Boiler Efficiency and Steam Quality:
The Challenge of Creating Quality Steam Using Existing Boiler Efficiencies
The following article is a part of the National Board Classic Series. It was written by Glenn Hahn, technology manager, Spirax Sarco, Inc., and published in the May 1998 National Board BULLETIN.
Boiler efficiency measures how much combustion energy is converted into steam energy, while steam quality measures how much liquid water is present in the steam produced.
A major benefit of using steam as a heat transfer medium is the large amount of heat released when it condenses into water. With a latent heat of vaporization (or condensation) as high as 1,000 BTU per pound, it takes very little steam to carry a large amount of energy. Other advantages include the safe, nontoxic and nonflammable characteristics of steam plus its ability to deliver heat at a constant, controlled temperature. Steam can also be delivered to users with conventional piping and valve equipment that is inexpensive, is readily available, requires little maintenance, and has a long service life. Compared to other heat delivery and distribution systems, steam is less expensive to operate and is 100% recyclable.
In spite of these advantages, many steam users experience system safety problems, premature equipment failures, and poor steam system efficiency. Specific problems can include frequent boiler shutdowns from low-water level, damaged steam pipes and valves due to water hammer, vibration, corrosion, erosion, reduced capacity of steam heaters, and overloaded steam traps. These problems are most frequently caused by low steam quality, often called "wet steam" or "carry-over."
Steam quality is a measure of the amount of liquid water contaminating the steam. (For example, steam at 100% quality contains no liquid water and appears as a 100% clear gas, while steam at 90% quality contains 90% steam by weight and 10% water by weight in the form of a fog, cloud, or droplets.) Water droplets in high-velocity steam can be as abrasive as sand particles. They can erode pipe fittings and rapidly eat away at valve seats. And if a puddle of water is allowed to accumulate in steam pipes, it will eventually be picked up by the high-velocity steam and accelerated to near-steam velocity, increasing chances of it crashing into elbows, tees, and valves. This can lead to erosion, vibration, and water hammer. This water hammer will gradually – and sometimes catastrophically – loosen pipe fittings and supports.
Since steam is produced by the rapid boiling of water in high-heat flux boilers, it can entrain (or draw in and transport) water as it escapes from the water surface. This entrainment, while damaging to the steam system, is independent of boiler efficiency. Basically, both high- and low-efficiency boiler operation can produce – or not produce – excessive entrainment. While entrainment cannot be completely prevented, it can be minimized by proper boiler and steam system operation.
Case I, On-Off Boiler Feed
In a simplified explanation of boiler operation, a hot heat-transfer surface is covered with water. Steam bubbles are produced at the heat-transfer surface, rising through the water and then leaving the water surface to enter the steam system. Because of the heat of water, the pressure at the heat-transfer surface is slightly higher than the pressure at the surface of the water. Because of this higher pressure, the steam bubbles produced at the heat-transfer surface will either leave the boiler slightly superheated or be cooled to the saturation temperature of the water as it rises through the water. Under normal conditions, the steam bubbles tend to be cooled to saturation temperature as they rise through the water.
When feedwater enters the boiler, it enters between the heat-transfer surface and the surface of the boiling water. Even though the feedwater is pre-heated, it is still necessarily colder than the water in the boiler and creates a cold layer within the boiler water. As steam bubbles rise from the heat-transfer surface through this cold layer, they cool and some of the steam in the bubbles will condense. This causes two serious problems.
First, the steam bubbles leaving the surface of the water and entering the steam system will contain a mist of water. When a large amount of feedwater enters the boiler, the steam space above the water level becomes foggy. This fog and the resultant water-contaminated, low-quality steam continues until the water in the boiler becomes reasonably isothermal.
The second problem is the suppression of the rate of steam production. The addition of a large amount of cooler water slows steam production until the water reaches saturation temperature.
These problems can be prevented by using continuous boiler feed rather than on-off feed. Since modulating feed adds water at a very low rate compared to an on-off feed, the water in the boiler will remain relatively isothermal and no cloud will be formed.
Case II, Reduced Operating Pressure
"Operate the boiler at its maximum design pressure" is a common saying among boiler designers. But too often, this rule is not followed when energy cost reductions are needed. During periods of low steam demand, or when all the use points require pressure-reducing stations, boilers are often operated at substantially less than design pressure.
While operating at lower pressure can, in some boilers, provide slightly higher energy efficiency, low-pressure operation also reduces steam quality. This reduced steam quality can be demonstrated from basic engineering principles.
Lower Pressure Increases Entrainment
As a steam bubble rises through the water and reaches the surface, it finally breaks through the final layer of water and enters the steam space. This final act of leaving the water causes water entrainment in several ways.
Initially, the bursting of the steam bubble or the rupture of the thin layer of water surrounding it produces an initial rush of high-velocity steam that carries a small amount of that thin water layer into the steam space. Then, the loss of the steam bubble from the water surface briefly creates a crater on the water surface. Water rushes in to fill this crater, colliding with water rushing from the other sides of the crater, and produces a tiny splash near the center of the crater. The water droplets from these splashes are then easily entrained in the rising steam.
The size of the bubbles is directly related to steam pressure. Low-pressure operation requires a larger volume of steam to carry the required heat energy. This low-pressure operation produces more and larger steam bubbles and creates greater turbulence on the water surface. These bubbles produce more craters and larger craters, as well as more and larger splashes as they leave the water surface. In addition, low-pressure operation results in a higher vapor velocity which, when combined with the high turbulence of low-pressure operation, tends to carry water droplets into the steam systems rather than allowing them to fall out by gravity.
The solution is to operate the boiler at its maximum design pressure and use pressure-reducing valves at the point of use where required.
Case III, Rapidly Fluctuating Demand
In most industrial steam systems, steam demand fluctuates over a wide range. The rate at which these fluctuations occur can seriously affect steam quality. A rapid, short-term steam demand increase of only 15% can cause high entrainment of water in the boiler. Demand increases of 15% or more can occur quite frequently in industrial plants when steam valves are opened all at once at shift changes and as batch processes come online. For example, if a process of which steam consumption is only 5% of boiler output is turned on rapidly (such as with an on-off valve), the system demand can easily increase by 15% or more until the process reaches a steady state of operation.
When a steam valve opens, two problems occur in the boiler. First, steam pressure drops rapidly. The drop in steam pressure itself causes additional entrainment as explained in Case II above. Second, the interface between water and steam rises. This occurs because at the instantaneous lower pressure operation, the rapid production of high-volume steam bubbles can literally fluidize the water. (This phenomenon is often called "swell.") The water level can easily rise so high that water is literally sucked into the steam line. Eventually, the loss of boiler water can cause the low-water level alarm to sound. In some cases, this water loss can be so rapid that the boiler will shut down upon producing a low-level water alarm. In the meantime, the steam lines get filled with water.
Compact Boilers Can Magnify the Problem
Modern boilers are highly efficient and very compact. While this design has advantages, these compact boilers have little steam space to dampen changes in steam demand. If steam use increases only slightly, the pressure in the boiler can drop significantly. This lower pressure operation, combined with the shorter distance between the water/steam interface and the steam outlet pipe, further increase entrainment. Older boilers, while much larger, have a larger steam space which can tolerate greater changes in steam demand without severe changes in steam pressure or water level.
High Entrainment Fools Low-Water Level Alarm
In some circumstances, steam demand increases are so disruptive to boiler operation that boiler life as well as steam quality suffers. In some cases, the external water level indicator shows the water level as satisfactory, yet the actual level of the water/steam mixture in the boiler may be filling the steam space and water may be literally pouring into the steam lines by steam demand syphoning.
As water is lost rapidly and the steam/water mixture contains more and more steam, tubes may overheat. By the time the external water level detector eventually identifies a low-water level and shuts down the boiler, the steam distribution system will be laden with water and boiler tubes may have been damaged. Of course, the plant will now be without steam until the boiler is restarted.
The key to reducing this cause of poor steam quality is to prevent rapid increases in steam demand. Modern computerized control systems can accommodate this solution by measuring instantaneous steam flow or modulating demand based on a maximum allowable change in steam flow.
Case IV: High Total Dissolved Solids (TDS)
Conventional wisdom teaches that high total dissolved solids in boiler water increase tube corrosion and/or fouling. Indeed, that is true. High or highly fluctuating TDS will result in low heat transfer, reduced boiler capacity and efficiency, and shortened tube life. But it can also affect steam quality.
Increased TDS in the boiler water causes increased foam production on top of the water. This low-density, two-phase system's foam is produced and easily entrained by the steam rising out of the water. As rapid drops in steam pressure caused by demand increases, this foam can be drawn into the steam system, depleting the boiler of water before the level detector can identify the problem, while filling the steam lines with corrosive water.
The solution is obvious – keep TDS at least as low as that recommended by the boiler manufacturer. Unlike boiler feedwater, there is no definitive evidence indicating a steam quality difference between on-off or modulating blowdown to control TDS. However, given the adverse effect of rapid and intermittent inflows of make-up water, modulated blowdown would be preferred.
Steam quality is a measurement of the amount of water entrained in the steam. It depends not on the efficiency of the boiler but on the ability of the steam to separate from boiling water, without carrying liquid water particles with it throughout the entire range of boiler operations. Video camera studies of internal boiler operation indicate the following operating recommendations for preventing poor quality steam:
- Control steam usage to ensure that steam demand does not exceed boiler capacity.
- Control changes in steam usage to ensure that rapid changes in steam demand will not reduce steam quality.
- To affect either of the above, use modulating versus on-off valves at steam-use points.
- Add boiler feedwater with modulating versus on-off controls.
- Use TDS controls rather than time-based blowdown.
- Operate the boiler near its maximum design pressure.
- When any of these recommendations are not followed, reductions in steam quality can be dramatic. Low steam quality can damage steam equipment, control valves, and heat exchangers by water hammer, erosion, and corrosion, resulting in shortened equipment service life, steam loss, low operating efficiency, and even safety problems.
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.