The heart and soul of any boiler-based power generation system is the steam loop or circuit (Figure 1). Efficiency suffers if the appropriate amounts of water are not available. In more extreme circumstances too much water (carryover) or too little water (low water condition) can cause damage to other components, shortening a boiler's lifespan. In the most extreme situation a dry fire accident could occur, resulting in severe damage and personal injury.
 Figure 1. A diagram of a steam loop (Click image for larger version) |
Level indication in the steam loop is critical, yet the methods employed to measure it have been slow to evolve or change. Some of that has been due to code requirements (PG-60 of the ASME Boiler and Pressure Vessel Code) or a simple lack of confidence in new technology. In the past 15 to 20 years, technologies such as magnetic level gauges or differential pressure devices have replaced direct-reading glass gauges on applications such as feedwater tanks, high-pressure preheaters, hotwells, and also for drum-level indication. Guided-wave radar (GWR) is the newest technology addition and, used in conjunction with other technologies, it is considered a reliable, cost-effective choice for redundant level measurement in all steam loop applications, including drum level monitoring.
Theory of Operation
Patented by the British in 1935, radio detection and ranging or radar, was originally used for military purposes, to locate or identify ships and airplanes. Radar units for these purposes are very powerful and use a frequency-modulating continuous wave (FMCW) technology. By comparison, radar devices for level measurement use pulses of electromagnetic waves with frequencies from 1.5 to 26 GHz, known as microwaves. They are also available in two typical configurations: free space/noncontact or guided wave/contact.
These devices determine the level of liquid by identifying the time it takes for the microwave pulse (traveling at the speed of light) to travel from the measuring device to the liquid and back, as shown in Equation 1.
 |
(1) |
where:
| D | = | distance from measuring device to liquid |
| c | = | speed of light |
| T | = | amount of time for microwaves to travel from device to liquid and back |
Why use microwaves? Although process conditions such as vapors, temperature, pressure, material buildup, and condensate have little effect on radar devices, it is important to consider whether the process medium will reflect microwaves and this is accomplished by looking at its dielectric number. The dielectric number is a measure of the polarization power of an insulating material; it indicates how much charge can be stored in a type of material vs. the amount that can be stored in air. Water has a dielectric number of 80 and is considered a great reflector of microwaves. Air has a dielectric number of 1 and is considered a poor reflector of microwaves.
 Figure 2. A radar-based level sensor |
A GWR device (Figure 2) uses the same principle as noncontact radar devices in that it can transmit and receive reflected microwave energy. The primary difference between the two devices is the typical operating frequency (1.5 GHz for GWR vs. up to 26 GHz for free-space radar) and the presence of a wave guide, a metal rod or cable that guides the microwave pulses to the process media. The transmitter directs the pulse down the wave guide with approximately 80% of the available energy staying within an 8 in. radius.
Directing more of the energy on to the process media improves the signal to noise ratio; even with interference from nozzles or point-level devices, more energy directed at the process medium means more energy that can be reflected back to the transmitter. More energy also means an improved ability to work with lower-dielectric liquids or in other conditions that can attenuate signal strength, such as agitation or foam. Both noncontact and GWR devices are very accurate (±0.4 in.), with accuracy independent of the liquid's conductivity, density, and dielectric number, removing the need to reconfigure the instrument if changes in the process liquid occur.
Steam Loop Applications
We have discussed the general advantages a GWR can provide in level indication applications but what sets steam loop applications apart and makes them unique?
The propagation speed of the microwave pulse generated by a radar device is well defined and stable. Typical process conditions such as pressure (vacuum or high pressure) or temperature have minimal, if any, effect on this speed. The same is true of vapor blankets. While normally they are not a significant concern, there are specific circumstances when this is not the case.
If the vapor in question is considered a polar gas, (i.e., a vapor whose dielectric constant can change due to pressure or temperature) the propagation speed of the microwave pulse can be affected. Hydrocarbons experience very small changes in their dielectric constant even at very high pressures and temperatures. Steam, by contrast, is greatly influenced by the pressure and temperature of the application as we can see in the table in Figure 3.
 Figure 3. Dielectric constant of steam as a function of pressure and temperature |
A radar pulse travels at the speed of light, but only if the pulse is moving through vapor with a dielectric constant ≤1.0. If the dielectric constant of the vapor space in a vessel rises above 1.0, that propagation speed will be reduced and the pulse will take more time to reach the liquid surface and return to the transmitter. Referring to Equation 1 we can see that if the time increases due to a reduced propagation speed then the level measurement provided by the GWR will be lower than the actual level. At lower pressures for saturated steam, the measured error experienced (Figure 4) is relatively small. However, once an operating temperature of 400°F is required, the measured error could be as high as 3.5% to 4%. At 600°F that error can increase to 19% or 20%.
 Figure 4. Errors in the gas-phase of steam as a function of temperature and pressure |
Historically, the only way to offset the physical effect of a steam phase was to implement a correction factor in the transmitter electronics. While this method works, the correction factor is only valid for a given pressure/temperature setting; during startup or if the operating parameters for the application change, this adjustment will no longer be valid and could make the measured error worse than if you had no correction factor at all. The other method is to program a compensation table into the DCS or PLC to correct the raw signal from the GWR. A dynamic form of compensation is a better solution and ensures that your level measurement is accurate regardless of changes in the process. The best method for achieving this is to implement a reference section on your wave guide.
The reference section (Figure 5) provides the GWR with a known distance with which to compare a return signal. It always resides in the vapor or steam phase of the application and provides a small return signal to the transmitter. The transmitter compares this signal to the programmed reference distance. If the two match, the transmitter knows that the level signal it receives is correct. If it is greater than the known reference distance then the electronics in the transmitter know that the level signal will have a proportional offset and algorithms will automatically compensate for this and provide the DCS or PLC with an accurate level measurement.
 Figure 5. A diagram of the reference section of a GWR |
A GWR supplied with dynamic gas-phase compensation will provide a highly accurate level measurement. Properly designed, the unit will operate in saturated steam conditions up to 2900 psig at 690°F with no fear of damage to the electronics and will provide a reliable, accurate level indication directly from your GWR.
One additional aspect to consider when choosing your GWR is its mechanical design. In higher pressure and temperature steam loop applications you must take into account the unit's ability to withstand extreme conditions. For example, every GWR employs an isolation material between the wave guide and the electronics responsible for launching the microwave pulse and receiving the return signal. For standard units this isolation material is a polymer such as Teflon or PEEK and at lower temperatures (approximately 300°F for Teflon and 450°F for PEEK) these materials work well. At elevated temperatures, however, the steam will eventually bypass the polymers, resulting in damage to the electronics and causing the device to fail. Higher pressure and temperature applications need more temperature-resistant isolation materials such as ceramics and graphite to ensure longevity of service.
Comparison to Other Level Technologies
How does the accuracy and performance of a GWR with dynamic compensation compare to other more conventional technologies employed in steam loops?
Direct-reading glass gauges. For boiler drum level, direct-reading glass gauges are still the only true, direct level indication per PG-60 of the ASME Boiler and Pressure Vessel Code. A direct-reading glass gauge can be used in conjunction with other nondirect reading devices, but cannot be eliminated. Consequently, at least one operational glass gauge must be present. Over time the numerous gaskets present on a glass gauge will wear and fail. Glass without mica shields for higher-pressure and higher-temperature service may experience etching, causing it to weaken and fail under pressure. Belleville washers are also needed to help maintain seal integrity during pressure and temperature cycling. Many of these units have been slowly replaced with devices that require less intensive maintenance and that can be monitored from the control room.
Displacer transmitters. This is a semi-submerged float or weight connected to a spring balance. The complete assembly is typically enclosed in a chamber attached to the boiler drum. As the float weight rises or falls with the drum water level, the changing tension on the spring balance is converted to a level signal. Over time, metal fatigue will result in measurement drift and errors in the level indication. Corrosion can also impact the performance of the unit, especially if water quality is not tightly controlled. In addition, the setup for a displacer is based on a specific water density; should the pressure or temperature of the water vary, the level indication will be incorrect.
Water columns with conductivity probes. For a long time these devices have been used in conjunction with direct-reading glass gauges to provide a remote reading option to the control room. Typically, when a signal from the conductivity probes is received at a control panel it will illuminate a series of lights, providing a remote level indication. The control panel can also provide high/low relays as well as a 4-20 mA output. While not technically a continuous signal, when fully functional the devices do provide good control without the need to have a technician physically at the boiler drum. Maintenance is the most common issue with these instruments. The conductivity probes are subject to corrosion and require regular replacement. Considering that most systems have between 10–20 probes per column, this can be costly.
Magnetic level gauges. A magnetic level gauge is probably the most common replacement for most of the more traditional level gauges in a steam loop, including boiler drum level. Although a magnetic gauge has local visual level indication, it is not considered a direct-reading level gauge because the level indication depends on the magnetic attraction between the internal float and the exterior indication device. Like other displacer-type devices, a magnetic gauge is designed for a specific liquid density. If the float is not properly manufactured or the density of the process changes, measured error will occur.
Magnetic gauges can be very dependable when properly maintained. The strength of the magnetic coupling between float and indicator is directly proportional to the distance between them. Consequently, the radial clearance between the outside face of the float and the inside face of the chamber is very small. Dirt or debris that finds its way into the chamber can become lodged between these two components, preventing the float from following the change in liquid level.
Differential pressure transmitter. As its name implies, this level indication device measures the difference in pressure between the vapor space in a vessel or chamber and the liquid height. The net result is the pressure exerted on the lower sensor, independent of the pressure in the vessel. By comparing this value to a known density, the liquid height can be determined. Initially this provides an accurate and dependable level indication. Over time, the diaphragm or sensing component is subject to metal fatigue, resulting in measurement drift and the need for recalibration. Because a differential pressure transmitter is calibrated based on a specific liquid density, if the density changes then the level indication will be in error.
Guided-wave radar. GWR has no moving parts, has digital electronics, provides a linear output and is, therefore, not subject to drift or calibration issues unless the unit is physically damaged. GWR is also impervious to changes in pressure or temperature, reacting only to the change of impedance that occurs as the generated pulse contacts the liquid surface. Employing a dynamic form of gas-phase compensation largely eliminates any effect that a high-pressure or high-temperature polar medium might have on the unit's signal propagation.
Closing Thoughts
There is no shortage of technology choices for continuous level measurement in your steam loop. All have unique features and benefits that need to be carefully evaluated before you make your selection for your individual application.
In the case of GWR, one of the first considerations is the operating temperature and pressure. For saturated steam conditions <400°F, a standard GWR will provide excellent service. At saturated steam conditions >400°F the impact on measured error due to the effects of steam vapors will be more pronounced, requiring the use of dynamic gas-phase compensation for maximum accuracy as well as a more temperature-resilient design to protect the electronics.
Application flexibility, resistance to effects from process conditions, low maintenance requirements, and dynamic compensation all combine to make GWR an accurate and reliable choice for steam loop applications. These features also result in a low cost of ownership and tighter control over system operation.
REFERENCES
1. Win Van de Kamp, "The Theory and Practice of Level Measurement", Proceedings of the ASME 2011 POWER Conference, July 12–14, 2011, Denver, CO.
ABOUT THE AUTHORS
Keith Riley is product business manager with Endress+Hauser Inc., Greenwood, IN; he can be reached at 317-535-2169, [email protected].
Ravi Jethra is the power and energy industry manager at Endress+Hauser Inc., Greenwood, IN; he can be reached at 317-535-2147, [email protected].