by Harry de Jong
General
1. If the boiler is equipped with sootblowers, and any other fuel other than natural gas has been fired, the sootblowers should all be operated before taking the boiler off the line.
2. After the sootblowing operation is complete, the steam flow from the boiler should be gradually reduced and the burner run to the low fire position.
3. With the burner in low fire position, blow down the boiler along with the water column, gauge glass, and feedwater regulator. Turn the burner off in accordance with the burner manufacturer’s instructions. If the boiler is equipped with a flue gas outlet damper, it should be fully closed to allow the unit to cool slowly.
4. Remove and clean the burner oil gun. Place the fuel supply equipment in standby condition (for gas, shut main supply cock). Throw the main electrical switch, and take the feedwater regulator out of service. Hand operate the feedwater valves to keep the water level above one-half (1/2) gauge glass.
5. When the boiler pressure falls below line pressure, the boiler stop valve should be closed if the setting has cooled enough to prevent any pressure buildup. If the boiler is equipped with a non-return valve, the valve should close automatically when the boiler pressure drops below line pressure. This, of course, isolates the boiler from other units remaining in service. As the drum pressure falls below 15 to 25 psi, the manual closing device (handwheel) of the non-return (if equipped) should be closed and the vent valve on the boiler opened. This will prevent a vacuum on the boiler waterside, which will loosen well set gaskets and cause future problems. While there is still a small amount of steam pressure available, the boiler should be blown down and filled back to a safe level with freshly treated hot water in preparation for the next startup.
If the boiler is being shut down just for overnight or for the weekend, the foregoing procedure is generally all that is required. The primary concern is to make sure there is sufficient water in the boiler. If the boiler is only going to be shut down overnight, the boiler can be secured and will have pressure still showing the next morning. When the boiler is insulated well enough to permit steam pressure to remain overnight, there is no danger of pressures dropping low enough to start pulling a vacuum. Then the boiler vent valve can be left closed.
In those instances where the boiler may be left unattended long enough that the boiler cools down and does start to pull a vacuum, the piping can be equipped with a vacuum breaker valve. The valve can be installed close to the top of the steam drum or shell, preferably above the water line. As the boiler cools down and the condensing of the steam starts to form a vacuum, the valve permits air to enter the drum and atmospheric pressure will exist in the drum. One precaution must be taken in such an installation. When starting up the boiler each time, be sure the boiler vent valve is opened to evacuate all of the air that has been trapped in the boiler and boiler water.
Up to this point, we have been talking about shutting down the boiler for just a few hours or a few days. If the boiler is going to be taken out of service for several weeks or several months, then a different procedure must be followed.
Wet Storage
There are two (2) basic methods of laying up a boiler for extended periods of time. They are wet and dry storage.
If the unit is to be stored for no longer than a month and emergency service is required, wet storage is satisfactory. Wet storage is not generally employed for boilers that may be subjected to freezing temperatures. Several alternative methods may be employed.
1. The boiler to be stored should be closed and filled to the top with chemically treated feedwater or condensate, to minimize corrosion during standby storage. Water pressure greater than atmospheric pressure should be maintained within the boiler during the storage period. A head tank may be connected to the highest vent of the boiler to maintain pressure above that of atmospheric pressure.
For short periods of wet storage, the water or condensate in the boiler should contain approximately 450 PPM of caustic soda and 200 PPM of sodium sulfite. If the boiler is equipped with a superheater of the drainable type, it can also be filled with the above described treated water by overflowing from the boiler.
If the superheater is non-drainable, it should be filled with condensate or demineralized water containing no more than 1 PPM of dissolved solids. Before introducing the water into the superheater, sufficient hydrazine should be added to achieve a concentration of about 200 PPM. Sufficient volatile alkali should also be added to produce a pH of 10. The treated water may be introduced into the superheater through an outlet header drain until the water flows into the boiler. When the superheater is filled, close the vents and drains. This quality of water may also be used in the boiler. If the storage period should extend beyond a month, the concentration of hydrasine should be doubled.
2. As an alternative, the boiler may be stored with water at normal operating level in the drum and nitrogen maintained at greater than atmospheric pressure in all vapor spaces. To prevent in leakage of air, it is necessary to supply nitrogen at the vents before the boiler pressure falls to zero as the boiler is coming off the line. If boiler pressure falls to zero, the boiler should be fired to re-establish pressure and drums and superheaters thoroughly vented to remove air before nitrogen is admitted. All partly filled steam drums and superheater 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 could be produced in the superheater, or a minimum of 5 psi.
3. Rather than maintain the water in the boiler at normal operating level with a nitrogen cap, it is sometimes preferred to drain the boiler completely, applying nitrogen continuously during the draining operation and maintaining a pressure of nitrogen greater than atmospheric throughout the draining and subsequent storage.
Dry Storage
Dry storage is preferable for boilers out of service for extended periods of time or in locations where freezing temperatures may be expected during standby.
We would strongly recommend that large signs be placed in conspicuous places around the boiler to indicate the presence of moisture absorbing materials. The message to be conveyed can be as follows:
Alternatively, air dried externally to the boiler may be circulated through it. The distribution should be carefully checked to be sure the air Rows over all areas.
When a routine shutdown is scheduled, it should be planned so there is time to perform certain operations in the shutdown procedure.
All the recommended rules for operating the sootblowers should be followed. One of the most important rules is that the steam load on the boiler is 70% of boiler rating or greater. Operation at low load, with its resulting colder furnace may result in some unburned combustibles escaping from the furnace, and redepositing on the tubes or collecting in pockets. Agitation of the combustibles during sootblowing at low load may result in an occurrence. Therefore, It Is Important to have all of the areas of the gas passes thoroughly swept with flue gas, thus purging any combustible gas from pockets where it may collect.
The cleaned boiler should be thoroughly dried, since any moisture left on the metal surface would cause corrosion. After drying, precautions should be taken to preclude entry of moisture in any form from steam lines, feed lines, or air.
A moisture absorbing material should be used, such as quicklime, at the rate of two (2) pounds or silica gel at the rate of five (5) pounds for 30 cubic feet of boiler volume. It may be placed on desiccant trays inside the drums or inside the shell to absorb moisture from the air. The manholes should then be closed and all connections on the boiler should be tightly blanked. The effectiveness of the materials for such purposes and the need for their renewal may be determined through regular internal boiler inspections.
For long periods of storage, internal inspections should be performed to assess the condition of the moisture absorbing materials. Such inspections should be initiated monthly, unless experience dictates otherwise. The moisture absorbing material increases in volume as moisture is absorbed, making it necessary to use deep pans. Fresh material should be substituted as needed at the time of the inspection.
If the boilers are going to be stored in any place other than a dry, warm protected atmosphere, then steps should be taken to protect the exterior components also. Burner components that are subject to rust, such as jackshaft, linkage, valve stems, moving parts, etc., should be coated with a rust inhibitor and covered to protect them from moisture and condensation. Electrical equipment, electronic controls, relays, switches, etc., should be similarly protected.
Pneumatic controls, regulators, diaphragm or piston operated equipment should be drained or unloaded and protected so that moisture, condensation, rust, etc. will not damage the equipment during a long period of storage. Feedwater lines, as well as blowdown, sootblower, drain lines, etc., should all be drained and dried out. Valve stems, solenoid valves and diaphragms should all be protected by lubricant, rust inhibitors, plastic coverings or sealants.
This article was taken out of the “Boiler Room Guide” and is published by Cleaver Brooks. This guide provides assistance to supervisory, operating and maintenance personnel in understanding the operation of boiler equipment, and to provide guidance to achieve safe and efficient operation of this equipment.
For further information contact Harry de Jong at Canadian Boiler Industries Ltd. Phone: 403-255-0186 or Fax: 403-255-9605.
First published April 1998
