trend setting approach

8/6/2019 Trend Setting Approach

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India Bo iler dot Com

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Trend Setting Approach towards controllable Losses inSteam Generating System

By Sri Ansuman Sen Sharma; India Boiler dot Com

Introduction:

Steam generating system efficiency is defined as the heat added to the

working fluid expressed as a percentage of the heat in the fuel being burnt.Efficiency = Heat output in steam/ Heat Input through fuel. If someone does

this calculation and finds their boiler efficiency is to the tune of 85 to 88%, the

person quite naturally would be satisfied with the performance of their boiler

plant. In fact, it is quite often, one would tend to approximation in measuringperformance parameters in order to reach the efficiency figure of his/ her

preference.

This approach doesnt give any sort of idea about the performance nor does it

affect any change in actions to optimize performance of the system.When asked about the performance, some times you come across these

responses:

Our stack gas never looks black! In fact it looks a little whitish yellow. We are

very careful about supplying adequate air. We have Oxygen monitoring system; therefore our combustion must bepretty efficient!

Our stack temperature is 180oC, which is same as what other industries are

maintaining. We are using Natural Gas as fuel! We have an Economizer in line and therefore all waste heat is being

recovered

We have hundreds of steam traps in the steam distribution system. Hardly 4

to 5 are passing, which is less than even 0.5%!We have a large and complex steam distribution system. All steam pipes are

well insulated. Only few of the valve insulations are sometimes removed to

attend gland leakage (You know, Indian Steam Valves!). Since they frequently

leak, we keep them open to attend the leakage from time to time. However,its just a few valves!

Since we have a low pressure boiler, we are using softening plant for water

treatment..and so on.

With more focus on energy conservation these days, a number of technologieshave evolved and continues evolving, which can improve the performance

significantly. But if you are already satisfied with the performance of your

system, would you take the pain to introduce a new technology or change theexisting practice and risk rocking a steady ship? Therefore first we need to be

dissatisfied! As Sharukh Khan theatrically says in one of those Ads DONT

BE SANTUSHT!

Its not Heat .its Money!

The new trend setting approach is exactly that. Gone were the days, where

fuel bill was a necessary expenditure to achieve all important Production.And forget about passing on the fuel cost to the customers, because you

would be kicked out of the competition in that case. You will have to start

looking for the Controllable Losses and when you identify them, there are

lots of ways to plug them. One has to first change their approach towardsSteam Generating System.


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Performance of steam generator:

In order to achieve economic optimization in Boiler operation, one needs to

ensure maximum extraction of heat from the fuel and then transfer maximum

extracted heat to water and steam without causing any accident.Not all boilers are created equal each boiler system has its own specific

characteristics, be it a different design consideration, different fuels ordifferent operating conditions. Approach towards economic optimization

naturally differs from system to system.But the good thing is Every Boiler operates under the same fundamental

thermodynamic principles. If we take two Steam generating systems, one

with a capacity of 700 TPH steam at 510oC superheat and 110 bar pressureand another with a capacity of 8 TPH saturated steam at 12 bar pressure, the

fundamental for heat utilization remains same.Steam generating system efficiency to the greater extent depends on the skill

of designing but there is no fundamental reason for any difference in

efficiency between a high pressure and low pressure boiler. Large boilers

generally would be expected to be more efficient particularly due to designimprovements.

If we start looking for the controllable losses, if we can identify them, we can

definitely find means to stop them.

Change in outlook: Actuallythere are hundreds of areas, where we need tochange our outlook. However, for the sake of this discussion, let us consider

only those statements, I have mentioned earlier.

Statement #1

Our stack gas never looks black! In fact it looks a little whitish yellow. We are

very careful about supplying adequate air. Mind you; this kind of attitudelies with the operator of small and aged boilers, generally using lignite or

other solid fuel, firing manually most of the time, and guise what? They aremore in numbers in our country! The only priority for their operator is to

supply needed quantity of steam at required pressure to the user end. The

inlet Air Damper remains in 100% open condition all the time and at times, 80

150% excess air is being used. No wonder the smoke looks whitish yellow incolour! The owners, not the operators, in this case are required to change

their approach. This can be helped if some one uses a calculator and tell the

owner that even lowering the excess air just by 10% might lower the annualfuel cost by Rs. 60-70 Ks, even when you are using a cheap fuel like lignite.

Whereas, the scope is to lower down the excess air by nearly 40-50% in many

cases!Then perhaps they would start thinking about installing some combustion

control techniques. A little modification in the furnace and fuel feeding

methods can result into a lot of saving.

They may even start thinking about getting a new, more energy efficientBoiler all together! The mantra is. DONT BE SANTUSHT!

Statement #2

We have Oxygen monitoring system; therefore our combustion must be

pretty efficient! This is a slightly better scenario than the previous one.Here the intent of controlling excess air is present but the method of

application is doubtful in some cases. Simply installing a Combustion Control

unit by Oxygen Trimming may not achieve the desired control of excess air. In

most cases, when the process operator starts complaining about thetemperature drop (read pressure drop, since more steam is being drawn than

the boiler can produce), the poor operator at the boiler has no option other

than trying to get maximum..and the result? Inlet Damper in 100% open


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condition and to hell with what Oxygen meter is showing! In fact they are the

happiest when these Oxygen meters start giving faulty results (which they

eventually do) and then they are in no hurry to get it calibrated. In many

cases VFD is not installed, therefore losing in both combustion efficiency andfan power. In this case both the owner and the operator should be aware that

merely introducing Combustion Control Technology is not enough. You have toensure proper operation of the same. Just DONT BE SANTUSHT because

you have spent some money to ensure efficient combustion.

Statement #3

Our stack temperature is 180oC, which is same as what many otherindustries are maintaining. We are using Natural Gas as fuel! Now that is a

criminal waste of energy! When we can bring down that temperature to even100oC if possible and when lowering of 22oC can increase the efficiency by a

whopping 1%! One may not be able to change it that easily but at least one

should be always aware about it. If we keep in mind that there is a huge

amount of energy available, which can be recovered, we would be alwayslooking for the economically viable options to recover the same. There can be

so many options.installing an Economizer, installing an Air Heater, installing

a condensate heater, using the heat in an evaporative chiller, or one can eventhink of drying fuel by the hot gas. Till you get some solution to recover that

heat, just DONT BE SANTUSHT. See money going out of your chimneywhenever you look at the stack.

Statement #4

We have an Economizer in line and therefore all waste heat is being

recovered This is again a slightly better scenario than the previous one.Some thing has been done to recover the waste heat, therefore no one can

blame the intention but in many cases, the application is again doubtful. I willinterject with one of my personal experience here. One of the chemical

Industries, have asked us to review their system since they doubted the

adequacy of steam generation from one of their package type boilers. Their

stack temperature sometimes goes as high as 200-210oC. They are firingNatural Gas and sometimes FO. They also have an Economizer in line!

We came to know from the operators that the temperature pick up in the

economizer is around 5oC! Fortunately, during our visit, the Economizer wastaken off line because of a tube leakage and since repairing was going on, we

could get a look inside. What we found was the partition wall which was

designed to be there (it was there in the drawing alright!) to guide the hot fluegas across the tubes, was absent. The flue gas was going inside the

Economizer and short circuiting to the outlet without passing through the Eco-

tubes. The Economizer was practically serving the purpose of an ash collector

when they have been using FO (which was evident from the thick layer of ashcollected at the bottom). Now, this boiler was in operation for several years,

during which a number of times the Economizer was opened for tuberepairing. It never occurred to anyone that The Economizer is redundant, it is

not recovering any waste heat..why? Because everybody was

SANTUSHT, an Economizer was kept in line after all! This is a classical

example of the approach, which we are required to change.

Statement #5

We have hundreds of steam traps in the steam distribution system. Hardly 4to 5 are passing, which is less than even 0.5%! A definite understatement!

Much more than those numbers are likely to be in failed open condition. There

are very few organizations, which are seriously checking trap condition


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regularly. In most of the process industries, once the traps are fitted, since

they are self operated, they are usually forgotten. Particularly where

condensate is recovered through a closed cycle (which is of course

commendable), it becomes extremely difficult to identify a faulty trap. Itrequires a trained team of professional to identify a trap in failed open

condition. This is of course going to cost money, in fact, quite a lot of moneysince you are planning to replace a trap every time it is found faulty! Who is

willing to spend unnecessarily for such small things? Only large plants, whichare having large and complicated distribution system, may be able to afford

such expenditure.this could be the common reaction from most of the

small plant operators.Here is a simple variant of the Napier formula, which gives a pretty close

estimation of the Steam loss through an orifice.

Steam Flow (kg/ hr) W = 11x Pa x D

[Where Pa is Absolute Pressure, kg/cm2and D is diameter of Orifice in cm]

Formula for daily Fuel Cost per Trap - Using Cost of Natural Gas for steam

raising:

Q = W x 24 x h fg x C / (BE x CV)

Where: Q = Energy Lost per day in RupeesL = kg/Hr of steam lost

hfg = Latent heat of steam at corresponding pressure

C = Cost of fuel per sm3

BE = Boiler EfficiencyCV = Calorific Value in kcal/ sm3

When using some other fuel, CV and C are to be taken accordingly.Even if we consider a partially passing trap or valve or any other orifice, we

will see the cost of steam loss is reaching a staggering figure. Next time I see

a steam trap, I would start wonderingis it passing? I will see money

going down the drain (literally!) and I will not be SANTUSHT until I make

sure it is not.

Statement #6

We have a large and complex steam distribution system. All steam pipes are

well insulated. Only few of the valve insulations are sometimes removed toattend gland leakage (You know, Indian Steam Valves!). Since they frequently

leak, we keep them open to attend the leakage from time to time. However,

its just a few valves! That happens to be a fact! A steam line has to be

insulated and that is universally agreed. You ask whyand 8 out of 10answers would be Steam would get condensed, otherwise. There is

nothing wrong with the answer, but everything wrong with the approach.

Lets talk money.

The heat losses through bare pipes can be found by reference to Table given

by Spirax Sarco and using a simple equation. Against different temperature

difference, heat emission per meter of un-insulated pipe is to be taken fromthe below mentioned table. It is considered as heat emission from barehorizontal pipes with ambient temperatures between 10C and 20C and still

air conditions. The result may not be very accurate, but it would give a close

estimation of the actual loss taking place.


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Table 1

Pipe size (NB)

15 20 25 32 40 50 65 80 100 150

Temperature

difference

steam to airC W/ m

60 60 72 88 111 125 145 172 210 250 351

70 72 87 106 132 147 177 209 253 311 43280 86 104 125 155 171 212 248 298 376 519

90 100 121 146 180 196 248 291 347 443 610

100 116 140 169 207 223 287 336 400 514 706

110 132 160 193 237 251 328 385 457 587 807

120 149 181 219 268 282 371 436 517 664 914

130 168 203 247 301 313 417 490 581 743 1025

140 187 226 276 337 347 464 547 649 825 1142

150 208 250 306 374 382 514 607 720 911 1263

160 229 276 338 413 418 566 670 794 999 1390

170 251 302 372 455 457 620 736 873 1090 1521

180 275 330 407 499 497 676 805 955 1184 1658190 299 359 444 544 538 735 877 1041 1281 1800

200 325 389 483 592 582 795 951 1130 1381 1947

Using the temperature difference of the steam and atmosphere, it can be

converted as

Fuel cost per meter length per day of bare pipe =

HL (from the table) x 3.6 x 4.187 x 24 x C/ (BE x CV)

Even if we neglect the loss due to condensate, this would sum up to a

considerable amount if we use a calculator.

Next time, when we see an un-insulated steam pipe, we would not think ofsteam condensation, we will see MONEY DOWN THE DRAIN.

Here is an interesting piece of information. It is not only important to insulatethe bare pipe but all hot parts of the system with the exception of safety

valves should be insulated. This includes all flanged joints on the mains, and

also the valves and other fittings.

Equivalent lengths of pipe for valves and fittings can be taken as:

Pair o f mating flanges - 0.5 m Line size valve - 1.0 m

So if we have 4 or 5 line size valves un-insulated, those should be counted as4-5 m of bare pipe. We can not just wait for the annual turnaround to attend

the miscellaneous insulation jobs. As prefabricated insulating covers for

flanged joints and valves are now more widely available the loss by keepingthese parts un-insulated for maintenance point of view can be avoided. These

are usually provided with fasteners so that they can readily be detached to

provide access for maintenance purposes.

Effect of moisture

Insulation primarily depends on air-filled voids to function effectively.Exposure to moisture causes the displacement of insulating air by heat-

conducting water. Waterlogged insulation transfers heat 15-20 times faster

than dry insulation! Protecting insulation from moisture/water ingress is just

as important as selecting the most effective type of insulation and installing


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an economic thickness. The practical requirement, then, is to make

waterproofing an integral part of any insulating job.

Install adequate, leak-proof vapour barriers on the interior (warm) sideof walls, ceilings or floors.

Weatherproof exterior walls by cladding or other treatment thatprevents water infiltration.

Maintain the integrity of water-impervious roof membrane by regularinspection and maintenance.

Cover insulated pipes with suitable cladding (whether for indoor oroutdoor applications) with sealed joints, and maintain its integrity byinspection and prompt repair of damaged sections.

For high-temperature applications, choose a vapour-permeablecovering that will allow moisture to pass outward.

Statement #7

Since we have a low pressure boiler, we are using softening plant for water

treatment What is wrong in this statement? Apparently nothing, because a

good softening plant should be enough for a low pressure boiler, so far as

water quality control is concerned.Does a softening reduce TDS? The answer is No.Does the use of soft water reduce Blow Down? The answer is still No.

Does Blow Down affect efficiency? The answer is still No! Why? Because most

standards for computation of boiler efficiency, including IS 8753 and BS845 do

not include blow down as a loss in the efficiency determinationprocess.. and we areSANTUSHT!

Should we?

In a small boiler plant usually there is no flush steam recovery system and

here the cost of fuel loss comes toBw x (hg h) x 24 x C/ (BE x CV)

Where,Bw = Blow down quantity per hour

hg = Enthalpy of 1 kg of saturated steam produced under working pressure,

h = Enthalpy of 1 kg of feed water entering the boiler.

And Bw can be calculated as (considering no condensate return)Bw=(Generation in kg per hour x Feed Water TDS) / (permissible TDS in

Boiler Water Feed Water TDS)

if we consider use of return condensate as Feed water with some % age of

make up water from the softening plant, the blow down rate will decrease andwe can work it out from simple material balance.

It is evident that if we can keep the Feed water TDS as minimum as possible,

the required rate of Blow down will decrease. Though the permissible TDS in

Boiler Water is quite high for low pressure boilers (3500 5000 ppm), still the

blow down requirement would be considerably high. Usually once a shift blowdown is the most common practice in many boiler plants.Where as, a small RO plant or a DM plant is not costing very high these days.

Why dont we check out? The Mantra is once againplease DONT BE

SANTUSHT!

Conclusion:Energy Management is not a job for a designated group of professionals givenwith the responsibility to increase efficiency. When every individual handling

energy starts looking for holes to plug the controllable losses, there would be

tangible result. Therefore the trend setting approach for every operators of a


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steam generating system would be See money while seeing fuel, or

stack gas, or steam.. always try to identify the controllable losses. and

never, ever be SANTUSHT with the existing practice

trend setting approach

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