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August 2000
Process Industry Practices
Process Control
Process Analyzer System Design Criteria
In an effort to minimize the cost of process industry facilities, this Practice has
been prepared from the technical requirements in the existing standards of major
industrial users, contractors, or standards organizations. By harmonizing these
technical requirements into a single set of Practices, administrative, application, and
engineering costs to both the purchaser and the manufacturer should be reduced. While
this Practice is expected to incorporate the majority of requirements of most users,
individual applications may involve requirements that will be appended to and take
precedence over this Practice. Determinations concerning fitness for purpose and
particular matters or application of the Practice to particular project or engineering
situations should not be made solely on information contained in these materials. The
use of trade names from time to time should not be viewed as an expression of
preference but rather recognized as normal usage in the trade. Other brands having the
same specifications are equally correct and may be substituted for those named. All
Practices or guidelines are intended to be consistent with applicable laws and
regulations including OSHA requirements. To the extent these Practices or guidelines
should conflict with OSHA or other applicable laws or regulations, such laws or
regulations must be followed. Consult an appropriate professional before applying or
acting on any material contained in or suggested by the Practice.
This Practice is subject to revision at any time by the responsible Function Team
and will be reviewed every 5 years. This Practice will be revised, reaffirmed, or
withdrawn. Information on whether this Practice has been revised may be found at
© Process Industry Practices (PIP), Construction Industry Institute, The
University of Texas at Austin, 3208 Red River Street, Suite 300, Austin,
Texas 78705. PIP member companies and subscribers may copy this Practice
for their internal use.
Not printed with State funds
August 2000
Process Industry Practices
Process Control
Process Analyzer System Design Criteria
Table of Contents
1. Introduction................................. 2
1.1 Purpose ............................................. 2
1.2 Scope................................................. 2
2. References .................................. 3
2.1 Process Industry Practices ................ 3
2.2 Industry Codes and Standards .......... 3
3. Definitions ................................... 3
4. Design Criteria ............................ 5
General .............................................. 5
Safety................................................. 5
Process Interface............................... 6
Sample Transport Systems ............... 7
Sample-Conditioning Systems........... 8
Calibration.......................................... 9
Analyzer Shelters............................... 9
Status and Validation Signals .......... 10
Documentation................................. 10
Process Industry Practices
Page 1 of 10
Process Analyzer System Design Criteria
August 2000
The purpose of this document is to convey conventional and widely accepted design
practices used in industry for the engineering, design, and assembly of Process Analyzer
Systems. Practitioners of this discipline are aware that each Process Analyzer System is a
unique and customized entity. This document is intended as a prompt to an experienced
analyzer systems engineer to ensure that a safe, reliable, and maintainable system is
designed and engineered.
The other PIP analyzer documents in this series deal with proven means to specify,
purchase, and install Process Analyzer Systems in an engineering and construction
environment. These documents collectively provide a framework for obtaining preassembled analyzer systems from third-party vendors who specialize in fabricating
complete Process Analyzer Systems. These specialists are frequently called Analyzer
System Vendors (ASV).
Process Analyzer Systems are engineered, designed, and fabricated for one of the
1) Process measurement or monitoring
2) Effluent measurement or monitoring
3) Ambient atmospheric measurement or monitoring
A properly functioning system requires integration of multiple design tasks:
1) Selection of analytical technology
2) Safety
3) Sample extraction
4) Sample transport
5) Sample conditioning
6) Sample return/disposal
7) Utilities support
8) Analyzer shelter
9) Signal conversions and control system interfaces
10) Status alarms
11) Calibration facilities
A Process Analyzer System is not a commodity but is a custom-designed system to meet
a specific requirement.
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Process Industry Practices
Process Analyzer System Design Criteria
August 2000
Applicable requirements in the latest edition, revision, or addendum of the following codes,
standards, and references shall be considered an integral part of this Practice. Requirements
modified by this Practice, owner’s referenced drawings, or a purchase order shall take
precedence over published specifications.
Process Industry Practices (PIP)
– PIP PCEPA001 - Process Analyzer System Engineering Guidelines
– PIP PCEPA002 - Process Analyzer Project Implementation Guidelines
– PIP PCIPA001 - Process Analyzer System Field Installation
– PIP PCSPA001 - Process Analyzer Project Documentation Data Sheet Instructions
– PIP PCSPA01D - Process Analyzer Project Documentation Data Sheet
– PIP PCSPA002 - Process Analyzer System Data Sheet Instructions
– PIP PCSPA02D - Process Analyzer System Data Sheet
– PIP PCSPA003 - Process Analyzer Shelter Data Sheet Instructions
– PIP PCSPA03D - Process Analyzer Shelter Data Sheet
– PIP PCSPA004 - Process Analyzer Bid Proposal Data Sheet Instructions
– PIP PCSPA04D - Process Analyzer Bid Proposal Data Sheet
– PIP PCTPA001 - Process Analyzer System Acceptance Testing
Industry Codes and Standards
• National Fire Protection Association (NFPA)
– NFPA 30 - Flammable and Combustible Liquids Code
– NFPA 70 - National Electrical Code (NEC)
– NFPA 496 - Standard for Purged and Pressurized Enclosures for Electrical
– NFPA 497 - Recommended Practice for the Classification of Flammable Liquids,
Gases, or Vapors and Hazardous (Classified) Locations for Electrical Installations
in Chemical Process Areas
Process Analyzer System: A system that consists of an analyzer, a sample-conditioning system,
and one or more readout devices. These are designed and assembled to automatically take a
representative portion of a process stream and to identify and measure specific component
concentrations or physical properties of the sample.
Analyzer Project: A project to design and install one or more analyzer systems, which may
involve analyzer enclosures or shelters. Analyzer projects have multiple task requirements and
will involve a team of engineering disciplines, each discipline having roles and responsibilities.
Process Industry Practices
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Process Analyzer System Design Criteria
August 2000
Analyzer System Fast Loop: A part of the sample transport system that is designed to bring the
process sample close to but not through the analyzer. The purpose is to reduce the time lag in
getting sample from the process tie point. The term “bypass sample line” is sometimes used to
define the same part of the system.
Analyzer Flow: The sample flow (take off from the fast loop) that is delivered to the analyzer for
specific component analysis or for physical property determination.
Calibration and Automatic Calibration: The introduction of a standard to the analyzer. The
measured results are compared with the standard, and the response factors of the analyzer are
adjusted so that the measured results match the standard. Automatic calibration is the same
activity with no human intervention and is normally on a fixed time cycle.
Validation and Automatic Validation: The introduction of a standard to the analyzer. The
measured results are compared with historical data and reported, and the response factor is not
changed. Automatic validation is the same activity with no human intervention and is normally
on a fixed time cycle. Other terms commonly used are “bench mark” and “check peak.” When
the historical data trend of a validation sample clearly exceeds the acceptable limit, the
instrument should be calibrated.
Statistical Quality Control (SQC): A collection of quantitative data that are subjected to analysis,
interpretation, and presentation. With an analyzer system, the data are typically the following:
Measurement precision or repeatability – presented in the form of a normal distribution
Calibration or validation response factors – presented as a control chart with upper and
lower confidence limits.
Service factor or performance – presented as a percentage of the time that the analyzer
system was available.
Original Equipment Manufacturer (OEM) Tests: The acceptance tests that are performed at the
Analyzer Manufacturer’s facility. These tests must have defined and measurable parameters with
repeatability factors or tolerance factors that are stated in the data sheets or in engineering notes.
Examples of these tests are baseline drift, repeatability, resolution, signal ranges, etc.
Factory Acceptance Tests (FAT): The tests that are performed at an Analyzer System Vendor’s
(ASV) facility after complete fabrication and assembly of the analyzer system(s). These tests
must have defined and measurable parameters that encompass the complete analyzer system
including any support peripherals such as analyzer shelters or buildings and all ancillary
equipment. Reviewing documentation and conducting physical inspection for compliance to
codes and standards plus adherence to the analyzer system engineering specification must be part
of this acceptance test.
Site Acceptance Test (SAT): The transfer of ownership of an analyzer system following a defined
period of uninterrupted, on-line operation within acceptable performance parameters.
These tests, including the documentation requirements and standards compliance, must be fully
defined at the bidding stage of an analyzer project.
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Process Industry Practices
Process Analyzer System Design Criteria
August 2000
Commissioning: A field demonstration of the operation of an analyzer system and all the
associated hardware, software, and peripherals. This activity will include demonstrating that all
signals are the correct range and that all alarm status generates the correct sequence of operations
and actions.
Startup: A demonstrated operation of the analyzer system(s) on actual process samples.
Design Criteria
Physical and chemical properties data for each monitored sample stream are required for
the system design. Using these data will permit a design considering each of the
following aspects:
Sample disposal
Safety relief vents
Analyzer vents
Sample-conditioning system design
Materials of construction
Maintenance procedures
Safety monitors
Safety equipment
Safety shall be the foremost design parameter during all stages of the analyzer selection
and system design. All applicable codes and practices shall be adhered to. Examples of
safety considerations for some commonly encountered materials are as follows:
For highly toxic process samples, the following shall be optimized in the design
of the Process Analyzer System:
• The location of the analyzer with regard to the process sample source point
• Minimum quantity of sample needed for a reliable, safe, and speedy
• Sample disposal or return point for proper disposal of spent samples
• Safety support equipment, such as breathing air stations, safety showers,
• Emergency escape routes from the analyzer location.
• Toxic gas monitors and alarm devices
Process Industry Practices
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Process Analyzer System Design Criteria
August 2000
For highly flammable process samples, the following shall be optimized in the
design of the Process Analyzer System:
• Sample stream constituents lower explosive limit (LEL)
• Chemical reagent components lower explosive limit (LEL)
• Sample system hardware components surface temperature (T-Rating)
• Need and location of combustible area monitoring detectors and the
calibration of the monitors with regard to the sample stream
• Sources of static electricity such as air conditioner blowers, etc.
• Safety issues with any other of the process analysis requirements
The sample transport, conditioning, and disposal shall not create a hazard to
personnel, the environment, the process, or the plant that is being monitored.
All hazardous materials shall be properly identified, labeled, or tagged.
Process Interface
Sample Probes When a sample must be extracted from the process, transported, and
conditioned, a sample probe shall be installed in lines 3 inches or greater
in diameter. The sample point shall be selected to provide easy access,
maintainability, fast response time, and a representative sample. Sample
points that could be multi-phase must be avoided. Fixed-flange probes are recommended for any operating pressures.
These probes shall have the following features:
An arrow stamped on the flange indicating the direction of flow
in the process line
Capable of being rodded out or blown back to process
Welded shutoff valve in compliance with line specifications
Sized for approximately 30% immersion beyond the inside pipe
wall up to a maximum immersion of 3 inches. A typical fixedflange sample probe is shown in Annex A of IEC 61831,
(PIP PCEPA001). Approximate calculations are shown in
subsequent annexes. For high-maintenance service, retractable sample probes are also
acceptable if the following additional safety features and characteristics
are included:
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Arrow stamped for flow orientation
Marked for insertion depth (scoring tubing reduces mechanical
Marked for retraction depth to clear the process root valve
Process Industry Practices
Process Analyzer System Design Criteria
August 2000
Expanded section on end of probe to prevent blowout through
packing gland
Safety chain for blowout prevention Special-purpose, high-pressure, retractable probe devices are
commercially available for pipeline or similar demanding service. In any case, fixed or retractable sample probes shall meet the process
pipe specification and must have the mechanical strength not to break in
high-velocity flow conditions. Harmonic calculations are available to
predict the mechanical breakpoint of the sample probe.
In-Line Sensing Elements In-line sensing elements shall be installed such that they can be removed
for replacement or maintenance while the process is in operation. Sensing elements and analyzers that are mounted in process slip streams
shall be designed to be blocked in, depressurized, and flushed while the
process is in operation. All “wet electrolyte” sensing elements must be installed so that the
sensing element is constantly wetted to ensure that the sensor does not
dry out. In-line sensing elements shall be installed in a manner to ensure that the
sensing element does not become fouled with debris.
Sample Transport Systems
The primary objective of fast loop design shall be to minimize the sample
transport time and the mass of sample transported. One or more of the following
techniques may be utilized to minimize sample transport time or sample mass:
• Pressure reduction
• Phase change
• Minimum-bore tubing and components
• Eductors
• Pumps
• Heating
The sample return point selected directly influences fast loop design. The sample
return point shall be selected to minimize sample conditioning and maintenance.
Sample Phase Phase shall not be changed when the sample phase matches the phase
required by the analyzer. Phase shall be changed when required for analyzer system operation.
Process Industry Practices
Page 7 of 10
Process Analyzer System Design Criteria
August 2000 When a phase change is designed into the sample transport system, the
phase change must be maintained once it has occurred. If this is not
achieved, the analysis results will be erratic.
The sample transport system materials of construction are a significant factor in
design. Sample transport system components are normally stainless steel. When
the process sample is not compatible with stainless steel, other materials (as
defined in the plant piping specification) shall be required.
Sample transport systems shall be designed and fabricated without low spots,
loops, or pockets that can form traps.
Sample-Conditioning Systems
Many analyzer system problems can be traced back to the sample-conditioning system.
The sample-conditioning system shall operate as follows: The sample must be representative, and the sample-conditioning system
shall not alter the concentration of the measured component. Response time must be consistent with the requirements of control or
monitoring. The system must be maintainable and must provide sufficient
information to aid in trouble shooting the analyzer system.
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The sample-conditioning system shall be kept as simple as possible yet meet the
design objective, which may include such features as auto-calibration and/or
All materials, including elastomers, metals, and composites, shall be verified for
compatibility with the process sample.
Isolation valves shall be provided on all sample systems, both at the sample
probe/sample return points and at the sample-conditioning system. Frequently, a
flush valve is included to run the fast loop with the sample system isolated.
Dead-volume components contributing to increased lag time shall be minimized.
On multi-stream sample systems, cross-contamination from valve leaks is always
a possibility. Double block and bleed, or equivalent, shall be utilized for
prevention of incorrect analysis caused by leaks.
Pressure or temperature sensitive components or devices used in the sampleconditioning system shall be protected from exposure to excessive pressure or
Heated sample lines shall enter the sample-conditioning enclosure via heat
shrink entry seals to ensure that the sample temperature is uniformly maintained.
Ordinary bulkhead unions can be used when sample temperature integrity is not
a concern.
Sample-conditioning enclosures shall have windows suitable for viewing
measuring devices (e.g., pressure, flow, etc.) if enclosure cannot be opened for
viewing due to safety or operating conditions. When high temperatures must be
maintained, all adjustments shall be externally accessible.
Process Industry Practices
Process Analyzer System Design Criteria
August 2000
4.5.10 Sample-conditioning enclosures shall close and seal sufficiently using, at a
minimum, a single-handle, three-point latch mechanism.
4.5.11 All sample-conditioning systems, whether enclosed or plate-mounted, shall be
provided with a means for decontamination before maintenance.
4.5.12 The location of sample-conditioning systems for samples containing toxic or
flammable materials shall be determined by the following: For an analyzer located inside an analyzer building, the sampleconditioning system shall be on an outside exterior wall with flowlimiting devices to and from the analyzer. For field-mounted analyzer enclosures, normal practice shall be to
mount the sample-conditioning system adjacent to the analyzer
enclosure. For analyzers mounted in three-sided, free-ventilation shelters, the
sample-conditioning system shall be mounted where most convenient
for maintenance.
4.5.13 Components of all sample-conditioning systems, whether enclosed or platemounted, shall be mounted in such a way that each can readily be removed for
maintenance without removing any others.
4.5.14 All components and controls of sample-conditioning systems, whether enclosed
or plate-mounted, shall be clearly tagged and labeled for ease of understanding
and maintenance.
All Process Analyzer Systems shall be provided with a means for calibration.
The design of a Process Analyzer System influences its overall precision. The
calibration standard used on the system will determine only the relative
accuracy. PIP PCSPA002 defines and specifies these terms.
Calibration samples shall be introduced to the analyzer in the same phase as the
conditioned process sample.
Analyzer Shelters
The selection of a shelter for a Process Analyzer System shall be based upon the
• Analyzer manufacturer’s published specifications for operating conditions
• Ambient site conditions
• Criticality of analysis
• Owner company philosophy
The four basic types of analyzer shelters are as follows:
• Type I – Field mounted: an analyzer in an integral enclosure mounted on a
free-standing support
Process Industry Practices
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Process Analyzer System Design Criteria
August 2000
• Type II – Freely ventilated: typically a three-sided shelter
• Type III – Climate controlled: analyzer enclosure interior temperature
controlled for proper operation of the analyzer system(s)
• Type IV – Controlled for area classification reduction: pressurized and
purged to allow operation of analyzer system(s) that are not rated for the
exterior area classification of the shelter
In Types III and IV shelters, doors shall open outward and shall be equipped
with panic hardware, automatic door closers, and windows.
Closed shelters shall also be equipped with appropriate sensors for combustible
gas, toxic gas, oxygen deficiency, and loss of pressurization as required.
Alarm conditions shall be shown outside each entry door as well as transmitted
to a continuously manned location.
Status and Validation Signals
Regardless of how simple or complex a Process Analyzer System is, valid
analytical information shall be conveyed to the end user.
Wherever a Process Analyzer System is on closed loop control and has a
fault/status signal available, this signal shall be transmitted along with the
measured parameter to the host company computer.
As the complexity of a Process Analyzer System increases, other signals may be
required (e.g., calibration, purge fail, no sample flow, etc.).
Once a Process Analyzer System is shipped to its final destination, complete and
proper documentation shall be supplied.
The final documentation shall contain the following at minimum:
• Originals of all OEM documentation for all parts of the system
• All ASV drawings and documentation
• Original specifications with all agreed-upon changes documented by the
• Start-up and shut-down procedures
• Calibration procedures
• Maintenance procedures
• All software documented in a manner to allow changes by the end-user
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PIP PCSPA001 provides a convenient means for specifying and tracking
Process Industry Practices