External Floating Roof Tanks

No gap until liquid reaches low level. Then, yes gap.

Wiki is good.

Dear Lyn, Thanx for the comment could you please elaborate it a bit more.

External floating roof tank - Wikipedia, the free encyclopedia

Farhan,

You did not say what part of the world this tank is located. However even in the driest part of the world there is some chance of rain.

You do not normally want this rain to mix with your product. To take care of this problem, "Floating Roof" tanks are normally fitted with seals that prevent most of the water from entering the tank. The rain water is captured by the floating roof and drained via flexible lines inside the tank from gutters in the floating roof down to outlets near the base of the tank.

It is not perfect "no water" method but is intended to catch a very high percent of the rain water that hits the top of a tank.

it is located in Oman (middleast)..thing is the is offstream and subjected to inspection there we notice seal gap of around 4 in.

I do know that Anaerobic Digester tanks in wastewater treatment plants utilize a gap between the top if the liquid column and the bottom of the floating tank, so as to collect methane gas (resulting from the bacteria metabolizing the waste).

OP, are you describing an Anaerobic Digester used in a wastewater treatment plant?

no its a crude oil tank with 96 feet diameter having single deck pontoon external floating roof.

You shouldn't have any gap....fix it...

Sounds like your tank diameter is slightly larger at the bottom then the top. Since it is probably not feasible to modify the tank, modifying the lid is the way to go. Might try something like a flexible ring (rubber like flat material fastened at the edge of the lid) that lays flat, up the wall, then as the lid settles it flexes to take up the increasing gap toward the bottom. You should also have a valve at the bottom of the tank wall to drain accumulated water out of the tank. I worked on some tank farms some years back and they would drain the water from the bottoms of the tanks from time to time, even though they had covers. But minimizing water/dirt infiltration is always a good idea.

Gentlemen all the replys you made are appreciated, but still my question is in normal conditions is there any gap between Roof deck and product liquid, because as per

API 650 : APPENDIX C-EXTERNAL FLOATING ROOFS
C.3.3.5 The deck of single-deck pontoon floating roofs shall be designed to be in contact with the liquid during normal operation,

regardless of service. The design shall accommodate deflection of the deck caused by trapped vapor.

No gap. That is what "in contact with the liquid" means.

You have provided your own reference.

Are you OK with that?