POWER February 2013 - 14

The work environment near boiler drums can be harsh.
Traditional incandescent or halogen lighting required routine
maintenance on potentially hot lighting fixtures. The marketplace
now offers LED models that provide a bright image
and consume a fraction of the power while greatly reducing
the maintenance risks associated with their predecessors.
Additionally, LED illumination requires little maintenance,
and 10-year lamp life is very commonly available.
Other industry improvements contribute to increased water
gauge glass service life, including high-quality original equipment
manufacturer (OEM) components and well-documented
instructions. The ready availability of OEM maintenance instructions
on the web eliminates excuses for missed critical maintenance
requirements.
Additional Rules for Remote Indicators
The rules in the Code are clear when it comes to remote indication
for the control room. If the operator in the control area
cannot see a gauge glass level there must be two independent
(remote) indicators on continuous display for the operator.
The logic behind this requirement is simple. If there are two
completely independent indication systems that track with
each other as the water level rises or falls, then the operator
has confidence in the gauge glass level. If an operator has
only one level indication, then how does the operator know
that it is displaying properly?
Electronic displays have similar rules. If one of the displays
is on a computer screen, it must be dedicated to this image or
at least set up with the drum level similar to a " tool bar " so the
screen can be manipulated without loss of display of the drum
level information.
Given the importance of the measurement to staff and
equipment safety, no compromise should be made when selecting
boiler water level measurement equipment. The most
common technologies used for remote indication of drum level
include:
■ Conductivity multi-probe systems that sense and remotely
display the level with a panel indicator and also serve as
a trusted technology for alarm and trip circuits. Differential
pressure technology that relies on a pressure head and
sensing lines near the drum to activate a remote display.
■ Guided wave radar technology is the newest technology for
these applications but also requires programming to ensure
accurate level indication.
■ Magnetic level gauges consisting of a stainless steel chamber
with an internal float containing a magnet that attracts
an externally mounted indicator. However, the Code limits
use of these systems to 900 psi, due to risks for inaccuracy
at higher and sliding operating pressures.
All of the above technologies have application in the boiler
room but must be properly installed to prevent indication errors.
Recent Developments in Conductivity Probe Systems
Conductivity probe systems, popular since the 1960s, have
benefited from improvements in low-voltage technology. New
systems have the intelligence to automatically monitor probe
condition and to notify the control room that a blowdown of
the probe column to remove residue and mineral buildup is
necessary to maintain accuracy (Figure 3). These intelligent
systems can reduce staff exposure to hazardous areas while
extending probe and valve life. Additionally, system intel14
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POWER
| February 2013
ligence can identify probes that have become unstable and
need to be replaced, further simplifying maintenance while
improving system reliability.
The placement of the indication range for any instrument
must also meet strict requirements. The upper end of the range
must not intersect the elevation of the lower outside edge of the
steam pipe diameter exiting the drum. Also, the lower end of the
indication range must not intersect the upper outer edge of the
water connection pipe exiting the drum. Furthermore, the lowest
edge of the indication range must be at least 2 inches above the
lowest safe operating level that will not cause any harm to the
drum internals.
The minimum pipe size requirement to feed instrumentation
from the drum is 1 inch for water columns, standpipes, and
low water cutout devices. The minimum drain pipe size is ¾
inch. However, the minimum drain pipe size for remote indication
systems is ½ inch. Further, the minimum drain size from
the lower valve associated with a water gauge glass can be
3/8 inch, as long as the inside diameter of the drain provides
at least a ¼-inch diameter unrestricted bore. However, most
installations use ½ inch as their smallest drain piping.
Valves Are Critical, Too
Mating piping and valves from the drum, field wiring, and sys3.
Remote maintenance. The modern control room display
for a conductivity probe type system is shown. New " intelligent "
systems automatically monitor probe conditions and instruct operators
precisely when to blow down the column to clean the probes to
maintain accuracy and reliability. Courtesy: Clark-Reliance Corp.
http://www.powermag.com
POWER February 2013

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