How do you calculate the attemperator water consumption in Boilers?

 

1-What do you mean by attemperation?

Attemperation is a method employed for controlling the super-heated steam temperature

Attemperators are used to control the main steam (super-heated steam) temperature in Boilers.

2-What are the two basic types of attemperators?

Spray type attemperator

Surface type attemperator

3-What is the main differences between attemperator & Desuper heaters?

Sl No.	Attemperator	Desuper heater
1	An attemperator controls steam temperature	Desuperheater removes whatever superheat there is in steam and reduces the temperature to a point at or nearly at saturation temperature
2	Attemperators are generally found in and/or associated with boiler steam, in zones where too high temperature affects something downstream of that point	Desuperheaters are used for downstream use of saturated steam
3	Outlet of the attemperation will be superheated steam only	Outlet of desuperheater will be saturated steam
4	Used in super-heated lines	Used in MP or LP steam lines

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4-What type of attemperation method is used in modern high pressure boilers?

In modern high pressure boilers variable nozzle and spray type attemperators are generally used

5-What is the percentage of attmepration used in High pressure Boilers?

Generally varies from 8 to 15%

 6-What is the reason behind using stainless steel sleeve inside the attemperator header?

Generally SS sleeves are fitted inside the attemperator header, this sleeve performs following functions.

 

Acts as a thermal barrier, separates hot and cold working elements to mitigate the intensity of thermal cycles experienced by critical components.

Protects steam pipe from thermal shock, helps to improve secondary atomization.

 

7-What precautions should be taken during attemperator liner or sleeve design?

•  Minimum length of straight pipe upstream of the liner should be three times the pipe diameter.
• Length of the liner downstream from the spray nozzles should be between 3 and 6 ft, depending on the particular installation.
• Length of straight pipe downstream of the liner should allow a residence time of 0.067 second for spray water to evaporate before the first elbow.
• Location of the temperature sensor should be at a distance downstream of the liner that allows 0.2 seconds of residence time to ensure complete mixing of the evaporated water and superheated steam. However, if the mass flow of spray water is greater than 15% of the mass flow of superheated steam, the residence time should be increased to 0.3 seconds.

 8-What factors are considered for designing attemperator?

Following factors are considered:

• Feed water pressure, flow rate, and temperature at the spray water control valve during various load conditions
• Locations of temperature sensors at the upstream & downstream ends
• Water chemistry
• Residence time of steam & water mixture for sensing temperature at downstream end
• Type of attemperator spray nozzle
• Rate of atomization & size of droplets

9-What is the distance of temperature sensors from attemperator spray nozzles?

For proper controlling of steam temperature, the upstream & downstream distance of sensors should be minimum of 5D & 20D respectively for straight pipe, where D is the diameter of attemperator header

 How do you calculate the Attemperator water consumption?

1. An attemperator is used to control the 95 TPH super-heated steam temperature from 425 deg C to 395 deg C by using 110 deg C feed water. Consider the main steam & feed water pressure 87kg/cm2 & 100 kg/cm2 respectively. Calculate the quantity of attemperator water

Given data,

Mass of steam, Ms = 95 TPH

Enthalpy of steam before attemperation at pressure 87 kg/cm2 & temperature 425 deg C, H1 = 762.41 kcal/kg.

Enthalpy of steam after attemperation at pressure 87 kg/cm2 & temperature 395 deg C, H2 = 741.81 kcal/kg

Feed water enthalpy at temperature 110 deg C, Hf = 111.65 kcal/kg

For calculation of attemperator water Mw

We have the relation,

Heat lost by the super-heated steam = Heat gained by the feed water

Ms X (H1-H2) = Mw X (H2-Hf)

95 X (762.41-741.81) = Mw X (741.81-111.65)

Mw = 3.1 TPH

2. A 125 TPH Boiler having variable type attemperator control valve for controlling main steam temperature from 495 deg C to 425 deg C at pressure 67 kg/cm2. The feed water is used for attemperation is 105 deg C, calculate the quantity of water required for de-superheating.

In the above example, Boiler feed pump having head 1000 meter & efficiency 62% supplies attemperator water, then calculate the extra power consumption for attemperation. Consider motor efficiency 95%

Given data,

Mass of steam, Ms = 125 TPH

Enthalpy of steam before attemperation at pressure 67 kg/cm2 & temperature 495 deg C, H1 = 812 kcal/kg.

Enthalpy of steam after attemperation at pressure 87 kg/cm2 & temperature 425 deg C, H2 = 771 kcal/kg

Feed water enthalpy at temperature 105 deg C, Hf = 106 kcal/kg

For calculation of attemperator water Mw

We have the relation,

Heat lost by the super-heated steam = Heat gained by the feed water

Ms X (H1-H2) = Mw X (H2-Hf)

125 X (812-771) = Mw X (771-106)

Mw = 7.7 TPH

For calculation of power required for pumping 7.7 TPH of water, we have

Motor input power = Flow in m3/sec X Total head X Density of water X 9.81 / 1000 X Pump efficiency X Motor efficiency)

Density of attemperator water at temperature 105 deg C = 960 kg/m3

Attemperator flow in m3/sec = 7.7 X 1000 X / (960 X 3600) =0.022 m3/sec

Now,

Motor power = 0.0022 X 1000 X 960 X 9.81 / (1000 X 0.62 X 0.95)

Motor power = 35.17 KW

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What do you mean by Regenerative system in power plants???

 

Thermal power plants efficiency is in the range of 30 to 40%, the improvement of thermal cycle efficiency can be done either by increasing the inlet steam pressure & temperature or decreasing the turbine exhaust pressure. But improvement of Boiler parameters & decreasing exhaust pressure lead to increase in cost & also have limitations related to metallurgy & risk.

Another way is to increase the thermal efficiency is by using regenerative cycle. This cycle is more efficient than Rankine cycle.

What do you mean by Regenerative process or cycle???

In this process, steam is extracted from turbine at one or more points during steam expansion. This pressure is high, medium & low. Steam after some expansion cycle gets feed water heating ability. Such extracted steam is utilized to heat the feed water going to the Boilers nearer to its saturation temperature. Heating the Boiler feed water temperature ultimately increases the overall thermal efficiency.

How does the increase in feed water temperature increase the overall thermal efficiency??

Increase in feed water temperature at economizer inlet reduces the work done by boiler to generate the steam & hence consumes less fuel. As a thumb rule, on every 6-7 degree C rise in feed water temperature reduces Boiler fuel consumption by 1%.

What are the advantages of Regenerative cycle???

Regenerative cycle helps to increase in power plant thermal efficiency

Amount of steam condensed in steam condenser per KW decreases

Cooling water consumption decreases

Condenser size reduces

Auxiliary power consumption of cooling system reduces

Heat rate of the plant drastically reduces

Due to less fuel consumption load on fuel handling, fuel feeding & Boiler fans decreases & hence saving in plant auxiliary power consumption.

What are the major disadvantages of Regenerative cycle or process?

There are no much disadvantages except the requirement of HP, LP heaters, relating piping & controlling equipments.

What are the major parts of Regenerative cycle?

HP heaters, LP heaters & Deaerator.

What are the HP heaters?

HP heaters are the shell & tube type of heat exchangers situated between Boiler feed pumps & economizer.

The main design purpose of the HP heater is to heat the feed water coming from Boiler feed pump.

 

Why the name HP heater?

Because it I situated in high pressure zone that is at Boiler feed pump discharge feed water circuit

What are the heat transfer areas present in HP heaters?

Main heat transfer zones are

De-super heating zone

Condensing zone

Sub-cooling zone

What is the function of de-super heating zone in HP heaters?

It is separate heat exchanger placed within the shell, its main function is to remove the super heat from extracted steam

What is the function of sub-cooling zone in HP heaters?

It is another separate counter flow heat exchanger placed within the HP heater shell, its main function is to sub cool the condensate formed in condensing zone

What are LP heaters & where they are placed?

The condensate formed in the surface condenser is pre heated to elevated temperature before it goes to deaerator are called LP heaters.

LP heaters are placed between deaerator & ejector or CEP

LP heaters are also having 3 zones as like HP heaters

Why the name LP heater?

LP heaters are placed at low pressure zone from CEP to deaerator hence called LP heaters

Schematic diagram of power plant regenerative system

 

How do you prove that, regenerative cycle will increase the thermal power plant efficiency?

This can be explained by taking an example

Sl No.	Particular	UOM	Boiler-1	Boiler-2
1	Boiler steam generation	TPH	100	100
2	Steam pressure	Kg/cm2	87	87
3	Steam temperature	Deg C	515	515
4	Boiler efficiency	%	69	69
5	Fuel GCV	Kcal/kg	4300	4300
6	HP heater available	YES/NO	Yes	No
7	Feed water temperature at economizer inlet	Deg C	160	110

 

Looking at the above example, both boilers seem to be safe, except Boiler-1 has HP heater that is feed water heater & Boiler-2 has no HP heater.

Based on this we shall calculate fuel consumption of both the Boilers.

Enthalpy of steam at above parameters Hg = 818 kcal/kg

Enthalpy of feed water at temperature 110 deg C, Hf1 = 111kcal/kg

Enthalpy of feed water at temperature 160 deg C, Hf2 = 162kcal/kg

Now we shall calculate the fuel consumption of Boiler-1

Boiler1 = Steam flow X (Steam enthalpy-Feed water enthalpy) / (Fuel GCV X Boiler efficiency)

Boiler 1 =100 X (818-160) / (4300 X 0.69)

Boiler 1 fuel consumption = 22.17 TPH

Boiler2 = Steam flow X (Steam enthalpy-Feed water enthalpy) / (Fuel GCV X Boiler efficiency)

Boiler 2=100 X (818-110) / (4300 X 0.69)

Boiler 2 fuel consumption = 23.86 TPH

Looking at the fuel consumption of both the Boilers, Boiler 1 consumes less fuel as it has HP heater & hence more feed water temperature than Boiler-2

 Less fuel consumption in the sense less heat rate & less heat rate is nothing but more efficiency of the plant

 

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How do you calculate the Deaerator & HP heaters steam consumption???

DEAERATORS:

What is the function of Deaerator?

Functions of deaerator

To remove the dissolved oxygen from deaerator & bringing down to <0.005 ppm

To heat the feed water

To provide NPSH to Boiler feed pumps

Acts as surge for feed water storage device

2-Which steam is used for deaerator?

Saturated steam having pressure from 0.3 to 3 kg/cm2 and temperature 130 deg C to 200 deg C

3-What are the pipe lines connected to Deaerator & storage tank?

Inlet lines:

• Make up water line
• Condensate water line from CEP
• Return condensate water from process
• Recirculation water line from BFP
• Balance leak off water lines from BFP
• Condensate water from HP & LP heaters
• Steam line from turbine extraction or PRDSH

Out let lines:

• Feed water line to BFP suction
• Deaerator overflow line
• Deaerator storage tank drain line

Understanding with examples.

1-Calculate the steam consumption of deaerator based on following given data

Sl No.	Particular	UOM	Value
1	LP steam flow to deaerator at pressure 1.5kg/cm2G & temperature 135 deg C t	TPH	?
2	Feed water outlet temperature from deaerator	0C	105
3	Feed water outlet flow from deaerator	TPH	125
4	CEP water flow to deaerator at temperature 50 deg C flow	TPH	120
5	Make up water to deaerator at temperature 25 deg C	TPH	5

        

 

 

 

 

 

 

 

 

 

Solution:

Enthalpy of steam at pressure 1.5 kg/cm2 & temperature 135 deg C = 653 kcal/kg

Enthalpy of CEP water at temperature 50 deg C = 50 kcal/kg

Enthalpy of makeup water at temperature 25 deg C = 25 kcal/kg

Enthalpy of deaerator outlet feed water = 106 kcal/kg

Mass of steam = (Feed water flow X Enthalpy –CEP flow X Enthalpy-Makeup water X Enthalpy) / (Enthalpy of steam-Enthalpy of deaerator outlet water)

Mass of steam      = 125 X 106 -120 X 50 -5 X 25 / (653-106)

Mass of steam = 13.02 MT

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2-Calculate the steam consumption of deaerator based on following given data

Sl No.	Particular	UOM	Value
1	LP steam flow to deaerator at pressure 2.5 kg/cm2G & temperature 150 deg C t	TPH	?
2	Feed water outlet temperature from deaerator	0C	128
3	Feed water outlet flow from deaerator	TPH	210
4	CEP water flow to deaerator at temperature 45 deg C flow	TPH	30
5	Make up water to deaerator at temperature 25 deg C	TPH	20
6	Process return condensate at temperature 95 deg C	TPH	160

        

 

 

 

 

 

 

 

 

 

 

 Solution:

Enthalpy of steam at pressure 2.5 kg/cm2 & temperature 150 deg C = 658.4 kcal/kg

Enthalpy of CEP water at temperature 45 deg C = 45 kcal/kg

Enthalpy of makeup water at temperature 25 deg C = 25 kcal/kg

Enthalpy of deaerator outlet feed water = 129 kcal/kg

Enthalpy of return condensate water = 96 kcal/kg

Mass of steam = (Feed water flow X Enthalpy –CEP flow X Enthalpy-Makeup water X Enthalpy-Return condensate X Enthalpy) / (Enthalpy of steam-Enthalpy of deaerator outlet water)

Mass of steam      = (210 X 129 -30 X 45 -20 X 25-160 X 96) / (658.4-129)

Mass of steam = 18.66 MT

HP & LP heaters:

1-What are the applications of HP & LP heaters?

HP & LP heaters are used to heat the feed water & to improve the cycle efficiency

 

2-What are the various pipe lines connected to HP heaters?

Inlet lines:

1-Feed water inlet line

2-Bleed steam line from Turbine

Outlet lines:

1-Feed water outlet line

2-Condensate outlet line

Understanding with examples.

A HP heater is used to heat the 105 TPH feed water from 105 °C to 150 °C by using turbine bleed steam at 8 kg/cm2 and 200 °C. The condensate returning from heater is at 125 °C, calculate the quantity of steam used.

 Given that,

Qf = 105 TPH

Tf1 = 105 °C

Tf2 = 150 °C

Hg at pressure 8 kg/cm2 & temperature 200 °C =677 kcal/kg

Enthalpy of condensate water Hf = 126 kcal/kg

Heat lost by the steam = Heat gained by feed water

Ms X (Hg-Hf) = Mw X (Tf2-Tf1)

Ms X (677-126) = 105 X (150-105)

Ms = 8.57 TPH

 

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Power plant & Calculations