XXVII. EFFICIENCY TESTINGOF BOILERS

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Contents1. INTRODUCTION

2. DIRECT METHOD AND INDIRECT METHOD EVALUATION OFEFFICIENCIES

3. TEST PROCEDURE

4. EFFICIENCY CALCULATION -BS-2885(1974) /IS : 8753 : 1977

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XXVII. EFFICIENCY TESTING OF BOILERS

1. INTRODUCTION

As a part of contract obligation a guarantee clause for the performance of the equipment isincluded in each and every contract. The text of a particular contract reads as follows:

Performance tests shall be conducted by the company on the proposed boiler. The purchaserat his own expense, shall make all preparations, furnish all operating and testing personneland incur all expenses with such tests. The date for such performance tests shall be mutuallyagreed upon by the purchaser and the company.

Performance tests and performance calculations shall be made in accordance with BS 2885standards. The measure of performance shall be the results of such tests. Performancecalculations shall be based on the latest edition of the IS[ Steam tables.

Subject to the performance conditions specified in preceding paragraphs 1 thro 11, the companymakes the following guarantees.

The company guarantees the steam generator to be continuously operated at the rated outputcapacity, when firing the specified coal. The company makes the following performanceguarantees at the rates MCR capacity of 180 T/Hr. and with a feed water temperature of200oC.

Overall efficiency = 85%

Superheated Steam Pressure at Main Steam Stop Valve of each Boiler = 96 Kg/sq.cm.

Superheated Steam Temperature at Main Steam Stop Valve =510 + 100c

The above guarantees are based on the higher calorific values of design coal specified withambient air temperature of 400C and relative humidity of 60% and with normal surfaceconditions in furnace.

Having seen a typical contract condition, it is but natural you may get some doubts.

Is it all the four guaranteed?

Yes : The first one guaranteed is efficiency, the second is pressure, the third one steamtemperature and the fourth is the low.. But indirectly the evaluation of efficiency takescare of all other parameters.

What is BS 2885 Standard is the next question before you?

Each country has its own standard. For example India has IS, the British BS, the AmericansASME-PTC, and the Germans DIN Standard. All standards are basically the same. Dependingon the origin of the manufacture the standards also are referred.Could it be true that all standards are the same ?

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Yes, really. There are minor differences in the method of evaluation of efficiency betweenthese standards; but the end result normally does not change by more than 0.05% betweenthese standards.

Then why BS Standards is referred in this contract.The reasons are:

i) The influence of B.S. on various countries.

ii) it is the only standard which speaks about both direct method and in i rect method ofevaluating the efficiencies. All other sandards normally mention only indirect method.

iii) Recently Indian Standard Institution has come out with IS : 8753-1977 with thesimilar guidelines as that of BS : 2885-1974

2. DIRECT METHOD AND INDIRECT METHODEVALUATION OF EFFICIENCIES

In the direct method, the output (Steam) and input (Coal) are measured and the ratio output/input in terms of heat values is called efficiency. In, the indirect method, the input is assumedto be 100% and the various losses encountered are calculated and substracted from 100. Thevarious losses considered for this type of treatment are :

i) Exhaust Gas loss

ii) Air Moisture loss

iii) Radiation loss

iv) Fuel Moisture loss

v) Unburnt Carbon loss

vi) Unaccounted loss

vii) Loss due to hydrogen in fuel.

Which method is more reliable ?

Technically speaking both are same. Direct method physically satisfies the customer as he isable to see the output and input. Direct method, gives an edge for the manufacturer as theunaccounted loss and radiation loss considered at the design stage give him some advantage.

Will a manufacturer conduct the guarantee tests as per direct method? Yes ; But the purchaserhas to provide him facilities to measure positively the fuel burnt and the water evaporated.In general for small oil/gas fired boilers, an attempt can be made to measure the efficiency bydirect method. But for large boilers, it is advisable to go for losses methods as the combursomemethod of weighing tons of fuel and water can be avoided. BHEL normally does the guaranteetest by losses method.

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In general guarantee is given for maximum continuous rating load of the boiler. Hence thetest will be conducted at boilers MCR Ratings. It may so happen in a utility boiler the MCRRating cannot be achieved due to the reason that the turbine may not be able to consume allsteam. In such cases the boiler will be loaded according to turbine limitations and the boilermanufacturers cannot be compelled to go for MCR load.

3. TEST PROCEDURE

3.1 Instruments

All important instruments are calibrated by customer engineers in the presence of suppliersrepresentatives. The important instruments are:

i) Flow meters like steam flow meter, feed water flow meter, auxiliary steam flowmeter.

ii) Temperature indicators like, . main steam temperature at superheater outlet, feedwater temperature, auxiliary steam temperature.

iii) Pressures like, steam pressures at super-heater outlet, feed water inlet pressure,auxiliary steam pressure

iv) Flue gas outlet temperature after air heater.

v) Orsat analysis at air heater outlet.

For the meters, the DP transmitter square root extractor, and indicator/integrator should beindividually and collectively calibrated.

Ali indicators for temperature must be calibrated.

All pressure gauges must be tested by dead weight pressure gauge tester.

Flue gas outlet temperature must be measured at number of points.

Fresh KOH ; solution and Pyrogallic acid must be filled in orsat apparatus.

All these instrument calibration must be attested by the customer and BHEL before the startof the test.

3.2 Sampling

Fuel sampling is an important factor in the guarantee test. Raw coal is collected at periodicalinterval from the feeders. The sampling point is predetermined and the samples thus collectedwill not lose any inherent moisture. The normal interval at which the samples are collectedcan be 15 minutes.In the case of an oil fired boiler a conti. nuous dripping arrangement for the oil will be madeso that a portion of the fuel going to the burner will get collected.In gas fired boilers, it is not possible to store the gas in a container and then analyse it in alaboratory. Hence normally it is analysed as the gas is fired in boiler.

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3.3 Points to be Agreed

The customers representative and the suppliers representative must agree on certain points.

i) The extent of the duties and responsibilities of the designated competent person.

ii) The specific objects of the tests.

iii) That the specified working conditions can reasonably be met and that the appropriatefuels can be burned at the necessary rates.

iv) Correction curves or tables for deviation of test conditions from stipulated operatingconditions.

v) The number and duration of test.

vi) That the specified fuel-to be used in the tests is available in adequate quantity tomeet the requirement.

vii) In the event of plurality of fuel being burned, the ratio of the different fuels to beused.

viii) The method of evaluating efficiency.

ix) The method of expressing the thermal efficiency (i.e.) direct method or losses method.

x) The general method of operation and responsibility for operating the plant duringthe test.

xi) If various manufacturers are involved, their, respective responsibilities for theperformance of the component plant items and of the plant as a whole.

xii) The laboratory to make the necessary analysis of fuel and refuse.

xiii) The provision of reserve fuel samples and refuse samples.

xiv) Any departure from the method of measurement prescribed.

xv) The steam tables to be used.

xvi) The state of the plant, (i.e.) cleanliness of contact surfaces, wear and tightness andthe operation of cleaning equipment.

xvii) The operation of blow down and if to be used, the method of measurement.

xviii) The operation of integral ash quenching equipment.

xix) The method of measurement of fuel.

xx) The method of sampling fuel.

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xxi) The method of fuel grindability determination.

xxii) The method of determining the mass of refuse.

xxiii) The method of assessment of heat tossers from water filled ash hopper and fromsolid residues of combustion.

xxiv) The method of sampling refuse.

xxv) The method of flue gas analysis.

xxvi) The method of measuring feed water.

xxvii) The instruments to be used for temperature measurement.

xxviii) When testing according to losses method, the value to be adopted when necessaryfor radiation and convection losses.

xxix) The method of steam purity measurements and correction and o f boiler waterconditioning.

xxx) The location of instruments.

xxxi) Tile measurement intervals.

xxxii) The condition under which a test should be terminated as unsatisfactory.

3.4 Duration of the Test

The duration of the test is normally only 4 hours in the case of pulverised fuel fired boilers, oilfired boilers and gas fired boilers. But however the boiler will run in the test load conditionatleast for one hour prior to the test and for one hour after the test. During these periods, theparameters will be maintained at test load condition. But however it is expected that theboiler should run for a period of 9 hours prior, to the control period under steady state conditions.

3.5 Frequency of reading

The aim of this is to have as many readings as possible, so that a better average will beobtained. As per this standard, atleast 25 readings are necessary for better average. BHEL asa policy takes all readings at every five minutes duration except flow meter readings. Due tovarying nature of our industrial load and utility load, flow meter reading are taken at everythree minutes.

The above readings are attested by the persons who recorded it, the customers representativeand the suppliers representative. A copy of these readings will be given to the supplier and aswell as the customer too.

All samples collected will be made into three equal parts, each weighing not less than one kg.in weight. One part will be given to the customer, the other will be given to the supplier and

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the third one will be sealed and kept at the customers premises.

The customer and supplier will independently analyse the sample and agree upon the valuesto be used. In case they are not able to come to an agreement, the neutral sample will beanalysed in an agreed laboratory and the results will be binding both of them However, theresults of such samples analysed should be intimated to the supplier within one month fromthe date of test in order to enable the supplier to prepare the report.

3.6 Important Precautions

During the period of guarantee test, it is very much necessary that the heat transfer surfacesare fairly clean. Hence at the start of the guarantee test, the supplier has the right to clean theheating surfaces like economiser, water wall, superheaters and airheaters. The customer willprovide the necessary facilities including the man power etc.

The supplier will also use the soot blowers to clean the water wall surfaces just before thestart of the test. He may also use the blow downs to bring the salts under control. But, however,he is prohibited to use these during the test period.

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4. EFFICIENCY CALCULATION -BS-2885(1974) /IS : 8753 : 1977

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Contents1. INTRODUCTION2. DIRECT METHOD AND INDIRECT METHOD EVALUATION OF EFFICIENCIES3. TEST PROCEDURE4. EFFICIENCY CALCULATION -BS-2885(1974) /IS : 8753 : 1977