Surging

Turbocharger Surging

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SURGING Surging of the turbocharger is also known as coughing, snorting, barking and howling. When heard for the first time, it can be quite frightening, especially on a large blower. It happens when for one reason or another, the engine cannot use the air at the rate it is being delivered, or if the pressure differential between suction and discharge is too high. The pressure of the air drops in the diffuser, and then the air surges back through the compressor wheel. Repeated surging can cause damage to the bearings, turbine blades, compressor wheel and silencer filter. In normal operation, air is discharged from the compressor wheel with a velocity of ‘a’. This is combined with the rotary velocity ‘b’, giving a resultant velocity and direction ‘c’ The diffuser vanes are matched to this air direction, and so the air enters the diffuser with a velocity ‘d’ which is approximately equal to ‘c’ The area increases through the diffuser, and so the velocity decreases to ‘e’, and the energy is converted from kinetic energy into pressure energy. If there is now a restriction in the air flow in the system, ‘a’ will be reduced. Because the rotary velocity ‘b’ remains the same, the direction of resultant ‘c’, and therefore ‘d’ will change, and not be matched to the diffuser. Because of the resulting turbulence in the diffuser caused by this, the effective area through the diffuser will not increase and thus ‘e’ will remain the same as ‘d’; i.e. no conversion of kinetic into pressure energy. At a certain point, the pressure built up in the air manifold will blow back through the compressor, causing the surge in turbo charger revolutions, the noise and vibration. The system will now stabilise itself, pressure will be built up again, and if the condition causing the surge is still present, the cycle will repeat itself. Depending on the degree of restriction, the frequency can be anything from a few seconds to a number of minutes. Causes of Turbocharger Surging Dirty air filter. Dirty air cooler (on air side) Scavenge ports choked with carbon Scavenge air temp after air cooler too high Turbine side of turbocharger dirty Back pressure in exhaust system too high Sudden changes in engine load (heavy weather) One or more units not operating properly (fuel injection, exhaust valve) Any changes to engine /TC matching parameters ( emergency running with one unit out of service for example). If repeated surging occurs, reduce the load on the engine, open scavenge dump valve or open turbocharger wastegate (if fitted)

06-02-2017 22:47

Turbocharger Surging

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MATCHING OF THE TURBOCHARGER TO THE ENGINE

The lines of constant speed in the diagram opposite show that if the air flow increases, the pressure rises and then falls. If a line is drawn through the peak pressures, a surge line is produced. If the air flow is reduced for a particular compressor speed so that the air flow is at the left of the surge line, then there is a reduction in pressure. Once the delivered pressure falls below that in the inlet or scavenge manifold, then flow reversal will occur and surging takes place.

The engine operating line is placed far enough to the left of the surge line so that in the event of a change in engine operating conditions under normal operation, surging will not take place. For instance, at higher ambient temperatures, the density of air is reduced, and the mass flow for a given volume is less. Because there are fluctuations in manifold pressure and turbocharger RPM, the full load point is not fixed, but is within the shaded area as shown. Broadly speaking, the further the operating line is from the surge line, the lower the efficiency of the turbocharger. The turbocharger may be matched to the engine to produce full output at 85% full load. This is so that higher pressures can be obtained at low powers, increasing engine efficiency and reducing thermal loadings. This means that over 85% load, the engine is producing more exhaust gas than is needed for optimum running.. To overcome this an exhaust waste gate is fitted on the system, and some of the exhaust gas bypasses the turbocharger, its energy being utilised in the exhaust gas economiser. A charge air waste gate may be fitted to dump excess air when necessary, and a charge air bypass may be fitted to engines running under variable load. This puts air into the exhaust gas side and helps avoid surging when the engine is running at lower loads.

06-02-2017 22:47

Turbocharger Surging

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Waste Gates on an MAN B&W V48-60 Medium Speed Engine

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06-02-2017 22:47