ASHRAE Journal - November 2014 - 42

TECHNICAL FEATURE

room is required per the rules of 7.4, at least one approved selfcontained breathing apparatus, suitable for the refrigerant used,
shall be located outside of, but close to, the machinery roam. A
second, backup, self-contained breathing apparatus shall also be
provided.-ASHRAE Standard 15-1994
The removal of Section 11.6 on SCBAs in its entirety
seemed to run counter to safety; however, the hazards
associated with untrained/unqualified personnel attempting to don this personal protective equipment in order
to make entry into a machinery room outweighed any
apparent benefit this equipment attempted to provide.
The other major change included in the 2001 edition of
Standard 15 related to the technical basis for sizing the
vent piping for safety relief devices. Equation 1 shows
the formula for the maximum equivalent length of a
discharge pipe connected to the outlet of a relief device
as it appeared in versions of Standard 15 prior to 2001.
Although simple, this equation included a number of
inherent assumptions or simplifications that really limited its applicable use in refrigeration system applications. As a result, the Standard 15 committee took action
to update the line length equation.

L=

9P2d 5
16 Cr2

(1)

where
Cr = rated discharge capacity as stamped on the device
by the manufacturer in pounds of air per minute
d = internal diameter of pipe in inches
L = length of discharge pipe in feet
The new line length formula shown in Equation 2
addressed many of the limitations associated with the
old formula but its comparative increase in complexity raised the eyebrows of professionals responsible for
engineering safety relief systems. One of the factors
that contributed to the initial distrust of the new line
length formula was the result of the calculated maximum allowable line length, L, when large relief valve
capacities were combined with small outlet piping. In
this case, the calculated length by Equation 2 turned out
to be a negative number which gave engineers pause as
to the believability of this "new" equation. As discussed
elsewhere,2 the result of calculated negative line lengths
does not stem from an error in the equation but, rather,
an artifact of the maximum line length equation not
being constrained for choked flow that will occur under
those circumstances. Rather than further modify a
somewhat already complicated equation, the Standard
42

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N OVEM BER 2014

PHOTO 1 The original B9 code covered both industrial and commercial refrigeration

systems.

15 committee issued an interpretation that made it clear
a transition to a larger pipe diameter on the outlet of the
relief device was permitted to enable reasonable equivalent lengths of vent piping to be attached (IC Standard
15-2001-1).
L=

(

0.2146 × d 5 × P02 − P22
f ×C

2
r

) − d × ln ( P ÷ P )
0

6× f

2

(2)

where
L = equivalent length of discharge piping, feet
Cr = rated capacity as stamped on the relief device in
lb/min or in SCFM multiplied by 0.0764, or as
calculated in 9.7.7 for a rupture member or fusible plug, or as adjusted for reduced capacity due
to piping as specified by the manufacturer of the
device, or as adjusted for reduced capacity due to
piping as estimated by an approved method
f = Moody friction factor in fully turbulent flow
d = inside diameter of pipe or tube, inches
ln = natural logarithm;
P2 = absolute pressure at outlet of discharge piping, psi;
P0 = allowed back pressure (absolute) at the outlet of
pressure relief device, psi
Since 2001, ASHRAE has faithfully published Standard
15 on a three-year interval with relatively minor changes
being made from edition to edition. The 2004 edition
was a republished version of the 2001 edition with minor
changes made to the requirements for the application of
flammable refrigerants as well as pressure vessel relief protection. Historically, ASHRAE Standard 15 would include
specific content from ASHRAE Standard 34 that related to
the safety classifications of each individual refrigerant. As
ASHRAE Journal - November 2014

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