coal flow ability report - bunker coal flow study for anpara-d

Job no: S/BHEL- Noida/Flow Lab/16/2010-11 Confidential Not For Publication

Draft Report

Bunker Coal Flow Study for

2x500 MW Thermal Power Station of Anpara-D Project, U.P.

Sponsored by

M/s BHEL, Noida

December, 2010

Prepared by Research & Development Centre

NMDC Ltd (A Government of India Enterprise)

Uppal Road, Hyderabad - 500 007 India

Bunker coal flow study – TPS Anpara-D

I N D E X

Sl.no. Contents Page no.

Executive Summary 2

Glossary of Terms 5

Symbols and Abbreviations 7

List of Figures 8

List of Tables 9

1. INTRODUCTION 10

2. OBJECTIVE 14

3. EXPERIMENTAL WORK 15

3.1 SAMPLE PREPARATION 15

3.2 MOISTURE DETERMINATION 16

3.3 BULK DENSITY 16

3.4 BULK DENSITY VARIATION WITH CONSOLIDATION 17

STRESS

3.5 SHEAR TESTS 18

3.5.1 RING SHEAR TESTER 18

4. EXPERIMENTAL DATA GENERATED 21

5. RESULTS & DISCUSSION 27

6. RECOMMENDATIONS 30

R&D Centre, NMDC Ltd Page 1


Executive Summary

M/s. BHEL,Noida vide work order no. PW/PE/PG/ANP/P-307/10, dt:16.06.2010

have awarded the work to R&D centre, NMDC Ltd. for comprehensive flowability

studies on the coal sample of the proposed 2 x 500 MW Anpara-D thermal power

station, UP, to provide relevant parameters for the design of reliable gravity flow coal

silos. The silos are required to promote Mass Flow without choking and rat holing

problems. Accordingly approx. 400kg of coal sample was received from TPS Anpara at

R&D Centre of NMDC on 15.09.2010.

A representative sample was drawn from the lot to establish the size analysis. The coal

sample has been crushed to -5mm size and homogeneously mixed. The fine sized coal

of typically less than 2.36mm size, which is primarily responsible for flow related

problems has been screened and was used for conducting the shear tests. The

flowability characteristics of coal and its interaction with five different liners i.e. Stainless

steel SS409M(2D Finish), SS304(2B finish), IS 2062, Mild steel(rusted) and UHMWPE

(ultra high molecular weight polyethylene) were established using Ring Shear Tester at

four different moisture levels of coal by physically altering the moisture content. As most

coal samples exhibit high yield strength between 55% to 85% saturation moisture, it

was decided to conduct tests on coal sample at 55%, 65%, 75% and 85% saturation

moisture levels (SML). The corresponding moisture content (mc) of each saturation

moisture level for coal is respectively 16.4%, 19.4%, 22.3% and 25.3%. The coal

sample exhibited highest cohesive strength at 25.3% mc which is considered the critical

moisture. The undisturbed storage time tests were conducted at 25.3% mc for storage

up to 24 hours and 72 hours to establish the effect of undisturbed coal storage in the

bunker/silo. The shear test data has been processed to establish the minimum slopes

and outlet sizes required to generate Mass Flow in coal bunkers.

The following are the salient points.

• The results of testing indicate that the tested coal is compressible and has

moderate angles of internal friction and moderate bulk strength. Based on the

Jenike’s classification, the coal can be classified cohesive at 16.4% mc and

very cohesive thereafter.



• The flow function curves indicate that coal exhibits highest yield strength at a

moisture content of 25.3%. Hence the storage time tests were conducted at

25.3%, which is considered the critical moistures. There is a significant

increase in the bulk strength of coal after 24hours and 72 hours of storage.

• The coal at 25.3%mc, requires a critical (minimum) outlet dia. of 0.70m to

prevent ‘cohesive arching’ at instantaneous condition (without storage) in

case of conical hoppers.

• The storage time test at 25.3%mc indicate that the critical (minimum) outlet

dia. required is 1.0m for 24 hours and 1.26m for 72 hours of storage to

prevent the formation of cohesive arching above the outlet in the conical

hoppers.

• The minimum conical hopper slope (with horizontal) required for Mass Flow

at a typical outlet dia. of 0.914m (0.914m was chosen out of general practice in coal bunkers of 500MW thermal power plant) is 73.50 with stainless steel SS304 (2B finish) liner and 71.30 with SS409M (2D finish) to handle coal at all moisture levels. The minimum slopes for other outlet

dimensions are also presented in the report.

• The slope of the hopper (with horizontal) decreases normally with the

increase in hopper outlet size for different liners. The minimum slope required

to promote Mass Flow against different outlet sizes has been provided in

tabular form in the report which may be utilised in the functional design

process.

• The typical outlet dimension of 0.914m (dia) for the coal bunkers is sufficient

to prevent cohesive arching above the bunker outlet while handling tested

coal at all moisture levels and undisturbed storage in the bunker for less than

24 hours. However, upon extended undisturbed coal storage beyond 24



hours, the coal is likely to form an Arch at the outlet leading to no flow

condition.

• If prolonged storage of coal in silo beyond 24 hours is expected, then Arching

can be avoided by locating a shut off gate at hopper sectional dia of 1.26m

instead of placing at the 0.914m dia. During the shut down, the shut off gate

should be closed and coal below the gate is to be emptied by running through

the system. In such an arrangement, the effective outlet size would be 1.26m.

This will facilitate to initiate the coal flow satisfactorily from the silo after

extended period of undisturbed storage up to 72 hours.



GLOSSARY OF TERMS

Cohesive Arching:

A no – flow condition caused by bridging of the material over the hopper outlet

Bin:

Usually vertical section of a storage container (Sometime used synonymous to a

Bunker/Silo)

Bunker:

Storage container having both vertical and convergent sections

Expanded Flow:

Combination of Mass Flow in converging section and a Funnel Flow bin on top

Flow Function:

Plot of unconfined yield stress versus major consolidation stress for specific bulk

solid. It is a bulk solid parameter

Flow Factor:

It is a flow channel parameter. Flow factor is the ratio of major consolidation

stress in a bulk solid flowing in a channel to the major principal stress that would

cause it to cease flowing. The value of flow factor depends on the geometry of

the hopper, especially on the slope of the channel walls, the angle of wall friction

and the effective angle of friction.

Funnel Flow:

A flow pattern in which the material flows primarily in the central part of the bin or

hopper in the form of a funnel



Gravity Flow:

The flow of a bulk material is induced by gravity alone

Hopper:

Converging section of a storage container

Instantaneous condition:

No storage at rest (Filling of bunker followed by extraction)

Mass Flow:

A flow pattern in which all the material in the bin or hopper is in motion and flow

occurs along the walls of bin or hopper

Rathole/Piping:

A restricted flow condition in which the material flow is limited to Vertical central

cylindrical core above the hopper outlet

Plane Flow:

A flow pattern characterized by flow trajectories that are symmetric about the

vertical plane through the longitudinal axis of the outlet slot

Silo:

Tall storage container, usually with a centrally located opening

Time storage:

Bulk solid stored undisturbed in the bunker for specified time



SYMBOLS AND ABBREVIATIONS

Dc: Minimum diameter of circular discharge opening for a Mass Flow Silo, m

Ds: Minimum side of a square discharge opening for a Mass Flow Silo, m

Dp: Minimum width of a slot discharge opening for a Mass Flow Silo, m

Df: Critical rathole dimension (Funnel Flow), m

θc: Minimum angle (from horizontal) for a conical hopper walls and end walls

of a transition hopper for Mass Flow, Degrees

θp: Minimum angle (from horizontal) for a wedge shaped (plane flow) hopper

and side of transition hopper for Mass Flow, Degrees

FF: Flow Function

FFt: Time Flow Function

σ (Sigma): Normal stress, pa (Pascal)

σ1: Major consolidation stress, pa

FC: Unconfined Yield Stress, pa

δ (Delta): Effective angle of friction, Degrees

φ x (Phix): Kinematic angle of wall friction, Degrees

φ(Phi): Angle of internal friction, Degrees

sml: Saturation moisture level

mc: Moisture content



LIST OF FIGURES

Sl. no.

Contents

Page no.

1 Mass Flow and Funnel Flow Patterns 12

2 Expanded Flow Bin 13

3 Variation of bulk density with major consolidation stress 17

4 Jenike – Schulze Ring Shear Tester

19

5 Photographs of Liners Tested 20

6 Typical treatment of yield Locus (25.3%mc, Level-2) 21

7 Kinematic angle of wall friction (16.4% & 19.4%mc) 22

8 Kinematic angle of wall friction (22.3% & 25.3%mc) 23

9 Flow functions 24

10 Flow function and Time Flow function (25.3%mc, 24hrs and 72hrs) 25



LIST OF TABLES

Sl. no.

Contents

Page no.

1 Size analysis of as received sample 15

2 Percent moisture content with respect to saturation

moisture level 16

3 Bulk density and Angle of repose 17

4 Flowability parameters 26

5 Minimum outlet dimension to prevent cohesive arching at critical moisture 32

6 Minimum hopper slopes for Mass Flow at a given outlet dimension

33



1. INTRODUCTION

1.1 Gravity Flow Of Bulk Solids

Gravity forces in general, are utilized wherever possible to cause the flow of bulk

solids in bins, hoppers and stockpiles. Earlier, the designs for such systems were

based on the angle of repose concept of the material. However, this parameter does

not take into account the consolidation loads experienced by the bulk solid when

stored and extracted from bunkers. The cost of Bulk material handling operations is

very substantial and for this reason handling and storage facilities should be

designed to gain maximum reliability and efficiency.

Advances in this field have shown conclusively various factors other than angle of

repose, which influence greatly in establishing optimum flow condition for the

material with respect to bunker geometry and liner selection. The common

problems associated with material flow are segregation, flow blockage due to

arching, Rat holing, wall failures etc. These problems are in turn related to factors

like Effective angle of friction between particles, Kinematic angle of wall friction,

Angle of internal friction etc. Now a days, increased awareness amongst material

handling experts has emerged to consider various flow parameters like moisture

effect, liner effect, storage effect, bunker geometry, effect of wall pressure in

hopper, flow path and velocity fields etc., while designing the geometry of the

system as against conventional approach of angle of repose.

The design of storage bins for bulk solids involves

1. Determination of the strength and flow properties of the bulk solids for the

worst likely conditions expected to occur in practice. 2. Determination of the bin geometry to give the desired capacity, and reliable,

predictable gravity flow. 3. Estimation of loadings exerted on the bin walls and the feeder under

operating conditions.



1.2 Modes of Flow In order to appreciate the problems encountered with the bunker operation it is

important to know the Flow patterns that occur in bunkers during gravity flow.

There are two basic modes of flow, Mass Flow and Funnel Flow

1.2.1 Mass Flow Mass Flow pattern describes a condition in which all the material in the bin is in

motion whenever any of it is drawn out (Fig.1). It is not necessary that the velocity

across the cross section is constant only that all the material will be in motion.

Mass Flow bins require more headroom than the Funnel flow systems because

the hopper walls have to be smooth and steep. The flow pattern sequence is

“first-in, first-out”, Rat holes do not develop and fine powdery materials will have

time to deaerate after charged in to the bin. Material bulk density at the outlet is

relatively constant and segregation is minimized because particles at the centre

and sidewalls of the bin are discharged simultaneously. Mass Flow bins are

especially suitable for cohesive solids (including many fine powders) that degrade

with time, and where segregation should be minimized. But the disadvantage

associated with Mass Flow is wear of bin and hopper walls when handling

abrasive bulk solids.

1.2.2 Funnel Flow Funnel flow (or Core flow) on the other hand occurs when the bulk solid sloughs

off the surface and discharges through a vertical channel, which forms within the

material in the bin (Fig.1). This mode of flow occurs when the hopper walls are

rough and slopes are shallow.

It follows the “first-in, last-out” sequence of flow pattern. Flow rate tends to be

erratic with varying feed bulk density. Stable rat holes can form if the stored

material develops enough cohesive strength, resulting in severe loss of the ‘live

capacity’, besides pseudo-stable rat holes may develop causing erratic flow. Most

fine powders exhibit flushing in a Funnel flow bunker system, because they can

support a stable rat-hole. Funnel flow bunkers also exhibit the problems of

segregation.



Fig.1 Mass Flow and Funnel Flow Patterns

1.2.3 Expanded Flow Bin Where large quantities of the bulk solid are to be stored, the expanded-flow bin

(Fig.2) is often an ideal solution. This bin combines the storage capacity of the

Funnel flow bin with the reliable discharge characteristics of the Mass Flow

hopper. It is necessary for the Mass Flow hopper to have a diameter at least

equal to the critical pipe or rathole dimension Df at the transition with the Funnel

flow section of the bin. This ensures that the flow of material from the Funnel flow

or upper section of the bin can be fully expanded into the Mass Flow hopper. The

Expanded flow bin concept may also be used as advantage in the case of bins or

bunkers with multiple outlets.



Fig.2 Expanded Flow Bin



2. OBJECTIVE The objective of the assignment is to establish the flow properties of coal sample for the

proposed 2 x 500 MW Anpara – D thermal power station, UP, to provide relevant

parameters for the design of reliable gravity flow coal silos based on the test data

obtained at different moisture contents along with storage time effect up to 72 hours.



3. EXPERIMENTAL WORK 3.1 Sample Preparation

Approximately 400Kg of coal sample has been received from the Thermal power

plant, Anpara, U.P. The sample has been uniformly mixed and representative

sample was drawn from the lot to establish the size analysis. The as received

sample was coarse in size and containing lumps as large as 40mm. The size

analysis of as received sample is presented in Table-1. About 100 Kg of

representative sample has been further drawn from the lot which was subjected

to stage crushing and reduced to size below 5mm. The crushed sample was

screened through an 8 mesh (2.36mm) aperture screen. About 60 kg of

representative sample of -2.36 mm was cut from the lot and the same is used for

shear testing to generate flowability test data.

TABLE-1 SIZE ANALYSIS OF AS RECEIVED SAMPLE

Nominal Screen aperture Size

Tyler Mesh no. mm

Cumulative weight percent passing

--- 40 94.59 --- 20 85.51 --- 10 72.74 --- 3 51.78 18 1 33.89 20 0.833 30.50 28 0.589 26.09 35 0.417 21.40 48 0.295 17.45 65 0.208 13.68

100 0.147 11.45 150 0.104 11.29 200 0.074 9.93 250 0.061 9.66

325 0.043 9.46



3.2 Moisture Determination

Moisture content was determined on -8# (-2.36mm) size fraction by drying small

quantity of samples at 107o C until dry in a forced convection oven. The loss in

weight of each sample divided by its original (wet) weight before drying is

denoted as moisture content.

After determining the moisture content of the air dried coal, the saturation

moisture level of coal is also established by gradually adding small quantities of

water to a known quantity of coal sample until the coal reaches a 100%

saturation level. The total quantity of water added is noted. The moisture of the

resultant sample at 100% saturation is determined.

The percentage moisture of the coal with reference to various saturation levels

are shown in Table-2.

TABLE- 2

PERCENT MOISTURE CONTENT WITH RESPECT TO SATURATION MOISTURE LEVEL

Moisture level

Moisture content (%)

Air dried sample 8.68

55% saturation moisture level 16.4





3.3 Bulk Density

The bulk density of the as received coal sample is determined along with repose

angle and is shown in Table-3.



TABLE-3 BULK DENSITY AND ANGLE OF REPOSE

Sample Size of coal Bulk density (kg/m3)

Angle of Repose (deg)

Condition

Coal As received 977 35.50 10.25% moisture

3.4 Bulk Density Variation with Consolidation Stress

The bulk density is an important parameter in calculation of bunker/silo loads,

bunker capacities, opening sizes and material flow rates. The bulk density

variation with major consolidation load at different moistures is established using

Ring shear tester and is presented in Fig.3.

700

800

900

1000

1100

1200

1300

0 10000 20000 30000 40000 50000

Major Consolidation Stress, Pa

Bul

k D

ensi

ty, K

g/m

3

16.4% mc19.4% mc22.3% mc25.3% mc

Fig. 3 Variation of bulk density with consolidation stress



3.5 Shear Tests The influence of moisture content on the flowability of coal is very significant. For

most bulk materials like coal, the bulk strength tends to increase (flowability

drops down) with increased moisture content, reaching a peak between 55% to

85% saturation. Beyond this peak value the bulk strength generally reduces with

further increase in moisture content. To find the moisture level at which the coal

attains maximum bulk strength, shear tests were conducted at moisture contents

16.4%, 19.4%, 22.3%, and 25.3%mc, by altering the moisture content of

received sample. The above moistures correspond to 55%, 65%, 75% and 85%

saturation respectively. The methodology of shear testing is based on the special

procedure of compacting the coal sample at different specified moisture levels to

obtain packing conditions expected in the bunkers/silos and then subjecting the

sample for shear testing using Ring shear tester.

3.5.1 Ring Shear Tester Ring shear tester (RST-01.pc) (Fig.4) is used to evaluate Effective angle of

friction and the Flow function (FF) of the coal sample at various moisture levels.

The standard shear cell is homogenously filled with the sample of -2.36mm size

by avoiding large voids and the excess material is scraped off in level with the

top of the shear cell. It is carefully placed on the driving axle of the ring shear

tester and the sample is subjected to shearing (Fig.4). Bunker storage time tests

were carried out in a Consolidation Test Bench to evaluate undisturbed storage

effect for 24 and 72 hours.

The wall friction tests were also carried on Ring Shear tester using the wall liners

Mild steel (rusted), SS304 (2B), IS2062, SS409M (2D) and UHMWPE. The liners

(Fig.5) were cut to the required shape and dimensions and placed in the

appropriate shear cell. The sample to be tested is homogeneously filled up to the

top of the shear cell. The cell is placed on the driving axle of the ring shear tester

and the sample is subjected to shearing against the wall liner under different

stress conditions.



Fig.4 Jenike-Schulze Ring Shear Tester (RST-01.PC)



SS304(2B) Mild Steel (rusted)

UHMWPE SS409M(2D)

IS 2062

Fig.5 Photographs of Liners Tested



4. EXPERIMENTAL DATA GENERATED

The interactions of coal flowing within itself and against different bunker wall/

liners are determined from the shear test data. The data generated at 16.4%,

19.4%, 22.3%, and 25.3%mc for coal sample was analyzed using RST-

CONTROL 95 software for plotting yield loci and constructing Mohr stress circles

to evaluate the relevant flowability parameters which forms the basic design

criteria for suggesting bunker/silo configuration for Mass Flow. Typical treatment

of yield loci, wall friction curves (phiX) and flow function (FF) curves are

presented in Fig. 6 to 10. The flowability parameters determined are presented in

Table-4.

Fig. 6 Typical treatment of yield Locus (25.3 %mc, Level-2)



Fig.7 Kinematic angle of wall friction (16.4%&19.4%mc)



Fig.8 Kinematic angle of wall friction (22.3%&25.3% mc)



02000400060008000

10000120001400016000180002000022000

0 10000 20000 30000 40000 50000

Major Consolidation Stress, Pa

Unc

onfin

ed Y

eild

Stre

ss, P

a

16.4% mc19.4% mc22.3% mc25.3% mc

Fig.9 Flow functions



Fig.10 Flow function and Time Flow functions (25.3%mc, 24hrs and 72hrs)




TABLE-4 FLOWABILITY PARAMETERS

Kinematic angle of wall friction, deg, Phix

Moisture content

Effective angle of friction,

deg,

δ IS2062 Mild Steel(rusted) SS304(2B) SS409M(2D) UHMWPE

16.4% 49Arctan[71/Sigmaw

+Tan(18.1)]

Arctan[173/Sigmaw

+Tan(20.3)]

Arctan[198/Sigmaw

+Tan(18.1)]

Arctan[74/Sigmaw

+Tan(17.0)]

Arctan[10/Sigmaw

+Tan(14.5)]


+Tan(18.7)]

Arctan[269/Sigmaw

+Tan(20.6)]

Arctan[519/Sigmaw

+Tan(18.2)]

Arctan[407/Sigmaw

+Tan(17.4)]

Arctan[25/Sigmaw

+Tan(19.2)]


+Tan(16.7)]

Arctan[554/Sigmaw

+Tan(19.1)]

Arctan[586/Sigmaw

+Tan(16.3)]

Arctan[536/Sigmaw

+Tan(15.9)]

Arctan[328/Sigmaw

+Tan(19.4)]


+Tan(17.5)]

Arctan[575/Sigmaw

+Tan(19.6)]

Arctan[527/Sigmaw

+Tan(16.6)]

Arctan[449/Sigmaw

+Tan(17.7)]

Arctan[379/Sigmaw

+Tan(18.6)]


5. RESULTS & DISCUSSION

5.1 Assessment of Coal Flowability The Flowability of coal at different moisture levels is characterised by their flow

function curves (Fig.9). Based on these curves along with wall yield loci, an

assessment of flowability of coal tested is given below.

• The results of testing indicate that the tested coal is compressible and has

moderate angles of internal friction and moderate bulk strength. Based on the

Jenike’s classification, the coal can be classified cohesive at 16.4% mc and

very cohesive thereafter.

• The flow function curves indicate that the coal exhibits highest yield strength

at a moisture content of 25.3% particularly at low consolidation stresses

(below 12 Kpa). Hence the storage time tests were conducted on coal at

25.3%, which is considered the critical moisture. There is a significant

increase in the bulk strength of coal after 24 hours and 72 hours of

undisturbed storage (Fig.10).

• Majority of the wall yield loci are not passing through origin and exhibit some

cohesion/adhesion particularly at elevated moisture contents. In other words,

the wall friction angle depends on normal stress (Fig.7&8). It implies that in

such cases, the minimum slope of the hopper to promote Mass Flow will vary

with the outlet dimension of the hopper.

• The average effective angle of internal friction (delta) varies from 490 to 560

depending on the moisture content of coal (Table-4). This forms one of the

factors for further evaluation of design parameters.

5.2 Mass Flow Design Parameters Based on the Jenike theory, the Mass Flow bin design parameters like minimum

slope (θc or θp) and outlet dimension (Dc or Dp) to prevent cohesive Arching in



Hoppers were established using the flow parameters like effective angle of

friction, wall angle of friction and flow function. The results are presented in

Table-5&6. The minimum slope of hopper for Mass Flow varies with outlet size

(slope decreases with increase in outlet size) in some cases, which is due to the

cohesion/adhesion exhibited by bulk solids against the liner tested. The variable

slopes with different outlets are given in Table-6. This may be used in the bunker

design process. The above given results are minimum dimensions and hence

steeper angles and larger outlets than given above are permitted. Some of the

salient points are as follows.

• The coal at 25.3%mc, requires a critical (minimum) outlet dia. of 0.70m to

prevent ‘cohesive arching’ at instantaneous condition (without storage) in

case of conical hoppers (Table-5).

• The storage time test at 25.3% mc for 24hrs and 72hrs of storage indicate

that there is significant effect of storage on the coal flowability. The

storage time test at 25.3%mc indicate that the critical (minimum) outlet dia.

required is 1.0m for 24 hours and 1.26m for 72 hours of storage to

prevent the formation of cohesive arching above the outlet in the conical

hoppers (Table-5).

• The minimum conical hopper slope (with horizontal) required for Mass Flow

at the outlet dia. of 0.914m (0.914m was chosen out of general practice in

coal bunkers of 500MW thermal power plant) is 73.50 with stainless steel SS304(2B) liner, 71.30 with SS409M(2D finish) to handle coal at all moisture levels (Table-6).

• The Mass Flow slopes for a wedge shaped (slot outlet) hopper is 100 less

(minimum, varies depending on the outlet size and wall friction) compared to

a conical hopper and the minimum outlet dimension is typically half that of the

conical hopper. The Mass Flow slopes and minimum outlet dimensions for a



wedge shaped hopper are also presented in Table-5&6 as a part of this

study.



6. RECOMMENDATIONS The following recommendations have been proposed based on the test results.

• It is recommended that the proposed coal bunkers of the thermal power plant be

designed to promote Mass Flow. The design parameters presented in the report

should be followed to ensure Mass Flow of coal.

• For a typical case of a conical hopper with outlet dimension of 0.914m, the

minimum slope (with horizontal) should be 71.30 with SS409M (2D) liner in the

converging hopper portion to promote Mass Flow. Whereas the SS304 (2B) liner

is calling for a minimum slope of 73.50.

• The minimum outlet size required to prevent cohesive arching in case of a

circular outlet (dia) is 1.0 m and 1.26m to initiate coal flow after 24 and 72 hours

of undisturbed storage at critical moisture.

• The typical outlet dimension of 0.914m (dia) for the coal bunkers is sufficient to

prevent cohesive arching above the bunker outlet while handling tested coal at all

moisture levels and undisturbed storage in the bunker for less than 24 hours.

However, upon extended undisturbed coal storage beyond 24 hours, the coal is

likely to form an Arch at the outlet leading to no flow condition.

• This problem can be overcome by any one of the following options. 1) To Increase the outlet size to the required 1.26m dia. at the feeder

interfacing. This needs change of gravimetric feeder design which is

however generally not preferred due to increase in belt width. 2) To locate a shut off gate at hopper sectional dia of 1.26m instead of placing

at the 0.914 m dia. During the shut down, the shut off gate should be closed

and coal below the gate is to be emptied by running through the system. In

such an arrangement, the effective outlet size would be 1.26m. This will

facilitate to initiate the coal flow satisfactorily from the silo after extended

period of undisturbed storage up to 72 hours. This option is the most feasible

one in the present case.



• If a different liner like UHMWPE and geometric shape is chosen by any reason,

the design parameters presented in the report for the liners concerned should be

followed strictly to ensure Mass Flow.



TABLE-5

MINIMUM OUTLET DIMENSION TO PREVENT COHESIVE ARCHING AT CRITICAL MOISTURE

Sample Moisture Content Condition Dc

(m) Ds (m)

Dp (m)

Instantaneous 0.70 0.63 0.35

24 Hours storage 1.0 0.9 0.5

Coal

25.3%

72 Hours Storage 1.26 1.14 0.63



TABLE-6 MINIMUM HOPPER SLOPES FOR MASS FLOW AT A GIVEN OUTLET DIMENSION

Mass Flow Hopper slope (degrees, with horizontal)

IS 2062 MS (Rusted)

SS304 (2B)

SS409M (2D) UHMWPE

Moisture Content

(%)

Outlet Dimension

(Dia or Width)

(m) θc θp θc θp θc θp θc θp θc θp

0.5 66.7 54.3 72.2 58.8 70.5 56.4 65.6 52.9 60.7 48.4

0.914 65.8 53.8 70.0 57.6 67.8 54.9 64.6 52.7 60.5 48.4

1.5 65.3 53.5 68.9 57.0 66.6 54.2 64.1 52.0 60.5 48.3

2.0 65.2 53.4 68.5 56.8 66.1 53.9 63.9 51.9 60.4 48.3

16.4

2.5 65.1 53.3 68.3 56.6 65.8 53.8 63.8 51.9 60.4 48.3

0.5 70.9 57.1 75.8 61.1 80.5 62.6 76.6 59.7 67.0 55.1

0.914 68.6 55.7 72.2 59.1 73.5 58.4 71.0 56.4 66.7 54.9

1.5 67.4 55.0 70.6 58.1 70.4 56.5 68.4 54.8 66.5 54.8

2.0 67.0 54.8 69.9 57.7 69.1 55.7 67.4 54.2 66.4 54.8

19.4

2.5 66.7 54.6 69.5 57.4 68.3 55.3 66.8 53.8 66.4 54.7

0.5 76.9 59.6 81.9 64.1 80.3 61.5 78.6 60.2 76.2 60.7

0.914 70.8 55.9 74.8 59.8 72.4 56.7 71.3 55.8 71.8 58.1

1.5 68.1 54.2 71.7 57.8 69.0 54.6 68.1 53.8 69.8 56.9

2.0 67.0 53.6 70.4 57.0 67.6 53.7 66.8 53.0 69.0 56.4

22.3

2.5 66.3 53.2 69.6 56.6 66.7 53.2 66.0 52.3 68.6 56.1

0.5 75.5 59.2 80.7 63.7 76.9 59.6 76.1 59.7 75.3 59.7

0.914 70.5 56.2 74.5 59.9 70.9 55.9 71.0 56.6 71.0 57.2

1.5 68.3 54.9 71.7 58.2 68.2 54.3 68.5 55.2 69.0 56.0

2.0 67.4 54.3 70.5 57.5 67.1 53.6 67.7 54.6 68.2 55.5

25.3

2.5 66.8 54.0 69.8 57.0 66.4 53.2 67.1 54.3 67.8 55.2



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coal flow ability report - bunker coal flow study for anpara-d

Documents

thermal power

thermal power

provide relevant

stainless

ring shear

major consolidation

shear test

ensure mass