Level measurement 3

[stratford]

Hi to all. i want to ask about the level measurement of dark colered oils and chemical solutions as NaOH, H2SO4, HNO3 and HCOOH. Reading form documentation and from some personal experience i would choose the radar level measurement. What i am trying to find is what are the reasons and the benefits and also which limits of technical data(viscosity, conductivity, density etc) determines if i should use radar level measurement instead of microwave or differential pressure. Thank you very much for your time.

 

[ilan7]

From my experiance, the Differential pressure works the best in almost all aplications. Only when the liquid density is changing during the process, it can not be used.
Lately we started using Radar transmitters, and still on the learning curve: we have a lot of problem with combination of Radar transmitters and VFD pumps.

 

[stratford]

Hi ilan7. The problem with the liquids is that the oils especially are of great density and very thick, so i am afraid of the differential pressure sensors. From the other hand the radar measurements have the restriction that the liquid or solid has to be of a dieelctric constant higher than 1.7 and these kind of oils have 2.1-2.2. That is a thing for me to solve.

 

[ilan7]

Stratford
Use the DP transmitter with 2 or 3" diaphragm and capillary tube, and all those problem will be eliminated. We had an aplication with havy crude oil and the only level that is working there is the DP with the remote diaphragm. We use SMAR, Yokagawa and Rosemount. (best bet is on SMAR). It just a bit tricky setting it up, and need to be done ONLY after installation.

 

[jsummerfield]

Consider several technologies. DP has been replacing displacer technology over several decades. Besides the need for constant density, the addition of diaphragm seals with capilaries adds a temperature problem. Two radar technologies are popular. Guided wave radar uses a probe and works with lower dielectric constant than open radar with cone/rod type antennas. Another technology is magneto-strictive. We often buy the magnetic level indicators integrated with a guided wave or magneto-strictive level transmitter instead of float/displacement switches. Watch the metals and soft parts for material compatibility with each technology. I have many vessels including a caustic tank application with a nitrogen blanket being installed now.

John

 

[stratford]

jsummerfield
Thanks for your reply. The problem with radar level measurement depends on the low dieelectric constant of the oils. It is approximatelly 2.1-2.5 and the radar sensor needs a minimum of 1.7, so as you understand we are at the margin. The guided wave is a good but extremely expensive sollution. The pulse and continuous wave makes me wonder. Can you tell me more or give me a link for the magneto-strictive level transmitter ?

 

[boldfish]

Try

http://www.ktekcorp.com

A dielectric as low as 1.3 is advertised for their guided wave radar level transmitter.

 

[controlnovice]

Stratford,

Usually, the radar level measurement state the minimum dielectirc constant. If the minimum is 1.7 for your instrument, a dielectric in your material of 2.1 should be good for the instrument.

 

[khan101]

I would suggest differential head method not with one d/p transmitter but with three seperate pressure transmitters. One at the bottom, one a little bit higher and one at top. The bottom and top pressure will determine the level in the colum and bottom and middle will determine the density in the column. The density then can be used for correcting the errors in level due to density changes.

 

[JimCasey]

Stratford:
If you're worried about viscosity of the product affecting the response of a DP cell, they make units with flush diaphragms that are exposed directly to the contents of the vessel....no possibility of viscous fluid causing slow response.

Khan's trick with three transmitters is self calibrating for density swings. In olden days we used to make a densitometer by taking a displacement level transmitter and keeping the displacer submerged all the time. As the density of the fluid changed, the output of the transmitter would then change proportionately.

 

[djs]

If there is no foam at the top of your liquid, perhaps ultrasonic level meter would work?

 

[Floatingproduction]

Stratford
I am totally agree with you as I never see any other sensor then Tank radar while measuring this kind of product level. since I am calibrating installing allot of different kind of level sensors on shore and off shore and over the tankers too cheers there is bubbler method can be used but you don’t' get accuracy due to a lot of leakage in the sounding pipe and other tubes which leads pneumatic signal to the Control Room Cheers

 

[itdepends]

Has anyone actually used three separate pressure transmitters for level measurement as suggested by Khan? I would be concerned about the need for accurate calibration and drift on the three separate instruments throwing your results out.

IME (please note I primarily work with fluids with a high dielectric)
DP cell- clean service only
Ultrasonics- no foam and low temperatures (particularly prone to interference from steam)
Radar- guided or otherwise- high steam and/or enclosed environments

 

[ilan7]

Itdepends
We used few application with three level transmitters, adding program at the PLC for:
1. Calculating the average
2. If there is more then 15% diferential - using only two
3. Issue a warning in case of one level transmitter reads different then the other two.
4. When only two transmitters online - taking the higher (or the lower - according to the application) reading for the HMI and the control.

 

[Ashereng]

Most of the oil I am currently dealing with is about API 30-40. Pretty light.

Most of the tanks use DP for level. Some older and mostly smaller ones use floats in bridles.

DP is fairly accurate, and except for density changes, accurate. For most of our application, level is to ensure we don't overfill or suck out. For that, DP suffices. The density range is not enough to affect us. For custody transfer, we use positive displacement meters.

For hi/lo level switches, we use floats.

A float in a bridle works well, if it doesn't gunk up. Floats in a bridle is also cheap. Add a magnetic pick up and you get a remote signal too! On most grades, it works. We do have some heavier oils, and dirtier oils, and in those cases, things gunk. The other problem with bridles is they get dirty and freeze - so you need to wipe them once in a while and heat them.

Radar is relatively newer. In new tanks, they work and we are seeing mostly radar on new tanks. In old tanks, with internal floating roof, they are hard to retrofit, so, back to DP.

Hope this helps.

 

[itdepends]

Ilan7,
Khan's post above suggests using three separate pressure transmitters for combined level/density (i.e. three transmitters=1 level instrument with density compensation). I get the impression from your post that you are talking about three seperate level transmitters for redundancy/safety considerations.

 

[danw2]

Correct. There are two different multiple transmitter schemes being discussed.

One involves the use of multiple level transmitters with a voting scheme where the level is chosen as the average of all, or 2 of 3, if one is way out from the others.

The other scheme involves the use of multiple pressure transmitters for level measurement in large storage tanks and is a conventional technique called Hydrostatic Tank Gauging. It is typically used for the mass storage of petrochem products.

For HTG, 2 transmitters are mounted at the lower part of the tank, but separated from each other in elevation. Each measures the liquid head pressure. The difference in head pressures can be used to calculate density, since the elevation difference between the two is a known constant.

A 3rd transmitter is mounted up at the top to measure vapor pressure.
A temperature transmitter is usually installed to provide a variable for density compensation due to temperature.

The pressures and temperature signal are processed by a control system to provide level, volume and even an indirect mass measurement.

API (American Petroleum Institute) addresses HTG in

Chapter 16.2 Mass Measurement of Liquid Hydrocarbons in Vertical Cylindrical Storage Tanks by Hydrostatic Tank Gauging. First Edition, November 1994

ISO addresses HTG in 11223

ISO Abstract:
ISO 11223:2004 gives guidance on the selection, installation, commissioning, maintenance, validation and calibration of hydrostatic tank-gauging (HTG) systems for the direct measurement of static mass in petroleum storage tanks. It is intended to cover custody transfer applications, although details of other, less accurate, measurements are included for information. It also gives guidance on calculations of standard volume from measured mass and independently measured reference density. Information is also included on measurements of observed and standard volume using density measured by the HTG system itself.

ISO 11223:2004 is applicable to hydrostatic tank-gauging systems which use pressure sensors with one port open to the atmosphere. It is applicable to the use of hydrostatic tank gauging on vertical, cylindrical, atmospheric storage tanks with either fixed or floating roofs.

HTG has been implemented since the early 1990's with the advent of "smart" transmitters that exhibit extremely low drift characteristics.

Dan