By most manufacturers' standards, high-quality steam is 99.5-percent dry saturated steam. For a boiler to provide an appropriate amount of such steam safely, efficiently, and at the correct pressure, the amount of water inside of the boiler must be carefully controlled.
This article will discuss problems associated with high- and low-water-level conditions, the effects of steam pressure and load on water level, components of an effective water-level-control system, the importance of maintaining a mass/heat balance, boiler-level-control strategies, and types of level-measurement sensors.
Repercussions of Poor Control
When a boiler operates with an excessive amount of water, carryover and priming are likely to occur.
As water rises above the horizontal center line of a boiler drum, the water-surface, or steam-release, area decreases. When water-surface area is not sufficient enough for steam to be released at a controlled velocity, carryover of water droplets through the steam header occurs. Other causes of carryover include poor control of boiler-water total dissolved solids (TDS) and contamination of boiler feedwater. Water that is carried over contains dissolved and suspended solids, which can contaminate controls, heat-transfer surfaces, steam traps, and steam itself.
Priming takes place when a large, rapidly applied load results in a sudden reduction in steam pressure, which, in turn, causes boiler water to be pulled into piping.
When a boiler operates with an insufficient amount of water, severe damage and, in extremely rare cases, explosions can occur. With low water, tubes are uncovered and no longer cooled by boiler water. The temperature of the metal increases rapidly, the metal's strength is reduced, and collapse or rupture occurs.
Effects of Pressure and Load
Departing from a boiler's design operating pressure raises the potential for water-level problems, risking damage to both the distribution system and boiler.
When steam pressure is lowered, water level rises as steam bubbles expand, or swell. Conversely, when steam pressure is raised, water level falls as steam bubbles compress, or shrink.
Under swell conditions, water level rises with increasing load because more steam resides below the water. As water rises above the horizontal center line of a boiler shell, the water-surface area decreases. Boiler designers allow for a water level higher than the normal working level (NWL), providing sufficient area for the release of steam at a velocity low enough to avoid carryover. This ensures that the steam off-take does not exceed a specific height above the NWL.
As subcooled feedwater enters a boiler drum, it cools the drum water to below the operating-pressure boiling point. This acts to condense the steam bubbles in the drum water and collapse the steam blanket. Thus, a drum should be large enough to absorb subcooled water without being overly affected by it.
Even the best-controlled steam-generating system can produce carryover under certain load conditions. Therefore, a separator installed close to a boiler is recommended. By means of inertia, a separator removes the denser water particles from steam for elimination by a float-and-thermostatic steam trap.
Steam-outlet control valve
A control valve installed after a main shutoff valve provides better control of flow rate and, thus, warmup rate. A pulse timer can be used to slowly open a control valve. In a large distribution system, a line-size control valve often is too coarse to provide the required slow warmup. Under such circumstances, a small control valve in a loop around an isolation valve could be used. Where parallel slide valves are used for isolation, pressure can be equalized on either side of a control valve prior to opening, making the valve easier to open and reducing wear.
Multiple-boiler systems employ a steam manifold usually referred to as the main distribution header (Figure 1). Proper flow to and from a steam header is necessary for efficient warmup, good steam quality, and proper steam distribution. The design of a distribution header should allow boilers to share a load equally. Pressure drops between each boiler outlet and the header outlet to a plant should be within 1.5 psi. This minimizes carryover and helps to prevent overload and lockout of boilers. A lack of balance can cause excessive steam-outlet velocity, resulting in an extremely volatile water surface and failure of a level-control system.
The maximum velocity of steam through a header under full-load conditions should be 50 fps. Gravity and low velocity cause condensate to fall and exit from the bottom of a header so that it can be drained by a steam trap.
Header check valves
Steam-distribution headers should have main shutoff valves with integral check valves. Alternately, spring-loaded disc check valves can have a dampening effect on oscillation tendencies.
Condensate should be removed from a header as soon as it forms. A float-and-thermostatic-type steam trap usually comprises the first trapping point. This type of trap has an infinite turndown ratio on flow and pressure, so it operates effectively at high and low load and drains condensate immediately. Because removed condensate can contain carryover particles, this trap should drain to a boiler-blowdown vessel, rather than a feed tank.
Maintaining the Mass/Heat Balance
Level detection and control has three purposes:
Identify low-water situations and take appropriate action, such as shutting down or locking out burner operation.
Identify high-water situations and throttle down feedwater supply to reduce the chance of carryover and water hammer.
Maintain water level within design set points.
A level gauge and an operator are not enough for the optimal water level in a steaming boiler to be maintained. A level gauge indicates a lower-than-average water-surface level in a boiler shell because of such factors as:
Height of the gauge connection.
TDS and chemicals.
Size of the boiler shell.
Waves caused by sudden load swings.
Level-control systems with sensors or probes that fit inside of a boiler shell or steam drum provide a higher degree of safety than level-control systems installed externally. Actions based on probe signals include:
Sounding an alarm.
Opening or closing a relay.
Starting or stopping a pump.
Opening or closing a valve.
The objective of a boiler-drum level-control strategy is to provide a continuous mass/heat balance by replacing every pound of steam leaving a boiler with a pound of feedwater, thus maximizing the quality of the steam leaving the boiler.
Depending on load variations and the size of a boiler, one of three principal level-control strategies typically is employed: single element, two element, or three element.
In a single-element strategy, one device measuring instantaneous drum level provides a control signal to a feedwater regulator (Figure 2). Used in both on/off and modulating modes, this strategy is practical only for smaller boilers with moderate load changes because level phasing can lead to over- or underfilling.
As demand increases and drum water level rises, the device sends a signal to decrease feedwater flow when, in fact, feedwater flow should be increased to follow the mass of water leaving the drum as steam. With rapidly increasing demand, the level could get critically low.
Conversely, as demand decreases and drum water level falls, the device sends a signal to increase feedwater flow when, in fact, feedwater flow should be decreased to maintain mass balance. With rapidly decreasing demand, the level could get high enough to cause carryover into the distribution system.
Because it is the least expensive option, single-element control is practically universal among boilers with steam-generation rates below about 11,000 lb per hour. In practice, however, even moderate load swings can lead to excessive fuel and maintenance costs. A high rate of flow of feedwater into a drum can cause burner firing rate to vary as the pump switches on and off. For instance, calculations show that with high-temperature feedwater, burner firing rate may have to be 40-percent higher with a feed pump on. This continuous variation causes:
Wear on burner controls.
A “saw-tooth”-type steam-flow rate.
Practical for moderate load swings and rates, the two-element strategy can be used with boilers of any size. One element follows drum level, while the other meters steam flow to provide a mass balance of the water in and out of the drum (Figure 3). The first element tracks the error between the measured level and the set point, which is sent to a signal summer as a process variable. The second element measures steam flow, which also is sent to the signal summer. The summed result is the control output to the feedwater control valve.
Two-element control senses changes in demand before water level changes. The controller adds or subtracts output to moderate the signal from the drum-level controller to the feedwater control valve. Because steam flow normally is the larger variable, it can override the trim effect of the drum-level measurement amid moderate load changes, ensuring an accurate response. Under steady load conditions, the drum-level controller influences the feedwater control valve and acts to trim the level to the set point.
Two-element control cannot account for pressure or load disturbances in feedwater supply. It also cannot cope with phasing because only relatively slow-responding drum level is controlled. Amid rapidly rising demand, this shortcoming can cause drum-water subcooling by allowing more feedwater to enter a drum than a boiler can accommodate thermally.
A feedwater flow element can be added to the two-element control strategy (Figure 4) to address phasing and accommodate rapid load swings. The summed output of the two-element controller is sent to a second feedwater flow controller as a remote set point.
The fast-acting feedwater flow controller uses feedwater flow as its process variable and steam flow as its set point. For every pound of steam leaving a boiler, a pound of feedwater is added, the loop acting on the feedwater valve. The remote set point from the two-element level control changes with steam-flow and drum-level variations, such as those that occur during blowdown. The feedwater controller modulates its output to maintain drum level in a mass/heat-balanced state.
Accounting for the thermal dynamics of a boiler's recovery rate is important when tuning a feedwater controller. If the controller reset is set too fast, subcooling of the drum will occur, resulting in cyclic phasing like that found in a two-element system.
Three-element control is a must with multiple boilers sharing the same feedwater header and supply system. This feedwater system experiences variations in feedwater flow with two or more boilers online.
Types of Sensors
Float control is the simplest form of level measurement. Mounted in an external chamber or inside of a boiler shell, a float moves up and down as water level changes. At the opposite end of the float rod is a magnet that moves inside of a virtually non-magnetic stainless-steel cap. Typically, there are two magnetic switches — one providing on/off pump control, the other providing an alarm. This arrangement can be used to provide level alarms.
A more sophisticated system providing modulating control uses a coil wrapped around a yoke inside of a cap. As the magnet moves up and down, the inductance of the coil changes. This is used to provide an analog signal to a controller and then a feedwater-level control valve.
Level-signal output usually is via a vertically or horizontally mounted magnetically operated switch (mercury type or “air-break” type) or an inductive coil. In either case, a magnet acts through a non-magnetic stainless-steel tube.
Multiple conductivity probes (Figure 5) are used to give point measurements of water level. When water touches the probe tip, current flows through the sensor's circuit, triggering an action through an associated controller. Two probes are necessary to switch a pump on and off at predetermined levels.
A standard conductivity probe is used to provide low-water alarm. Probes are sheathed with a polytetrafluoroethylene (PTFE) insulator to minimize the risk of contamination. These probes typically have on-board diagnostics, such as a comparator tip that continuously measures and compares the resistance to ground through the insulation and through the probe tip and a current leakage detector between the probe and the insulation.
The use of differential-pressure cells (Figure 6) is common with pure-water systems, such as those in pharmaceutical processing. In those applications, the conductivity of water is very low, which can mean that conductivity and capacitance probes will not operate reliably.
Differential-pressure cells are installed with a constant head of water on one side and a head that varies with boiler-water level on the other. Variable-capacitance, strain-gauge, or inductive techniques are used to measure the deflection of a diaphragm; from that measurement, an electronic level signal is produced.
About the Author
A steam-trap product manager for Spirax Sarco Inc., Joe Radle has 35 years of experience in building and testing steam-trap products, research-and-development engineering, applications engineering, and steam-system troubleshooting. He is listed as the inventor on one U.S. patent and has one patent pending. Radle attended Northampton County Community College and Lafayette College and is a member of the Fluid Controls Institute, serving on a steam panel that develops industry standards.