Category Archives: Scavenging and Turbocharging

U-Tube Manometer: Crucial Instrumentation for Main Engine Air Coolers and Turbochargers

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In the intricate world of marine engineering, the efficient operation of main engine air coolers and turbochargers is paramount to ensure the smooth functioning of a vessel’s propulsion system. These critical components are responsible for optimizing the combustion process and maintaining engine performance. To monitor and maintain these systems, marine engineers rely on essential instruments like U-Tube Manometers.

If you are familiar with marine diesel engines, you may have noticed that some of the components, such as the main engine air coolers and the turbochargers, are equipped with U-shaped glass tubes filled with liquid. These tubes are called U-tube manometers, and they are used to measure the pressure difference between two points in a fluid system.

Example of U-tube manometer on turbocharger

In this article, we will delve into the significance of U-Tube Manometers, why they are preferred over pressure gauges, their maintenance requirements, and troubleshooting tips. Understanding the importance of functional U-Tube Manometers on air coolers and turbochargers is vital for the safety and performance of a marine engine.

The Role of U-Tube Manometers

A U-tube manometer is a simple device that consists of a U-shaped glass/plastic tube containing liquid, usually water or oil.

The liquid level in each leg of the tube depends on the pressure applied to that leg. If both legs are exposed to the same pressure, such as atmospheric pressure, the liquid levels will be equal. However, if one leg is connected to a point of higher pressure, such as the inlet of an air cooler or a turbocharger, and the other leg is connected to a point of lower pressure, such as the outlet of an air cooler or a turbocharger, the liquid level in the high-pressure leg will drop, while the liquid level in the low-pressure leg will rise. The difference in liquid levels indicates the pressure difference between the two points.

So, the principle behind their operation is simple: as the differential pressure across the component changes, the liquid level in one arm of the U-tube rises while the other falls, providing a visual indication of the pressure difference.

U-tube manometers are used instead of pressure gauges for several reasons:

  • Direct Reading: One of the primary advantages of U-Tube Manometers is that they provide a direct reading of the differential pressure. Unlike pressure gauges that rely on mechanical elements, U-Tube Manometers offer a clear and instantaneous visual representation of the pressure difference, making them highly reliable. They are accurate and reliable, as they are not affected by temperature changes or mechanical vibrations.

  • Accuracy: U-Tube Manometers are known for their accuracy and precision in measuring pressure differentials. Pressure gauges may drift or require recalibration over time, while U-Tube Manometers maintain their accuracy as long as the liquid column remains stable. They do not require any external power source or calibration. They can measure both positive and negative pressures, as well as vacuum.

  • Durability: U-Tube Manometers are robust and durable instruments that can withstand harsh marine environments. They are less prone to damage compared to fragile pressure gauge dials and needles. Also, they are simple, cheap, and easy to install and operate.

Importance of Functional U-Tube Manometers

U-tube manometers are important because they provide a visual indication of the pressure difference across the air coolers and the turbochargers. This pressure difference reflects the performance and efficiency of these components, as well as the condition of the engine.

Example of U-tube manometer in main engine air cooler

For example, the main engine air cooler is a heat exchanger that cools down the compressed air from the turbocharger before it enters the engine cylinders. This increases the density and oxygen content of the air, which improves the combustion process and reduces emissions. The U-tube manometer connected to the air cooler shows the pressure drop across the cooler, which is proportional to the amount of heat transferred from the air to the cooling water. A low pressure drop indicates a low heat transfer rate, which means that either the air cooler is dirty or fouled, or that there is insufficient cooling water flow. A high pressure drop indicates a high heat transfer rate, which means that either the air cooler is clean and efficient, or that there is excessive cooling water flow.

U-Tube Manometers act as early warning systems. A sudden change in the pressure differential could indicate a problem with the air cooler or turbocharger, allowing engineers to take corrective actions before the issue escalates, potentially avoiding costly repairs and downtime.

By monitoring the U-tube manometers regularly, one can assess the performance and condition of the air coolers and turbochargers, and take appropriate actions to maintain or improve them.

Maintenance of U-Tube Manometers

U-tube manometers require little maintenance, but proper maintenance of U-Tube Manometers is essential to ensure their accuracy and reliability:

  • Liquid Column Inspection: Regularly inspect the liquid column in the U-tube for signs of contamination, evaporation, or air bubbles. Any irregularities can affect the accuracy of the readings and should be addressed promptly.

  • Leak Checks: They should be checked periodically for any leaks, cracks, clogs, or contamination. Ensure that the connections between the U-tube manometer and the monitored equipment are leak-free. Leaks can lead to false readings and should be sealed immediately.

  • Calibration: Periodically calibrate the U-Tube Manometer to confirm its accuracy. This calibration process may involve adjusting the liquid column height to a known reference value.

If any problems are detected with the U-tube manometers, they should be repaired or replaced as soon as possible.

Troubleshooting U-Tube Manometer Issues

When U-Tube Manometers are not functioning correctly, it can lead to inaccurate pressure readings. If there is any discrepancy between the readings of the U-tube manometers and other indicators of the engine performance, such as power output, fuel consumption, exhaust gas temperature, or emissions, one should investigate the possible causes and solutions.

Here are some common troubleshooting steps:

  • Check for Blockages: Inspect the tubing and connections for any blockages or obstructions that might impede the movement of the liquid in the U-tube.

    If both legs of the U-tube manometer show equal liquid levels, it means that there is no pressure difference across the component connected to the tube. This could indicate that either the component is blocked or bypassed, or that there is no flow through the component. One should check the valves, pipes, filters, and pumps related to the component, and ensure that they are open, clean, and working properly.

  • Verify Liquid Integrity: Ensure that the liquid inside the U-tube is in good condition and free from contamination. Replace the liquid if necessary.

  • Recheck Connections: Confirm that all connections are secure and that there are no leaks. Tighten or replace fittings as needed.

    If one leg of the U-tube manometer shows a higher liquid level than the other, it means that there is a negative pressure difference across the component connected to the tube. This could indicate that either the component is leaking or damaged, or that there is a backflow or reverse flow through the component. One should check the seals, gaskets, flanges, and clamps related to the component, and ensure that they are tight, intact, and aligned correctly.

  • Verify Liquid Column Stability: If the liquid column is fluctuating excessively, it could indicate air bubbles or evaporation. Replenish the liquid and remove any trapped air. If the liquid level in the U-tube manometer fluctuates or oscillates rapidly, it means that there is a pulsating or unstable pressure difference across the component connected to the tube. This could indicate that either the component is vibrating or resonating, or that there is a surge or stall in the flow through the component. One should check the mounts, supports, dampers, and silencers related to the component, and ensure that they are rigid, secure, and effective.

In conclusion, U-tube manometers must be always functional because they provide vital information about the pressure difference across the air coolers and turbochargers, which affects the engine performance and efficiency. If the U-tube manometers are not functional, one may not be able to detect any problems or faults with the air coolers and turbochargers, which could lead to serious consequences such as engine damage, power loss, fuel wastage, or emission violations. Therefore, it is essential to keep the U-tube manometers in good working condition and monitor them regularly.

In the challenging and dynamic environment of the open sea, having reliable instrumentation is not just a matter of convenience; it’s a matter of safety and operational efficiency.

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What causes fouling of the air coolers?

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The turbocharger uses the exhaust gas from the engine to compress new air and charge the engine with a positive pressure higher than ambient conditions.
Exhaust gas temperature convection and compression raises the temperature of the air and this cannot be delivered directly into the engine due to operational limitations exceeding.
As a result, the engine is equipped with a cooler that returns the air temperature to ambient relatively close levels.
Because hot air has a lower density, the mass of air charged into the engine is smaller than when the air is cold.
Thus, the charge air cooler increases the density and lowers the temperature of the charge air.
The compressed charged air exiting the charge air cooler will have a temperature of around 40 to 50 degrees Celsius, down from approximately 150 degrees Celsius.
At cold temperatures, the lower temperature of the air increases the density of the charge air.
Increased charge air density increases scavenging efficiency and allows for more air to be compressed inside the engine cylinder, allowing for more fuel to be burnt inside the combustion chamber, resulting in increased power.
Additionally, the engine is kept at a safe operating temperature. Compression temperature reduction alleviates stress on the piston, piston rings, cylinder liner, and cylinder head.
Additionally, the charge air cooler lowers the exhaust gas temperature.
It has been demonstrated that a one-degree Celsius decrease in scavenging air temperature results in a five- to ten-degree Celsius decrease in exhaust temperature.
This does not mean that cryogenic temperatures may be used to charge the air.
If very cold air reaches the cylinder liner, a severe thermal stress occurs, resulting in cylinder liner failure.

Charge air coolers are installed between the turbocharger compressor outlet and the engine intake or scavenging manifold.
The figure below illustrates the precise position of a charge air cooler.

When the air cooler become fouled, the heat transfer between air and cooling agent, which is normally fresh water, is decreasing and this is indicated by the raise in air temperature after cooler and a raise in the pressure drop across the cooler (U-tube manometer).

Fouling of the air passages in air cooler, which is part of the engine turbocharging system, is usually due to oil film and oily-water films collected on the sides of the tubes and tube fins. Lint and similar material adheres to these films of oil or emulsion. The presence of oil may be caused by faulty air filters which allow the air to pass by the side of the filter element.

Sometimes the oil is drawn from bearing at the blower end.

The presence of moisture is usually the result of high humidity, when the engine is operating in warm air temperatures in conjunction with low sea water temperature.

So, the signs of air coolers on air side are: higher pressure drop across the cooler, higher air temperature after the cooler, higher cooling water outlet temperature, higher underpiston scavenging air temperature, higher exhaust temperature on all cylinders and in worst case scenario turbocharger surging.

If the fouling is on the water side of the cooler the signs are: higher air temperature after the cooler, higher cooling water outlet temperature, higher underpiston scavenging air temperature and higher exhaust temperature on all cylinders.

As a remedy, fouling can be prevented through proper plan maintenance by cleaning the coolers at regular intervals.

The air side of the cooler can be water washed and/or chemically cleaned. On most of the 2 stroke main engines there is a possibility of periodically cleaning the air coolers even during engine operation at low load. On some of the vessel there is designated tank available where a mixture of water and Air Cooler Cleaner chemical are mixed and recirculated through the air cooler for a certain period of time. It is very important to rinse with fresh water the air cooler very well after cleaning and blow with air if possible in order to prevent chemical corrosion of the cooler parts.

For the water side a soft brush can be used to clean inside tubes and in case of hard deposits a long drill bit can be used, but with caution in order not to damage the tubes.

However, if there is no improvement after cleaning then the cooler needs to be removed from the engine and dipped into a chemical bath for several hours.

Source: Jazzrel Clark Gella

Usually this procedure, especially for main engines are performed during every vessel special survey (dry docking), when the coolers are dismantled and taken into yard facilities for proper cleaning into chemical ultrasonic bath.

ME air cooler after ultrasonic cleaning in Dry Dock

In conclusion, keeping the air coolers clean is very important and must be part of the vessel plan maintenance. The engineers must be familiar with monitoring of coolers performance, manufacturer’s maintenance manual, cleaning procedure and assembling/disassembling procedures. Failure to do so can lead to engine breakdowns, expensive repairs and vessel stoppage which incurs high operational costs for the ship owner.

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Why turbochargers are surging?

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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.

Source: Sriram Balasubramanian

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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