Chemical Engineering June 2010 - 38

PTFE
envelope
Feature Report
ing standards, the codes and standards
mentioned above provide generalized
requirements that touch on such key
aspects of safety as relative equipment
location, mass volume versus
risk, electrical classifications, valving,
and so on. They cannot, and they are
not intended to provide criteria and
safeguards for every conceivable situation.
Designing safety into a particular
piping system containing a hazardous
liquid goes beyond what should be expected
from an industry-wide code or
standard and falls to the responsibility
of the owner or EOR. As ASME
B31.3 states in its introduction, " The
designer is cautioned that the code is
not a design handbook; it does not do
away with the need for the designer or
for competent engineering judgment. "
When designing piping systems to
carry hazardous liquids, the design
basis of a project or an established
protocol for maintenance needs to
incorporate a
mitigation strategy
against two worse-case scenarios: (a)
A leak at a pipe joint containing a
hazardous liquid, and (b) The rupture
or loss of containment, during a fire,
of surrounding hazardous piping systems,
not otherwise compromised that
would add fuel to the fire.
The occurrence of those two failures,
one initiating the incident and
the other perpetuating and sustaining
the incident, can be minimized or
eliminated by creating a design basis
that provides the following:
* Added assurance against the potential
for joint failure
* Added assurance of containment
and control of a hazardous liquid
during a fire
* Safe evacuation of a hazardous liquid
from the operating unit under
distress
Fire prevention through design
Piping joints. When designing piping
systems to contain hazardous liquids,
one of the key objectives for the
design engineer should be taking the
necessary steps to minimize the threat
of a leak, steps beyond those typically
necessary in complying with the minimum
requirements of a code. There are
certainly other design issues that warrant
consideration, and they will be
touched on much later. However, while
Primary
sealing
element
Monel*
windings
Flexible
graphite filler
* Monel is a registered trademark of international Nickel
Carbon steel
outer ring
Figure 3. If flanged
joints are necessary, it is
suggested that fire-safe
spiral-wound type gaskets
with graphite filler
be specified
the pipe, valves, and instrumentation
all have to meet the usual criteria of
material compatibility, pressure, and
temperature requirements there are
added concerns and cautions that need
to be addressed.
Those concerns and cautions are
related to the added assurance that
hazardous liquids will stay contained
within their piping system during
normal operation and for a period of
time during a fire as expressed in such
standards as API-607, FM-7440, and
BS-6755-2. Designing a system, start
to finish, with the intent to minimize
or eliminate altogether the potential
for a hazardous chemical leak will
greatly help in reducing the risk of fire.
If there is no fuel source there is no
fire. In the design of a piping system,
leak prevention begins with an assessment
of the piping and valve joints.
There are specified minimum requirements
for component ratings,
examination, inspection, and testing
that are required for all fluid services.
Beyond that, there is no guidance
given for fire safety with regard to the
piping code other than a statement in
B31.3 Para. F323.1 in which it states,
in part: " The following are some general
considerations that should be
evaluated when selecting and applying
materials in piping: (a) the possibility
of exposure of the piping to fire and
the melting point, degradation temperature,
loss of strength at elevated
temperature, and combustibility of the
piping material under such exposure,
(b) the susceptibility to brittle failure
or failure from thermal shock of the
piping material when exposed to fire
or to fire-fighting measures, and possible
hazards from fragmentation of the
material in the event of failure, (c) the
ability of thermal insulation to protect
piping against failure under fire exposure
(for example, its stability, fire resistance,
and ability to remain in place
during a fire). "
The code does not go into specifics on
this matter. It is the engineer's respon38
ChemiCal engineering www.Che.Com June 2010
sibility to raise the compliance-level
requirements to a higher degree where
added safety is warranted and to define
the compliance criteria in doing so.
Joints in a piping system are its
weak points. All joints, except for the
full penetration buttweld, will de-rate
a piping system to a pre-determined
or calculated value based on the type
of joint. This applies to pipe longitudinal
weld seams, circumferential welds,
flange joints and valve joints such as
the body seal, stem packing, and bonnet
seal, as well as the valve seat.
For manufactured longitudinal weld
seams, refer to ASME B31.3 Table
A-1B for quality factors (E) of the
various types of welds used to manufacture
welded pipe. The quality factor
is a rating value, as a percentage, of
the strength value of the longitudinal
weld in welded pipe. It is used in wall
thickness calculations as in the following
equations for straight pipe under
internal pressure:
(1)
(2)
Where:
c = sum of mechanical allowances
D = outside dia. of pipe
d = inside dia. of pipe
E = quality factor from Table A-1A
and A-1B
P = internal design gage pressure
S = stress value for material from
Table A-1
t = pressure design thickness
W = weld-joint strength-reduction
factor
y = coefficient from Table 304.1.1
Also found in Para. 304 of B31.3 are
wall thickness equations for curved
and mitered pipe.
With regard to circumferential
welds, the designer is responsible
for assigning a weld-joint reduction
factor (W) for welds other than longitudinal
welds. What we can do, at
Profiled
inner ring
Secondary
sealing element
http://www.Che.Com
Chemical Engineering June 2010

Table of Contents for the Digital Edition of Chemical Engineering June 2010
