Spectacle Blind & Spade+Spacer

You wrote:
"When do we go for Spade & when do we use Spectacle Blind? As per me, line size drives the selection...hmm.....When the line size increases above 12" we prefer spades over spectacle blinds. Right? Can somebody shed more light on this matter"

Size is a contributing factor to the decision making process but Weight is the real issue. If the Figure "8" Spectacle Blind becomes too heavy for the maintenance team to handle then you start doing things to make it easier.

Where I worked we considered the following"
- The changing of all "Blinds" was handled by a two person team (think Safety).
- Two men could handle lifting and moving 100 lbs (45 kg).
- When the Figure "8" exceeded 100 lbs then we changed to the Paddle & Spacer.
- When the Paddle exceeded 100 lbs then we included some type of assistance for lifting and handling the weight. Remember, a smaller thicker paddle might be heavier than a larger thinner paddle.
- Lift assistance might be a small Davit, a Monorail or open overhead clearance for mobile crane.

abhijit,
I would like to revisit some of your original points.

-Spectacle Blind & Spade+Spacer both are used for positive isolation so as to ensure complete safety of the workers during maintenance.
Please note, in this case the block valve must be shut off and everything downstream muse be depressurized, drained and (depending on the commodity) steamed out. This also must be done in a safe manner.

-Positive isolation so as to avoid toxic release to atmosphere.
If a Blind is really required in a system that is Toxic then you would be better off installing a Double-Block(DBB)and-Bleed set. This would be two Line Class Valves, a Flanged Spool piece with two small valved connections and a Spec Blind between the two valves. With a DBB set like this you can shut off both Block valves, hook up a closed line to the two Bleed valves and evacuate the space between the two Block Valves of the Toxic material and steam it out to a Blow Down system or Flare. With this done now you can open the flanges (now in a safe zone) and spin the Spec Blind.

-Positive isolation might be used to divert fluid to elsewhere.
I am not sure what you are thinking of here. A Spec Blind is not a good choice for simple diverting flow. Possibly all you need is a valve on the branch and the main line.

-Positive isolation to quickly stop the fluid flow in case of emergencies.
In order to "spin a figure "8" Blind (or change a disk & Donut) you need to open the flanges. If this is a true emergency you do not want to be spending time removing a bunch of stud bolts to open a line to spin a Blind.

-Positive isolation for physical separation between systems when desired/required.
This is the same as the first item.

-Positive isolation of individual equipment or it can be the whole process unit as a whole.
This is also a repeat

-Both of the positive isolation methods are safest & most efficient when compared to other positive shut-off methods like Valve, Valve+Blind Flange.
I am not sure what you mean here. You only mentioned "Blinds" what is the other method?

-Ensures safety & avoids down time
Single isolation w/Blinds and DBB w/Blinds do provide an increased level of safety if used and used properly but I fail to see how they "avoid down time".

-Shape of a spectacle blind (Figure "8") resembles with spectacles, one side is having full blind plate and other side is having ring.
Right!

-Spade looks like a frying pan with a handle like bar for handling it.
Correct! And in almost all situations where you intend to use a "Spade" you must also have a "Spacer". The spacer has an open hole through the center but a ring that fills the area between the flanges where the "Spacer" is to be inserted.

-Spectacle blinds or Spades should have the same rating as of the piping.
Spectacle Blinds and Spade Blinds need to have the thickness calculated to meet the maximum operating (Pressure & Temperature) conditions of the line and the material must be compatible with the commodity.