Boiler and Steam Generator Operation

3

Learning Outcome

When you complete this learning material, you will be able to:

Describe in detail the typical procedures for operation of a steam generator.

Learning Objectives

You will specifically be able to complete the following tasks:

1. 1. Describe the detailed hot and cold startup procedures for a steam generator including safety precautions..
2. 2. Describe the detailed shutdown procedure for a steam generator including safety precautions.
3. 3. Describe the detailed lay-up procedures for a steam generator including safety precautions.
4. 4. Describe the detailed refractory dry out procedure for a new steam generator including safety precautions.
5. 5. Describe the detailed boil out procedure for a new steam generator including safety precautions.

Objective 1

Describe the detailed cold and hot startup procedures for a steam generating unit including safety precautions.

The instructions and recommendations in this module apply primarily to natural circulation watertube boilers with fully water-cooled furnaces. Some remarks will be added concerning other steam generating units both larger and smaller. The unit to be discussed is a field-erected, pulverized coal-fired watertube boiler.

Note: always follow the manufacturer's recommendations for the specific boiler being operated.

COLD STARTUP

The startup procedure used will depend on how the boiler was laid up when it was shut down and the length of time it was out of service.

There are two different cold startup procedures for a steam generator:

• • Steam generator startup after a long-period shutdown (more than 2 months)
• • Steam generator startup after a short-period shutdown (less than 2 months)

Steam Generator Startup After a Long-Period Shutdown

If the steam generator has been shut down for more than 2 months, then the instructions for commissioning a new boiler should be followed (with the exception of boiling out) before the steam generator is returned to service. This is important because of the possibility of scale becoming detached from various internal surfaces, oxide formation in the tubes, and the cumulative effect of other slow but persistent processes of deterioration. Therefore the steps involved in this type of a startup are:

• • External inspection
• • Internal inspection
• • Hydrostatic test
• • Safety valve setting
• • Putting a steam generating unit into service

• • To ensure a bent tube is clear, use compressed air to blow a wooden ball through the tube. A fibre optic camera can be used to view the inside condition of tubes as well.
• • Once the headers and drums have been cleaned, the boiler can be closed in preparation for the hydrostatic pressure test

Hydrostatic Test

New steam generators or steam generators that have undergone major repairs or have been out of service for an extended period of time, are subjected to a hydrostatic test of 1.5 times the design pressure. The water used for the test should be at a temperature no lower than the surrounding atmosphere and in no case lower than 21°C. This is necessary to prevent condensation forming on the outside of the tubes and plates that makes the detection of leaks difficult. The water temperature, however, must not be so high as to prohibit touching and close inspection of the various parts.

Good quality (mineral free, feedwater quality) water is used to avoid corrosion and fouling problems. Any sections of the steam generator, which are drained immediately after the test, can be filled with clear, filtered water. Non-drainable parts are filled with distilled or demineralised water adjusted to neutral pH and chemically treated to remove oxygen.

When filling the various parts of the steam generator, ensure all air is vented otherwise a dangerous condition develops. The superheater is often tested with the boiler (there may not be a stop valve between the boiler drum and the superheater). After the drum is full of water (water comes out of the vent), the drum vent is closed. When water comes out of the superheater vent, it is also closed.

Air trapped by the water compresses and, in the event of a leak, the air expands producing a hazardous condition. A guide to the effectiveness of the air venting during the filling of the steam generating unit is the time taken to raise the pressure after the steam generating unit shows full. An excessive time taken to raise pressure indicates that the test pump is compressing air trapped in the boiler.

For a hydrostatic test at 1½ times the design pressure, gags (Fig.1) must be used to prevent safety valves from opening. Blank flanges may be used in the case of flanged valves. Blank flanges are preferable to gags as they provide a better seal and prevent damage that can be caused by gagging the safety valves. To prevent excessive pressure on the safety valve spindle and to protect the valve seating surfaces, the gag should not be applied until the pressure has reached at least 80% of the valve set pressure. The gags and blank flanges should be removed after completion of the test. Instrumentation, such as transmitters and sight glasses that could be damaged by the hydrostatic pressure should also be isolated until the hydrostatic test is completed.

After satisfactory completion of the hydrostatic test the steam generator can be drained. The vents are opened to aid the draining of the steam generator.

Figure 1
Test Gag

1. 2. A hydraulic testing device to open the valve when boiler pressure is at its standard value.

Fig. 2 shows the hydraulic testing device arrangement for testing safety valves.

Referring to Fig. 2, the safety valve lifting gear is removed and the hydraulic jack (hydroset) yoke is set on top of the safety valve compression screw. The safety valve spindle is secured to the hydroset unit with a turnbuckle. The safety valve set pressure and the pressure at which the boiler is operating are both known, so the hydroset pressure required to open the valve can be determined. Then, the pump applies the necessary hydroset pressure, and the safety valve should simmer or weep. If the valve is

During the pressure-raising period, water is added only when required. Therefore, there is no flow through the economizer and, depending upon the economizer location, steam may be generated and trapped within the economizer. This can be avoided by allowing boiler water to circulate through the economizer using the economizer recirculation valve or by opening a boiler blow-down valve. Economizer recirculation valves, where fitted, will connect the economizer to the boiler bottom drum or blowdown system, thus enabling a natural circulation of water to be established due to convection. This will protect the economizer from overheating while there is little or no flow of feedwater.

As the steam generator is brought on line and the controls are put into the automatic mode, pay extra attention to the function of the various controllers to ensure proper operation.

The boiler operator must see the water level directly. It is poor practice to depend on an indirect or remote water level indicator until standard working conditions on the steam generator have been established. If the water gauge glass is not directly visible at operating floor level, or by projection in the control room, an operator should be stationed between the gauge and the feedwater regulating valve to maintain the closest possible control over boiler water level.

Do not rely on high and low water alarms, automatic feedwater regulators, or other safety devices until the boiler is steaming steadily.

When the pressure reaches 170 kPa, close the drum vents. This ensures that all air is out of the boiler or steam generating unit.

As the pressure is raised in the steam generator, conduct an inspection tour of the steam generator to check for leaks on the gas side, and the waterside. Ensure that the fans and pumps are working properly.

There is no steam flow through the superheater or the reheater during the pressure-raising period. Therefore, the temperature of the flue gas entering these sections must be controlled to keep tube temperatures below 480°C for carbon steel tubes and below 500°C to 540°C for alloy tubes. Control of the flue gas temperature is achieved by regulating the firing rate and selecting burners located higher or lower in the furnace wall. Excess air variation and gas recirculation are also methods of control.

The temperature of the flue gas entering the various furnace sections is measured with retractable thermocouples that can be withdrawn after steam flow through the tubes is established.

In addition to the superheater and reheater, the drums and headers must also be protected from thermal stresses. For components with rolled tube joints, the rate of temperature change must not be greater than 40°C per hour. This temperature change refers to the saturation temperature of the water which changes as the pressure is being raised. For

1. 3. Ensure that the feedwater pump, fuel gas supply system, coal supply system, and ash handling system are all ready for service.
2. 4. Open the steam generator drum vents and drain the steam generator drum vents to a level of one-third of the sight glass.
3. 5. Open the superheater and reheater drains.
4. 6. Follow the procedure from the previous heading – Putting a Steam Generator into Service .

HOT STARTUP

Hot startup is used when a steam generator has been shut down for only a short period of time – twenty four hours or less.

Once the steam generator is ready to be restarted:

1. 1. Check the water level in the sight glass. The level may need to be adjusted before lighting the unit. Start the chemical feed pumps, making sure the boiler water contains the desired amount of chemicals.
2. 2. If the drum pressure is below 170 KPa, open the drum vent and the superheater drains.
3. 3. If the steam generator is supplying steam to a single user, such as a turbine, then open the drain valve at the turbine stop valve as well as any other drain valves on the steam line between the steam generator and the turbine.
4. 4. If the steam generator is tied into a pressurized header, open the drain valve on the non-return valve and crack open the bypass around the header valve.
5. 5. The next step is to start the combustion air fans and purge the furnace. If the furnace has multiple burner units, the purge is done with 25 percent of full-load airflow until five volume changes are made. If there is only a single burner, the purge is done with 70 percent airflow until eight volume changes are made.
6. 6. Then raise the steam pressure slowly, using the recommended guidelines as stated under the heading - Putting a Steam Generator in Service .

When the steam pressure reaches 170 KPa, close the drum vents.

As the steam generator pressure is increasing, the operator does a thorough inspection of the steam generator to ensure there are no leaks on the gas side or the waterside, and that the fans and feed pumps are working properly.

Objective 2

Describe the detailed shutdown procedure for a steam generator including safety precautions.

STEAM GENERATOR SHUTDOWN

Steam generators must be taken out of service at regular intervals for cleaning, inspection, repair, or because the steam user (turbine) must be shut down for repairs.

Before the steam generator is taken out of service, follow these steps:

1. 1. Operate all soot blowers
2. 2. Close the continuous blowdown
3. 3. Blowdown the steam generator to remove sludge from the bottom drums
4. 4. Shut off the chemical feed pumps

Follow these steps to safely shut down a steam generator:

1. 1. When reducing load during shutdown, the combustion controls must be switched from automatic to manual before the lower limit of the automatic system is reached, usually at about 25 percent load.
2. 2. Pulverized coal mills and feeders must be emptied as the coal-fired burners are shut down.
3. 3. If the steam generator is gas-fired, shut off the burners sequentially as the load is reduced.
4. 4. When the steam generator load is sufficiently reduced, shut off the remaining burners.
5. 5. After firing has been stopped, the setting should be thoroughly purged with an airflow of 25 percent of full load for at least 5 minutes. The fans are shut down and the burner registers closed. The steam generator setting should be cooled down slowly using the manufacturer's recommendations. Some units have thermocouples to accurately regulate the rate of cooling.
6. 6. If the steam generator is supplying a turbine, open the drain valve at the turbine stop valve, as well as other drains on the steam supply line to the turbine. If the steam generator is feeding a pressurized header, turn down the valve stem on the non-return valve, then close the header block valve, and open the drain between the header block valve and the non-return valve.

Objective 3

Describe the detailed lay-up procedures for a steam generator including safety precautions.

STEAM GENERATOR LAY-UP

When a steam generator is laid up (mothballed) and not required for service or standby use, it must be protected against internal corrosion.

When a steam generator is taken out of service, it must be cooled, drained, cleaned internally and externally, and inspected. All instruments must be drained, to prevent damage due to freezing temperatures. The inspection determines the repair work that is necessary and what mechanical and chemical cleaning should be done. A decision should then be made on whether to employ dry storage or wet storage.

Dry Storage

For boilers that will be out of service and exposed to freezing ambient temperatures, dry storage is used. The cleaned boiler must be thoroughly dried, because any moisture left on the metal surfaces promotes corrosion. The drum, superheater, economizer, and other waterside vents are opened to drain the boiler completely. A small flame is used to evaporate any water left in the boiler. The source of the small flame can be from one of the burners, or a small portable heater can be placed in the bottom of the furnace. The vapours at the drum vent are analysed for moisture content, using a portable instrument made for this purpose. The vapour analysis indicates if the boiler waterside is moisture free. The flue gas temperature in the stack should not exceed 200°C or the temperature recommended by the steam generator manufacture.

After drying, the entry of any moisture or air into the waterside of the steam generator must be prevented. Moisture absorbing material, such as quicklime at the rate of 1 kg for 1 m ³ of boiler volume or silica gel at the rate of 3 kg for 1 m ³ of boiler volume, can be placed on trays inside the drums to absorb moisture from the air. The manholes and all connections on the boiler should be tightly closed. If it is readily available, a supply of an inert gas, such as nitrogen, can be connected to the drum vent to provide a positive pressure. The nitrogen pressure should be maintained at approximately 35 kPa. Warning signs and tags must be attached stating that the boiler is stored under nitrogen pressure.

The steam generator can now be filled through the feedwater or other filling line with condensate. Clean service water, which has been treated with hydrazine and additional volatile alkali, can also be used. If the storage period is expected to exceed three months, the concentration of hydrazine should be doubled. A professional boiler water treatment service will advise on the chemical content.

After filling the superheater with condensate treated with hydrazine and additional volatile alkali, the steam generator may be filled using feedwater or condensate either of which has been previously treated with caustic soda and sodium sulphite.

Once the steam generator has been filled to the top vent, the water pressure in the steam generator must be maintained at a pressure slightly above atmospheric by connecting a head tank to the highest vent on the boiler.

Steam Generator Partially Filled

The boiler may be stored with water at normal operating level in the drum, and nitrogen maintained at greater than atmospheric pressure in all vapour spaces. To prevent air leaking into the boiler, it is necessary to supply nitrogen at the vents before the steam generator pressure falls to zero as the boiler is coming off the line. If steam generator pressure falls to zero, the steam generator must be fired to re-establish pressure, and superheaters and reheaters thoroughly vented to remove air before nitrogen is admitted. All partly filled steam drums and superheater and reheater headers should be connected in parallel to the nitrogen supply. If nitrogen is supplied only to the steam drum, nitrogen pressure should be greater than the hydrostatic head of the longest vertical column of condensate that can be produced in the superheater.

Steam Generator Empty

When using this type of wet storage, nitrogen is added to the steam generator at 35 kPa on the boiler. This prevents oxygen from getting into the waterside of the steam generator. Then as the steam generator is drained, nitrogen is added continuously. Once the steam generator is completely drained (dry nitrogen is blowing from the drains), the drain valves are closed and the nitrogen is used to maintain 35 kPa for the duration of the storage.

Objective 4

Describe the detailed refractory dry out procedure for a new steam generator including safety precautions.

DRY OUT

This commissioning step takes place after the hydrostatic test has been successfully completed on the new steam generator, and all the required refractory has been put on the steam generator.

The purpose of this step is to remove all moisture that is imbedded in the refractory. Failure to do so can result in damage to the refractory.

Various shutdowns, interlocks and controllers must be proven to be in correct operation during the dry out.

This includes testing of the fuel system trips to ensure the main fuel valve closes if any of the following occur:

• Low steam drum level
• Loss of any draft fan
• Loss of main flame

Also, the feedwater level controllers can be checked for correct operation.

Follow these steps for dry out:

1. 1. Ensure that the low water level shutdown alarm is on, indicating a low water level condition.
2. 2. Open the drum vent and begin slowly filling the boiler with water in preparation for the refractory dry out.
3. 3. Ensure the gauge glass is in service. This gauge glass must be a temporary glass, because the chemicals for the dry and boil out period produce a caustic solution in the water that damages the gauge glass.
4. 4. The feedwater should be treated for proper pH and oxygen removal. The temperature of the feedwater should be such that the temperature differential between the feedwater and the metal temperature is no more than 40°C. This is to avoid undue stress to the boiler metal.

Objective 5

Describe the detailed boil out procedure for a new steam generator including safety precautions.

STEAM GENERATOR BOIL OUT

This step is done after the refractory dry out has been completed.

The purpose of the boil out is to remove any oil, grease, or other contaminating materials from the internal surfaces of the waterside of the boiler. Chemicals are added to the boiler water and the steam pressure is raised and maintained in the boiler for a predetermined period of time. This dissolves any deposits present on the drum and tube surfaces.

For example, add the following chemicals for each 45 000 L of boiler water:

• • 18 kg trisodium phosphate
• • 2.25 kg caustic soda
• • 2.25 kg sodium silicate
• • 22.5 kg soda ash

Note: When working with these chemicals, be sure to follow all safety precautions and wear the appropriate safety equipment. If you are not familiar with these chemicals, consult the MSDS (Material Safety Data Sheet).

The following steps are required to complete the boil out of the steam generator.

1. 1. Ensure that the gauge glass is in service. The gauge glass for the boil out must be a temporary glass, because the chemicals for the drying and boiling out period produce a caustic solution in the water that damages the glass material.
2. 2. Open the drum vent. All superheater drains should be wide open unless there are special recommendations for adjustment of drains when boiling out and/or pressure-raising.
3. 3. Open economizer and air heater bypass dampers, if provided.
4. 4. Be sure that all the blowdown valves are closed.
5. 5. Confirm that the boiler feedwater pump and the combustion air fans are ready for service.

1. 14. Once the boil out is complete, shut down the burners and the combustion air fans. Cool the boiler down according to the manufacturer's recommendations.
2. 15. While the boiler is still under pressure, open all blowdown valves for a few seconds to discharge the concentrated sludge and dislodge loose scale. Apply a short blow down to each location at intervals corresponding to approximately 170 kPa drop in boiler pressure.
3. 16. When the drum pressure is at 170 kPa open the drum vent.
4. 17. Open all blowdown valves to drain the steam generator. Once the water level is below the manhole, open the manhole door.
5. 18. Once the steam generator is drained, flush the tubes and headers with high-pressure water to remove any sludge or slime before it dries out or bakes onto the heating surfaces.
6. 19. Inspect the drums and headers for traces of oil, grease, and scale. If there is still a large amount of oil or grease left, another boil out is required. If there is a significant amount of scale, then the boiler has to be chemically cleaned before it can be put into service. Be sure to obtain a vessel entry permit.
7. 20. When a satisfactory boil out is complete, all manhole door covers, hand-hole caps, and blowdown valves should be carefully reinstalled, employing new gaskets. All joint seats should first be carefully cleaned.
8. 21. Keep a careful look out for defects of any nature in the boiler, combustion equipment, and auxiliaries which can only be rectified while the unit is shutdown and cold. Defects should be remedied while the boiler is undergoing its final cleaning and before fires are restored for the next operation.

Chapter Questions

A3.3

1. 1. Describe how to carry out a hydrostatic test on a steam generator.
2. 2. Describe the procedure for drying out a steam generator
3. 3. Describe the procedure for boiling out a steam generator
4. 4. Describe the shut down procedure for a steam generator you are familiar with.
5. 5. Describe how to lay-up a steam steam generator for long period of time. List activities carried out while the steam generator is shut down.