check valves and selection

1. I compare wafer type swing check valves.There are 2 types.One with spring loaded other is without spring.

Which one is better selction. Is the spring causes any pressure drop? .I need to know which one will be better.

2.Again there are 2 types one is unicheck(single disc) and other is double disk.

Some says double disc check valves are causing less pressure drop than others.Is it true? and how.Please give me some examples.

If I given to select out of these two which one shall I proceed ?.Though cost impact is there which application provides less pressure drop and what are its other applications.

3.Is there any disdvantages of wafer types over conventional swing chack valves?

4.Some applications says bronze mounted iron body check valves. wht does it means?

Give some websites for see the catalogue and pictures. HOW IT LOOKS?

5.What is non slam in check valves ?.what are the types available in non slam?.