From BSL-1 to BSL-3: Choosing the Right Bolting for Your Application

In critical industries like oil & gas, petrochemicals, and energy, fasteners may be small components—but they carry massive responsibility. A bolt failure in a high-pressure flange or critical joint can lead to catastrophic results: system downtime, environmental damage, or even loss of life.

To prevent this, the American Petroleum Institute (API) introduced strict standards—API 20E (for alloy and carbon steel bolting) and API 20F (for corrosion-resistant bolting)—which define three levels of bolting specification: BSL-1, BSL-2, and BSL-3. Each level requires progressively stricter quality, testing, and documentation controls.

But how do you know which BSL level is right for your application?

This guide will walk you through the key differences between BSL levels and how to choose the right one for your project.

What Are BSL Levels?

BSL stands for Bolting Specification Level, which is a classification system used to define the quality requirements for bolting materials used in pressure-containing equipment.

These specification levels are defined in:

• API 20E: For carbon and alloy steel bolting
• API 20F: For corrosion-resistant bolting materials (e.g., stainless steel, Inconel)

Each level—BSL-1, BSL-2, and BSL-3—represents an increasing degree of testing rigor, traceability, and quality assurance.

Overview of BSL Levels

BSL Level	Intended Use	Traceability	Testing Requirements	Documentation
BSL-1	Non-critical applications	Basic heat traceability	Minimal NDE and mechanical testing	Material Test Reports (MTRs)
BSL-2	Moderately critical	Full lot traceability	Enhanced NDE, hardness, tensile tests	MTRs + Lot-specific CoC
BSL-3	Most critical (e.g., HPHT, sour service)	Full traceability to each part	Full NDE, mechanical testing, PMI, QA oversight	Full test data + traceability reports

BSL-1: For General, Non-Critical Use

When to Use BSL-1:
Use BSL-1 bolting for non-critical components where failure does not pose a significant risk to personnel, environment, or equipment. This includes:

• Utility piping and structural applications
• Low-pressure service
• Equipment not subject to fatigue, extreme pressure, or sour service

Key Features:

• Basic material documentation (MTRs)
• Manufacturer certification without third-party testing
• Limited or no nondestructive testing (NDT)
• Lower cost and shorter lead times

Best for: Standard maintenance, low-risk projects, and general-purpose fasteners.

BSL-2: For Moderately Critical Applications

When to Use BSL-2:
BSL-2 bolting is suitable for applications where higher reliability is required, but the risk profile doesn’t demand the most stringent controls. Typical use cases include:

• Moderate-pressure flanges and valves
• Non-sour service in upstream and downstream operations
• Structural components under dynamic loads

Key Features:

• Enhanced documentation with lot traceability
• Heat treatment control and verification
• Mechanical testing (tensile, hardness, impact testing if required)
• NDT such as magnetic particle or ultrasonic testing

Best for: Pressure-boundary bolting, pipe flanges, rotating equipment, and mid-tier refinery or offshore applications.

BSL-3: For High-Criticality and Severe Service Conditions

When to Use BSL-3:
BSL-3 is reserved for critical service environments, where failure could lead to severe consequences—either from high pressure, sour environments (H₂S exposure), or environmental hazard.

Common in:

• High-pressure, high-temperature (HPHT) systems
• Subsea installations and wellheads
• Sour service conditions (NACE MR0175/ISO 15156 environments)
• Safety-critical joints in blowout preventers (BOPs), flanged joints, and pressure vessels

Key Features:

• Full traceability to raw material heat and part level
• Independent third-party or in-house lab testing
• Positive Material Identification (PMI)
• 100% NDT on each part
• Detailed documentation package, including MTRs, CoCs, heat treatment charts, and inspection records

Best for: Mission-critical systems where bolt failure is unacceptable.

Key Factors in Choosing the Right BSL Level

When selecting the appropriate BSL level, consider the following:

1. Service Pressure and Temperature

• Higher pressures or temperatures increase the risk of bolt failure due to stress or creep.
• Use BSL-2 or BSL-3 for pressure-rated or heat-exposed equipment.

2. Corrosive or Sour Environments

• H₂S environments can cause sulfide stress cracking.
• BSL-3 is often required along with NACE-compliant materials.

3. Safety and Environmental Risk

• What’s the consequence of failure?
• For any safety- or environmentally critical application, err on the side of BSL-3.

4. Project or Operator Specifications

• Many oil majors have internal standards requiring minimum BSL levels.
• Always cross-check against client specs or project-specific documentation.

5. Budget and Lead Time

• BSL-1 bolts are generally faster to deliver and cheaper.
• BSL-2 and BSL-3 require more time for manufacturing and testing but offer significantly higher assurance.

NP Energy Tip: Ask These 3 Questions Before Ordering Bolting

1. What is the pressure rating and temperature of the application?
   (This determines material grade and testing level.)
2. Is the equipment in a sour service or offshore/subsea location?
   (If yes, BSL-3 is typically required.)
3. Does your end-user specify API 20E/20F and which BSL level?
   (Never assume—check the datasheets, P&IDs, or vendor specs.)

Why Trust NP Energy for API 20E/20F Bolting?

As a trusted API 20E/20F bolting distributor, NP Energy helps customers across upstream, midstream, and downstream sectors navigate bolting requirements with:

• Expert guidance on BSL level selection
• Full compliance with API 20E (BSL-1, 2, and 3) and API 20F
• Complete documentation: MTRs, CoCs, heat treatment charts, and NDT results
• In-stock inventory of B7, L7, B8, Inconel 718, and exotic alloys
• Rapid fulfillment with traceable, tested fasteners from approved manufacturers

Whether you’re building a pipeline, servicing a refinery, or planning an offshore installation, NP Energy ensures the bolting you choose is the bolting you can trust