• On the order of 40 to 50% of the capital cost of the
equipment—extremely variable.
• A full set of PIP Process Control Practices documents
cost U.S. $6500 in 2002.
For process measurements to achieve the targets of safety, accuracy,
reliability, and economy, more than measuring equipment
is involved. The entire system—from the process fluid characteristics,
the ambient conditions, legal and regulatory requirements,
and operations/maintenance requirements—must be
coordinated to ensure that the equipment can be installed, calibrated,
operated, recalibrated, maintained, and, if necessary,
rebuilt or replaced while meeting the above primary criteria.
This section attempts to provide guidance to persons who
are unfamiliar with current industrial practice; it does not
attempt to cover all industries and all measurements. Specifically,
it cannot cover the multitude of legal and regulatory
requirements mandated by bodies such as the Occupational
Safety and Health Administration (OSHA).
INSTALLATION DOCUMENTATION
The primary installation document is commonly called the
instrument index
(see Figure 1.7a). This tabulates all the
tagged physical devices and commonly also includes tagged
software devices. Each of the physical devices is then referenced
to the associated installation drawings, such as the
physical location plans, installation details (mechanical support,
piping and wiring), cable ladder and conduit routing
diagrams, and the connection diagrams. The instrument index
is usually one of many documents from a large database,
which also keeps track of calculations, specifications, and
procurement documents and may also interface with a threedimensional
CAD model of the plant.
In a plant being designed with three-dimensional modeling,
many of the dimensional drawings that otherwise would
have been made previously are generated on demand by selection
from the model. This enhances the quality of the design
by flagging and eliminating clashes between equipment, piping
and electrical/instrumentation space requirements and permits
virtual walk-through reviews for operations and maintenance
personnel.

Physical vs. Schematic Documents
The physical or scalar documents are the location plans (often
sectional plans), cable/conduit routing plans, and the room layout
drawings. These are based on the mechanical or piping
layouts, commonly with the instrument information available
as an overlay. The instrument tapping locations will be defined
on the vessels and piping, and the final location for the various
instruments becomes a matter for negotiation between the various
groups to balance the requirements for operability with
accessibility for maintenance. Traditionally, the instrument
installation details have been essentially schematic, being used
largely for material take-off. But with the growing use of threedimensional
CAD techniques, there is a tendency to produce
approximately scale models for the common details to ensure
that access requirements are addressed. Connection diagrams
(electronic, electrical, pneumatic, hydraulic, and process) are
purely schematic. These are now largely automated, with a
minimal amount of input data being fed to a database loaded
with connection rules for the various types of equipment.
SAFETY IN DESIGN
The instrument connections to the process are commonly the
least mechanically secure components in the system. Consider
the relative strength of a 1/2NS (DN15) Sch. 160 pipe as used
by the piping designer to the usual 0.5-inch (12.7-mm) OD
seamless 316L tube with 0.049-inch (1.24-mm) wall used for
equivalent duty by the instrument designer. Yet this material has
in fact an adequate strength for most applications within the
range of Class 600 piping, provided that it is adequately protected
and supported. Supported not only when the equipment
is in service, but when any components are removed for maintenance.
Many installations can be found with long runs of tube
run to an absent transmitter, with the tube supported at best by
a rope or wire. Not only are long tubing runs a significant source
of measurement error, the lack of support is inherently hazardous.
Modern installation details will anchor the tubing runs by
supporting the instrument manifold, which remains in place if
the transmitter is removed, and minimize any hazard from the
temptation to use tubing runs as a hand (or foot) support.
The first valve off the process (known as the “root valve”)
has traditionally been the province of the piping designer.
More recently, the selection of this valve has become a joint

temperature any chance of a leak should be obviated. DBB
provides this by providing two isolation valves between the
technician and the process, with the space between vented to
a safe place. The definition of where DBB is required is
normally part of the operating company’s standards, but Class
600 (and higher) piping should always be covered by it. Toxic
materials call for more stringent techniques, with tubed vents
and designed-in decontamination methods.
Pipe and Tube Material
Current minimum design practice is to use a stainless steel
meeting both 316 and 316L for tubing and fittings for both
pneumatic and process connections. The pneumatic tubing
may be 0.25 inch (6.35 mm) or 0.375 inch (9.53 mm) OD,
while process connections are usually 0.375 or 0.5 inch (9.53
or 12.7 mm). The wall thickness of pneumatic tube is commonly
0.035 inch, while process tubing is a minimum of
0.048 inch, with heavier (0.064 inch) used for pressures
above about 1000 psi (6800 kPa). This is the heaviest wall
tube that can conveniently be bent and fitted off without using
hydraulic benders and setters.
Plants using metric standards may use either metric or
inch series tube but mixing the two in the same plant should
be avoided, as accidents can be caused by mismatching.
12 mm OD tube will fit in a half-inch compression fitting but
will rapidly disassemble itself under test. Always use seamless
drawn tube for compression fitting installations, as electric-
resistance-welded (ERW) tube has a small flat on the
outside that makes for difficulty in achieving a leaktight
connection.
316 stainless is a good general-purpose material, but it is
prone to chloride attack at temperatures above 140

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