I believe that most of the marine engineers encountered, at least once in their career a turbocharger surging. This is not a pleasant experience, especially if you a in the close proximity of the turbocharger when at such happens, due loudly noise and backpressure effect.
Turbochargers are designed to match the engine and balance the latter rate of air consumption over the whole working range and it should not fall into the surging limit area. Therefore when various engine part do not perform in synchronization with the turbocharger, this will lead to surging .
Surging can have a multitude of reasons with which most of the engineers are more or less familiar, but sometimes finding the cause of surging can be troublesome and time consuming. Surging must be avoided as much possible as it reduces the turbocharger’s efficiency and performance and continuous surging can cause damage of its bearings and compressor failure.
The most common causes can be:
- Rapid change of engine load – this often happens during bad weather when the vessel is pitching heavily and propeller comes out of the water and in this case the engine rpm must be reduced concomitantly with changing of vessel course in order to slow down the vessel pitching. Most of the engine governors have a rough sea mode that will slow the governor response in such cases in order to prevent turbo surging. Another reason of change in load can be a defective engine governor or high wear and lag in governor command link to the engine. This can be easily observed especially during good weather if sudden and frequent rpm change occurs in the engine.
- Improper power distribution between engine cylinders. In this case one unit is producing more or less power than the others, thus leading to engine unbalance and variation on exhaust gas pressure. This is affecting mainly the impulse type turbochargers as the exhaust is pumped directly into the turbine. In order to remedy this issue a power card must be taken on the engine to determine and remedy the faulty cylinder. The fault in the cylinder may be: excessive wear of the cylinder, unit misfiring, leaking exhaust valve, improper adjusted valve timing and leaking injectors.
- Fouled turbocharger parts like: dirty filters (can be observed visually or by checking the U-tube manometer installed on the compressor silencer), damaged silencer, dirty and clogged nozzle ring, dirty turbine impeller, worn out turbocharger’s bearings etc.
- Dirty and clogged scavenging air cooler and/or water mist catcher prevents the air flow and creates back pressure into the compressor. This can be observed on the U-tube manometer installed on each scavenge air cooler. The value must be compared with the one’s from the engine shop trial at different loads. Hence the importance of having a fully functional U-tube manometer, although you can find plenty vessel where nobody pays attention to this. As a remedy the air cooler must be chemically cleaned, although this must be a frequent activity and part of the engine plan maintenance system.
- Damaged or blocked scavenging air flaps.
- Restriction in exhaust gas flow due fouling of economizer, nozzle ring dirty, obstruction of exhaust manifold, damage gratings inside exhaust manifold. As a remedy U-tube manometer readings on economizer must be checked and compared with the shop trial and economizer cleaned if necessary. Due nowadays engine slow steaming it is a good practice to soot blow the economizer twice a day using soot remover despite engine load, as long as the engine runs at constant rpm. Similarly, exhaust manifold must be inspected for any obstructions and gratings checked and repaired as found necessary.
- Problems in engine fuel supply system like: leaking injectors, cold fuel which leads to improper injection, engine fuel starvation, seized or defective fuel injection pump etc. As a remedy fuel temperature must be checked and monitored and power card must be taken in order to check the performance of engine cylinders.
- In the worst case scenario scavenge or exhaust fire.
These are the most common causes, but sometimes there is some unexpected reason for turbocharger surging. Few years back I’ve been on a vessel with prolonged turbocharger surging issue. The support from the office was really disappointing as they kept told vessel to open and inspect the air coolers for cleanliness. However, after several inspecting and cleaning of air coolers (although Δp was in very good range compared with shop trial), we continued our own investigation by checking the scavenging flaps, water trap, ME exhaust valves, ME performance and compression pressures, exhaust gas economizer Δp, exhaust gas manifold for any obstructions and everything found in good order.
After presenting a full report to the office with our investigation and where we explained that something is wrong with the turbocharger (even though as per our records they have been overhauled by a third party company in close relations with superintendents) they decided to send a ME maker representative onboard as they didn’t trust our investigation. I don’t want to blame anyone but it seems that many of the superintendents are simply clerks with no deep engineering knowledge but good at office politics. However, the attending engineer have done the same investigation as we have and in their report specified that everything is in order with ME and the turbocharger have to be opened and inspected (imagine the reaction from the office😊). However, after long debates the office decided to sent someone else to investigate the turbochargers and we discovered that the nozzle ring’s fin blades were heavily worn (change in thickness and shape) and some of them with hard soot deposits. It seems that the nozzle rings were not replaced at last overhaul as we found few sheared bolts and most probably to remove it would have taken quite a lot of time and would have delayed the vessel. The wear and deposits on nozzle ring’s fins had a negative impact on turbocharger’s efficiency and functioning, as restricted the gas flow and changed the flow pattern, thus leading to turbocharger‘s continuous “barking” and surging.
As a conclusion, in order to prevent turbocharger’s surging you need to follow the specific plan maintenance, to keep your filters clean, regularly chemically clean the scavenge air coolers, check the engine performance, clean and/or water wash the economizer regularly and inspect main engine units, scavenging space and exhaust manifold regularly.
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