Shaft couplings & selection guide

A shaft coupling is a mechanical device used to connect rotating shafts and absorb misalignments between them. A coupling is a mechanical device, which is used to connect driver and driven shaft permanently or semi permanently.

Couplings can be rigid or flexible depending on the alignment accuracies of the system and torque requirements. Shaft couplings are used for power and torque transmission between two rotating shafts such as on motors and pumps, compressors, and generators.

Functions of couplings:

• Connects the shafts of two units, which are manufactured separately.
• Transfers motion, power and torque
• To reduce transmission shock loads from one shaft to another
• Protection against overloads.
• Introduces mechanical flexibility.

Design considerations for couplings:

• Type of drive & driven equipments
• Alignment accuracy
• Operating & surrounding temperatures
• Type operation (Intermediate, continuous, frequent ON/OFF etc)
• Shafts diameters to be connected
• Maximum & minimum bores size
• Operating & design power
• Maximum & peak loads
• Space available
• Operation & Maintenance cost
• Service factor (Generally, for medium duty use a service factor of 1.5. For heavy duty use a factor of 2 and for extra heavy duty a factor of 3 should be used)

Types of couplings:

Main types of couplings: Rigid coupling and Flexible coupling

Regid couplings:

Rigid coupling is used to connect two shafts which are perfectly aligned. Most of the rigid couplings are made of aluminum, steel, or stainless steel.

Types of Regid couplings:

• Sleeve or muff coupling
• Clamp/compression
• Flange coupling

Considerations for regid coupling selection:

• Angular misalignment tolerance
• Parallel misalignment tolerance
• Axial motion allowed
• Dimensions like Bore diameter,Coupling diameter,Coupling length & Design units

No.of coupling bolts for flange couplings

• 3 if shaft size is up to 40 mm

• 4 if shaft size is 40–80 mm

•  6 if shaft size is 80–180 mm

Flexible Couplings:

• Pin bush coupling (Protected and unprotected type)

• Jaw/spider/love joy coupling

• Gear coupling

• Bibby/grid coupling

• Metaflex/flexible disc coupling

• Tyre coupling

• Fluid coupling

• Oldham’s coupling

Dimensions available Pin bush coupling catalogues:

• Coupling size

• Coupling flange diameter

• Hub diameter

• Coupling gap

• Coupling maximum & minimum bore diameters

• No.of pins or bolts

• Maximum speed

• Torque

Gear Couplings:

Gear couplings also transmit high torques. They have misalignment capabilities generally about 0.01-0.02 inch in parallel and 2 degrees in angular. Gear couplings are often used in pairs with spacer shafts to span the distance between the driving and driven equipment. They generally require lubricant although some designs intended for lighter duty use lubricant free nylons or other polymers for the center sleeve.

Grid Couplings:

Grid couplings employ spring-like connecting elements that weave between slots machined in the coupling hubs. They are capable of high torque transmission with an added bonus of shock absorption and torsional vibration dampening. They operate without lubricant. They are appropriate for power transmission and capable of handling parallel misalignment up to 0.30 inch and angular misalignment of about ¼ degree.

Disc Couplings:

Disc couplings use single or multiple discs and single or double stages which bolt to the shaft hubs. They are used for power transmission and rely on the flexibility of their thin metal discs to transmit torque and accommodate angular misalignment. They are not especially good at managing parallel misalignment. They are capable of transmitting high torques and are often used to couple high horsepower motors, gas turbines, etc. to loads.

Oldham Couplings:

Oldham couplings handle high degrees of parallel misalignment owing to their sliding element design. Use of an elastomer center element instead of metal is popular in modern versions. Some manufacturers claim an ability to tolerate up to 5-degree angular misalignment through the use of cylindrical, rather than rectangular, sliders.      

Fluid Couplings:

Fluid couplings or hydraulic couplings work on the hydrodynamic principle. In drives consisting fluid couplings, there is no mechanical contact between the driver and the driven machine and power is transmitted by means of a fluid. Due to the mechanical separation between the driver and the driven machine, a fluid coupling enables to achieve two separate value of acceleration in the drive, the fast value of acceleration for the driver and simultaneously the slow value of acceleration for the driven machine. 

         Fluid couplings are often used to drive large inertia machines in combination with squirrel cage motors. They permit a load free acceleration of the motor and consequently with increasing oil fill, provide a soft/gentle quasi steady state start-up of the machine. The maximum torque occurring during the start-up process is restricted to lowest possible level. As fluid coupling allows quick acceleration of the motor and short duration of high value starting current, it results into economical design for electrical system. In addition, systems that use multiple motors can be switched on in a staggered sequence to limit the current demanded during the motor acceleration. This avoids grid overloading caused by simultaneous motor starts.

Fluid couplings are used in drives for conveyor systems such as belt conveyors, bucket elevators and chain conveyors. The smooth application of fluid coupling torque provides a smooth start-up of belt conveyor to protect the belt from damaging stresses. In heavy industry, they are used for applications such as crushers, roller presses, mixers, large ventilators, boiler feed pumps, large compressors, centrifuges, etc

Types of fluid couplings:

Constant fill type:

Constant-fill Couplings Couplings of this type are mainly used for start-up (to limit torque) and to cushion the torsional vibration of the drive chain. In this type of couplings, various designs mainly differ through adjoining chambers, who’s automatically controlled filling and emptying have a significant influence on the start-up behavior. Constant-fill couplings are sealed to the outside. Filling of the operating fluid in a coupling is carried out before its commissioning.

Drive requirements determine the design and filling quantity. The ratio of the operating fluid volume filled to the overall volume of the coupling is called the fill level.

Variable-speed Couplings:

Couplings of this type are used to control or regulate the speed of the driven machine over a wide range below the drive speed. These couplings have devices that seamlessly change the transmission behavior during operation. This mainly occurs by changing the fill level. The fill level can be changed during operation either via a radially movable scoop tube or by controlling the operating fluid inlet and outlet via valves and nozzles. These couplings always have an external fluid circuit for filling changes that can also aid cooling.

Missalignment tolerances for angular & parallel alignments

Speed (RPM)	Angular misalignment in Mills/inch of coupling diameter		Parallel misalignment Mills
	Good	Acceptable	Good	Acceptable
600	1	1.5	5	9
900	0.7	1	3	6
1200	0.5	0.8	2.5	4
1800	0.3	0.5	2	3
3600	0.2	0.3	1	1.5
7200	0.1	0.2	0.5	1

Calculations:

Example-1:What is the size of muff coupling, which is required to fit on 50 mm shaft Outer diameter of muff or sleeve = 2 X shaft diameter + 13 mm

                                            = 2 X 50 + 13 = 113 mm

                Length of sleeve = 3.5 X shaft diameter =3.5 X 50 =175mm

Example-2: Calculate the flange coupling dimensions required to fit on a shaft of 65 mm diameter.

Based on shaft diameter we can calculate the following dimensions of flange coupling.

Outside diameter of hub = 2 X shaft diameter (d) ==2 X 65 = 130 mm

 Length of the hub = 1.5 X d = 1.5 X 65 = 97.5 mm

Pitch circle diameter (PCD) of the bolts = 3 X d = 3 X 65 = 195 mm

Outside diameter of the flange = 4 X d = 4 X 65 = 260 mm

Thickness of flanges = 0.5 X d = 0.5 X 65 = 32.5 mm

Example-3:What is the maximum torque developed on a gear coupling mounted for pump & motors of power rating 525 KW & speed 3000 RPM

Torque = 9550 X Power/Speed

         T = 9550 X 525/3000

         T = 1671.25 Nm

Basic calculations on fuels & combustion

Fuel:
It is a substance which releases heat energy on combustion. The principal combustible elements of each fuel are carbon and hydrogen.

Fuels classification:

Primary Fuels: These fuels directly available in nature, Ex: Wood, Peat, Lignite coal, Petroleum and Natural gas.
Secondary Fuel: These are prepared fuels, Ex: Coke, Charcoal, Briquettes, Kerosene, fuel oil, petroleum gas, producer gas etc.

Fuels are also classified as Solid fuels, liquid fuel and Gaseous fuel.

Different types of coals: Peat, Lignite, Bituminous coal, Anthracite coal and Coke.

Gaseous fuels:Natural gas, Coal gas, Coke oven gas, Blast furnace gas, producer gas, Water gas and Sewer gas.

Energy producing elements in fuel: Carbon, oxygen, hydrogen and Sulphur.

Formation of Charcoal & Coke:

Charcoal:It is obtained by destructive distillation of wood.

Coke is formed by destructive distillation of certain types of coal.

Calorific value of fuel: Calorific value (GCV) is the amount of heat released on complete combustion of unit quantity of fuel.It is measured in kcal/kg.

GCV is also known as Higher Calorific Value (HCV),it is given by following formula

GCV or HCV = (8084 X C% + 28922 X (H2%–O2%/8) + 2224 X S%)/100…kcal/kg

Where C, H2, O2 and S are percentage of Carbon, hydrogen, oxygen and Sulphur respectively in fuel.

Net Calorific Value (NCV) or Lower Calorific Value (LCV):

The total heat released by fuel during combustion is not completely utilized. Some heat is taken out by water vapour which is produced during combustion of hydrogen. Such heat value taken by considering heat taken away by water vapour is called NCV or LCV.

LCV = HCV – (9 X H2% X 586), Where H2 = Hydrogen% in fuel and 586 is latent heat of steam

in kcal/kg.

Useful heat Value (UHV) of coal:

NCV = GCV - 10.02 X Percentage of total moisture.

Ultimate analysis:Ultimate analysis indicates the various elemental chemical constituents such as Carbon, Hydrogen, Oxygen, Sulphur etc.

Proximate analysis of coal:Proximate analysis determines fixed carbon, Volatile matter, moisture and percentages of ash.

Volatile Matter (VM) & is its significance:

VM is generally a composition of methane, hydrocarbons, hydrogen and carbon monoxide and other incombustible gases like CO2 and Nitrogen. It is the indication of presence of gaseous fuel in the fuel.

Significance:

• Helps in easy ignition of coal by increasing the flame length
•   Sets minimum limit on the furnace height and volume.

Properties of Coal:

·         Caking Index: Indicates binding property of coal.

·         Swelling Index: Indicates caking capacity of coal.

·         Slacking Index: Indicates the stability of coal when exposed to open atmosphere.

·         Grinding Index: It gives the idea of ease of grinding of coal.

·         Abrasive Index: Indicates the hardness of coal.

Combustion:

Combustion is the rapid oxidation of fuel accompanied by the production of heat

Oxygen is the major element on earth, making up to 21% (by volume) of our air. Carbon, hydrogen and Sulphur in the fuel combine with oxygen in the air to form carbon dioxide, water vapour and Sulphur dioxide releasing tremendous heat.

Oxygen is the major element on earth, making up to 21% (by volume) of our air. Carbon, hydrogen and Sulphur in the fuel combine with oxygen in the air to form carbon dioxide, water vapour and Sulphur dioxide releasing tremendous heat.

Basic requirements of combustion:Fuel, Oxygen and 3T’s

C + O2 = CO2+ Heat 8084 kcal/kg of Carbon.

2C + O2 = 2CO + Heat 2430 kcal/kg of Carbon.

2H2 + O2 = 2H2O 1 Heat 28922 kcal/kg of Hydrogen.

S + O2 = SO2 + Heat 2224 kcal/kg of Sulphur.

Products of Combustion: CO2, CO, O2, SO2 and ash.

Spontaneous combustion: is a phenomenon in which coal bursts into flame without any external ignition source but by itself due to gradual increase in temperature as a result of heat released by combination of oxygen with coal.

Major contents of ash:

• Silica (SiO2)
• Aluminum oxide (AlO3)
• Iron Oxide (Fe2O3)
• Sodium Oxide (Na2O)
• Potassium Oxide (K2O)
• Calcium Oxide (CaO)
• Magnesium Oxide or Magnesia (MgO)

Fly ash & Bottom ash :

Fly ash 70–80%, Bottom ash 20–30%.

Fly ash at Economiser : 7–8%, APH: 10–12% and ESP: 80–82% of total fly ash.

Oxygen & Nitrogen present in the air :By weight Oxygen 23% and Nitrogen 77% and by volume Oxygen 21% and Nitrogen 79%.

Stoichiometric air fuel ratio: A mixture of air and fuel, which contains sufficient amount of oxygen for complete combustion.

Rich mixture: Mixture with deficiency of air

Lean mixture: Mixture with excess air

Power generation phenomenon in steam Turbo generators

Theoretical air of combustion:

Minimum amount of air that supplies the sufficient amount of oxygen for the complete combustion of all carbon, hydrogen and any other elements in the fuel that may oxidize is called theoretical air.

Theoretical air required for combustion of carbon:

We know that Carbon on oxidation with Oxygen forms Carbon dioxide.

C + O2 = CO2

12 + 32 = 44 (Molecular weights of Carbon and Oxygen are 12 and 16 respectively)

1 + 2.67 = 3.67

So, 1 kg of Carbon requires 2.67 Kg of Oxygen for complete combustion into 3.67 kg of carbon dioxide.

Similarly

Hydrogen on oxidation forms Water

2H2 + O2 = 2H2O

4 + 32 = 36 (Molecular weights of Hydrogen and Oxygen are 1 and 16 respectively)

1 + 8 = 9

So, 1 Kg of Hydrogen requires 8 Kg of Oxygen for its combustion & forms into water.

Excess air: Amount of extra air given to ensure complete combustion is called excess air.

3T’s of combustion:

• Temperature: High enough to maintain the ignition of the fuel.

• Turbulence: Is the mixing of fuel and air.

• Time: Sufficient enough for combustion.

Specific heat Cp and Cv:

Specific heat is the amount of heat in kcal needed to raise the temperature of 1 kg of substance by 1 °C. Cp and Cv are the specific heat at constant pressure and constant volume of gas.

Solved examples:

 Example-1:A Biomass (Bagasse) contains 23% of carbon, 22% of oxygen, 3.5% of Hydrogen and 0.05% of Sulphur, then calculate the theoretical air required for its combustion.

We have, Theoretical air requirement = (11.6 X %C + 34.8 3 (%H2 - %O2/8) + 4.35 X %S)/100… 

Kg/kg of fuel.

Therefore, Th air requirement will be = (11.6 X 23 + 34.8 X (3.5 – 22/8) + 4.35 X 0.05))/100

                                                             = 2.9 kg of air/kg of fuel.

Example-2:What is the quantity of excess air, if O2 measured in the Boiler outlet flue gas is 6%?

We have, Excess air = (O2%/( 21 - O2%)) X 100

                                 = (6/(21 – 6)) X 100 = 40%

Example-3:A complete combustion requires 2.9 kg of theoretical and 20% of excess air, then calculate the total air consumed for complete combustion per kg of fuel burnt.

Actual quantity of air supplied = (1 + Excess air %/100) X theoretical air

                                                   = (1 + 20/100) X 2.9 = 3.48 kg of air/kg of fuel.

Example-4: A flue gas has 10% of CO2 and theoretical calculated CO2 is 12%, then calculate percentage of the excess air.

                                         % of Excess air = ((Theoretical CO2%/Actual CO2%) - 1)) X 100

                                                                   = (12/10) - 1) X 100 = 20%

Example-5: A Coal sample contains 10% of ash, coal required is 300 MT/day, assuming 100% combustion calculate the mass of ash generated in a day.

                                                   Mass of ash generated = (300 X10)/100

                                                                                        = 30 MT

Example-5:A Indonesian coal contains 58% of Carbon, 4.2% of Hydrogen, 11.8% of Oxygen and 0.5% of Sulphur, also needs 20% of excess air for its complete combustion. Calculate the Total air required for complete combustion and O2% in flue gas.

We know that,

Theoretical air required for combustion is = (11.6 X 58 + 34.8 3 (4.2 - 11.8/8) + 4.35 X 0.5)/100

                                                                    = 5 7.7 Kg/Kg of Coal

                                                      Total air = (1 + EA/100) X Theoretical air

                                                                    = (1 + 20/100) X 7.7

                                                                    =  9.24 Kg/Kg of Coal

                               Also we know that EA = (O2 %/(21 - O2%)) X 100

                                                               20 = (O2/(21 - O2)) X 100

                                                               O2= 3.5%

Safety valves and its basic concepts

Pressure Relief Valve:This device is generally fitted on liquid lines like water, oil line. In this, valve the opening is proportional to increase in the line or vessel pressure. Hence the opening of valve is not sudden, but gradual if the pressure is increased gradually. In relief valve valves may not open 100%, as the line pressure reduces valves closes gradually. Pressure relief valves have higher flow capacities

Pressure Safety Valve: It is fitted on compressible fluid or gas lines. For such a valve the opening is sudden. When the set pressure of the valve is reached, the valve opens almost fully.

Pressure safety valve & relief valves are used for system, equipment & man power protection.

Pressure reducing valve: These may be of hydraulic or pneumatic type used for water lines. This valve reduces the pressure of the water that goes through it, and is used to obtaining a regulated and constant value at its outlet.

Pressure control valves: These may be of hydraulic or pneumatic type used for steam lines

Safety valve:

A safety valve must always be sized and able to vent any source of steam so that the pressure within the protected apparatus cannot exceed the maximum allowable accumulated pressure.Here the valves sizing, manufacturing, installation, positioning & setting are more important.

Factors to be considered for selection/design of a pressure safety valve:

• Connection size and type

• Operating pressure 

• Operating Temperature

• Back pressure

• Service

• Required capacity

• Thermodisc

• Thermal compensation

• Blow down & Operating gap

Terminology used in safety valves:

Set pressure: It is the pressure at which safety valve lifts or pops up.It is usually 106-107% of operating pressure.

Reseat pressure: It is the pressure at which Safety valve seats.

Blowdown: It is the Blowdown is the difference between set pressure and reseating pressure of a safety valve expressed as a percentage of set pressure.

Blow down of safety valve = (Set pressure – Reseat pressure) X 100 / Set pressure

Blow down of safety valves is in the range of 2 to 5%.

Chattering: Excessive pressure loss at the inlet of the safety valve will cause extremely rapid opening and closing of the valve, this is called as chattering.

Chattering may result into lowered capacity as well as damage to the seating surface of the valve. Continuous chattering may result into damage to the other parts.

Following recommendation wil assists in eliminating the chattering

• The area of the inlet nozzle should be equal to the the inlet area of safety valve & that nozzle should be short as possible

• Inlet nozzle corners must be rounded to a radius of not less than ¼ of the diameter opening

Sonic Vibrations: Flashing, choked flow sudden flow or cut/off of steam in safet valves & related lines may result into  sonic vibration. This velocity is usually reached when the valve pressure drop rises to 50% of the upstream pressure. Vibration of long pipelines can also occur due to mechanical damage.

IBR acts, regulations & forms used

Precautions to avoid sonic vibrations:

• Safety valve should be installed at least 8D to 10D of pipe diameter down stream from any bend in steam line.

• Safety valves should not be installed closer than 8D to 10D to pipe diameters either upstream or down stream from the diverging or converging  “Y” fittings.

• The safety valve nozzle should never be installed in a steam line in a position directly opposite a branch line of equivalent size.

Accumulating test pressure: The accumulation test is done on boilers to limit the excessive pressure rising while the safety valve is in open. The test is carried on new boilers or new safety valves with full firing condition with MSSV and feed water valves closed. It is conducted as long as water in drum permits generally 7 minutes for water tube boilers.

25-Questions & answers on Boilers safety valves

Boiler safety valves overhauling steps

Consideration for installation of safety valve:

• Exhaust drain & cover plate vent piping must be installed so that they will not impose under pressure on the safety valve.

         Note: Do not plug the cover plate hole or do not reduce the hole piping size.

• Discharge pipe of the safety valve should not be supported on the valve body

• Clearance between the valve exhaust piping and the discharge stack should be sufficient to prevent contact when considering thermal expansion of the boiler valve

• Steam flowing vertically out of the discharge elbow produces a downward reaction on the elbow, in proportion on the quantity of steam flowing & its velocity.
  
  
• In no case should discharge piping smaller than  the outlet valve
  
  
• For optimum performance safety valves should be serviced regularly

• Valve assembly should be within 1⁰ vertical alignments.

• Gaskets fitted should be of correct size, should not close the valve inlet opening

10-Tips to reduce LOI in Boilers

Adjustment of set pressure:

Safety valves are set +/- 1% of set pressure.Set pressure should not be changed without the permission of manufacturing unit.

Before proceeding to check the popping (lift) pressure, ensure the pressure gauges used are calibrated. To adjust the popping pressure, remove the lifting gear, exposing the adjusting bolt lock nut. Loosen the lock nut if the opening pressure is low tighten (turn clockwise) the adjusting bolt, if it is high loosen (turn counter clockwise) the bolt. After each adjustment the lock nut should be securely tightened to prevent loosening of the bolt.

Adjustment of blowdown:

If the blow down is not as desired when the set pressure has been obtained, it is must to adjust the rings. The guide (adjusting) ring is the principal blow down control ring. To change its position, remove the guide set screw on the back of the valve body. Insert a screw driver or similar tool and engage one of the notches (these can be seen through set screw hole). The ring can then be turned to the right or left as desired. Turning the guide (upper) ring to the right raises it up and reduces the blow down. Turning the guide (upper) ring to the left lowers it and increases the blow down. After each adjustment always replace and tighten the set screw being careful that its point engages a notch and does not rest on the top of the tooth.

Note: Do not attempt to adjust blow down with lower ring

Power Transformers QnA

Factors which cause safety valve to damage or failure:

• Quantity & quality of the steam

• Discharge piping stress and back pressure

• Variation in ambient temperature

• Improper gagging

• Improper bolting of flanges

• Foreign material in the steam

• Improper method of assembly & disassembly

Guidelines for Boiler safety valve setting:

Preliminary checks

• Ensure calibrated pressure & temperature gauges are fitted.

• Gauges for each individual valves should be fitted

• Discharge piping has to be inspected for binding on the valves,supports and welds on piping.

• A rope appx. 6-7 meters with a hook one end should be attached to the valve lifting lever before starting the pressure rise. It will help in operating the lever to avoid chattering & over pressure

• Have the correct tooling available

• Establish the good communication system

Guidelines:

• If the unit has Electromatic safety valve, this valve should be in operation firts for more safety of the unit.

• Drum valves to should be tested first: Possibilities of valve part damage because of GIRL BLASTING are grater on superheated valves in contrast to the drum valves .If super heater valve is gagged after seat damage while testing other valves, the total valve damage will increase.
  
  
• Boiler temperature increases during the testing cycle of the Drum valves. Consequently higher temperature steam will be available for super heater steam valves and produce accurate results

• Keep water level low as possible, if drum level is high the safety valves may slugged with water causing long blow down & also may result damages to seat & disc.

• Maintain pressure rising in the range of 2-3 kg/cm2 per minute, slow pressure rising may result into simmering of the valve.

• If fuel feeding system fails at nearer set pressure, then reduce the boiler pressure at least 10% & raise again. Holding the boiler pressure nearer to set pressure for long time may result into simmering & valve lift erratically.

• If a valve has to be lifted several times, cooling off period is very must. Cooling period is around 20-30 minutes.

• If valves have not been tested with hydro test prior to the steam condition, it is recommended to hand lift before steam actuation.

Challenging situations & troubleshooting during boiler light up & start up

Safety valve floating procedure:

• Normally the highest set pressure valve is the valve floated first. While setting this valve other safety valves are gagged.

• Start the boiler as per cold start up procedure by modulating the firing.                
• When the drum pressure reaches about 60–70% of operating pressure gently tighten gage on other safety valve. 

• Raise pressure slowly by throttling start up vent valve. When 80% of popping up pressure is reached manually operate the safety valve under test. This will blow off any debris or dust left over in the valve internals.

• Raise the boiler pressure by modulating the firing

• When the pressure reaches nearer to the set pressure close the start up vent. While the safety valve pops (lift), open the start up vent valve and note down the lifting/set pressure value.When the valve sits back, note down the reset pressure

• Control of drum level is important to avoid possibility of water carry over from drum to the super heater.

• The set pressure is adjusted by either tightening or loosening the adjusting nut. Tightening the nut increases the set pressure and vice versa

•  Blow down is adjusted by upper rings adjustment.
• After setting the set pressure and blow down, bring down the boiler pressure to operating level.

Examples-1: A boiler steam drum safety valve lifts at 125 kg/cm2 and reseats at 120 kg/cm2, then calculate its blow down percentage?

                                   BD% = (125 – 120) X 100/125

                                                 =4.0%

Example-2: A boiler super heater safety valve has blow down 3% & has been set at 70 kg/cm2, calculate the reseat pressure.

3% = (70-P2) X 100/70

Reseat pressure P2 =67.90 kg/cm2

Precautions shall be taken during Super heater safety valve set at lower operating temperature than actual:

Safety valves blow down should be set more than required, as blow down percentage decreases as the steam temperature increases. An approximate rule is to add 0.5% of set pressure to the blow down for each 56.5 °C rise in SH steam temperature.

Example-3:
If a Super heater safety valve lifts at 189.5 kg/cm2 & reseats at 180 kg/cm2 at the temperature of 400 deg c, then calculate the blowdown calculation at 540 deg c

We have,

Lift pressure        = 189.5 kg/cm2

Reseat pressure =180 kg/cm2

Difference           =9.5 kg/cm2

Difference in temperature =540-400 = 140 Deg C

Asper above condition for every 55.6 deg c  rise in steam temperature blow down percentage increases by 0.5% of set pressure

140/55.6  X 0.5 X 189.5/100

=2.385 kg/cm2

Hence, blow down  at 540 deg c =9.5-2.385 =7.115 kg/cm2

Available @ Flipcart/Amazon/Notion press