Continuous oil/water interface monitoring--consensus technology 1

[LaSalle1940]

I'm looking for a consensus/recommendation for technology for monitoring the oil/water interface level in a storage tank (dimensions: ~140 ft diameter x ~50 ft high) that contains mostly water but has a relatively shallow (6" - 18") layer of oil floating on top. Naturally we're talking about a fixed device that provides continuous monitoring (as opposed to a testwell-type device that's used intermittently).

The operating conditions are not excessive (ambient outdoor temperatures in the northeast US; barometric pressure). However, this would be a refinery environment so that devices suitable for hazardous (explosion-proof) areas are appropriate.

My sense is that a two-pronge approach would be applicable: that is, a radar detector to monitor the overall level (and thus the position of the free surface of the oil layer) and a floating device to monitor the position of the water layer would thus yield all the necessary level data (overall level/oil layer top level; interface level; thickness of oil layer by difference).

 

[dpc]

An explosion-proof radar transmitter might be hard to find. You could use a float with a low density to float on top of the oil and another float of greater density to float on the water.

 

[Ashereng]

I don't know about consensus.

If all you need is the interface, then a single float will work.

If you also need the overall level, then a second float will be needed.



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[JLSeagull]

Both open path and guided wave radar are available for hazardous areas.

Open path could accommodate the total level looking at the oil level. Guided wave could measure the interface if the dielectric constants are suitable. For the guided wave radar to work on the interface the upper fluid cannot be conductive. That should normally be the case for hydrocarbons. The dielectric constant for the lower fluid (water) must be an order of magnitude higher than for the upper fluid (oil). If the water is ambient temperature the dielectric constant may be around 80. The oil is likely somewhere between 2 and 3. Thus, guided wave radar could fit the water level well.

 

[danw2]

I put a Siemens Mercap capacitive unit with a solid rod on short bypass (5' in height) aa an interface level transmitter for oil/water and it's worked wonderfully for 4-5 years now.

I was told the Mercap was replaced with an LC300 or LC500. At 50 ft height you'd have to use the flexible cable/rope rather than a rod.

Non-contact radar (for the top oil level) usually takes AC power, at a 50' shot, it usually isn't loop powered. But the market changes weekly. Maybe someone's got one now. Do you have power on the top of the tank?

Never used guided wave radar, so I don't know what its power requirements are.

My experience is that all these level instruments are available EXP or I/S for the loop powered devices.

Dan

 

[JLSeagull]

Both guided wave and open path are available 4-20 mA loop powered from some vendors. I have used each from Ohmart-Vega. K-Tek may be loop powered. Some manufacturers may require more ampacity than 4 mA but use 3-wire with +24 Vdc, signal return and +0 Vdc (power supply common, ground, earth, etc.)

 

[JimCasey]

This is a particularly challenging application because there are actually 2 interfaces to monitor: oil-water and air/oil.

I like the radar to monitor the upper liquid level. Ultrasonic would also be appropriate. Be careful about using a wave-guide tube because it is possible to get a different combination of water and oil inside the guide tube and to have a different level in the tube than in the tank.

The floating unit could be a capacitance probe. Since the dielectric constant of water and oil are so different, capacitance probes can see the interface. (Endress & Hauser, Drexelbrook )

 

[jgv]

I recomend you to use servo-operated level measurement, it can measures the interface with relative facility.
You can find this equipment at:

http://www.endress.com.

You need to consider when you have emulsions oil-water that would dificult the level measurements