TATA CONSULTING ENGINEERS LIMITED COVER PAGE AN OVERVIEW OF BOILER DRUM & ITS LEVEL MEASUREMENT & CONTROL SHEET i OF i AN OVERVIEW OF BOILER DRUM & ITS LEVEL MEASUREMENT & CONTROL Date: 02-02-2011 TCE Office Code: DK Author: Guided By- Mr. Arun Kumar Maiti (M5M124) Mr. Sugata Bnadyopadhyay (M4B329) Mr. Amitava Sengupta (S1S837) Mr. S.B. Mukherjee (M6M227)
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
TATA CONSULTING ENGINEERS LIMITED
COVER PAGE
AN OVERVIEW OF BOILER DRUM & ITS LEVEL
MEASUREMENT & CONTROL
SHEET i OF i
AN OVERVIEW OF BOILER DRUM & ITS LEVEL
MEASUREMENT & CONTROL
Date: 02-02-2011
TCE Office Code: DK
Author: Guided By-
Mr. Arun Kumar Maiti (M5M124) Mr. Sugata Bnadyopadhyay (M4B329)
Mr. Amitava Sengupta (S1S837)
Mr. S.B. Mukherjee (M6M227)
TATA CONSULTING ENGINEERS LIMITED
CONTENT SHEET
AN OVERVIEW OF BOILER DRUM & ITS LEVEL
MEASUREMENT & CONTROL
SHEET i OF i
SECTION TITLE NUMBER OF
PAGES
1.0 INTRODUCTION
1
2.0 PURPOSE OF DRUM IN BOILER
2
3.0 DRUM INTERNALS & FITTINGS ARRANGEMENTS
4
4.0 CODES & STANDARDS
5
4.1 PG-60 Requirements for Miscellaneous Pipe, Valves and Fittings
6
5.0 DRUM DISHED END INSTRUMENT TAPPING POINTS
9
6.0 METHODS OF DRUM LEVEL MEASUREMENTS
11
6.1 Direct Reading Devices
11
6.2 Indirect/Remote indication
14
7.0 DENSITY ERROR CORRECTION IN DRUM LEVEL INDICATION
16
8.0 DRUM LEVEL CONTROL SYSTEM
21
8.1 Measurement
22
8.2 Control
22
9.0 INSTALLATION PROCEDURE
32
10.0 CONCLUSION
33
11.0 REFERENCES
33
TATA CONSULTING ENGINEERS LIMITED
AN OVERVIEW OF BOILER DRUM & ITS LEVEL
MEASUREMENT & CONTROL
PAGE 1 OF 34
1.0 INTRODUCTION
Boiler steam drum water level is one of the most important power plant parameters to
measure and control. If the level is too low, boiler tubes will be damaged by overheating.
If the level is too high, the super heater tubes and the turbine may be damaged by
moisture or water treatment chemical carryover. As boiler operating pressures and boiler
drum wall thickness have increased, many boiler steam drums have become smaller.
This reduced boiler drum volume demands even more accurate level control. A variety
of instruments is available and approved by the ASME for power boiler level indication.
However, simply specifying one or more of these instruments will not guarantee that the
boiler level will be indicated accurately. So it is required to thoroughly understand the
operating principles of each instrument, instrument installation requirements and the
boiler operating scheme. A difference in the water density between the level instrument
and the boiler is the major source of level error. The National Board of Boiler Inspectors
has repeatedly reported that low water conditions in boilers is the number one cause of
boiler related accidents.
Fig. 1. Steam Drum.
TATA CONSULTING ENGINEERS LIMITED
AN OVERVIEW OF BOILER DRUM & ITS LEVEL
MEASUREMENT & CONTROL
PAGE 2 OF 34
2.0 PURPOSE OF DRUM IN BOILER
The boiler water/steam drum, or steam drum, is an integral part of the boiler’s design.
This vessel has three specific purposes.
Provide a volume space to hold the boiling water in the boiler.
Provide enough water volume to allow for good thermal mixing of the
cooler bottom drum water with the hotter surface interface water.
Provide surface area and volume for the efficient release of the entrained
steam bubbles from the boiler water.
The boiler water/steam drum also provides a logical location for
Addition of feedwater,
Addition of chemical water treatment
Surface blowdown, which helps reduce the surface tension of the
water/steam interface to allow better steam release.
The steam drum (fig. 1) is a cylinder, located at the top of the boiler. It runs length wise
from the Left side to the Right side of the boiler front side. The steam drum provides a
space for the saturated steam generated in the tubes and for the separation of moisture
from the steam. (Saturated steam is steam that has not been heated above the
temperature of the water from which it was generated). The steam drum also serves as
a storage space for boiler water, which is distributed from the steam drum to the down-
comer tubes. During normal operation, the steam drum is kept about half full of water.
The steam drum either contains or is connected to many of the important controls and
fittings required for the operation of the boiler.
Made from high Carbon Steel with high tensile strength and its working involves
temperatures around 390oC and pressures well above 350 psi (2.4MPa). The separated
steam is drawn out from the top section of the drum and distributed for process. Further
heating of the saturated steam will make superheated steam normally used to drive a
steam turbine. Saturated steam is drawn off the top of the drum and re-enters the
furnace in through a super heater. The steam and water mixture enters the steam drum
through riser tubes; drum internals (fig. 2) consisting of demister separate the water
droplets from the steam producing dry steam. The saturated water at the bottom of the
TATA CONSULTING ENGINEERS LIMITED
AN OVERVIEW OF BOILER DRUM & ITS LEVEL
MEASUREMENT & CONTROL
PAGE 3 OF 34
steam drum flows down through the down-comer pipe, normally unheated, to headers
and water drum. Its accessories include a safety valve, water-level indicator and level
controller. Feed-water of boiler is also fed to the steam drum through a feed pipe
extending inside the drum, along the length of the steam drum.
Fig. 2. Steam Drum Internal Arrangements.
A steam drum is used without or in the company of a mud-drum/feed water drum which
is located at a lower level. A boiler with both steam drum and mud/water drum is called a
bi-drum boiler and a boiler with only a steam drum is called a mono-drum boiler. The bi-
drum boiler construction is normally intended for low pressure-rating boiler while the
mono-drum is mostly designed for higher pressure-rating.
TATA CONSULTING ENGINEERS LIMITED
AN OVERVIEW OF BOILER DRUM & ITS LEVEL
MEASUREMENT & CONTROL
PAGE 4 OF 34
Low water levels affect the internal thermal recirculation of the boiler water resulting in
cold spots in the boiler water and steam collapse. This lack of circulation also reduces
the effectiveness of the chemical water treatment and can cause precipitation of the
chemicals as chemical salts or foams.
High water levels raise steam exit velocities and result in priming or boiler water
carryover into the distribution system. Priming results in wet dirty steam while carry-over
can result in dangerous water hammer and pipe or equipment damage.
3.0 DRUM INTERNAL & FITTING ARRANGEMENTS
The internal fittings in the steam drum help distribute the water evenly throughout the
drum, separate the generated steam from the water and remove moisture from the
steam before it leaves the boiler (fig. 3).
a) Lower baffle plates or apron plates separate the incoming feed water and
generated steam and direct the steam to the separators.
b) Primary separators (cyclone separators) separate most of the water from
the steam by giving it a cyclone or rotary motion so that the water
particles are expelled from the steam by the centrifugal forces. These
separators are vertically mounted in the steam drum so that the steam
rises out the top and the water falls back into the steam drum.
c) Secondary separators (chevron dryers) remove additional moisture from
the steam by changing the direction of steam flow several times. The
steam passes on but the moisture cannot make the direction change with
the steam. These separators are mounted above the primary separators
and direct steam to the dry box which collects the steam at the top of the
steam drum, directing it to the steam outlet piping to the super heater.
d) Feed water leaves the economizer and enters the boiler through the
internal feed pipe and becomes "boiler water." Perforations along the side
of the feed pipe allow water to be distributed evenly throughout the steam
drum.
TATA CONSULTING ENGINEERS LIMITED
AN OVERVIEW OF BOILER DRUM & ITS LEVEL
MEASUREMENT & CONTROL
PAGE 5 OF 34
e) Since suspended solids may accumulate on the surface of the water in
the steam drum, there must be means of removing them. The surface
blow pipe is used to remove these light suspended solids from the
surface of the water and to reduce the total dissolved solid content of the
boiler water. Suspended solids usually consist of oil, salt contaminants, or
excessive treatment chemicals which can cause foaming on the water
surface. Dissolved solids usually consist of salt contaminants and
treatment chemicals that are in solution
Fig. 3. Arrangement of Internal fittings in a Single Furnace Boiler.
TATA CONSULTING ENGINEERS LIMITED
AN OVERVIEW OF BOILER DRUM & ITS LEVEL
MEASUREMENT & CONTROL
PAGE 6 OF 34
4.0 CODES & STANDARDS
The requirements for power boiler drum level indication are clearly stated in the ASME
Boiler and Pressure Code Section I, Paragraph 60.1.1. In addition to the direct reading
gauge glass, remote liquid level indicators of various types are permitted under certain
design conditions. These guidelines give the minimum level of protection considered
suitable for safe equipment operation.
4.1 PG-60 Requirements for Miscellaneous Pipe, Valves and Fittings
60.1 Water Level Indicators (Refer Fig.4.a,b,c)
60.1.1 Each boiler, except forced-flow steam generators with no fixed steam and water
line, and high temperature water boilers of the forced circulation type that have no
steam and water line, shall have at least one water gauge glass. Except for electric
boilers of the electrode type, boilers operated at pressures over 400 psi shall be
provided with two water gauge glasses which may be connected to a single water
column or connected directly to the drum.
Two independent remote level indicators may be provided instead of one of the two
required gauge glasses for boiler drum water level indication in the case of power boilers
with all drum safety valves set at or above 900 psi. When both remote level indicators
are in reliable operation, the remaining gauge glass may be shut off, but shall be
maintained in serviceable condition.
When the direct reading of gauge glass water level is not readily visible to the operator
in the area where immediate control actions are initiated, two dependable indirect
indications shall be provided, either by transmission of the gauge glass image or by
remote level indicators. A gauge glass image transmitted to the operator’s working area
by means of a fibre optic cable, with no electrical modification of the optical signal is
considered to provide direct reading of the gauge glass water level.
The lowest visible part of the water gauge glass shall be at least 2” (50mm) above the
lowest permissible water level, as determined by the boiler manufacturer. When remote
level indication is provided for the operator in lieu of the gauge glass, the same
TATA CONSULTING ENGINEERS LIMITED
AN OVERVIEW OF BOILER DRUM & ITS LEVEL
MEASUREMENT & CONTROL
PAGE 7 OF 34
minimum level reference shall be clearly marked.
PG-60.2 Water Columns
PG-60.2.1 The Water Column shall be so mounted that it will maintain its correct
position relative to the normal waterline under operating conditions.
PG-60.3 Connections
PG-60.3.1 Gauge Glasses that are required by PG-60.1 shall be connected directly to
the shell or drum of the boiler or to an intervening water column.
PG-60.3.2 The lower edge of the steam connection to a water column or gauge glass in
the boiler shall not be below the highest visible water level in the water gauge glass.
There shall be no sag or offset in the piping which will permit the accumulation of water.
PG-60.3.3 The upper edge of the water connection to a water column or gauge glass
and the boiler shall not be above the lowest visible water level in the gauge glass. No
part of this pipe connection shall be above the point of connection at the water column.
PG-60.3.4 Connections from the boiler to the water column shall be at least NPS 1.
Connections for gauge glasses connected directly to the boiler shall be at least NPS 1/2.
Connections from the boiler to the remote level indicator shall be at least NPS 3/4 and
including the isolation valve and from there to the remote level indicator at least 1/2 in.
O.D. tubing. These connections shall be completely independent of other connections
for any function other than water level indication.
TATA CONSULTING ENGINEERS LIMITED
AN OVERVIEW OF BOILER DRUM & ITS LEVEL
MEASUREMENT & CONTROL
PAGE 8 OF 34
Fig. 4. ASME Boiler and Pressure Vessel Code Requirements.
TATA CONSULTING ENGINEERS LIMITED
AN OVERVIEW OF BOILER DRUM & ITS LEVEL
MEASUREMENT & CONTROL
PAGE 9 OF 34
Some typical specifications have been provided relating to various different boiler
pressures and types of trim equipment. Each of these specifications is followed with a
short list of issues relating to the system design or equipment selected (refer Table-1).
This section might be worth considering when choosing a specific design concept.