As such, the turbo will operate in the surge region even with the throttle open. This can occur when mating a large compressor to a small turbine to enhance low RPM response. When surge occurs in these conditions, it can be particularly severe and care should be taken when selecting a turbo that it will not operate in the surge region at full throttle at any RPM.
This is where a good understanding of the compressor map comes in handy. In these conditions, ideally, a different turbo should be used, but it is possible to eliminate the surge with a variety of methods which are more related to engine tuning and are beyond the scope of this article.
First of all, engine damage is highly unlikely, unless the turbo fails catastrophically and sends bits of compressor through the engine, but even then it would have to make it through the intercooler first. In these conditions, the diverter valve has no role to play as it should be shut under WOT conditions. The next level down in severity is having no diverter valve of any type.
In this case, the turbo will surge every time the throttle is closed if there is boost pressure in the intercooler. Such turbo systems however were generally running relatively low boost pressures, and it would be fair to say that wear would be greater on a more heavily boosted turbo. The lowest level of severity would be cars fitted with a diverter valve that sometimes encounters surge at lower RPM, before the turbo has fully spooled up.
In these cases, wear associated with the surge could be considered negligible — other factors such as increased boost or even differences in driving style are more likely to have a greater impact of the overall life-span of such a turbo than low RPM surge.
Whilst this rocket attachment did create some mild compressor surge, the entertainment value was deemed to be worth it…. A correctly set up GFB diverter valve is like having your cake and eating it, too.
Turbo Management System, or TMS for short, is a name applied by GFB to valves with design features that help ensure the best throttle response, and more information on this can be found at www. The jumps are highly disruptive and damaging due to high axial thrust and radial vibration. Surge cycles damage bearings and decrease efficiency with each cycle. For axial compressors, the damage may be measureable after a few surge cycles. The total number of surge cycles provides a good metric of the total loss in compressor efficiency.
It is imperative to prevent the surge and to ensure sustained recovery. The surge controller setpoint should be offset to the right of the surge curve on the compressor map, as shown in Figure 3. If the surge setpoint follows the shape of surge curve, the offset can be optimized to be on the longitudinal axis of the efficiency ellipses.
The size of the offset depends on the speed of the automation system and the tuning of the surge controller. Some plants may think of the surge curve as being the point where the surge valve opens.
In this case, integral action must be greater than the proportional action. However, the extra integral action causes a larger overshoot of the surge setpoint, necessitating the setpoint offset to be increased accordingly, which generally corresponds to lower operating efficiency.
Most other plants see the surge setpoint as being the best operating point, when surge valves are open through tuning so that proportional action dominates integral action, preventing overshoot. Using higher controller gain rather than a lower reset time gives a faster correction. For closer operation to the surge curve and to reduce dire consequences from surge, the total must be less than 1 second.
How fast the automation system really needs to be and the required tuning of the surge controller is best determined by running a first-principle dynamic model that includes a momentum balance as well as material and energy balances.
Even a fast feedback controller is unable to get a compressor out of severe surge because of the huge jumps in flow. What is needed is an open-loop back up that forces the surge valves to immediately open and holds them open for sufficient time to sustain operating point stability before allowing the feedback controller to start to close the surge valves. The open-loop backup is triggered by a large predicted overshoot of the surge setpoint to prevent surge, or a precipitous drop in flow indicating an actual surge.
An innovation uses a predicted overshoot via a fast future value that is generated by the rate of change of a decreasing flow, with a good signal-to-noise ratio multiplied by the total loop dead time, with updates every controller execution. The open-loop backup simply puts the feedback controller into a remote output mode that is seen by operators. The remote output is immediately stepped up to a position that typically prevents surge, but is incremented every execution until the future value stabilizes, putting the surge controller bumplessly back in cascade with the surge setpoint computed to sustain an offset from the surge curve.
Many suppliers of standalone compressor controllers have proprietary control strategies providing feedback control, with a backup requiring special expertise and tuning.
External-reset feedback ERF , also known as dynamic reset limit, in the surge controller, with a fast readback of actual valve position, enables up and down setpoint rate limits in the analog output blocks to provide fast opening and slow closing of the surge valves without the need to retune the surge controller.
The surge control system principles basically are the same for surge vent valves and surge recycle valves. At least two valves in parallel are used to provide redundancy, particularly because surge valves might not open after sustained operation in closed position, where stiction from seal or seat friction is greatest.
When surge takes place inside the compressor the flow of gas actually reverses for an instant. If the flow drops for any reason eg blockage in the intercooler piping the impellers cannot add as much energy into the gas, so the developed pressure drops. If this carries on the compressor reaches a point where the developed pressure is the same as the discharge pressure — at this point flow STALLS.
At stalled flow condition the gas flow is zero momentarily so there is zero pressure developed and the compressor pressure collapses. In fact, in many cases, the flow can be reduced further before the actual stability limit is reached. At flows lower than the flow at the stability limit, practical operation of the compressor is not possible, and the compressor cannot produce the same head as at the stability limit. Therefore, the compressor is no longer able to overcome the pressure differential between suction and discharge side.
Because the gas volumes at the compressor discharge are now at a higher pressure than the compressor can achieve, the gas will follow its natural tendency to flow from the higher to the lower pressure: The flow through the compressor is reversed.
Due to the flow reversal, the system pressure at the discharge side will be reduced over time, and eventually the compressor will be able to overcome the pressure on the discharge side again. If no corrective action is taken, the compressor will again operate to the left of the stability limit and the above-described cycle is repeated: The compressor is in surge.
The observer will detect strong oscillations of pressure and flow in the compression system. It must be emphasized that the violence and the onset of surge are a function of the interaction between the compressor and the piping system.
For fast changes in conditions, if the discharge pressure imposed on the compressor exceeds the capability of the compressor, it will slide towards surge. What is compressor surge?
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