Level tank transmitter fluctuations

[mabbympb]

Hello,

We have a fuel oil tank that is approximately half full (300,000) litres.

This tank is only there for use as a standby, should our gas supply become interupted.

The tank is measureing the level using 3 rosemount level level transmitters. Looking at PI trends over a period of ten days (any ten days) the level measurements 'change' by a few thousand litres.

The trends show peak high levels at approximately 4pm each afternoon, and peak low levels at approximately 4am each morning.

can anyone help explain this please?

http://img266.imageshack.us/img266/8184/fueloillevel.jpg

 

[magoo2]

What is the approximate temperature at both times and what effect do you think it might have on the tank?

 

[mabbympb]

ambient temp in the day is about 18 degrees C while at night is about 12 degrees C

With the sheer volume of liquid contained within this tank i do not see how ambient temperatures can alter the level indications so quickly (12 hours each way)

It is a real strange one for me, as the tank has not used the fuel consistently for years, or routinely in the last year.

To me, the trends should be flat lined

 

[PHovnanian]

What is the sensing technology that these Rosemount level transducers use? Given the tank geometry, what sort of level change does a volume change of 1000 liters represent? Any idea how the fuel temperature changes during the day? What's the temperature coefficient of expansion of fuel oil? And how is the tank vented w.r.t the level sensor reference (if they are pressure transducer types)?

 

[mabbympb]

We are using 3 x rosemount 2088 level transmitters that use capilliaries from the base of the tank at the tapping point to the transmitters which are installed adjacent to these on the bund wall.
There is no temperature measurement measurement on this tank, but one must guess that the temperature would be somewhere around ambient.

(I cant get my head around this, as if you have 1 kg column of liquid, and this expands in temperature throughout the day, you will still only have that 1kg column of liquid, even if the actual volume varies.)

 

[jraef]

The Rosemount 2088 is a pressure transmitter, so you have something else interpreting level based on pressure. If that system has no temperature compensation, and/or an anaeroid bellows to account for external ambient pressure changes, you could very well see wide swings in volume. Could be that given your system's status of being a back-up supply, nobody thoughjt it important enough to spend the extra money to do it right. If it were a revenue based system, i.e. like the tanks in a Gas Station, you bet that would be there.


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[xnuke]

Do the capillaries seal to a diaphragm at the base of your tank and use an intermediate fluid to transmit the pressure to the 2088s, or are the capillaries connecting the liquid in the tank directly to the 2088s?

xnuke
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[Thealanator]

Is the top of the tank sealed or open to atmosphere? Does atmospheric pressure vary through the day? I think a differential pressure transmitter would help. High side would connect to the tank bottom and low side would connect to the empty space at the top of the tank (ullage space).

 

[NCTHAI]

What variation you have is becuase of change in temperature. The change in temperature changes the densiy and hence pressure being measured changes reflecting in change in level.

NC

 

[PHovnanian]

I cant get my head around this, as if you have 1 kg column of liquid, and this expands in temperature throughout the day, you will still only have that 1kg column of liquid, even if the actual volume varies.

Well, now that we know the transducers are the pressure type, we can eliminate some sources of this effect. Ultrasonic or float level sensors would be sensitive to volume changes, pressure (as you have surmised) would not.

But there are some variables that can affect pressure transducers. As xnuke hypothesized, a different working fluid in the sensing capillaries (or contamination in them by moisture or air bubbles) could introduce errors. Also, the configuration of the transducer's reference inputs and tank venting configurations could introduce some errors. In fact, the expansion of air (and resulting pressure) due to ambient temperature changes would be greater than that of the fuel oil. So I'd be looking at tank venting or bubbles in the tubing.

 

[BJC]

Are you sensor lines comming out of the bottom of the tank or the side. I can't read your chart but it appears the three sensors are different.
Check your sensor lines for sediments, check your isolation valves to see if they are all open, check ths installation to assure it follows good instrumentation practice.
Oil can grow things and oil tanks can rust even with oil in them.
Does the sun hit the sensing lines at the same time of day?
My guess you instruments aren't responding to tank leve but sensing pressure in the connecting tubing.
Spray water on the lines when the pressure peaks.