|Steam drum boiler installation
Credit: Emerson Process Management
In large boiler installations, the water level in the boiler steam drum must be precisely controlled to optimize steam production, maximize boiler efficiency and maintain safe operation. If the water level is too low, there is a risk of damage to the steam drum and the tubes, as well as a risk of the boiler exploding. If the level is too high, the water could be carried over into the turbine which relies upon the supply of clean dry steam. Even a small amount of water can cause catastrophic damage to the turbine blades, the cylinders and the housing.
|Figure 1: Magnetic level indicators eliminate problems with coating, plating and fouling|
To monitor boiler water, operators need access to a visual indication of the drum level. This is both an important safety check and also a requirement of the boiler and pressure vessel code (BPVC) issued by the American Society of Mechanical Engineers (ASME). This direct and independent indication helps protect against damage caused by low and high water levels.
Traditionally, visual indication has been provided by level gauge (sight) glasses. These would be installed at each end of the boiler for inspection during operator rounds. However, as the operators required to perform these rounds will testify, a problem usually occurs around one month after the sight glasses have been installed. Contamination and discolouration makes the sight glasses very hard to read – compromising safety.
To address this issue, power plant operators are installing magnetic level indicators on boiler drums and feedwater heaters. These provide better visibility and also reduce maintenance. Magnetic level indicators utilize a magnetic coupling to a float housed within an external ‘float’ chamber. The float moves up and down as the water level changes, activating a series of lightweight magnetized indicators or flags. The distinctive red colour of the flags makes it easy to determine the level of difficult-to-see colourless fluids, with the indicator being visible to the operator from more than 30 metres away.
|Figure 2: Triple redundancy on boilers and feedwater systems is becoming an industry standard|
Magnetic level indicators offer a clear indication of liquid level and are usually installed in a chamber next to the boiler drum. Because the contents of the steam drum do not come into contact with the glass, problems with coating, plating and fouling are completely eliminated, ensuring reliable level indication.
Because magnetic level indicators rely on a float to indicate the level, they are not considered a ‘direct’ indication of level like a gauge glass. Gauge glasses are still required, but because they can be hard to read, they are often valved off until needed. Since magnetic level indicators are much easier to see they are often accepted as the secondary indication device.
Electronic steam/water gauging systems can also be used to monitor water levels within boilers. Emerson’s Hydrastep consists of a number of electrodes installed within a water column attached to the boiler. The electrodes are positioned above and below the water level and a step change in resistance between two adjacent electrodes identifies the water level. Local and remote displays provide operators with high visibility of boiler levels. These systems feature a wide, accurate measuring range and are virtually maintenance-free. Hydrastep systems provide continuous monitoring of steam drum level.
Boiler drum level control
Drum level is critical for safety, but controlling and maintaining the correct water level is also important to maximize boiler efficiency. However, there are several factors that make accurate water level measurement difficult.
For example, the steam drum contains a turbulent mixture of water with entrained steam bubbles and steam. As steam demand decreases, the drum pressure will increase and compress the entrained steam bubbles. So even if the amount of water is actually increasing, the drum level will appear to shrink.
|Figure 3: Density and dielectric properties of water and steam change with pressure and temperature|
Conversely, as steam demand increases, drum pressure decreases and the gas bubbles expand, causing the drum level to appear to swell. To account for shrink and swell, power plant engineers use complex control systems to maintain a constant liquid water level in the steam drum.
In addition to the requirement for a visual indication of drum level, the BPVC stipulates the need for fail-safe modes, diversity, self-diagnosis and measurement redundancy. The type of level measurement technology used for any boiler application is based on the hazard analysis, but the code recommends two local level indicators, combined with two independent level transmitters to provide boiler drum level control and indication.
Based on these requirements and users’ operational experiences, triple redundancy on boilers and feedwater systems is becoming an industry standard. Combinations of differential pressure and guided wave radar transmitters can meet this requirement, with acceptable combinations being two Guided Wave Radar (GWR) + one Differential Pressure (DP); two DP + one GWR; or three devices of the same technology.
DP transmitters are widely used for the control of boiler level as they meet the application’s needs for accuracy and reliability. DP transmitters measure both liquid level and static pressure. The high-pressure side of the transmitter measures liquid head pressure and static pressure, while the low-pressure side measures static pressure only.
The net result is that the static pressure is automatically compensated out, leaving only the liquid head measurement. This measurement, combined with the liquid density, provides the level measurement.
DP transmitters are calibrated for level based upon a specific liquid density. If that density changes, errors will be introduced into the level measurement. These changes are especially common during startup, when both the liquid and steam phases of the system will have density changes as the system reaches operating temperature and pressure. A compensation method based on operating conditions is necessary for the density adjustments of the DP level measurement.
To provide density compensation for DP level measurements, a pressure measurement is used to determine the appropriate density values based on the operating pressure and the established density values from the saturated steam tables.
Guided wave radar transmitters
GWR transmitters are providing an alternative choice for steam drum applications as they offer a number of important advantages over DP technology.
For example, GWR transmitters are completely independent of density and able to withstand extreme temperatures up to 400°C and pressures up to 345 bar. They provide accurate and reliable measurements even when exposed to mechanical vibration and high turbulence. And GWR transmitters have no moving parts, providing the added benefits of low maintenance and high reliability.
In a typical installation, the GWR is mounted on top of a chamber with a probe extending to the full depth of the chamber or vessel. A low energy microwave pulse is sent down the probe and when it reaches the media surface, a reflection is sent back to the transmitter.
The transmitter measures the time taken for the pulse to reach the media surface and be reflected back, and an on-board microprocessor calculates the distance using Time Domain Reflectometry principles.
Transmission speed depends on the dielectric constant of the media. As can be seen from Table 1, in the case of water, this changes in both the liquid and steam phases. In high-pressure steam applications, the effect is to slow down the speed of propagation and this can result in an error of up to 20 per cent over temperature.
As the temperature increases, the dielectric of water decreases and the dielectric of the vapour increases. Accurate level measurements can only be obtained if the water dielectric remains sufficiently high to provide a reflection back from the surface. However, there reaches a point between 2610 psi (180 bar) and 2900 psi (200 bar) when the dielectric difference between steam and water becomes too small to offer reliable level measurement. In this case, level transmitters based on GWR technology are no longer suitable.
To maintain the accuracy of level measurement it is necessary to apply a compensating factor to the GWR transmitter. If the conditions are stable, the dielectric of the vapour at the expected operating pressure and temperature can be entered manually during the configuration of the transmitter. This allows the unit to compensate for the dielectric at the expected operating conditions.
For higher-pressure applications, which may have more variations in the operating conditions or where the users want to be able to verify the unit under near ambient conditions, such as during startup and shutdown, dynamic compensation is required.
For example, Emerson’s Rosemount 5300 Series GWR transmitters incorporate Dynamic Vapour Compensation technology, which provides a highly accurate level measurement across a range of operating conditions.
Dynamic Vapour Compensation works by using a GWR probe with a target at a fixed distance (see Figure 4). The transmitter knows where the pulse reflected from the target should be with no vapour present.
|Figure 4: Signal curve of GWR with Dynamic Vapour Compensation|
When there is vapour in the tank, the electrical distance of the reflector pulse appears beyond the actual physical distance of the reflector point. The distance between the actual and apparent reflector points is used to continuously calculate the vapour dielectric.
The calculated dielectric is then dynamically used to compensate for vapour dielectric changes – eliminating the need to introduce compensation in the control system. For applications below 180 bar, GWR with dynamic vapour compensation will provide highly accurate level measurements in saturated steam conditions.
The Rosemount Dynamic Vapour Compensation integrated still pipe vapour probe with a long reflector is recommended for best practice. Depending on the application, probe type, reflector length and internal as well as external conditions, the accuracy errors can vary, with accuracy errors down to 2 per cent being achieved.
Application and installation conditions, such as a lower temperature in the bypass chamber, can cause changes within the measured media. Therefore, the error readings can vary depending on the application conditions and may cause an increase of the measuring error by a factor of two to three.
Figure 4 illustrates the radar signal curve before and after vapour compensation. Without compensation, the surface pulse appears to be beyond the actual level. After compensation the surface appears at the correct surface level point.
To meet the requirements for indication and redundancy in drum level applications, a recommended combination is a magnetic level gauge with Guided Wave Radar installed in an adjacent chamber. Together these devices provide a low-maintenance solution that provides high accuracy and local indication.
The BPVC issued by the American Society of Mechanical Engineers recommends two local level indicators combined with two independent level transmitters to provide boiler drum level control and indication. Triple redundancy on boilers and feedwater systems is becoming an industry standard.
Magnetic level indicators provide a clear indication of liquid level, but because they rely on a float they are not considered a ‘direct’ indication of level such as, for example, a glass gauge. However, magnetic level indicators are much easier to see, and are often accepted as the secondary indication device. Electronic steam/water gauging systems can also be used to monitor water levels, providing operators with a high visibility of boiler levels with virtually zero maintenance and exceptional reliability.
Differential pressure transmitters are widely used for boiler level control as they meet application needs for accuracy and reliability. However, because differential pressure transmitters are calibrated based upon a specific liquid density, errors are introduced into the level measurement as the density of the water and steam changes. These errors can be compensated within the control system to calculate the best possible level.
Sarah Parker is an application manager at Emerson Process Management.
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