Lizzette,
Are you trying to design a Vapor Recovery Unit abbreviated as "VRU" or for your storage tanks since you are saying that the aim is not to vent to atmosphere?
I am providing some general guidelines for design of such a system:
1. If it is practically feasible (no complicated loops or bends in piping) you direct the the vent pipes from each of the tanks to a common pipe manifold. The pipe manifold then gets routed to the vapor recovery unit or if you plan to destroy the vapors then to a flare or incinerator.
2. The individual vent connection / pipes from the individual tanks to be sized for the maximum filling rate of the tanks multiplied by the vapor growth factor (generally 1.2 to 1.3)
3. The common manifold pipe to be sized based on the maximum simultaneous filling of your 14 tanks. If we assume that 8 tanks are being filled simultaneously at identical filling rates then the design flow rate to the common manifold would be:
8*Max. filling rate of one tank*vapor growth factor
4. Now coming to the pressure drops in your system. There will be 15 pipe and piping components, inline equipment such as flame arresters and inline instrumentation. Let us say P1 is the pressure drop for your tank 1 vent line, P15 is the pressure drop in the common manifold, Pequip the pressure drop for the in-line equipment and Pinst the pressure drop for the in-line instruments. Then
P1 + P15 + Pequip + Pinst = 'Tank backpressure'
This calculated backpressure should be less than the design pressure of your tank for the selected vapor piping size. If it is not, increase the vapor line size and / or the manifold size and re-check. Keep on repeating this exercise till the calculated BP is less than the tank design pressure.
5..If the True Vapor Pressure (TVP) of your products is very low, then the possibility that the flow might not be adequate to operate the VRU or incinerator or flare. In such a case a blower or fan may be required to be installed at the inlet of the VRU / incinerator which will suck the vapors from the tanks subjecting the tanks and vapor piping to vacuum. You need to check whether your tanks are also designed for vacuum.
The aforementioned are some overall guidelines for design of VRUs. Some well know international companies in Oil & Gas / Refining / Petrochemicals have developed standards for Vapor Recovery units which include vapor recovery from tank farm installations. Try to find them out.
Hope this helps.
Regards,
Ankur.
Edited by ankur2061, 26 February 2014 - 07:53 AM.