by Joe Farsetta, Certified Master Inspector® and Certified Commercial Inspector
While there are several types of steam boilers, their basic operating principles are nearly identical. Water is heated to a gaseous state, generating pressure and steam. The steam is then transported through pipes via that pressure, which is used to power radiators, turbines, steam rooms, sterilization chambers, cooking equipment, etc.
Once water is converted into steam, minerals and other sediment that can’t be converted to a gaseous state will be left behind and drop to the bottom of the boiler. This process, known as blowdown, is then used to help remove the sediment. If this process didn’t occur, the boiler would not function properly.
There are two methods of blowdown: surface blowdown and bottom blowdown. Here are the differences.
Surface blowdown is a means by which low volumes of water are bled off from inside the boiler to get rid of some impurities. The fastest way to catch and jettison those impurities is at the point nearest where water is turned to steam. Surface blowdown is often sent to what is called a flash tank. Remember that super-heated water hitting a cooled surface doubles its velocity. Therefore, the flash tank catches this surface blowdown. Along with the flash tank, there may be a heat exchanger. Steam can transfer heat to the exchanger and capture lost heat. Surface blowdown happens continually, whereas bottom blowdown occurs infrequently.
Bottom blowdown is required where those impurities resulting from the conversion of water to a gaseous state can’t be captured and eliminated via surface blowdown. These insoluble particles find their way down and settle at the bottom of the boiler tank. Absent of elimination of these particles, they will eventually mix with the water, get pumped around, re-circulated, and wind up on all sorts of surfaces, including heat exchangers.
Water treatment helps mitigate this problem, but it’s intended to allow these impurities to gather inside the boiler in a place known as a mud drum, which is located at the very lowest part of the boiler tank. Valves allow steam pressure to force the sludge out.
Shorter blowdown durations enable the water level inside the boiler to remain relatively unaffected, but multiple short events are preferable to a single longer-duration blowdown. The flash tank also helps prevent scalding anyone performing the blowdown. This will help remove the sediment from the bottom of the tank.
Inspectors should look for bottom blowdown piping and where the water is captured, as well as for signs of corrosion or leaking at these valves. There may be more than one valve visible in a series. This helps ensure a proper seal, should one valve start to corrode and leak.
As steam cools, it reverts back into a liquid, which is referred to as condensate. This condensate has to make its way somewhere. Preferably, there are no leaks in the condensate return lines as it finds its way back to the boiler. This can happen via gravity (in a condensate line pitch), pumps, or a combination of both. Steam traps are used to efficiently collect the condensate for return to what is referred to as a feedwater tank (sometimes also called a de-aerator). Oxygen is introduced along the way, and if it’s not removed from the water, it can cause problems inside the boiler, including scaling and rust. Therefore, the de-aerator or feedwater tank is an essential functional element of a steam boiler system, as it removes oxygen and other dissolved gases from the feedwater.
Fresh make-up water is also dumped into the de-aerator and encounters the same process before mixing with the condensate. The mixture of the two now becomes the feedwater, which gets pumped back into the boiler, when necessary, to make up for any water loss resulting from steam generation.
Heat is a necessary element for steam boilers, especially in cooler climates. Even where physical heat is not needed for comfort survival, it is used for a variety of other tasks, including cooking and mechanical processes. Artificial heat is used everywhere and in virtually every industry. When it cannot be feasibly generated onsite, it is purchased. The commercial inspector will encounter sources of artificial heat frequently, such as in office buildings, apartment buildings, warehouses, hospitals, and factories. Some smaller apartment buildings use purchased steam. In these applications, the source of the steam is not to be inspected, but the distribution of steam is subject to the inspection.
Steam as a Heat Carrier
A heat carrier in a forced warm-air system is one part of a two-part process: the air which is warmed and conveyed to heat the building’s occupants; and the medium by which the heated air is transported. Steam is an optimum heat carrier. Piping is the transport medium. As a pressure cooker provides a sealed and pressurized system, allowing the water to boil at a higher temperature and allowing for the production of hotter heat, a steam boiler is also a sealed and pressurized system.
Steam as a heat carrier is easily transported via its own pressure, which eliminates the need for pumps, making it more cost-effective. Condensing water is collected and returned to the boiler to be re-heated and used again.
Before the advent of condos with individual heating systems, there may have been one central boiler for multiple buildings. In many instances, these were steam boilers. A large boiler operating at capacity may consume a relatively small amount of fuel required to meet the heating needs of many homes. This is why you may not see a steam boiler in a multi-tenant setting like a strip mall, where each tenant is responsible for their own heating bill. Remember this while examining the sources of heat in any type of property you are inspecting. You will need to be able to recognize this and approach your inspection accordingly.
Large steam systems are designed and constructed to withstand high pressure. If poorly constructed or maintained, these boilers can explode with the force of a stick of dynamite. In some configurations, multiple boilers are inter-connected. Up to five units can be connected. Steam boilers are all relatively similar in design, such as a boiler pressure tank, which may be a horizontally placed cylindrical tube (though often hidden from view), with a flame tube and burner. There are tubes within the boiler. Steam boilers are never completely filled with water, as opposed to hydronic or hot-water systems. Open space in the boiler tank allows for steam, which produces the pressure within that space and outward.
Steam Is Not Just for Heat
A commercial inspector may find himself examining multiple sources of heat within a structure or series of structures. For that matter, the inspector may be examining a specific type of facility associated with a specific business for a client. As such, the inspector must be prepared to observe and recognize the specific application that the steam boiler may interact with.
A commercial laundry is a good example of steam used as a non-heat-related application in commercial use. Steam can be used to not only heat the water on a large scale, but steam production can be used at the pressing stage. In some older units, steam was utilized to inflate and press buttoned dress shirts. The user secured the waist area of the shirt to a base, stepped on a pedal, and steam inflated the shirt. Within three seconds, the shirt was perfectly ironed, with all creases removed, and was ready for folding and packaging.
The industrial food industry is another example of commercial steam use. Food processing on a large scale can find many uses for steam. Whether in a process for peeling tons of tomatoes for canning, or processing potatoes, steam’s application is common.
In healthcare, steam may be used for sterilization or even humidification. Steam may be used within air handlers in place of a hot water coil to produce heat.
As a commercial inspector, your focus is to examine those systems that are critical to the client’s overall operation. The steam plant may handle far more than heat.
Its conceptual design and functional purpose may be affected by multiple observed conditions, including:
- fuel type and the number of systems;
- whether the system is mixed-use or dedicated to sections;
- fuel supply, which may include underground storage tanks, propane tanks, and/or natural gas;
- method of transport of steam and condensate;
- age and condition of the pipes;
- insulation (or gaps) and condition of the insulation;
- obvious signs of corrosion;
- the condition of the boiler itself;
- when it was last refurbished;
- when it was installed;
- who designed the system;
- when the piping was installed; and
- blowdown frequency and methodologies.