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Ultrasonic Level Detector Selection

01/25/2018 7:32 AM

I couldn't see how to create my own blog on CR4, so I am posting this helpful information in this relevant discussion group. hope you all like and share.

ULTRASONIC LEVEL DETECTORS

Consider these experiences when selecting ultrasonic level detectors...

Overview: Most visiting this forum already know what an ultrasonic level detector is, but for those who don't...

These instruments use high frequency sound bounced off a surface to determine distance. They are often used as level detectors in tanks and silos. Long-lived, useful service depends on configuration of the tank roof and internals, material of construction compatibility with process chemicals, position on the roof, stillness and orientation of the surface being measured.

Ultrasonic level detectors (ULD’s) work on the principle of measuring the time delay between emitted high frequency sound and its reflection from a surface. In many applications these devices have been used successfully as level detectors. However in the situations discussed below they did not give satisfactory long-term service.

POWDERED PRODUCT SILOS

One application that proved difficult for the model of ULD selected was in measuring the level of powdered product in vertical silos. It appeared that the dust created during filling caused interference with the reflections and resulted in false level readings. The dust also coated the surface of the detector, thereby causing intermittent spurious readings during the normal operating mode.

A further complication that occurred in the silo was the inversion of the product surface shape. This situation arose because the product experienced core flow as it was removed from the bottom of the silo. Figure No. 1 highlights the change in surface geometry from conical pyramid to inverted cone as the product was removed from the bottom of the silo.

FLAT ROOF TANKS

Fictitious levels have been experienced when ULD’s were installed in the roof of a flat roof, liquid storage tank. The same model sensor, when used in the roof of a conical roof liquid storage tank, produced reliable results. Figure No. 2 shows a method that proved successful in overcoming the spurious results. It was to mount the sensor at the apex of a fabricated cone sitting over the tank manway entrance.

When the same cone mounted sensor idea was used on a flat top reactor vessel the results were mixed. The major difference between applications was that the liquid surface in the storage tank was still, while in the reactor the liquid surface was undulating and turbulent with a great deal of vapor generation. This environment did not appear suited to the use of ULD’s. Putting the detector at the top of a dip pipe may have improved the situation by removing the turbulence and reducing some of the generated vapor.

MATERIAL OF CONSTRUCTION COMPATIBILITY

One consideration that affects the selection of instrumentation in direct contact with process chemicals is the compatibility of the materials of construction with the product being monitored.

The use of one manufacturer’s ULD to measure the liquid level in storage tanks for 30% hydrochloric acid (HCL) eventually resulted in the detector’s plastic housing dissolving from acid attack. The plastic used for the housing was not suited for long term application in HCL vapors.

PROCESS TEMPERATURES and CHEMICALS

The ULD in the flat top reactor previously mentioned had an operating temperature limitation of 70o C. The process involved mixing chemicals with HCL during which the high temperature generated from the reaction was to be cooled to 50o C through a heat exchanger. The manufacturer’s specifications were apparently satisfied. However the local temperature at the surface of the liquid in the reactor was higher than the design temperature of the bulk liquid and it was to this temperature which the ULD was exposed. Not long into service the ULD was removed due to chemical attack from the vapors aggravated by the high temperature in the reactor.

A better selection would have been made had consideration been given to the local environment in which the equipment and its housing were located and not the expected average conditions.

VACUUM SERVICE

Since ULD’s require sound to be transmitted and reflected they will only operate in environments which contain sound transmitting atmosphere. As such they will not be reliable in a process conducted under vacuum conditions.

INTERNAL STRUCTURES

The pulse emitted from a ULD will be reflected back by any object in its path. If there are internal structures within a tank below the ULD they will reflect the pulse and produce a false signal. Relocate the ULD to a place where there is clear space below it.

CALIBRATIONS

It is necessary to check and calibrate the high and low levels and the span when the ULD is in its final position. With tanks this is done by filling them with water and physically dipping and measuring the levels and comparing results. In situations where water cannot be used it may be possible to use the product and dip it, or take the ULD to a ‘dummy’ test set-up on another item of equipment where water can be used and then replace it back in its required position.

To download and share with others, a PDF copy of this article, see Ultrasonic Level Detectors PDF

Hope some of you gained some additional insight.

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#1

Re: ULTRASONIC LEVEL DETECTOR SELECTION

01/25/2018 7:40 AM

In addition, a couple of notes from personal experience:

  • Be wary of situations where there is a gas other than air in the space above the measurable substance: nitrogen blanketing over a flammable is one commonly-found possibility.
  • Be wary of situations where the gas above the measurable substance is at other than atmospheric pressure: an 0.3barg over-pressure to exclude moisture, for example.

In these circumstances, ultrasonic can be used with appropriate care, though the fall-back measuring technique is radar.

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#2
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Re: ULTRASONIC LEVEL DETECTOR SELECTION

01/25/2018 10:24 AM

Thanks, great addition.

Cheers

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#3
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Re: ULTRASONIC LEVEL DETECTOR SELECTION

01/25/2018 3:48 PM

Also if the air/vapor in the headspace contains corrosive agents, as it will in sulfuric acid storage tanks.

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Re: ULTRASONIC LEVEL DETECTOR SELECTION

01/26/2018 5:54 AM

Good case for radar, that one.

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Re: ULTRASONIC LEVEL DETECTOR SELECTION

01/26/2018 7:01 AM

Radar Level Detectors

Radar is a popular technique for tank level detection of liquids. The radar sends a sub-millionth of a second pulse from an emitter and the return signal is detected at a receiver. The reflection’s time lapse is measured and electronically converted to a distance from the liquid surface.

FIGURE 1 Rod and horn style radar detectors

REF: https://www.scribd.com/user/390881492/Poppy-Fairley

Usually the radar is mounted in a flanged nozzle or socket welded to the roof. It can also be mounted in a pressure balanced standpipe in those tanks with undulating surfaces. The radar can be mounted directly to the roof provided the liquid full height is below the bottom of the antenna. If the antenna gets below the liquid surface false indication results.

Care must be taken to position the radar to prevent unwanted reflections off tank internal structural beams, wall welds, rivets, etc. Spurious reflections can be electronically separated from the true level reflection but this introduces programming complications better avoided if possible.

Keep the receiver below the bottom of the nozzle so that it is in clear space. Pulses can bounce off the nozzle walls to the receiver and produce false signals.

Foam on the liquid surface deadens the return pulse.

If located inside vigorously agitated tanks the risk exists that the opening of the antenna cone can get blocked by splashed product. This is a problem with products that sublime (evaporate then solidify on surfaces) like sulphur or that crystallise.

Check material compatibles and chemical resistance. Especially over many years so that you don’t have reoccurring operating problems.

REF: Feed Forward Publications - In house maintenance training program.

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#13
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Re: ULTRASONIC LEVEL DETECTOR SELECTION

07/25/2018 3:01 AM

We are using Radar Transmitter Sitran LR560 for measuring level in Powder Silos Tank.

Following picture shows the use of Radar Transmitter in cement Silos.

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#11
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Re: ULTRASONIC LEVEL DETECTOR SELECTION

01/28/2018 1:37 AM

Ultrasonic level detection has been used for many years in the maritime industry, particularly for sounding liquid storage in fuel, oil, and water tanks, especially for the remote reading of these tanks.......and as you correctly pointed out, in tanks containing hazardous cargoes. Tanks in machinery spaces used for the daily requirements of fuel, water, and oil are usually dipped, once a day, with a dip tape to ensure correct calibration of electronic gauging........such is the nature of the beast (electronics). Some of the machinery space tanks may have gauge glasses fitted......these are generally fitted with automatic self closing valves or cocks, in the event of gauge glass breakage, (they prevent the entire tank contents leaking out). I still used to ensure that these tanks were dipped manually once/day as well.

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#12
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Re: ULTRASONIC LEVEL DETECTOR SELECTION

01/28/2018 5:37 PM

Wow, once a day huh. I guess that is understandable given the criticallity of being stored in the confined space of a ship. (Where humans are confined to the same general space.) Thanks for the interesting addition of your experience.

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#6

Re: Ultrasonic Level Detector Selection

01/26/2018 3:32 PM

I thought this was an interesting post. I didn't realize how complicated it really is.

I went to Google.

I typed in : silo core flow inversion reduction.

Lots of white papers, if you have access. I don't.

Here is a couple of images, the math is beyond my understanding.

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#7

Re: Ultrasonic Level Detector Selection

01/26/2018 4:19 PM

Most ultrasonic frequencies will not shoot through CO2 (carbon dioxide).

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Re: Ultrasonic Level Detector Selection

01/26/2018 4:46 PM

That is an interesting factoid.

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Re: Ultrasonic Level Detector Selection

01/26/2018 4:58 PM

What is frustrating is that every ultrasonic level supplier knows it, but not one will publish the fact, so it becomes a matter of one's membership in or relationship to the supplier's oral tradition as to whether the information gets passed on to whomever should have it.

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Re: Ultrasonic Level Detector Selection

01/27/2018 12:00 AM

Yep, that's true with most industrial suppliers. We specialize in PLC training, college instructors and individuals go to OEM for training, but of course PLC vendor is not going to teach them about short comings of their software/hardware, so then they go on to teach others, and you got as a result, most people finding out the hard way. after machine crashes of thousands of dollars in downtime. Because we have specialized in PLC training for over 2 decades, we know the ins and outs to teach/warn customers about. But what we see is even the most experienced professional, 9 out 10 don't know of the short coming. often 10 our 10.

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