comp air design guide
TRANSCRIPT
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DESIGN GUIDE
FOR
COMPRESSED AIR SYSTEM
PROJECT ENGINEERING MANAGEMENTNEW DELHI
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INDEX
S.NO DESCRIPTION PAGE NO.
1.0 AIM OF THE DESIGN GUIDE 2
2.0 SCOPE OF THE DESIGNGUIDE 2
3.0 SYSTEM DESCRIPTION 3
4.0 SIZING OF COMPRESSOR 4
5.0 TYPE OF COMPRESSORS 9
5.1 RECIPROCATING V/s SCREW COMPRESSORS 9
5.2 LUBRICATED V/S. NON-LUBRICATED 10
5.3 SINGLE V/S. MULTI STAGE 10
6.0 COMPRESSOR OUTLET PRESSURE 11
7.0 CAPACITY CONTROL 11
8.0 SIZING AND NO. OF AIR RECEIVERS 13
9.0 AIR DRYING PLANT 14
10.0 DISTRIBUTION PIPING AND JOINTS 15
11.0 DRAIN POINTS 16
12.0 VALVES 17
13.0 CODES AND STANDARDS 17
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1.0 AIM OF THE DESIGN GUIDE
With the growth of automation in Power Engineering Instrumentation practice, Measurement
and process control has attained relevant importance in the design of systems. This control
system can be actuated either by pneumatic or electronic controllers. By far, the pneumatic
controllers are preferred for its simplicity and economy.
In addition to the compressed air being used for control (termed as Instrument Air - IA), it is
also used for purposes such as atomising fuel in a fuel gun, scavenging a fuel gun for
maintenance purposes power for driving pneumatic appliances (termed as Service Air-SA).
The aim of this design guide is to identify the compressed air requirement of major associated
equipment in a Thermal power station, Gas turbine plant and Combined cycle power plant
and to evolve guidelines for selection of design parameters of the compressed air system.
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2.0 SCOPE OF THE DESIGN GUIDE
This design guide with the different parameters of a compressed air system, which are
listed below: -
- Sizing and No. of air compressors.
- Types of Compressors.
- Compressor Outlet Pressure.
- Capacity Control of Compressors.
- Sizing and No. of air Receivers.
- Air Drying Plant.
- Drain Points.
- Valves
.
This design guide is discussed under the following heads:
- State of Art.
- Analysis & Recommendation.
State of art
This includes a brief discussion on the practices being followed specially by Indian
consultants, namely DCL and NTPC in the selection of the various design parameters
of a compressed air system. A station capacity of two (2) units of 210/500 MW each
has been considered while working out an example on compressor sizing.
Analysis & Recommendations:
The merits and demerits of the various prevalent practices that are controversial in
nature and discussed above under state of art are analysed. Based on this analysis
recommended practices to be adopted are made for implementation. It is suggested
that BHEL's view point in this matter are impressed upon the customer during
discussions with them.
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3.0 SYSTEM DESCRIPTION:
A compressed air plant comprises of the following:a) Air compressor
b) Compressor drive, which is generally an electric motor.
c) Inter and after coolers.
d) Air receiver
e) Air drying plant.
f) Distribution piping with valves, drain traps, etc.
The compressor is the heart of the plant which sucks ambient air through suction
filter-cum-silencer. The compressor can be horizontal or vertical, single or multistage,
reciprocating or screw and can be air or water cooled. The compressor is driven by an
electric motor either directly coupled or through `V' belts.
When ambient air is compressed in the compressor, enormous heat will be added to
the air due to compression. The resulting air temperature may be as high as 160C,
which, if directly used, will be harmful to the equipment. Hence in case of a
multistage compression, the compressed air is cooled in an intercooler and after final
compression, is cooled in an after cooler. The inter and after coolers can either be air
or water cooled and if water cooled, can either be horizontal or vertical.
Immediately next to the aftercooler, a moisture separator which can be integral part to
aftercooler or separate is provided to remove the condensed moisture from the cooled
compressed air. An automatic drain trap is provided at the bottom of the separator,
which drains out periodically, the collected water.
The air, thus compressed and cooled, is stored in an air receiver before distribution.
The receiver, in addition to storage, also dampens the pulsations in air and will
condense and drain as much moisture as possible through auto/manual drain traps
provided.
From the air receiver, service air is taken to the various consumer points directly
whereas the instrument air is taken through an Air drying plant. The purpose of the
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Air drying plant is to remove the moisture from the air, which otherwise gets
condensed on the working parts of the actuators and hampers the equipment. Now a
days to maintain the interchangability of air in case of emergency is envisaged and
subsequently the service air is also made moisture free. The air is dried to a dew
temperature of - 40o C.
Air Drying Plant generally used in power station are of following type
1. Reactivated Blower Type
2. Heat of Compression
3. Heatless Dryer Type
4. Refrigerated type
.
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4.0 SIZING OF COMPRESSOR.
The practice of sizing of on I-A compressor for a station capacity of two units (2x210 MW) isto consider the total continuous & intermittent requirement for each unit. Simultaneity factor
of 0.4 is considered for intermittent requirement. In addition to this a margin of 25% for
leakage, wear & tear and contingency is taken for sizing the compresses.
Service air requirement is generally less than Instrument air requirement, but compressor of
service air is select same as that of instrument air keeping in mind the contingency
requirement in case of emergency.
Analysis & Recommendation:
For the 2x100%/200 MW unites, it is recommended to provide capacity compressors, so that
one will act as a standby. Reciprocating v/s screw compressors. In power plants, generally
positive displacement compressors are installed for compressed air system because of large
flows and high pressure characteristics.
Wherever possible, it is advisable to provide same capacity compressors for IA & SA duties
for interchangeability and advantage of common spares.
Various Consumption Points in Power Plant: -
Typical Instrument Air Requirement for 500 MW
(Continuous and Intermittent)
S.NO DESCRIPTION Eqpt.
per
boiler
Cont./
Inter
QTY
Inter(per
opn.)
QTY Cont
(NM3/ min)
Total QTY
Cont (NM3/
min.)
1 Burner Tilt P/C 4 Cont - 0,22 0,88
2 Sec. air damper 88 Cont - 0,086 6,88
3 I/P converter for SADC 22 Cont - 0,006 0,12
4 Scanner air emer. Damper 1 Inter 0,018 - -
5 Scanner air fan dis. Damper 2 Inter 0,018 - -
6 Seal air fan dis. Damper 2 Inter 1,018 - -
7 Seal air filter DP controller 2 Cont. - 0,03 0,06
8 Hot air regulating dampers 10 Cont - 0,09 0,81
9 Cold air regulating dampers 10 Cont - 0,09 0,81
10 Hot air shutoff gate 10 Inter 0,018 - -
11 Cold air shut off gate 10 Inter 0,02 - -
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12 LO trip valve 1 Inter 0,015 - -
13 HO pressure control valve 1 Cont - 0,03 0,06
14 HO pressure control valve I/P
converter
1 Cont - 0,0025 0,005
15 HO temperature control valve 3 Cont - 0,03 0,18
16 HO temperature control valve I/P
converter
3 Cont. - 0,03 0,18
17 HO cooler temperature control valve 1 Cont. - 0,03 0,03
18 HO cooler temperature control valve
I/P converter
1 Cont. - 0,03 0,03
19 Atomising steam press. reducing
valve
1 Cont - 0,03 0,06
20 HO recirculation line trip valve 1 Inter 0,015 - -21 HO trip valve 1 Inter 0,015 - -
22 HO flow control valve 1 Cont - 0,03 0,06
23 Corner nozzle valves 68 Inter 0,03 - -
24 Ignitor advance - Retract mechanism 20 Inter 0,006 - -
25 SH spray control valve 4 Cont - 0,033 0,132
26 SH Block Valve 1 Inter 0,0174 -
27 RH block valve 1 Inter 0,0174 - -
28 RH spray control valve 4 Cont - 0,033 0,132
29 SB steam PRV 2 Cont - 0,033 0,066
30 AH DA Head valve 4 Inter 0,015 - -
31 SB steam drain temp. control valve 5 Cont - 0,03 0,18
32 LO pressure control valve 1 Cont - 0,03 0,06
33 LO pressure control valve I/P
converter
1 Cont - 0,0025 0,005
34 LO flow control valve 1 Cont - 0,09 0,18
35 Seal Air to mill discharge 10 Inter 0,09 - -
36 Seal Air to mill 10 Inter 0,09 - -
37 Seal Air to Feeder 10 Inter 0,09 - -
38 Feeder outlet gate 10 Inter 0,18 - -
39 Purge Meters 20 Cont - 0,028 0,504
40 AH outlet regulating damper 4 Cont - 0,22 0,88
41 Purge Air for Flue gas & mill
pressure tappings
90 Cont. 0.028 2.52
42 Boiler Frame Analysing System 36 Cont - 0,18 6,48
C & I Requirement ( EDN Bangalore )
1 IA for oprn. of control valve/ E-P linestor Cont. 4.688
2 IA for SO2/NOx probe purging Inter 0,283
PIPING CENTRE ( MADRAS )
1 Primary SCAPH steam controlvalve 2 Cont 0,05 0,1
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I/P Converter 2 Cont 0,16 0,32
2 Secondary SCAPH steam controlvalve
2 Cont 0,05 0,1
I/P Converter 2 Cont 0,16 0,32
3 CBD level control 1 Cont 0,05 0,05
CBD I/P Converter 1 Cont 0,16 0,164 Primary Scaph drain level control 1 Cont 0,05 0,05
I/P Converter 1 Cont 0,16 0,16
5 Secondary Scaph drain levelcontrol
1 Cont 0,05 0,05
I/P Converter 1 Cont 0,16 0,16
BFP(Hyderabad)
1 BFP Recirculation ValvePneumatic Actuator
3 Cont. 0.066 0.198
Miscellaneous
1 Ash Handling Plant Cont 5
2 Mill Reject System Cont 1.25
3 DM Plant Cont 3
Typical Service Air Requirement for 500 MW
(Continuous and Intermittent)
S.NO DESCRIPTION No.. Per
boiler
Cont./
Inter/
startup
QTY per
eqpt
(NM3/hr)
Total QTY
per Boiler
(NM3
/ Hr)
1 Atomising air for LDO firing 4 During
Start
up
350 1400
2 Pulveriser Coal sampler 2 Inter 35 70
3 HEA Ignitor 20 Cont 27 540
4 Feeders bulls eye cleaning. 10 Inter 1 10
5 Furnace temp. probe 2 Inter 600 1200
6 Regenerative air heater air motor -
-primary 2 Emerg 405 810
- secondary 2 Emerg 405 8107 Regenerative air heater air blower -
-primary 2 Inter 640 1280
- secondary 2 Inter 640 1280
8 Air Heater fire sensing device 4 Cont. 1 4
9 Acoustic Pyrometer Cont - 408
10 Furnace Observation door
- Bleed Air 90 Cont. 2 180
- Air Curtain 4 Inter 20 ---
C & I Requirement ( EDN Bangalore )
1 Purging of transmitter lines in boiler linese Inter. 8.55
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Miscellaneous
1 Mill Reject System Cont. 5
CUSTOMER REQUIREMENT
TOTAL (Cont.)
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Notes:
1. Air consumption listed above is typical for any 2X 210/500 MW Thermal power station
with requirement varying from case to case.
2. Air requirement during initial start up & commissioning has not been consumption in the
consumption for sizing of compressor for e.g.,
For purging operation (generator at stand still)
For air leakage during commissioning
For pressure testing after terminal box welding
Atomising air for LDO firing
Regenerative air heater air motor
Regenerative air heater air blower
3. Acoustic Pyrometer requirement is considered in calculation of Service air requirement
4. Air requirement for Ash Handling plant and Mill reject system may be considered in the
sizing of compressor, in case customer asks for.
5. Coal Flow measurement requirement is also to be considered in calculation of instrument
air Requirement (Excluded in this case)
6. Customer Requirement is also to be taken into care.
7. For sizing of compressor capacity, we take a factor of 0.4 for intermittent requirement
with an overall margin of 25 % on the total air requirement. For e.g., In Bakreswar, we
have compressor of 15NM3/ min and for Simhadri 30 NM3/ min capacity.
TO CONCLUDE THE FOLLOWING ARE RECOMMENDED:
1. The IA and S.A compressor shall be sized for the total of all continuous
requirements plus the intermittent requirement of one unit.
2. For each stage of power station two (2) 100% IA and two (2) 100% SA
compressors shall be proposed (one (1) working and one (1) standby).
4. 25% margin of compressor wear & tear, leakage etc., shall be added to the
requirement in all cases.
5. Identical and equal capacity compressors will be considered, wherever
possible.
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5.0 TYPE OF COMPRESSORS
5.1 Reciprocating V/s Screw compressor.5.1.1 State of art
(a) Reciprocating compressor - In this type of compressor, we generally have a
decrease in volume of air, resulting in increase of pressure by positive
displacement of piston. Due to reciprocating action of piston, gas (air)
compresses and pressure is increased.
(b) Screw compressor - Screw compressor to basically a twin Helical lobe
compressor, in which both lobes rotate simultaneously and compress the air/gas
in between. The advantage of the screw over reciprocating compressors, is the
economy with regard to
Lesser space requirement.
Lesser expensive foundation.
Lower creation time.
Lower maintenance cost.
5.1.2 Analysis and Recommendation
In present scenario consultants like NTPC and DCL are preferring Screw
compressors over reciprocating compressors due to aforesaid reasons.
5.2 Lubricated v/s Non-lubricated:
5.2.1 State of Art:
Instrument air compressors are required to be non-lubricated type since pressure of
oil is harmful and affects the performance of the diaphragm operated controllers. TheI.A. compressors are designed to be oil free by provision of Teflon rings instead of
the conventional oil rings.
5.2.2 Analysis & Recommendations:
It is essential that the instrument air compressors should be oil free type. In case of
service air, even though oil free air is not a must, still certain advantages as under will
be obtained if made oil free like Instrument air compressors.
a) The traces of oil carried over settles in the distribution piping and in course of
time may choke the lines.
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b) The inter-connection between the IA system & SA system is possible without the
necessity to include the oil separator, which increases the multiplicity of the IA
system.
c) In case of identical service and instrument air compressors, spare inventory is
reduced.
Hence it is recommended that the service air compressor also may be of non-
lubricated type.
5.3 Single stage vs. Multiple stage:
5.3.1 State of art
The pressure of say 8.0 kg/cm2 (g) can be achieved with a single stage compression
from atmospheric to 8.0 kg/cm2 (g) or compress it to an intermediate pressure of
about 4 kg/cm2(g) in the first stage, cool it, then compress it to 8.0 kg/cm2 (g) in the
second stage.
5.3.2 Analysis & Recommendation:
The advantage obtained in two stage compression is that because of the interstage
cooling, and the lesser compression ratio in both the stages, the temperature of air
attained will be about 140C, whereas in case of single stage compression the
temperature attained will be about 210C. Handling of air at lesser temperature has its
own advantages and results in better life and better performance of the machine.
Another advantage of two-stage compression is the lesser HP required at the shaft of
the compressor.
It is therefore recommended that a two-stage compressor be provided for both LA &
SA compressors with a discharge pressure greater than 7 kg/cm2 (g) except for very
small capacity compressors.
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6.0 COMPRESSOR OUTLET PRESSURE:
6.1 State of art:Most of the controllers and other consumer points of the compressed air system
require a pressure of 7 kg/cm2 (g) for their proper operation. Allowing a line pressure
drop of 0.5 kg/cm2 upto the farthest point (for 2 units) and a further pressure drop of
0.5 kg/cm2 in the air drying plant, after cooler, inter cooler, vales etc., discharge
pressure of the compressor is to be around 8.0 kg/cm2 (g).
6.2 Analysis & Recommendation:
It is recommended that both IA & SA compressor (s) shall have a discharge pressure
of 8 kg/cm2 (g).
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7.0 CAPACITY CONTROL OF COMPRESSORS:
7.1 State of art:(A) Reciprocating Compressor
The capacity of a compressor can be controlled by two methods termed as `Dual
Control'.
Load/unload method
ON/OFF method
Load/unload method:
In this mode the compressor runs continuously but its capacity is controlled by
loading and unloading a set of cylinder by lifting the suction valve plates off their
seats, which is affected through an electric means. Once the desired pressure setting
in the pressure switch (s)/, these cylinders are loaded again on fall of pressure. The
Drawing `Scheme of Controlling Air Compressor' indicates a scheme where in the
capacity control is achieved in steps of 100%, 50% and 0% through the solenoid
valves SV1 & SV2 which are energised to open at set valves by means of pressure
switches installed on the outlet header of each compressor. This allows the air from
the air receivers to the unloading valves of the compressor thus unloading the
compressor 50% & 100% respectively. Similarly, the loading in steps of 50% &
100% is achieved when the solenoid valves SV1 & SV2 are closed successively.
On-off method
In this mode the compressor is started or stopped automatically depending on the air
pressure. The pressure switches mounted on each compressor outlet header give
signal to compressor motor starter thereby starting and stopping compressor motor.
(B) Screw Compressor
Capacity Control for Screw Compressor.
a) The capacity is regulated by throttling the air inlet and at the same time, opening a
blow off valve. Under load the blow-off valve is closed and the throttle valve fully
open. The unloading device is operated by a spring loaded automatic air relay of the
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same type as that used.
b) Slide valve control for screw compressor.
This type of control incorporates a third bars in the compressor casing, at the cusp of
the rotor bars. In this bars is positioned a slide valve at the discharge side of the rotor,
enabling recycling of the partially compressed air.
As the compressor is off-loaded, the valve moves progressively opening up parts,
which connect the compressed air back to inlet. This form of control is only suitable.
when the compressor is driven by a constant speed device such as an electric motor.
In case of screw compressor, we have normally 0-100% control system as compared
to reciprocating compressor where we have 0-50-100% capacity control.
7.2 Analysis & Recommendation:
In case of intermittent demand of compressed air the fall and rise in pressure will be
very rapid and in case of the ON/OFF mode the compressor motor is subject to
frequent start and stop which will naturally be harmful to the motor and cable and the
compressor will have reduced life in view of the frequent stress imposed on it.
The IS. No. IS-6206-1971 entitled Guide for Selection, installation and maintenance
of air compressor plants with operating pressures up to 10 Bars. states Automatic start
- stop operation of motor is also possible but its use shall depend upon the size of the
motor, the nature of the application, and local conditions of power supply.
The following are recommended.
a) `Dual Control' with the load/unload control being, `Main' and ON/OFF as
`Standby' shall be provided.
b) The Load/Unload system shall have both electric and mechanical means of
load/unload, the mechanical as a standby to the electrical one. In case of failure of
the pressure switch or the solenoid valve associated with the electrical system
(which is quite frequent), the air regulator path i.e. `Mechanical' control can be
opened manually through a selector, switch.
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c) Automatic changeover to `Mechanical' in case of failure of control supply is not
recommended and an alarm then will be initiated to indicate `Control supply
failure.
d) Loading & unloading in steps of 0%, 50% & 100% are only recommended.
e) The compressors in whatever mode of capacity control shall be able to start only
in unloaded condition.
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8.0 SIZING AND NO OF AIR RECEIVERS:
8.1 State Of Art:
When the boiler/turbine unit trips due to power failure the compressors also trip. But
however, compressed air is required for bringing down the essential control/trip
valves to their original position and also emergency running of regenerative air heater
air motor. These requirements are met from the air receiver.
.
8.2 Analysis and Recommendation:
The following are recommended:
i) The total live I.A. and S.A. receivers(s) capacity shall be sized based on storing
ten (10) minutes of total working Instrument air compressor (s) capacity or as per
customer requirement taking into account the resultant increase in volume of air
due to fall in pressure from receiver pressure of 8 kg/cm2 (g) which being the
minimum pressure at which most of the instruments can operate.
ii) Each compressor whether IA or SA shall have an individual receiver attached to
it for dampening the vibrations of compressed air. There shall be a common air
header after the receivers and no valve shall be provided on the inlet side to the
receivers.
iii) All receivers shall preferably be located outdoors, adjacent to the compressor
plant.
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9.0 AIR DRYING PLANT:
9.1 State of art:
The air Drying Plant is needed for providing moisture free compressed air which is
required for the instrument controls. The present day air drying plant are capable of
drying air to a dew point of -40C. This quality of air is generally acceptable to all
the instruments and controllers.
.
(a) Blower Regenerated No loss Type Drier - The regeneration air from centrifugal
blower at low pressure is allowed to pass through an electrical heater of pre-
determined rating. The hot air which has got very high moisture holding capacity
picks up the absorbed moisture from saturated desiccant bed and vented to
atmosphere.
(b) Heat of compression: -
(i) Full flow - Hot air from last stage compression is bed into the absorber vessel
for regeneration and then same air is cooled in a specially designed combination cooler. After
cooling; this air goes through the second absorber where the moisture goes absorbed in
activated bed and dry air goes out.
(ii) Split Flow: - The main compressed air stream passes through after cooler,
then into drying section, and finally out of the dryer into the dry compressed air net. All
moisture is removed through adsorption by the silica gel powder on the glass fibre based
paper drum. The regeneration airstream by passes the after cooler and in lead is shunted into
the regeneration section. The regenerated airflow is mixed with the main flow in the ejector
nozzle.
.
9.2 Analysis & Recommendation:
Earlier non-purged air flow, desiccant type, air drying plant with separate blower and
suction filter arrangement was recommended but know a days due to advent of screw
compressors in the compressed air system, Heat of compression type Air drying plant is being
asked by the customer.
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10.0 DISTRIBUTION PIPING & JOINTS:
10.1 State of Art:
The compressed air piping can either be of mild steel galvanised. It is an obvious fact
that galvanised pipes are superior to black pipes, because of its corrosion resistance. The cost
difference between a black and galvanised compressed air pipework, which are of small bore,
is very insignificant and hence galvanised piping for both IA and SA pipework are preferred.
.
To conclude the following are recommended:
a) Galvanised pipes for both IA & SA with screwed joints as per BS: 21:1973
only with Teflon tapes, if necessary.
b) For site threading a coat of zinc epoxy shall be applied both inside and
outside after due cleaning of the part.
c) The distribution piping shall be run over ground with drain points at suitablelocation.
11.0 DRAIN POINTS:
11.1 State of Art:
At convenient intervals (preferably 30 m) in the distribution pipework, drain points
are to be provided at operating level, this is with a view to drain out periodically, the water
collected in the pipework, this can be through a normally closed globe valve, which can be
opened manually at periodic intervals, or an automatic drain trap.
In case of auto drain traps, they get clogged due to dirt in course of time and hence do
not function properly. It, therefore, becomes necessary to provide manual bypass
arrangements.
11.2 Analysis & Recommendations:
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It is recommended to provide only Manual Drain Traps in compressed air system at,
accessible & visible locations at periodic intervals not exceeding 30 M at the lowest points.
The pipe should be given a fall of not less than 1 M in the direction of air flow.
However, in the compressor house where the piping is run in trenches, auto drain
traps with manual Bypass shall be provided.
12.0 VALVES:
12.1 State of Art:
The following types of valves are being used in the compressed air lines:
a) Gate valve
b) Globe valve
c) Plug valve
d) Ball valve
13.0 CODES AND STANDARDS:
1. IS- 2825/1969: Code for unfired pressure vessels.
2. IS- 4503/1967: Shell and Tube Type Heat Exchanger
3. IS- 5456/1985: Code of practice for testing of positive displacement type air
compressors and exhausters.
4. IS- 5727/1981: Glossary of terms relating to compressors and exhausters.
5. IS- 1239/1990: Mild steel tubes, tubular and other wrought steel fittings (Part - 1)
6. IS- 1239/1992: Mild steel tubes, tubular and other wrought steel fittings (Part - 2)
7. IS- 6206/1985: Guide for selection, installation and maintenance of air compressors/
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