×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Level tank transmitter fluctuations

Level tank transmitter fluctuations

Level tank transmitter fluctuations

(OP)
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  

RE: Level tank transmitter fluctuations

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

RE: Level tank transmitter fluctuations

(OP)
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

RE: Level tank transmitter fluctuations

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)?

RE: Level tank transmitter fluctuations

(OP)
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.)

 

RE: Level tank transmitter fluctuations

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.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Level tank transmitter fluctuations

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
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Level tank transmitter fluctuations

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).

RE: Level tank transmitter fluctuations

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

RE: Level tank transmitter fluctuations

Quote (mabbympb):

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.

RE: Level tank transmitter fluctuations

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.  

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources