Level measurement
Level measurement
(OP)
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.





RE: Level measurement
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.
RE: Level measurement
RE: Level measurement
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.
RE: Level measurement
John
RE: Level measurement
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 ?
RE: Level measurement
A dielectric as low as 1.3 is advertised for their guided wave radar level transmitter.
RE: Level measurement
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.
RE: Level measurement
RE: Level measurement
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.
RE: Level measurement
RE: Level measurement
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
RE: Level measurement
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
RE: Level measurement
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.
RE: Level measurement
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.
RE: Level measurement
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.
RE: Level measurement
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