Interlocks and alarms on a carbide-to-water acetylene plant: what trips why it fails safe, and how to prove it works

An acetylene generator holds a reaction that releases about 429 kcal for every kilogram of carbide, a gas with a minimum ignition energy of about 0.02 mJ, and a tank of lime slurry that can set like cement. The interlocks exist so that when any of the conditions holding those in balance is lost, the carbide stops before an operator has to notice. On KIGL plants they are simple, few and air-operated, and the design rule is that a failure of the protection itself leaves the plant safe.

Level is measured by a differential-pressure transmitter, not a glass gauge; KIGL generators carry no glass level gauges at all, and the reading is confirmed by hand at the upper and lower level check valves, which should give gas and water respectively. The temperature is sensed by an RTD. The panel's trip lamp shows which fault has acted; the hooter can be acknowledged but the lamp stays lit and the feed will not run until the condition is cleared.

Generation is controlled, not tripped, by pressure. Each hopper's screw feed is started and stopped by a pressure switch: one set at 0.7 to 0.74 kg/cm²g and the other at 0.88 to 0.92 kg/cm²g, with the panel interchanging the switch connections on changeover so that only one hopper is ever on line. The maximum generation pressure is 0.92 kg/cm²g. When a hopper runs empty, pressure falls, the second hopper starts automatically and the first is recharged in manual, then returned to auto.

The carbide valve between each hopper and the tank is a soft rubber seat held closed by a spring; screw-feed rotation drives a hydraulic pump whose pressure overcomes the spring and opens it. Stop the feed and the hydraulic pressure bleeds away and the spring closes it. The hydraulic relief is set at 12 kg/cm²g. The hopper vent valve is mechanically interlocked so that the lid can be opened only when the vent is open.

Relief valves are not part of control. IS 8471:2003 requires a generator to run at 130 % of its rated capacity without its generating-chamber relief valve venting when use is stopped (clauses 19.3 and 19.6), and clause 11.6 forbids any shut-off valve between a relief device and the generator. Relief is the reserve; the switches and the trips do the work.

The trips, the water inlet valve and the slurry drain valve are operated by instrument air, and every one is arranged so that losing air sends it to its safe state. The water inlet valve opens on rising temperature and spring-closes on air loss; the carbide valve closes on its spring; and the electrical circuit stops all motors except the agitator when the air-pressure switch drops. A minimum of 2 kg/cm² on the regulator discharge is confirmed before every start.

The agitator is excepted on purpose. After a trip the danger is a settled lime bed that coats the next charge and makes hot spots, so the agitator runs intermittently on its timer, 24 hours a day, trip or no trip.

The compressor has its own low and high pressure switches. The low-pressure switch stops it before suction falls below 0.1 kg/cm²g and will not let it start until suction is back to 0.2; this is the protection against pulling a vacuum on the generator and drawing air in, and it is tested at the end of every shift by letting the machine stop on it at shutdown. The high-pressure switch stops the machine at the charging maximum, and the manual is explicit that no valve may sit between its connection and the compressor; otherwise a closed valve can isolate the switch from the pressure it is meant to see. Each stage also has a safety valve venting through the compressor vent piping.

On plants that use a low-pressure gasholder instead of generator-pressure control, the 1968 C.P.I. code required the compressor to be stopped automatically when the bell fell to a set level at least 30 cm above its rest position, before air could be drawn into the circuit, and the bell to be checked periodically for free movement.

An emergency stop at the generator room exit door stops all charging equipment; the reset and pilot light are on the master control panel outside the generating area. The C.P.I. code asked for stop buttons near every exit of the compressor building and in the immediate vicinity of the generating room, because the first action in any incident is to stop compressors and generators feeding gas into the high-pressure system. See the emergency response guide.

Across five carbide-to-water plants audited by the KIGL group, the control panel was found modified beyond its schematic at four of five, which means no interlock on it could be certified to trip; the correct answer was to restore or redraw and re-certify the panel before restart, with no partial fix. Other findings, each from at least one plant: the compressor low-pressure switch jumpered; generator high-pressure setpoints raised well above the maker's maximum, identically on more than one generator; a level transmitter scaled with lime and reading low, so the trips and slurry valve were working from a false level; a transmitter swapped for an incompatible one so the plant ran in manual; and the hopper changeover left permanently in manual, which is evidence of a panel fault, not an operating choice. At one plant with excellent logs and housekeeping, the pressure switches had been removed. Interlocks are invisible when they are missing, which is why they are tested on a schedule and protected by a permit-to-modify.