Boiler accidents and injuries are on the rise. An incident report issued by the National Board of Boiler and Pressure Vessel Inspectors states that 90 injuries were caused by boilers and pressure vessels in 2003 (a 300-percent increase from 2002). Most of the recorded accidents were caused by low-water conditions, poor maintenance practices, or operator error.
Inpart, this is because of the neglect or lack of drum-level instrumentation installed at a growing number of plants. Many installations with code violations have been observed over the years, and personnel at these plants have been advised to take the necessary corrective actions.
Section I of the American Society of Mechanical Engineers' (ASME's) International Boiler and Pressure Vessel Code has specific minimum requirements for direct and indirect level-indicating instruments, which must be installed on every power boiler manufactured in accordance with the code. In addition, all boilers operated at up to 400 psi must have at least one direct-reading water-gauge glass in service at all times.
Boilers operated at pressures greater than 400 psi may have either two directreading water-gauge glasses in service or one water-gauge glass (which may be valved off and kept in serviceable condition). This is only if there are two independent or indirect level-indicating devices on continuous display for an operator.
Increasingly, there seems to be confusion among users, specifying engineers, and sales people concerning the various types of levelindicating instruments and minimum code requirements. The only types of direct indicating (reading) devices are gauge glasses. The various types of gauge glass are:
Tubular, used for pressures of up to 250 psi and displaying the meniscuses (water levels).
Prismatic, or reflex, used for pressures of up to 350 psi and appearing black below a meniscus and white above a meniscus.
Flat, or transparent, used for pressures of up to 2,000 psi and displaying meniscuses. The water below and steam above a meniscus appear clear. For this reason, the ASME code requires gauges with multiple sections to overlap by a minimum of 1 in. to prevent loss of visibility of an actual water level.
Ported (glass-disc), which display green for water and red for steam utilizing light refraction. This is accomplished by strategically introducing two color sources into glass discs opposed by 160 degrees. The ASME code requires this type of gauge to be installed properly with a light fixture to create the appropriate distinction between indications for water and steam.
These instruments are designed to permit an operator to observe an actual water level without an interface mechanism or sensor that could misrepresent the drum level. Tubular glass provides the least amount of safety because of industry standards for glass thickness. However, prismatic and flat-glass gauges provide an approximately 3-1 ratio between the thickness of the glass and the width of a sealing gasket.
With high-pressure applications, actual drum level may be slightly higher than indicated in a gauge glass. This is because of minor changes in the density of the gauge glass' water, compared with the boiler drum's slightly warmer water.
Indirect indicating devices
There are several types of indirect indicating devices, including conductivity-probe level indicators, differential-pressure transmitters, magnetic level indicators, and guided-wave radar. Conductivityprobe and differential-pressure indicators have been the most widely specified technologies.
While differential-pressure transmitters provide a fine resolution of a drum level for input to control valve actuation, conductivity-probe systems provide multiple independent sensors in a chamber at specified locations, which provide reliable level indication with typical safeguards against single-component failure. Operators have reported improved confidence with independent stand-alone conductivity instruments with a light-emittingdiode (LED) level indicator, which brightly illuminates levels in typical control rooms.
Magnetic level indicators are based on float technology with magnetic coupling. This type of instrument offers options for remote indication output and point level switches.
Magnetic level indicators will be recognized in the 2007 edition of the ASME code, with an application pressure limit of 900 psi. Users should consider the quality of their boiler water if considering this type of instrument.
Because the ASME code does not distinguish between new and old boilers and their water chemistry, a high amount of iron particulate in boiler water may cause operational inaccuracy if an excessive amount of particulate builds up on a float. Magnetic level gauges often are applied to feedwater heaters, which tend to be reliable applications because of their clean-water environments.
Some users mistakenly have violated the ASME code by replacing water-gauge glasses with magnetic level gauges. While this is a common practice in the petrochemical industry, the code requires a direct-reading water-gauge glass on every power boiler manufactured to meet Section I. The decision to eliminate all directreading water-gauge glasses for this application is a violation.
Guided-wave-radar technology is rather new and is not yet widely accepted by the industry. This is because of operational irregularities among various manufactures, field-application data-programming requirements, and mystical perceptions of an instrument that has to calculate a level based on the interpretation of measurement data.
Proper maintenance procedures, as outlined by original equipment manufacturers, must be followed to maximize the service life of level instruments and maintain applicable regulatory-agency approvals. The introduction of inferior components and the possibility of incorrect repair procedures may have a detrimental effect on any level-indicating instrument. Cross training between shift personnel can reduce the risk of maintenance errors.
To achieve optimum safety for boiler operation and plant personnel, any individual responsible for the selection, specification, and replacement of a level instrument on a Section I power boiler must understand the applicable code requirements and the instruments being used and specified in his or her plant.
It is wise for management to consult with a plant's insurance carrier to verify if additional instrumentation beyond minimum code requirements is required.
Personnel in many plants perform impeccable maintenance on these instruments, while others are in serious condition. By conducting a survey of existing drum-level-indication instrumentation and consulting with operators, a user can identify and rectify any instruments of concern for a safer boiler-room atmosphere.
About the author
A product manager for ClarkReliance Corp., James W. Kolbus has 24 years of experience in research and development, application engineering, and field projects.