SURGING IN A CENTRIFUGAL COMPRESSOR
'Surging' is defined as 'A momentary back-flow' through the compressor from the discharge to the suction. This can occur when the mass flow of gas to the compressor falls below a critical level with a high pressure difference across the machine. When there is not enough gas to replace that being pushed forward, discharge gas will flow backwards through the compressor towards the suction side. This back-flow will tend to decrease the speed of rotation while the speed controller will try to maintain the correct speed. The gas flowing backward provides more volume to the suction side and the compressor picks up and begins to push gas forward again. The machine speed will tend to increase and the governor will again try to maintain the correct, set speed. All of this takes place very quickly; the backward and forward gas flow together with the speed control action causes rapid fluctuations in the flow and pressure of the system. This 'Surging' in the machine can be very damaging to the compressor and associated piping and equipment due to heavy vibrations set up in the system. Generally, if the suction flow drops too low, a 'Low-flow Trip' will shut down the machine. Also, surging can cause the machine to 'Overspeed' before the control system can react. This can also cause damage and is prevented by an 'Overspeed Trip Mechanism' which will again shut down the machine.These problems of Surging are usually prevented by an 'Anti-surge' system and control valve which, externally, recycles discharge gas back to the suction side in order to maintain a 'Minimum Flow Rate' to the machine. Because compression causes temperature increase, this recycle gas is normally taken from the discharge side, downstream of the after-cooler, in order to prevent greater and greater temperature increase at the discharge side.
In large, powerful machines, the anti-surge system is computer controlled and depends upon data received from the following: - Gas Flow rate, Suction pressure and Temperature, Discharge Pressure and the Density (or S.G.) of the gas entering the machine.
Figure: 8, on the following page gives a general idea of an anti-surge system.
In the above picture the Combustion Gas Turbine consists of an Axial Flow air compressor, a High Pressure (HP) Compressor Turbine and a Low Pressure (LP) Load Turbine. Six fuel fired Combustion Chambers, (3 each side), provide the heat input into the air compressor discharge and the super-heated air provides the required energy to drive the HP turbine. The hot gases give up energy in driving the HP turbine but, sufficient remains in the hot gases to drive the LP (load) turbine. The LP turbine is connected by a shaft to the 'Load' which, in this case, is two, multi-stage, natural gas compressors driven in 'Tandem'.
C. AXIAL FLOW COMPRESSORS
Where the discharge of the wheels of a centrifugal compressor leaves at right angles to the wheel, in an axial flow machine, the flow of gas is parallel to the rotor. Imagine a series of fans placed in front of each other. When they are operating, the flow of air from the first fan is fed into the second fan which further increases the flow of air. This is repeated through the series of fans tending to give a much increased air flow from the final fan.The rotor of an Axial Flow compressor consists of a number of stages (or rows) of many ‘ROTOR’ (Rotating) blades fitted at an angle into the rotor body – similar to the blades of a fan. The blades in each row become smaller and narrower from stage to stage. This arrangement allows for the decrease in volume of the gas as its pressure increases from stage to stage. However, as the gas leaves each stage, it is moving at high velocity and in the opposite direction to the rotation of next stage. In the casing of the machine, rows of ‘STATOR’ (Static) blades are fitted in diaphragms placed at an opposite angle to the rotor blades between the rotor stages.
These Stator blades have two purposes;
- To decrease the velocity of the gas – thereby increasing pressure (Bernoulli's Principle).
- To redirect the flow of gas into the blades of the next stage.
The following pictures (Figures: 9, 10, & 11) show some internal arrangements of Axial Flow
Compressors.
Figure: 9 - Axial Flow Compressor – Horizontally Split – Open
Figure: 10 - Axial Flow Compressor Stator Blades
Figure: 11 - View of Axial Flow Compressor Internals
Figure: 12 Simplified Diagram of an Axial Flow Compressor
D. RECIPROCATING ( PISTON ) COMPRESSORS (Positive Displacement Compressors)
'Reciprocation' means 'Backward and Forward Motion'. A 'Reciprocating' compressor therefore, is one with a forward and backward operating action. The simplest example is the 'Bicycle pump', which everyone at some time or other will have used to re-inflate their bike tyres. The name 'Bicycle PUMP' is not really the correct term because it causes compression.It is essentially a hand operated compressor and consists of a metal or plastic tube called a 'Cylinder' inside of which a hand-operated piston rod or 'SHAFT' is pushed back and forth. On the shaft end, inside the cylinder, a special leather or rubber cup - shaped attachment is fixed. When the piston is pushed forward, (this is called a 'Stroke'), the cup flexes against the cylinder walls giving a seal to prevent air passing to the other side. As the handle is pushed, air pressure builds up ahead of the cup and is forced (discharged ) into the tyre through the tyre valve which also prevents air escaping when the pump is disconnected or when the piston is pulled back. When the pump handle is pulled back, (called the 'Suction' stroke), the cup relaxes and the backward motion causes air to pass between it and the cylinder wall to replace the air pushed into the tyre. This 'reciprocating' action is repeated until the tyre is at the required pressure. Because the air is expelled from the pump during the forward stroke only, the compressor is known as 'Single Acting, Reciprocating'.
( See Figure : 13 )
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