che_december-2024 - 18

Facts At Your Fingertips
Steam Trap Operation
Department Editor: Scott Jenkins
S
team is commonly used across
the chemical process industries
(CPI) for a variety of end uses,
including process heating, mechanical
drive, moderation of chemical
reactions and fractionation of hydrocarbons.
Heat exchangers, turbines,
fractionating towers, stripping columns,
and reaction vessels are examples
of equipment types for which
steam is used. As the steam's energy
is transferred to the system (for example,
by the transfer of its latent heat
to a process fluid, or by transforming
its energy to mechanical work to
drive rotating machinery), the steam
condenses and is removed as a liquid
via a steam trap. This one-page reference
reviews the operation of steam
traps and outlines the different types
of steam traps available.
Steam trap function
Steam traps are automatic valves
that remove condensate (condensed
steam) and non-condensable gases
(such as air) without allowing steam
to escape. Steam leaks sap the efficiency
of the steam system. In general,
steam traps work by keeping
steam in the system's " steam loop "
while extracting air and condensate
(water) and redirecting it to the " condensate
loop. "
If steam traps do not function
properly, excess steam will flow
through the end-use device or the
condensate will back up into it. Excess
steam loss will lead to more
costly operation, while condensate
backup will reduce performance and
may lead to waterhammer [1].
Steam traps should be located at
the lowest natural and incidental low
points (low points created by closing
a valve) in the steam system. Steam
traps are often installed at the following
locations: Placed at 50- to 150-ft
intervals of straight pipe; after every
heat exchanger; and at every place
where there are changes in elevation
or pressure.
Steam trap purposes
Steam traps serve several important
functions in industrial operations, in18
cluding
the following:
Preventing steam waste.
Steam traps automatically
drain condensate from steam
systems while keeping live
steam from escaping.
Saving energy. Steam traps
help prevent steam leaks,
which can waste energy.
Removing non-condensing
gases. Steam traps remove air and
other non-condensing gases from
the system.
Shutterstock
Thermostatic air vent closed when
steam is present
Steam
Inlet
Float
Outlet
Condensate
Valve
FIGURE 1. This cutaway view shows the internal components
of a float-and-thermostatic steam trap
Protecting equipment. Steam traps
protect steam systems from damage
caused by prolonged exposure to
condensate, such as corrosion and
inconsistent heat delivery.
Ensuring efficient heating. Removing
air from the system ensures even
heating and delivers steam in as dry a
state as possible.
Steam trap types
Steam traps generally fall into one of
three main categories: mechanical,
thermodynamic and thermostatic.
These categories are described in
the International Organization for
Standardization (ISO; Geneva, Switzerland;
www.iso.org) standard ISO
6702: 1982 [2].
Thermostatic traps. Thermostatic
traps use temperature differential to
distinguish between condensate and
live steam. This differential is used to
open or close a valve. Under normal
operating conditions, the condensate
must cool below the steam temperature
before the valve will open. Common
types of thermostatic traps include
bellows and bimetallic traps [1].
Mechanical traps. Mechanical traps
sense the difference in density between
condensate and live steam
and use that to produce a change in
the position of a float or bucket. This
movement causes a valve to open
and close. Several mechanical trap
designs are based on this principle,
including ball float, float and lever, inverted
bucket, open bucket and float
and thermostatic traps (Figure 1).
Thermodynamic traps. Thermodynamic
traps operate according to
changes in fluid dynamics, using the
difference in kinetic energy (velocity)
between condensate and live steam
to operate a valve. The disc trap is the
most common thermodynamic trap,
but piston or impulse traps are sometimes
used.
Steam trap failure modes
When steam traps fail, it is usually in
one of two ways: by leaking live steam
or by failing to discharge condensate,
either partially or fully.
Hot-leaking failures. Steam traps
can develop leaks if dirt prevents the
valve from fully closing. This reduces
the efficiency and effectiveness of the
steam system.
Cold-blocked low-temperature
failures. If the valve fails to open,
condensate can back-up into steam
headers, resulting in wet steam and
waterhammer. In process heating
applications, a backup of condensate
can cause the heating temperature
to drop. This, in turn, can affect
product quality.
Condensate impurities. Often, a
significant portion of the steam condensate
is recovered and returned to
the boilers. Depending on the plant
processes the steam serves, the condensate
may contain impurities that
include acidic compounds, organic
compounds or mineral salts. These
impurities can cause corrosion and
failure of condensate-return piping
and equipment, and they can potentially
result in steam-generator corrosion
and fouling.
n
References
1. Department of Energy, Energy Efficiency & Renewable Energy
Office, " Improving Steam System Performance: A Sourcebook
for Industry, 2nd ed., " U.S. Department of Energy, 2012.
2. International Organization for Standardization, Automatic Steam
Traps, ISO 6704:1982, www.iso.org.
3. Risko, J.R., Why Bad Things Happen to Good Steam Equipment,
Chem. Eng., March 2015, pp. 50-58.
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
DECEMBER 2024
http://www.iso.org http://www.iso.org http://WWW.CHEMENGONLINE.COM
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