lecture 2 machine deterioration and failure

7/27/2019 Lecture 2 Machine Deterioration and Failure

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Lecture 2

Machine Deterioration and

Failure


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Deterioration The act or process of becoming worse or lower in quality, value

or character.

Breakdown

Adjustments

Down Time

Increament

Defects

ReworksEffect of

Deterioration


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Ferrous Metal Wrought Iron - An iron alloy with very low carbon content

containing fibrous inclusions caused by the integration of slagduring the manufacturing process. Historically, it was known as"commercially pure iron", however it no longer qualifiesbecause current standards for commercially pure iron require acarbon content of less than 0.008% by weight.

Steel - Alloy of iron and carbon, whereby between 0.07- 2.1 %carbon, by weight, is dissolved in the iron. Varying the amount

of alloying elements and form of their presence in the steel(solute elements, precipitated phase) controls qualities such asthe hardness, ductility, and tensile strength of the resultingsteel.


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Ferrous Metal Cast iron Ferrous metal containing between 2.1 4% carbon and

often contains between 1-3% silicon. The melting point of cast iron is

about 300oC lower than pure iron. Cast iron tends to be more

resistant to corrosion than wrought iron or steel.

Stainless steel - A steel alloy with a minimum chromium content of

10.5% by weight. The chromium forms a passive surface layer of

chromium oxides on the surface, which prevents corrosion.

Depending on its designed use, stainless steels can also containnickel, manganese, molybdenum, sulphur, phosphor, lead,

aluminium, titanium, silicon, nitrogen and copper, plus others.


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Ferrous Metal Deterioration The main deterioration problem with ferrous metals is rust. Rust is caused

by the oxidation of iron in the presence of moisture.

Red/orange rust is chemically unstable, and porous, which allows moisture,

oxygen, chlorides, sulphides and sulphates to rear bare metal, so aprotective layer is not formed. Dark brown/black rust (typically magnetite) is

usually chemically stable.

When rust forms, it can also cause mechanical damage, because as it forms

it occupies a larger volume than the metal, so in some situations it will

cause parts to bend or even break.


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Prevention of Ferrous Metal Deterioration Ferrous metals are often plated to provide corrosion protection.

Typical platings are :

1. zinc (galvanizing)

2. tin (tinplate)

3. lead (ternplate)

4. chrome

5. nickel.


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CuprousMetals

Cuprous metals include copper, bronze (copper/tin alloys) and brass (copper zincalloys).

These alloys are fairly corrosion resistant. A protective layer ofoxide is usuallyformed, giving them a dull, dark appearance. If exposed to organic acids, ormoisture and carbon dioxide, pale green and/or blue copper carbonates may beformed. These are generally stable and are not an ongoing corrosion concern,although they may appear unsightly.

Sulphidescause tarnish, which on copper and its alloys is typically a thinrainbow coloured patina. Copper sulphides are generally chemically stable.

Brass and bronze in conditions where they are subjected to mildly acid flowingwater, are liable to de-alloying. This is known as dezincification anddestannification respectively. The zinc and tin are gradually leached out of thealloys, leaving a porous, physically weak copper matrix.


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Copper Alloys Copper alloys - vulnerable to active corrosion by chlorides. In the right

conditions chlorides will result in pitting, and it is often referred to as

Bronze Disease since it is possible for the corrosion to infect other

nearby parts. Chloride corrosion of copper and its alloys is a cyclic process. The chlorides

react with copper to form cupric chloride, which then in turn reacts with

moisture to form hydrochloric acid, which then attacks un-corroded copper


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Aluminium alloys Alluminium alloys - include copper, magnesium, zinc, silicone.

Aluminium alloys have very good corrosion resistance, because of a thin

layer of aluminium oxide that forms rapidly on the surface. Chlorides and

sulphates are the most common causes of corrosion encountered. If theprotective oxide layer is interrupted in the presence of chlorides, pitting

usually results. Chloride pitting of aluminium is similar to bronze disease, in

that it becomes a self-propagating cycle.


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Machine Failure Faulty design

Material defects

Process and manufacturing deficiencies

Assembly of installation defects

Off-design or unintended service conditions

Improper operation

Maintenance deficiencies


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Machinery FailureMany causes of failure in machinery exist and their predominance will vary tosome degree from industry to industry. However, the most common causes, inorder, are:

Misalignment Unbalance

Resonance

Bearings

Looseness Flow-related problems

Electrical


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Misalignment Misalignment exists when the centre lines of two adjacent machines

deviate from each other.

Misalignment is universally recognised as the leading contributor to

machinery failure. For the same level of vibration it is much more seriousthan unbalance for its effect in reducing bearing life, largely because of the

parasitic axial thrust.


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Principal Causes of Misalignment Lack of appropriate Standards and Specifications

Poor Tolerances and Poor Methods

Good Methods, but bad practices

Lack of understanding of precision process

Dynamic movement (thermal growth, pipe strain etc.)

Mis-diagnosis with Unbalance or Looseness


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Unbalance Unbalance exists when the mass center line of a rotor is not coincident with

the geometric center line. The resultant orbital motion has a severe impact

on the life of bearings and on seals.


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Principal Causes of Unbalance Accumulation of assembly tolerances

Poor attention to assembly

Inappropriate use of standards

Non-homogeneous materials (casting blow holes)

Operational uneven build up of product

Erosion of rotor material


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Resonance Resonance is a condition which occurs when a machine forcing frequency

(related to rotational speed) becomes coincident with, or close to, the

natural frequency of a machine component or appendage.

The condition is like that of a mechanical amplifier and can result in a verysevere vibration at that frequency when the forcing vibration is already too

high.

Tacoma bridge collapses due to resonance


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Bearing Failure The most common cause of bearing failure is loss or contamination of the

lubricant. Mechanical defects such as misalignment and unbalance have a

severe impact upon bearing life.

There is a cubic relationship between fatigue (vibration velocity) andbearing life; thus a halving of vibration velocity can theoretically give 8X the

bearing life.


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Bearing Failure Catagories


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Looseness Looseness may be considered as being where parts are not fitted tightly

together. Typically this fits into three categories;

Structural looseness/weakness of machine feet, baseplate or foundation.

Also by deteriorated grouting, loose hold-down bolts at the base anddistortion of the frame or base. This may be seen as Soft Foot.

Loose Plummer block bolts, cracks in frame structure or bearing pedestal.

Improper fit between component parts, e.g. excessive clearance in a sleeve

or rolling element bearing, impeller loose on its shaft.


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Plummer Blocks


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Flow Related Problems Flow related problems such as cavitation and recirculation are caused by pumps

operating outside of their design parameters.

When a centrifugal pump is operated below its design capacity, or with extremesuction conditions, it is likely to experience cavitation. Cavitation occurs when

vapour bubbles are formed in the pump low pressure regions and then collapsein a higher pressure region; the collapse leads to erosion of the impeller, andsometimes the casing.

Another common problem with centrifugal pumps is recirculation. Liquid returnsfrom the impeller discharge either externally to the impeller suction throughworn wear rings, or may return internally, and impacts on the impeller vanes.The problem is worse when the discharge is throttled or when the impeller isaxially displaced so that it does not align with the pump discharge. The result isincreased vibration at vane passing frequency and there may be a significanttemperature increase. The reverse flow in the pump and mixing of the liquidresults in a random vibration not unlike cavitation and can lead to erosion of theimpeller.

lecture 2 machine deterioration and failure

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