Saturated Steam (VS) Superheated Steam

 Saturated Steam 

Raise the temperature of water to its boiling point (Sensible Heat Transfer), after this liquid phase of water, will transfer to vapor phase or in simple water will start vapourising (Latent Heat Transfer). Now as long as the liquid water left temperature of the steam is same as that of water. So, saturated steam is formed when these both phase coexists ( liquid water and its vapor).

 Superheated Steam 

When all the water is vaporized, any subsequent addition of heat raises the steam’s temperature. Steam heated beyond the saturated steam level is called superheated steam.

  WHY IS SATURATED STEAM GENERALLY PREFERABLE TO SUPERHEATED STEAM ??  

Industries normally use saturated steam for heating, cooking, drying or other procedures. Superheated steam is used almost exclusively for turbines. The various types of steam have different energy exchange capacities and this justifies their different uses. Energy transfer capacity, also known as the heat transfer coefficient (U), is used to compare types of steam. Its value is determined by the number of watts that goes through per surface unit and per degree of temperature difference. The greater this value, the greater the heat transfer for a given situation.

Superheated steam only yields sensible heat in an exchanger. It must, therefore, cool down before heating another substance. Superheated steam stuck to a surface cools down while yielding energy to the exchanger. However, superheated steam farther away from the surface cannot easily cool down and yield its energy, because superheated steam is an insulator (a bad conductor of heat), as all gases.

 REFERENCES 

1)  http://www.systhermique.com/steam-condensate/services/troubleshooting/superheated-steam/

2)  http://www.tlv.com/global/TI/steam-theory/types-of-steam.html#superheat

Difference between Surging and Choking

 COMPRESSOR 
Mechanical device which imparts kinetic energy to the fluid in impeller and conversion of this energy into pressure by decreasing velocity in diffuser.

  SURGING    
According to the standard operating curve of compressor, maximum discharge pressure is obtained at minimum flow rate. So, if maximum head capacity is reached, then pressure in diffuser will be greater than pressure at impeller outlet, this will prevent fluid from moving towards discharge and cause the fluid in diffuser to flow back towards impeller. Reversal of flow takes place.

 IT CAN 

• Damage bearings.
• Damage other rotating parts.
• Cause high vibrations.

 REMEDY 

• Anti-surge valve.

 CHOKING 

According to the given below graph, compressor run in between of surge point and stonewall point which also known as Choking point. As it is described in the above para that if flow is decreased and pressure is increased Surging will occur. Now, flow increases and discharge pressure decreases which means less back pressure on fluid and less resistance to flow as a result flow increases to a velocity approachable to MACH=1 also known as Sonic velocity and choking occurs.

 IT CAN 

• May cause very severe damage to the compressor.

 REMEDY 

• Anti-choke valves.

Scaling and Fouling

In Short :

Scaling is depositing of dissolved minerals on equipment surface.

Fouling can be caused by garbage collection for the pump suction grid in the pits, the growth of algae in lighted areas, and mucus in the shadows or dark areas of water systems. The material can clog pipes or other parts of the system, after he has escaped and migrated to the flow of water.

Difference between Flashing and Cavitation

DIFFERENCE
Flashing happens in the control valve if the liquid pressure drops to the value below its vapor pressure hence it may form vapor component (like bubble). Similar to flashing, the vapor is formed in cavitation in the same way, the difference is that the liquid pressure is increasing to a value over its vapor pressure during pressure recovery in which makes the vapor turn back into liquid state afterwards.
REMEDY
Flashing could be anticipated by hardened material trim therefore to overcome flashing, the selected control valve shall have hardened material trim. However, hardened material trim is not sufficient to handle cavitation. Anti-cavitation trim shall be used otherwise the system design itself should be changed so  cavitation is avoided.

Difference between gas and vapour?

    The term gas is used when the substance is in its gaseous form. The term vapor is used when the substance is in equilibrium between two phases, usually liquid and gaseous.

 All matter on earth exists in any of the three states: solid, liquid or gaseous. ‘Gas’ refers to a substance in the gaseous state. Gases do not have either shape or volume. However, the term ‘vapor’ or ‘vapour’ refers to a substance in equilibrium between two phases, usually liquid and gaseous. Vapor is not a state of matter but is a specific type of gas.

 Gases are in a gaseous state at room temperature. The molecules in a gas can expand to occupy any available volume as there is very little inter molecular attraction. On the other hand, the molecules of a vapor gain energy and vaporize from a substance which is either a solid or liquid at room temperature.

A gas is a substance above its critical temperature but below its critical pressure, while a vapor is a substance above its boiling point temperature.  

Summary:
1. A gas has one defined state at room temperature whereas a vapour is a substance that is in gaseous and liquid equilibrium at room temperature, at a given pressure. A gas can also refer to a single element of matter with its own unique physical and chemical properties, for example oxygen, nitrogen and neon.
2. Because vapour is actually the gaseous state of an element, vapour particles will be of a single element and may have a definite shape whereas gas particles when observed under microscopic view do not have a definite shape and will be a collection of atoms, ions, electrons and molecules. However both gas and vapour particles are in a random motion, colliding with each other or the walls of the containing vessel when observed under a microscope.
3.Gas is one of the 4 states of matter which are solid, liquid, gas and plasma, vapor does not classify as such. It is just a substance it is gaseous form.
4.Vapor can turn back and forth into liquid and solid states but a gas cannot
5.Gases cannot be see while vapors are visible
6.Vapors settle down on ground while gases do not

Surface Science

https://en.wikipedia.org/wiki/Surface_science

Types Of Corrosion

http://www.corrosionclinic.com/types_of_corrosion/uniform_corrosion.htm