Is there any problem using dual plate check in wafer type design in sizes large sizes like 16" and 18" due to extra long stud bolts ?.Should one go in for lug type or double flange design?

The quick answer is NO! But of course you did not give any relevant information about the system where this valve will be used. Information such as: Commodity, Operating Pressure, Operating Temperature, Material, etc.

At this link (http://www.metraflex.com/double_disc.php) you will find "Double Disk" Wafer Check Valves up to and including 24" And if you snoop around the web site you will find all sorts of information about these valves such as they are actually made up to 30".

Now, if someone makes this valve and your operating conditions do not exceed the Design Criteria of the valve shown then you should be able to consider these valves acceptable.

It just takes a little homework.

Valve will be used in hydrogen,steam and naphtha service.With MOC as A216WCB and max rating of 600#.
read somewhere that during a fire, long, exposed bolts around the outside of a wafer-style component will heat up first, causing the bolt to loose strength so extra long bolts are avoided generally.

Your comment about the risk for Stud Bolts during fire is valid and should be a prime consideration.

You have now given us more information about the operating conditions. So you have some decisions to make. Before you make that decision you need to do your home work.

Alternates:
1. Threaded Lug type Check Valve (Two sets of 20 Cap Screws)
2. Double Flange Type Wafer Design Check Valve (Two sets of 20 standard Stud Bolts)
3. Wafer Type Check Valve with Fire Resistant Insulation (20 Long Stud Bolts*)

Have you checked the Price of each type of Check Valve type?
Have you considered the cost of the Bolting for each of the three types of valves?
Have you checked with the Client about the possibility of using Fire Proofing?

(*) - 16" Class 600 Wafer Type Check Valve requires 20-1 1/2"-20 1/2 " long Stud Bolts
- 18" Class 600 Wafer Type Check Valve requires 20-1 5/8"-24 1/2 " long Stud Bolts