POWER November 2012 - 62

PLANT DESIGN
The Evolution of Steam
Attemperation
The fundamental design principles and process for modern steam desuperheating,
or the attemperation of superheated steam in the power generation
industry, have been evolving since the early 1930s. Meeting the
requirement for steam quantity, quality, and temperature consistency is
the foundation of traditional attemperator component design, particularly
for fast-response combined cycle plants.
By Martin-Jan Strebe and Arvo Eilau, Tyco Valves & Controls
I
ncreases in steam and combustion turbine
operating temperatures and capacity that
are inherent in the quest to increase steam
cycle efficiency are advancing metallurgy
technology. At the same time, diverse operational
requirements-including cycling and
low-load and load-following operations-
have added complexity to the design of today's
combined cycle (CC) plants. Increased
final superheated steam volumes and temperatures
coupled with these diverse operational
modes are, in turn, challenging many other
vital plant components and systems, particularly
the steam attemperator system.
Attemperator Design Overview
An excellent attemperation system for a modern
CC plant requires a balance of design efficiency,
component flexibility, and system
reliability. Rapidly varying load conditions
place strenuous duty cycles on steam attemperation
components and downstream apparatus.
On average, the attemperator system
will experience 700 to 1,000 thermal cycles
per year of normal operation. The thermal
cycles can double in a cycling unit.
Most modern heat-recovery steam generation
(HRSG) superheated steam attemperator
component designs can be characterized as
either circumferential, probe, or a combination
of both technologies. As with many
complex engineering components, designs
evolve from functional requirements derived
from expected plant operations. Each of these
design categories has a unique set of requirements
that must be met to achieve expected
levels of plant performance and efficiency.
One of the more common superheater attemperator
designs used in the HRSG CC market
today is a circumferential spray design (Figure
1). The primary function of this design is to
inject water perpendicular to the steam flow
through multiple fixed or floating spray nozzles
via a penetration in the main steam pipe wall and
62
1. Circumferential in-line attemperator. In this design, water is injected perpendicular
to the pipe steam flow through spray nozzles to desuperheat steam. Source: Tyco Valves
& Controls
DCS
TIC
In-line
Steam flow
attemperator
2. Probe-style IU desuperheater. In this design, an integrated flow control valve is
inserted into a pipe through which water is injected into the flowing steam. A downstream
probe measures the downstream temperature and is used to control the water flow. Source:
Tyco Valves & Controls
DCS
TIC
Steam flow
the attemperator's inner spray liner or protective
shield located inside the pipe. The nozzles produce
mechanical atomization of the water droplets
into the superheated steam flow. This design
will often utilize external circumferential piping
to the main steam pipe for water supply to the
individual spray nozzles in conjunction with a
remote spraywater control station.
An alternative design for steam temperature
control integrates a probe unit within the
pipe. This design is divided into two major
www.powermag.com
categories: integrated units (IU) and separated
units (SU). Integrated probes incorporate
the spraywater control valve function within
the component (Figure 2). SUs offer a probestyle
spray for water atomization with a remote
spraywater control valve and external
water supply piping (Figure 3). The probe
application, whether of IU or SU design, employs
single or multiple spray probes into the
superheated steam flow, spraying water droplets
parallel with the steam flow.
POWER | November 2012
http://www.powermag.com
POWER November 2012

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