Mechanical Seal

Parts of a mechanical seal
1.Rotary seal ring
2.Stationary seal ring
3.Rotary seal ring secondary seal
4.stationary seal ring secondary seal
5.spring
6.Gland Plate
7.Clamp Ring

Mechanical Seal 4 main points sealing system.
Primary Seal:
The sealing Between Rotating and stationary Faces is know as primary seal

Secondary Seal
The seal between the rotary part and shaft with o ring
The seal between the statioary part and gland seal plate
The seal between gland plate and stuffing box is usally o ring or gasket

Stationary seal ring
The stationary seal ring is usually attached to the gland plate and is made of softer material like carbon graphite

Rotary seal Ring
The Rotay seal ring is fixed to the rotating shaft and is made of harder material usally tungsten carbide and silicon carbide.

The fluid film:
In mechanical seal faces are lubricated by thin film between rotating and stationary

The distance between the stationary and Rotary Faces are 0.0001mm

Essential Requirement of proper operation of MechanicSeal
1.Seal faces must be flat and polished.
2.seal faces must be perpendicular to the shaft.
3.sufficient spring force for maintaining the contact
4.sufficient lubrication between seal faces

Flatness of seal faces 0.0008mm obtained by lapping

Mechanical seal faces material:
Usually dissimilar materials are used for seal faces. one face will be soft material like carbon and another material will be harder like silicon carbide and tungsten carbine. However for abrasive fluids both faces are harder materials are used.

Requirement of spring in mechanical seal
A spring member is used to maintain the contact between the faces.It pushes one face to contact with the other.
1.single spring
2.multiple springs
3.metal bellows

Types of Mechanical seal:
Pusher
Non pusher or bellow seals
Balanced
Unbalance seal
inside mounted
outside mounted
catridge design

Seal Orientation
1.face to back used in tandem un pressurized dual seals
2.Back to back: both flexible seal elements between seal faces used in pressurized dual seal
3.face to Face : Both seal faces are between flexible elements used in pressurized dual seal

Codification Based on API 682
Based on API 682 all mechanical seal shall be cartridge type
C2 A1 C 11 62

C2  category
A1  seal arrangment
C   Type
11   Flush Plan
62   Quench Plan 62

Category 1 seal chamber temperature from -40 to 260c and pr upto 22kg2
category 2 seal chamber temperature from -40 to 400c and pr upto 42kg2

A1 Single seal
A2 Dual un pressurized seal
A3 Dual pressurized seal

Dual pressurised seal
On this kind of seal the lubricating fluid is generated by the barrier fluid, which is 1.5 - 2kg  more than the product pressure. If there is any leakage the barrier fluid penetrates the product.

Dual Mechanical seal Application:
In high pressure and temperature aggresive fluids
acid fluids
vaccum lines
chemical
food processing industry.

Mechanical Seal selection Basis
Metal parts: must be corrosion resistant steel, stainless steel , haste alloy, bronze.
Mating Faces: Must also be corrosion and wear resistant ceramic, carbon ,tungustun carbide, silicon carbide
pressure: Depending on pressure seal may be balanced or unbalanced.

Seal flushing
Seal Flushing is based on the operating condition of the pump. commonly used seal flushing plan are plan 11 and 23
Plan 11 - seal flush from pump discharge through orifice
plan 23 - seal flush plan from internal pumping device in seal chamber through cooler. its is used in hot water service
plan 32 - seal flush plan of injecting cool and clean water from external source.

seal flushing:
It prevents abrasive particles from entering seal faces.
It prevents sediments build up in seal faces.

seal quenching:
It prevent leaked water from contacting the atmosphere
It lubricates and cools the seal faces.

Identification of mechanical Seal Failure:
There is excessive loss of sealed product
Excessive loss of barrier fluid
There is excess process contamination

Common causes of Mechanical Seal Failure
Incorrect Fitting and Alignment
Loss of barrier fluid for lurication
incorrect seal selection
Most of mechanical seal fail due to process change or fault.
Shaft runout
Bearing failure

Single Mechanical Seal:
A single mechanical seal has one mating surfaces which is pressurized and lubricated by the sealing product. Its is the preferred solution for many applications.

Tandem Mechanical seal
It consist of two mechanical seal arranged in succession. seal near product is called primary seal or inboard seal.primary seal is lubricated and pressurized by product fluid. The seal near atmosphere is called outboard or secondary seal.The outboard seal is lubricated by unpressurized buffer fluid.If primary seal fails , secondary seal will protect the leakage.The secondary seal is used only for additional protection

Tandem mechanical seal Application:
Volatile liquids, chemicals, hazardous liquids, food processing and pharmaceutical field.


Double Mechanical Seal:
It consist of two Mechanical seal with presurized barrier fluid circulating between them.The barrier fluid is always higher than the product fluid.The inboard and outboard seal are pressuried and lubricated by barrier fluid.IF inner seal fails barrier fluid leaks into product . If outer seal fails barrier fluid leaks to the atmosphere, In both cases the product never leaks. This type of seal is used in hazourdous products.

Double Mechanical Seal application
High toxic and Explosive Hazardous fluids.

Most commonly used seal faces:
Carbon & silicon carbide

Where carbon faces cannot be used;
In most areas carbon faces are used. but in some areas two mating faces are metal like silicon and tungsten carbide.seal faces are selected according to the product characteristics.In abrasive and acidic area, volatile fluids areas carbon cannot be used.

Reasons for selecting two hard faces:
carbon faces are succeptable to acid attacks
carbon faces cannot be used in abrasive applications like slurry, sea water,crude oil
food processing and pharmaceutical were carbon wear are not accepted
Duty temperature exceeding the carbon temperature range
Higher viscosity will tend to bond the faces together.

Causes of component Damage:
Mechanical Damage to seal face due wrong alignment and installations.
Chemical and corrosion attack due to incompability
cracks may devolpe in seal ring and o ring may get harder and brittle due to high temperature.
wear due to excessive misalignment and abrasives

Mechanical Seal life:
Normally a mechanical seal life will be five years when operated in clean stable media , service life may exceed five years, when operated in viscous and abrasive media it may fail within few months

Springs used in mechanical seal:
Single spring
Multi spring
Wave spring
bellows

mechanical seal types by arrangment
single seal:
inside mounted
outside mounted

Dual seal
pressurised (double seals)
un presurized seal( tandem seals)

hot cyclone & cold cyclone in CFBC Boiler

 In hot cyclone super heaters located before cyclone. 

In cold cyclone superheaters located ofter cyclone.

Thermal Power plant Questions & Answers

1-The following is the correct order of energy conversion in thermal power plants

(A) Chemical energy – Mechanical energy – Electrical energy

(B) Mechanical energy – Chemical energy – Electrical energy

(C) Wind energy – Mechanical energy – Electrical energy

(D) Heat energy – Electrical energy – Mechanical energy

(Ans: A)

2-In thermal power plant, turbine is placed

(A) before boiler

(B) in between boiler and generator

(C) after generator

(D) any of the above

(Ans: B)

3-In the steam condensing power plants

(A) The amount of energy extracted per kg of steam is increased

(B) the steam, converted into water, can be re-circulated with the help of pump

(C) Both (A) and (B)

(D) None of the above

(Ans: C)

4-In thermal power plants, the dust of flue gases is trapped by

(A) Precipitator

(B) Economizer

(C) Superheater

(D) Air preheater

(Ans: A)

5-The path of flue gases in Thermal power plant is

(A) Boiler – Economizer – Superheater– Air preheater

(B) Boiler – Superheater – Air preheater – Economizer

(C) Boiler – Air preheater – Superheater – Economizer

(D) Boiler – Superheater – Economizer – Air preheater

(Ans: D)

6-The following is not a component of Thermal power plant

(A) Condenser

(B) Cooling tower

(C) Turbine

(D) Fuel tank

(Ans: D)

7-With the increase in _____ the efficiency obeys the ‘law of diminishing returns’

(A) Pressure

(B) Temperature

(C) Volume

(D) All of the above

(Ans: A)

8- With the increase in _____ the efficiency obeys the ‘straight line law’

(A) Pressure

(B) Temperature

(C) Volume

(D) All of the above

(Ans: B)

9-Fluid fuels are handled by

(A) burners

(B) stokers

(C) both (A) and (B)

(D) None of the above

(Ans: C)

10-For steam boilers, the fuel(s) is (are) mainly

(A) Bituminous coal

(B) Fuel oil

(C) Natural gas

(D) All of the above

(Ans: D)

11-The most common method(s) used for burning of coal is (are)

(A) Stroker firing

(B) Pulverized fuel firing

(C) both (A) and (B)

(D) None of the above

(Ans: C)

12-A ‘stroker’ is a power operated fuel ___ mechanism

(A) Burning

(B) Feeding

(C) Handling

(D) Storage

(Ans: B)

13-The spreader stroker, secondary air is supplied

(A) through holes

(B) through nozzles

(C) from bottom side

(D) any of the above

(Ans: B)

14-The following is not a pulverized fuel burner.

(A) Tangential burner

(B) Turbulent burner

(C) Cyclone burner

(D) Radial burner

(Ans: D)

15-In which of the following type of burner, liquid fuel is raised by capillary action?

(A) Wick burners

(B) Re-circulating burner

(C) Rotating cup burner

(D) All of the above

(Ans: A)

16-A Fluidised bed may be defined as the bed of ______ particles

(A) Liquid

(B) Solid

(C) Both (A) and (B)

(D) None of the above

(Ans: B)

17-The following is (are) ash handling system(s)

(A) Hydraulic system

(B) Pneumatic system

(C) Steam jet system

(D) All of the above

(Ans: D)

18-The following is dry type dust collectors

(A) Spray type

(B) Packed type

(C) Impingement type

(D) cyclone separator

(Ans: D)

19-The major constituent of fly ash is

(A) Silicon dioxide

(B) Aluminium oxide

(C) Calcium oxide

(D) Magnesium oxide

(Ans: A)

20-The draught produced by the chimney if due to the ____ difference between the column of hot gases inside the chimney and the cold air outside.

(A) Temperature

(B) Potential

(C) Density

(D) None of the above

(Ans: C)

Forging Types

Forging operations:

1: Drawing:

This is the operation in which metal gets elongated with a reduction in the cross sedation area. For this, a force is to be applied in a direction perpendiaulant to the length axis. 

2:Up setting:

This is applied to increase the cross seat ional area of the stock at the expanse of the length. To achieve the length of upsetting force is applied in a direction parallel to the length axis, For example forming of a bolt head.  

3: Fullering:

It a similar to material cross-section is decreased and length increased. To do this; the bottom fuller is kept in angle hole with the heated stock over the fuller .the top fuller is then kept above the stock and then with the sledge hammer, and the force is applied on the top fuller.  

                           

4:Edging:

It is a process in which the metal piece is displaced to the desired shape by striking between two dies edging is frequently as primary drop forging operation.

5:Bending:

Bending is very common forging operation. It is an operation to give a turn to metal rod or plate. This is required for those which have bends shapes.

6:Punching:

It is a process of producing holes in motel plate is placed over the hollow cylindrical die. By pressing the punch over the plate the hole is made.

7:Forged welding:                                                                                               It is a process of joining two metal pieces to increase the length. By the pressing or hammering then when they are at for ging temperature.Itis performed in forging shop and hence is called forged welding.

8:Cutting:

It is a process in which a metal rod or plate cut out into two pieces, with the help of chisel and hammer, when the metal is in red hot condition.

9: Flating and setting down:

Fullering leaves a corrugated surface on the job. Even after a job is forged into shape with a hammer, the marks of the hammer remains on the upper surface of the job. To remove hammer marks and corrugation and in order to obtain a smooth surface on the job, a flatter or set hammer is used.

 10: Swaging:

Swaging is done to reduce and finish work for desire size and shape, usually either round or hexagonal. For small jobs top and bottom swage pair is employed, where as for large work swage block can be used.

Economiser and its types

An economiser is a mechanical device which is used as a heat exchanger by preheating a fluid to reduce energy consumption. In a steam boiler, it is a heat ex-changer device that heats up fluids or recovers residual heat from the combustion product i.e. flue gases in thermal power plant before being released through the chimney. Flue gases are the combustion exhaust gases produced at power plants consist of mostly nitrogen, carbon dioxide, water vapor, soot carbon monoxide etc. Hence, the economiser in thermal power plants, is used to economise the process of electrical power generation, as the name of the device is suggestive of. The recovered heat is in turn used to preheat the boiler feed water, that will eventually be converted to super-heated steam. Thus, saving on fuel consumption and economising the process to a large extent, as we are essentially gathering the waste heat and applying it to, where it is required. Nowadays however, in addition to that, the heat available in the exhaust flue gases can be economically recovered using air pre-heater which are essential in all pulverized coal fired boiler.

Working Principle of Economizer

As shown in the figure above, the flue gases coming out of the steam boiler furnace carry a lot of heat. Function of economiser in thermal power plant is to recover some of the heat from the heat carried away in the flue gases up the chimney and utilize for heating the feed water to the boiler. It is simply a heat ex-changer with hot flue gas on shell side and water on tube side with extended heating surface like Fins or Gills. Economisers in thermal power plant must be sized for the volume and temperature of flue gas, the maximum pressure drop passed the stack, what kind of fuel is used in the boiler and how much energy needs to be recovered. When the water is boiled in steam boiler, the steam is produced which is then super-heated after which it is passed to the turbines. Then the exhausted steam from turbine blades, is passed through steam condenser of turbine in which the steam is condensed and this condensed water then is pre warmed first in feed water heater then in it before re-feeding in boiler. It is placed in the passage of flue gases in between the exit from the boiler and the entry to the chimney. In this a large number of small diameter thin walled tubes are placed between two headers. The flue gases flow outside the tubes usually in counter flow.

Classification of economizers: 

1. Based on construction 

(i) Plain tube type economizers: The external surface of tubes is kept clean and free from soot by soot scrapers moving up and down the economizer tubes. Otherwise heat transfer resistance is increase and efficiency of economizer decreased.

(ii) Gilled tube type economizers: Rectangular gills are provided on the bare tube walls to increases the heat transfer surfaces .

2. Based on part of steam generation

(i) Steaming type economizers: some part of the water (about 5 to 7%) to be converted into steam during its passage through the economizer.

 (ii) Non-steaming type economizers: While in case of non steaming economizer feed water is heated within 75% of the saturation temperature of the boiler.

3. Based on location of economizers 

(i) Independent economizers: economizer is installed outside the boiler house at any convenient place

(ii) Integral economizers: economizer is installed close to the boiler 

4.Types of Economiser Based on boiler Efficiency

(i) Non condensing Economiser
     The most common type of economiser used in thermal power plant is the non condensing thermal power plant. They are Heat exchanger tubes with fins around them in the form of spiral and they are loacated inside the duct at the exit region of the boiler.In this economiser used for reducing the flue requirement of boiler by transfering the heat from the flue gas to the feed water inside the tubes. These are used in coal fired thermal power plant where the temperature of the flue gas can be reduced to 120 deg c and not beyond it.This type of Economiser increases the efficiency by 3-6%

(ii)Condensing Type Economiser: They are used in nautral gas fired thermal powerplant. They reduced the fluegas temperature below condensation temperature as low as 25 deg c. They increase the boiler efficency by 10-15%. while using the condensor the fuel should not contain sulphur.

Advantages of economizers:

 (1) It improves the boiler efficiency. It has been found that about 1% efficiency of boiler is increased by increasing temperature of feed water by 6 °C with help of economiser. 

(2) It reduces the losses of heat with the flue gases, The temperature of flue gases is about 370°C to 540°C at exit of last superheater or rehea ter, having large amount of heat energy which otherwise otherwise would have been wasted. 

(3) It reduces the consumption of fuel. It has been estimated that about 1% of fuel costs can be saved for every 6 °C rise in temperature of the boiler feed water. 

(4) It reduces thermal stresses in the boiler due to reduced temperature differential in the boiler.