Deaerator Troubleshooting and Preventive Maintenance

Topics on this blog

  1. Theory of Deaeration
  2. Spray and Tray Cascading System
  3. Deaerator Trouble Shooting
  4. Preventive Maintenance of Deaerator

Deaerator is an equipment used for the removal of impurities from feed water like dissolved oxygen, carbon monoxide, carbon dioxide and content of Sulphur.  In addition to removing free oxygen and dissolved gases, a Deaerator provides the advantage of heating boiler feedwater. Expansion and contraction of heating surfaces and the thermal shock are reduced and also a reduction in fuel consumption by adding hot feedwater to a boiler.

 

THEORY OF DEAERATION

  • Dissolved gas is removed by reducing the partial pressure of that particular gas in the surrounding atmosphere.
  • The solubility of any gas dissolved in any is directly proportional to the partial pressure of gas above the liquid.
  • The heating system is provided with trays, steam header inside the Deaerator.
  • The first row of trays are designed to act as a condensing stage which allows only the non-condensable gases to escape and vent losses of steam is appreciably controlled.

The Deaerator is made of two parts:

– In the upper tank, deaeration is done and the water is heated

– In the lower tank, deaerated feed water is stored.

SPRAY & TRAY CASCADING SYSTEM

  • In order to accomplish a high degree of efficient deaeration, water is sprayed through various spray nozzles arranged below the water header.
  • When water enters the nozzle at pressure, fine mist generated by whirling action of vanes.
  • Mist so created passes through various stages of trays arranged in rows.
  • Incoming steam not only heats the tray but also removes dissolved gases.

By the time water reaches

  1. Stage 5 – Devoid of O2& CO.
  2. Stage 7 – Co2
  3. Stage 8 – So2
  • Condensation of steam is reduced to the negligible degree because of already preheated water and most of the steam remains at vapour.

 

DEAERATOR TROUBLESHOOTING/ PROBLEMS IN DEAERATOR

Why Oxygen O2 Level is High in Deaerator?

A heater is specified to ensure minimum O2 levels of 7 ppb.

The Oxygen level can go high in the Deaerator if there are any loose fittings which result in the air infiltration.

The other reasons for High O2 can be because of Trays not installed properly in place, O² inlet not in accordance with specified designed conditions, Water inlet temperature can be too low, Spray valves not installed correctly.

Even if the stabilization period is insufficient the O2 levels will be high. The improper venting can also cause High Oxygen level in a deaeration system.

 

Why the outlet temperature of Deaerator low?

The calibration of the thermometer can be wrong which can show the incorrect Thermometer reading.

If the steam flow is insufficient, Heater will be flooding or if the Inlet Flow is not piped correctly the temperature can indicate low temperature.

The ratio of the Steam and water should be correct for the correct outlet temperature.

If the Pipe and Valve sizing in not correct the spray valves will not function properly to give proper temperature.

It is good to check all valve and control settings, and inlet flows and temperatures periodically to prevent damage.

 

 

Why there is Excessive Pressure fluctuation in Deaerator?

Steam Pressure Reducing Valve, downcomer, and equalizer may be improperly sized or calibrated.

The Inlet steam pressure can be too high or too low or there could be excessive inlet temperature variation to cause fluctuation.

Even if the heater is flooding there could be excessive pressure fluctuation. The design should be within the design range to avoid pressure fluctuation.

 

Why Water Hammering in Deaerator?

Water hammering can happen when the designing is not as per the parameters.

The inlet flows mixing just prior to Deaerator inlet can cause water hammering it is better to design to mix flows farther upstream of Deaerator.

High inlet velocities and Improper pipe design is also the reason for water hammering.

It is good to redesign as per the HEI parameters.

 

Why Carbon Di Oxide CO2 is high in Deaerator?

High CO2 at the inlet then the CO2 would be higher in the Deaerator.

If the ph level of incoming water is High the CO2 will be higher, than, it is good to lower the ph level by controlling the level of dozing in the DM plant to reduce the CO2.

CO2 will be high if the venting is improper.

 

Why Iron Oxide in Deaerator?

A possible reason for Iron oxide in Deaerator could be due to incoming Condensate or system corrosion which can happen if positive pressure is not maintained in the Deaerator.

Frequent shutdowns can cause Iron Oxide in Deaerator.

Higher Oxygen levels in Deaerator can also result in Iron Oxide.

The vent piping should be short and as vertical as possible to avoid Iron Oxide in Deaerator.

The vent flow should be proper to avoid water carryover which could also be the reason for Iron oxide.

 

Why Sudden/ unexpected Deaerator storage tank level excursions?

The reason for sudden storage tank level excursions can be due to the malfunctioning of the control systems.

It is also good to check the overflow level and boiler feed pump operation to rectify the malfunctioning overflow or improper boiler.

Even if there is a Pressure fluctuation the Deaerator tank would be flooding.

 

Why Deaerator Tray Upsets/ Tray Abuse?

Tray abuse is created when much colder condensate suddenly enters the feed box and is sprayed in the spray chamber, rapidly lowering pressures just above the trays. This results in flashing and both steam and water want to move-up rapidly or even violently through the trays.

The typical tray design features a series of level trays having multiple slots with raised lips. In this arrangement water falling vertically in a thin film to the tray below hits the tray surface and must travel horizontally before falling to the next tray.

The Tray Upset problem may arise when the partial pressure above the water in the storage tank drops enough to permit flashing of the stored water allowing a mixture of steam and water to shoot quickly.

 

PREVENTIVE MAINTENANCE OF DEAERATOR

  1. Check the ph level in DM plant
  2. Check for the Thermometer calibration
  3. Check the fittings all valve and control settings.
  4. Check the blocks in vents
  5. Check all inlet flows and temperatures
  6. Check steam supply
  7. Check Steam and water ratio and check for restrictions
  8. Check all valve and control settings
  9. Check Pipe and valve sizing
  10. Check flow level and boiler feed pump operation
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Fin Tube Economiser

Economisers with extended fins increases heat transfer surface.

When Fins are attached to plain tube heat transfer surfaces they extend the surface available for heat transfer. While the extended surface increases the total transmission of Heat, its influence as the surface is treated differently from simple conduction to convection. The heating medium in Economiser is flue Gas.

 

Fin Tube Economiser

Fin Tube Economiser

Fin Tube Economiser

Gas-to-liquid heat transfer is also used for the recovery of waste heat from near-atmospheric combustion gases such as extended surfaces Economisers or otherwise called as Fin tube Economisers.

Transverse Fin Economisers in crossflow are used when the heat transfer coefficients of the fluids passing over are low. This applies particularly to gases and air at low and moderate pressures. Tubes are also available which have many very small fins integrally shaped from the tube metal itself and which are usable in conventional 1-2 exchangers with the baffled side-to-side flow. Transverse fin Economisers are also used in larger gas cooling and Heating services, tempering coils for air-conditioning, air-cooled steam condensers for turbine and engine work.

Transverse Fin Economiser

Transverse Fin Economiser

Based on the temperature and application High-Pressure Heaters, Medium Pressure Heaters and Low-Pressure Heaters are designed, manufactured and supplied.

In the Pharma Industry for drying the Tablets, various High Pressure or Low-pressure Economisers are used.

In Paper & Pulp Industry and Milk Powder drying medium Pressure, Economisers are used.

Low-Pressure Fin Tube Economiser is selected for the agro-based industries.

Engineering Equipment India is designing, manufacturing and supplying Plain tube and FinTube Economisers.

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Cement Manufacturing Process and Equipment Required

Steps in manufacturing cement

  1. Quarrying and transportation of Raw materials to process plant
  2. Manufacture and processing of raw/ recycled materials
  3. Transportation of finished product

Cement Manufacturing.

A raw mill is an equipment used to grind raw materials into “Raw Mix” during the manufacture of cement. Dry raw mills technology allows minimization of energy consumption and CO2 emissions. Crushed raw material and additives are transported to raw mill hoppers through the belt converyors. The raw material contains a certain amount of natural moisture. This Moisture is removed through Hot-air furnace or kiln pre-heater before grinding the materials. After the Dry chamber the Raw materials enter various stages of grinding chamber before it enters the Grit separator. Here the powdered raw material and the unwanted materials are separated and sent to cone shaped cyclone separator which separates fine and unfine materials. The unfine materials are sent to baghouse for reprocessing. The fine materials are sent through pneumatic conveyor to Silo.

Some equipment required for Cement Manufacturing. The equipment in blue is manufactured by us.

  1. Coal yard and raw coal hopper:

Coal is stored in a yard is dumped in a hopper and supplies the coal to the coal crusher. This hopper vibrates to sends the coal in batches through a belt conveyor.

  1. Coal crusher:

The large coal is crushed to the required small size.

  1. Raw coal hopper and table feeder:

Raw coal hopper stores the crushed raw coal material. Table feeder is attached at the bottom of the raw coal hopper to supply the coal in batch quantity to the coal mill.

  1. Coal grinding ball mill :

Raw coal materials are fed into rotating cylinder through hollow shaft neck. Due to centrifugal force and friction, the medium and raw material is pushed to move from feeding side to discharging side.  The Induced draft fan is connected with discharging side to form negative pressure so that the pulverized coal will be taken out with the pumping air by Blowers.

  1. Classifier:

The grinded coal from mill fed into the classifier. Classifier separates the coal as fine and unfine material.  unfine material is send back to coal mill with the help of girt return belt conveyor for grinding. Fine material is sent to Cyclone separator.

  1. Cyclone and dust collector:

After the fine material enters the cyclone separator from classifier fine coal is sent to the dust collector.

  1. Screw conveyor:

Through screw conveyor, the fine coal from the cyclone and dust collector are sent to Fine coal hopper, And through weigh feeders the coal are send to kiln and precalciner. 65% coal is sent to calciner and 35% coal is sent to the kiln.

  1. Preheater

Pyro processing is a process in which materials are subjected to high temperatures in order to bring about a chemical change. The raw material mix is fed to a kiln where pyro processing takes place.

Preheaters:

To save energy, modern cement plants preheat the materials before they enter the kiln rising more than 200feet, hot exit gases from the kiln heat the raw materials. 3 or more preheaters are installed to raise the temperature of the material before it enters the kiln.

  1. Kiln

Kiln is the heart of the cement making process a horizontal sloped steel cylinder lined with fire brick turning from about one to three revolutions per minute. High degree of heat triggers chemical and physical changes that converts the calcium and silicon oxidesin to calciumsilicates, cement’s primary constituents. At the lower end of the kiln, the raw materials emerge as a new substance: red hot particles called clinker.

  1. Gate cooler:

The clinker is cooled by forced air through the Induced draft fan. Several fans are also used in the cooling process. Waste heat recovery system recovers the excess heat from this cooling process and recirculated back to the kiln or preheater tower.

  1. Clinker breaker & silo storage:

The bulk clinker material is crushed into required size and stored in the silo.

  1. Clinker grinding (cement mill)

A cement mill is an equipment used to grind the hard, nodular clinker from the kiln area into the fine grey powder that is Cement.

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Boiler Steam Drum Internals

The steam drum internals is necessary to reduce the moisture content of the saturated steam leaving the drum by mechanical means. By moisture reduction, the solids in the boiler water are reduced in the steam.

Normally the drum is divided into two sections. The rear section has the most active boiler riser tubes, the unheated downcomer tubes, and one-half of the furnace riser tubes. The front section contains the heated downcomer boiler tubes, the least active boiler riser tubes & one-half of the furnace riser.

Tubes with the upward flow are called ‘risers’ and the tubes with the downward flow are called ‘downcomers’.

Wet Steam entering the rear half of the drum from the riser tubes is collected in a sealed off compartment formed by horizontal and vertical baffles. From this compartment, the steam is led through openings in the horizontal steam collecting box/ chamber and pass through a row of cylindrical ducts or baffles.

Steam flows upward from the front & rear steam ducts to the final stage of separation & enter the drier box where remaining entrained water gets separated. Since the velocity is relatively low, this water cannot be picked up again and, therefore, drops inside.

If the internals is not secured or fitted properly there is a possibility of carryover which affects the super heater performance and also premature failure can happen further the drum level cannot be maintained due to the process of carryover.

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How to choose between chiller and heat exchanger?

Heat exchangers are devices used to transfer heat energy from one fluid to another that is hot fluid to cold fluid. The fluids are separated by a solid wall to prevent mixing.

Chillers are devices used to transfer cold fluid to another liquid medium.

The easiest way to answer this question is to ask a few more; Do you have a facility water supply available? If no, you need a chiller. If yes, then ask another question, will my process temperature be above my facility water temperature? If no, you need a chiller. If yes, a heat exchanger may suit your application. It is generally less expensive than a chiller and more reliable as it has less mechanical components to fail, and if water consumption is not an issue it is more energy efficient as well.

A chiller is always required for temperatures below the normal water temperature or whenever the temperature required is much lesser than ambient temperature.

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What is the need for fume extraction system?

Health and laws state that it is the employer’s responsibility to protect the health, welfare and safety of their employees. Failure to do so can result in expensive legal action, potential fines and poor employee relations.

The Need for Fume Extraction System

Many types of dust and fume are hazardous to health if inhaled. Employees who have continued exposure to such atmosphere may get asthma attacks or other respiratory diseases.

Fume extraction system will help to:-

Provide a better working environment.

Protect employee health.

Ensure compliance with Health & Safety laws

Reduce complaints by operators due to odours, dust and vapours.

Avoid the possible cost of health compensation claims.

Reduce the cost & time to cleaning the equipment.

Increase production speeds.

Reduce downtime.

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What is a Deaerator?

The Deaerator is vertical spray cum tray with counter flow of steam & water and with horizontal feed water storage tank.

In order to accomplish a high degree of efficient deaeration, water is sprayed through various spray nozzles arranged below the water header.

Trays are arranged in a triangular fashion to increase the resident time for fine deareation.

This arrangement will cut down the amount of phosphate additive and eliminate the need for sulphate or Hydrazine additives. This will also reduce the amount of blow down and also reduce the cost of chemical treatment.

For initial start – up / heating up of water, Sparger type arrangement is provided in the feed Water storage tank.

This arrangement will also prevent hammering effect when the deaerator is actually charged with steam.

At the deaerator storage tank outlet nozzle, a sample point is provided to measure dissolved oxygen content.

The pump suction/ outlet nozzle of the deaerator storage tank is extended up to the perforated inlet header running along the length of the storage tank as shown in the drawing. Due to this arrangement, no vortex breaker is required. Hence less pressure drop.

LP Dosing line with nozzles at definite intervals have been provided inside the deaerator storage tank.

In our design, no separate vent condenser is required. The first row of trays is designed to act as vent condenser.

The deaerator is designed to take care of all working conditions. The turn-down is 10 %.

The only place for flashing steam to go is upwards to the heater. For this to happen safely, the equalizers the balancing pipes between tank and heaters must be adequately sized.

The vents actually have a dual purpose. Not only do they provide a convenient exit for oxygen lanten steam, they also permit the steam in the spray chamber to approach as close to atmospheric pressure as possible. The reason is that the lower the partial pressure, the more readily the sprayed water gives up the dissolved gases it contains.

Problems in deaerator functioning -Troubleshooting

Reasons for poor removal of dissolved oxygen:-

Sudden water demands in the system may create a partial vacuum and if the steam control valve response is not quick enough, vacuum breakers may open allowing air to enter into the deaerator. Check Pr.gauge to see that required

control pressure is always maintained.

Poor atomization of water.

Proper sizing and opening of vent.

Check positioning of trays.

Many factors affect the operating requirements and opting for a customised deaerator may pay off in the long run.

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