Alarms and Trips

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 

A less common type of fuel-gas trip to a heater is a low-pressure trip. A pressure transducer generates a milliamp output from a boiler feedwater pump. Should this milliamp output fall below a certain level, the instrument air signal to the fuel-gas regulator actuator will be shut off. These fuel-gas valves are air-to-open, meaning that loss of instrument air flow causes the valve to close. 

Some fired heaters, especially boilers, have a device called a purple peeper, which is simply an optical device that looks at a flame. If it does not detect light with a wavelength in the high-frequency (i.e., purple) end of the optical scale, it interprets this as a flame-out. The fuel-gas regulator is automatically shut. 

Some heaters also have a low fuel-gas pressure trip on the fuel gas itself. The idea here is that if fuel-gas flow is lost, we do not want it to surge back into the heater too quickly if the fuel-gas pressure is suddenly restored. 

Our home circuit breakers, or fuses, are, of course, trips to prevent overheating electric circuits or electric motors. The only difference is that at work our electric circuit breakers have a built-in time delay. This is needed to allow the motor driver to overcome the starting torque inherent in most large pieces of rotating process equipment. 

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 

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The minimum alarms and trips reeommended for eaeh major driver and driven maehine should be a low oil pressure alarm, a low oil pressure trip (at some point lower than the alarm point), a low oil level alarm (reservoir), a high oil filter differential pressure alarm, a high bearing metal temperature alarm, and a metal ehip deteetor. See Table 15-1. 

Plant engineer responsible for mechanical maintenance such as testing of alarms and trips, as well as installation of new instruments. 

Alarms and Trips Can Make Overfilling More Likely... 

Many accidents have occurred because instrument readings or alarms were ignored (see Sections 3.2.8, 3.3.1, and 3.3.2). Many other accidents, including Bhopal (see Section 21.1), have occurred because alarms and trips were not tested or not tested thoroughly, or because alarms and trips were made inoperative or their settings altered, both without authority. These and some related accidents are described below. 

All protective equipment should be tested regularly, or it may not work when required. While it is sufficient to test relief valves every year or every two years, instrumented alarms and trips are less reliable and should be inspected every month or so. 

Number of alarms and trips found out-of-order when tested... 

It follows that it is good practice to separate chemical plant safety systems from control systems, so that the number of components common to both is minimised. Whilst it is quite possible to specify that control systems should raise alarms and trips when measured variables move out of bounds, a likely reason for this is that a part of the control system failed in the first place. So the control system cannot be relied upon to raise alarms reliably. 

Critical process safeguards as alarms and trips. A Process Hazards Analysis may have questioned the lack of a loss of heat transfer fluid flow alarm and burner firing shutdown for a flow loss condition. 

First, safety critical systems must be reliable. These systems control releases in the event of accidents. It s necessary to have a critical analyzer, instrument and electrical system test program. This should consist of preventive maintenance and alarm and trip device testing for panel alarms, emergency isolation valves and other critical components. [7]... 

Class 1 safety instrumentation loops include alarms and trips on storage tanks containing flammable or toxic liquids, devices to control high temperature and high pressure on exothermic-reaction vessels, and control mechanisms for low-flow, high-temperature fluids on fired heaters. Other Class 1 instruments include alarms that warn of flame failure on fired heaters, and vapor detectors for emergency valve isolation and sprinkler-system activation. All of these alarms, shutdown valves, and other critical instruments are regularly proof-tested to a well-defined schedule. 

Cause and effects charts (SAFE charts) with schedule of alarm and trip settings. ... 

Check all alarm and trip actions by varying the loop controller input signals and adjust as necessary. 

Control of the process. Prevention of hazardous deviations in process variables (pressure, temperature, flow), by provision of automatic control systems, interlocks, alarms and trips together with good operating practices and management ... 

The gas accumulates in a special chamber which is fitted with two float switches, and these operate alarms and trips when the gas accumulates slowly or rushes in during internal explosions. This system is called a Buchholz relay, and is normally used only on transformers fitted with conservator tanks. Transformers below about 1600 kVA are often sealed type liquid-immersed units. Internal explosions are released by using a special blow-off valve. Often the space above the liquid level in this type of unit is filled with an inert gas such as nitrogen. 

The result of the oil ingress was an 18-month shutdown while PSP valve and filter assemblies were removed and new filters were fitted. The pump seal oil systems were modified to prevent any further possibility of oil ingress, and alarm and trip systems were added to prevent blockage of the pump filters in order to protect the subassembly filters. 

The addition of safety equipment to a process may lead to an increase in the number of shutdowns. For example, adding fire eyes to a furnace will improve its safety because they will warn of situations where a burner flame has gone out. However, fire eyes may fail internally, thus leading to spurious alarms and trips. 

RECOMMENDATIONS FOR SAFE MANUFACTURE The main hazard which could arise in this process is the decomposition of the reaction product. The experimental investigation has shown that this will not occur at normal process temperatures but could occur if the batch was overheated. Even though no decomposition of the reaction product was detected in the adiabatic test at 140°C, the increase in temperature needed to initiate selfacceleration is relatively small and this possibility cannot be excluded. The options for safe manufacture include the design and installation of a suitable emergency relief and disposal system or the fitting of alarms and trips to ensure that overheating does not occur. 

Provision of photoelectric flame monitor with warning alarms and tripping system for fuel supply shall also be made if flame is extinguished due to any reason. 

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