Surge limit of a turbocharger

Surging in a turbocharge happens when a reversal of gas flow through the turbo charger compressor happens. When an engine cannot use up all the air delivered by the turbo compressor, the pressure inside the scavange reciever increases rapidly. This air will be blown back through the turbo compressor through the diffuser ring. It can cause imbalance of the rotor shaft and vibrations in the system which is not at all good for the plant.

Turbocharger characteristic curve is a plot between pressure ratio and air flow through the turbo compressor. This curve is defined for each RPM of the turbocharger and shows how the turbocharger will behave under such RPMs.  Turbocharger will have different efficencies under different pressure ratios for same RPM. 

As the efficency of the turbocharger increases for the same RPM, the turbocharger reaches more and more near to the surge point. Surge point is a praticular pressure ratio at which a turbocharger will surge under a particular RPM.lets say a T/C starts surging at a pressure ratio of 2.0 for an RPM of 15000. The surge point for 15000 RPM can be said as 2.0 bar under this condition. Above 2.0 pressure ratios, the T/C mass is defintiley going to surge, be it 2.1 or 200 for 15000 RPM.

If u draw a line connecting all the pressure ratios  or surge points for all the T/C RPMs possible, u get what u call the SURGE LINE .See the image below for more clarity.

Please comment below for more discussion.

How charge air coolers of marine engines works

Charge air coolers or scavenge air coolers are used to cool down the charge air, usually after supercharger or turbocharger compressor in diesel engines. In marine engines, the charge air temperature after the T/C compressor can rise upto 145-160 deg Celsius.

The increase in temperature of air is not at all a good thing for the efficiency of the plant, as efficiency increases proportionally with charge air density. Thus, this air needs to be cooled to the lowest possible temperatures, to increase the density of the air, and thus the efficiency of the plant.

The volume of air handled is so huge that the LT system of the ship ( central fresh water system ) cannot handle the heat transfer. A LT system which can cool the charge air supplied to ME needs huge plate coolers, which are not worth it. So, a conventional tube type cooler with sea water as cooling medium is used for this purpose.And please note that the lowest charge air temp here possible is also sea water temperature , which is always less than the LT temperatures.

Direct cooling using sea water onboard ships is used only in Main Charge Air coolers for Scavenging due to this.

Sea water is very corrosive and is not usually used for cooling other engine parts directly.

Boiler water treatment- What ph level to maintain??

Boiler water should be always alkaline, as acidic medium accelerates corrosion. only in a alkaline environment, steel will stay as steel in contact with water. Steel and water in touch is like petrol and matchbox together. Always they have huge affinity to each other. Acidic environment just lights the fire and ur boiler inside will start rusting, giving dark brown water when u take samples for tests.

Boiler water should be treated to a ph level of 9.5-11.5 as UNITOR, a big player in boiler treatment chemical market, recommends. 

Alkalinity is normally maintained by adding sodium hydroxide, branded by different chemical companies in different names. UNITOR brands it as autotreat, VECOM brands it as BOILER TREAT, and so on...

Sodium hydroxide provides a highly alkaline environment in the boiler. This is about the only environment where water and steel get along well. Heat magnifies the normally corrosive effect water has on steel, since it speeds up chemical reactions. Maintaining the correct alkalinity range minimizes this highly corrosive effect of water. Alkalinity also plays a critical part in various chemical reactions in the boiler. Frequently, most of boiler water alkalinity comes from the addition of sodium hydroxide in the chemical program. Some of the alkalinity comes from naturally occurring alkalinity found in raw water supplies. If it is present naturally, it contributes to the required alkalinity in the boiler and decreases the amount of sodium hydroxide needed.

Jerk type fuel pumps having injection timings retarded- Why?

Fuel pumps plungers are machined to very fine tolerances, as is the matched barrel in which it reciprocates. This means that a plunger from one fuel pump cannot be used into another barell from a different fuel pump, of same make and model.

Wear due to erosion (due to high pressure fuel as it spills)  takes place on the top edge of the plunger and the edge of the helices and spill ports. This, together with the wear in the plunger and barrel, will lead to the injection timing becoming retarded, for which adjustment may have to be made.

 a fuel pump plunger and barell.

Labyrinth seal fitted on the back surface of a compressor wheel of a turbocharger:

A labyrinth arrangement is  fitted to the back of the compressor impeller to restrict the leakage of air to the gas side. It also prevents the oil being drawn into the compressor. Labyrinth seals use high rotational speed of the shaft to its advantage.

Labyrinth seals or glands are fitted to the shaft and casing to prevent the leakage of exhaust gas into the turbine end bearing, or to prevent oil being drawn into the compressor. To assist in the sealing effect, air from the compressor volute casing is led into a space within the gland. A vent to atmosphere at the end of the labyrinth gives a guide to the efficiency of the turbine end gland. Discoloring of the oil on a rotor fitted with a roller bearing will also indicate a failure in the turbine end gland.

How centrifugal compressors handle air, where centriful pumps do not

Centrifugal pumps and compressors operates on the same principle of converting kinetic energy of the rotating impreller to pressure energy of the fluid it handles.The fluid particles are thrown out from the tip of the impeller with huge kinetic energies, which is coverted to pressure energy by the special volute design of the casing. Both cent. pumps and compressors operates on same principle but have a different clearence levels, and imepeller dimensions.

The perfomance of a centrifugal pump will be largely dependend upon the wear ring clearence. Wear ring clearence is the clearence between the impeller and the casing at the inlet side of the pump. If this clearence is high, the fluid inside the casing will be pumped back to the suction eye, reducing pump efficency. For a normal centrifugal pump handling liquids, this clearance is relatively higher.
For a centrifugal compressor, the wear ring clearence will be extremely small. This will allow handling of fluids of very low densities i.e, air .

A large impeller can create very high angular velocites at the impeller tips, which traslates into high pressure energy of the fluid handled. The Auxilliary blowers for main engines and T/C blowers are centrifugal with large imepller diamters because of this reason. Large rotational speeds also increases the rate of energy transfer to the fluid handled, which also results in handling of light fluids.

Why nimonic coating is provided on valve stem of exhaust valves of marine engines??

The purpose of Nimonic coating in exhaust valve of modern marine engines is to prevent hot corrosion of the valve stems, increase valve life and to minimize the effect of valve burning.

Modern marine engines, operaing on HFO with a bore greater than 300mm, the one propelling my current vessel is MAN B&W 6G50ME, operates with a relatively high exhaust tempertaures of 350-400 deg celsius. Under this temperature the uncoated valve stems may give up ther strength, leading to overall reduction in valve life. Also under high temperatures the valves stems are subjected to hot corrosion. Nimonic have a very high temperature stability and resists hot corrosion.

Full nemonic coated valves are also available from engine manufaturers , which will increase overall lifetime of the valve, but increase the cost marginally.

Valve seats are usually treated with a harder alloy compound, stellite.

MAN B&W are now introducing DURA SPINDLE valves with W seats to still increase the overall life, with a promised run time of 20000 hours.