Combined-Cycle Water/Steam Monitoring11/14/2013
Email
Print
0
Facebook
1
Twitter
0
LinkedIn
1
Share
By Brad Buecker, Contributing EditorEarlier in this series, I
wrote about modern techniques to minimize impurity ingress,
scaling, and corrosion in heat recovery steam generators (HRSG).
But without accurate sampling and good data collection, the
conditions within an HRSG may be or become harmful without the
knowledge of operators or other technical personnel.This is another
of those cases where an "ounce of prevention is worth a pound of
cure." This article outlines important samples required for
successful HRSG operation.Monitoring the Entire SystemOrganizations
such as the Electric Power Research Institute (EPRI) have written
entire manuals regarding HRSG sampling [2], so obviously a complete
overview is not possible in a single technical article. However, it
is possible to discuss the most important samples and the benefits
derived thereby.HRSGs come in many different designs, but a common
general arrangement is shown in Figure 1. We will use it for this
discussion.
Let's begin at the condenser and condensate pump discharge
(CPD), with the note that many HRSGs are not equipped with
condensate polishers. For units with water-cooled condensers, the
condenser is the primary source within the system for impurity
ingress. Even a minor leak from one condenser tube can introduce
enough impurities to cause significant damage in the evaporator
circuits. In the opinion of many, the CPD sample point is the most
important of any within the entire water/steam network.Where the
condensate is treated by a polisher, the effects of a condenser
tube leak are dampened. However, it is still important that any
leak be detected as quickly as possible to prevent premature
exhaustion of the polisher and subsequent carryover of contaminants
to the boiler.Recommended on-line analyses include: Cation
Conductivity (or becoming popular, degassed cation conductivity)
Sodium Dissolved OxygenThe name for cation conductivity has
undergone an evolution, and research groups have now begun
referring to the technique as conductivity after cation exchange
(CACE). For this article, we will continue to use cation
conductivity. The technique has been adopted at most plants for
detecting impurity in-leakage. If a condenser tube fails or
impurities enter from another source (a makeup water treatment
system failure is the next most likely source), the sodium,
calcium, and magnesium salts in the water are converted to their
respective acids, primarily dilute sulfuric and hydrochloric acids,
by the cation exchange column.
The dilute acids that emerge (HCl and H2SO4, primarily) are more
conductive than their respective salts and have an immediate
influence on conductivity, thus providing a quick indication of
upsets. The cation column also removes ammonium ions (NH4+), which
are formed by conditioning chemicals added to the feedwater. If
ammonium ions are not removed from the sample, they can mask
impurities. In general, the cation conductivity of a clean
condensate sample should be less than 0.2 micromhos (microsiemens)
per centimeter (S). This limit is mandatory for systems that
operate on all-volatile treatment oxidizing [AVT(O)], which has
been developed to minimize flow-accelerated corrosion in
economizers and evaporator tubes. [3]So, how does degassed cation
conductivity improve upon the method? Air that leaks into
condensers of course contains a small percentage of carbon dioxide.
At significant air in-leakage rates, the CO2 that enters can
increase the condensate conductivity and mask other impurity
ingress. Degassed cation conductivity utilizes either a reboiler or
purge vessel (with nitrogen gas feed) to remove carbon
dioxide.Direct sodium monitoring of the CPD is also very effective
for detecting condenser leaks and other impurity ingress. With a
tight condenser, sodium levels in the condensate should be very low
(
