2.8.18 Instrument Air Compressors and Drives
Air compressors shall be designed for continuous operation with high efficiency to satisfy the system requirements. Satisfactory operation in parallel shall be ensured without any uneven load sharing, undue vibration, noise etc.
The design shall incorporate every reasonable precaution for the safety of all operation and maintenance personnel. Each compressor unit should have all moving parts protected by a guard.
The scope of supply shall include all the necessary compressed air plant complete with compressor coolers, necessary isolating and non-return valves, pressure relief valve, water separators, silencers and vent pipe work, which shall be ducted to a safe discharge outlet.
4×100% ( 2 working + 2 stand by) capacity instrument air compressors shall be provided for both units.
The output from each set of working compressors (as applicable), expressed as free air delivered (FAD) at ambient condition, shall be sufficient to meet the instrument air requirement for one unit and silo area air requirement such as silo bag filter cleaning valve actuation. etc. At least 10% margin shall be provided on compressor capacity over and above the maximum flow requirement. The bidder shall furnish detailed calculations of compressor sizing with the bid. A discharge pressure of 8 kg/cm2 with three steps of unloading and design ambient of 50 degree C ambient air temp and 100% RH shall be considered for sizing of the compressors.
Compressors shall be of reciprocating oil free type. All instrument air compressors and their drives provided under this contract shall be completely interchangeable.
The wall thickness of the cylinders shall be sufficient to withstand the maximum cylinder pressure and to permit successive re-boring without hazard.
Piston shall be provided with piston rings to seal the leakage of compressed air. The piston rings shall be self-lubricating material preferable graphite impregnated teflon. In case the cylinder is horizontal, suitable slipper rings shall be provided to avoid rapid wear and/or distortion of the piston ring.
Each compressor shall have an inlet filter (dry type) unit to protect the compressor. The filter inlet area should be large enough to ensure frequent filter changes are not required.
A crank shaft driven positive displacement lubricating pump shall be used for lubricating the bearings and any other parts needing lubrication.
Cooling water requirement, if any, shall be arranged by the bidder.
Bidder shall include an inlet and/or outlet silencer so that each compressor unit complies with the noise requirements.
2.8.18.1 Air Receivers
Each compressor shall be provided with an air receiver of ample size of so that delivered air pressure is kept within ±5% of rated pressure without excessive start/stop operation in the working cycle.
Air receivers shall be located convenient to compressor discharge. The receivers and associated fittings shall comply with BS:5169 and BS:1123 or other approved standards.
The receivers shall each be supplied complete with test certificate, safety relief valve, fusible plug, pressure gauge, flanged inlet and outlet connections, inspection manhole (in accordance with Clause 25.2.4 of BS:5169-1975, or similar international equivalent), screwed drain connection, automatic drain trap with balance line, drain trap isolating valve, balance line isolating valve and manual drain valve, nameplate, support frame or legs, and a set of suitable foundation bolts.
As far as possible the design, manufacture and performance of air receivers shall be in accordance with the following latest applicable standards :
1) IS : 2825 – Code for unfired pressure vessels.
2) ASME-Sec. VIII Div.-1 Code for unfired pressure vessels.
3) BS : 487 – Fusion welded steel air receivers.
4) IS : 7936 – Air receivers for compressed air installations.
2.8.18.2 Water Separators
The Bidder shall supply water separators complete with automatic drain traps. Each separator shall be rated for twice the individual compressor flow at 1 bar above maximum operation pressure. Each water separator shall have flanged connections.
2.8.18.3 Air Drying Plant
One no. each for each compressor Air drying plant shall receive compressed air saturated with moisture. They shall be capable of operating continuously to provide reliable, moisture free compressed air supply. Dew point of the outlet air measured at the operating pressure shall be as mentioned in the data sheet or lower throughout the working of the drying plant.
The adsorber tower shall be sized for a drying cycle (i.e. the time any particular tower is adsorbing) of period as specified (8 hours). After this period the adsorber tower will be put under reactivation operation while the other will take over the duty.
The above stated change-over from drying duty to reactivation or vice-versa shall be fully automatic with manual over ride facility.
The four way valves shall not be used in the air drying plant piping circuit for changeover facilities.
Air drying plant shall be complete with all pipings, valves, instruments and other accessories as required for safe and satisfactory operation and reactivation of the plant. Open through system shall be used for reactivation.
Atmospheric air is filtered and is passed through the wet desiccant by a blower with the help of proper valve arrangements after heating it to required temperature by an electric heater. This moist air is exhausted to the atmosphere. Suitable solenoid and pressure reducing valves for slow depressurisation and re-pressurisation of the adsorber tower shall be provided for such case.
The filtering media for prefilter shall be cable/candle type ceramic filter designed to withstand at least 50% of the static pressure applied as differential pressure. Pre-filter shall be provided with isolation valves.
Quantity of desiccant shall be calculated giving the consideration to the residual moisture content at the end of regeneration cycle. Bidder shall submit detailed calculations in support of material and quantity of desiccant offered alongwith curves (with ref. to the source of these curves) indicating variation of adsorption capacities with temp. at various R.H. at working pressure of 8 kg/cm2(g). Bidder shall also indicate the residual moisture left in the desiccant after regeneration. For calculating the desiccant quantity, adsorption capacity and density to be considered shall not exceed 10% and 550 kg/m3 respectively. Dew point of outlet air shall be considered as (-) 40 degree C at 1 ata pressure.
Desiccant beds shall be designed with sufficient cross-sectional area for air flow resulting in low air velocity and consequently low pressure drop.
The adsorbent material shall be supported at top and bottom by stainless steel (AISI 304) 40 mesh strainer, on mild steel plates.
Adequate free board (minimum 20% of desiccant bed depth) shall be provided to dislodge the entrained particles from the adsorber.
The adsorber tower shall be provided with suitable inspection window made of glass/pespex for observation of the adsorbent conditions and desiccant filling and removal connection.
Each air drying plant shall be complete with suitable after filters in the down stream of the adsorber towers. After filters shall be provided with individual isolating valves, 100% standby or bypass valves. After filter shall prevent any carry-over of desiccant particles with the effluent air stream.
For reactivation air drying plant shall be provided with the air blowers and drive of sufficient capacity and pressure for supplying air.
Dry type air filter at the suction of blower shall be provided.
Heater shall be used to heat the air upto the required temp. for the purpose of reactivation of wet desiccant bed. Suitable temp. and pressure indication shall be provided at the outlet of the heater shell. The heater shall be fabricated as per IS:2825 or approved equal. The heater shall be thermostatically controlled, electric type and shall preferably of flanged type so that replacement burnt out heating element or other maintenance work is easy.
All pressure vessels such as moisture separator, prefilter and after filter casings, adsorber towers etc. shall be designed as per IS:2825 or approved equal. All vessels shall be Class-II type as per the said standard. Adequate provisions for manhole/handhole, relief valve, draining trap station shall be made. Internal surfaces of all the vessels shall be suitably protected against corrosion and rusting.
Material of construction shall be as follows:
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Compressor
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To suit service condition and as per relevant codes/standards
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Air receiver
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IS:226/IS:2002/IS:2062
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Inter Cooler and After Cooler
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Tube
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S.S.
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Shell
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SA 285 Gr. C
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Air drying plant
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Retention mesh for Silica Gel and other components coming in contact with moisture.
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SS
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2.8.18.4 Drive Motor
Drive motor shall be connected to the air compressors directly or through any suitable type of power transmission system.
The continuous motor rating (at 50°C) shall be at least fifteen per cent (15%) above the maximum load demand of the compressor in the entire operating range to take care of the system frequency variation. Total maximum load demand shall include air compression power plus any power consumed in auxiliaries like cooling fan etc. When the driver is not directly coupled to the compressor, due account shall be made for losses in power transmission in addition to the above 10% extra margin.
2.8.18.5 Instruments
Each Compressor shall be provided with the following instruments as a minimum:
(i) Pressure Gauges
a) with each air receiver for air pressure : 1 No.
b) after each stage (LP & HP) for air pressure : 1 No.
c) at the lubricating oil inlet for oil pressure : 1 No.
d) at the prefilter (air drying plant) inlet and
outlet for air pressure : 1 No. each
e) at the adsorber tower inlet & outlet for
air pressure : 1 No. each
f) at the after-filter (air drying plant)
outlet for air pressure : 1 No.
g) at the blower (air drying plant) outlet
for air pressure : 1 No.
h) with the air receivers on instrument
air line near each unit for air pressure : 1 No.
j) at the cooling water inlet to each
compressor unit for C.W. pressure : 1 No.
k) at the instrument air ring header
for air pressure : 1 No.
l) at the inter-cooler outlet for C.W. pressure : 1 No.
m) before air receiver for each compressor
for C.W. pressure : 1 No.
n) at the after-cooler inlet for C.W. pressure : 1 No.
(ii) Temperature Gauges
a) after each stage (LP & HP) for
air temperature : 1 No. each
b) at the C.W. outlet of each cylinder
for C.W. temperature : 1 No.
c) at the inlet of HP cylinder for
C.W. temperature : 1 No.
d) at the after-cooler outlet for
C.W. temperature : 1 No.
e) at the after-cooler outlet for air
temperature : 1 No.
f) with each air receiver for air temperature : 1 No.
g) at the adsorber tower inlet & outlet
for air temperature : 1 No.
h) at the outlet of heater for air drying
plant for air temperature : 1 No.
i) at the lubricating oil inlet for
oil temperature : 1 No.
j) at the cooling water inlet to each
compressor unit for C.W. temperature : 1 No.
k) at the lubricating oil in crank case
for oil temperature : 1 No.
(iii) Pressure Switches
a) with each air receivers
i) for air pressure high : 1 No.
ii) for air pressure low : 1 No.
iii) for load/unload : 1No. each
iv) for start/stop : 1 No. each
b) at the lubricating oil inlet for oil pressure low : 1 No.
c) at the compressor discharge
- for air pressure high : 1 No.
- for air pressure low : 1 No.
d) at the air drying after filter outlet for
instrument air pressure low : 1 No.
e) at the instrument air ring header
for instrument air pressure low : 1 No.
(iv) Temperature Switches
a) with the cylinder jacket for water
temperature high : 1 No.
b) before the after-cooler for air
temperature high : 1 No.
c) after the after-cooler for air
temperature high : 1 No.
d) after the inter-cooler for air
temperature high : 1 No.
e) with the reactivation air dryer for
reactivation air temperature low. : 1 No.
(v) Flow Indicators
a) at the cylinder C.W. outlet for C.W. flow : 1 No.
b) at the after-cooler outlet for C.W. flow : 1 No.
c) at the inter-cooler outlet for C.W. flow : 1 No.
d) at the lubricating oil cooler outlet
for C.W. flow : 1 No.
e) at the cooling water supply line to
Air Compressor for C.W. flow : 1 No.
(vi) Level Gauges
a) at the inter-cooler moisture separator : 1 No.
b) at the after-cooler moisture separator : 1 No.
c) at each air receiver. : 1 No.
(vii) Vacuum gauge at the suction filter : 1 No.
(viii) Flow switch in the cooling water supply
line to air compressor unit to trip the
compressor in case of low flow of water. : 1 No.
(ix) Indicating type flow switch in the
instrument air line to air receiver for
each unit to alarm high flow due to leakage. : 1 No.
(x) Differential pressure gauge across oil filter. : 1 No.
The bidder shall include any other instrument required to make the system safe, efficient and reliable.
2.8.18.6 Gear Boxes
Gear boxes containing helical gears, totally enclosed and lubricated shall be used for speed reduction between motor drives and driven Equipments. (i.e. Compressors, blowers, clinker grinders, combined ash slurry disposal pumps, scrapper conveyors, etc.)
Gear boxes shall be totally enclosed and oil lubricated type. Positive lubrication shall be provided to all the bearings and particularly to the working surface of the gear teeth.
The gear unit shall be specifically designed for continuous service at the speeds required by the application. Precision and accuracy of manufacture is essential to minimise problems of vibration, impact stresses, noise distortions and lubrication.
Dynamically balanced, helical gears shall be used.
i) Casing
a) The gearing shall be enclosed in an oil and dust proof gear case made of closed grained cast iron or fabricated steel.
b) The gear case shall be horizontally split and stress relieved before machining. The faces of adjoining sections shall be accurately machined to provide oil tight joints. Oil level gauges, drain, fill vent, thermometer, dipstick and other required connections equipped with approved mountings, valves and fittings shall be furnished for the gear casing.
c) The housing shall be generously proportioned and of rigid construction to provide permanent alignment of rotating parts and protection from possible external vibration. The arrangement shall be such that it will be possible to lift gear case cover without disturbing the alignment of shaft, gear and pinion.
The gear box shall be provided with suitable bearings of conservative design to meet the worst duties under transients.
ii) Rating of Gear Box
a) Gear box shall be designed for heavy shock loads and while selecting a gear box a service factor of minimum 2.0 shall be considered.
b) The nominal mechanical KW rating of each gear box shall not be less than the name plate KW rating of the selected drive motor.
c) Thermal rating of each speed reduction gear box at 500C ambient temperature shall not be less than the name plate KW rating of the selected drive motor.
d) For cooling of gear box, in case thermal capacity without auxiliary fan is not sufficient to meet the thermal rating of gear box, necessary auxiliary cooling fan shall be provided. Cooling coil for this purpose is not acceptable.
e) The selection of gear box shall be subject to Owner’s approval. The Bidder shall furnish necessary calculations and selection charts for this purpose for Owner’s review and approval.
2.8.19 Wetting Head
Adequately sized wetting head shall be provided above collector tank for wetting the dry fly ash and separating it from conveying air, if any.
The wetting head shall be provided with a water supply of sufficient quantity and pressure for feeding the nozzles that will produce fog spray of water, ensuring rapid wetting of the incoming ash.
Nozzles shall be so directed as to prevent spray from entering the ash discharge pipe.
The bidder shall elaborate as to have water vapour generated by the rise in water temperature shall be prevented from rising from the wetting head into the discharge chute to avoid the possibility of dry ash discharge pipe chute blockage.
Wetting head shall be compact in design and provisions shall be made for easy replacement of nozzles. Flanged connections shall be provided for all inlet and outlet connections of the wetting head.
Material of construction shall be as follows:
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(a) Body
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Close grained 2% Nickel cast iron minimum hardness 250 BHN.
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(b) Nozzle
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Body of MS/Cast Iron and replaceable tip of Anti corrosion tool steel or stainless steel of hardness 500-550 BHN. OR Ceramic material.
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2.8.20 Collector Tanks
The collector tank shall be located below the wetting tank and serve the purpose to separate and vent out air and discharging the ash slurry to the combined ash disposal sump through pipe lines. This will also serve the purpose of separating and venting out the entrapped air discharged by wetting head and before discharging the entrapped air into atmosphere, it shall be passed through an air washer to achieve a sustained outlet dust emission of 50 Mg/Nm3 (Max.) with the ash handling system operating at designed capacity. The dust emission figure shall be guaranteed.
The collector tank shall be located at an elevation which will provide sufficient static head to overcome frictional and other losses in the gravity flow discharge line upto ash slurry sump.
For maintenance purposes suitable stair case, maintenance platform, walkway with handrails at necessary levels shall be provided.
The sizing of collector tank, inlet/outlet connections etc. shall be corresponding to the maximum anticipated discharge flow requirements. The collector tank shall be complete with drain, overflow and vent pipe connections alongwith pipes and requisite size and lengths.
The tank shall be internally lined with alloy C.I. liners of thickness not less than 20 mm.
The material of construction shall be as follows:
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Tank
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Tested quality, M.S. of minimum 10 mm thickness, as per IS:2062.
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Baffle
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Tested quality M.S., of minimum 12 mm thickness, as per IS:2062.
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Liners
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Alloy C.I. (300-350 BHN), 20 mm, thick minimum.
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shivajichoudhury
