Can installing redundant instrumentation mean the difference between achieving optimized boiler control, driving up energy costs, or possibly even preventing a safety incident?
Issues related to optimum boiler operation can be many and varied. When operating problems occur, a work order is often issued to “tune up the boiler” – but the chance of fully resolving boiler issues with only a tuning effort is often not high. In a recent workshop, Bob Sabin from Emerson stressed that loop tuning and curve adjustment will not overcome field device issues at the root of many boiler operation difficulties, and the way to build optimal boiler control is typically to begin with the measurement and actuation field devices on which the boiler depends.
To look at a specific example, some key boiler instrumentation on the water side is shown in the picture to the left. As most realize, drum level control is key for safety and reliability, and choices regarding measurement of drum level can greatly impact operations.
High drum level can cause water carryover that lowers heat transfer efficiency and possibly damages downstream equipment such as steam turbines. Low level exposes tubes to excessive heat, resulting in tube damage and unplanned shutdown. Typical challenges in drum level measurement include the fact that water density fluctuates and water/steam dielectric can change with changes in pressure and temperature.
Another issue is the shrink and swell phenomenon. As steam demand decreases, drum pressure increases, which compresses entrained steam bubbles and can cause the drum level to appear to decrease even though it actually increases. Conversely, as steam demand increases, drum pressure decreases, and the gas bubbles expand, often causing the drum level to appear to increase.
To help compensate for this phenomenon, boiler control engineers employ three-element control strategies that simultaneously look at steam flow, the rate feedwater is flowing to the steam drum, and the water level in the steam drum. In addition, compensation for pressure and temperature must be made either at the level instrument or in the computer control system.
Redundant drum level measurements are recommended to achieve safety and reliability and one way to obtain measurement redundancy is by combining differential pressure (DP) and guided wave radar (GWR) level technologies. GWR is independent of density, but steam dielectric (DC) can cause error with pressure changes. For this reason, compensation must be made for DC when using this level technology.
With a standard GWR, obtaining a DC value for what the GWR sees is often difficult. A more direct approach is to work with a GWR device that carries out this compensation internally. Called Dynamic Vapor Compensation (DVC), it works by inserting a fixed reflective object in the path of the radar waves, well above any expected liquid level. The GWR compares the measured distance to the reflector with its known distance to create a compensation value that it applies to all readings. Because it determines this correction value continuously, it corrects measurement errors under all conditions and reduces the error rate to less than 2 percent.
This is just one example, boiler operations are also impacted by field devices including fuel flow, air flow, steam flow, oxygen, valves, dampers, furnace pressure, and more.
Have you used redundant boiler drum measurements? For more on boiler optimization at all levels, read “Four Key Measurements for Optimal Boiler Control." Let us know your experiences or questions in a comment below!