Reasons for Gaps Between Direct and Indirect Efficiency

As we are aware, boiler efficiency can be stated in two different ways, one being direct and another, indirect. Each method has its own advantages and disadvantages.

The direct method gives us more realistic efficiency values but in order to understand where the losses are taking place, indirect efficiency will be more helpful.Constant topic of contemplation among the steam users is the difference between direct and indirect efficiency. Studies done at a large number of plants show that there exists a huge gap between direct and indirect efficiency depending on the type of the boiler and boiler operating practices followed. If one tries to find out the reasons behind this gap, we can find out where these unaccounted losses are taking place and think of possible solutions.

This article tries to find out the reasons behind this wide gap between direct and indirect efficiency. First of all, we will have a look at both the methods of calculating boiler efficiency.Direct EfficiencyIn the direct method, efficiency is calculated by dividing energy delivered by the boiler by energy input as fuel, using the equation:% Efficiency = F (hs - hw) / NCV X f

hrhs = enthalpy of steam at operatin pressure in Kcal/Kghw = enthalpy of feed water in Kcal/KgNCV = Net calorific value of fuel in Kcal/Kgf = actual fuel flow in Kg/hrIndirect Efficiency

The method most standards follow is the indirect efficiency calculation method. In this method, each loss is individually calculated, and the sum of these losses is then subtracted from 100 to give efficiency %.This method has one advantage - since each loss is individually measured, we have quantitative data which we can use to actually reduce an individual loss, thereby increasing efficiency. The losses which occur during the operation of the boiler are Stack losses, Radiation losses and losses due to water and hydrogen in fuel.So, % Efficiency = 100 - (L1 + L2 + L3+L4)Where,L1= Stack LossL2= Losses due to enthalpy in water vapour in flue gasesL3= Radiation LossL4= Unburnt Loss

Why does the difference exist between direct and indirect boiler efficiency?

1. Indirect method is snapshot after boiler is tuned

Indirect efficiency gives us the efficiency of the boiler at a particular time. It does not give us the overall picture of the boiler over a period of time. The boiler is tuned to operate under certain specific conditions, but these conditions are never constant. For e.g the boiler is set to operate at a certain ambient temperature. This temperature is never constant and changes during the course of time. Hence the efficiency of the boiler also changes with the change in conditions.2. Efficiency is at 100% load condition

The indirect efficiency of any boiler is specified at 100% load. But practically no boiler operates at 100% load. Hence in actual the committed efficiency will never be achieved. In reality the efficiency of the boiler falls down at lower loads. Since boilers rarely operate at full load conditions the actual efficiency figures achieved are always less than that specified in indirect method. This adds to the gap between direct and indirect efficiency.3. Start up and shut down losses

Start up and shut down losses are mandatory in every boiler. Burners are incorporated with pre- purge and post-purge which actually is a safety measure. During start-up, the burner does not start firing immediately. Instead it purges air for a period of 30 seconds before the actual atomization. The purpose of Pre-purge is to blow away the residual exhaust flue gases that exist in the furnace and the boiler tubes since the boiler is shut down. Similarly a post purge cycle is carried out after the shutdown. These purging cycles blow away hot flue gases which actually is a loss. This problem gets aggravated when boiler is operated on lower loads. Reason: if the loads drop below the turn-down ratio, the burner trips and the boiler shuts down. This would get reduced if the loads are higher and do not fluctuate.4. Blowdown Losses

Blowdown has to be carried out from the boiler to maintain the correct TDS levels in the boiler. Water contains certain level of TDS. When the water is heated it leads to increased concentration of the TDS which is not good for the boiler. Hence after a certain period of time some quantity of water is removed and fresh water is charged. This is actually a loss as usable heat is being drained out. The fresh charge leads to lowering of water temperature and hence higher fuel quantity will be required. Hence blowdown affects the steam fuel ratio.5. Ambient temp variationEach boiler is set to operate under certain temperature conditions. But the ambient temperature is not constant and varies during the day. This leads to the change in steam fuel ratio.In case of solid fuel fired boilers, following additional factors come in to the picture further reducing the gap between direct and indirect efficiency.1. Ash & Grit Losses:

Ash is generated when any solid fuel is burnt. The quality of solid fuel is not consistent at all the times which leads to higher losses due to unburnt particles. Thus the fuel consumed is higher for generating a specific amount of steam. The quality of fuel leads to the change in the steam fuel ratio.

2. Inconsistency in fuel firing:

In case of manual solid fuel fired boilers the fuel feeding rate is not even. This depends purely on the experience and the assumption of the boiler operator. This leads to inconsistent fuel feeding and will lead to lower steam fuel ratio. In case of indirect efficiency the fuel firing rate is considered constant and hence it does not show the real picture.3. Consistency of fuel quality in terms of calorific value:The calorific value of the fuel is not constant at all times. It changes with the change in season. The moisture / impurities change from time to time leading to an inconsistent steam fuel ratio. This is immediately reflected in direct efficiency because the steam generated to the heat input ratio will vary.4. Fouling nature of fuels

All the solid fuels have fouling tendency. The flue gases generated will foul the tubes during the course of time. This leads to reduces heat transfer area and will increase the fuel consumption.From the above points it evident that direct efficiency shows the real picture as against indirect efficiency. Any change in external factors and conditions will have a direct impact on the fuel consumed for generating a specific quantity of steam. In reality the fuel consumption is much higher than what is committed due to the above reasons. Hence the direct efficiency is lesser than indirect efficiency. It is always advisable to aim at reducing the gap between direct and indirect boiler efficiency.

Understanding Indirect Boiler Efficiency

Indirect Efficiency Calculation & BS845

The method most standards (including IS8753, BS845 etc.) follow is the indirect efficiency calculation method. In this method, each loss is individually calculated, and the sum of these losses is then subtracted from 100 to give efficiency %.

So, % Efficiency = 100 - (sum of all losses)

This method has one big advantage - since each loss is individually measured, we have quantitative data which we can use to actually reduce an individual loss, thereby increasing efficiency. So this method tells us where we are, and how to get where we want to be.

In a typical oil fired boiler, there are three losses to consider :

1. Loss due to water and hydrogen in fuel:

This is the difference between GCV and NCV of a fuel, and needs to be considered if efficiency is calculated on GCV. Not much can be done to reduce this loss, as it is a function of fuel constituents alone.

2. Stack loss:

Improper combustion is responsible for this loss. In most burners, the manufacturer specifies a minimum level of excess air required to ensure that complete combustion of the fuel takes place. However, typically, excess air levels are higher than this specification, so fuel is being spent to heat air from ambient to flue gas temperature. Further, since the amount of air required depends on amount of fuel (which in turn depends on load on the boiler), it varies continuously, making it that much more difficult to ensure that the excess air levels are kept within specified levels. This loss presents the greatest opportunity for energy conservation schemes, whether manual or through automation. Stack loss can increase if the damper is not correctly positioned, or if the burner nozzles need cleaning, or in the case of oil, even if oil temperature is not controlled.

3. Radiation loss:

This is a function of temperature gradient between the boiler water and the ambient, quality of insulation and surface area of the boiler. It is typically specified by the boiler manufacturer at full load conditions (say 1% for a packaged boiler). However, since it is a constant loss, at half load it will be double as a percentage. Accordingly, if steam flow is known, we can work out the instantaneous radiation loss.

Boiler efficiency and blowdown

Most standards for computation of boiler efficiency, including BS845 and IS8753 are designed for a spot measure of boiler efficiency. Invariably, they ask that the blowdown valve be kept shut throughout the efficiency determination process, and therefore remove blowdown from the perspective. However, depending on feed water quality, boiler blowdown can be between 2 and 5 % of steam generation, and is a huge loss by itself. As utility managers, we are not really as interested in the absolute value of efficiency as per some specified method, but more in the steam/unit fuel figure. Accordingly, the blowdown loss is of utmost importance in reduction of a boilers operating cost.

Understanding Boiler Stack Losses

What are stack losses?

Constant combustion takes place inside a boiler furnace. In order to burn the fuel, oxygen is required. Hence, the burner is supplied with air. Air normally has 21% Oxygen, 78% Nitrogen and moisture content depending upon the location. The fuel which is burnt inside is composed of hydrocarbons mainly. After combustion, Carbon dioxide and water vapor are formed due to combustion of carbon and hydrogen as follows-

C+ O2=CO22H2 + O2 = 2H2O

Air comes inside a boiler at ambient temperature, i.e. around 25- 40 Degree Celsius depending upon the location. After combustion, combustion products, i.e. Water vapor, Carbon dioxide and other gases depending on fuel composition are formed. These flue gases heat the water and then leave the system through a Chimney. These flue gases are at high temperature and carry a large amount of heat (m*cp* ?t). This heat energy is taken from the fuel being burnt and along with the flue gases it escapes un utilized. These losses are termed as stack losses.

Excess Air and stack losses

From the equations mentioned above, it can be seen that one molecule of oxygen is required for the complete combustion of one unit carbon to form one molecule of carbon dioxide. Similarly, one molecule of Oxygen is required for combustion of two Hydrogen molecules. So, there is a fixed ratio of Oxygen which should be supplied in order to achieve complete combustion. In reality, the perfect mixing of fuel and Oxygen never takes place and if we supply just the necessary amount Oxygen (air), due to improper mixing, complete combustion of Caron does not take place. This results in partial combustion of the fuel as follows

2C + O2 = 2CO

It can be seen that due to insufficient amount of Oxygen, instead of Carbon dioxide, Carbon monoxide is formed. This process releases significantly less amount of heat as compared to the previous one. Hence, additional Oxygen should be supplied in order to achieve complete combustion of Carbon. This is achieved by sending in some quantity of excess air.

One more reason to supply excess air is, there are always fluctuations taking place during combustion. Oxygen content of air changes seasonally. Apart from that, in case of increase in boiler load, oxygen starvation might take place. Considering all these parameters, it is always recommended to allow certain buffer or excess air to pass in.

Heat carried per hour by stack gases can be calculated by-Q= m*Cp*tWhere,m= mass flow rate of flue gases (kg/hr.)Cp= Specific heat capacity of flue gasest = Difference between inlet and outlet temperatures of flue gases.

Stack losses are directly proportional to mass flow rate of flue gases and the difference between inlet and outlet temperatures of flue gases. Mass flow rate of flue gases and the outlet temperature of flue gases should always be monitored to keep a check on stack losses.

Controlling mass flow rate of flue gases

For efficient combustion, we need to supply a certain amount of excess air. Only required amount of excess air should be supplied. Depending upon the type of fuel which is being used and burner type, standard percentage of allowable excess should be found out and these values should be followed strictly.Controlling outlet temperature of flue gases

A common trend observed is, as boiler is operated continuously, the measured outlet temperature of flue gases increases. After operation for a few months, significant rise in flue gas temperature and hence stack losses is observed. The reason behind this is, as a boiler is operated, scales are formed on the boiler tubes. As the thickness of the scales increases, the coefficient of heat transfer goes down. As a result, heat from flue gases is not transferred to the water side and it is carried away by the flue gases. The best way to tackle the situation is monitoring flue gas temperature just at the boiler outlet and cleaning the tubes when it goes beyond acceptable limits. By treating feed water, the frequency of cleaning can be brought down.Heat recovery from flue gases

Heat which is being carried away by flue gases can be recovered up to a certain limit. This reduces the temperature of flue gases and saves the cost of cooling the gases before leaving them into the atmosphere. The easiest way of recovering heat from flue gases is to pass them through a pre heater. Pre heater heats the air going inside the furnace. This elevated temperature of air results in reduced value of ?t hence reduces the stack losses.Dew point condensation

Depending upon the type of fuel being used, acidic vapors can be present in the flue gases (like SO2). If the temperature drops below the dew point for these acidic vapors, corrosion of metal walls takes place. This puts a restriction on up to what level heat from flue gases should be recovered.

Installing a pre heater system and constantly monitoring excess oxygen and flue gas temperature can help to reduce the stack losses and keep them minimal.

Variation in Boiler Efficiency with Load

It is a general observation that boiler efficiency tends to decrease as the boiler load decreases. Both practically and theoretically, for any boiler, the efficiency is highest at maximum load conditions. At part loads, considerable drop in the boiler efficiency is noticed. This article briefly explains why efficiency of a boiler drops down when it is operated at part loads.

Reasons for drop in boiler efficiency at low/part loads-

Radiation losses

The boiler is designed to transfer a specific amount of heat through the designed surface area. For this the fuel should be fired at the specified rate. Radiation losses depend on the heat transfer area. Heat transfer area is constant for a given boiler and hence, remains constant at different load conditions. This implies that some part of the generated heat is always lost as radiation losses. Therefore when the boiler is operated at lower loads, lesser amount of fuel is fired. As a result, lesser amount of heat is generated. But some part (Which is constant) is going to be lost. Hence radiation losses increase at lower loads.

In simple words,Suppose the boiler heat transfer area is designed for 1000Kcal/hr. The radiation losses for this surface are say 10 Kcal/hr i.e. 1%. So when the boiler is operated at lower loads, lesser fuel is fired and in turn lesser heat is generated say 500 Kcal/hr. But the radiation losses remain 10Kcal/hr, as the surface area doesnt change. This means 10Kcal (2%) of total 500Kcal generated is lost. Thus radiation losses increase. Thus it is advised not to run boilers on lower loads.

Low Fire Operation:During the operation at lower loads the combustion is less efficient. This is because of the increased oxygen % in the flue gases. Generally the oxygen % is 2-3% greater at low fire operation than what at high fire. Thus the oxygen % rises above the ideal 3-3.5% and efficiency drops.

Start up and shut down losses:Start up and shut down losses are mandatory in every boiler. Burners are incorporated with pre- purge and post-purge which actually is a safety measure.

During start-up, the burner does not start firing immediately. Instead it purges air for a period of 30 seconds before the actual atomization. The purpose of Pre-purge is to blow away the residual exhaust flue gases that exist in the furnace and the boiler tubes since the boiler is shut down. Similarly a post purge cycle is carried out after the shut down. These purging cycles blow away hot flue gases which actually is a loss. This problem gets aggravated when boiler is operated on lower loads. The reason for this is, if the loads drop below the turn-down ratio, the burner trips and the boiler shuts down. Occurrence of such situations can be brought down if the loads are higher and do not fluctuate.

At the low load conditions, all of the above 3 parameters come into the picture which in turn bring down the boiler efficiency. To ensure that boiler operates at high efficiencies, it should always be operated at full load conditions.

Benefits of On-Line Boiler Efficiency Monitoring

Why maintaining boiler efficiency is crucial?

In case of boilers, cost of buying a new boiler is small compared to the amount of money spent in operating the boiler year on year. The following diagram shows the break-up of total cost of ownership for both oil/gas fired boilers and solid fuel fired boilers. It can be easily noticed that the fuel cost is the biggest contributor in the overall ownership cost for both oil/gas and solid fuel fired boilers.Amount of money spent on fuel is closely related to the efficiency of the boiler. Although boiler efficiency is an important parameter in the boiler specification when buying a new boiler, little attention is given to it once the boiler is steaming. The myth is that boiler will always continue to generate fuel at its rated efficiency. Truth is, the actual delivered boiler efficiency tends to be lower than 3%-12% than the rated efficiency.

A survey conducted by CII shows that scope of savings in boiler house ranges from 28% to 46% for different industries (out of total scope of savings in the entire steam and condensate loop).At the same time, as following illustration clarifies, steam consumption increased by 16% when boiler efficiency dropped by 10%.This makes putting check on boiler losses even more crucial.

The cost of steam generation has gone up by almost 2.5 times in the last decade. The cost of generating steam using a solid fuel fired boiler today is greater than an oil fired boiler ten years ago. In the next sections of this article, we will explore how online efficiency monitoring can help us to tackle the situation in a much better way.Online boiler efficiency monitoring- Why is it important?It can be observed that the efficiency of a boiler is not static- but it is dynamic. Users take it for granted that the boiler is operating at the rated efficiency. Monitoring across large number of boiler houses shows that there are many variations which affect the boiler efficiency. It is important to adjust the boiler for these variations regularly to ensure optimum boiler performance.

A few of the factors which affect boiler performance are-1. Shift variations

Typically in plants, load pattern changes from shift to shift. If the boiler response to these load variations is not changed, the efficiency of the boiler drops. The required changes in settings can be as simple as changing firing rate or pressure limit of the boiler. The changes can be easily done from the boiler control panel itself in oil, gas or solid fuel fired boilers. Online efficiency monitoring helps to identify what needs to be changed at what time.2. Daily variations

Load variations may occurs from day to day too. Again, the boiler needs to be adjusted to cater to this efficiently.3. Weekly variations

As the fuel quality is changing continuously, adjustments need to be made to the combustion system to take care of these variations. In oil or gas fired boilers, the quality of oil being received may vary from tanker to tanker. The moisture content in solid fuel changes from time to time and the source of purchase. This changes in fuel quality makes adjustments necessary.4. Seasonal variations

Boiler loading pattern is an important factor here too. The production requirement of the plant may be affected by seasonal demands. This calls for adjustments again. In solid fuel fired boilers, the fuel available may change depending on the seasons. The ambient air temperatures and humidity will also change from season to season. The combustion system needs adjustment too.Online efficiency monitoring How does it help?

Online boiler efficiency monitoring works through measuring several critical parameters. Some of the measured parameters are- stack temperature, steam temperature, feed water temperature, combustion air (inlet) temperature, Stack oxygen, furnace pressure, steam flow, fuel flow, blowdown, blowdown TDS etc.

The strength of online monitoring is not only limited to measuring the parameters as they also calculate different losses, efficiency and generate valuable trends and analysis. The losses which such systems can calculate include blowdown losses, radiation losses, and enthalpy losses and stack losses.Efficiency and Steam: Fuel Ratio Calculation

Real time efficiency monitoring makes available its users the boiler efficiency at any time. This helps the plant personnel to relate any changes in the boiler efficiency to the reasons behind the changes. This particularly helps in identification of factors which are preventing the boiler from running at the optimum efficiency and hence help the personnel to take the correct action.Patterns/ Trends and Analysis

Many times, observing data over a long period of time can give valuable insights. Observing trends and pattern of a boiler over months can help to spot important trends and take diagnostic actions. It is good to have a real time monitoring system which intelligently suggests corrective action by analysing the trends over a long period.

Typical Boiler Questions

What is Scale Formation ?How do I stop Scale Formation from happening ?What is corrosion ?How do I stop Corrosion ?Why all the concern about Condensate Treatment and Monitoring ?How do I prevent the Most Common Boiler Problems ?A More In-depth LookWhat is the meaning of Horse power ?What is Scale Formation ?Scale formations in boilers are responsible for lost efficiency, increased maintenance and operating costs not to mention lost revenue due to outages and downtime. Most scale formations in boilers can be traced to the presence of hardness in the make-up water. This hardness reacts in the high temperatures environment within the boiler to form and insoluble scale. This insoluble scale coats the heat transfer surfaces, acting as an insulator to impede heat transfer.Hardness isn't the only cause if scale formation in boilers, other impurities such as iron, silica, copper, oil, etc. are often found in samples of boiler scale. In fact, it is rare to find scale which isn't the result of several of these impurities.Normally pre-softening the water before feeding it to the boiler is the first step in eliminating scale formations. Even when the make-up is soft, there is still a need for chemical scale inhibitors inside the boiler. With proper treatment the problems of lost efficiency, tube damage and lost production can be avoided or greatly reduced. Proper treatment requires the right balance of chemical treatment and control.How do I stop Scale Formation from happening ?The first and foremost aspect of stopping scale formation is to have a good idea of the make-up water that is feeding your system. If you aren't sure, have a certified laboratory complete a fully analysis on this water so you can make an informed decision on what exactly the potential problems you may encounter.After determining these specific aspects of your make-up water then your water treatment expert can guide you through a program that fits your situation.Just a few items that may be of concern when putting together a good water treatment program for your boiler. A complete program will include sludge build-up, pH levels, oxygen removal, condensate treatment, and alkalinity levels.Back to TopWhat is corrosion ?Corrosion in boilers can almost always be traced to one or both of two problems. The most common cause is dissolved oxygen entering the system via the feed-water. The oxygen causes very localized corrosion to occur in the form of pitting. The pits are small but deep pinpoint holes which eventually can penetrate tube walls and cause their failure. Another common cause of corrosion in boiler systems is low pH within the boiler. This reduced pH may result from carbon dioxide infiltration or form contamination by other chemicals.Oxygen corrosion is normally controlled by driving the oxygen from the feed-water in a deaerating heater or by chemically removing it with an oxygen scavenger such as sodium sulfite.There are many contaminates which can infiltrate a boiler system and cause low pH levels to develop. Manufacturing wastes such as sugar or acids from plating operations which can be returned to the boiler with condensate can be a source of problems because they concentrate in the boiler. Oxygen can infiltrate the boiler system at virtually any point. When dissolved, oxygen is present in boiler feed water attach on feed lines, pumps and economizers can be expected. The severity of the attach depends upon the concentration of the oxygen and the temperature of the water.Back to TopHow do I stop Corrosion ?You can use a deaerator which is defined as a piece of equipment which heats water with steam to insure essentially complete removal of dissolved gases. There are several types of deaerator available, each having its own advantages and disadvantages.Internal treatment for dissolved oxygen corrosion is normally accomplished by the addition of sodium sulfite. Most oxygen scavengers contain a catalyst which speeds the reaction of the sulfite with the oxygen. In systems equipped with a deaerator the sulfite should be fed to the storage tank of the deaerator or to either the suction or pressure side of the feed water pump. In systems which do not have a deaerator, the sulfite can be fed at almost any point in the feed water system, including the condensate tank.Internal treatment for carbon dioxide is normally accomplished by the use of a volatile amine. "Amine" refers to any of a number of chemicals derived from ammonia. There are two major groups of amines in practice as water treatment chemicals today. There are normally referred to as "neutralizing amines" or "filming amines" depending upon whether they neutralize the acid formed by carbon dioxide or form a protective film on the metal.Filming amines do not neutralize the carbonic acid which forms in condensate systems. Instead, they form a film on the metal which is non-wettable, or impervious to water. this protective film prevents the corrosive impurities from contacting the metal.Neutralizing amines function by increasing the pH of the condensate. Normally they are fed at such a rate that the pH of the condensate is maintained slightly above 7.0. Satisfactory reduction of carbon dioxide corrosion is possible with the use of a neutralizing amine. it is necessary to supplement this type of condensate protection with an oxygen scavenger to remove dissolved oxygen.Whether condensate corrosion is controlled by chemical treatment or a combination of mechanical and chemical methods, it is important that careful checks and testing be incorporated as a part of the treatment program. No treatment can be better than the way in which it is applied. Consult a water treatment expert to get you started on the right foot.Back to TopWhy all the concern about Condensate Treatment and Monitoring ?

You Condensate is very important to your facilities overall operation, ignoring this unseen component will soon cause failures costing bottom-line dollars. Therefore, condensate must be treated with the proper chemistry. Treating your plants steam condensate is critical for several reason, but these are the most important two reasons:

1. To insure the integrity of your equipment.2. To keep the amount of condensate corrosion minerals that are returned to the boiler's makeup water in check.

Corrosion in your steam lines occurs when the carbonic acid builds up and begins to breakdown the metallic surfaces throughout the system. When the Carbonic acid is allowed to build, localized attacks occur due to the simple increase in CO2, which is the breakdown product of carbonate alkalinity in the boiler, condensing with water to form H2CO3. This results in the "pitting" of condensate piping, which usually shows up by visual leaks at threaded junctions. Oxygen pitting occurs as steam condenses and the vacuum created pulls air into the system. Due to the localized nature of oxygen pitting, it can cause relatively quick failure in a condensate system.The most common method of dealing with this problem is through the use of neutralizing amines. These chemicals, better known as morpholine and cyclohexylamine, neutralize the carbon acid, and increase the pH of the condensate. Corrosion of mixed metallurgy condensate systems is minimized when the pH is maintained between 8.8 and 9.0. Due to high alkalinity in boiler makeup water elevating the pH to this level may not be economical. In this case the pH should be maintained at 8.3 or higher, or a filming amine applied.A filming amine, such as octyldecylamine, provides a non-wettable protective barrier against both carbonic acid and oxygen. When utilizing a filming amine, the pH is usually maintained between 6.5 and 7.5, so a neutralizing amine may still be required.

In order to minimize oxygen pitting one can utilize a filming amine as previously mentioned, or a volatile oxygen scavenger such as DEHA (diethylhydroxyamine.) DEHA provides better results as it scavenges oxygen and passivates or coats the condensate system, making it less susceptible to corrosion.Depending on the treatment method chosen, condensate monitoring can vary. In all cases the following tests should be performed.

1. Soluble and insoluble iron levels.2. pH levels at various points in your steam condensate system. It is extremely important that pH measurements be made on cooled samples. If the sample is taken hot, carbon dioxide will gas off, which results in artificially high pH measurements.

If a filming amine is utilized, the residual should be measured. The same is true if DEHA is used asan oxygen scavenger. In the latter case, a residual of 100 to 150 ppb is usually targeted. Note thatthis may take time (as much as 6 months) since much of the DEHA will be consumed passivatingthe system.Back to TopHow do I to Prevent the Most Common Boiler Problems ?

A regular inspection schedule is critical and should cover four areas: boiler, burner, controls, andsystem.

Preventive maintenance is the most widely used means of minimizing common problems in boilers.Unfortunately, most maintenance programs do not properly address the needs of the boiler and itsrelated systems. Statistics indicate about two-thirds of all boiler failures and nearly all unscheduledshutdowns are caused by poor maintenance and operation.

Boiler inspection and maintenance are critical. It covers four basic areas: boiler, burner, controls, and system.Regardless of boiler design, application, or size, the basic maintenance criteria remain the same.

Maintaining the BoilerThere are eight primary areas of the boiler itself that should he examined or inspected regularly.

Water level. The most important maintenance inspection is to check the boiler water level daily.Insufficient water causes pressure vessel damage or failure. At a minimum, steel in the pressurevessel could overheat. The condition could change the pressure withholding capabilities of thevessel, necessitating vessel repair or replacement. More seriously, a low water level could damagethe equipment or building. or even cause personal injury.

Boiler blow down. Steam boilers should be blown down daily to maintain recommended dissolvedsolids levels and to remove sludge and sediment. Hot water boilers generally take on no makeupwater and, therefore do not need to be blown down.

As the boiler takes on makeup water the solids concentration builds up. Solids accumulate in eitherdissolved or suspended form. Unless they are controlled dissolved solids promote carryover ofwater with the steam causing water hammer and damaging piping, valves, or other equipment.Carryover also raises the moisture content in the steam, affecting proper operation of equipment thatuses steam.

Suspended solids, which cause sludge or sediment in the boiler, must be removed because theyaffect the heat transfer capabilities of the pressure vessel. Sludge buildup leads to problems rangingfrom poor fuel-to-steam efficiency to pressure vessel damage.

Water column blow down. Water columns on steam boilers should be blown down once each shiftor at a minimum once a day. This action keeps the column and piping connections clean and free ofsediment or sludge. The water column also must he kept clean to ensure the water level in the gaugeglass accurately represents the water level in the boiler. The gauge glass and tricocks connected tothe water column are the only means of visually verifying boiler water level.

The low-water cutoff should be checked once a week by shutting off the feed water pump andletting the water evaporate under normal steam conditions at low fire. The gauge glass should heobserved and marked at the exact point at which the low water cutoff shuts down the boiler. Thetest verifies operation of the low-water cutoff under operating conditions. The low-water cutoff alsoshould the removed and cleaned every six months.

Water treatment. Proper water treatment prolongs boiler life and ensure safe and reliable operation.Treatment programs are designed around the quality and quantity of raw water makeup and systemdesign. They should be directed by a qualified water management consultant. Flue gas temperature.Flue gas temperature is a good indicator of boiler efficiency changes. The temperature should berecorded regularly and compared to those of a clean boiler under the same operating conditions.Accurately determining the affect on efficiency requires that the firing rate and operating pressure bethe same.Back to Top

A rise in flue gas temperature usually indicates dirt on the fireside of the boiler or scale on thewaterside. As a rule of thumb a 40-deg F rise in temperature reduces boiler efficiency 1% The costof fireside cleaning should be compared to those of lower operating efficiencies to determine theminimum temperature rise at which the fireside should be cleaned. Other factors also affect flue gastemperature. For example, a rise in stack temperature may indicate a baffle or seal in one of theboiler's passes has failed.

Waterside and fireside surfaces. Waterside and fireside surfaces should be inspected and cleanedannually. A visual inspection provides an early warning that the vessel needs repair or watertreatment or that combustion needs adjustment. Inspecting and cleaning water-column connectionsshould receive special attention. Soot in the breeching is a fire hazard and can cause severecombustion-related problems.

Safety valves. Safety valves are the most important safety devices on the boiler They are the last lineof defense for protecting the pressure vessel from overpressure. Once a year. operating pressureshould be tested by bringing the relief valve to its setting. Valves should pop and reseat according tothe valve stamping.

Refractory. Refractory protects steel not in direct contact with the water from overheating. It alsohelps maintain proper burner flame patterns and performance. If the boiler remains on all the time,refractory should be inspected twice a year. If the boiler cycles more frequently or is turned on andoff daily, refractory should be inspected more often.

Heating and cooling refractory a lot shortens its life considerably. It cracks and eventually fails. Hotspots on the steel that the refractory protects indicate refractory or gasket failure. If a hot spot isfound, the cause should be determined and repaired immediately to prevent the steel from failing.

Maintaining the BurnerAlthough burners vary by design, application, fuel, regulations, and insurance requirements, the samebasic maintenance criteria must be addressed. Burner maintenance generally focuses on safety.efficiency, and reliability. Adjustments should be made only by a trained service technician using theproper instrumentation and tools.

Combustion. Poor combustion is unsafe and costly. Changes in combustion air temperature andbarometric pressure, for example, impact burner performance (see table). Low excess air levelsresult in incomplete combustion, sooting, and wasted fuel. High excess air levels raise stacktemperatures and reduce boiler efficiency. Maintaining steady excess air levels with an oxygen trimsystem helps ensure optimum efficiency at all times.

Visually inspecting combustion is the easiest way to detect changes that affect safety and efficiency.Changes in flame shape, color, and sound are among early indicators of potentialcombustion-related problems. Changes may be due to:

Large fluctuations in ambient temperaturesChanges in fuel temperature, pressure, heating value, or viscosityLinkage movement dirty or worn nozzleDirty or distorted diffuser dirty fanDirt on the boiler firesideFurnace refractory damage.

Visual combustion inspection should be compared to flame characteristics observed at similar firingrates with efficient combustion. However, combustion efficiency is verifiable only with a flue gasanalyzer. Even if a flame appears to be good, it should be checked with an analyzer and adjustedonce a month.

Fuel and air linkage. Changes in fuel and air linkage affect the combustion fuel-to-air ratio. Flamefailure or a hazardous fuel rich condition may result. Proper linkage settings should be physicallymarked or pinned together. Linkage should be checked for positioning, tightness, and binding. Anynoticeable changes should be remedied immediately.Back to Top

Oil pressure and temperature. Pressure and temperature directly affect the ability of oil to properlyatomize and burn completely and efficiently. Changes promote flame failure, fuel-rich combustion,sooting, oil buildup in the furnace, and visible stack emissions. Causes include a dirty strainer, wornpump, faulty relief valve, or movement in linkage or pressure-regulating valve set point. Oiltemperature changes typically are caused by a dirty heat exchanger or a misadjusted or defectivetemperature control.

Gas pressure. Gas pressure is critical to proper burner operation and efficient combustion. Irregularpressure leads to flame failure or high amounts of carbon monoxide. It may even cause over orunder firing, affecting the boiler's ability to carry the load. Gas pressure should be constant at steadyloads, and should not oscillate during firing rate changes.

Usually, pressure varies between low and high fire. Therefore, readings should be compared tothose taken at equivalent firing rates to determine if adjustments are needed or a problem exists.Gas pressure irregularities are typically caused by fluctuations in supply pressure to the boilerregulator or a dirty or defective boiler gas pressure regulator.

Atomizing media pressure. When oil is burned, an atomizing medium, either air or steam, is neededfor proper, efficient combustion. Changes in atomizing media pressure cause sooting, oil buildup inthe furnace, or flame failure. Changes result from a regulator or air compressor problem or a dirtyoil nozzle.

Fuel valve closing. If a fuel valve leaks, after burn may occur when the burner is turned off, or rawfuel could leak into a hot boiler and cause an explosion. When the burner is turned off, the flameshould extinguish immediately. Prolonged burning is a hazard and demands immediate action.

Maintaining the ControlsControls are often used to protect the boiler against unsafe operation. Flame safeguard, operating,limit, and safety interlock controls are among the most common. Of course, controls only protectthe boiler if they are maintained and adjusted properly.

Flame safeguard control. Also called the primary control or the programmer, the flame safeguardcontrol ensures safe light-off, operation, and shutdown of the burner. The control regulates purgingthe boiler of all gases prior to trial for ignition. It also verifies that there is no flame in the boiler priorto lightoff, and checks for a pilot before allowing the main flame to light. The control provides proofthat the main flame has ignited before releasing the boiler to the run (modulation) mode. Mostimportantly it does not allow any action to occur if operating controls, limits, or safety interlocks areopen.

In addition, this control initiates a post purge upon shutdown to remove all gases from the boiler.And it often provides a means for detecting a problem elsewhere in the system. Although the flamesafeguard is designed for fail-safe operation and is quite reliable, a faulty device can be catastrophicand should not be ignored.

Operating and limit controls. These controls tell the boiler at what temperature and pressure tooperate. Proper settings minimize boiler cycling, maintain proper limits for efficient system operation,and ensure the boiler shuts down when predetermined limits are reached.

Improperly set operating controls cause the burner to operate erratically and stress the pressurevessel. All these controls should be checked weekly. The scale of the control for temperature orpressure settings should not be relied upon. Settings should be verified with the actual operatingtemperatures and pressures on the boiler gauges.

Safety and interlock controls. Safety and interlock controls vary with state, local, and federal codesand insurance requirements. They must be operational at all times. Among the consequences ofinoperable safety interlocks are personal injury, equipment or property damage, and liability forlosses or damages. All interlocks should be checked weekly for proper operation. A defectivecontrol should be replaced immediately. A control should never be bypassed to make a boiler run.

Indicating lights and alarms. Indicating lights and alarms are part of the control circuit. They alert theoperator to specific boiler conditions. Unfortunately, they are often neglected and do not providethe intended information. Many control circuits have test buttons to verify proper operation. Circuitsthat do not should be checked by simulating conditions that activate a light or alarm.

Maintaining the SystemAll too often, when a boiler problem occurs, the system is overlooked. The emphasis falls on theequipment and not the equipment's function in the overall system. An effective maintenance programmust be based on an understanding of the entire system and the function of each piece of equipment.Only an understanding of the system provides the means for preventing the causes of system-relatedproblems and reducing the time spent on the symptoms.Back to Top

Operating conditions. Operating parameters of the boiler room system should be recorded daily.The data provide a means for evaluating boiler operation trends that affect efficiency, downtime, andmaintenance planning. The following data should be recorded.

Feed water pressure/temperature. Changes in feed water pressure affect the system's ability tomaintain proper boiler water levels. A pressure drop may be caused by a leaky check valve on astandby pump or a worn pump impeller. Changes in feed water temperature are indicative of aproblem in the deaerator, potential pump seal damage, loss in efficiency, dirty economizer, dirtyblow down heat recovery exchanger, or excessive or insufficient condensate returns.

Boiler water supply/return temperatures. On hot water systems, supply and return temperatures tothe boiler are a means for evaluating the system's effect on the boiler and vice versa. The desiredoperating temperature set point and temperature differential across the boiler should be evaluatedagainst the system design to determine if a potential problem exists. High temperature differentialscaused by excessive load or a control malfunction could cause thermal shock and subsequentlypressure vessel damage.

Makeup water use. Records of the amount of makeup water used help determine the presence ofleaks or losses in the system. They also assist in developing a more effective chemical treatmentprogram. Excessive water use indicates a change in system operation and, therefore, a change inefficiency.

Steam pressure. Steam pressure operating set points usually are based on system design and type ofsteam use. Pressure changes are typically caused by problems with control settings, burneroperation, boiler efficiency, or, most commonly, changes in steam demand.

Leaks, noise, vibration, and unusual conditions. Checking for leaks, noise, vibration, and the like is acost-effective way to detect system operational changes. For example, a small leak is repaired bytightening connections. By the time a leak becomes large, sealing surfaces usually are worn andmajor repairs are needed.Back to TopA More In-depth LookA maintenance program must focus on prevention to be an effective tool. Whether the maintenanceprogram is motivated by safety, cost, reliable operation, or all of these, it is the best means ofpreventing common, boiler-related problems.Automatic low-high water control equipment must be serviced on a daily basis when the boiler is in operation. A high frequency of boiler failures is the result of low water, and can be attributed to a careless boiler operator. A procedure must be established at your school to regularly clean the glass gauge column by "blowing down" the column at the start of the school day, during non-peak operating periods, and at the conclusion of the school day or shift. This ensures ability to determine the level of water in the boiler.

Low WaterA major reason for damages incurred to low pressure steam boilers is the low water within the boiler. If the condition of low water exists it can seriously weaken the structural members of the boiler, and result in needless inconvenience and cost. Low pressure boilers can be protected by installing an automatic water level control device.

Steam boilers are usually equipped with automatic water level control devices. It must be noted, however, that most failures occur due to low water on boilers equipped with automatic control devices. The water control device will activate water supply or feed water pumps to introduce water at the proper level, interrupt the gas chain and ignition process when the water reaches the lowest permissible level, or perform both functions depending on design and interlocking systems. No matter how automatic a water control device may be, it is unable to operate properly if sediment scale and sludge are allowed to accumulate in the float chamber.

Accumulations of matter will obstruct and interfere with the proper operation of the float device, if not properly maintained. To ensure for the reliability of the device, procedures must be established in your daily preventive maintenance program to allow "blow-down" the float chamber at least once a day. Simply open the drain for 3 to 5 seconds making certain that the water drain piping is properly connected to a discharge line in accordance with local Codes. This brief drainage process will remove loose sediment deposits, and at the same time, test the operation of the water level control device. If the water level control device does not function properly it must be inspected, repaired and retested to guarantee proper operation.

Low Water Cutoff - Tests and MaintenanceThere are two very effective tests for low water controls on steam boilers. The first is the quick drain. or blow down test, which should be performed at a time other than a peak steam generating period. As the water is drained from the column the firing sequence is interrupted, the low water alarm signal activates and the boiler operation shuts down.

The second, and more costly method is the slow-drain test. By opening the blow down valves the water level can be checked to determine the water level in the column, the gauge glass, and the boiler. The boiler should shut down while you determine the level in the gauge glass.

As a safety precaution, the low water float chamber of hot water boilers should be tested daily, at the beginning of the shift, at the end of the shift, and once during non-peak firing periods. Time of tests and the boiler controls tested should be recorded on your Boiler Room Log.

Annually, or as required, a thorough inspection of all low water control parts shall be performed. The annual inspection should include opening and cleaning the water chamber.Back to TopFeed Water PumpsOld, worn and obsolete feed water pumps are sometimes overlooked as potential problems. A centrifugal pump may have worn seal rings that allow the water to chum between the suction and discharge openings.

An indicator of the latter problem is low pressure discharge. Also, by comparing the time it takes to raise the boiler water level to a predetermined level or the time to empty the condensate tank to the time it formerly required, it is possible to determine if a pump is operating properly. Also, a pump that operates quietly does not mean it is functioning properly.

OverpressureSafe operation of a boiler is dependent on a vital accessory, the safety valve. Failure to test the safety valve on a regular basis or to open it manually periodically can result in heavy accumulations of scale, deposits of sediment or sludge near the valve. These conditions can cause the safety valve spring to solidify or the disc to seal, ultimately rendering the safety valve inoperative. A constantly simmering safety valve is a danger sign and must not be neglected. Your preventive maintenance program includes the documentation and inspection of the safety valve. A daily test must be performed when the boiler is in operation Simply raise the hand operating lever quickly to its limit and allow it to snap closed. Any tendency of a sticking, binding or leaking of the safety valve must be corrected immediately.

Steam Traps - Care and MaintenanceSteam traps have play a very important role in steam distribution systems. The service performed by steam traps is primarily to discharge condensate. Normally a steam trap can be easily and quickly selected by considering only the average operating conditions. However, an exact analysis of these conditions will give the proper data necessary for selecting the type and size for greater savings and proper plant operation. After the careful selection of the steam trap, it must be properly installed, tested, periodically inspected, cleaned and maintained to keep it operating efficiently.

Traps need cleaning periodically. A simple way to prevent dirt from entering is to drop a short length of pipe vertically below the supply to the trap (called a dirt leg) which can be cleaned easily and frequently.

Traps can be seriously damaged by scale or pipe comings in lines. A good practice is to install strainers ahead of the traps which should be inspected and cleaned frequently.

Traps are subject to severe wear if steam blows through continuously. They should be inspected for worn valve parts or a change in operating conditions.

When a steam trap fails to discharge, inspect the heating system and be certain that all units are drained with separate traps, thus guarding against short circuiting, loss of energy, and reduction of operating efficiency.

Traps operating under high pressure or superheated steam are often insulated in a manner similar to adjacent pipe lines. In such instances, they shall be fitted with dirt pockets, test valves, and drains.

Steam traps installed in areas exposed to climatic conditions will lose heat if not insulated and may freeze unless adequately protected. Discharge lines should be short and self draining and traps should be fitted with a drain tapping and valves.

Steam traps handling large volumes of air require more frequent inspection and proper venting for efficient operation. Vents shall be used to avoid air binding and ensure positive drainage. Gauge glasses shall be kept in proper repair, for they indicate whether or not the trap is working. Periodic cleaning and gauge glass replacement shall be considered as a high priority in the maintenance of steam traps.

All steam traps require protection from corrosion to prevent unnecessary deterioration. All valves, joints, and gaskets should be kept tight to avoid steam leakage and ultimate energy losses. For continuous and efficient operation. steam traps require periodic inspection and maintenance for purposes of eliminating foreign matter and obstructions in supply and discharge lines. Each steam trap at an assigned work station should be inspected as specified by the preventive maintenance program.Back to TopSteam Trap - TroubleshootingIt is important to inspect the operation of steam traps frequently. There are many conditions under which traps may fail to operate property. The following are some of the most common reasons for trap failures:

1. Condensate does not flow into the trap:

A. Obstruction in line to trap inlet. B. Valves leading to trap are closed. C. Bypass open or leaking. D. Trap may be air bound. E. Insufficient pressure to blow condensate through orifice. F. Improper installation of trap. G. Accumulation of foreign matter within the trap. H. Trap held closed by defective mechanism. I. Strainer may be blocked.

2. Condensate fails to drain from trap.

A. Discharge valve may be closed. B. Trap may not be large enough to handle condensate. C. Pressure may be too low to blow the condensate through. D. Improper installation for draining. E. Check valve may not be holding. F. Obstruction in return line or the line may simply be too small.

3. Trap does not shut off.

A. Trap is too small for the condensate load. B. Trap held open by defective mechanism, C. Overload due to excessive boiler foaming or priming. D. Submerged steam coils leaking. E. Differential pressure exceeds design of trap. F. Scale or foreign matter lodged in orifice.

4. Steam blows through trap.

A. Valve mechanism does not close due to wear or defective valve. B. Mechanism is held open by foreign matter. C. Trap has not been properly primed or reprimed after clean-out or blow-off. D. Bypass is open or leaking. E. Excessive pressure for design of trap.What is the meaning of Horse Power ?Horse Power is a unit of measurement of the ability of a boiler to evaporate water, usually given asthe ability to evaporate 34 lb. (15.6 kg) of water an hour, into dry saturated steamfrom and at 212F (100C).Back to Top