In our first couple of blog posts, we took a look at feed water, the chemistry of that water and how it transforms in low pressure steam boilers due to it being concentrated or cycled. We also saw its scaling and corrosivity potential and this affects both steam boilers and hot water heat systems or hydronic boilers.
A hydronic system is considered a closed loop due to the fact that it is not creating steam. Instead, this system uses water as a conductor of heat and this heat is transferred or radiated through heat exchangers, coils, enhanced piping, shell and tube and plate and frame exchangers or radiators. Sometimes these hydronic systems can contain either ethylene or propylene Glycol as freeze protection. They are still treated with iron and yellow metal corrosion inhibitors and tested, however being a closed loop, they should not lose water and therefore treatment and protection is more predictable. If these systems lose treatment, it can show that there is a leak in the system which can be easily found by adding a fluorescent dye to the water in the loop. You can then easily spot the leak when doing inspections. These systems typically run year-round. Below we will talk about what the function of each system is and what reasons they are selected. Also, since we have covered what they are, what water they are fed with, how they are tested, treated, and controlled, as well as blowdown preventative maintenance and cleanings, we will also look at the length of their operation over the course of a year. We will look at how they should be stored when taken offline. This method of storage is referred to as lay-up. There are 2 options for this Lay-up. One is a wet Lay-up where the boiler is filled to capacity with water then it is hyper dosed with corrosion inhibitor to maintain passivity of the metal surface during this off-line period. The second is a dry Lay-up, and that involves completely draining and opening all points to the vessel, adding desiccants to insure there is no remaining moisture and in some cases spraying surfaces with inhibitors, this is usually done if it is unknown as to when it will be used again or if it is an emergency backup.
Unraveling the benefits of Low-Pressure Steam Boilers and Hydronic Heat Loops
In the world of heating systems, low pressure steam boilers and hydronic heat loops stand as reliable and efficient options. These two methods play crucial roles in maintaining optimal indoor temperatures in residential and commercial buildings. While they operate on distinct principles, their combined functionality contributes to a comfortable and energy-efficient living or working environment. In this blog, we will delve into the inner workings of low-pressure steam boilers and hydronic heat loops, exploring their benefits, applications, and why they continue to be popular choices in the realm of heating systems.
Low Pressure Steam Boilers: Low pressure steam boilers are powerful heat generators used for various heating applications. They operate by heating water in a closed vessel until it turns into steam. The resulting steam is then distributed through pipes to radiators or other heat emitters located throughout the building. The heat released from the steam as it condenses back into water warms the surrounding air, providing comfortable indoor temperatures.
Advantages of Low-Pressure Steam Boilers:
- Energy Efficient: Steam has a high latent heat of vaporization, meaning it releases a significant amount of heat during condensation. This property ensures efficient energy transfer, leading to lower fuel consumption.
- Zoning Capability: Low pressure steam systems can be easily zoned, allowing for precise temperature control in different areas of the building.
- Durability: Steam boilers are known for their longevity and resilience, making them a cost-effective long-term investment.
- Versatility: They can be used for various heating applications, including space heating, domestic hot water production, and even certain industrial processes.
Hydronic heat loops: also known as hot water heating systems, use heated water instead of steam to provide warmth. Benefits of Hydronic Heat loops:
- Even Heat Distribution: Hydronic systems offer uniform heating throughout the space, eliminating hot and cold spots commonly found in forced-air systems.
- Quiet Operation: Unlike forced-air systems that may produce noise from blowers and vents, hydronic heat loops operate silently.
- Compatibility with Various Heat Emitters: Hydronic systems can be paired with different types of heat emitters, including radiators, baseboard heaters, and radiant floor systems, providing design flexibility.
- High Efficiency: Hydronic systems are highly efficient and can be easily integrated.
The basic components of a hydronic heat loop include:
- Boiler: A boiler is the heart of the hydronic heating system. It heats the water to the desired temperature and maintains it within a specific range. The most common types of boilers used in hydronic systems are gas-fired boilers, oil-fired boilers, or electric.
- Circulating Pump: A pump is used to circulate the hot water throughout the system. The pump helps maintain a consistent flow of heated water.
- Piping: The system consists of a network of pipes that carry the hot water from the boiler to the heating units (radiators, baseboard heaters, or radiant floor heating systems) and then return the cooler water back to the boiler for reheating. The pipes are usually made of copper, PEX (cross-linked polyethylene), or other suitable materials.
- Heating Units: which can be radiators, baseboard heaters, or embedded in the floor for radiant floor heating. These units transfer the heat to the surrounding air
- Expansion Tank: Since water expands as it is heated, an expansion tank is included in the system to accommodate the increase in water volume reducing pressure on the system.
When the Heating season is over, 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 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 practices in place, 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.
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.
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, Sodium Nitrite should be added until its level in the boiler water reaches above 1,500 ppm. If the superheater is of the drainable type, it can also be filled with the same treated water by overflowing from the boiler.
- If preferred, the boiler may be filled using feedwater or condensate treated with caustic soda and sodium nitrite after first filling the superheater with condensate treated with hydrazine and additional volatile alkali.
When the 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.
Should I perform a wet lay-up or a dry lay-up?
There are two primary methods of laying up a boiler – wet lay-up and dry lay-up. As you can imagine there are advantages and disadvantages to each. Aspects that you will need to consider in the selecting your choice of lay-up procedure are as follows:
- the size of the boiler
- the type of boiler
- the length of time the boiler will be offline.
- the temperatures that the boiler will be subjected to while offline.
- the resources that will be required to refill the boiler with treated water
- the resources that will be required to monitor the boiler while offline.
Choosing the best lay-up method for your boiler is not always so apparent and it sometimes makes sense for you to consult with an experienced water treatment service provider.
A dry boiler lay-up protocol should be used if the boiler will be shut down for an extended period or when there will be no foreseeable urgency to restart the boiler. This method is also preferable where the idle boiler may be exposed to subfreezing temperatures. Unlike a wet lay-up method, the dry lay-up also has a “set and forget it” component; it does not require constant monitoring, testing or circulation of the water.
Dry Lay-up Procedures
This procedure is preferable for boilers out of service for extended periods of time or in locations where freezing temperatures may be expected during standby. It is generally preferable for reheaters.
- The cleaned boiler should be thoroughly dried, since any moisture left on the metal surface would cause corrosion. Precautions should be taken to preclude entry of moisture in any form from steam lines, feed lines or air.
- A moisture absorbing material, such as quicklime (2 lb. per 30 cu. ft.) or silica gel (5 lb. per 30 cu. ft. of boiler volume) may be placed on trays inside the drums to absorb moisture from the air. The manhole should be then 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 boiler inspections.
- Alternatively, air dried externally to the boiler may be circulated through it. The distribution should be carefully checked to be sure the air flows over all surfaces.
- In the case of a high humidity area or a boiler that has been prone to off season corrosion in the past, there is another method of dry lay-up that utilizes an oil-based boiler treatment product to coat the inside surfaces of the boiler. This dry lay-up method is a little more substantial and it does add some extra steps, however the results are usually worth it. If this method is used the oil-based corrosion inhibitor must be removed before the boiler is brought back online. This is accomplished by filling the boiler, adding caustic and performing a high alkalinity boil out to remove the oil-based lay-up product. Once the boiler is flushed out and refilled with properly treated make-up water it can be slowly brought back up to working temperature and operated as normal.
For boilers that run year-round, water treatment testing and chemical application is crucial. While this system is not producing steam all year, water in a cold state is corrosive, water in a heated state increases corrosion rates as it gets hotter.