February 2008

Special section: Flow/Level

All in one glass

Alternate approach to gage-glass maintenances leads to more accurate drum level measurements

Fast Forward

  • Glass and metallic components lead to high maintenance for boiler steam drum level gage glasses.
  • One electric company developed alternate approach involving one gage glass, two logic solvers.
  • New approach eliminates confusing electrode column type level indicators.
 
By Dale P. Evely

Boiler steam drum level gage glasses have a tendency to break, especially on outdoor units. They have historically required a high level of maintenance, most likely due to their combining glass and metallic components that must hold back steam and water at high pressure and temperature. Thermal cycling of these devices, as the boiler starts up and shuts down, also contributes to the maintenance problems. Such issues have led maintenance personnel to wonder why they could not replace gage glasses with other drum-level indication technologies that are not as prone to leakage.

Southern Company, a producer of electricity, fiber optics, and wireless communications, developed an alternate approach to address gage glass maintenance issues as well as measurement uncertainties associated with water-column-type measurements, and added outdoor installed heat recovery steam generators with large steam drums and measurement tap spacing.

We installed one gage glass and kept it serviceable (valved out most of the time). We installed three differential pressure-based level transmitters and three pressure transmitters using three separate sets of drum taps. We then used electronic signal isolators to wire all six transmitters to two separate sets of digital control system (DCS) analog input cards, with each set of input cards associated with a different digital control system logic-solving processing unit.

The next step was to provide redundant transmitter power supplies and redundant logic-solving processing units as a part of the DCS equipment. We performed drum-level pressure compensation calculation algorithms in the separate logic-solving processing units. We then had each logic-solving processing unit drive a separate analog output and a separate panel-board-mounted indirect indicator expressing the median of the three sets of pressure-compensated transmitters. We performed the steam drum-level control, alarm, and trip functions using the median of the three pressure compensated signals within one of these two logic-solving units.

We kept the option to install a magnetic level indicator on the drum end opposite the installed gage glass for local level indication purposes. We implemented this approach on all combined cycle units the company has since added to its fleet, and retrofit it on most of the older units of that type.

This approach eliminated control-room-mounted electrode column-type level indicators, which had been a continuing source of confusion due to the sliding pressures of the steam drums that caused the electrode indicators to rarely agree with the pressure compensated drum-level indications. These level indicators also saw use as controls for the steam drum levels.

The installed base of Southern Company fossil steam units recommended this approach to address concerns with gage glass maintenance issues and the potential inaccuracies of uncompensated direct and indirect drum level indications available within some of our older control rooms. You can implement this approach at a minimal cost where you have triple redundant pressure and level transmitters already installed, and where DCS equipment with multiple processors and spare input/output cards are already in place or planned for installation. 

Boiler, pressure code

Section I of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC) defines the rules for construction of power boilers. A power boiler in this context is any boiler that generates steam at a pressure of more than 15 PSIG. Part PG of Section I defines the general requirements for all methods of construction and the main menu requirements for water-level indicators. When a boiler operates at no more than 400 PSIG, it is only required to have one gage glass. The code describes a gage glass as "a transparent device that permits visual determination of the water level." If the boiler is meant to operate at more than 400 PSIG, then it requires two gage glasses, unless there are additional measurement systems. If at least one gage glass is not visible to the operator in the area where water level is controlled, then additional provisions are necessary.

Here is a list of necessary ASME additional measurement provisions:

  • If you have above 400 PSIG, you need two gage glasses per steam drum.
  • You may replace one of these two gage glasses with two independent (indirect) remote indicators; if both remote indicators operate reliably, you may valve out the gage glass but keep it serviceable.
  • One gage glass (or two indirect remotes) must be readily available to the operator in his operating location.
  • You can use mirrors and fiber optics to direct read the gage glass image remotely (considered a direct remote).
  • The operator must be able to view indirect remotes at all times if he uses them to replace a gage glass.

The ASME gage glass requirements have prompted those subject to the BPVC to ask for formal interpretations over the years in order to eliminate gage glasses and their associated mirrors or fiber optic systems. Some of these ASME formal interpretations include:

  • You may use a computer terminal to provide a remote boiler water level indication.
  • You will need continuous, uninterrupted indication of boiler water level from two remote level indicators if the gage glass is valved out.
  • Even though a level indicator does not require a power supply, that does not make it a gage glass.
  • Viewing a gage glass via a system of mirrors is a direct indication of boiler water level.
  • A remote computer terminal that displays boiler water level indication only on demand is not a continuous, uninterrupted indication of boiler water level from a remote indicator.
  • A magnetic level indicator (flipper gage) is an indirect indication of water level.
  • A magnetic level indicator is not a boiler gage glass.
  • A pressure compensated dP transmitter remote readout is an indirect indication of boiler water level.
  • A distributed DCS that displays boiler water level graphically but requires a keystroke to become visible is not a continuous, uninterrupted indication of boiler water level from a remote indicator.
  • A device that provides water level indication must comply with the rules of Section I of the BPVC.
  • When you replace one of two gage glasses with two independent remote indicators, each indicator must be an independent system that continuously measures, transmits, and displays water level. You can use the same signal processing algorithm for each of the two indicators.

The ASME BPVC review tells us, for  boilers with maximum allowable working pressure above 400 PSIG, you can remove one of your two gage glasses and throw it away, as long as you have two independent, remote water-level indicators that continuously measure, transmit, and display boiler water level. Even better, if your two independent remote water level indicators are operating reliably, you can valve out your gage glass and keep it valved out, but you must maintain the gage glass in a serviceable condition. It should, however, be easier to keep a gage glass in serviceable condition if it spends most of its time in a valved-out state.

Magnetic level indicators

The popularization of magnetic level indicators in the early 1990s led many to consider them to be a replacement for gage glasses. Even though ASME issued formal interpretations related to magnetic level indicators in 1993 and 1994, they did not add to the code description of a gage glass as "a transparent device that permits visual determination of the water level" until the 2004 edition of the document.

Many people today who do not look at the code itself are not aware ASME does not consider magnetic level indicators to be a direct indication of water level. It is possible the ASME opinion is based on moving parts within the device that can hang up or the possibility of demagnetization of the device in the presence of electromagnetic fields. You do not directly view a water level with these devices. Magnetic level indicators, however, are available from a number of manufacturers and are a useful technology for use as indirect water level indication in boilers as long as you remember their limitations.

Pressure compensation needed

Gage glasses, magnetic level indicators, electrode column type level indicators, and any other technology that uses measurement of a level in some type of a water column external to the steam drum all have an error associated with their level indication. 

The ASME code does not discuss this directly for gage glasses, but regarding the water columns the gage glasses connect to, it does mention the water column must be correctly positioned "relative to the normal water level under operating conditions." Most boiler manufacturers' gage glass installation drawings will show two different gage glass readings for normal water level cold and normal water level at normal operating drum pressure. We typically see boiler gage glasses mounted with their centerlines at the elevation that matches normal water level at normal operating drum pressure. This, of course, gets more complicated if the boiler drum is outdoors, where ambient temperatures vary more widely, or if the steam-drum pressure slides such that there is no normal operating drum pressure.

The physics behind gage glass error is, at operating pressures, the leg of water in the gage glass is somewhat cooler than the water in the steam drum. Cooler water is denser, so it will take a shorter leg of water on the outside of the steam drum to balance the leg of water on the inside of the steam drum.

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Even though you understand the physics, you might believe the temperature effect is minor. A typical arrangement for many large power boilers is as follows:

  • Drum level measurement tap spacing: 30 inches
  • Normal operating steam drum pressure: 2550 PSIG
  • Drum temperature at this pressure: 672ºF

Water level indication errors due to the temperature effect for this example would calculate like this:

  • Gage glass 10ºF cooler than the steam drum gives a 1.1 inch indication offset
  • Gage glass 20ºF cooler than the steam drum gives a 1.8 inch indication offset
  • Gage glass 30ºF cooler than the steam drum gives a 2.4 inch indication offset
  • Gage glass 100ºF cooler than the steam drum gives a 4.5 inch indication offset

From these calculations, we can conclude gage glasses are not only maintenance issues, but they also have uncertainties associated with the level they indicate. For very large steam drums with tap spacing greater than 30 inches, and for drums in outdoor service, the measurement uncertainty associated with an uncompensated water column type of a measurement could be much greater than this example.

The panel board mounted large case indicators that meet the ASME BPVC requirements to allow the elimination of one of the two gage glasses required for a power boiler that normally operates above 400 PSIG. This is because each of these remote indicators is a part of an independent system that continuously measures, transmits, and displays water level. Keeping both of these remote indicators operating in a reliable fashion should also meet the ASME BPVC requirements. This says you can operate with a gage glass or gage glasses in a valved-out but serviceable state. Implementing this approach also provides two indirect remote indicators in the control room that should always agree with the drum level signal in use for control purposes and thus eliminate a source of confusion for the boiler operator.

ABOUT THE AUTHOR

Dale P. Evely, P.E. is principal engineer, instrumentation and controls, at Southern Company in Birmingham, Ala. (dpevely@southernco.com). 

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