Spectacle blinds are safety devices used to quarantine sections of a line or a section of equipment in the event that either may require inspection or need to be disconnected. Unlike a typical valve, a spectacle blind acts as a perpetual or long-standing isolation solution.
Manufactured from a piece of cut-to-size metal, a spectacle blind is fitted in-between 2 pipe flanges and needs an extra gasket when installed. In addition to this bolts will also have to be lengthened; this is subject to the class of pipe and the size of the blind. The depth of a spectacle blind depends on line pressure as well as the size of the pipe.
The measurements of a spectacle blind are determined by the specification ASME B16.48 – Line Blanks. One that conforms to this specification will be denoted by the following:
- NPC – Nominal Pressure Class
- NPS – Nominal Pipe Size
- Manufacturer Name
- Ring number (if RTJ)
At one side of the blind is an opening to permit movement through the pipe whilst in operation; the other side is completely solid to prevent flow throughout maintenance work.
Hydrogen Induced Cracking
Here at Brown McFarlane we use steel grade HIC (Hydrogen Induced Cracking) to manufacture spectacle blinds. HIC happens when carbon steel is introduced to hydrogen sulphide (H2S) and is a result of electrochemical corrosion reactions between the wet H2S environment and sour service. Hydrogen induced cracking resistant steel is created via the electric arc furnace with dephosphorisation, desulphurisation, vacuum degassing and ladle refining to deliver untainted and standardised steel. HIC steel is predominantly appropriate for pressure apparatus where wet H2S corrosion can be an issue.
HIC is particularly common in the oil and gas industry where the employment of pressure vessels, made of steel, is extensive. Risks should be taken into consideration when hydrogen sulphide partial pressure is 3.5mbar or higher. Generally speaking, a carbon steel pressure vessel is not influenced by HIC damage when H2S levels are below this figure in a typical facility life cycle. Hydrogen induced cracking is predominant in wet H2S environments where it is often referred to as sour service. Failure to inspect this cracking over a long period will result in a crucial point where a steel component (such as the pressure vessel) can easily fail. To put it simply, HIC damage heightens as the level of hydrogen sulphide rises.
Benefits of HIC Tested Steel Plate
Hydrogen induced cracking tested steel plate is the most practical and cost effective answer to resisting hydrogen induced cracking. Standard carbon steel plate is definitely more susceptible to hydrogen sulphide corrosion in sour service and wet H2S environments. HIC tested steel plate is also a lot more cost effective than using stainless steel.
Brown McFarlane’s HIC Plates
Here at Brown McFarlane, our stocks of HIC plate are compatible with the obligations of the world’s most challenging specifications. In harsh refining and manufacturing environments, Brown McFarlane’s products do not allow for any compromises in terms of safety.
HIC resistant steel plates are available as ASTM/ASME A/SA 516 grades 60, 65, and 70. All of our material is verified to EN10204 3.2 specifications. Plates from mill or stock production are delivered:
- Between 6mm – 105mm in thickness;
- Up to 4,000mm in width;
- Up to 14,000mm in length.
This steel has the most refined chemistry with the following properties:
- Carbon equivalency of 0.41% maximum;
- Up to and including 50mm thickness;
- Sulphur at 0.001% maximum;
- Phosphorus at 0.008% maximum.
Low sulphur levels means there is no need for calcium treatment or the addition of any unnecessary elements. The extremely low phosphorus levels also provide enhanced in-service resistance. The entirely killed, vacuum degassed, fine-grain practice production route (including segregation mitigation during casting and deep deoxidisation) guarantees that this steel is one of the cleanest available on the planet.