Crude Oil Liquid Tank Measurement 1

[Duanester]

I am looking for a cost effective way to measure liquid level on a 2000bbl Tank. In the past I have had success using the DP Wellmark Series 685 EA with a 4 - 20 mA output for my PLC.
On my larger Tanks 50,000 bbls and above I have had success using Varec and Radar measurement.
The cost difference between the Wellmark and Radar is quite a bit.
My question is how accurate is the Wellmark DP if I have an internal floating foof( not vented to atmosphere).
Am I better off with a DP Transmitter that senses both the lower Psi and the upper, above floating roof psi to get an accurate differential?

Thanks
Duane

 

[danw2]

Yes. If the internal floating roof is unvented so that pressure or vacuum can build up in the space above the liquid, then that pressure/vacuum buildup will produce an error if the measurement is not 'differential'.

Differential measurement measures the head pressure at the bottom and the pressure above in the vapor area, subtracts the two (high side at the bottom minus low side at the top) and provides a liquid head pressure only. It requires tubing/pipe from the top to the low side of the transmitter (with a drop leg so condensed moisture doesn't buildup and exert its own pressure)

Tank level has been done for decades with industrial pressure transmitters (PT) that use a 24Vdc power supply for a 4-20mA output.

'cost effective' is relative and I don't know where industrial PT's line up with mechanicals. The industrials are almost bullet proof in that kind of service.