Spectacle Blind

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Spectacle blind, abbreviated as SB, also called spec blind, figure-8 blind or a paddle blind, is a safety device used to isolate a section of line or piece of equipment when the line or equipment needs to be inspected or removed from service.  It is different than a valve in that the blind is a permanent or long term isolation device.

DSC06237Spec Blind - line openDSC05310Spec Blind - line closedA spectacle blind is machined from a single piece of metal that is cut to match the pipe size, fit between two pipe flanges and requires an additional gasket when it is installed.  Also, the bolts will need to be lengthened depending on what piping class and size blind is used.  The thickness of the spectacle blind is specified based on the line pressure and pipe size.  When the spectacle blind is placed between two flanges in a pipeline, it can be rotated to align the hole with the pipe, allowing flow to pass through.  When it's rotated 90 degrees, the solid part of the spectacle blind blocks the flow, effectively isolating the two sections of the pipe.

The primary purpose of a spectacle blind is to provide a safe and convenient way to isolate a section of a pipeline for maintenance, inspection, or repair.  It's important in situations where simply closing a valve or using a blind flange is not practical or safe.  The advantage of a spectacle blind is that it can be easily rotated to open or close the pipeline, making it a versatile and reliable solution for temporary isolation.

In addition to its isolation function, spectacle blinds are also used to prevent foreign materials from entering a pipeline during construction or maintenance activities.  They are typically made from materials that are compatible with the pipeline's contents, such as carbon steel, stainless steel, or other corrosion resistant alloys.



spectacle blind Design Classification

  • Ring Spacer  -  Ring spacers are bored to the matching pipe ID and are the same thickness as the "single blind" that it replaces.  When removing a "single blind", either the flange and associated piping must be pulled together to seal the line, or a "ring spacer" must be installed to fill the gap.  Thick single blinds or rigid piping systems normally require ring spacers.
  • Single/ Line Blind or Blank  -  A positive shut-off device normally installed adjacent to, or in conjunction with, a valve.  Their purpose is to prevent accidental flow through a pipeline to a vessel.  With the exception of cast iron, plastic, or fiberglass services, they are not drilled with bolt holes, but fit inside the bolt circle of mating flanges.  Pipeline blinds or blanks are not the same as bolting blind flanges.  Single blinds use standard gaskets.
    • A combination of a "single blind' and a "ring spacer" can be fabricated for field convenience as a single unit.  Weight consideration and the associated difficulty of handling heavy pieces in the field are a primary consideration in specifying a "spectacle blind" or a combination of blinds.  Spectacle blinds are meant to be rotated to change blind/spacer orientation.
  • Spectacle Blind  -  A spec blind is a combination of a ring spacer and single blind.  They are usually permanently installed in a piping system and rotated as needed.
  • Test Blank  -  A test blank is specially designed blank used for hydrostatic or other incompressible fluid testing purposes only.  Their advantage is cost and weight savings since higher allowable stress values (or lower safety factors) are used in their design.
  • Vapor Blind  -  Similar to a "single blind", but thinner, normally 1/8" (3mm) to 5/16" (8mm) thick.  These are positive sealing devices intended to prevent accidental flow or leakage of vapors into a pipeline or vessel, usually while the system is in service.  Vapor blinds are not to be subject to differential pressure.

spectacle blind specification

The specification that determines the dimensions of a spectacle blinds is ASME B16.48 - Line Blanks.  A line blank conforming to this standard will be marked in the following way:

  • B16
  • Nominal pipe size (NPS)
  • Nominal pressure class
  • Material
  • Manufacturer's name
  • If RTJ, the ring number


Spectacle Blind Advantages and Disadvantages

  • Spectacle blinds are primarily used for isolating sections of a pipeline.  They provide a positive shut-off, allowing for maintenance, repairs, or modifications to be performed on one section of the line without affecting the rest of the system.
  • Spectacle blinds are a reliable means of ensuring the safety of personnel and equipment during maintenance or repair activities.  By completely blocking the flow in a controlled manner, they help prevent unintended releases of fluids and ensure a secure working environment.
  • Spectacle blinds are versatile and can be used in a wide range of industries, including oil and gas, petrochemical, chemical processing, and power generation.  They are suitable for various types of fluids and can withstand high pressures and temperatures.
  • Compared to other types of line isolation devices, spectacle blinds are often considered cost effective solutions for temporary or permanent blocking of pipelines.  They are relatively simple in design and are reusable in many instances.
  • When a spectacle blind is installed to isolate a section of the pipeline, there is downtime associated with the installation process.  This downtime may be a disadvantage in situations where continuous operation is critical.
  • Spectacle blinds provide a complete blockage of the pipeline, and there is no option for partial opening or control of flow.  If partial flow control is required, other types of valves or control devices may be more suitable.
  • Improper installation of spectacle blinds, such as incorrect orientation or handling, can lead to operational issues or safety risks.  It is essential to follow proper installation procedures and guidelines.
  • The installation of spectacle blinds may require additional space in the pipeline compared to other types of isolation devices, and this can be a concern in tight or confined spaces.


Spectacle Blind Standards

  • ASME Standards

    • ASME B16.5 - Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24 Metric/Inch Standard
    • ASME B16.20 - Metallic Gaskets for Pipe Flanges: Ring-Joint, Spiral-Wound, and Jacketed
    • ASME B16.47 - Large Diameter Steel Flanges: NPS 26 Through NPS 60 Metric/Inch Standard
    • ASME B16.48 - Line Blanks


Spectacle Blind Datasheets

Class Spectacle Blind Raised or Flat Face Class Datasheets
 ANSI 300  Spectacle Blind - ANSI Class 300 (in)  ANSI 75 - 2500  Pipe Flange Datasheets
 ANSI 600  Spectacle Blind - ANSI Class 600 (in)    
 ANSI 900 Spectacle Blind - ANSI Class 900 (in)    
 ANSI 1500 Spectacle Blind - ANSI Class 1500 (in)    
 ANSI 2500 Spectacle Blind - ANSI Class 2500 (in)    


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