How to determine the required blanketing gas rate

[123engr]

Dear All,

A produced water surge vessel is designed to collect treated produced water from the production platform before it is transported to the Water Injection platform via a dedicated PW pipeline pumps. Two pumps are designed to be operating simultaneously at 25,000 BWPD per pump (total outflow from vessel is 50,000 BWPD). Each taking its suction from the surge vessel.

Blanketing gas (fuel gas) is designed to maintain the pressure within vessel when it drop below 1 psig through PCV-820B. PCV-820C is expected to open and relief excessive pressure from the vessel when the pressure exceeds 3 psig. See attached drawing.

Please, how can I determine the required blanketing gas flow rate for the Surge Vessel? What criteria should I use?

Note:The gas line size are estimated pending detail result on gas flow rate required.

Thanks for your anticipated cooperation.

I am what I am by His grace

 

[Compositepro]

The worst case scenario is when the tank is almost empty, sitting in the hot sun, and then being hit with rain.

 

[Montemayor]

I do not believe that the worst case scenario is when the tank is almost empty, sitting in the hot sun, and then being hit with rain. That case is valid for a CONDENSING film heat transfer coefficient. This is not a condensing case inside the tank. When the tank is almost empty and has a volume of blanket fuel gas – no matter what the pressure – the coefficient of heat transfer due to natural internal convection currents is very, very low. A gas film coefficient – especially one that is essentially static – is for all practical purposes an INSULATOR because of its low value. All insulation materials work on this same principle of a static gas film coefficient to reduce the heat transfer.

The worst case scenario will be when the tank is being pumped out by both pumps running simultaneously with their performance rated at the end of their respective curves. Under these conditions, the rapid outflow of the liquid will try to create a partial vacuum if the incoming makeup blanket gas is insufficient to fill the volume void being created by the liquid outflow. However, if the tank is rated for full vacuum, it is not in jeopardy. Nevertheless, the pumps would start cavitating and lose suction. Sudden, reverse flow through the centrifugal pumps is possible unless discharge check valves are in place.

The attached P&ID does not identify the tank as being rated for full vacuum, so I assume that it isn’t. Therefore, the worst case is a threat to the tank’s mechanical integrity and the blanket fuel gas control valve should be sized for supplying the maximum flow rate of liquid displacement inside the tank – which is described above.