Conservation Vent Sizing 1

[mp08]

Hi everyone.

I'm working on evaluating relocating some conservation vents for our equipment that are not in an easy to access area. The inlet piping to the CV would be extended with more a few more bends. I know there are rules around 3% inlet pressure drop for pressure relief valves, but I didn't know if there was something similar for conservation vents. Does anyone know of one or would it just play into backpressure calculation? The API standards have me a little bit confused on this.

Thanks

 

[don1980]

There isn't a pressure loss rule for weight loaded tank vent devices. The 3% Rule is aimed at preventing pop valves from chattering, and that's not a concern with modulating tank vent devices - they don't chatter. However, remotely locating a tank vent device is generally a bad idea because of the risk of exceeding the tank's pressure limits (internal pressure and vacuum). If the device isn't mounted directly on the tank nozzle, then you need to consider whether line losses could cause the tank to exceed these limits. On the pressure side, you can compensate for this line loss by decreasing the set pressure, and thus prevent the tank from exceeding its max limit. On the vacuum side, that becomes very difficult (and very risky) when the PVRV or VRV is mounted remotely. An API tank can be designed for up to 15 psi of internal pressure, but these tank can seldom tolerate more than a couple of oz/in2 vacuum. Because vac devices are set so low, you generally don't have the ability to adjust the set pressure to compensate for line loss. Even in the rare case that you can compensate by adjusting the vac set pressure, that's a risky thing to do because the margin for error is razor thin.