The importance of steam in manufacturing dates back to the dawn of the industrial revolution in the 18th century. While its use in transportation has declined, its use in other industries has grown, broadening the range of applications. For instance, steam provides mechanical power by driving turbines. It is a versatile heat source for processes, and on the comfort side, it is widely used for district heating.
Although common, steam generation and distribution systems are complex and can be expensive to operate. Depending on the processes running at a facility, it is not uncommon for steam to be the most energy intensive and costly utility, whether produced on-site or purchased from a third party. Given that reality, this article will concentrate on steam system efficiency by looking at three key points:
- Multiple factors affect efficiency.
- Instrumentation and the data it produces are the primary tools for determining efficiency and identifying problem areas.
- Instrumentation is critical at the boiler, but it also must be deployed throughout the distribution system.
At the outset, we will assume that the steam system being examined is currently effective and operating safely to deliver the required amount and quality of steam when needed to all applications. If this is not the case, trying to increase efficiency is putting the cart before the horse.
Without adequate instrumentation, efficiency is difficult to quantify. Plant managers may settle for simply determining how much money is being spent on fuels and decide whether the figure seems tolerable based on the plant’s production.
The difficulty of improving efficiency is that many variables can affect it. In fact, when looking at a complete steam system, virtually everything affects efficiency in one way or another, from combustion air temperature at the boiler to a malfunctioning steam trap on the edge of a distribution line.
To the greatest extent possible, process plant steam systems are closed to retain heat and water (figure 1). Escaping steam reduces efficiency as does condensate leaking from a steam trap. Exhaust steam from the various processes must be saved and condensed back into water to recirculate as feedwater. It must be kept as hot as possible so it can be reintroduced to the boiler at a temperature that requires the least fuel to turn it back into steam.
Where condensate is still hot enough to flash into low pressure steam, it can be used for appropriate applications, recovering all available energy. Even the heat of blowdown water warms the raw water replacing it. Heat retention must be measured and monitored in all parts of the system, therefore.
Instrumentation for Boilers
A boiler is the starting point for any steam system, and a firetube design is the most common configuration (figure 2). The following variables affect, or at least indicate, efficiency:
- Steam pressure and flow.
- Fuel volume, which must be converted to total energy content.
- Combustion air volume and temperature.
- Flue gas temperature.
- Feedwater flow and temperature.
- Blowdown volume.
Most of these measurements report directly to the boiler control system so that it can provide consistent performance even when loading and other operating conditions vary. Naturally, the combustion process is the greatest concern because it represents the main cost driver. The primary measurement determines how much fuel is being consumed. This must be translated to energy content because different fuel types are used based on the boiler design.
Thermal mass flowmeters measure mass better than volume measurements for this application. They can be programmed to the fuel type in use — a particular fuel oil grade, for instance — to reflect its calorific value. Other situations may call for other technologies such as Coriolis, vortex or ultrasonic flowmeters to monitor additional parameters such as temperature and pressure.
Combustion airflow must match fuel flow rates to provide the most efficient combustion. It is also important, however, to monitor temperature between the preheater and burner. Using residual heat from the flue gas to warm combustion air increases efficiency. Similarly, a feedwater heater draws heat from the flue gas, so it is critical to monitor the temperature, flow and pressure of the feedwater where it enters the boiler.
Steam flow is paramount because it is the main output of the boiler. Vortex flowmeters are widely used as the measurement technology in these applications because they can measure temperature, flow, pressure and even dryness fraction with a single instrument (figure 3). These values can be used to calculate total energy output from the boiler, yielding an accurate picture of efficiency. Given the importance of temperature and pressure, however, separate instruments usually are used. In fact, multiple instruments often are employed for control and safety instrumented functions.
Feedwater flow is challenging to measure because it is at high pressure and temperature. Vortex flowmeters often are used for this because they can tolerate the harsh process conditions. Their multi-variable ability also provides temperature and pressure readings. Ultrasonic flowmeters are another good alternative due to no pressure drop and wide turndown capability. Magnetic flowmeters may be practical in situations where water treatment methods have not reduced conductivity below the required threshold. The same considerations apply for measuring blowdown water flow because it comes directly out of the boiler.
Flue gas temperature provides insight into heat recovery — and, therefore, the efficiency — of the boiler, feedwater heater and air preheater. Measurements made after each of these stages make it possible to determine whether all the BTUs possible are being captured, or whether expensive unrecovered energy is being blown out the stack.
With all these variables, calculating boiler efficiency can be a challenge. An effective system must record and historicize the variables so that it is possible to determine which could be affecting overall performance (figure 4).
For the boiler itself, basic fuel-to-steam efficiency is the main concern. This figure will vary for understandable reasons such as overall loading, but it also may change due to abnormal conditions. Some of these might be visible in the variable history while others may not. Where efficiency changes and there is no clear attribution, such conditions indicate that an unmonitored element is at work, reducing heat transfer. This could be fouling on the water side, soot accumulation on the fire side, or other factors.
Determining which of these issues might be the cause requires digging into multiple other factors. For example, an increase in flue-gas temperature could indicate a reduction of heat transfer due to soot deposits. The key is to understand when fouling might be happening, making it possible to perform predictive maintenance instead of allowing accumulation to escalate until an unplanned shutdown is necessary.
Instrumentation for Distribution
Once steam leaves the boiler and moves to applications via the distribution piping, much can happen to support or hinder efforts to make the steam system more efficient. Large distribution headers and feed pipes to major steam-consuming applications should have flowmeters installed to monitor consumption and steam quality. Vortex flowmeters are suited for such uses due to their ability to measure temperature and dryness fraction. This can help ensure that every user is getting the required amount and quality of steam.
It is equally important to monitor temperature on the condensate return lines to determine heat transfer. Flowmeters and temperature sensors should be deployed in the largest numbers possible to pinpoint what is happening in individual applications. Monitoring a header feeding several pieces of equipment may provide insufficient detail to identify bad actors.
Instrumentation for Heat Exchangers
One of the most common uses for steam is driving heat exchangers. Where a process calls for raising the temperature of liquids (and to a lesser extent, gases), a heat exchanger provides a mechanism to move heat from steam without directly mixing steam into the liquid. The two most common designs are shell-and-tube and plate heat exchangers (figure 5).
Shell-and-tube designs pass the process liquid through a bundle of parallel pipes encased inside a shell. Steam passes through the shell interior, where it contacts the pipes and transfers its heat into the process liquid. By contrast, plate designs create a multiple-layer stack of separated plates held in a frame. Alternating spaces between the plates are fed either steam or process liquid.
Looking at the process from the steam side, maximizing efficiency calls for feeding perfectly saturated steam with the highest possible dryness fraction, so its latent heat transfers into the process liquid, turning the steam completely into liquid condensate. No live steam should reach the outlet.
The steam line to the heat exchanger should have instrumentation to measure mass flow, temperature and pressure (figure 6). Dryness fraction also is critical because it provides an accurate picture of the available energy. A vortex flowmeter can provide these measurements.
Measuring condensate temperature at the outlet can verify overall energy transfer. The main automation system likely will be monitoring the process liquid temperature and flow — at least at the outlet if not also the inlet. If these measurements are not readily available, it may be necessary to add instruments.
Instrumentation Improves Profitability
Effective control and optimization of steam systems, and the processes they support, depend on instrumentation. It is necessary to determine what is happening in the boiler and steam-distribution systems as well as how fuel is being transformed into steam as a useful source of heat and power.
The solutions discussed will result in quick payback when comparing the costs of instruments against the ongoing costs of inefficient steam system operation. Squeezing even a few percentage points out of these operational costs by improving steam system efficiency, along with reliability and safety, will deliver improvements to the bottom line.