When Spring arrives, it is time to consider how your boiler system will spend the next few months while it sits dormant offline. If you own and/or operate a facility with a commercial steam boiler that only gets seasonal use, then this topic is for you.
When a boiler is not required to produce heat for extended periods of time, it often makes sense to take that boiler “offline” and prepare for a time of “safe inactivity.” The process is known as “laying up the boiler.” The main reason for doing this is to extend the overall life of the boiler and reduce maintenance costs and downtime. In some cases, proper end of season lay-up can mean the difference between a boiler lasting 10 years or lasting for more than 30. This post covers the features and benefits of performing a WET lay-up. (The next post in this series will cover a DRY boiler lay-up procedure.) The wet lay-up protocol is the one that you want to use if you have a boiler that is sitting idle, but that may need to be ready to go online in a hurry. This is also known as a standby boiler.
For the purposes of this post we will assume that you have the best water treatment company in your area and, during heating season, your boiler water treatment levels are maintained within specific optimal operating ranges to effectively inhibit corrosion and prevent scale. However, for seasonal boilers, during the non-operating months, even the best boiler water treatment program must be supplemented to prevent corrosion.
There are two primary methods of laying up a boiler – wet lay-up and dry lay-up. As you can guess there are benefits and potential downfalls to each. Choosing the correct method is not always obvious and so it may make sense for you to get some guidance from an experience water treatment service provider. Some factors in the selection of lay-up include the size and type of boiler, the length of time the boiler will be offline, the temperatures that the boiler will be subjected to while offline and the resources needed to refill and monitor the boiler with treated water.
Why do I need to lay-up my boiler anyway?
When a boiler is taken off-line and allowed to cool down for extended periods of time, a boiler lay-up program is highly recommended, to offset the increased levels of oxygen in the boiler. Oxygen can quickly produce pits in tube sheets and boiler tubes. Once oxygen pitting starts, it can easily corrode through a boiler tube in a very short period of time. When it is time to start your boiler back up in the fall, you first have to make a call to a mechanical contractor to plug or replace tubes. Not good for your boiler and very expensive.
What do I need to do when my boiler is taken offline?
When a boiler is taken out of service, the boiler should be cooled until the water is below the atmospheric boiling point, but not below 180°F, and then the boiler should be emptied and flushed out. An inspection should be made to determine what repair work is necessary and what cleaning should be done. A decision should then be made on whether to employ dry or wet storage techniques.
WET LAY UP PROCEDURES
A wet procedure may be used for a boiler that is taken offline completely or for one that has been placed in a “standby” condition. Wet storage is particularly useful if the standby boiler may be required to go back online at short notice or if it is impractical to employ a dry storage procedure. The method is not generally employed for reheaters or for boilers which may be subjected to freezing or sub-freezing temperatures.
There are a few alternative procedures that may be employed in a wet lay-up. The following is the most typical:
The clean empty boiler should be closed and filled to the top with water that has been conditioned chemically to minimize corrosion during standby. It is important that 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 help maintain pressure above that of the atmosphere.
- For short storage periods, caustic soda and sulfite should be added until their levels in the boiler water reach 450 ppm total alkalinity and 200 ppm sulfite. If the superheater is of the drainable type, it can also be filled with the same treated water by over flowing from the boiler.
- If the superheater is non-drainable, it should be filled only with condensate or demineralized water containing a minimum of dissolved solids, not more than 1 ppm. Before introducing the water into the superheater, mix in uniformly about 200 ppm of hydrazine and sufficient volatile alkali, such as ammonia, cyclohexylamine or morpholine to produce a pH of 10. The treated water may be introduced into the superheater through an outlet headed drain until the water over-flows into the boiler. When the superheater is filled, close the drains and vents. The boiler can now be filled through the feedwater or other filling line with condensate, feedwater or clean service water treated as described, with hydrazine and additional volatile alkali. If the storage period is expected to exceed three months, the concentration of hydrazine should be doubled.
- If preferred, the boiler may be filled using feedwater or condensate treated with caustic soda and sodium sulfite after first filling the superheater with condensate treated with hydrazine and additional volatile alkali.
During the Time That Boiler is Offline
The boiler water should be circulated periodically to prevent the chemicals from stratifying or falling out of solution. The burner may be used to warm the water in the pressure vessel to stimulate natural circulation.
It is important to routinely test the water while the boiler offline to monitor the chemical concentrations. If there are any leaks in the system it can cause make-up water to be introduced to the boiler. Untreated make up water will be high in oxygen and carbon dioxide and will aid in corrosion.
Before Boiler Comes Back Online
Before starting a steam boiler that has been in wet lay-up, perform a bottom blow off to reduce the alkalinity. This reduces the chance of carryover. Confirm that all tags and locks are removed, and closely monitor the system cycles for a minimum of three to five cycles to ensure proper functioning of the boiler before allowing it to run automatically.
When is appropriate to use a dry lay-up procedure?
Great question! You can read all about that in the second part of this series: Springtime Boiler Maintenance: Dry Boiler Lay Up.
I want to learn more about other important boiler water treatment services. What can I do?
Well you’re in luck! The first step in efficient and safe boiler operation is knowledge. Learning more about what potential issues your steam boilers could face is always a good idea. There is a treasure-trove of informational resources regarding steam boiler operation and water treatment available online. Another great resource is our free boiler operation eBook that is available for instant download: “10 HUGE Mistakes Facilities Make in Boiler Operation and How to Avoid Them!” which you can download for free at the link below.
As always, if you have any specific questions or concerns regarding your facilities water treatment program, please feel free to contact one of our water treatment experts.
Thanks for reading and please check out our free eBook!