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I have a Boiler Feedwater tank(3825 m3) using FG fuel gas(natural gas) for blanketing set @ 0.86 kPag. Tank is design for 2 kPag with a PVRV set @ 1.5 kPag and a PRV set @ 2 kPag. Before the FG enters the tank there is tee to a VRU system. My immediate task is to determine the FG flow rate to the PCV tank blanketing pilot operated regulator i.e. either a Fisher or Anderson Greenwood. My 1st thought is figure out the displaced volume when the level changes inside the tank but at what rate and what criteria do a choose for minimum, normal & maximum flow?..wannabe Process Eng

[Art Montemayor]

Valene:

With 1,000,000 gallons of Boiler Feedwater (BFW) stored, you have a mighty large storage tank – and probably a mighty large boiler house!

Your blanketing natural gas (NG) flow rate is set by the negative displacement of the BFW in the tank. The faster the BFW is taken out, the faster you must make up the need for additional NG.

Therefore, it is incumbent on you to find out (usually this is done by putting on your safety boots and NoMex coveralls) and getting out in the tank farm to take down the empirical field data on inflows and outflows related to the tank. This is probably very taxing, boring, and tedious. But it has to be done unless you can obtain the correct and accurate data in another way.

You need this accurate data because it is the basis for the sizing of the NG feed valve’s orifice to the tank. Otherwise you run the risk of undersizing this critical valve and could directly cause the “implosion” of the tank due to excessive partial vacuum being created. I wouldn’t trust anyone’s word on what the flows are. And I would stress that you size the feed valve for the highest possible draw-out rate – and then put a contingency on that figure.

Thanks Art.

Since this is a grass roots project at Feed stage I only need preliminary data until we go into detail engineering. I know the inlet flow rate and I have the estimated pump design flow rate for the BFW recycle and the BFW pump discharge rate. I presumed my worst case will be when all the pump discharge are at maximum rate.

I forgot to mention my PVRV vacuum set pressure is -0.25 kPag. The Pressure Vacuum Relief Valve PVRV and Pressure Relief Valve PRV will both be size & supplied by the tank vendor but we will provide them the process data.

[pleckner]

I presumed my worst case will be when all the pump discharge are at maximum rate.

You answered your own question!

[deeras]

you can use API 2000 as a guide for atmospheric storage tanks venting/blanketing design. blanketing gas rate will be based on tank geometric volume, pumping in/out rates and type of blanketing gas used.