POWER February 2013 - 15

tem maintenance are also important parts of the boiler drum
level sysem. The installation and type of isolation valves that are
installed between the drum and level instrumentation are also
critically important.
When isolation valves are installed between the water column
and the drum (which is a Code option), they must be
locked open. This action prevents actuation of these valves
or false trips caused by unauthorized personnel isolating the
actual water level from the instrumentation without operations
staff knowledge.
The types of isolation valves used include gate and globe
types. Globe-type valves may provide longer service life in this
application. However, the specific design requirements in Section
1 for globe-type valves for instrumentation mandate at
least one-quarter of the inlet port diameter must be above the
seat when the valve is open. This provides a straight line flow
path within the valve and greatly reduces the risk of blockage
caused by sediment buildup, which could result in a trapped
level in the instrument. This important update to the Code
(Section 1, Subsection PG-60) occurred in 2007.
The most common materials used for construction of the
pressure components on level instrumentation are carbon
steel pipe (SA106- Grade B), SA105 forged steel for valve bodies,
SB-61 bronze for low-pressure water gauge valves (rated
up to 450 psi), and various grades of stainless steel that are
listed in the Code (Section 1, Subsection PG12). Other acceptable
grades of carbon steel pipe and plate materials are
identified in Section 2.
Proper Water Level Improves Heat Rate
While staff safety remains foremost, protection of the turbine
and plant efficiency are also directly related to boiler drum
water level. Properly maintained water level optimizes fuel
efficiency. However, the added assurance against operating
with a too-high water level may prevent saturation of internal
drum steam separators and ultimately prevent water droplets
(which carry particulate) from entering the steam outlet and
the turbine.
There is much more to level indication instrumentation than
meets the eye. The best place to learn more is by reviewing Section
7 of the Boiler Code or Recommendations for the Proper Care
of Power Boilers. This section of the Code includes important
information, such as:
■ Boiler startup fundamentals
■ Verification of water level in two or more devices
■ Checking instrumentation and piping for leaks
If you have any doubts about the proper operation of the level
instrumentation in your plant, consult with your safety department
or your local jurisdictional inspection authority (state or
insurance) to verify the requirements for your specific application.
Finally, always follow the OEM guidelines for the installation
and operation of level instruments to ensure that you have the
most up-to-date and accurate information. ■
-Contributed by James W. Kolbus (jkolbus@clark-reliance
.com), Clark-Reliance Corp.
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February 2013 | POWER
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