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In this project, the mission which needs to be accomplished is to investigate the relentless root cause of such failure and to give useful recommendations which can help to avoid such failures in the reciprocating compressor under study in the future. An engine driven auxiliary air compressor failed, the air compressor is located in the Hawke workshop. The first stage will be to establish details about the compressor, the manufacturer and how the compressor works. Classification of compressors and types will be also discussed and of course the theory of operation. Collecting components failed in order to be examined subsequently for the failure cause or causes this will be held using a variety of methods. NDE (non-distractive examination) or NDT (non-destructive testing) is valuable way to use for preliminary inspection of the failed parts without damaging it.
Discussion of the probable causes for the failure well be performed in the following chapters as well as the conclusion obtained from study conducted.
This project will investigate and analysis a failure happened in an air compressor. This investigation will be done in several ways to figure out the cause of the failure. Air compressor is a machine has the ability to compress the air and release it to convert the electrical power to kinetic energy to use it in several industrials and usages. Air compressor is consisting of many components. These components are electrical motor, pistons, air pump, air receiver, air drier, filters, air pressure regulator and pressure switch. All of these components will be discussed in details in the next chapters. There are also types of air compressor will be discussed later.
The case which had here is a failure in an air compressor. From the first sight on the damage happened in it, it seems to be that the failure happened in the connecting rod and it’s obvious from the below figure number 1. This figure shows that the failure happened in the connecting rod from the ring part which connected to a crank shaft. But the reason for this failure is unknown as there are several scenarios could be adopted. The first scenario is the fatigue is the reason for this failure. The second scenario is a very heavy pressure loaded to the compressor which led to that failure. These are the most expected scenarios which will be investigated later in this project to figure out the reason of the failure.
Figure 1: Failure of compressor connecting rod
Amis and objectives
The main aim of this graduation project is to investigate and discover the cause or the causes of failure of diesel engine driven auxiliary reciprocating air compressor made by Knorr-Bremse, model number is LP 4865. This will be achieved by carrying out the below objectives, once its achieved, the main aim of the project will be achieved.
Collecting valuable information about, model number is LP 4, this will include: design of compressor, design of each part, construction materials of each part, operation principles, working theory, production range..etc. collecting data will help to recognize main failure cause, and accordingly to prepare new procedure to eliminate the problem, to decrease the opportunity of failure occurrence in the future, use greater safety factor and to recognize the person or the entity responsible for failure and to make mistake proofing for future avoidance of the failure. Collecting this background information is not limited to numbers only but it is should be extended to everything such as pictures, charts and graphs and samples.
Investigating operational parameters, history of operation, nature of operation..etc in order to be able to determine precisely the root cause of such failure and which failure mode it follows.
Physical investigation or macroscopic inspection should be carried out. This mission includes photo capturing, code of the product, model number, lot number, batch number and serial number.
Perform compressor disassembly in order to dismantle each component, part, assemblies and sub-assemblies to perform through investigation and inspection of each. It’s should be mentioned that necessary tests and analyses should be performed; mainly the manufacturer usually has his own procedures and tests for test and inspection.
What is the air compressor?
Air compressor is a machine that converts electric power to kinetic energy (motion). This energy comes from pressurizing and compressing air. After that, the compressed air goes into a chamber, and the air kept in the chamber by unidirectional valve. There are many types of compressors these types are: reciprocating compressor, rotary screw compressor and centrifugal compressor. These types will be explained in details below.
Reciprocating air compressor is positive displacement compressor. This compressor sucks a volume of air and presses it with high pressure. This pressure done by using a piston and cylinder as movement part and displacement part. The compressor could be single acting or double acting according to its accomplish by using one side or both side. When the pressure difference between the cylinder and the receiver became proper, the valves open. Inlet valves open when the pressure in the cylinder is slightly below the intake pressure. Discharge valves open when the pressure in the cylinder is slightly above the discharge pressure. The compressor could be a single stage when the compression process done parallel.
Rotary screw compressor:
Also Rotary air compressor is a positive displacement compressor. Single stage helical or spiral lobe oil flooded screw is the most common in rotary air compressor. This compressor is consisting of 2 rotors located in a case where the air is compressed internally without any valves. The cooling for these compressors done by oil. As the cooling for the working parts happened inside the compressor, this type of compressor will not experience over heat due to operation, so it could operate without stop. Due to the simple design of the rotary screw compressor and it’s parts it can be maintained easily and installed any where could handle its static weight. The 2 stages rotary air compressor uses 2 rotors with a combined air end. This 2 rotors installed in series to share the compression. This raise up the efficiency with 50%. This 2 stage rotary compressor combined the maximum profit from the rotary screw with its simplicity and flexibility and the reciprocating compressor with its effective energy when it works 2 stages with double acting. The 2 stages of this compressor could be cooled by water, air or oil. The cooling system for this compressor could be oil, air and water.
The centrifugal air compressor is a dynamic compressor that depends on transfer of energy from a rotating impeller to the air. This done by changing the air’s momentum and pressure. By slowing the air in stationary diffuser the momentum converted to pressure. This compressor is oil free compressor. There is a separation between the lubricated oil and air by shaft seals and atmospheric vents. Where the high volume of dry air is required a centrifugal compressor is the proper compressor as it has a few moving parts and could operate continuously.
The reciprocating air compressor (knorr-bremse, model lp 4865)
The main study object in this project as said before is to study the reciprocating air compressor (knorr-bremse lp 4865). By searching the internet and try to get information about the air compressor which needed to complete this study but unfortunately the required information couldn’t be found. So the general information about the reciprocating air compressor will be mentioned instead of the specific air compressor.
There are 9 main parts for the reciprocating air compressor these parts are:
Crank case: Crank case is a close rigid body where the crank shaft and the bearing house where located. This body could by rectangular or square shape. Mainly crank case formed by using cast iron.
Crank Shaft: Crank shaft in one of the most important parts in air compressor. It is a motion transferor. It designed as one piece has balance in its dynamics and tries to avoid any twisting. To ensure a long life for bearings, the polishing and crank pin will be done. Generally crank shafts have a fly wheel. This crank shaft manufactured mainly of high grade S.G iron.
Connecting rod: This part is responsible of transfer the motion from the crank shaft to the piston and changing the direction from the rotating motion come from the crank shaft into reciprocating motion in piston. This part is mainly forged of alloy steel.
Bearings: Bearings are made to provide the rotating gear with rigidity. It’s mainly manufactured of copper lead alloy.
Cross slide: to get a perfect running for cross head, this part is responsible for reduce the inertia. This part is commonly manufactured of high grade S.G. iron.
Cylinder: This part is where the air flow passes and compressed. Cylinders are manufactured with water jacket to reduce the generated heat from the compression process. This cylinder mainly made of cast iron.
Pistons: Piston is the main part in the compression process, which move forward to compress the air and backward to intake a new air amount to compress. There are 2 types of piston according to its lubrication system, the first type is non-lubricated which made from aluminium alloy, and the second type which is lubricated is made from cast iron. Due to moving of piston in the cylinder, so there must a space exist between them, to ensure there is no leak happened for air a piston rings located on piston.
Piston rod: Piston rod is this rod which connects the piston to the connecting rod. The piston rod is manufactured of alloy steel.
Intake and discharge valves: These valves are responsible for the amount of air sucked or discharged. When the piston in the retraction position the suction valve open to allow the air to get in. when the piston move forward and reach the maximum pressure the discharge valve open. These 2 valves are adjusted to be opened and closed according to the pressure difference between inside the cylinder and the outside. These valves are made from stainless steel even a plate type or spring type.
The first problem in this study is that damage which happened in the connecting rod from ring which connected with crack shaft. As said in the background, there are 2 scenarios leads to this damage. The first scenario is a fatigue accrues in the connecting rod with lead to this damage as it couldn’t stand the motion and the pressure any more. The second scenario is an excessive pressure more than the maximum pressure which could be hold by the compressor. At the same time a malfunction in the discharge happened, and this could be taken in consider as it must opened at slightly below the maximum pressure. In both scenarios, the damage in the connecting rod lead to another greater damage in the piston and the cylinder, as the piston start to move in non-liner motion and hitting the cylinder wall. This leads to a serious damage in the compressor’s piston and cylinder.
The second problem is gathering the information about the air compressor Knorr-Bremse Lp 4865. This problem happened as the manufactured company keep the information about this compressor not listed and its material and properties to figure out the reason of the damage, maybe by knowing its maximum pressure or material type to check the fatigue on it.
To overcome the second problem a deeply internet search is made with trying to contact with the manufactured company through their website www.knorr-bremse.com, but with no result. So another technique implemented, which is gathering common information about reciprocating air compressor and its common parts and materials. This could be useful but not accoutre.
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