1 reply to this topic

[ShaunHill]

This issue was raised at a hazop I attended recently.
The operating forman mentioned an incident where a frozen theif hatch was believed to be a factor in a tank explosion. Fortunately this occured at a remote unmanned facility so no-one was injured.

The tank contained oil/ water emulsion with vapours flowing to a vapour recovery unit (VRU) compressor.
The VRU had apparently shut down and the tank continued to fill. I believe that production should have stopped at that point, but that was not the operating philosophy since the VRU is considered non-essential for production.

Normally the theif hatch would open to relieve the excess pressure. However, it seems that the theif hatch had frozen shut. So, pressure built up in the tank to the point where the roof of the tank blew off and landed a fair distance away.

Fortunately, no-one was injured and the tank was the only equipment damaged.

What the operations group in that area proposed to do was to install a secondary venting device on any tanks that were not directly vented to atmosphere. The secondary device was just a clear plastic U-Tube that is filled with anti-freeze. The level in the tube approximates a 5 oz. relief pressure. It does require maintenance to fill the tube if it ever relieves, but this is prefered to a tank roof blowing off.

Has anyone else heard of issues of tank theif hatches freezing shut?
Any other methods of dealing with this problem?

I realize the southern US and other warm countries may not have this issue, but I live and work in Canada and the areas I deal with have freezing temperatures for 5 months out of the year.

I think there were other factors that lead to the freezing of the theif hatch. Generally the oil water emulsion flowing to the tank is quite warm (say 30-40C) so there could be a fair bit of moisture in the vapour which will condense on the cold roof of the tank (and the thief hatch), which then leads to the opportunity for freezing. I don't think this should be a problem if there is no moisture in the tank (ie pure liquid hydrocarbon storage). Also, we may not notice this problem in tanks that are normally vented to atmosphere simply because the normal venting does not require the theif hatch.

Shaun

[MIsh Rose]

Hawk-eye Industries located in Edmonton, AB has developed a thief hatch that is a non-freezing . The "Marsh Hawk Thief Hatch" details can be seen at www.hawk-eye.com. It was developed for the northern climates of Canada and problems operators were having with their sour service tank hatches freezing shut in the winter. Most hatches are made in the south not taking into account the colder temps up North.

It can also have some sweet service applications. Most tank farm operators leave their hatches open in the winter to prevent freezing, losing heat from the tanks in the process. These hatches will stay closed all winter and will save some tank heating costs in the process.


Mish Rose
mish@hawk-eye.com