Low-flow trips

A furnace was protected by a relief valve on its inlet line (Figure 10-1). A restriction developed after the furnace. The relief valve lifted and took most of the flow. The flow through the furnace tubes fell to such a low level that they overheated and burst. The low-flow trip, which should have isolated the fuel supply to the furnace when the flow fell to a low value, could not do so because the flow through it was normal. 

The relief valve should have been placed after the furnace or, if this was not possible, before the low-flow trip. 

A furnace feed pump tripped out. The flowmeter was frozen, so the low-flow trip did not operate. Two tubes burst, causing a long and fierce fire. The structure and the other tubes were damaged, and the stack collapsed. 

There was no low-flow alarm or low-flow trip on the furnace. 

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 ... 

As previously described, temporarily prevent the low-flow trip from shutting down other parts of the process by use of hand jacks, gags, etc. 

Losing flow for a few seconds won t significantly upset downstream equipment, provided one has taken the precaution to bypass any low flow trips. The exception to this is the radiant tubes in a furnace, which might coke up or overheat quickly when flow is lost. 

Paddle-type devices are sometimes used for low-flow trips. These have a somewhat lower reliability than orifice-flow trips. 

Instrumented channel flow High Reduce low flow trip effectiveness. 

With alarm management, the high pressure trip would alarm, but the other associated alarms would be suppressed, since they are the expected result of the high pressure trip. On the other hand, a low flow condition from a different cause would be alarmed. Plants with managed alarms are inherently safer than those without since it is easy to silence and overlook a critical alarm in the midst of an alarm shower. However, the benefits should be balanced with the increased complexity and maintenance requirements. 

Trip of one feedwater pump (or condensate pump) 24. Feedwater low flow... 

Aeration was controlled at one of three rates by connecting either a dissolved oxygen (DO) or redox electrode, as appropriate (Section 2.4), to a meter containing a trip amplifier. The trip amplifier operated at two set points and switched two solenoid valves to allow the entry of air at a high or low flow rate through a pipe that emerged under the... 

Ionization smoke detectors contain a small radioactive source (Americium 241) which ionizes air in a small chamber. The ions flow to a charged plate giving a measurable current. Products of combustion in the chamber are not easily ionized and absorb the radiation and reduce the current. The low current trips the alarm circuit. The size and composition of the particles are crucial to successful detection so that some types of smoke or vapor are detected at very low (invisible) levels. 

An example of a causal factor chart for a relatively simple incident is shown in Figure 9-8. In this example, there are two redundant pumps, one of which is required to supply feed to a reactor downstream. The operator is requested to change-over operation from Pump A, which is running, to Pump B, which was previously shutdown. Instead of opening Pump B suction valve, the operator opens the wrong valve, causing the Reactor to trip on low flow detection. 

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. 

The caustic flow to the electrolyzer feed header is measured and monitored to ensure that there is always a flow to the cells. Magnetic flow meters are standard. The low-flow switch depicted in Fig. 11.53 starts a delay timer when it trips. After the delay set on the timer, the rectifiers will shut down. If the caustic flow recovers in time, the timer resets automatically. 

Carry-over from the coke drum will plug the heater charge pump suction screen. As the pump loses suction, flow will fall off and the stagnant resid in the tubes will coke. This difficulty is discussed in the previous chapter on delayed coking cycles. Trip off the heater charge pump on low flow (see Fig. 3-2). 

The flow of fuel to the boiler should have been slopped automatically when the water level fell to a dangerous level. Unfortunately, the low-level trip failed to function. (Many newer boilers come equipped with backup low-level trips.) The operatingengineer investigating this incident determined that this trip had not been tested for years. He set up the following program to prevent a recurrence. First, the fuel gas control valve was locked into its normal operating position with its hand jack. Then the low-liquid-level trip pot (see Fig. 17-1) was blocked in and drained down. Finally, an operator verified that the pneumatic signal to the fuel-gas control valve fell to zero (this valve was AFC—air failure closes). 

On a sulfur recovery plant, excessive tripping of the reaction furnace feed was reducing unit reliability. The process engineer was asked to investigate. He found that all of the tripping incidents were due to the low-air flow trip. When the compressed air flow dropped below 1,000 SCFM, both the hydrogen sulfide (i.e., fuel) and air flow would automatically be stopped. The trip was designed to prevent the formation of a combustible mixture in downstream vessels when air flow to the reaction furnace stopped. With no air, there would be no combustion in the furnace. 

In reality, however, there was never any loss of combustion until the trip shut off the air and H2S. The indicated low air flows were just instrument-and meter-related malfunctions. Therefore, minor instrument problems were causing major process problems by activating the low-air-flow trip. 

The process engineer concluded that the response of the operators to the false low-air flow trip was creating the very hazard that the trip was supposed to prevent, i.e., the accumulation of a combustible mixture in downstream vessels. 

If the circuit is being made up in preparation for a reactor startup, all of the contacts in both the reset primary circuit strings will be closed. If the circuit is being made up during shutdown for testing purposes, most of the contacts in the reset circuit will have to be bypassed, plus those contacts in the primary strings which are tripped open, for example, by low flow or low pressure indications. 

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. 

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