Typical P&ID arrangem arrangement ent for for Centrifugal Centrifugal Compressor Compressor Systems Systems | Enggcyc... Enggcyc...
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Enggcyclopedia Enggcyclopedia > Blog > Process Proce ss Design Design > Process Deli De liverables verables > P&ID Typicals and Symbols Symbols > Typical P&ID arrangement for Centrifugal Compressor Systems Systems
Typical P&ID arrangement for Centrifugal Compressor Systems
Figure Figure 1 – Typical Typical P&ID for a centrifugal cen trifugal compressor compressor system 1. Proper centrifugal compressor symbol should be selected first, as shown in figure-1. Normally Normally, a centrifugal compressor is accompanied by a Knock Out Drum (KOD) at the compressor suction and an aftercooler at the compressor discharge, discharge, as per a typical compressor PFD. Symbols Symbols for these th ese equipments should also be placed on the P&ID before proceeding ahead. All the equipment symbols should should be selected from the legend sheets of a particular project. 2. All the nozzles on the compressor, suction drums and aftercooler should then b e correctly correct ly represented with size and flanges. flanges. This includes includes inlet and outlet out let nozzles nozzles and equipment drains / vents vent s as shown in the typical P&ID in figure-1. 3. Centrifugal compressor compressor suction KOD is intended for removing the entrained liquids before sending gas to the compressor. compressor. Demister, Demister, mesh pad etc. are used in the knock out drum to efficiently remove the liquid liquid droplets. 4. Compressor aftercoolers are generally air coolers and the related fan, motor etc. etc . should be clearly indicated on the P&ID. If cooli c ooling ng water is used, proper symbol for a heat hea t exchanger excha nger should be used. 5. Inlet, outlet lines for each equipment, anti-surge line, line, drain/vent lines, lines, line to the relief valve etc. are the next to be be drawn up. Line number, material class, size size etc. is to be correctly cor rectly assigned assigned to each of the t he lines. 6. Isolation Isolation valves, spectacle blinds, spacers etc. to be used for maintenance should be drawn dra wn up next on various lines between the equipments. Requirement for isolation valves, spectacle blind, spaces etc. depend on the project standards, which should be followed while indicating these on the P&ID. Sometimes, Sometimes, to minimize minimize the number of isolation isolation valves between t he equipment, they can c an be placed only at the suction suct ion KOD inlet inlet which is inlet inlet of the centrifugal compressor compressor system and discharge of the aftercooler afte rcooler which turns out to be the outlet ou tlet of the centrifugal c entrifugal compressor system. Spectacle Spectacle blinds or spacers space rs can be used for fo r isolation isolation between betwee n individual equipments equipments for quick
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maintenance. This is simply a guideline and project standards need to be followed when indicating the isolation requirements. A check valve should be normally provided on the compressor discharge to avoid reverse flow when the pump is not in operation. Pressure relief valves can be provided on the compressor discharge line, downstream to the check valve, to protect the equipments downstream of compressor. Pressure gauges should be provided on suction and d ischarge of the compressor. Level gauges need to be located on the compressor suction knock out drum and temperature gauges on inlet, outlet lines for the aftercooler. Pressure transmitters should be provided on compressor suction and discharge line. A flow transmitter should be provided on compressor suction line. Signals from these transmitters are sent to an ‘Anti-Surge Controller’. Based on the gas flow and differential pressure head developed by the compressor, the anti-surge controller operates the anti-surge valves to prevent compressor surge condition. An anti-surge line from the aftercooler discharge to the suction KOD inlet should be provided for anti-surge control. When the compressor approaches surge condition (low flow, high differential head), the anti-surge valves open up to lower the pressure differential and circulate higher gas flow. Sometimes, a performance controller can be included in the centrifugal compressor system to control the rotating speed (RPM) of the compressor based on inlet pressure, flow etc. in order to achieve optimum performance. Performance controller will typically adjust the motor/turbine speed. Level transmitters provided on the suction knock out drum are responsible for liquid level control in the drum. Alarms are usually provided for high and high high liquid level conditions. Temperature transmitter can be provided on aftercooler for temperature control by sending a signal to adjust the fan speed of the aircooler. Emergency Shutdown (ESD) valves can be provided on inlet / outlet lines of the compressors system to isolate whole system in case of a shutdown. The inlet line of the suction KOD corresponds to inlet of the compressor system. Aftercooler discharge and liquid outlet of suction knock out drum correspond to the outlet lines of the compressor system. Shutdown valves can be located on these lines as shown in figure-1. Drains and vents to be provided on the suction / discharge lines, compressor casing, suction Knock Out Drum, air cooler body etc. for completely draining/venting compressor and associated piping, for maintenance. For purging the compressor system, a nitrogen connection can be provided right after the first isolation valve on the suction KOD inlet line. All the guidelines given here are very general and may be modified as per specific requirements of any particular project.
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