Why Gasket Bore Size Is the Most Overlooked Risk in Pipeline Isolation Systems

By C. HERRERA and I. SOKAIRAN, Saudi Aramco 

(P&GJ) —  In pipeline applications, insulating gasket kits are used for two primary cases. The first case involves preventing galvanic corrosion, when dissimilar metallic flange materials are in contact and the process fluid or external environment acts as an electrolyte. These materials are electrically connected not only through the bolting of the flanged joint but also via the gasket surface. In general, most pipeline gaskets are either metallic, spiral-wound gaskets or ring-joint gaskets, which create sufficient electrical conductivity between the gasket contact surfaces of the flanges. Galvanic corrosion depends on the difference in electrochemical potential between dissimilar metals—the greater the potential difference, the more severe the corrosion. 

The second case occurs in scenarios where insulating gasket kits are employed in pipeline applications, when it is necessary to isolate a cathodically protected (CP) pipeline. This occurs when, for example, a flange-joined CP pipeline section (e.g., a buried pipeline) connects to a non-cathodically protected (non-CP) section, such as an aboveground segment of the pipeline system. It also applies when CP current intended for a specific pipeline section might flow to other underground infrastructure or to equipment unintentionally connected to the pipeline. If these adjacent facilities are not intended for CP protection, significant current loss—commonly termed a "current drain"—can occur. This loss can be mitigated through electrical isolation of the pipeline, achieved by installing insulating gasket kits. 

To ensure effective electrical isolation in a flanged joint, an insulating gasket kit must interrupt conductivity through two critical means: 

1. Bolted fasteners: Non-conductive sleeves and washers prevent electrical continuity along the bolts, avoiding galvanic coupling between dissimilar metals in the flanges. 

2. Flange interface: The gasket itself must provide both a sealing barrier and electrical insulation, requiring careful material selection (e.g., non-conductive composites). 

While the fundamental design of non-metallic dielectric sleeves and washers has remained largely unchanged since its introduction in the 1980s, gasket technology has evolved significantly over the decades. Early gaskets were relatively simple in design and material, whereas modern iterations offer enhanced sealing performance, improved resistance to chemicals and high temperatures, low permeability to process fluids and, when required, fire-safe properties. In addition, they provide superior electrical insulation. These advancements enable more reliable and durable isolation across a broader range of operating conditions, enhancing the overall effectiveness and service life of flange isolation systems. 

Insulating gaskets currently available for pipeline applications typically feature a metallic core, often constructed from flat or serrated (grooved) stainless steel alloys 316 or higher. These cores are combined with non-metallic components, such as dielectric linings, sealing layers or coatings, to achieve electrical isolation and sealing performance. Design variations may include secondary sealing rings and/or high-temperature-resistant materials to meet fire-safe requirements, as specified in international standards like API SPEC 6FB. 

Failures in insulating gasket systems often stem from deviations in material specifications or improper installation, which can create unintended pathways for stray electrical currents. Additionally, these systems are highly sensitive to bolting torque specifications, resulting in two categories of torque-related failures: 

1. Excessive torque may crush non-metallic components (e.g., sleeves, washers or sealing layers), compromising their electrical insulation properties. 

2. Insufficient torque can result in inadequate compression, allowing conductive contact between flange faces and establishing unintended electrical pathways. 

This dual risk underscores the importance of adhering to manufacturer-recommended torque values and material standards, to maintain both mechanical integrity and electrical isolation. 

While these factors are well understood by practitioners, the most critically overlooked aspect of specifying pipeline insulating gasket kits is the gasket's internal diameter (ID). According to NACE SP 0286-Electrical Isolation of Cathodically Protected Pipelines, para. 7.2.3.1, gaskets may protrude into the bore of the pipe by 1.5 mm (60 mil), to prevent electrically conductive bridging over the isolation material. 

The issue arises when the pipe schedule and gasket ID are not specified in the gasket purchase order. In such cases, the gasket may be delivered with default dimensions, per spiral wound gasket (SWG) standards. SWG ID dimensions are inherently larger than standard pipe IDs, as they are not required to match the pipe bore, resulting in a gap of several millimeters between the pipe and gasket in some cases. This gap increases with thicker pipeline walls, leaving a larger portion of the flange face surfaces exposed. The exposed area creates a risk for electrically conductive bridging between these surfaces. This risk of galvanic corrosion may be further exacerbated by sludge, deposits and liquid accumulation in the gap, particularly at the bottom (6 o’clock position) of the pipeline. 

To avoid this often-overlooked issue, practitioners must clearly indicate the wall thickness of the pipeline in purchase orders to the gasket manufacturer, as well as state this detail explicitly—a nuance often overlooked in procurement and design (FIG. 1).

FIG. 1. Cross-section of a flanged joint showing ID mismatch between the gasket ID and the pipe ID (left) vs. optimal alignment (right). Note the gap at 6 o’clock, where conductive debris accumulates.

About the Authors 

CARLOS HERRERA SIERRALTA is a piping and pipelines engineering consultant with more than 30 yrs of international experience in the oil and gas sector. He currently serves in Saudi Aramco’s Consulting Services Department and as Vice Chairman of the Aramco Standards Piping Committee, providing expert guidance on piping and pipeline design, integrity and reliability. Herrera’s career includes senior roles with major oil and gas companies worldwide and in major onshore and offshore projects. He has published multiple technical peer reviewed papers, contributed to Pipeline & Gas Journal, and holds patents related to piping technologies. 

IBRAHEEM ALSOKAIRAN is a piping and pipeline specialist with more than 18 yrs of experience in the oil and gas industry. He serves as a consultant in the Consulting Services Department at Saudi Aramco, focusing on technical assessments and integrity evaluations that support the reliability and safety of critical piping and pipeline systems. Throughout his career, Alsokairan has co-authored several technical articles published on international platforms.