Main purpose and advantages:
Better sealing properties, more precise location and exact compression af sealing material, utilization of other, more suitable sealing and spezialized sealing material (O-rings)

Disadvantages:
Commercial availabillity and cost. Normal raised faced is far more common and ready available both regarding valves, flanges and sealing material.

Other arguments regarding mounting and machining could be added.

Typical common used is O-ring groove on one side for flanges (valves) for hydro-electrical power plants for higher pressures and longterm thightness against water.

Another complexity is that some rigid rules must be applied to the piping design .... Do you order valves to be female end both sides, or on one side maybe, in which case do you 'point all male ends in the flow direction' (pardon the expression)  ... or what. Same applies to any flanged joint / vessel conn. of course.

You could always consider an RTJ flange instead.

The main disadvantage is that these flanges are rare and someone might mistake it for a raised face.  That I am aware of, we have two tongue and groove flanges in our refinery.  They are in seal oil service to a large centrifugal compressor.  At an overhaul many years ago, a mechanic assumed they were raised faces and dropped in a standard spiral wound gasket.  The gasket was cut but sealed for a while.  After the unit was up and running, it started leaking.  We had to shut down an FCC at a cost of more than $1 million US.  I plan to remove these two flanges at the next opportunity.  I would suggest that you stick with standard raised face and RTJ flanges.  

Johnny Pellin

onrush:

In the past I've used tongue-and-grooved flanges for many years - primarily in ammonia refrigeration service.  The main attribute of using these type of flanges (although not mentioned yet) is that they inherently prevent a gasket "blow-out".  I have never come across, seen, or heard of any shortcomings regarding mounting and machining.  In fact, I've never had a leaky tongue-and-groove flange - and that is quite an achievement in ammonia service.

The obvious trade-offs include price and weight.  I always used Vogt's design (found in their old F-11 catalog) and I've applied the type up to 3,000 psig working pressure.  I always found them available and in stock - like other conventional raised face flanges when I bought them from Vogt.  The material of construction I've used is forged steel.  I would not hesitate using them if they are from a recognized supply - except that the price and the weight will be higher.  I would certainly prefer them over RF type in ammonia service - based on my experience.

Like ring joints and lap joints, I believe they have had their "niche" in certain applications.

To quote the Navco Piping Datalog:
"The small tongue and goove type of face was developed originally for hydraulic service to effect a high unit gasket pressure in a confined area.  It has the disadvantages of excessive gasket pressure under temperature change and pipe deflection, mushrooming of tongue under excessive bolt loading, and requires special attention in machining and assembly.  The large tongue and groove type face has the merit of gasket retention with lower unit gasket pressure than the small tongue and goove.  The gasket area is nearer the bolt circle making it slightly more resisitant to flange stresses and bending moments.  Special care must be exercised in machining and assembly."

My experience concurs with Navco's opinion.