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. 

Purpose of an economiser on board a ship

An economiser, as the name suggests increases the overall economy of the main propulsion plant.
The heat from burning fuel in main engine is mostly used to move the piston and thereby create the main propulsion onboard. Some of this heat energy cannot be used in the process, as the process is never isothermal. This heat energy is used to produce steam onboard, using economisers.

The process is simple. The unused heat from the main engine exhaust is transferred to water in the economiser, which produces the necessery steam for the propulsion plant. When economiser is absent, this head energy need to be provided by burning extra fuel in boilers, which costs $$$. So economiser in the end, increases the overall heat efficency or the ECONOMY of the propulsion plant.

Why a diffuser is given in the atomizer of a pressure jet boiler

A diffuser is a Round plate with a circular hole in the middle, and a number of radial grooves,which is placed in front of a jet atomizer of a boiler.

The function of diffuser or as normally called, swirler plate is to mix the air and fuel properly. There are radial cuts in the diffuser plate which will create a vortex flow of the air . Atomizer is directed into the middle hole of the diffuser. When the Combustion fan directs air into the diffuser, it creates a vortex flow. The atomizer nozzle directs the fuel into this vortex, which gets evenly mixed with the air. The flame stablity is greatly related to the cleanliness of the diffuser plate.

When the diffuser plate becomes dirty, mostly due to carbon and fuel particles sticking to it, the radial cuts in the plate are blocked. In this condition, the air supply to the boiler burner becomes low, and Boiler starts tripping at high loads. The air- fuel ratio cannot be maintained under a dirty diffuser condition. This further creates more carbon accumulation which may eventually fully block the diffuser swirler action, and failing the boiler even at low loads. This is a major reason why a boiler fires reasonably good under low load and trips under high loads, mainly above 70%. This is my personal experience onboard my vessel, where my boiler was tripping at 70%+ loads. Inspection of diffuser revealed considerable cabron accumulation. Once cleared boiler was firing normally under all loads.

To prevent the blockage of diffuser plate, always keep good fuel- air ratios. A small carbon accumulation can accelerate the spoiling of the plate.

Keep ur diffuser plates clean and have a happy boiler my sailor friends:) happy sailing.

How to Blow down a boiler

Boiler blow down is one of most important routine jobs done on a main or aux boiler onboard a ship. I have a AALBORG MISSION OC- 2000 Kg/hr steam composite boiler and ALBORG MISSON OC- 20000 Kg/hr aux boiler onboard the vessel i am currently onboard. The boilers are being blown down regularly, when the water tests show an increased chlorine level , when conductivity increases ,or when the boiler water colour changes to reddish brown.

Blow down is needed to remove the sediments which get accumulated in the water ring on the boiler or when chlorine levels increase due to bad quality feed water ( when fresh water  generator salinty is high).

PROCESURE TO BLOW DOWN:

1. Keep boiler firing ( this way you will have a better circulation of water inside the boiler)
2. Normally 2 valves are given on boiler side and 2 valves on the sea side for blowdown purpose. The idea behind giving two valves for blowdown in each of the places is that, The valves attached to the ships hull and the boiler body are very difficult to overhaul, when ship is sailing or in port. These valves can be overhauled when ship is in dock only( by adjusting list or blowing down entire boiler you can do this , but you know how hard it is :P). So normally dont play around with these valves and dont keep them throttled( throttling a steam valve or a high pressure line valve causes wire drawing effect which destroys valve seats and it starts leaking). Now, you need to control flow rate for blowing down, and how do u achieve that??? Throttle the valve which is not attached to the boiler body or the ships hull!! This is why 2 valves are given .
3.Open the valve attached to the ships hull and the boiler body fully first.
4.Open the sea side valve .
5.Throttle the flow rate using the second valve given after the valve attached to boiler body.Simple!!!
6. When water level reaches the normal level on guage glass, close the throttled valve first( see which valve is throttled from point no.5)
7. Close other valves.
8. Finish the blowdown.

This is my practise.. if u guys have any new ideas or suggestion please dont forget to leave them in comments. Happy sailing :)

Refrigeration and Air Conditioning systems onboard Ships- Vapour Compression systems

Refrigeration And Air conditioning systems

Moving heat from a lower temperature to higher temperature requires work to be done , according to second law of thermodynamics and this work can be done in many ways in different systems. The most widely used techniques to move heat from lower temperature to higher tempreatures are 

1. Vapour Compression systems

2. Vapour absorption systems.

3. Peltier systems

4. Magnetic refrigeration systems

Vapour Compression systems are used onboard because of its high efficency and co-efficient of perfomance compared to vapour absorption systems. Vapour absorption cycles are not used now a days and has been phased out towards the beginning of 20th century.

Vapor Compression systems Explained:

Change of state is always associated with a latent heat of phase change and is absorbed from the surroundings. This is the principle behind vapour compression systems. The vapour used is cycled between liquid and gaseous phases. The liquid to gas phase change absorbs heat from the surrounding( the evaporator) and vapour to liquid change needs work to be done, and is supplied by a compressor. 

Thus , in a vapour compression system, we must have a compressor which Compresses the refigerant vapor into high pressure vapour, a condesnor which cools the sub cooled liquid vapour into liquid phase, a TEV or thermostartic expansion valve or a capilliary tube, which regulates the flow of liquid and an evaporator in which the liquid refrigerant absorbs the heat and forms into saturated vapour. The vapour which comes out from the evaporator coils is not really saturated, but superheated , the reason of which is explained below.

The compressor: 

Marine refrigeration sysetms widely use a multi cylinder electric motor driven reciprocating compressors. The advantage of such compressors is that the control of load can be employed using cutting in and cutting out of individual cylinders, which adds to be overall efficiency of the system. 

The Air condition compressors crankcase is pressurized

The Air condition compressors crankcase is pressurized because of a bypass connection between compressor suction and the crankcase. This line is given because we cannot afford to lose even a small amount of refrigerant from the system. The piston- liner surface is not prefectlly sealed, even if high tech seal rings are used.There is always a little leakage from this interface, as in any other reciprocating systems.If the crankcase was vented to atmosphere, the refrigerant would escape into atmosphere. This gas which leaks from the interface is collected into the crank case of the compressor and is used back in the next cycle, by connecting crankcase and compressor suction side. Also oil carry over will be there in this arrangement which needs to be coped for.

In latest systems where oil ingress into the system is very much unacceptable, we use teflon or PTFE rings, which sweeps out the scope for piston lubrication. Such systems use a cross-head type piston, and isolates the liner and crankcase. 

This is one of the major difference between Air condition compressor and Air compressors. Air compressor crankcase are open to atmosphere , as there is a breather given for the crank-case. 

Varying the load on compressors:

The A/C compressor is having more than one unit when load control is required. The multi stage system is used so as to run the compressor at different loads. usually stages are set at 33%,66% and 100% loads. Each unit of compressors put in load according to the load on the system. A solenoid operating valve will keep the suction port depressed always when a unit is not in use. This removes the load on that particular unit. When the particular unit is needed to be on load, and add to the cooling capacity of the system the soleniod valve operates and removes the constraint placed earlier on the suction valve. 

A compressor stages are usually setup to cut-in and cut-out based on the L.P or low pressure reading in the compressor suction side. This is explained well below in respective sections.

L.P and H.P sides of a compressor- The distinction -Why ??

The compression disharge line will be always pressurized compared to the suction side, and thus is called High Pressure side. Usually the line till the Thermostatic expansion valve is called H.P side. HP pressure refers to the pressure in this side, and is read by a guage fixed in the compressor discharge. 

The compressor suction side from TEV to the suction port is at a Low Pressure, compared to the H.P side and this called  L.P. evaporator pressure is always referred as L.P pressure, and is of very much importance in the refrigeration system. 

How Temperature of evaporator is controlled in refrigerators.- L.P CUTOUTS 

The pressure in the evaporator is a measure of the evaporator temperature according to the gas laws. The vapor absorbs heat from the evaporator and its pressure rises at a constant temperature, as the phase change is taking place here without any super-heating. When the evaporator pressure or L.P falls to the pressure of the gas at that particular set temperature, the compressor cuts-out and system is at set-point. When the pressure in the evaporator increases, the compressor cuts in and starts the cycle. 

Compressor stages are set at to cut-in at diffeerent L.P pressures. If a particular temperature cannot be achieved by a particular no of stages, the L.P pressure increases and at the cutting in point the next stage cuts in. 

Vegetable room at 4 Degrees and Meat room at  -18??? Same compressor??? :O

This is achieved by using back pressure valves in the individual lines from each of these spaces. L.P in each of the evaporator is controlled by this back pressure valve, and thus controls the temperature.Each of the back-pressure valve maintains a particular pressure inside the system, and thus ensures a particular temperature in the system. So, L.P of vegetable room will have a higher valuse than L.P of meat room. Each of the evaporator is having its own expansion valves. 

Thermo static Expansion valves

These are flow control devices used in refrigeration systems.The degree of superheat of vapour after the evaporator to the compressor suction side is maintained by the  There is an orifice as the main part of the valve, which will open and close based upon demand of the evaporator. This is NOT the device which maintains pressure gradient in the system , and it is the compressor which is doing so. 

How does TEV control degree of superheat?? Why vapour is superheated ???

A saturated vapour is always appreciated inside the evaporator coils as heat transfer is most efficient during phase change. When superheating happens, lesser amount of energy is absorbed and thus is inefficient. Even though superheating is unfavourable, we always keep a small degree of supreheat for the vapour coming into the compressor suction side. This is done in order to avoid entry of any liquid into the compressor, as it may badly affect it.

TEV's sensing bulb is placed near to the compressor suction where vapour is present in the supreheated form inside the L.P tube. The vapour inside evaporator is now superheated and the feeler bulb has saturated vapour inside it. Saturated vapour pressure is always greater than superheted vapour pressure at the same temperature. This pressure difference is directly related to the degree of superheat. Thus TEV operates on this pressure difference as input, and not the pressure inside the feeler bulb as input. When degree of superheat falls below a value, this pressure gradiant lowers, and TEV remains closed for a longer period, lowering flow of refrigerant liquid into evaporator coils, and pulls up the degree of superheat.

In large installations an equlizing line is given to balance the feeler bulb pressure in the TEV. This is in order to compensate for the pressure loss in evaporator coils due to friction of the coils. This pressure loss will create an increase in degree of superheat making the liquid refrigerant vapourizing completely before the end of evaporator coils. The effect is nullified by using a equalizer line.

Engine room Funnel- Construction, Regulation and Uses of Engine room funnel

A funnel which forms a part of the engine room , acts as a chimney for the exhaust of engines, boiler steam or engine room gases. It acts as a casing for the uptakes from engine room. It serves the purpose of a normal chimney, and has added modifications for meeting various pollution regulations and safety standards. It clears the exhaust gases off ships deck, and thus keps in check fouling of ships structure or decks. The funnel houses Main engine exhaust, Generator engine exhausts, Boiler exhausts, and also funnel flaps.

The funnel size and shapes will be largely dependent on the volume of exhaust gases produced by the main propulsion engines and auxilliary engines. Funnel volume is also counted with engine room volume, and therefore should be included in all calculations involving engine room volume, for example calculating volume of CO2 needed for a fixed CO2 system.

The funnel also acts as a casing for the Main engine and auxilliary engine exhaust silencers. Catalyric convertors also finds its place inside the engine room funnel.

Funnel Flaps Working, Construction and regulations

Funnel flaps in a ship refers to the movable closures or windows present in the aft head of the funnel ( generally). Light weight gases from the engine room escapes to the outer atmosphere through the grills in the funnel or the funnel flap openings.

Hydraulic or pneumatic arrangements are provided for closing and opening the funnel flaps. By regulation, the funnel flaps should have provisions to be opened from a remote location. Usually, Fire control stations houses the remote control arrangements for funnel flap operation.

Funnel flaps are to be closed before releasing of C02 or doing a composite boiler or exhaust gas boiler cleaning by pressurizing the engine room.

Funnel flaps are to be always kept clean and lubricated well, It is a main component of the engine room, a favourite choice of psc inspectors and various other surveyors. A poorly maintained funnel flap will invite bad reports and will pose a serious threat to safety of the engine room.

It is a part of saturday routines to check the remote and local operataion of the funnel flaps , and is usually done by the fourth engineer or third assistant engineer. 

Fire Cabinet or Fire Box- Contents

Fire box contains the following items

1. Fire hose

2. F-Spanner for opening the hydrant valves

3. Nozzle- Either Standard or Dual purpose;  Standard nozzles will produce only jets, whereas dual purpose nozzles will produce both jet as well as Spray depending on the position of the turn key.

4. Hook spanner

5. Instructions pasted on the box

Difference between purifier and clarifier- GRAVITY DISC

1. The presence of a gravity disc in the purifier- There is a general misconception that gravity disc generates the interface between oil and water in the purifier. this is not correct. Interface is formed dude to the centrifuging action, the lighter oil particles will be in the central portion and the heavier water and sludge particles will move to the outer periphery. Thus, What is gravity disc doing here??? NOTHING.! Gravity disc is just giving a surface of seperation between oil and water flow.  In alfa laval alcap purifiers which are newer models of the renowed manufacterer, There is no gravity disc present. Instead of gravity disc a water in oil detector is used. 

2. Clarifier has a sealing ring wich seals the water outlet, as water and impurities remain in the until desludging. 

3.The conical discs in a clarifier usually don’t have feed holes in them but if they do, then a disc without any holes is fitted at the bottom of the stack.

4. Sealing water is not present in clarifiers for the generation of a seal that prevents the oil to leave through the water outlet at starting. Purifiers and Clarifiers differ only in that clarifiers are not set up to remove water.

Gangway safeties in ships. Things to be checked before using the gangway.

The accommodation ladder and gangway must:

1. be firmly secured to the bulwark;

2. be properly aligned with the means of access to the vessel;

3. have treads that are at least 600 mm in width and 200 mm in depth, with a permanent non-slip surface;

 4. be equipped with two handhold stanchions that are not less than 40 mm in diameter;

5. extend not less than 1.2 m above the top of the bulwark;

 6. be fitted at the point of boarding or disembarking the vessel not less than 700 mm and not more than 800 mm apart.

 Every accommodation ladder and gangway must:

1. be maintained in a safe condition;

2. be installed in a manner that reduces movement;

3. be suitably rigged and maintained to compensate for the movement of the vessel;

 4. be adequately lighted;

5. as far as practicable, be not be more than a 40° angle to the horizontal plane;

6. be provided with a lifebuoy that has an attached line and is strategically placed and ready for immediate use; and

 7. have the mechanical, electrical, gearing, hydraulic and pneumatic systems in good working order.

Freeboard explained ,and why it is given.

In sailing and boating, free board means the distance from the waterline to the upper deck level, measured at the lowest point of sheer where water can enter the boat or ship.

Freeboard defines the reserve bouyancy of a vessel. It is to be there to satisfy the reserve bouyancy regulations by IMO and other conventions. Reserve buoyancy is important for the vessel’s safe operation at sea.

Sufficient free board is required at all times to prevent the vessel being swamped and overwhelmed. For ocean going vessels, it is important to note that those structures above the waterline that are not watertight will not contribute to the reserve buoyancy of the vessel. Again, this makes sense – an open wheelhouse, or a cabin with the doors left open will not offer much protection if the vessel begins to ship water over the side.

Another factor affecting the freeboard to take into account is the trim of the vessel. Trim is defined as the difference between the draft forward and the draft aft. Draft is the depth of the hull below the water. If the aft draft is greater, the vessel is described as being trimmed by the stern, if the forward draft is greater, she is trimmed by the bow.

Length between perpendiculars and its Significance

Length between perpendiculars, often abbreviated as p/p, p.p., pp, LPP, LBP or Length BPP is a term describing the length of a ship. LBP refers to the length of a vessel along the waterline from the forward surface of the stem, or main bow perpendicular member, to the after surface of the sternpost, or main stern perpendicular member. When there is no sternpost, the centerline axis of the rudder stock is used as the aft end of the length between perpendiculars.

Measuring to the stern post or rudder stock was believed to give a reasonable idea of the ship's carrying capacity, as it excluded the small, often unusable volume contained in her overhanging ends. On some types of vessels this is, for all practical purposes, a waterline measurement. In a ship with raked stems, naturally that length changes as the draught of the ship changes, therefore it is measured from a defined loaded condition.

The significance lies in the fact that this length gives the length of the waterline of the vessel, in loaded condition, a very crucial parameter.

Annex 1 discharge criteria outside special area and inside special area for marine vessels

Inside special areas or outside special areas within 50 nm

ANY DISCHARGE IS PROHIBITED with the exception of clean or segregated ballast

Outside Special areas: 

ANY DISCHARGE IS PROHIBITED, with the exception of clean or segregated ballast, or except when:

1. the tanker is proceeding en route, and

2. the instantaneous rate of discharge of oil does not exceed 30 litres/nm, and

3. the total quantity of oil discharged into the sea - does not exceed - for tankers delivered on or before 31 December 1979 - 1/15,000 of the total quantity of the particular cargo of which the residue formed a part - and - for tankers delivered after 31 December 1979 – 1/30,000 of the total quantity of the particular cargo of which the residue formed a part, and

4. the tanker has in operation an oil discharge monitoring and control system and a slop tank arrangement as required by regulations 29 and 31, respectively

Cylinder Head - Cylinder head mountings, Function and Combustion chamber shape.

Cylinder head or cylinder cover houses various fixtures called cylinder head mountings. They are positioned and designed very carefully, as they are subjected to very high combustion pressures, and temperatures. Cylinder head mountings and functions  are explained below in a brief sense.

Functions of cylinder head:

1. Forms a part of the combustion space- Cylinder head covers the top part of liner thus providing a space for combustion to take place. The shape of the cylinder head will greatly influence the combustion space shape, and thus characterstics and peak pressures developed in the engine. The efficiency of swirl and injection  also depends on the Shape of this space.the three main Combustion chamber designs are Hemispherical pent roof, Bath tub and Wedge.

2. Cylinder head mountings- It houses various running parts like Exhaust/inlet valves, Fuel injectors,  Relief valve,  Air starting valve, Rocker arm, indicator cock, cooling water and fuel connections etc.