reciprocating compressor – What Is Piping: All about Piping Engineering

Why do we need a Compressed Air System? Compressed Air (CA) is a major prime-mover for the modern industry. Compressed Air is referred to as the fourth utility after electricity, gas, and water. A properly managed Compressed Air system can: save energyreduce maintenance & decrease downtimeincrease throughputimprove product quality depending on its end-use Compressed Air Quality CA quality ranges from Plant air to Breathing air depending upon the end-use. Quality is determined by the dryness & the contaminant level. Higher the quality, the higher the cost. The following figure (Fig. 1) gives the applications of compressed air with respect to quality. Fig. 1: Application of Compressed Air. Components of a Compressed Air system Compressed air systems consist of: a supply-side which includes compressors and air treatmenta demand side, which includes distribution and storage systems and end-use equipment. A Compressed Air system broadly consists of: CompressorPrime moverControlsTreatment equipment and accessoriesDistribution system. Air Compressor Types (Fig. 2) The Air compressor is the heart of any CA system.There are two basic compressor types: positive-displacement and dynamic. Fig. 2: Basic Compressor Types Reciprocating Compressor (Fig. 3) This is a very versatile type of compressor and can be used for nearly all industrial applications. These are constant-capacity machines & deliver the air in pulses. Characteristics are: High discharge pressures & relatively low to moderate volumetric flows (600-3000 cfm with a pressure range of 2000-5000 PSIG)Generally more maintenance intensive due to the many wearing partsIt can be single-acting or double-acting, single-stage or multi-stage, air-cooled or water-cooled and lubricated or non-lubricated.High efficienciesOccupy larger footprintHigher capital costControl is usually by Load-unload with 3 or 5 step capacity control Fig. 3: A typical reciprocating compressor Multi-staging Multistage machines are used in place of single-stage ones for the following reasons: Single-stage compression would generate excessive heat of compressionMOC would have to be of high grade and hence expensivePower consumption of a single machine would be higherBetter efficiency Rotary Compressor (Fig. 4) The most common type of rotary compressor is the helical-twin, screw-type. Less common types include sliding-vane, liquid-ring, and lobe. Characteristics are: Not usually suited for high discharge pressures & are most efficient for moderate air flows & low pressures (3000-6000 cfm with a pressure range of 300-400 PSIG)Low initial cost, compact size, low weight, and are easy to maintain.Dry or oil-flooded typeLower efficiencyVSDs provide good capacity control Fig. 4: A typical rotary compressor Centrifugal Compressor (Fig. 5) Centrifugal compressor develops pressure by increasing the velocity of the air going through the impeller & then recovering the velocity in a controlled manner to achieve the desired flow and pressure. Best suited for continuous air flows in large quantities. Characteristics are: The heat generated & power consumption is lower.The space requirement & maintenance is minimum.Inherently non-lubricated.Available for flows ranging from 300 to more than 100,000 cfm, but the common ones are 1200-5000 cfm with a pressure range up to 125 PSIG.Capacity control by inlet valve/guide vane throttlingSurge/choke phenomena Fig. 5: A typical Centrifugal Compressor Characteristic curves The characteristic curve (Fig. 6) of a compressor plots its discharge pressure as a function of flow. Fig. 6: A typical characteristic curve Selection criteria Flow rateDischarge pressureEnd-use of the airEnergy efficiencyReliability Compressor sizing Estimation of Compressed air consumption: Instrument air requirement 5 nm3/hr per CV0 nm3/hr per ROV The plant (Utility) air requirementCompressed air requirement for pneumatic equipment, etc. operating in full load condition Compressor Discharge pressure end-use pressure (for instrument air, minimum 7 bar g)plus all the pressure drops in the system. Compressor Controls Compressed air system controls serve to match compressor supply with system demand. Proper control is essential to efficient operation & high performance. System controls include: Start/StopLoad/UnloadDual controlModulatingSpeed variationPressure/Flow controls Minimum Instrumentation required Indications/alarms/trips consist of 3 major systems: Compressor, Lube oil & Cooling Water Lube Oil system Minimum alarms Low oil pressureLow oil level in the reservoirHigh oil filter differential pressureHigh oil temperatureThe high thrust bearing metal temperature Temperature gauges Oil piping to & from coolersThe outlet of each radial & thrust bearing Pressure gauges Discharge of the oil pumpOn bearing headerOn control oil line & seal oil line Compressor Pressure indicator at inlet, inter-stage & dischargePressure switch at dischargeTemperature indicator/alarm at inlet, inter-stage, and outletThe temperature gauge on bearingsVibration switchesA safety valve on each stage (for reciprocating type only)Flowmeter (if required) CW System Pressure & temperature gauge on CW inletThe temperature gauge on CW outletThe thermal relief valve on CW outlet Contaminants in Compressed air The 3 major contaminants in Compressed Air are: WaterOilDirt Compressed Air System Components/Accessories The standard components/accessories include: Prime Mover The prime mover is the main power source providing energy to drive the compressor. This power can be provided by any of the following sources: Electric motors: Economic, reliable, efficientDiesel/natural gas engines: Fuel availability, higher maintenance, high cost/uncertainty of powerSteam turbines & combustion turbines: Inexpensive steam availability Intercoolers/Aftercoolers Air-cooled/water-cooledMinimum pressure dropRegular maintenance Moisture/oil separators Separation of condensed moisture/oilTypes – Impingement baffle type, Centrifugal type, with Demister pads. Pulsation Dampeners Reduce/eliminate pulsations of reciprocating machinesInstalled at the outlet of each stage Receivers Storage for utilization at peak loadDraining of condensed waterReduce pulsations from reciprocating machinesReceiver sizing is based on hold-up for a drop in pressure level, say 10 minutes for a pressure drop of 3 bar.Provided with standard accessories. Air filters Suction filters or Post-compression filters. Felt cloth filters are used for suction. Compressed air filters can be: Coarse particle filters (filter media can be a ceramic candle, felt cloth, etc)Coalescing & activated carbon filtersMicrofilter (high efficiency for special uses such as breathing air, etc) Minimum pressure drop Drain traps ManualMechanical float typeElectronic timer operatedAuto drain traps – condensate sensingRegular maintenance required to avoid CA wastage Lube oil coolers To remove heat from the lube oilUsually Shell & tube type with CW Air distribution piping Least pressure drop in the system to reduce operating costs. The maximum pressure drop between the compressor and farthest end of compressed air consumption should be around 0.3 barVelocities between 6-10m/s in air mains; this will limit the DP & thus energy consumption and also allow moisture to precipitateMinimum bends & joints (long radius bends to be used)Arrangement for draining of moisture at regular intervals, slope provisionMinimum expanders/reducersLeakproof joints, proper piping supportsGauges to be provided at different locations to monitor the system pressure & temperature. Compressor Cooling system Cooling plays an important role in energy efficiency, two types are: Air-cooled – In-efficient, preferred only for low capacity compressorsWater-cooled – Efficient, used for high capacity compressors Water is circulated in a jacket around the cylinders to remove the heat resulting from compression & friction due to sliding of pistons. Water is also required for the inter/aftercoolers and lube-oil coolers. CW consumption for inter/aftercoolers can be estimated based on the compression ratio per stage assuming an adiabatic efficiency. Generally, per thumb rule, Compressed air plant layout and distribution Plant layout: Centralized layout All compressors installed in a single house, cost-effectiveness as maximum plant space utilizationMore pressure drop expected Decentralized layout Suitable for large industry, different levels/pressures of airCompressor situated at the maximum user location, less pressure drop, max energy utilizationCan lead to noise and heat inside the plant Compressor location The location should be such that compressor can induct clean, dry and cool airInteresting fact:- every 4°C reduction air intake temp reduces power consumption by 1%Points to be remembered while selecting the location of the air compressor: Low humidity to reduce water entrainmentAdequate ventilation especially for air-cooled unitsMinimum suction pipingMinimum bends Compressed air distribution (Fig. 7) Fig. 7: Different compressed air layouts Codes API 617 Centrifugal compressorsAPI 618 Reciprocating compressorsAPI 619 Rotary type positive displacement compressorsAPI 672 Packaged integrally geared centrifugal air compressors