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ASME Performance Test Code - 19.3, Fundamentals of Temperature

Measurement

4.7.6Primary Air Flow Calibration

4.7.6.1Purpose

Calibration of primary airflow through the pulverizers is the first step in balancing &el and air to the burners. It is important to ensure that each primary air flow instrument is indicating the actual primary air flow through the mill before proceeding with other pulverizer tests by calibrating the station flow indication against the flow measured in a grid using a standard pitot or a calibrated 'S' type pitot.

The data collected in this test may also be used to draw clean air curves.

4.7.6.2 Frequency

Time based, typically every two years, but the other triggers can take precedence:

Evidence of coal setting in horizontal pipe runs

*Poor coal fineness

Excessive mill spillage of coal

High pressure drop across pulverizer

4.7.6.3Unit Conditions

Primary air flow calibrations are normally done with the unit in service, so the air temperature during the test is the same as during normal operation. (Test can be conducted with the unit off-line, but on-line is preferable.) Steady conditions on the pulverizer and PA fans needs to be maintained during test period. Calibration should be done at minimum three different operating flows.

4.7.6.4 Data to Be Collected.

The control room data on Pulverizer to be tested should be logged on a control room log sheet to ensure that test has been conducted under stable conditions, and to compare the station flow to the actual (measured via the pitot and grid) flow. Readings should be taken every 10 minutes. See Vol. I1 for an example.

The air flow should be determined by dividing the duct into a grid, and measuring the temperature, static pressure and pitot (usually "S"e) differential in the centroid of each area. The number of points to be used should be determined according to Section 4.7.13. It is not very often that in a power plant requisite straight length would be available in ducting to a Pulverizer to measure air flow with an accuracy of +\- 2.0% which is

4.7.7.1 Purpose

The second test in a rnill pe~ormanceprogram is to check to see if each coal pipe is receiving the same amount of primary air. Clean airflow tests are also used to calibrate primary air flow measurement device wherever straight lengths are not available in PA ducting to the pulverizer.

4.7.7.2 Frequency

Time based, typically every year before the unit outage, but the other triggers can take precedence:

Slag formation around certain burners of the same pulverizer. CO imbalance at the hrnace exit

Temperature imbalance at the hrnace exit

Large O2spread at AH inlet to maintain equal steam temperatures. High content of combustiblesin fly ash

Poor flame appearance

4.7.7.3 Unit Conditions

Clean air tests are done over the normal range of airflow through the mill, without feeding any coal, but preferably at the normal mill outlet temperature (the unit should be in service to provide hot primary air, but the rnill being tested will be out of service).

Prior to conducting tests, details of work undertaken during the unit over haul should be recorded. Change of burner nozzles, fbel pipe orifice inspection and their replacement etc. should form part of detailed database to be maintained. Clean airflow tests in a running unit can be conducted when unit is operating at part load during off peak hours when more stable operating regime is likely to prevail. Calibration and checking of instruments associated with the specific pulverizer should be carried out before carrying out clean air flows tests. Such tests are run at a minimum of three different operating points.

4.7.7.4 Data to Be Collected

Control room data on Pulverizer to be tested should be logged on a control room log sheet to ensure that test has been conducted under stable conditions. Readings to be taken every 10 minutes. Control room log sheet and data sheet for recording velocity traverse data are annexed to Pulverizer test procedure No: CENPEEP/EFF/TP/201 forming part of Volume No.II.

The velocity in each coallair pipe should be measured with a pitot (usually an "L" type) by traversing a minimum of two cross sections (measuring the same locations where coal is sampled for a fineness test, see Section 4.7.13.3). The traverse point should be covered in both the inward and outward movement of pitot. The air temperature is to be measured using K type of thermocouple.

Data on static pressure in &el pipe and barometer pressure to be taken during the test period.

Fuel pipe area to be taken fiom engineering drawings for computation of flow.

4.7.7.5Calculations

Velocity in each fitel line is calculated to ascertain clean air balance. Clean air balance is expressed as a deviation fiom the mean velocity of a1 the pulverizers individual coal transport pipes. The following formulae process dirty air traverse data.

BP +-SIP

% Deviation = Ava. Velocity - Velocitv x 100%

Avg. Velocity

Volumetric Flow (Q)= Velocity x Pipe Cross-sect. Area

Mass Flow (W) = Q x 60 midhour x Density

Bp = Barometric Pressure ("Hg); Sp = StaticPressure ("WC)

K = Calibration Coefficient of the pitot (standard "L" type pitot is 1)

4.7.7.6 Analysis

The air flow through each coal pipe is measured and the flow through each individual pipe is compared to the average flow through all pipes. If each pipe is not within +I- 2% of the average, the coal pipe orifices should be adjusted or replaced. Database obtained can be used to plot clean air curves for the pulverizer.

Velocity traverse data obtained in each fuel pipes where ever adequate straight lengths are available should be plotted on a curve to ensure that profile across the cross section of pipe is regular. To improve the accuracy of measurement ,traverse cross section could be more than two cross sections.

4.7.7.7 Reference

Test Procedure for Routine Performance Testing of Pulverizer Volume It of this document.

4.7.8Pulverizer Dirty Air / Coal Flow Test

4.7.8.1Purpose

The dirty air/coal flow test is usually the third rnill performance test (run after obtaining satisfactory results fiom both the primary air calibration and clean air tests. This test is used to:

Detect imbalance, if any, in the air and coal flows between the discharge pipes of a Pulverizer.

Collect a representative, iso-kinetic, sample of pulverized coal from dierent pipes for determination of fineness fractions

Cross-check the readings of the station instrumentation e.g. primary airflow through the mill, mill outlet temperature, coal flow through the feeder etc.

4.7.8.2 Frequency

Time based, typically after every pulverizer overhaul and before the unit outage, but the other triggers can take precedence:

Slag formation around certain burners of the same pulverizer.

CO imbalance at the krnace exit

Temperature imbalance at the hrnace exit

Large 02,spread at AH inlet to maintain equal steam temperatures.

High content of combustibles in fly ash

Poor flame appearance

4.7.8.3 Unit Conditions

Prior to conducting tests, details of work undertaken during the equipment over haul should be recorded. Change of burner nozzles, he1 pipe orifice inspection and their replacement etc. should form part of detailed database to be maintained.

Tests on a specific pulverizer should be done when the mill parameters outlet temperature and primary airflow are steady and mill is running around "nominal loading".

The definition of "nominal loading" must not be changed fiom test to test. Before the first test on a pulverizer, the test engineer should select that are as close to the maximum capacity, but, shall always be achievable.

4.7.8.4Data to Be Collected

A member of the test crew logs the mill parameters every fifteen minutes during the test to ensure the stability of test conditions.

Two or more ball valves are installed on straight section of each mill discharge pipe as per ASME code depending on the upstream and downstream lengths available from the nearest bend or disturbance. (See Section 4.7.13.3)

Dustless connectors are used with the ball valves to ensure dust free working. Conditions of mill internals with reference to running hours, overhauls, pipe orifice replacement etc. are noted for each mill to be tested.

Traverse points on the pitot tube are marked on an equal area grid in accordance with ASME Pedormance Test Code for traversing circular ducts or pipes.

Two equal lengths of tubing are cut to required length and then taped or bound

A 5" to 10" vertical inclined manometer is set up on a level and stable work area. The tubing is connected to the high and low side taps on the manometer.

Dirty pitot tube is inserted in each sampling port and traversed across the pipe to measure differential pressure at 12 pre-determined points as per ASME code. Based on these values, the sampler dP is obtained using formulae for iso-kinetic sampling.

Static pressure and temperature are measured using static pressure probe. The following data should be recorded for each test.

Coal pipe designation

Individual velocity head for each traverse point (For 2 ports - 24 points) Temperature and static pressure for each coal transport pipe

After determination of the dirty air velocities in their associated coal pipes, isokinetic coal samples are extracted. The coal sampling probe is marked identical to the dirty air test probe.

The sampler differential pressure obtained in step 8 will result in a velocity through the sampler tip that is equal to the velocity of the coal and air mixture through the coal transport pipe (i.e. iso-kinetic sampling).

The power consumed by the mill during the,test can be computed by connecting energy meters

4.7.8.5Calculations

Velocity in each fbel line is calculated to ascertain d i i air balance. Dirty air balance is expressed as a deviation from the mean velocity of all the pulverizers individual coal transport pipes. The following formulae process dirty air traverse data.

% Deviation = Ava. Velocity - Velocity x 100%

Avg. Velocity

Volumetric Flow (Q)= Velocity x Pipe Cross-sect. Area

Mass Flow (W) = Q x 60 rnin/hour x Density

Bp = Barometric Pressure ("Hg); Sp = Static Pressure ("WC)

K - Calibration Coefficient of the pitot

Iso-kinetic fuel sample is collected, weighed & analyzed for fineness data using four standard mesh screens.

After completing the testing of all the coal transport pipes of a mill, following formulae are used to calculate the coal flow through each coal pipe.

The performance data obtained can be put on a feedback format to circulate for necessary corrective action.

TcTime of sample collection per pipe in minutes

Air to fuel ratio = Air Flow in kg per hour

Coal Flow in kg per hour

Gross Coal sample is collected from feeder inlet chute for determination of coal characteristics namely size of raw coal, moisture, HGI for applying corrections to the measured test data.

Table 4.11 Example of Dirty Pitot SurveySummary Data (MillX)

Measured Mill outlet temperature matches with the control room value but the Mill operating PA flow differs by 20 tlhr.

High +50 mesh retention could be ascribed to the high PA flow through the mill.

4.7.8.6 Analysis

Acceptable Values arefiom a dirty air test are:

Pipe to Pipe Dirty Air Flow Variation +/- 5%

OJAL PIPE

M R T I C M I INalNED

USTLESS CONNECTOR

Figure 4.2 Dirty Air Probe

Reproducedwith Permissionfrom Literatureof American Boiler Construction, Inc.