Troubleshooting Centrifugal Pumps

                    Because most of the pumps on board ship are centrifugal pumps, we will examine the symptoms and common causes of centrifugal pump problems here.

Centrifugal pumps work, as the name suggests, on the centrifugal forces acting on the liquid being pumped.  

The centrifugal force is imparted to the liquid through the high-speed rotation of a specially shaped impeller.

Whenever we find a centrifugal pump not performing well, the causes can usually be traced back to 3 common reasons:

• Suction related problem
• System related problem
• Mechanical related problem

Now you can easily pinpoint the trouble. Download free DiagnoPump, the pump troubleshooter software Here.

 










Suction Related Problem

The liquid inside the impeller must behave like a solid column in order for the centrifugal pump to work. More liquid must replace whatever liquid forced out by centrifugal force. If the solid column of liquid is broken, by the presence of air, the pumping effect is reduced or broken, and the pump does not perform properly. Some of the common suction related faults are:

• Pump not primed
• Pump suction pipe not completely filled with liquid
• Suction lift too high
• Insufficient margin between suction pressure and vapor pressure
• Excessive amount of air or gas in the liquid
• Air pockets in the suction line
• Air leaks into the suction line
• Air leaks into the pump through the stuffing box or gland
• Foot valve too small
• Foot valve partially clogged
• Inlet of suction pipe insufficiently submerged
• Water seal pipe plugged
• Seal cage improperly located in stuffing box, preventing sealing fluid entering space to form a seal
• 
  

System Related Problem

Most of the system related problems occur because of design flaw. For example, the designer may have chosen the wrong pump whose characteristic does not match the system requirement. For the Marine Engineer working on board ship, these problems are rare. Perhaps sometimes, when replacing motors, and starting up. Below are some of the common reasons:

• Speed too low
• Speed too high
• Wrong direction of rotation
• The total head of system higher than design pump head
• The total head of system lower than design pump head
• Specific gravity of liquid different from design
• Viscosity of liquid differs from that for which the pump was designed
• Operation at very low capacity
• Parallel operation of pumps unsuitable for such operation

Mechanical Related Problem

This group of problems is most often encountered on board ship. The effects of mechanical related problems could manifest themselves as suction related problems, like air leaks in the system, worn out impellers, and mouth rings, but the most common occurrence is the presence of vibration and abnormal noise in the equipment.

All equipment on board ship, including centrifugal pumps will encounter mechanical related problems. The practice of good maintenance, and vigilant watchkeeping will lessen the damage to the equipment. Some of the common mechanical related problems are:

• Foreign matter in the impeller
• Misalignment
• Foundations not rigid
• Shaft bent
• Rotating part rubbing on stationary part
• Bearing worn
• Wearing rings worn
• Impeller damaged
• Casing gasket defective, permitting internal leakage
• Shaft or shaft sleeves worn or scored at the packing
• Packing improperly installed
• Incorrect type of packing for operating conditions
• Shaft running off-center because of worn bearings or misalignment
• Rotor out of balance, resulting in vibration
• Gland too tight, resulting in no flow of liquid to lubricate the packing
• Failure to provide cooling liquid to water-cooled stuffing boxes
• Excessive clearance at bottom of stuffing box between shaft and casing, causing packing to be forced into pump interior
• Dirt or grit in sealing liquid, leading to scoring of shaft or shaft sleeve
• Excessive thrust caused by a mechanical failure inside the pump or by the failure of the hydraulic balancing device, if any.
• Excessive grease or oil in anti-friction bearing housing or lack of cooling, causing excessive bearing temperature
• Lack of lubrication
• Improper installation of anti-friction bearings
• Dirt getting into bearings
• Rusting of bearings due to water getting into housing
• Excessive cooling of water cooled bearing, resulting in condensation of moisture from the atmosphere in the bearing housing

How are Generators Synchronized on a Ship?

                        Synchronization of generators is an activity that is carried out quite often on ship. It is a prerequisite that each and every engineer on the ship knows the procedure thoroughly. In emergencies, the engineers are required to carry out the process manually in extremely limited amount of time.

Introduction

                  It's a known fact that marine generators are the heart of any type of ship. Maritime law requires that every ship should have at least two generators. Nowadays all the ships have around 2-3 generators on board. More number of generators are used to facilitate load sharing and to prevent wear down due to excessive load.
             Maintenance of generators at regular interval of time is extremely important. In this article we will learn the process of generator synchronization when multiple machines are required or one of the generators needs to the stopped and the other started in its place
Say for example if a ship has three generators on board, two are used under normal working conditions and one is kept as stand-by.                  
              Whenever a requirement to service a running generator arises , the standby generator is brought in line and the desired generator is taken off line. For bringing the standby generator in line, the generator is synchronised with the other running generators.

The main things that are kept in check for synchronizing a generator are :

• Frequency
• Voltage
• Load
• Phase

Let's have a look how the synchonization of generators is done manually

Generator Synchronization Procedure- Before starting

A step by step method for synchronizing generators in provided below.

1. When a decision of synchronizing generators in taken, first the bridge should be notified about the scheduled activity
2. Start the generator that has to be synchronized. Before starting, prime the engine with fuel using hand pump. Make sure the engine block heater is turned off.
3. Open the air valve and then turn on the engine.
4. Once the engine starts, check if the oil pressure and cooling water pressure is adequate. Check if the cooling water pump is working properly by feeling the pipes. Once the check is done, close the air valves.

Synchronizing procedure

Once the engine starts running properly, synchronization is carried out.

1. In the Engine control room, Check the pressure gauges.
2. On the generator control panel, check if all the ground lights are working properly with adequate brightness.
3. Also check the synchronizing relays for open position. Bring the running or the lead generator to the desired optimum parameters: 480 volts and 60 hertz
4. Bring the generator that is to be synchronized(0n-coming) to the desired parameters. Now turn on the synchronizing relay and keep a close look at the needle.
5. The needle in the synchroscope will move at a varying speed initially. Adjust the speed of the generator by obtaining a steady slow motion of the needle in the clock wise direction.
6. Once the needle is moving at a steady speed, depress the breaker close button when the needle has traveled three-fourth of its way. Energize the breakers when the needle reaches a position similar to the 11' o clock position of a clock.
7. After doing this, check the parameters of the on-coming generator. They should be same as those of the leading generator. i.e 480 Volts and 60 hertz

 

 

 

After synchronizing

After the main job of synchronizing, the following steps are to be carried out.

1. Change the governer control to the off-going generator.
2. Now the load shown in the guages by this generator should be removed off the system as soon as possible before it starts acting as load(reverse power). This can be done by quickly pulling the trip breaker as soon as the generator goes off-line.
3. Once the generator is offline, stop the engine using a toggle switch.
4. After turning off the engine, turn on the engine block heater.
5. At the end, take a proper look at the control panel guages for adequate pressure and even distrubution of load.

           It must also be noted that load distribution can be adjusted by varying the fuel supply to the generator via its governor but for current sharing to be equal you would need to vary the excitation current which changes the power factor of the generator.

Image Credits

http://powergenelectrics.com/yahoo_site_admin/assets/images/synchroscope_2.291110319_std.jpg
http://www.seg-me.com/UNATRAC%20Project1.jpg
http://www.o-t-s.com/synchroscope.jpg

Centrifugal Oil Purifiers - Starting and Stopping Procedures

Centrifugal Oil Purifiers - Starting and Stopping Procedures

               We have already know about the basic principle of operation of purifiers...! Lets know how to start and stop purifiers.., necessary safety precautions before starting, de-sludging procedure, and emergency stopping....etc.....!

Recap...!

We all know that centrifuges are an important type of auxiliary equipment on board ships and that they are classified into two operating functions.
      
      One is Clarifier, which seperates solids from liquids.
      Other type is a Purifier, which seperates liquids of different density.  

The Purifier operates on the principle of seperation by centrifugal force. But in order to optimize the purification process, certain parameters should be adjusted before purifier is started. Out of those parameters, very important parameters are...

1. Feed inlet oil temperature, 
2. Density of Oil, 
3. R.P.M of the rotating bowl,
 4. Back Pressure, 
5. Throughput of oil feed.

Insight Of the Parameters...!

1. Feed inlet oil temperature: Before entering the purifier, the dirty oil passes through the heater, which increases the temperature, thus reducing the viscosity of the oil to be purified. The lower the viscosity, the better will be the purification.

2. Density of Oil: As the dirty oil entering the purifier is heated to reduce the viscosity, the density also reduces. The lower the density, better the seperation.

3. R.P.M of the rotating bowl: If the purifier has not achieved full rpm(revolutions per second), then the centrifugal force will not be sufficient enough to aid the seperation.

4. Back Pressure: The back pressure should be adjusted after the purifier is started. The back pressure varies as the temperature, density, viscosity of feed oil inlet varies. The back pressure ensures that the oil paring disc is immersed in the clean oil on the way of pumping to the clean oil tank.

5. Throughput of oil feed: Throughput means the quantity of oil pumped into the purifier/hr. In order to optimize the purification, the throughput must be minimum.

Pre-checks before starting a Purifier...!

Before starting a Purifier, following checks are very essential:

1. If the Purifier is started after a overhaul, then check all fittings are fiited in right manner. The bowl frame hood locked with hinges.

2. Check the Oil level in the gear case. Ensure that it is exactly half in the sight glass. Also ensure the sight glass is in vertical position, as there is a common mistake of fixing it in horizontal position.

3. check the direction of rotation of the seperator, by just starting and stopping the purifier motor.

4. Check whether the brake is in released position.

Starting a Purifier...!

1. Ensure the lines are set and respective valves are open. Usually the lines are set from settling tank to service tank.

2. Start the purifier feed pump with the 3-way re-circulation valve in a position leading to settling tank.

3. Open the steam to the heater slightly ensuring the drains are open so that the condensate drains. close the drains once steam appears.

4. Start the Purifier.

5. Check for vibrations, check the gear case for noise and abnormal heating.

6. Note the current (amps) during starting. It goes high during starting and then when the purifier bowl 
picks-up speed & when it reaches the rated speed, the current drawn drops to normal value.

7. Ensure the feed inlet temperature has reached optimum temperature for seperation as stated in the Bunker report & nomogram ( bunker delivery note gives the density of the fuel and using this we can get the seperation temperature and gravity disc size from the nomogram)

8. Now check whether the bowl has reached the rated speed by looking at the revolution counter. The revolution counter gives the scaled down speed of the bowl. The ratio for calculation can be obtained from the manual.

9. Now, after the bowl reaching the rated rpm, check for current attaining its normal value.

De-sludge procedure:

10. Open the Bowl closing water/Operating water, which closes the bowl. (Ensure sufficient water is present in the operating water tank)

11. Now after 10 seconds, open the sealing water to the bowl.

12. The sealing water should be kept open till the water comes out of the waste water outlet.

13. Once the water overflows throught the waste water outlet, stop the sealing water.

14. Now open the de-sludge water/bowl opening water. (this is done to ensure the bowl has closed properly). During de-sludge we can hear a characteristic sound by the opening of the bowl.

15. Repeat the steps 10, 11 ,12 & 13.

16. Open the 3-way re-circulation valve such that the dirty oil feed is fed into the purifier.

17. Wait for the back pressure to build up.

18. Check for overflowing of dirty-oil through waste water outlet & sludge port.

19. Now adjust the throughput to a value specified in the manual. Correspondingly adjust the back pressure too.

20. Now the purifier is put into operation. Change over the clean-oil filling valve to service tank.

After-checks & stopping of purifier...!

Checks after starting the purifier during regular watches:

1. Adjust the throughput, back pressure, temperature of feed inlet if necessary

2. gear case oil level, motor amps, general leakages, vibration have to be monitored

3. De-sludge every 2 hours for heavy oil purifiers & every 4 hours for lubricating oil purifiers.( refer manual or chief engineer instructions)

Stopping of Purifiers:

1. De-sludge the purifier after stopping the feed inlet.

2. shut down the steam inlet to the oil.

3. Stop the purifier after filling up the bowl with water.

4. Apply brakes and bring up the purifier to complete rest.

5. If any emergency, the purifiers has emergency stops, on pressing it, will stop the purifiers immediately shutting off the feed.
                                        
       Thus we have seen in detail how to start the purifier after carrying out all safety checks and we have also seen how to stop it...! But today the system is of ALCAP and much more......

What is Puncture Valve in an engine ?

What is Puncture Valve in an engine ?

                           
                  The puncture valve consists of a piston which communicates with the control air system of the engine. In the event of actuation of the shut-down system, and when 'STOP' is activated, compressed air causes the piston with pin to be pressed downward and 'puncture' the oil flow to the fuel valve. 
                  
                   As long as the puncture valve is activated, the fuel oil is returned through a pipe to the pump housing, and no injection takes place. I have also added a few more comments and attached a file to further explain the function(s) of the puncture valve. 

                                           

    MAN B&W reversing: Reversal of the fuel pump follower only takes place while the engine is rotating. If the engine has been stopped from running ahead and started astern, the fuel pump follower will move across as the engine starts to rotate and before fuel is admitted by venting the fuel pump via the "puncture" valve.


WHAT SHOULD I  DO IF AN AIR START VALVE STICKS OPEN WHILE MANOEUVRING: 
           The fuel pump of the effected unit should be "lifted" (fuel rack zeroed, puncture valve operated or whatever)...................

THE INVENTOR OF THE STEAM TURBINE: Charles Parsons

                                          Charles Parson's yacht Turbinia, pictured here, was powered by his steam turbine. He dramatically demonstrated its speed at the British Navy Review before Queen Victoria in 1897 when it was easily the fastest vessel on view. The British naval establishment was impressed and soon adopted the turbine in its latest battleships (credit: Wiki)

                                A plentiful supply of cheap electricity, and much faster passenger steamships and military battleships. These were some of the things made possible by Charles Parsons, who grew up in Birr, and invented the steam turbine in 1887.
                            Charles was born in 1854 and came from a brilliant scientific lineage. His father was the famous astronomer, William Parsons, who had built the world’s largest telescope on the grounds of Birr Castle in the 1840s.
                             The steam turbine invented by Charles, hugely increased the power that could be harnessed from a steam engine. The invention made him a rich man, and it changed the world.