What is a Spectacle blind and Spacer
Spectacle blind and Spacers are a pressure-retaining device used in the piping/pipeline industry to temporarily or permanently blind (shut) or isolate part of the piping or pipeline system.
- A spectacle blind is a forged product and combination of a flange spade and a ring spacer in one single product.
- One end of the spectacle blind will have an opening that allows fluid flow through the pipe during operation and the other end is made solid to block flow during isolation or maintenance time.
- One can simply rotate the spectacle blind by 180 degrees to close or open the fluid flow through the piping system.
- However, when there is space constraint, Flange spades and ring spacers can be used as separate products.
- As one of the two discs is always outside the pipe, one can easily understand if the blind is in an open or closed position.
As it looks like the numeric number Eight or a pair of eyeglasses, Spectacle blind (Fig. 1) is also known as a figure 8 blind or figure 8 spectacle blind.
Design Code and Standards for Spectacle Blind
ASME B16.48 is the governing code for spectacle blind design. This specification provides the dimensions, pressure-temperature ratings, materials, marking, dimensional tolerances, and testing requirements of the Spectacle blind. In general, Blinds are available in sizes NPS 1/2 – NPS 24 and these are installed between ASME B16.5 flanges in the 150, 300, 600, 900, 1500, and 2500 Pressure Classes. They are manufactured from steel plates and are available in forged material grades like ASTM A105 (high-temperature carbon), A350 (low-temperature carbon), and ASTM A182 grades (alloy, stainless steel, duplex).
Bigger size spectacle blinds with diameter >24 NPS are also available, but this is custom made. Spectacle blinds of Carbon steel Material are normally manufactured from a single piece of steel, whereas alloy / stainless steel blinds can be produced by welding more pieces together.
Definition of Terms used with Spectacle Blind
Ring Spacer or Spacer Ring or Paddle Blank or Paddle Spacer
Spacer Rings are made to match the pipe ID with the same thickness as the “single-blind” that it replaces. During the installation of spacers the flange and associated piping should be pulled together and a “the ring spacer” has to be installed to fill the gap. Normally, a handle is attached to the blank by welding.
Single/ Line Blind or Blank or Spacer Blind or Flange Spade or Paddle Blind
Single-blind is a positive shut-off device that is generally installed along with a valve so as to prevent accidental flow. In general, they fit inside the bolt circle of mating flanges. However, for Plastic, Fiberglass and Cast Iron, these are bolted. Single blinds use standard gaskets.
To differentiate between a spacer and blind in the installed line, the handle of the blank is made solid whereas a formed eye or drilled hole is made for the spacer.
A combination of a ring spacer and a single-blind is used as a spectacle blind. As required that are rotated to serve the purpose. When spectacle blinds become too heavy, a pair of paddle blank and paddle spacer is used instead of a single spectacle blind.
Test (Hydrotest/Pneumatic Test/Service Test) Blank
During leakage testing, test blanks are used. Test blanks are specifically designed blanks and used only for testing purposes for the test duration.
Similar to a “single-blind”, Vapor Blinds are positive sealing devices used for preventing vapor leakage into a pipeline or vessel. Generally, they are thinner comparative to single-blind and normally 1/8″ to 5/16″ (3-8mm) thick.
Jack Screw Flanges
For larger size blinds, Jack Screw Flange is required. Jackscrew help in spreading the flanges and replacing the spacer/blind or turning of spectacle blind.
Use of Spectacle Blind, Spades and Spacers
As Spectacle Blind, Spades and Spacers belong to the family of pipeline isolation devices, they are used to provide flexibility during operation and maintenance. Hence, it is required to consider this during the design of the Piping system. Blanks are normally installed in a horizontal line. On average, spectacle blinds are installed on the following
- At inlets and outlets of static and rotary equipment
- In bypass line
- At unit battery limit
- Where double isolation required for high-pressure lines.
Considerations in Piping Material Specification (PMS)
In the PMS following details need to be clearly specified:
- Nominal Pipe Diameter
- Pressure Rating
- Material of construction
- Dimensional Standard and
- Facing type.
Common Spectacle blinds are available in the following facing types
- Raised Face (for high-pressure applications)
- Flat face (for low-pressure applications)
- Male and Female Ring-Joint Blanks (used in conjunction with ring joint gaskets)
Few important Considerations
- Spectacle blinds must be accessible from grade, platform or if below 4500 mm by a portable ladder, or temporary scaffolding.
- The proper weight of Spectacle blind needs to be considered during stress analysis when near to the equipment nozzle.
Few more Resources for you…
“Pipe Coupling”-A short Introduction for the piping professionals
A Literature on Piping Nipples for piping and plumbing industry
Piping Elbows and Bends: A useful detailed literature for piping engineers
Reducers used in Piping Industry: A short literature
Tee Connection: A short literature for piping engineers
Difference between Stub-in and Stub-on Piping Connection