MOTOR ORAL QUESTIONS & ANSWERS PART-3

0
1576

Q – Explain TCA and TCR type of turbocharger.

Ans – Before starting with TCA & TCR, first I will tell you what is turbocharger and what is the importance of it.

Earlier we use supercharger which supply the air at high pressure so due to high pressure density of air increase and more fuel we can efficiently burn inside the engine, size of engine becomes compact and we can produce more power because of the power to weight ratio of the engine has been increased.

But if supercharger have that much of benefit then why we will use the turbocharger instead of supercharger because the biggest disadvantage of superchargers is that they suck engine power simply to produce engine power. They are run off an engine drive connected to the crankshaft, so you are essentially powering an air pump with another air pump. Because of this, superchargers are significantly less efficient than the turbochargers. So supercharger take energy from the engine itself so due to this loses will be there at the same time turbocharger has been driven from the exhaust gases which is one of the biggest advantage of it.

Note – Some time surveyor might ask that why we cool the air which came from the turbocharger compressor side the reason for this is that the temperature of the air is 120 degree Celsius so due to this high temperature the air expansion will take place and due to this density of air becomes low and we unable to send large quantity of air inside the engine during combustion due to which effective combustion will not take place that’s why we use cooler to reduce its temperature and increase its density

Turbocharger is the heat conversion device which convert heat energy to K.E.

TCA Turbocharger

We try to cover all the points in TCA turbocharger but to understand the working of TCA

turbocharger please check the diagram side by side –

1. Rotor of the turbine side is forged to the shaft

2. Turbine blade is fitted to the rotor by the arrangement which is called fir tree arrangement

3. One wire is pass through all the blades and welded at the ends just to reduce the vibrations which is called the dampen wire.

4. Shroud Ring – the work of shroud ring is same as just like the work of wear ring in the centrifugal pump. Shroud ring is stationary and very small clearance is there and it wont allow the exhaust gas to pass

5. Nozzle Ring – Exhaust gas from engine first pass through the nozzle ring then to the turbine blade, so some of the energy of exhaust (heat) converted to K.E in nozzle ring and remaining conversion will take place in blade of the turbocharger

6. When exhaust come out from the engine it will hit the blade due to which its direction keep on changing and it will give thrust and due to this shaft starts rotating

7. Blower we make of aluminium alloy because that side was not exposed to high heat

8. Air side we have the inducer and the blower, inducer is hydraulically press fit and it guide the air towards the blower.

9. As blower is rotating with the high RPM so due to this rotation air come out from the periphery of the blower with high kinectic energy but in the end we only want the pressure energy so that’s why we fit diffuser in the end to get the pressure energy from the kinectic energy

Note – 1. Both end we have the bearing to take the thrust

Slower side = deep groove ball bearing – which takes the axial thrust

Turbine side = roller bearing – in which expansion is allowed

These two bearing require lubrication so separate lubrication is been provided and attach pump is there to give the lubrication, which is been run by the shaft itself

2.Whether turbocharger is radial type or axial type that depend upon the flow of exhaust gas over the turbine side (important point to note).

3. Just like in this case if you see the diagram that exhaust gas enters parallel to the shaft Blower side always be the same it is of mixed type mostly, only the turbine side changes that’s why it is TCA type of turbocharger

4 Turbine side casing is water cooled but blower side is normally air cooled because it cannot deal with high temperature.

5.Labyrinth seal – the purpose of the labyrinth seal is to make sure that exhaust should not go to the oil side and one air sealing connection is also provided just to give the cooling and to maintain the positive pressure so that exhaust should not come to the oil side under any circumstances.

Q – How fouling take place in turbocharger and procedure of turbocharger washing.

Ans – This question is very frequently asked in MMD Orals because you need to understand the basic concept of fouling

If 1 mm layer is deposited on the blower then it may drop the efficiency of your turbocharger so according to the instruction given in the manual clean it at regular intervals so at fouling of turbocharger should not occur.

Note – 1. If you did not perform the turbocharger washing from a long time then don’t do it unless you done a complete overhaul of turbocharger because after a long time if suddenly do a washing then complete deposits from the blade could not be removed and it make a turbocharger imbalance and it create a heavy vibration and noise which is difficult to handle and you may break important part inside the turbocharger

2. For blower side washing of turbocharger a separate water connection has been given

3. A very important point to note is that turbine side (exhaust) fouling is less as compared to the blower side (air) because turbine side is exposed to high heat and high exhaust pressure. so all the deposits will be removed because of this, but exhaust gas passes over the turbine should contain large amount of impurity like ash, calcium carbonate & carbon deposits.

4. Turbine efficiency we also get to know out with temperature difference at the inlet of the turbocharger and at the outlet because it give you an idea that how much heat energy is been transferred to the kinetic energy.

Procedure of washing the turbine side of turbocharger —

1. If you are going to perform the turbocharger washing of the turbine side then you have to reduce the rpm of the engine else cold water(normally we use warm water @ 4.5 bar pressure) and hot surface cause cracking of the blade of the turbocharger because of the quenching effect

2. Inform the bridge that you are going to perform the water washing of the turbocharger

3. Don’t suddenly drop the rpm at least take 30 minutes to drop the rpm gradually and keep dropping unless you will not get 200-230 degree temperature at the inlet of the turbine.

4. Once you reached to the desired rpm, then wait over there for at least 10-15 mins to stabilize the temperature, then open the drain

5. Open valve for water washing and continue water washing for at least 10 minutes 90 percent of the water is evaporated and only 10 percent came out from the drain)

6. Check the condition of water which came out from the drain if you found the clean water coming out it simply means that your turbine washing take place properly and your turbine is fully cleaned, so close the drain now.

7. Then close the water valve and keep running the turbine at the same rpm for at least 10 mins this is called by washing of the turbine and at the same time check for any abnormal noise or vibration from the turbocharger.

8. Now again gradually start increasing the rpm and keep checking for any abnormal noise or vibration

Note – Water washing has only one advantage that cleaning is very good and if carried out properly efficiency of the turbocharger has been increased

Disadvantages:

  • More time is required for cleaning
  • Chance of corrosion will be there because sulphuric acid is present in the exhaust gas
  • if not done properly chance of turbine blade cracking will be there (only to be done by the expert people)

So, that was the reason we normally use dry washing of the turbine side for that we will use grit or wall nut grit ( at the time of doing dry washing of turbine side make sure your drain has been shut)

Q – Explain TCR type turbocharger of your ship.

Ans-

Before I start with the TCR type first you need to understand that this type of turbocharger is normally used in small engine like generator and TCA type we mostly use in big engines like main engine.

TCR type turbocharger has certain advantages because it is compact and bearing life is way high and TCA type turbocharger bearing is always under fatigue stresses.

TCR type turbocharger bearing has more load carrying capacity

Note –If you use this type of turbocharger for M/E then you have to use two or three

turbocharger.

In this type of turbocharger there will be a header tank because if in case sudden black out takes place so lube oil pump for bearing lubrication has been stopped and no lubrication is been provided in that case your bearing has been gone, but this problem was not there in TCA type of turbocharger because attached lube oil pump has been there in that so till the time your turbocharger rotates it provided the bearing lubrication automatically.

In this type of turbocharger we have floating type of bearing so there is hydrodynamic

lubrication between the bearing and shaft/ housing and shaft ( 50 small header tank can able to full fill your purpose)

Regular maintenance on turbocharger

1. Check and replace the filter if pressure drop in manometer more than 50 mm

2. Regular washing of the turbine and blower side with respect to the instructions given in the manual

3. Change the oil of the bearing after certain number of running hours with reference to the PMS or turbocharger manual

Q-What are the different types of turbocharging with advantage and disadvantage?

Ans – There will be the two type of turbocharging

1. Pulse type

2. Constant pressure type

Pulse type – As soon as exhaust v/v opens, the exhaust start coming out in the form of pulse that pulse energy use to rotate the turbine

In pulse type turbocharger has to be fit near to the cylinder to avoid the expansion of pulse energy

Exhaust pipe should be small in diameter so that exhaust should not expand in the line.

Due to all this requirement exhaust piping system in pulse type turbocharger is little complicated.

How to do the grouping of exhaust pipeline towards the turbocharger in pulse type –

We have to do the grouping in such a way that pulse energy of one unit could not interfere with the pulse of another unit so we never ever make the grouping according to the firing order of the engine.

In main engine we don’t have any problem because mostly we have two turbocharger so we can easily make the grouping but in generator we use only one turbocharger so in that case we give two inlet to the turbocharger so that one unit pulse should not interfere with the other one.

Above we just only talk about the disadvantage of pulse type turbocharger now lets talk about its advantages –

1. It does not require auxiliary blower because it has high efficiency at low loads and

give a quick response when load changes suddenly.

2. In generator load is constantly fluctuating so in generator we mostly use pulse type

turbocharger

Note – But its efficiency is very less for those engine which normally run at high load (like main engine only at the time of manoeuvring we have the problem for that we have auxillary blowers so that was the reason we use constant pressure type of turbocharger in case of main engine).

In pulse type turbocharger pulse energy is used to rotate the turbine so when every time this pulse hits the turbine blade its increases the maintenance and also reduce the bearing life one of the biggest disadvantage

Constant pressure type – As the name indicate that the exhaust gas pressure at T/C inlet is always constant.

1. In constant pressure type we can put turbocharger at any place no limitation

2. No complexity in pipeline and manifold size, we can use big dia pipeline without any problem

3. Efficiency of the turbocharper is high at higher loads so it used at a place where load is not fluctuating much or engine should not run at slow speed most of the time.

Disadvantages – Now lets talk about the disadvantage of this type of turbocharger

1. Low efficiency at low loads that’s why it require assistance at low load (auxiliary blowers)

Auxiliary blower take suction from the blower only so that it also try to increase the

efficiency of the blower Due to this scavenge manifold pressure keep on increasing and at one point of time blower automatically cuts off when required scavenge manifold pressure has been reached by the activation of the pressure switches (at the time of manoeuvring blower keep on running!

Note – When the engine is running normally at high speed so sometime you notice that

auxillary blower is rotating with a small speed so it will give you an indication that air has been leaking from the flap valve.

Q – What is lambda control in turbocharger?

Ans – This question is very frequently asked now a days in MMD Orals as we already discuss that pulse type have so many disadvantages and due to this reason it was not much efficient because of high maintenance and less bearing life and at the same time in constant pressure type turbocharger it has only one disadvantage that at low load it won’t work properly so that’s why Man B&W design a special type of turbocharger which remove this demerit of constant pressure type turbocharger

If we put constant pressure type turbocharger in generator then at the time of starting and quick load changing we also need a quick response from the turbocharger so for that reason a connection of compressed air with a solenoid valve in the blower side is provided. So the compressed air @ 7 bar directly hit the blower side blades and increase the RPM of the turbocharger instantly and this solenoid valve operates by a electric signal which keep sensing the rpm and load changing fluctuation with in the engine so this is all about the lambda control in turbocharger.

Q – Exhaust from the main engine come out at a temp of 360 degree Celsius and temp. of exhaust gas at the inlet of the turbocharger is 460 degree Celsius how?

Ans – Sometime surveyor wants to check you basic understanding of thermodynamics so try to give the best at these type of question there could be the number of reasons for this which are given below-

1. Because of adiabatic compression – if we compress the gas quickly so that it doesn’t have a chance to exchange heat with its environment, the temperature will change. This sort of compression is called adiabatic compression ( exhaust of all unit come in the manifold because of this adiabatic compression will take place)

2. Exhaust come out from the engine have lot of KE which convert to P.E in the manifold

3. The sensor fitted on the outlet of the unit which sense the exhaust gas temperature will only give you the average temperature because with the exhaust gas some of the scavange air also go out whose temperature is nearly 40-50 degree Celsius.

Q – What do you understand by the term matching of turbocharger and what is VTA & VIG in new turbocharger?

Ans – Matching of turbocharger simply means that with respect to the lost it can able to supply the sufficient quantity of air that is called matching.

Our main priority is to match the turbocharger for full load means to check that the

particular turbocharger should have that much of capacity so that it can able to cope up with the full load and can able to supply the sufficient quantity of air.

You heard the term stoichiometric ratio this ratio simply tell you that 14 kg of air is required to burn 1 kg of fuel but we try to give 36 kg because losses will be there so that effective utilization of the power and fuel will take place

So to produce this much quantity of air we need blower and blower is been run by the

turbine so our main aim is to rotate the turbine with sufficient rpm)

Turbine rpm is mostly depend upon the nozzle ring, so we have to design the nozzle ring in such a way that it can full fill our requirement for all the loads

So the conclusion is that with the nozzle ring we can alter the rpm but nozzle ring is the fix ring, so it is not easy to alter the nozzle ring. So that’s why we design the nozzle ring in such a way so that it can perform well at full load also

So that’s why new technology came into action which is –

VTA –Variable turbine area

VTG — Variable turbine geometry

VTA & VIG can perform well at any rpm and fulfill the air requirement even whatever be

the turbine rpm at that time.

So with this new technology we can vary the angle of nozzle ring and we can easily match the turbocharger for different loads and it also help in reducing the Nox content

Turbocharger efficiency = Turbine efficiency X Blower efficiency X Mechanical efficiency

Q – what do you understand by the term turbocharger surging? Give reason for it.

Ans – At particular pressure ratio the manifold start supplying or rush into the opposite

direction to the compressor side (blower side), because compressor supply pressure

as compared to the manifold pressure. So because of this air come out on the blower side and expand with the huge sound that is called turbocharger surging.

OR

Instead of the turbocharger blower supplying the air to the motor, there throw of

flow so that air from the manifold will blow out into the atmosphere through turbocharger blower with a huge sound.

But the question is why this happens – there could be a number of reasons for this which later on I tell you but it mostly occur when you suddenly drop the rpm of the engine because in my previous article I told you about the matching of turbocharger, to better understand this I consider one example –

Let say your engine is running at 110 rpm and your scavenge manifold pressure is 1.5 bar and now you suddenly come to 50 rpm, may you give the signal to engine to come to 50 rpm and governor suddenly reduce the fuel but turbocharger is the rotating machine it still role with the same rpm and supplying the same quantity of air to the engine but now engine refuse to take that much quantity of air so manifold pressure keep on increasing and at one point of time pressure of manifold increase with respect to the supply pressure so due to this reversal of flow will take place this is basic of surging.

Note: – Frequent surging is very bad for the engine or for the turbocharger itself, it may damage the bearing of the turbocharger so try to avoid the surging as much as possible which give you a idea about stable and unstable operation.

It is clear when suddenly the requirement of air is not required then we approaches towards the surge line.

Now lets talk about other reasons of turbocharger surging –

1. It can also happen due to the sudden change in the engine load or speed.

2. Imbalance in cylinder power or faulty injectors

3. Un-cleaned turbine nozzle ring.

4. Damaged blades

5. Dirty or choked filter

6. The capacity of turbocharger is larger than required.

7. Increased back-pressure at the turbine side.

8. Improper power distribution between the main engine cylinders may cause turbocharger surging as one unit is producing more power & other is producing less. Due to this the air consumption needed by both the turbochargers differs, which leads to surging

9. Fouled compressor on the turbine side – In this case if the inlet filters are dirty then enough air cannot be provided for the combustion, which leads to surging. Similarly if the turbine side is also dirty i. e nozzle, blades etc enough air cannot be produced for the combustion.

10. Highly fouled exhaust i.e. economizer, if installed may cause back pressure in the turbocharger and thus finally lead to surging.

11. Bad weather – This is one of the reason for surging. Due to bad weather the engine suddenly starts racing & sudden load change takes place. This happens because during bad weather or pitching the propeller moves in & out of the water, causing the change in load on the engine.

12. Scavenge space fire / Exhaust trunking fire

13. Poor scavenging or leaky exhaust valve

How to Prevent Turbocharger Surging –

The following are the ways to prevent turbocharger surging. However, it is to note that some points may vary with design and construction of the turbocharger.

1. Keep the turbocharger intake filter clean.

2. Water-wash the turbine and the compressor side of the turbocharger.

3. Proper maintenance and checks should be done on turbocharger periodically.

4. Soot blow should be done from time to time in case of economizer or exhaust boiler.

5. Indicator cards to be taken to assess cylinder and power distribution of individual units

Note – So finally the conclusion is that mismatching is the reason for turbocharger surging.

Q-Draw the diagram of labyrinth seal and explain its principle of working.

Ans – Labyrinth-type seals are used to minimize recirculation losses within the compressor. A labyrinth seal consists of a number of teeth(knife-edges) that can be either stationary or rotating. Stationary labyrinth teeth are fixed to the compressor stationary components very close to the compressor rotor(see Fig. below). Sealing action is the due to the flow resistance caused by repeated throttling across the labyrinth teeth. Labyrinth seals are designed so that one of the two adjacent parts(labyrinth teeth and rotor) is relatively soft. The softer material yields on contact without any damage to the harder material. Compressor manufacturers select labyrinth seal clearances that are as tight as practical to minimize leakage while avoiding heavy rubbing with the rotor.

Q – Explain the different parts in mechanical seal with diagram.

Ans – Introduction

The mechanical seal acts as a check valve & a slider bearing. The obvious function is that of a check valve to restrict liquid under pressure from leaking out of the pump, or from drawing air into the pump when under vacuum conditions.

Seal Life

Since the seat must function as a slider or friction bearing, the seal has an unpredictable life Span. The seal of a centrifugal pump is generally replaced many times during the life of a pump. All bearing need lubricant & the seal lubricant is the liquid being pumped. Liquid infiltrates between the contact faces of the primary & the mating rings. Some of this liquid does find its way through to the atmosphere but is so slight as to only be noticed as corrosion of build up on the pump adapter. The condition of the pumped liquid will affect seal life.

Abrasives

The number one enemy of the mechanical seal is abrasive particles in the liquid being pumped. Abrasives may be anything from dirt to dissolved impurities in the liquid precipitating out of the solution. These abrasive particles infiltrate with the liquid between the seal faces & grind away the carbon primary ring. The normal shiny face of the primary ring & mating ring

Heat Damage

Excessive heat can damage the seal in two areas – the primary ring & the elastomer parts. The primary ring is made generally of carbon. Should the pump be operated without liquid even for a very short period of time – the primary & mating ring faces are denied lubricant. This causes the faces to become very hot. The binder mixed with the carbon breaks down & the face of the primary ring turns to a dull black powder.

The O-ring, or cup, & flexible diaphragm of the seal are made of one of many types of rubber-like substance called an elastomer. The type of elastomertic material is selected to match the temperature limit & types of material being pumped. When the temperature limit be exceeded, the diaphragm & O-ring will become hard & sometimes crack. The seal will then start to leak.