Using Ultra-Low Water Content, On-Demand Boilers
Traditional high pressure steam boilers serve effectively, but they present safety hazards should a pressure-vessel breach occur. Boiler designs with low water content and on-demand operation hold less water and, therefore, potential for steam flash off.
For many years, the advantages of ultra-low water content, on-demand, high pressure industrial steam boilers have been touted. Features such as a reduced footprint, quick startup and on-demand responsive performance allowed multiple steam boilers to be successfully operated in a modular arrangement. Further, the modular approach proved capable of delivering improved performance over industrial watertube and firetube designs. In addition, the efficiency gains were accompanied by lower emissions.
Given all that, this next line might shock you: While the benefits are significant, the gains they represent are not enough to justify such a radical redesign of a technology that has changed very little in the last 100 years.
So, why build an ultra-low water content, on-demand, high pressure industrial steam boiler in the first place? In a word: safety.
Most hot water (hydronic) boilers and low pressure steam boilers for heating are operated without any type of licensed supervision. However, high pressure steam boilers require a licensed operator to run in many jurisdictions. Why is this? Think about latent heat and flash steam.
First, it is important to understand that a pound of water takes up a different volume than a pound of steam. That ratio is about 1:1,600 between water and steam. That is, a pound of steam takes up 1,600 times the volume of a pound of water. That is a big difference.
Second, it is important to understand the amount of energy — or latent heat — that is required to change water into steam. For this discussion, latent heat is important to understand as the heat energy stored in saturated water and steam.
Finally, it is critically important to understand the effects of pressure and temperature on the physical state of water. 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. I will refer to these phenomena 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).
This also works in the opposite direction with pressure — which brings us, finally, 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 latent 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 (121°C) pressurized water by reducing pressure from 15 psig to 0 psig, as previously mentioned, 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 firetube boiler capable of operating up to 150 psig. Suppose it contains a little more than 1,000 gallons of water during operation — that is more than 7 million BTUs of stored energy in the form of latent heat.
Now, imagine a pressure-vessel breach. In an instant, 162 gallons (16.2 percent of 1,000) of that water is suddenly trying to occupy 1,600 times that volume — the equivalent of 259,200 gallons of water. (That is roughly 40 percent of the volume of an Olympic sized swimming pool.)
The energy behind that explosive force is stored within the volume of every operational high pressure steam boiler. The recent Loy-Lange Box Co. explosion, though it involved a steam generator, is one example. For others, just search the Internet for “steam boiler explosion.”
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 things:
- The volume of saturated water present.
- The pressurization of that volume above atmospheric pressure.
So, how can a high pressure steam boiler be made safer? The high pressure steam boiler industry, generally speaking, has not responded to this question by redesigning their equipment. Instead, efforts to improve operational safety have focused on codes and regulations for automated safety devices and operator licensing. Yet, boiler explosions still occur and cost millions of dollars in property damage worldwide.
As time has passed, repeated efforts have been made to replace the need for steam — especially high pressure steam — with safer, more energy-efficient, less maintenance-intensive methods of energy transfer. In many instances, hot water generators and industrial thermal fluid heaters have taken over applications previously served by steam. Despite the continuous improvements in those technologies, however, their inherent limitations cannot completely eliminate the need for or surpass the suitability of high pressure steam for many applications.
Ultra-Low Water-Content Boiler Designs
As noted earlier, inherent boiler safety is directly related to the volume of water present at saturation conditions and the pressurization of that volume above atmospheric pressure. Practically speaking, only one of these two factors — that of water volume — can be addressed without completely negating the value of high pressure steam.
So, water volume then becomes the target. Reducing water volume in a steam boiler poses a few challenges. The water volume in a steam boiler serves dual purposes. First, and most obviously, steam cannot be generated without boiling 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. Second, and just as importantly, the furnace side of the boiler — where the flame and hot flue gases are generated — is very hot. The water content of the boiler also serves to cool the pressure vessel. The key is to 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 in substantial excess of the steam pressure desired.
In fact, controlling heat input, water level and flow are the greatest challenges to building an ultra-low water content, on-demand boiler. As the buffer of water volume is removed from the steam boiler design, burner and water-level controls must be designed to react quickly — nearly instantaneously — to steam demand changes, and they must be designed to act in concert.
The primary tradeoff for significantly reducing high pressure steam boiler water content is that without that added buffer volume, everything speeds up. From a design standpoint, in order to deliver the quick output changes required to match steam demand, the pressure vessel itself must be capable of heating up and cooling down quickly without risk of thermal shock.
With components such as the burner controls, the requirement for quick response allows for simplification. Because system steam pressure changes can occur rapidly, and no accumulator effect is present, gradual modulation of the burner would be too slow to bring a boiler from low fire to full output. Instead, the ability to shift from low fire to full capacity instantaneously, while less sophisticated, provides a superior operational response.
From a water-level control perspective, with two stages of output, and no fixed water line, water level is maintained for each stage by a series of probes in a side-stream, liquid-volume control column and accompanying control logic. Should a fault in the control occur, the boiler will quickly trip offline and alarm, requiring the operator to physically reset it.
Another often cited and vitally important concern is the way in which water chemistry is affected by the reduction in operational water content. Repairing pressure vessel heat transfer surface damage due to poor water treatment in any high pressure steam boiler is expensive. Low water-content, on-demand steam boilers, due to necessarily tighter design tolerances, are not designed with incremental repairs to these surfaces in mind but rather wholesale replacement if required.
As a result, manufacturers of ultra-low water content, on-demand style boilers typically provide a proposal that includes water treatment equipment such as softeners, reverse osmosis and chemical feed.
The ability that ultra-low water-content steam boilers have for being operated on demand has opened up the possibility of operating them in a modular, multiple-boiler system. This keeps the water content of each individual boiler low while allowing these systems to match or exceed the performance and steaming capacity of traditional industrial watertube (IWT). The key to successful operation of such a system is the master control and the logic contained within it. The controls automatically start, stop, change the fire rate and rotate the firing order of the individual boilers to match actual steam demand as measured by main steam header pressure.
So, while the technology that is necessary to reliably operate an on-demand boiler did not exist, and was not fully developed, until a few short decades ago, that technology is now well advanced, and available for mainstream adoption.
Just how safe are on-demand boiler designs? Go back to the example that was discussed earlier of a 200-hp boiler operating at 150 psig saturated steam conditions. While the firetube boiler held more than 1,000 gallons of water while operating, an ultra-low water content, on-demand boiler of the same output capacity, maintains only up to 75 gallons of water — less than 7.5 percent of volume of the firetube. That is less operational water content by more than half (46 percent) of the volume (162 gal) estimated to flash off were a breach to the firetube’s pressure vessel to occur. Also, direct-flame impingement on heat transfer tubes and the feeding of cold feedwater into a hot boiler are not an issue for the on-demand design. In fact, they commonly occur by design during normal operation. Among other benefits, this feature can contribute to lower emissions (NOX).
With hundreds of thousands of operational on-demand industrial steam boiler units installed worldwide, over the span of decades, there is yet to be recorded a single death in association with their operation. If the safety of your people and property is a top priority in your business, consider on-demand high pressure steam boilers.