Steam is used to provide heat for process heating, sterilization and many other applications that are increasingly important to industrial facilities, among others. The Department of Energy cites several reasons for the popularity of steam in process heating, including lower toxicity, higher efficiency and lower costs when compared to other heat sources. These advantages, along with reliable performance, apply to both types of steam boiler designs: firetube and watertube.

From a mechanical design perspective, the primary difference between these two systems is:

• A firetube boiler passes combustion gas inside of a series of tubes,which are surrounded by water, in a vessel to produce steam.
• A watertube boiler passes water inside a series of tubes, which are surrounded by combustion gas, to transfer energy to produce steam.

Watertube boilers come in many shapes and sizes. They can be larger or smaller than firetube boilers with equivalent output, depending on the designs compared.

Modular, Once-Through Boiler Design

One type of watertube boiler — a modular, once-through design — has a compact configuration. Water flows through the tubes and changes from liquid to steam in a single pass within the tubes. By contrast, some traditional watertube boilers include an upper steam drum partially filled with water and a relatively complex tube and feedwater recirculation system.

Due to the once-through design’s reduced thermal mass, steam generation reaction time is less than traditional watertube designs and significantly less that traditional firetube designs. Multiple once-through boilers can be installed and configured to work together to provide a boiler system that optimizes the use of space and fuel.

A benefit of a modular, once-through watertube design is the ability to rapidly generate full steam even from a cold start (on-demand steam). On-demand steam can improve steam generation operations by matching the actual load demand with boiler output in near real time. While traditional boilers typically are placed in a standby or idle mode when not in use, such operating techniques are not required with on-demand watertube boilers. Keeping larger, traditional boilers ready for action, however, can add hours of run time and increase fuel costs while requiring personnel attention.

Modular, once-through boilers also are much different in size than traditional boilers. Some once-through watertube boilers can fit through a standard doorway. A traditional boiler will take up more floor space and can be more difficult to transport and install.

With once-through watertube boilers, if one watertube bursts, the pressure in the entire pressure vessel is bled off slowly. The tendency of the large body of water to flash into steam is minimized.

Elimination of Fireside Explosions

When it comes to safety, it is generally agreed that all current industrial steam boilers are safe. Catastrophic explosions are extremely rare.

At the same time, many industry experts agree that watertube boilers have less intrinsic risk than firetube boilers. This is based on the reduced potential energy stored within the smaller water content as well as due to the physical design and day-to-day concerns. In his book, “Boiler Operator’s Workbook,” the author R. Dean Wilson writes:

Modular, once-through watertube boilers combine a low water-content design with effective boiler geometry to almost eliminate catastrophic vessel failure. In the case that a failure does occur, the pressure vessel design allows the waterside failure to be contained within the boiler. In addition, the boilers are engineered with numerous safeguards beyond primary vessel safety and industry-standard features to ensure safe operations. These include backup high pressure cut-offs, watertube overheat cut-off, and flue-gas overheat cut-off. Preventive controls include scale monitoring based on tube temperature; monitoring for air temperature and feedwater pump status; and flame sensor self-checks. Fully automated operations include automatic surface blowdown, automatic bottom blowdown and automatic water shut-off valve.

When it comes to safety, it is generally agreed that all current industrial steam boilers are safe. Catastrophic explosions are extremely rare. In a firetube boiler, a sudden crack in the shell or a flue pulled loose at the end causes the entire body of water to be subjected to a sudden and substantial drop in pressure. The large volume of flash steam that results can cause an explosion of tremendous force.

Low Water-Content Design Maximizes Safety

With industrialization comes safety regulations to protect the lives and well-being of personnel and those in nearby buildings. This is exemplified with the founding of ASME in 1880 as a response to increasing steam boiler explosions.

There are several reasons for requiring steam boiler safeguards. Remember that a pound of water takes up a very different volume than a pound of steam — the ratio is about 1:1600 between water and steam. That is, one pound of steam takes up 1,600 times the volume of one pound of water. That is a big difference.

Saturated steam contains both sensible and latent heat. Heat added to liquid water first raises its temperature. Once the specific boiling temperature is reached, additional heat is needed to produce the phase change to steam.

Sensible heat is responsible for raising the liquid temperature to the boiling temperature, which is related to pressure. (More on this on a moment.) The latent heat of vaporization is responsible for the phase change from liquid to gas. (It may be helpful to think of this heat as hidden heat absorbed by the water.) This latent heat is in addition to the sensible heat.

Finally, it is also important to understand the effects of pressure and temperature on the physical state of water. When you boil water as part of a classroom science experiment, for example, you are boiling water at atmospheric conditions. While atmospheric conditions vary with altitude above sea level, that variation is not important for our discussion. It is worth noting, however, that water tends to boil at a slightly lower temperature in Denver (higher altitude) than in Los Angeles (lower altitude).

Low water-content steam boilers combine boiler geometry with a modular, once-through design.

Similarly, as pressure increases above atmospheric pressure, boiling occurs at even higher temperatures than 212°F (100°C). In fact, there is a direct correlation between the temperature at which water will boil and the pressure enacted on that water. We’ll refer to this phenomenon as saturation conditions. For instance, at just 15 psig, or approximately 2 atmospheres, water will not boil until it reaches approximately 250°F (121°C).

Notably, this also works in the opposite direction with pressure — which brings us to the subject of flash steam. Should pressurized water at saturation conditions of 250°F and 15 psig suddenly have a pressure drop to 0 psig, or atmospheric pressure, the sensible energy present will cause a portion of that water to immediately flash to steam. Imagine if you will, an entire gallon of water instantly flashing to steam, and suddenly trying to occupy the equivalent volume of 1,600 gallons of water.

Another important thing to know about flash steam is that the proportion of pressurized water volume that will flash instantly to steam increases as the pressure differential from atmospheric conditions increases. For instance, in order to flash off an entire gallon of 250°F pressurized water by reducing pressure from 15 psig to 0 psig, there would need to be 25 gallons present. This is because the percentage of flash steam resulting from that pressure drop is approximately 4 percent. Should saturation conditions corresponding to 60 psig be present, that percentage jumps to approximately 10 percent. At saturation conditions corresponding to 250 psig, the percentage of flash steam would be approximately 20 percent of available pressurized water volume.

To bring some perspective to the situation, consider that a small 200-hp Scotch-Marine type firetube boiler capable of operating up to 150 psig contains a little more than 1,000 gallons of water during operation. That is approximately 2.7 million BTUs of stored energy in the form of sensible heat. Now, imagine a pressure vessel breach: In an instant, 160 gallons (16 percent of 1,000) of that water suddenly tries to occupy 1600 times that volume, the equivalent of 256,000 gallons of water. That is roughly 40 percent of the volume of an Olympic-sized swimming pool.

To put it another way, if 160 gallons occupies approximately 21 ft³ of volume, then when that same volume flashes to steam, it would be trying to take up 34,000 ft³. Most likely, that is much more volume than is contained within the boiler room. That is quite an explosive force.

So, as you can see, there is inherent danger present when water exists in saturated conditions at pressures well above atmospheric pressure. The amount of danger is directly related to two factors:

• The volume of saturated water present.
• The pressurization of that volume above atmospheric pressure.

Practically speaking, however, only one of those factors — water volume — can be addressed without completely negating the value of high pressure steam in the first place.

Modular steam boilers take up less space in the boiler room.

Water Volume in a Steam Boiler

Reducing water volume in a steam boiler poses a few large challenges. These limits can be used to highlight some of the differences in the low water-content, modular, once-through watertube steam boiler design.

The water volume in a steam boiler serves dual purposes. First, and most obviously, steam cannot be generated without boiling the water present. So, there must be enough water present in the pressure vessel to produce the required capacity of steam at the required temperature and pressure.

Secondly, and just as importantly, the furnace side of the boiler, where the flame and hot combustion gas are generated, is very hot. Flame temperatures typically exceed 2,000°F (1093°C) at the burner. The materials the pressure vessel is constructed from — typically plain carbon steel — cannot maintain structural integrity at temperatures of even less than half that. For that reason, the water content of the boiler also serves to cool the pressure vessel.

To successfully design a low water-content boiler, one must design a boiler that can contain the minimum amount of water required to produce a given capacity of steam while still having enough cooling capacity to maintain the integrity of the vessel.

The modular, once-through watertube boiler also was designed with a primary goal of maximum safety. Though advances followed that initial design, the modular, once-through watertube boilers can generate steam quickly from a cold start and are designed for safety.

While traditional boilers typically are placed in a standby or idle mode when not in use, such operating techniques are not required with on-demand watertube boilers.

Just How Safe Are Some Once-Through Boiler Designs?

Let us return to the example of a 200-hp boiler operating at 150 psig saturated steam conditions.

While the firetube discussed earlier held more than 1,000 gallons of water, a low water-content, on-demand boiler of the same output maintains 75 gallons of water maximum. In other words, the on-demand boiler holds less than 7.5 percent of the volume of the firetube. In addition, direct flame impingement on the heat transfer tubes and the feeding of cold feedwater into a hot boiler — two dangerous operational “no-no’s” when discussing traditional boiler types — are not concerns for once-through boilers. Instead, they occur by design during normal operation.

Consider a commonly cited worst-case scenario: All controls and safeties have failed, and the water level is dropping below the low water cutoff (LWCO) while the burner remains firing. There is not enough water remaining to cool the tubes, and a rupture occurs. Under these conditions, using a once-through, low water-content boiler, the volume of water flashing to steam could be contained within the furnace volume of the boiler, and safely vented up the exhaust stack. The worst-case scenario could be handled without an explosion and loss of life.

Day-to-day safety concerns as well as downtime issues also are reduced in modular, once-through watertube boilers. Simple math suggests that if you have one or two large firetube boilers rather than several modular boilers, when one of the firetube boilers goes down or requires maintenance, downtime and loss of production can become a major concern.

Firetube maintenance also can result in heightened safety concerns. While there is an accepted belief in the boiler industry that modular, once-through watertube boilers have less intrinsic risk than firetube boilers due to physics, they also offer several advantages due to physical design.

One area is doors. During routine maintenance of traditional firetube boilers, the doors must be opened and closed. Mounted on a davit arm with heavy steel on the front and refractory in the back, firetube doors can weigh thousands of pounds. Often, jacks are required to line up the doors so bolts can be screwed in using an impact wrench. Modular, once-through watertube boilers do not have doors.

Firetube boilers also require continual maintenance procedures to replace the tubes and refractory, adding to the potential for injury.

Modular watertube boilers offer other safety and efficiency advantages. Using heavy-duty threaded plugs instead of hand holes reduces the potential for steam leaks and subsequent severe burns. The modular boilers’ clean, open environment helps provide access to clear gauges and reachable water checks while requiring simplified testing methods. Also, because modular watertube boilers can provide full steam in 5 minutes, operators can shut off the boilers when they are not in use for greater efficiency and overall safety.

In short, modular, once-through watertube boiler technology reflects safety and efficiency advances in industrial steam generation.