Start-up procedure of boiler Power the burner switch and prove the pilot and main flames. Check to see that the boiler vent and drain between the main stop valve(s) and the non-return valve are open. This is done to bleed any air from the system. If installed, open the equalizing valve around the main steam-stop valves. Open the main steam-stop valve(s). When the pressure reaches 10 to 15 psig, close the boiler vent. Test the low-water cutoff by securing the boiler feed and let the boiler steam off naturally to see if the switch cuts power to the burner at the proper low level. If it does not, break power to the burner immediately and take corrective action. With a positive result, reopen the boiler feed valve. Open the non-return valve when the steam pressure reaches 75 to 85 percent of the system-line pressure. Manually test the safety valve for proper operation and reseating. Bring up the boiler pressure slowly during low fire, allowing the non-return valve to automatically cut the

Excess air optimization If combustion air supply is in deficient, proper combustion of fuel may not take place and  hence loss due to unburned fuel will be more.  If combustion air supply is in excess, loss due to unburned fuel is negligible; whereas  heat loss due to heat energy carried away be flue gas will increase.  Furnace losses are mainly divided into three parts.  Loss due to unburned fuel or incomplete combustion loss  Loss due to heat energy carried away by flue gases or flue gas loss  Loss due to radiation and wall losses  Incomplete combustion loss   When air deficiency is there, the fuel atoms do not find enough oxygen atoms to  get burned.  Then some combustibles are left unburned.  This unburned fuel quantity gets reduced when enough or excess combustion air  is supplied.  The loss due to incomplete combustion will be almost negligible in the excess air  region.  Flue gas loss  The heat losses occurring due to the heat energy carried away by flue gases is  called flue ga

Boiler control Boiler control used  Increase uptime and availability The primary objective of most boiler operations is maintaining availability, or uptime. Many facilities have more than one boiler on-site running in parallel. It is essential to maintain and upgrade the boiler control systems to assure steam availability. Modern controls are more reliable and can readily adjust to load swings caused by varying overall plant operations.  Reduce flue gas emissions Failure to comply with current emissions regulations can be as costly as lost utilities. Government mandates enforced by fines, threat of closure, or imprisonment will usually provide sufficient incentive to comply with the regulations and modernize controls if necessary. Improved combustion efficiency reduces unwanted combustion by-products. Anything that goes into the manufacture of a product (raw materials, fuel, air, water, etc.) that is not in the final product is wasted cost. This can also create added waste disposal pro

Boiler steam pressure control Steam pressure is the key variable that indicates the state of balance between the supply and demand for steam.  If supply exceeds demand, the pressure will rise.  A steam flow feedforward signal is used with steam pressure control.

Boiler Master Control  With several boilers supplying a common header in parallel, it is generally desirable to provide a way to adjust the load distribution among the boilers. Depending on the load and the performance of the individual boilers, the most efficient operation may be achieved with some boilers shut down, some boilers base loaded (constant firing rate), and the remaining boilers allowed to swing with the load (variable firing rate).

Furnace Pressure control  A basic boiler has a steam water system and a fuel-air flue gas system.  In the fuel-air-flue gas system, the air and fuel are mixed and ignited in the furnace.  Air and fuel flow into the furnace and flue gas flows out.  The force driving this flow is the differential pressure between the gases inside the  furnace and those outside the furnace.  Furnace pressure is commonly referred to as draft or draft pressure.  The draft is maintained slightly negative to prevent the combustion products and ash  from being discharged from the furnace into surrounding areas through inspection ports,  doors, feeders, etc.  For greatest efficiency, the controlled pressure should be as close as possible to  atmosphere, thereby minimizing the ingestion of "tramp air" or excess air drawn through  the openings in the furnace duct work that cool combustion gases.  Furnaces are classified by the method for moving air and other gases through the  system.

Superheater It is used to remove the moisture from saturated steam coming out of boiler to increase its temperature above saturation point It avoids condensation of steam and avoids the erosion of blades in turbine The metal used for super heat tubes must have high temperature strength, creep strength and resistance to oxidation Normally carbon steel and chromium Molybdenum alloys are used they withstand temperature of 510ºC and 650 ºC Temperature of combustion is approaching the fusing temperature of ash in the coal and therefore there is tendency of the ash to collect in the fluid form on superheat tubes. This is called as slagging. Methods to avoid slagging Locate close to the furnace to develop required steam temperature A bank of screen tubes before superheater to limit slag accumulation Limiting constant temperature of 60 to 65% of load rating Use of combined convection-radiation superheater It effects improvement and economy in the following ways The super heater increases the c

Oil Pressure Drop Relay This is a device designed to monitor a process pressure and provide an output when a set pressure (setpoint) is reached. A pressure switch does this by applying the process pressure to a diaphragm or piston to generate a force which is compared to that of a pre-compressed range spring. A pressure switch is used to detect the presence of fluid pressure. Most pressure switches use a diaphragm or bellow as the sensing element. The movement of this sensing element is used to actuate one or more switch contacts to indicate an alarm or initiate a control action. Pressure switches have different designs with different sensing elements. One of the most common is the one with diaphragms or bellows as the sensing elements The one I will discuss here uses a piston as the pressure sensing element. In any case, the operating principle for this piston type is the same with a diaphragm or bellow type pressure switch.

Turbine Classification  The turbines may be classified based on the process condition, they are Condensing turbine Non-Condensing turbine Extraction turbine Condensing turbine The input to the turbine may be from a single source or from a number of boilers connected to a steam bus and then supplied The output may be condensing in which the exhaust pressure is sub-atmospheric (vacuum) The condensed water is recirculated back into feed water stream to avoid treatment of more freshwater Here the makeup water requirement is kept at minimum Non-Condensing turbine The input to the turbine is similar to that of condensing turbine, but the output may be ‘non-condensing’ in which the exhaust pressure is greater than atmospheric Such a turbine is also called backpressure turbine The steam that exhausted with some pressure may be utilized as process steam for other purposes in the plant Such a steam is called process steam or intermediate pressure steam or low pressure steam Extraction turbine Bo

Steam Turbines  A steam turbine converts the energy of high-pressure, high temperature steam produced by a steam generator into shaft work. The energy conversion is brought about in the following ways The high-pressure, high-temperature steam first expands in the nozzles emanates as a high velocity fluid stream. The high velocity steam coming out of the nozzles impinges on the blades mounted on a wheel. The fluid stream suffers a loss of momentum while flowing past the blades that is absorbed by the rotating wheel entailing production of torque. The moving blades move as a result of the impulse of steam (caused by the change of momentum) and also as a result of expansion and acceleration of the steam relative to them. In other words they also act as the nozzles. A steam turbine is basically an assembly of nozzles fixed to a stationary casing and rotating blades mounted on the wheels attached on a shaft in a row-wise manner. Steam turbines are employed as the prime movers together with

Boiler Drum Level (Feedwater) Control The cylindrical vessel where the water-steam interface occurs is called the boiler drum. Boiler drum level is a critical variable in the safe operation of a boiler. A low drum level risks uncovering the water tubes and exposing them to heat stress and damage. High drum level risks water carry over into the steam header and exposing steam turbines to corrosion and damage. The level control problem is complicated by inverse response transients known as shrink and swell. Simply put, shrink and swell refer to a decreased or an increased drum level signal due to the formation of less or more vapor bubbles in the water, and no change in the amount of water in the drum. This condition produces level changes during boiler load changes in the opposite direction of what is expected with a particular load change. Although only temporary, this can cause severe control system overshoot or undershoot.  Benefits of Boiler Drum Level Control Maximizes steam qualit

Boiler steam temperature control Accurate steam temperature control is necessary for avoiding the over stressing of superheater tubes and turbine front stages and to maintain overall efficiency as high as possible. Heating the steam further from saturation temperature is called superheating. Saturated steam from the boiler is passed through superheaters, where the heat energy from combustion gases is added to it to generate superheated steam. Water side steam temperature control Desuperheater Attemperator Diverting part of the feed water through attemperator for condensing partially saturated boiler steam Fire side steam temperature control Excess air control Flue gas bypass control Adjustable / tilting burner control  Water side steam temperature control Desuperheater action is just reverse of superheater action. The temperature and heat content of steam is reduced here unlike the superheater which increases the temperature and heat content of steam. Desuperheater may be located eithe

Combustion control The primary function of combustion control is to deliver air and fuel to the burner at a rate that satisfies the firing rate demand and at a mixture (air/fuel ratio) that provides safe and efficient combustion. Insufficient air flow wastes fuel due to incomplete combustion and can cause an accumulation of combustible gases that can be ignited explosively by hot spots in the furnace. Too much air flow wastes fuel by carrying excess heat up the stack. Combustion controls are designed to achieve the optimum air/fuel ratio, while guarding against the hazard caused by insufficient air flow.  

Excess air in boiler  Excess air means that amount of air supplied in addition to the theoretical quantity necessary for complete combustion of all fuel or combustible waste material present. Excess air ensures that there is enough air for complete combustion.  Excess air is expressed as a percentage of theoretical air required. Thus, 10% excess air indicates that 110% total air is being supplied.  In boiler operation, excess air represents a heat loss. This loss must be balanced against losses from incomplete combustion. Boiler efficiency is highly dependent on the excess air rate. So operators should optimize excess air to increase system efficiency. To ensure combustion is complete, they also should provide more combustion air than theoretically is required for boilers. This tactic helps ensure safe boiler operation. Technicians also should keep excess air levels as low as possible about 15 percent excess air, equivalent to 3 percent oxygen to reduce the quantity of air to be heated

Non return valve A non-return valve allows a medium to flow in only one direction and is fitted to ensure that the medium flows through a pipe in the right direction, where pressure conditions may otherwise cause reversed flow. There are different types of non-return valves, such as spring-loaded, swing type, and clapper type valves. Non-return valves are for example used with mixing loops in heating and cooling systems to ensure proper operation, and with domestic water systems to prevent backflow.  Non return valves basically allow the flow to move in just one direction. As a result, they’re also known as one-way valves. NRVs are also considered a kind of two-port valve as they have two openings: One opening is for exiting and the other is for entering fluids. NRVs usually don’t require manual assistance and function automatically. So most NRVs don’t have stems or handles. Types of Non-Return Valve Lift Check Valve Swing Check Valve Folding Disc Check Valves Tilting Disc Check Valve