Can Somebody please explain me few things about Suction Pressure and How it varies from Pumps to Pumps (Does it really varies???). What is the exact suction pressure/value for centrifugal pumps

It is quite simple:

If your suction pressure is too low, your pump won't pump :(

If your suction pressure is too high your pump will explode :\

Seriously. I'd recommend reading Laura's guide and then Google to learn basic concepts like this since you really need them to be effective in any role supporting the sale of pumps.

Good luck

Thank you Laura, the link was very useful and gave new ideas and doubts. And Thanks to you to Mr.Simon,,,,

I agree with the above comments.

Here are a couple good generalizations about suction pressure.
-Remain within the recommended values as offered by the pump manufacturer
-be very careful with mechanical seals and in some cases bearings, as they are often the reason for the max suction pressure of a pump. casings can always be manufactured to meet a pressure rating. With some pumps such as ANSI or API different seals can be used. Simply changing the mechanical seal style can (in some cases) change the max suction pressure of a pump.
-Suction pressure can often be used to artificially increase the NPSHa of an application so that a pump operates properly where it may not otherwise. NPSH calculations are required first to determine if suction pressure is desireable.