Concept of Liquid cavitation - A better understanding about boiling,flashing and cavitation

Concept of Liquid cavitation:

Cavities in the liquid body is the result of partial vaporization of liquid. Although the term cavitation can
mean the formation of cavities of gas or vapor in a liquid, the cavitation process consists of both liquid cavity formation, and liquid cavity deformation. It is therefore a reversible, double change of state phenomenon. On the hand the terms boiling and flashing are special categories of vaporization. Boiling is defined as the specific vaporization point of a liquid in the presence of local atmospheric pressure .
Flashing involves a fluid’s rapid phase change from liquid to vapor without the return of the fluid to the liquid phase.
One should not mean that cavitation, boiling,and flashing all are the same thing and therefore can be used interchangeably.

There are also occurrences in which relative flow arrangement, entrainment/dissolution, or chemical reaction can lead to cavity formation and later collapse in what is known as pseudo-cavitation. Liquids exposed to air or other gases can absorb a portion of that gas. Liquids often therefore become a solution of the parent liquid and a dissolved gas, with a different vapor pressure from that of the pure liquid. The mixture of liquid and vapor is now a compressible fluid.  When pressure increases, the mixture passes through its vapor pressure level and the vapor pockets instantly collapse like tiny balloons. Dissolved air or gases coming out of solution and imploding has been referred to as pseudo-cavitation.

It is general belief that Cavitation is always problematic and detrimental and therefore always should be eliminated or at least minimized. This belief is absolutely correct in perspective of process equipments of process industry. But there are some application such as subsurface drilling operation or process mixing applications where the associated turbulence is advantageous. A recent development in mixing micro technology called cavitation micro streaming, whereby a gas bubble inside a liquid is made to oscillate at a various frequencies, greatly enhances the mixing of blood samples with reagents.
The process of cavitation begins when the pressure on portions of the liquid decrease to a point low
enough for the fluid to change states, from a liquid to a gas. This occurs at the vapor pressure of the
liquid.

Alignment Rectification after process pipings are connected with Compressor nozzles results in catastrophic failure.

Care must be taken to ensure that alignment readings does not vary during alignment in both the circumstances i.e.
1) when pipe flange is not connected to compressor nozzles
2)when pipe flange is connected with compressor nozzles.
It is good practice to rectify piping and its companion flanges instead of rectifying alignment after piping is connected.This will result in catastrophic failure caused by undue loading and moments on machines casing and other internals.
There should not be load transfer from piping end to machine end after flanges are bolted with compressor nozzle in cold condition.However there may be axial and transverse loading which may occur during actual operation of plant load.This is being taken care during alignment by incorporating  axial and radial values as mentioned in OEM manual

Why maintenance work planning for a confined space entry needs stringent focus or attention ?

  

Let us understand first what is a confined space? Spaces or locations just enough to enter, difficult to enter or exit  and/or not designed for  continuous human occupancy are generally,to my understanding, called confined space.  For example,confined spaces in a process /chemical industry include but are not limited to  tanks, reactors,pressure vessels,columns, volume bottle ,ducts, pipelines, sumps, sewers,  inside culverts and vaults. Confined-space incidents/accidents are  significant cause of workplace death  and injury.  Associated hazards in confined space vary widely; the most common are: oxygen deficiency  presence of flammable materials/ constituents toxic gases or vapors Unless all the above hazards are completely eliminated by taking appropriate  mitigation measures work execution permit shall not be issued.

Whether RCM-single maintenance strategy applicable in all type of industry /organisations in same fashion.?

There is no one fix maintenance strategy which can be applied in all type of industry. To my understanding  maintenance strategy to be adopted successfully to achieve long term reliability depends on following:
1.type of industry(process,continuous,batch production,Hydrocarbon,Engineering
   manufacturing etc)
2.Age of industry.How old industry is?
3. Severity and consequences of failure of assets.(Impact :on assets,human
   being,surrounding/environment and reputation of organisation)

A continuous Process industry like Fertilisers,petrochemical,Oil-gas & Energy sector industry may adopt following Proactive good maintenance Strategy within RCM domain:
- PM :frequency based on past historical data/MTBF/OEM recommendation(capital
  machines)
-PdM or Condition based maintenance to identify failure detection threshold point
  to plan corrective measures to repair the item/equipment proactively before the hard
  failure happens.
-Capital overhaul based on running hours/nos of years/types of cyclic load/service being handled by equipment.

Effect on Rod Load if we increase compressor speed in reciprocating compressor.

To understand the effect of speed on "Rod Load" ,we must know first what is rod load? Actually Rod load is vector sum of Gas load and inertia forces (caused by all reciprocating parts including cross head assembly) acting in the direction of piston rod.The first part i.e.Gas load is function of cylinder chamber pressures and area of piston at crank end and head end.Therefore speed will have no effect on Gas load,however it will have effect on flow rate (capacity) and power absorbed by compressor.The second part i.e. inertia forces is function of mass of reciprocating parts and its acceleration/deceleration.Obviously change in speed will have effect on magnitude of acceleration/deceleration of piston rod assembly during each revolution of the crank.
Hence compressor speed will have effect on "Piston rod load".Its absolute value may increase or decrease depending upon crank angle during each revolution.

How to distinguish between Grinding wheel and Cutting wheel (Cut-off wheel) ?

Some useful tips to distinguish Grinding wheel and Cutting (or Cut-off) wheel are given as below:


1.      Remember: All grinding is cutting operation but all cutting is not grinding operation.

2.      Thumb rule : “THICKNESS”  is the the major difference between cutting and grinding wheels. A newly procured disc or wheel  up to 3 mm(1/8 inch max) thickness  is used as  Cutting wheel. Its radial periphery is utilised for cutting or slotting  operation as it can withstand a great deal of radial stress but not any side stress . Cutting wheel is not designed to take side load. Hence its use as grinding wheel is not recommended.

3.      Grinding wheel or disc start with minimum 6 mm thickness onwards. The shape and thickness of a grinding wheel decide a wheel's type. Grinding wheel can be used for side  as well as radial grinding as per job requirement.