Pls. give the difference between Isolation Valves & Block valves?

pls. tell me the differnce between isolation and block valves and also where it to be used .Pls. give the difference between Isolation Valves %26amp; Block valves?
*Terminology is differentiated on the basis of their application.





*Isolation Valves:





In some plants with fluid systems, some 2-way valves can be designated as normally shut or normally open during regular operation. Examples of normally shut valves are sampling valves, which are only opened while a sample is taken. Examples of normally open valves are isolation valves, which are usually only shut when there is a problem with a unit or a section of a fluid system such as a leak. Then, isolation valve(s) are shut in order to isolate the problem from the rest of the system.





*Block valve:





A valve that can block the flow of oil in a pipeline in both directions. Block valves include manual gate valves and remote gate valves.





Generally it is differing in the sense that block valve used in emergency but it is same as the isolation valve.





There are some line block valve and some combine systems like ';Block and Bleed valve';.





http://en.wikipedia.org/wiki/Block_and_b鈥?/a>





http://www.eng-tips.com/viewthread.cfm?q鈥?/a>Pls. give the difference between Isolation Valves %26amp; Block valves?
An isolation valve allows you to open one end of it, to allow material to flow, without coming in contact with another material, that is using the same plumbing.


http://search.globalspec.com/ProductFind鈥?/a>


A Block valve simply shuts off the supply.


http://www.plasticvalves.com/safety_bloc鈥?/a>


There's numerous systems that use isolation valves. Let's say you want to make some ice cream, you open the isolation valve to let the milk flow into the mixer, then open the valve to let in the cream. Depending on the valve type, you could then open it to allow in the vanilla, sugar, egg, etc. All with the same valve. It just keeps the ingredients from coming in contact . A block valve is used to shut off a supply, so you can perform maintenance, or just stop the flow.
The two terms are used interchangeably. They are valves that are used to completely interrupt flow as opposed to a valve that is used to throttle or control flow without stopping it.