ASHRAE Journal - August 2023 - 8

LETTERS
Re: June 2023 issue, " Steam
System Pipe Sizing for Industrial
Facilities " by Kevin R. LaPlante, P.E.
I congratulate Mr. LaPlante on
his article on the lost art of steam
pipe sizing, an important part of
the overall steam system design.
I understand how difficult it is to
convey all the nuances of steam
system design in a short article.
I wish to point out to the readers
some additional issues to consider,
which may differ from this article.
Figure 1 may be oversimplified.
Readers need to be aware that
high pressure condensate to the
plant from high pressure steam
drip traps may be limited by the
length depending on the available
vertical space for pipe pitch
and/or the backpressure imposed
by the boiler plant, which may
include boiler plant receiver elevation.
One or more intermediate
condensate pump stations may
be required depending on these
conditions.
The trap discharge from low
pressure steam loads may sometimes
be subatmospheric depending
on the pressure drop of the
steam pressure reducing valve and
the associated heat exchanger.
The discharge of these types of low
pressure loads should have connections
to the dedicated low pressure
pump receiver or be as close
as possible to the receiver inlet.
I agree that the equations in Table
1 are very useful and could replace
the cumbersome charts in the 2021
ASHRAE Handbook-Fundamentals,
provided that one has viscosity
data for steam. Dynamic viscosity
data for saturated steam is listed
8
ASHRAE JOURNAL ashrae.o rg
in the 2021 ASHRAE Handbook-
Fundamentals, Chapter 30, R-718
table, or values can be searched
online. Online data will most
likely be dynamic viscosity units of
centipoise.
The designer needs to convert
dynamic viscosity units to
kinematic viscosity units of ft2/s
the total system pressure drops
values appear to be from Table
29 in the 2021 ASHRAE Handbook-
Fundamentals, Chapter 22. The
designer needs to be aware that
the maximum pressure drop per
100 ft will usually apply for smaller
piping, resulting in velocities
much lower than the maximum
values listed. For larger pipe, the
maximum velocities will apply to
pipe sizing.
The total system pressure drop of
20% to 30% will usually only apply
to systems with an equivalent
length longer than 600 ft (180 m).
For longer systems consider using
the average system pressure for
each major pipe section in lieu
of pressure at the boiler header
or at the pressure reducing valve
outlet when determining steam
properties.
The description of pipe sizing
(m2/s) by applying the appropriate
conversion factors to get lbm/
ft·s (kg/m·s) and then multiplying
by the specific volume in units of
ft3/lbm (m3/kg). The equations can
also adjust for superheated steam
properties, which are different
than saturated steam properties
used for ASHRAE's tables.
I agree with Table 2 values for
maximum velocity, which are
more conservative than the
8,000 fpm to 12,000 fpm, with a
maximum of 15,000 fpm values
in the 2021 ASHRAE Handbook-
Fundamentals, Chapter 22. I would
only recommend a 10,000 fpm to
15,000 fpm maximum for industrial
applications where noise is
not an issue and where first costs
are important.
The pressure drop per 100 ft and
A U G UST 2023
between the equipment outlet (or
the steam drip leg) and trap inlet
may be incomplete. The slope and
velocity limits are important but
not as important as the total head
of liquid condensate upstream of
the trap. The article's suggestion is
of a pressure loss of 1.0 ft of water.
This may result in flash steam and
potential damage of the trap due
to water hammer. Hydraulic pressures
at the trap inlet should be
above the flash point.
The pipe sizing between the trap
outlet and the common drain line
may be designed differently than
described in this article. Flash
steam is expanding in this section
due to the high pressure drop of
the steam trap, thus transitioning
from bubble to slug flow to eventually
stratified flow. A designer may
want to keep flow in the slug flow
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ASHRAE Journal - August 2023

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