Air Compressor Receiver Prepared by CE Chew Eng

Air Compressor & Receiver Prepared by C/E Chew Eng Lai

Learning Objectives Uses of compressed air Installation requirements Air compression theory Multistage compression Volumetric efficiency Constructional details Operation and Maintenance Compressor lubrication Explosion hazard Air receivers

Uses of compressed air 25 – 30 bar Starting of main & auxiliary engines Ø Boiler soot blowing Ø 4 – 7 bar Service air Ø Whistle air Ø Pneumatic tools Ø Life boat, pilot ladder Ø 1. 5 – 2 bar Ø Instrumentation & control

Installation Requirements Compressors Minimum 2 or more compressors Sufficient capacity to charge air receivers within 1 hour from atm. to max. pressure to provide è 6 starts for non reversible engine è 12 starts for reversible engine 1 emergency air compressor & receiver for dead ship condition Air Receivers At least 2 air receivers with sufficient capacity without replenishment and able to provide è 6 starts for non reversible engine è 12 starts for reversible engine

Installation Requirements Pressure Relief devices Relief v/v to be fitted after each stage of compression Relief v/v to be fitted on the air receiver Relief v/v or bursting disc to be fitted on inter & after coolers Air Temperature Limited to 93 OC to prevent explosion Fusible plug or high temp cut out provided as safety measure Pressure Test Cylinders, cylinder covers, inter & after coolers are tested by hydraulic pressure to twice their working pressure. Casing of intercooler and after cooler is hydraulically tested to 1. 5 times the cooling water pressure

Air compression Theory Nearly perfect gas, as air is composed of mainly 23% oxygen and 77% nitrogen & rest other gases by mass Mixture of gases behave as near as perfect gases Follows perfect gas laws – Boyle’s and Charles’ laws During compression è Volume reduces è Pressure & temperature increases è Mass remains constant Air follows the combination of gas laws: PV = m. RT; PV / T = m. R = Constant

Types of Compression Isothermal compression (PV = C) no rise in temp during compression heat is removed during compression least power input & most efficient difficult to achieve in practice Adiabatic/Isentropic compr. (PV 1. 4 = C) no heat removed & temp. rises progressively during compression power input is max. & efficiency is low Polytropic compression (PV 1. 3 = C) actual compression process in practice heat is extracted during compression lies between isothermal and isentropic compression work done to compress air is reduced

Ideal P-V Diagram 4 -1 Air induction 1 -2 Air compression 2 -3 Air delivery 3 -4 Expansion of remaining air Vc Clearance vol. Vs Swept vol (V 1 – Vc) VI Induced vol (V 1 – V 4) Vol eff. = Induced vol/Swept vol Compr. ratio = V 1/ V 2 = P 2 / P 1

Actual P-V Diagram Similar to ideal except induction & del. process Which are modified by v/v action Waviness of the lines 4 -1 and 2 -3 is due to v/v bounce v/v inertia spring action variation in back pressure

Volumetric Loss Due to clearance Volume

Effect of increasing Clearance Volume

Multistage Compression As the pressure increases, more stages are required with intercooling due to following disadvantages of single stage compression: Low volumetric efficiency Ø As pressure ratio P 2 / P 1 is increased, vol efficiency drops High air delivery temperature Increase in pressure ratio results in higher delivery temp. causing excess coke deposits Ø thermal stress Ø Lubrication problem Ø Explosion hazard Ø Increased power input Ø Inter-stage cooling lowers the work done in compressing air Number of stages is governed by the required final pressure of the compressed air.

Multistage Compression Applied when delivery press. is high Compression is carried out in stages Ensures equal rise in temp in each stages to prevent v/vs & springs damage Requires minimum work; shaded area indicates the wok saved If stage pressures are P 1, P 2 & P 3, P 4 P 2 = P 1. P 3 & P 3 = P 2. P 4

Multistaging & Intercooling Advantages: Compression work is reduced Vol efficiency and hence compressor capacity is increased It approximates isothermal compression Vol of air delivered is reduced; as a result reduced size of cylinder, piston and delivery pipes is required Reduced air delivery temp gives less thermal &mechanical loading and better lubrication of piston & cylinder Due to less temp, suc & del valves remain clean Facilitates moisture separation Use of intercooler allows smaller receiver

Volumetric Efficiency It is the relationship between the Qty of air discharged and the swept volume. vol of air discharged as free air Vol efficiency = swept vol of L. P. piston Free air is air at atm. press and at 15 deg C Vol. eff. always refer to low pressure piston

Clearance Volume To provide for thermal expansion and prevent the piston from striking cylinder cover, a small clearance, called bumping clearance is maintained between cylinder cover & piston Bumping clearance must be as small as possible High bumping clearance reduces vol effn. of compressor Reduces compressor capacity & increases disch air temp. Bumping clearance is effected by bearing wear & gasket thickness Low bumping clearance can cause mechanical damage Bumping clearance must be checked after overhauling Clearance volume is about 6% of swept volume

Factors affecting volumetric efficiency Clearance volume - gaskets , bearing wear down Valves – dirty, leaky, spring tension Leakage past piston rings Insufficient cooling water due to dirty coolers High cooling water temp. High air inlet temp. Throttling of air intake due to Dirty intake filter Insufficient suction v/v lift Strong suction v/v spring

Compressed Air System

Two Stage Compressor

Two Stage Air Compressor

Components Air Filter Material – felt or paper, metal gauze or nylon strands Remove contaminant to prevent abrasive wear of piston rings, valves deposit on valves casing source of ignition Very dirty filter will cause Reduction in suction pressure Increase in compression ratio Increase in discharge temperature may cause coke deposit Less air flow and danger of overheating coke deposits In extreme case disch. temp. may exceed auto ignition temp of LO, resulting in an explosion

Components Suction & Delivery Valves Low inertia automatic valves which operate on low diff pressure Material used for Valve seat – 0. 4% carbon steel hardened & polished surface Valve plate – Nickel or stainless steel hardened & polished Spring – tempered hardened steel Usually of same type, only differences are spring load and the valve lift Excessive lift causes impact & v/v breakage Late closing of v/v affects volumetric efficiency Coke deposits make v/vs leaky & sticky causing overheating reduce vol. efficiency

Piston Trunk type piston is used in single acting compressor Tandem piston is used in multistage compact design Aluminium alloy material is used for good friction properties high strength with low coeff. of thermal expansion Piston rings material is alloyed cast iron HP piston rings are made thicker

Bearings Thin shell type in modern compressor Requires L. O. pressure above 2 bar Conventional thick shell can run even at 1. 0 bar Crankshaft Usually forged carbon steel integral with counter weight Cast steel being fatigue prone is not used

Inter-cooler & After-cooler Multi Tubular Copper tubes expanded in brass tube plates with allowance for expansion of tube nest Cast iron casing with bursting disc, to protect against sudden over pressure Drain pocket to remove moisture Relief v/v to protect against over pressure on air side In case of tube leakage, leaky tube is plugged / replaced

Inter-cooler & After-cooler Single Coil Type A single long copper coil is used Casing is protected by bursting disc in case of coil leakage, complete replacement of coil is required

Drain Trap Provided to drain off water & oil automatically from compressed air system Water must be drained at each stage to prevent Removal of oil film from cylinder wall, assisting wear of liner and piston ring Corrosion to form iron oxide, assisting degradation of oil Requires regular inspection

Unloader Prevents compressor to start on load Some of many methods to unload compressor cylinder throttling of suction speed variation by-pass discharge to suction depression to hold suction v/v plates on their seats Last two methods are most widely used in marine compressors.

Unloader Suction Valve Depression Fitted on each stage suction valves Actuated by solenoid or pilot air

Air Receiver Size depends on engine requirements Should have sufficient capacity to give 12 consecutive starts for reversible engine 6 consecutive starts for non reversible engine Fabricated from good quality mild steel with UTS 500 MN/m 2 with an elongation of not less than 23% to 25% having one longitudinal welded seam welded hemispherical dish at the ends with elliptical manhole door Must be cleaned internally & coated with paint or copal varnish Fitted with necessary mountings

Air Receiver

Air Receiver

Mechanics of Explosion Oxidation of oil Rate of oxidation depends on Temperature Air pressure Presence of catalyst – iron oxide Exposure time Heat generated due to oxidation is removed by air flow & CW Coke Deposits Due to oxidation coke deposit on leaky valves cause high temp. Max tolerable thickness of coke layer – 2 mm (for 10 to 30 bar press) Explosion Fire can initiate in unloaded compressor Burning coke could act as ignition point for air-oil vapour mixture Spontaneous ignition followed by weakening the metal wall can initiate explosion

Prevention of Explosion Design of compressor Installation Air flow rates & exposure time Prevent oil accumulation Air flow rate of 8 m/sec keeps the pipes clean by moving the large oil droplets to cooler areas Operation & Maintenance Lubricating oil – oil feed rate, viscosity, antioxidant, oil change, draining of excess oil Air filtration - contaminants Temperature – air flow, leaky valves, CW flow, clean coolers Catalysts – Iron oxides (rust) – requires regular draining of water

Function of Compressor Lubricant Lubricate bearing, gears & rubbing parts Remove heat due to compression & friction Provide sealing to minimise air loss Prevent corrosion Be suitable for long term use

Lube Oil Recommendation Lubricating oil is recommended according to compressor classification -- light, medium, or heavy duty depending upon parameters as: Compressor design Type of cooling Number of stages Air flow rate Oil retention time Ambient condition Ambient temp Coolant temperature Presence of dust or oil vapour Operating system Continuous or intermittent maintenance

Lubrication Selection According to normal lubrication requirement and to minimise coke deposit Plain mineral oil is not used as it oxidises easily at high temp. and pressure Lube oil must have anti-oxidant additives for good oxidation resistance Low viscosity – easy spreading, effective sealing, hydrodynamic lubrication with low friction Flash point – above 220 o. C Oil with minimum heavy ends If oil has too broad distillation range, the more volatile portion may tend to evaporate leaving behind heavier ends in the hot zone

Synthetic Oil Product made of chemical synthesis Raw materials or base stock may be mineral oil derived The chemical structure is planned and controllable The molecular structure is variable so that product performance may be modified Additive treated to produce lubricant with superior properties to mineral oil

Synthetic Base fluids n n n Hydrocracked Polyalphaolefin (PAO) Ester Properties n n n Exceptional low temperature flow Very high viscosity index Exceptional thermal and oxidation resistance Low volatility Exceptional film strength

Choice of Lubricant Full Synthetic Semi-Synthetic Price Semi-Hydrocracked Mineral Performance

Synthetic Oil Advantages Superior to mineral oil & excellent protection against corrosion Suitable for severe operation Good thermal & oxidation stability Operate over wide temperature range Good viscosity vs temp. properties Low pour point (-33 OC to - 48 OC) Good wear resistance with low viscosity Reduced maintenance Cleaner compressor due to minimum deposits Long service life Enhanced safety due to high flash point (246 to 266 deg C)

Synthetic Oil Disadvantages Less compatible with seals & paint Initial cost is high, but overall it may be cost effective Possible mineral oil incompatibility Potential toxicity

Operational Problems of Air Compressor Drop of compressor capacity / pressure Choking of air filter Valves leaking Increased bumping clearance Leaky piston rings Worn crank pin or journal bearing Leaky cooler / breakdown of cooler L. O. contamination Fire and explosion

Case Study Intercooler Casing Burst Vessel Type : Tanker Main Engine: Sulzer The Accident The vessel was in HK at outside anchorage. One of the electric motor driven main air compressor intercooler casing burst that resulted in the 4 th Engr. losing his life and 3 rd engineer seriously injured.

Investigation An immediate investigation revealed following facts: Heavy sludge deposits in intercooler cast iron casing Cooling water inlet valve was jammed in the open position and partially chocked This allowed restricted cooling water into the cooler The casing bursting disc had ruptured HP and LP relief valves were found in following conditions HP relief v/v lifted @ 235 bar! Relief v/v spindle was partially seized LP relief v/v was in proper order; but lifted at 10 bar The compressor was surveyed 14 months before the accident; but record showed no mention of the followings: testing of inter coolers and relief valves

Main Cause Poor maintenance 3 rd engineer had started the compressor with disch. valve shut Relief v/v was inoperable which eventually resulted in the bursting of the intercooler casing

Remedy It is essential all safety devices of a pressure unit are thoroughly inspected for their general condition, operation, settings etc at every inspection and survey Machinery eg air compressors, positive displacement pumps must not be started with discharge line blocked or shut