Chemical Process Industries widely employs reciprocating compressor in Various applications and hence it is important to have a better trip and safety system to protect the men and machinery in case of any abnormality. Let us see them in this article in detail.
Reciprocating compressor Trips
High Suction Temperature
Due to cooling water failure to inter coolers, valve passing problems, Fouling and Leak in inter coolers suction temperature of the gas will rise. Reciprocating compressor trip is provided for High suction temperature as this will result in damage of the compressor parts.
Low Suction Pressure Trip
In reciprocating compressor handling Hazardous flammable gases like Hydrogen, trip has been provided for low suction pressure. This is to avoid the atmospheric air ingression into the system through leaky flanges or valves which may cause explosion/fire in the discharge of the compressor where the temperature is high. In air compressors this trip is normally not necessary as it takes air from atmosphere.
High Discharge Temperature
High discharge temperature causes polymerization in some applications (like in Polyethylene) which will plug the flow and causes rupture in pipelines. In air compressors ignition of oil will take place at High discharge temperatures.
High Discharge Pressure
High discharge pressure in Reciprocating compressor may result in damage to pipes and machinery. It may also cause personal injury. Reasons for High discharge pressure include control valve malfunctions in consumption or in vent side, valve plate failures due to accumulation of foreign materials etc.
High Separator Level
Liquids are incompressible fluids which occupies space inside the compressor which reduces the mass flow rate of the compressor. Liquid slugging causes stresses which damages compressor connecting rods, piston, crank shaft etc. Therefore trip of the unit has to be provided for High levels in separators in all stages.
High Bearing temperature or High Lube oil Temperature
At High temperatures Lube oil will get deteriorated and increase in viscosity does not form oil film. This results in poor lubrication and causes failure of bearings. So trip for High Lube Oil/ Bearing temperature is provided.
High Vibration
Reciprocating compressors produce pulsations due to intermittent flow. Double acting machines and Pulsation dampeners will reduce the impact due to pulsation. During prolonged operation vibration of the equipment is prone to go high due to damage in internals, change in operating conditions etc. Therefore trip is provided for High Vibration in the reciprocating compressor.
Low Lube Oil Pressure
Adequate Lubrication is necessary for compressor moving parts. Therefore trip of the reciprocating compressor is provided for low lube oil pressure. Starting Interlock bypass timer is provided to bypass the trip during initial startup of the compressor. This time delay will allow the compressor to develop enough lube oil pressure during start of the machine. During normal operation the compressor trips as soon as low pressure is detected.
Cooling Water Failure
Cooling water is supplied to Inter coolers, Lube oil Coolers and Jacketing water coolers. Upsets in water flow will result in any one of the trips discussed here. Any way cooling water failure trip is also provided.
Safety Aspects in Reciprocating Compressor
Discharge Vent
Since Reciprocating compressors are constant volume compressors they keep on delivering the gas at the discharge. Therefore it is necessary to provide the vent in the discharge to avoid over pressurization and other impacts.
Inter stage Relief Valves
Relief valves in inter stage separators are helpful to prevent over pressurization due to Valve plate failures and inter cooler fouling.
Motor Protections
Excitation of Torsional critical speed in reciprocating compressors does not allow variable speed operations. Torsional critical speed is associated with angular or torsional deflection. Therefore in most of the cases driver is electric motor. Normal motor protections like Over Current, Under Voltage, Under Frequency trips are to be incorporated and executed with the help of relays.
Insulation
Thermal insulation has to be provided in the suction and discharge pipelines depending on the temperature. Cold insulation in Refrigeration systems protect the pipelines from corrosion and heat ingression into the system. Discharge pipelines contain gas at high temperature which is prone to cause burns and has to be properly insulated.
Packing Gas Vent
Packing gas vent to safe atmosphere has to be provided to avoid gas contamination with oil in crankcase and Fugitive emission.
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