Improving Steam Dryness Factor

Improving Steam Dryness factor

In Boilers the steam dryness constitutes a major milestone which cannot be measured online as no measurement systems are present

We have to buy the argument from the Boiler manufacturers, that the steam is 100% dry due to moisture separator efficiency in the drum

My view

Prologue :

1) In many power plants on identical turbine load the Steam / MW varies between 1% to 7%
2) In process boilers the steam trap losses account to 8% to 14% based on the distance travelled by Steam

The Steam velocity properties are closely associated with its dryness.
Increased dryness makes steam density lower & contributes to higher velocity & vice versa.

Look at the following questions
 
q1) Whether Steam velocity is relative to its enthalpy ? 
q2) Can the Steam velocity be increased for the same enthalpy ?
q3) What are the factors effecting the Steam velocity in the Boiler ? 
q4) Whether Steam is really 100% dry as portrayed ?
q5) What is the proof ?

If the Steam Velocity is relative to its enthalpy then at what dryness factor & if the dryness is reduced what would be the effect on steam properties ?

If the dryness factor varies, then Steam Velocity also should relatively change ?

Does Water pump, drum level, super heaters etc. the entire water circuit has a role to play regarding Steam velocity or it is just an assumed or concurred output ?

How does one come to knowledge instead of assumptions that Steam is Dry & at what % dryness ?

The drum is basically a cylindrically shaped construction with water injecting into it from the water pump, with a steam separator mounted at 80% level or height.

Drum Condition 1
The Drum level say considering average of 50%, the condition would be 

50% space for Steam
50% space for water
The steam separator is placed at 80% height

Water vapour will separate from the steam at the separator & pass through at the outlet

This means there is water vapour up to 25% height of the drum at mean value (space between water & dry steam) which is getting separated by the separator

Consider this,
1) Steam residence time in the drum is more due to 50% occupation of space
2) Water residence time is less due to its occupation of 50% space of the drum, its weight reduced due to decrease in density due to high feed water temperature
3) Water vapour formation is higher since higher residence time of steam will lead to its condensation i.e. increase its wetness

Drum Condition 2
The Drum level say considering average of 40%, the condition would be 

60% space for Steam
40% space for water
The steam separator is placed at 80% height

Water vapour will separate from the steam at the separator & pass through at the outlet

This means there is water vapour up to 35% height of the drum (space between water & dry steam) at mean value which is getting separated by the separator

Consider this,
1) Steam residence time in the drum is more than condition 1 due to 60% occupation of space

2) Water residence time is less due to its occupation of 40% space of the drum, its weight reduced due to decrease in density due to high feed water temperature
3) Water vapour formation is higher since higher residence time of steam will lead to its condensation i.e. increase its wetness
4) If Steam condensation increases due to not drawing the steam or any other reason, boiler stoppage etc., the steam residence time will further increase increasing the water vapour & water temperature & the possibility exists that all water, steam will convert into water vapour leading to water hammering

Drum Condition 3
The Drum level say considering average of 75%, the condition would be 

25% space for Steam
75% space for water
The steam separator is placed at 80% height

Water vapour will separate from the steam at the separator & pass through at the outlet

This means there is water vapour up to 5% height of the drum at mean value which is getting separated by the separator

Consider this,
1) Steam residence time in the drum is less due to 25% occupation of space

2) Water residence time is more due to its occupation of 75% space of the drum, its weight reduced due to decrease in density due to high feed water temperature
3) Water vapour formation is lesser since lesser residence time of steam will lead to its negligible condensation i.e. increase its dryness


Let us look into the physical equilibrium condition that exists in the Steam or water drum

Steam, Water Vapor & Water are all H2O, the same chemical substance but in different physical forms, all existing under one roof the Drum.

Equilibrium condition is when all the 3 phases merge into single phase either steam or water vapour or water. 

Of these possibilities, only water vapour possibility exists as the others are not possible to be achieved.

There are 3 conditions for the water drum

Water Quantity + Water vapor  Quantity > Steam Quantity
Water Quantity + Water vapor  Quantity = Steam Quantity
Water Quantity + Water vapor  Quantity < Steam Quantity

The 1st Condition : Water Quantity + Water vapor  Quantity > Steam Quantity
Steam is dry or with higher dryness factor due to its low residence time for condensation

This will improve its velocity & hence forth its kinetic energy driving the turbine
Increased velocity will lead to fewer losses in the steam traps

Equilibrium condition of Steam, Water vapour & water not achievable

The 2nd Condition : Water Quantity + Water vapor  Quantity = Steam Quantity
Steam is less dry or with higher wetness factor due to its increased residence time for condensation

This will decrease its velocity & hence forth its kinetic energy driving the turbine, leading to increased consumption of steam


Decreased velocity will lead to higher losses in the steam traps


Equilibrium condition of Steam, Water vapour & water in critical condition, which means it can swing either way


The 3rd Condition : Water Quantity + Water vapor  Quantity < Steam Quantity
Steam is much less dry or with increased higher wetness factor due to its maximized residence time for condensation

This will lower its velocity & hence forth its kinetic energy driving the turbine, leading to maximized consumption of steam


Lower velocity will lead to maximum losses in the steam traps

If steam condensation is high, then water vapour content will increase due to heat transfer between steam & water & the entire drum content will convert to water vapour, leading to water hammering.


Equilibrium condition of Steam, Water vapour & water is achievable


Now the key question is how to improve Steam Dryness ?

Simple, keep the steam residence time in the drum as mean or as low as possible

Example 1 :

A 6 TPH manual fired boiler was operating at 10.5 kg pressure & drum level of 40 to 50% in water pump auto mode, having reported Steam trap losses ranging from 9% to 12%

Correction was taken in the water level in the drum to run at 73% Max. & 63%. The Steam trap losses lowered by 75%

Example 2 :

A 90 TPH Stoker boiler operating at 65 kg pressure & 450 deg C temperature & drum level 35% to 40%, was having a unique problem that if the Steam load increased to > 50% i.e. 45 TPH, the steam temperature started to increase upto 490 deg C & pressure would drop to 56 kg. The boiler was never loaded to > 50% capacity in over 30 years of its installation

Correction was taken by raising the drum level to 75% & Steam Load achieved up to 100% of the Load, without any raise in Steam temperature or lowering of its pressure


Example 3 :


A 33 TPH with 42 Kg pressure rating, Stoker boiler was delivering Steam max. up to 22 to 27 TPH with 33 Kg pressure operation. The drum level was 40%, when the Steam load would increase, the boiler would tipsy turvy & compromise on pressure leading to higher demand. This problem was present for over 22 years of installation


Correction was taken by raising the drum level to 65% & boiler never had a problem in delivering load or pressure


Example 4 :

A client was complaining that their 8 TPH boiler was always over loaded & was never able to deliver steam properly when steam was in demand. The drum level was operated continuously at 30%

Analysis showed that they had demand of only 3.5 TPH, but the pressure dropped from 10 kg to 4 to 5 kg max. 


Correction was taken by raising the drum level to 75%, the pressure improved to 10 kg & steam demand dropped by 1 TPH for the highest production. The trap losses almost dropped by over 80%


There are scores of such cases, where correction of drum level improved the Generation as well as steam dryness properties.


Example 5 : A 70 TPH AFBC boiler had frequent tube failures & complained excessive erosion of bed coils even though the bed coils were studded


Reason was informed that they maintained very low drum level of 35% to 40% which had to be increased to 70 or 75%


What is happening ?
 
Where the drum level is low the Steam demand as well as steam parameters are unable to be achieved well. Even if achievement was possible, steam was wet & condensation losses also had to be produced increasing steam demand.


Most recent Boiler installations have witnessed reduction in Boiler Drum Sizes which are aggravating the above situation.


Small drum boiler installations are the easiest target for high erosion of tubes.


What are the dangers of small drum sizes, operating in Drum Condition 1 & Drum Condition 2 ?


The small drum size brings alive the situation where the water recirculation rate in the Boiler is low.

In small drum boiler installations the steam consumption per MW is higher & keeps wavering than prescribed values & such installations are more prone to tube failures.


Low water recirculation rate increases the possibilities of heat reception by the tubes & tube surfaces getting overheated & further opening up the possibilities of increased Erosion, Departure from Nucleate Boiling (DNB), leading to early tube failures ? Why ?


When Steel is in hot condition or tube surface is very hot, it becomes very soft & hence very vulnerable to erosion.


Alternatively if the tube surface is cold, it is always strong & has negligible vulnerability to erosion.

In almost all the cases where early tube failures have registered, they all lead to one common observation, low drum level operation or smaller size of the drum.

I myself have suggested many FBC & AFBC boilers to increase the drum level to 75% & operate. The result is they have no tube failures over past 5 to 7 years. 


The trick is to keep the tube surface cold i.e. enable increased water recirculation i.e. water should evacuate the heat & the tube should not be the recipient of the heat.


Increased Steam dryness


Ensure the drum size is large enough to hold 50 to 60% of generating capacity then @ 50% drum level 25 to 30% water will hold weight in the drum.


Operate drum level at 70 or 75% to increase water recirculation rates


Steam consumption will also lower & tube life also will be saved.

Hope you got some answers.

Thank you, for the attention
 
If you have any questions please write to me at  sap@chargewave.in

 

PS Anand Prakash

Director Technical

 

Chargewave Energykem Pvt. Ltd.