Chemical Engineering November 2013 - 40

Velan
Feature Report
DESIGN CONSIDERATIONS
With that said, we will take a look at
just a few of the variances required
by ASME B31.3 for piping systems
containing a fluid service that may be
considered lethal with human contact.
This portion of the article is intended
to provide examples of the types of
variances that are required for Category
M type fluid services beyond
what is stated in the base code.
Pipe fittings - M306.1
According to section M306.1.3 of the
ASME piping code, the following
equipment shall not be used for potentially
lethal fluids:
(a) fittings conforming to MSS SP-43
and MSS SP-119
(b) proprietary " Type C " lap-joint stubend
buttwelding fittings
Under M306.1.3(a), fittings manufactured
to MSS SP-43 (Wrought and Fabricated
Butt-Welding Fittings for Low
Pressure, Corrosion Resistant Applications)
do not qualify for use in Category
M Fluid Service due to the low-integrity
design of the fittings. Mentioned
also, under paragraph M306.1.3(a),
are fittings manufactured under MSS
SP-119 (Factory-Made Wrought Belled
End Socket-Welding Fittings; see Figure
4). These fittings also do not comply
due to the low integrity design of
the bell-and-socket joint.
With regard to section M307 on
Metallic Valves and Specialty Components,
Paragraph 307.1 applies,
subject to the requirements in Paragraph
M307.2. Paragraph M307.2
(Specific Requirements) states:
" Valves having threaded bonnet
joints (other than union joints) shall
not be used. " Threaded joints, in accordance
with paragraph 314, are to
be used for normal fluid service and
Category D fluid conditions only, but
can be used in severe cyclic-fluid service
conditions only when not subjected
to external moment loading,
such as with thermowells.
Section M307.2(b) states, " Only metallic
valves conforming to the following
requirements may be used:
(1) Special consideration shall be
given to valve design to prevent stem
leakage to the environment. " What is
implied by this is the suggested use
of a bellows-sealed valve to prevent
leakage of vapor or liquid
to the atmosphere
by way of the valvestem
gland packing.
" (2) Bonnet or coverplate
closures shall be:
flanged, secured by at
least four bolts, with
gasketing conforming
to paragraph 308.4; or
proprietary, attached
by bolts, lugs, or other
substantial means, and
having a gasket design
that increases gasket
compression as fluid
pressure increases; or
secured with a full penetration
weld made in
accordance with Paragraph M311; or
secured by a straight thread sufficient
for mechanical strength, a metal-tometal
seat and a seal weld made in
accordance with paragraph M311, all
acting in series. "
The italicized phrase above refers to
A/B Bonnet's tendency to move up and down
C System pressure
D Internal sealing forces against the spacer ring
E Bonnet take-up bolts
F Segmental thrust ring
G Outer row of body studs
H Gasket
FIGURE 5. The pressure-seal bonnet is designed to utilize
internal pressure to increase its sealing capability in
direct response to an increase in internal pressure
and miter welds, and of fabricated
lap- and branch-connection welds
comparable to those shown in figures
328.5.4E and 328.5.5 sketches
(d) and (e) shall be examined
* All connections are examined (instead
of 5% of them)
a pressure-seal bonnet design (Figure
5). Such a design, which will vary between
valve manufacturers, uses the
internal operating pressure of the fluid
as a sealing force; as the internal pressure
increases the sealing load on the
pressure-seal gasket increases. This
provides an increase in the sealing integrity
of the bonnet joints.
The above are just a few examples
of the added requirements stipulated
for Category M Fluid Service piping.
There are many other instances in
which the requirements for the construction
of potentially lethal piping
systems go beyond that of the base
code. And one more example of that
lies in the examination requirements.
Required examination - M341.4
For the degree of examination of equipment
required, Paragraph 341.4.1 applies
with the following exceptions:
(a) Visual examination: All fabrication
shall be examined (instead of 5%);
and all threaded, bolted and other mechanical
joints shall be examined (Instead
of random checks)
(b) Other examination
* The random radiography/ultrasonic
examination requirements of Paragraph
341.4.1(b)(1) apply, except that
at least 20% of circumferential butt
40 CHEMICAL ENGINEERING WWW.CHE.COM NOVEMBER 2013
* The in-process examination alternative
permitted in Paragraph
341.4.1(b)(1) may be specified on a
weld-for-weld basis in the engineering
design or by the inspector. It
shall be supplemented by appropriate
nondestructive examination.
This sheds light on the fact that these
types of piping systems demand a
much higher level of scrutiny when
being evaluated and are examined to
a much higher degree than required
for normal fluid services.
Designing for Category M
The need to integrate and instill personnel
safety and well-being into the
operation and maintenance of a CPI
facility is a basic essential. Such a core
philosophy extends from the conceptual
design of a facility to its ultimate
operation. But this, too, is applied in
degrees. The design and material specifications
for a chilled-water system
certainly would not apply to a system
containing a hazardous or potentially
lethal chemical. The added cost necessary
to attain the integrity required
for the more hazardous chemical service
would be money poorly spent on a
chilled-water system.
In designing a piping system for a
highly hazardous chemical, engineers
http://WWW.CHE.COM
Chemical Engineering November 2013

Table of Contents for the Digital Edition of Chemical Engineering November 2013
