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Thursday, 14 March 2019

BAG FILTER OR BAG HOUSE -COMPLETE DESCRIPTION-OPERATION MAINTENANCE AND TROUBLE SHOOTING

Bag Filter , Cooler
BAG FILTER OR BAG HOUSE -COMPLETE DESCRIPTION-OPERATION MAINTENANCE AND TROUBLE SHOOTING

BAG FILTER OR BAG HOUSE

1.0 GENERAL INFORMATION

A) INTRODUCTION

i) Bag Filters are designed for filtering contaminated air (air + solids) with fully automatic on-line cleaning system by means of compressed air impulses.

ii) Bag Filters come in various models viz. square, cylindrical,
modular type with side access or top removal arrangement.

iii) Bag Filters can be used as,

1) Dust collector for Dust Extraction Systems.
2) Product collector for Grinding Mills and Pneumatic Conveying Systems.
3) Vent filter for silo venting where Bag Filter is directly mounted on silo  top without its hopper.

B) DESCRIPTION

A bag filter is divided in three portions i.e. top portion called as Plenum (clean air chamber), middle portion called as Housing (dirty air chamber) and bottom portion called as Hopper (Refer fig.1.1). The plenum is separated from the housing by the bag plate where the bags i.e. the filtering elements are attached. The bag filter unit is a continuous self cleaning dust filter capable of removing dust particles as small as sub micron size from gaseous streams. High dust collection efficiency is maintained during continuous operating conditions without use of internal moving parts or rapping mechanisms. Versatile application and
simplicity of operation are inherent design features of bag filter.

2.0 - WORKING PRINCIPLE

Dust laden air under suction or pressure enters the lower section of the Bag Filter (BF). (Refer fig.1.1) The air travels through the filter bags (5), which retains the dust particles on the surface of the bag and the clean air passes through venturies (6) and Plenum (clean chamber) (7) to the outlet of BF.
Dust collected on the outside of the filter bags causes an increase in the pressure differential between the dirty and clean air sides of the BF. To control the pressure differential across the BF, a sequential timer (1) actuates a series of normally closed pulse valves (2) at preset intervals causing them to open. A momentary rush of high-pressure air (6-7 bar g) flows from the compressed air header (3) to the blow tube (4) and is expelled from the blow tube through venturies at a high velocity (primary air flow). Air from each ventury induces a secondary airflow. The combined effect of the primary and induced secondary air causes an instantaneous  pressure rise on the clean side of the filter bags, causing a reverse flow of air through the filter bags, thus dislodging the dust particles held on the outer surface of the bags sufficient for cleaning. Through this mechanism, the collected dust is released from the bags and falls into the hopper. Dust collected in the hopper gets discharged through suitable discharge device (8) i.e. Rotary Valve, Screw Conveyor etc. Since only a fraction of the total filter area of the BF is cleaned at any given time, continuous flow through the BF at rated capacities is assured.


3.0 - INSTRUMENTATION

A) SEQUENTIAL TIMER

An electronic sequential timer is provided for controlling the bag cleaning mechanism in conjunction with the pulse valves. The timer controls both, the ‘ON' time and the ‘OFF' time i.e. the duration for which the pulse valve is opened or pulse valve is closed. The ‘ON' time or ‘OFF' time are adjustable within a fixed range.
The recommended ‘ON' time is 50 msec and ‘OFF' time is 10 sec.,
however actual setting maybe varied depending on actual dust load,
pressure drop, nature of dust etc. Various combinations of input supply voltage and output voltage to pulse valves are available. Please refer the instrument / wiring diagram of the unit supplied with the bag filter prior to taking up any installation / electrical wiring activity. Wrong electrical connection or reversal of polarity could result in damage to the electronic circuitry.

B) PULSE VALVE

The valves are 2 way normally closed diaphragm valves operated by a pilot Solenoid Valve. These valves are designed for extremely fast opening / closing and high flow rates.
OPERATION:
The diaphragm divides the valve into an upper chamber and lower chamber. The air inlet leads into the lower chamber from where it passes into the outlet when the diaphragm lifts.
The diaphragm has a bleed hole through which the high-pressure inlet air bleeds into and fills up the upper chamber, creating the same pressure on both sides of the diaphragm. The pressure in the upper chamber acts on the full surface of the diaphragm whereas the pressure in the lower chamber acts on a lesser area of the diaphragm. This keeps the diaphragm pressed down on the outlet port, closing it. The upper chamber is connected to the atmosphere through air passage in the pilot valve base. The plunger in "normal" condition keeps the air passage closed, maintains the pressure in the upper chamber and hence the diaphragm valve remains closed. When the electric supply is given to the solenoid coil, the plunger lifts within the core tube and the compressed air in the upper chamber exhausts quickly to the atmosphere. Because of the exhaust air passage has a higher flow rate than the bleed hole on diaphragm, the pressure in the upper chamber falls suddenly. The pressure in the lower chamber then lifts the diaphragm, causing the compressed air to flow from inlet through outlet When the solenoid is de-energized, the plunger closes the air passage resulting in the diaphragm closing the valve outlet. Please ensure the power supply to the pulse valve is compatible with the coil rating before wiring up. Internal construction / components may vary from manufacturer to manufacturers.

4.0 TROUBLE SHOOTING

SN TROUBLE POSSIBLE CAUSES SOLUTION
a Excessive Emission from filter outlet
  1. Bag improperly installed
  2. Bag damps too loose
  3. Torn or clamage Bags
  4. ventury fasteners loose or missing
  1. Check bag installation, repair as necessary
  2. Tighten bag Clamps
  3. Replace Damage Bags
  4. Tighten or replace fasteners
b Bag filtering action rapidly impaired resulting in reduced air/gas flow through unit
  1. Inadequate cleaning air supply
  2. Improve pulse valve operation
  3. Defective timer card
  4. Excessive moisture entering & blinding bags
  1. Check air supply, correct to between 6 to 8 bar(g)
  2. Check pulse valves, continuous rush of air indicates open valve, No air pulse indicated plugged valve, repair as necessary.
  3. Replace timer card
  4. Check bag filter for excessive moisture, minor wetting is corrected by closing  damper and running cleaning mechanism (if not corrected within 24 to 3o hours, replace bags) correct moisture level in steam.
c High pressure drop (above 150 mmWG)
  1. Improper cleaning action
  2. Improper pulse valve operation
  3. incorrect timimg sequence
  4. Defective timer card
  5. Improper dust discharge from hopper
  6. Moisture blinding of filter bags
  1. Check cleaning air supply, correct to between 6-8 bar(g).
  2. Check pulse valve, repair as necessary.
  3. Check ON & OFF intervals, adjust as required.
  4. Replace timer card
  5. Remove cause of bloackage & clean the hopper. Check seal around airlock valve, reasel if leakage is occuring.
  6. Correct cause of excess moisture and replace bags
d Presure of cleaning air keep reducing
  1. Faulty or under sized compressor
  2. Leakage in main airline
  3. Pulse Valve sticking open.
  4. Defective timer card resulting in continuous output to pulse valve
  1. Check compressor manual or technical data sheet
  2. Locate & Repair leak
  3. Examine the valve. clean/reapir as necessary.
  4. Change connection to output if available or replace timer card

 Note:
1. It is recommended to use oil and moisture free air for the bag cleaning station.
2. avoid operating the unit at or close to dew point of the gas stream handled. special precaution needs to be taken in such cases.
3. For high temperature application take necessary care when starting-up system from cold, to avoid condensation.

5.0 MAINTENANCE

A. GENERAL

A sound preventive maintenance program will eliminate most breakdown situations and ensure long and trouble free operation of the filter unit.

B. PERIODIC MAINTENANCE PROCEDURE


PERIOD COMPONENT PROCEDURE
Daily
  1. Bag Filter
  2. Compressed air system
  3. D.P Gauge or D.P. Switch
  4. Pulse Valve and Timer
  1. Check exhaust for visible dust, refer to trouble shooting.
  2. Check for air leakage (low pressure) reapir as necessary. check valves.
  3. Check and record reading, if out of limits refer to trouble shooting, watch for a trend.
  4. Check timer and pulse valve's operation.
Weekly Hopper Check for material hanf-up on walls. clean the hopper wall from inside.
Monthly Filter Bags Check the bags for tears, holes, proper fastening. Repair or replace as necessary


C.  MAINTENANCE PROCEDURE

1. Bag Replacement
In the event of high emission from the filter unit due to damaged or torn bags. it will be necessary to remove all bags from the unit for inspection. The following procedure should be followed after isolating the unit.
A. For Side Access Units
a. Open the access door by standing on external platform.
b. Loose the bag clamps of the first nearest row.
c. Pull down the bag and retainer assembly with bag clamps.
d. Similarly remove bag and retainer assemblies accessible from the access door.
e. Place the portable internal platform on the two support provided in the bag filter housing.
f. Bet into the bag filter's housing with the help of safety cables.
g. Remove the remaining bag and retainer assemblies by standing on the internal platform as described in (b) and (c) above.
h. Inspect all bags minutely for pinhole, tears, punctures and excessive wear and tear especially at bottom of bags.
i. Remove damaged filter bags from retainer.
j. Refit all bags retainer assemblies as per instructions.

B. For Top Removal LTR/HTR Units
a. Open the access door on roof of bag filter and enter u=inot plenum. DO not stand / Step on the blow tubes. Remove the blow tubes by loosing coupling.
b. Lift the bag retainer and ventury assembly by the handle provided on the top Collar.
c. Remove all bag cage and ventury assemblies.
d. Remove filter bags by folding the top snap ring of the bag.
e. inspect all bags minutely for pinholes, tears, punctures and excessive wear and tear especially at bottom end of bags.
f. Replace the damaged bag
g. Carry out assembly of bags
h. Carry out installation of all bag retainer, clamp and ventury.

2. BAG WASHING
Inspect bags for hard caking/scaling and correct process parameters/operating parctice. Bags with hard scales can not be cleaned using the normal cleaning mechanism. Such bags need to be removed from the unit and be washed thoroughly as mentioned below:
Procedure for bag washing
a. Vacuum clean each bag prior to washing
b. Soak in cool or warm water at 60 Deg. C maximum temperature, using mild soap or detergent.
c. Rinse thoroughly to ensure all sealing have been removed.
d. Hand the washed bag and allow to dry completely before re-fitting.
Notes: 
1. The duration of soaking will depend on the nature of seals and needs to be established by the user.
2. Do not use any sharp instruments/knives to scrape off the scaling on the bag.

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Thursday, 7 March 2019

STACK MONITORING OR DUST MEASUREMENT IN ISOKINETIC SAMPLING

Calculation , Process Engineering
STACK MONITORING OR DUST MEASUREMENT IN ISOKINETIC SAMPLING

STACK MONITORING OR DUST MEASUREMENT IN ISOKINETIC SAMPLING 

By this method dust measurement or stack monitoring can be calculated in any straight ducts or ducts having laminar flow flow by developing isokinetic condition in equipment.

Standard Operating Procedure for Particulate Matter Determination


1. Pre-sample Activity

Weigh the properly conditioned thimble/filter and place it into the clean, air tight container. Designate appropriate label or ID No. to each thimble/filter container.
Stack Monitoring – Material and Methodology for Isokinetic Sampling Field activity starts with the collection of detail information’s from the industry about the products, raw materials, fuels, and stack dimensions.

2. Traverse Point Calculation

Calculate the traverse point and accordingly mark the distance from tip of the nozzle, on pitot tube and probe. Do not forget to add the collar length of port to the calculated traverses.

3. Composition of Flue Gases

Determine flue gas composition by orsat apparatus or multi gas analyzer. In case of Orsat analysis gas sample has to be collected in tedlar bag / non reactive bladder and allowed to cool before analysis. Gas analysis by multi gas analyser may be performed by direct insertion of sampling probe inside the stack and simultaneous estimation of all the components in pre-calibrated gas analyser. At least 3 observations should be taken for average percentile composition. Use gaseous composition data to calculate dry molecular weight of flue gas (Md).

Determine the Dry molecular weight (Md) by following equation.
Md = 0.44 (%CO2) + 0.32 (% O2) +0.28(% N2 + % CO) +……

4. Measure ambient temperature (ºC) and Barometric pressure in mm Hg

5. Check the leak in sampling instruments

The sampling train after having set up will be tested for leakage by plugging the inlet. The rotameter shall not give a reading beyond 5 lpm when the flow has been set 100 lpm. Also the dry gas meter should give a reading of less than 5 percent of the air flow.

6. Moisture Determination

Moisture can be determined by condenser method , in principle, involves extracting a sample of the stack gases through a filter for removal of the particulate matter, then through a condenser accumulating the condensate formed in process, and finally through a gas meter. The objective of the test is to collect and measure the volume of all the condensate formed at the condensing temperature from a measured amount of gas.
Calculation
Calculate equivalent vapour of condensate under sampling condition, m3
Vv = [ (Vc × 22.4) / (1000 × 18 ) ]  × [ Tm / 273 ]  ×  [ 760 / (Pbar - Pm) ]
Where:
Vv= Equivalent vapour of condensate under sampling condition, m3
Vc = Volume of condensate in condenser. ml 
Tm=Absolute meter temperature, ºK
Pm=Suction at meter, mm mercury column
Pbar =Barometer pressure, mm mercury column
Calculate the moisture content of the gases using the following equation:
Bwo = [ Vv / (Vv + Vm)]
M = [ Vv / (Vv + Vm)] × 100
Bwo= Proportion by volume of water vapour in stack gas.
M= Moisture in the flue gases, percent
Vv=  Equivalent vapour volume of condensate under sampling condition.
Vv= Volume of gas sampled (m3)

7. Wet Molecular Weight (Ms Determination

This equation can be used to determine the molecular weight of the stack gas on a wet basis
Ms = Md (1 – Bwo ) + 18 Bwo
Md = molecular weight of stack gas on dry basis, kg / kg –mole

8. Determine stack Gas velocity Pressure (ΔP) and Stack Temperature (Ts)

  • Check and adjust the upper miniscus of manometer fluid at zero.
  • Connect +ve and –ve end of the pitot tube in respective points.
  • Slowly insert the pitot and thermocouple upto the first traverse mark inside the stack. Keep the positive end towards the direction from which flue is coming. Hold it for stabilisation. Take the reading of fluid displacement in manometer and temperature.
  • Repeat the same in next traverse mark and so on.
  • Take average reading for Δ P and Ts
  • For measurement of static gas pressure pitot tube should be rotate by 90 Degree from the position of actual Δ P measurement. This would provide better accuracy

9. Determination stack Pressure (absolute stack gas pressure)

For the static pressure determination requires first to disconnect the positive end of the pitot tube then take the reading of velocity pressure at the traverse point in which the calculated average Δ P matches closely. For measurement of static gas pressure pitot tube Should be rotated by 90o from the position of actual Δ P measurement. This would provide better accuracy.
Calculate Ps
Ps = Pbar ± ( Δ Ps / 13.6 )
Where:
Pbar = Barometric pressure in mm mercury column
Δ Ps = Stack gas velocity pressure, mm water column
Ps = Static pressure mm Hg column.
Density of Hg = 13.6

10. Stack Gas Velocity Determination (US)

Connect pitot tube to the stack for velocity determination, calculate the stack gas velocity at all the traverse point by using the following formula. Consider the density factor for correction of velocity pressure and Δ Ps to convert water column manometer.

Us = Kp× Cp× ( Δ P ) 1/2 × [Ts / (Ps × Ms) ] 1/2
Where
Us = Stack gas velocity, m/s
Kp = Constant
Cp= type pitot tube coefficient.
Ts = absolute stack gas temperature, ºoK
ΔP = Stack gas velocity pressure, mm water column
Ps = Absolute stack gas pressure, mm Hg
Ms = Molecular weight of stack gas on wet basis, Kg / Kg –mole

11. Determination of Volumetric Flow rate / Discharge (US)

The following equation is used to calculate stack gas volumetric flow rate (m3/hr).
Qs = 3600 (US) × AS × AS (1-BWO) × [ Tref / TS ] × [ PS /Pref ]
Where
AS = Area of the stack (duct), m2
BWO = Proportion by volume of water vapour in stack gas.
Tref = 298 ÂºK
Pref = 760 mm
TS = Absolute stack gas temperature, ºK
PS = Absolute stack gas pressure

12. Determination of Flow Nozzles

Select the nozzle size, in such away that sampling rate a meter shall not exceed 70 % of pump capacity in any case. Cross sectional area of nozzle (mm) for different diameter is as follow:


SN Dia. of Nozzle (Inches) Cross Sectional Area(m2)
1 5/8 1.9783 X 10^4
2 3/4 2.8487 X 10^4
3 1/2 1.2661 X 10^4
4 1/4 3.16531 X 10^5
5 1/8 7.9132 X 10^5
Rs = (US * An) * 60 * 1000
Where,
RS = Sampling Rate at nozzle, LPM
US = Stack gas velocity, m/sec
An = Area of nozzle, m2
60 = Conversion Factor Seconds to Minute
1000 = Conversion Factor m3 to Litre

13. Determination of Sampling rate at gas meter

The meter for measuring the gas sample measures the gas at conditions of temperature, pressure and moisture content which are different than those in the flue. Therefore, calculate the sampling rate at the gas meter for each sampling points before starting the test and record on the log the required rate (Table 1).

Calculate the sampling rate at the gas meter as follows:
Rm = Rs × (Tm / Ts) × [ (Pbar - Ps) / (Pbar - Pm) ] × [ Vm / (Vm + Vv) ]
Where,
Rm = Flow rate through meter, m3/s
Rs = Sampling Rate at nozzle, LPM
Tm = Temperature at metering condition, ºK
Ts = Absolute stack gas temperature, ºK
Ps = Absolute stack gas pressure, mm mercury column
Pbar = Barometer pressure, mm mercury column
Pm = (Pm1 – Pm0) / 2 Suction at meter, mm mercury column
Vm = Volume of gas sampled at meter conditions, m3
Vv = Equivalent vapour volume of condensate at meter conditions, m3
Note: Take initial reading of vacuum guage (Pm0) in mm Hg at the staring of
sampling and final vacuum pressure (Pm1) in mm Hg just before putting off the pump when sampling is complete. Calculate average difference in suction
pressure, referred as Pm

14. Start the test after the sampling rate has been calculated and train assembled and checked for leakages. When equipment is ready in all respect, record the initial dry gas meter reading and push the sampling probe carefully into the duct to the point nearest to the back wall. Take the sample appropriately as per the requirement and with all the necessary precaution.

15. Determination of volume of gas sampled


Calculate the volume of gas sampled using the following equation:
Vstd = Vm × Y × [ (Pbar - Pm) / 760 ] × [ (273 + 25 ) / (Tm + 273) ]
Where,
Tm = Temperature of gas at dry meter condition, ºC
Vm = Volume of gas sampled at dry gas meter conditions, m3
(P bar - Pm) = Actual pressure in sampling train, mm mercury column.
Pm = Static pressure in sampling train, mm mercury column
Pbar = Barometeric pressure in sampling train, mm mercury column.
Y = Calibration factor of dry gas meter.

16. Sample Recovery

After the sampler has cooled, brush down the dust on the side of the nozzle carefully into the thimble using a small brush remove the thimble and replace it in same labeled container. In the case of filter holder is kept outside during the sampling, the dust from the sampling probe before the filter holder should be  brushed down into the filter.

17. Determination of dust concentration

Determine the mass of dust collected in the thimble by difference i.e weighing the  thimble before and after the run. Dry the thimble in an oven for about 2 hours at 120 ÂºC prior to sampling. After sampling, cool, dry and again weigh the thimble  along with dust maintaining the same condition as prior to sampling.

Calculate the dust concentration using the following equation:
Em = [ (W2 - W1) × 1000 ] / Vstd
Where,
Em = Dust Concentration in mg /Nm3,  ( 25 ºC, 760 mm Hg, dry basis)
Vstd = Volume of dry gas through the meter (25 ºC, 760 mm Hg), Nm3
W1 = Initial weight of filter paper in gram
W2 = Final weight of filter paper in gram

18. Correction of result at 11% O2

O2correction is only carried out, if O2 corrections > 11%. For O2 < 11% no  correction is allowed. It’s require to correct the value for the 11% O2 by following formula

Es = [ (21 - Os) / (21 - OM] × EM
Where,
Es = calculated emission concentration at the standard percentage oxygen concentration
EM = measured emission concentration
OS = standard oxygen concentration
OM = measured oxygen concentration

19. Determination of Emission Rate

Calculate the dust emission rate as follows:
Dust Emission rate (Kg/hr) = (Em × Qs) / 106
Where,
Qs = Flue gas flow rate (25 ºC, 760 Hg mm Hg), Nm3 / hr.
Note: Report the concentration as corrected at 11 % O2
TABLE-1
Plant Name & Address:
Date & Time:
Ambient Temperature:
Barometric Pressure (mmWG):
Moisture in Flue gas (%):
Flue gas composition:
Filter No and weight (Initial as well as Final):

Travers Point ΔP Ts Ps Us Qs Rs PM Rm Time DGM (m3) Vstd
Unit (mmWG) K mmWg m/sec m3/hr LPM Pm0 Pm1 LPM min Initial Final Nm3
1
2
3
4
5
6
7
8
9
10
11
12

Δ P = Stack Gas Velocity Pressure, (mm water column), Ts = Stack temperature (ºK),
Ps= Static pressure (mm water column), Us = Velocity of stack gas (m/s),
Qs = Volumetric Flow Rate/ Discharge, Rs = Flow at nozzle (LPM),
Pm = Vaccum Pressure Drop (mm mercury column),
Rm = Determination of sampling rate at gas meter. (LPM),
Vstd = Determination of volume of Gas Sampled 

Other Required Information
1. Feed rate
2. The nature, composition and quantity of the material being incinerated during monitoring
3. installed and operating capacity of the incinerator
4. No. of sample Port
6. Internal Diameter of the stack
7. Nozzle size selected for sampling
8. Pitot tube constant
9. ID fan capacity
10. Pollution control equipment installed and its status

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