Piping systems and circuits have been used for many years to organize effective piping inspection programs. Piping incidents have been a concern in the process industries for many years. Circuitizing piping improves inspection effectiveness. The process for developing circuits will be described. It is not a complicated process, but it requires skill and attention to detail. Circuitization points inspectors to vulnerable locations, promotes the proper inspection intensity, and provides input on the techniques best suited to find the anticipated damage. Common pitfalls are described.
Piping Deserves Attention
The mechanical integrity importance of piping cannot be emphasized strongly enough. In the 25th edition of Marsh’s “Large Property Damage Losses in the Hydrocarbon Industry,” 33 out of the 66 largest incidents outlined in the refining and petrochemical categories are directly attributed to piping failures.1
Several of the significant failures that are attributed to inspection-related causes are briefly described here:
- Monitoring potentially corrosive injection points (IPs) became vital in 1988 when the extrados of an 8-inch carbon steel elbow failed immediately downstream of an IP at a gulf coast refinery. The vapor cloud explosion cost an estimated $665MM in property damage as well as several lives.
- A similar incident four years later occurred at a refinery on the US west coast in which an explosion originating from a failed 6-inch carbon steel elbow was recorded as a sonic boom. The blast damaged buildings several miles away and caused a fire that burned for 3 days.
- A leak in 2000 in a condensate line at a Middle East refinery resulted in an explosion that damaged three crude units and destroyed two reformers. This failure, which resulted in $718MM in property damage, was attributed to a lack of inspection and maintenance of the line.
- In 1997 an explosion and subsequent fire that burned for 10 hours occurred at an olefins plant on the gulf coast when a 36-inch high-pressure light hydrocarbon line failed. The petroleum and chemical process industries continue to have more loss of containment incidents related to process piping than all other pressure equipment combined.2
The general industry approach to avoid a significant incident is to reduce the total number of incidents. For piping inspection-related incidents, this can be accomplished by implementing a robust inspection program that is described thoroughly in API 5703 with further specific guidance in API 5744. Piping circuitization is an important element of a
robust inspection program.Piping Systems and Circuits Demystified
API 570 defines a piping system as follows:
“An assembly of interconnected pipe that typically are subject to the same (or nearly the same) process fluid composition and/or design conditions.”
They are the first step in breaking down the miles of piping into more manageable blocks. Piping systems are usually identified by functionality. Systems are often broad such as
“reactor feed,” “absorber overhead,” “flare gas,” “rich amine,” or “furnace effluent.”
API 570 defines a piping circuit as follows:
“A subsection of piping systems that includes piping and components that are exposed to a process environment of similar corrosivity and expected damage mechanisms and is of similar design conditions and construction material where by the expected type and rate of damage can reasonably be expected to be the same.”
This categorization subdivides the system blocks into smaller buckets with the same chemical, mechanical, and corrosion characteristics. The elements of a systems and circuits project are as follows:
- System Drawings based on process flow diagrams (PFDs)
- System Legend with damage mechanisms (DMs)
- Damage mechanism review (DMR)
- Circuit Drawings based on piping & instrument drawings (P&IDs)
- Circuit Table that associates line numbers with circuits
For proper circuitization, a damage review is needed. The damage review can be documented at the PFD level in the system legend or in a more detailed fashion at the P&ID level in the circuit table. It is critical that the damage review be conducted by a corrosion SME to reap the benefits of a circuitization.
Other important terms are as follows:Corrosion monitoring location (CML):
CMLs are specific areas along the piping circuit where inspections are conducted. The nature of the CML varies according to its location in the piping system and the damage that is being investigated, such as local corrosion, corrosion under insulation (CUI), cracking, etc. Different inspection techniques are needed to find the specific damage.Thickness monitoring location (TML):
Discrete locations for measuring wall thickness. A subset of CML. Piping circuits are the building block for developing inspection isometric drawings, including the number of CMLs and CML placement. These activities will not be addressed in this article. What Piping Should be Circuitized?
A very important part of the work process is to establish which services and situations should be circuitized and which should not. Services and situations always circuitized are main process lines API 570 Class 1-3. Services that are sometimes circuitized include Class 4 services that are mission-critical to the process, higher pressure steam services, and utility-like Class 3 services, such as lube oil, hot oil systems, and tempered water systems like glycol water. Services that are generally not circuitized are Class 4 services like air and nitrogen.
Situations that are generally not circuitized are normally idle lines like start-up only, infrequently used regeneration lines. These situations might contain Class 1-3 fluids when
they flow, but they generally do not flow, and they are often ballasted dry. They can be service-tested for tightness before using them. Small bore piping that can be valved out
and tubing are also not circuitized.How Is it Done?
The services that will be included in the job scope are defined and delineated in the Systems agreed upon with the client. System drawings based on PFDs are colored to
show the system boundaries, and a system table is developed. The circuit break criteria are then defined by the Materials & Corrosion SME on the project considering the process. Potential circuit criteria are listed in API 570.
After establishing the circuit break criteria, work can begin to delineate circuits on the P&IDs. The circuits can be color coded by material or by system. The drawings are reviewed for mix points (MPs), IPs, and dead legs. The damage review can be documented at the PFD level in the systems table and drawings, or a more detailed damage review can be done in the circuit table. A cross reference between line numbers and circuits is provided.Benefits to Piping Inspection Efficiency and Spending
Circuitization simplifies inspection management and data analysis, improves damage detectability, and reduces costs. By combining lines into circuits, fewer components need to be managed in the inspection data management system. Managing inspections at the CML level would be a difficult task at best.
Larger plant sites may have tens of thousands of CMLs. If the circuits are properly selected, corrosion rates and pipe wall thicknesses should be similar for many CMLs in the circuit. This will lead to the same inspection frequency and ultimately similar repair or replacement schedules.
Damage detectability is enhanced. Where local corrosion is expected, more CMLs are typically defined and inspection techniques that are more effective than discrete TMLs are
used, such as RT or scanning UT. Costs are reduced in two ways. First, services and situations that do not require scheduled inspections are not circuitized. Second, the damage
review identifies circuits with general corrosion and lower corrosion rates. These circuits will require fewer CMLs. Likewise, if RBI is used, the reduced probability of damage may
reduce the inspection frequency. In the petrochemical industry where many dry non-corrosive streams exist, circuitization can provide significant savings via reduced inspections.Common Pitfalls
There is a lengthy list of pitfalls that can occur from incorrect circuitization process. A few of the more common ones are listed below:
Too Many Breaks:
- Too many circuit breaks
- Mixed metallurgy in same circuit
- Circuitizing and inspecting all piping
- Nonexistent or inadequate damage review
- MPs/IPs not identified
When breaks are made at every piece of equipment, this typically results in too many short circuits. Similarly, line number circuitization can result in too many circuits. Some of the inspection management benefits of circuitization are lost. On a recent job, 2500 line numbers were included in 320 circuits (excluding MP/IP and dead legs). On a smaller job, 250 line numbers were included in 60 circuits (excluding MP/IP).Mixed Metallurgy in the Same Circuit:
Metallurgy is always a circuit break criterion. It defines damage mechanisms and often influences the pipe schedule used. Nonexistent or Inadequate Damage Review:
A cornerstone of the circuit concept is to have the same damage mechanisms and understand the rates and morphology of damage. An inadequate damage review undermines the probability of detecting damage. It is still common to see circuits with only discrete TMLs. This is a sure sign of an inadequate damage review.Circuitizing and Inspecting All Piping:
Inspecting piping that is not included in the mechanical integrity program takes resources away from higher consequence piping. It also
costs more to create circuit drawings. Utility systems like low-pressure condensate can be defined as a single circuit. MPs/IPs Not Identified:
API 570 has requirements for MP/IP CML placement and the frequency of inspection based on API fluid class. An MP/IP list for each unit is considered recognized and generally accepted good engineering practice (RAGAGEP) by most operating companies.Summary
An effective piping inspection program is an important part of a mechanical integrity program. Establishing piping circuits will simplify inspection management and data analysis, improve damage detectability, and can reduce piping inspection costs. References
Submitted by ISAAC O’BRIEN & KENNETH KIRKHAM, P.E. – E2G | THE EQUITY ENGINEERING GROUP, INC.
- The 100 Largest Losses 1978-2017. (2018). Marsh Report, 25, 14–27.
- J. T. Reynolds, The 101 Essential Elements of a Pressure Equipment Integrity Management Program, Inspectioneering Journal, revised 2015.
- API 570 Piping Inspection Code: In-service Inspection, Rating, Repair, and Alteration of Piping Systems, 2017 Addendum.
- API 574 Inspection Practices for Piping System Components, 4th Edition November 2016