Floating Roof Level Measurement

[kudejewel]

Hi guys, this is my first posting.

I have this DP level application for crude oil in floating roof tank. The crude is highly viscous and waxy type with pour point @ 43degC. The tank are heated and insulated.

My problem is the DP level measurement is always off compared to the SAAB Tank Gauging which I believe is reliable and accurate.

I have calibrated the DP numerous times and still the measurement is always different and it drifts of after a while.

I have considered the actual SG and the level in the calculation and adjust the span accordingly but does not work.

Is there anything I am doing wrong?

 

[ilan7]

Think the DP transmitter will work on "real" liquid - not on a stiky crude. with that material you will not get the true hight of the liquid translated into pressure for the transmitter to read. Another problem is with the temperature, as diferent parts of the tank has different temperature - the specific gravity and the ability to flow is change with the temperature.

 

[NIMA2530]

Hello buddy
Since the liquid is viscos ,you need cappilary Tube or diaphragm seal,prevent it from chocking.When the tube are filled with crude it sticks and prevent accurate measurment.

http://www.gsiautomation.com/products_pressure.html

Go for diaghragm seal

 

[danw2]

If the suggestion for seals is for remote capillary seals, I would differ strongly. The measurement error due to temperature error in the capillaries would make ther the drift error now seen even worse.

Isn't the DP in use a flange mounted style used for level measurement? They've got good sized diaphragms.

You're probably reading head pressure accurately (unless you're spanning over too little a span and the span error increases with level). I suspect that the internal temperature variations are killing the conversion to level. With heated convection currents and resultant medium stratification the SG varies at different strata levels. The DP 'sees' the total head, but your single point temperature compensation doesn't account for the variations in temperature over time and those accumulated SG changes appear as 'drift'. It's the nature of DP.

There's gotta be a reason the oil storage & inventory people use multipoint temperature measurement - multipoint compensation?

 

[Mass44]

Hi there,

The SAAB radars are temp and pressure compensated that is why it will not give a exact same reading as your DP tx. If you are on a FPSO list and angle of the ship are taking into account as well in the compensation so you will not get the same reading with the DP transmitter.

To measure crude you need to use capillary type pad cell (chemical seal) DP transmitters since piped installations tend to block up after a while.
I assume you know how to work with capillaries but if you need some help with the calibration let me know and I will send you a procedure. The calibration is done different from a normal DP tx calibration and can be quite confusing to some people that is why I mentioned it.

 

[JLSeagull]

Storage tanks can also stratify with various density layers. This does not suit dp level measurement well.