhaps sale of equipment in this field. This may also be the case with nuclear installations in countries outside the OECD. There is some scope for further international conferences on the subject. Other areas for potential sales are fastacting control rods and systems to avoid operators by-passing essential safety trips. 

The expected value of the structure function and hence the unavailability of the operational safeguard and the safety trip amounts to... 

Some machines use a pressure-sensitive body bar, safety trip rod or trip wire, cable, or cord. Requiring a worker to reach over a pressure-sensitive body bar to perform some function near a point-of-operation will allow any excessive leaning to trip the control and stop the machine. Pushing or pulling on a trip rod or trip wire will also stop machines in an emergency. 

Instrumentation offered Steam and lubricating oil pressure gauges, bearing temperature indicator, electrical panel instruments (voltmeter, atruneter, KW, KWH, frequency meter, etc ). Tachometer with digital remote indication, ranning time totaliser, vibration meters, safety trips. 

Safety controls use involvement of the operator as a safeguarding method, and include safety trip controls, two-hand controls, and two-hand trips. For safety trip controls, if the operator or anyone trips, loses their balance, or is drawn toward a machine, the resulting pressure applied to a bar installed on the machine will stop the operation. The positioning of the bar, therefore, is critical. It must stop the machine before a part of the employee s body... 

There is a great temptation for operating personnel to bypass such trips permanently. Refinery management, however, takes a dim view of abandoning safety trips that are supposed to protect both lives and equipment. 

Which Is more important to reactor safety - Operating Procedures or Autcraatic Safety Trips ... 

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