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Naphtha Level Measurement
Naphtha level measurement in Drain/Drip oil tank

We observe in Naphtha level measurement (using DP-Capillary type), there is a level fluctuation with a uniform interval in respect to time. Naphtha density variation with respect to temperature (Temp range- 20 to 50degC) is negligible.

Construction of tank:

- The dimension of this cylindrical tank - Length: 1.5 meter, diameter:1 meter.
- Level measurement range - 0 to 850mmwc
- Inlet pipe without isolation from plant drain line.
- Outlet pipe connected to the pump with manual isolation.
- The tank consists of a flare line with a manual isolation.

The level increases up to 200mmwc (in the night time) and comes back to the initial point (in the daytime). This is cyclic event.

Comparatively purpose:
A Local sight gauges indicates no variation in the level as explained above.

Kindly advice for elimination of this fluctuation.


Any chance this question is for the same location discussed earlier this year, in May?

This is a classic case of unequal heating of the DP capillaries and seal fill fluids inside the capillary. A filled capillary is a 'closed system'. Either Charles or Boyles law tells us that the pressure in a closed system is proportional to temperature. This a classic example of the capillary pressure changing with daily temperature that bears no relationship to the liquid level being measured.

The DP transmitter subtracts the low side pressure from the high side pressure for 'reported' DP. When the low side pressure (which is supposed to be only the tank's vapor pressure) in the capillary increases due to heating during the day, the reported DP decreases.

When the low side capillary pressure increases (due to hotter temperature), the transmitter's reported pressure differential/level will drive proportionally downwards.

When the low side is capillary pressure decreases (due to cooler temperature), the transmitter's reported pressure differential/level will drive proportionally upwards.

Insulating the capillaries does not eliminate the heat difference between capillary and atmosphere, it only creates a lag time in heat transfer.

The solution to eliminating the temperature effect is to change to a different level measuring technology. Rosemount's ERS level measurement system uses two separate pressure transmitters, which communicate with each other via HART and then calculate and report the DP. There are no capillaries involved, only wires.

And there's physical floats and either non-contact or contact (guided wave GWR) radar technologies.

How is the incoming flow regulated? And how is vessel level controlled: discharge valve or pump rate?