Level Measurement

Chances are the DP is working as it is designed to, but the actual level varies from the indicated level due to temperature issues or specific gravity issues.

A remote seal capillary is filled with fluid. Boyles or Charles Law tells us that pressure is proportional to temperature in a closed system. A remote seal system qualifies as a closed 'filled system'. Temperature of the seal's fill fluid directly effects the pressure reading.

If the fill fluid boils, then there's additional error. One expects a different reading when the seals are cold than when they're hot. Minimizing temp error can involve steps like shielding capillaries from drafts, radiated heat, or sunlight exposure.

Head pressure measurements assume a constant, known specific gravity of the measured fluid. When the actual SG varies from the design SG, the result is error. Typically the SG correction factor is not applied in the DP transmitter, it is applied in whatever reads the DP signal. There are plants that have user variables for SG correction factors.

One time, I discovered that the water cleaning jet aimed at a Brix concentration analyzer probe worked on a timed period, once every 10 minutes. It produced a flow stream that impacted an adjacent DP remote seal diaphragm, causing a higher reading than expected for the duration of the cleaning spray. I can't recall where in the juice process line that was, though.

thanks for the immediate reply. in this regards, perhaps i can conclude that by using a remote seal d/p cell type in measuring level with changes in specific gravity is not applicable for this kind of measurement.

thanks and much appreciated

Before I agree that without a constant SG correction factor, DP is probably not the level sensing technology of choice, I have to ask, how critical is an accurate level measurement? Can the evaporation process tolerate the deviations due to fluctuating SG? Is the complaint of indicated-level-off-from-actual a whine or does it actually effect the process & product?

I used DP on a batch vessel for water evaporation from oil, with an attendant 5 inch (130mm) shift in actual level as the water evaporates over time. DP was the most cost effective to install, given the vessel. The shift in actual level does not affect the operation or the end result, other than indicated level varies from actual level over time.

An advantage of DP in food processing is that it can be accomplished with flush mount sanitary connections. Substituting alternative level technologies sometimes comes down to "what fits" and can be sanitized.

A specific gravity correction factor is relatively easy to apply:
measured DP (in distance level units; mm or inches, whatever) / SG = actual level distance

The problem is that an on-line SG density measurement needed for continual update is typically relatively costly to implement.

hi carl,

level measurement is critical on our part, since false level indication might cause a carry-over some entrained syrup in the vapor lines that could contaminate the condensate feed for the boiler. for one evaporator alone ( 4 sets ) the SG varies from 35 to 50 brix, the others from 40 to 70 brix.

any advice, suggestions and opinion are welcome. thanks and much appreciated

For this kind of application I would and have recomended a tube to near the bottom and measure the pressure of a gas flow "Bubbler Meter" these are common and easy to obtain all you need to do is determine the SG of the measured liquid.

A bubbler is still a head pressure measurement, it's just removed from the medium, rather than contacting the medium. The issue of changing SG still remains.

Assuming the Brix values are at 20°C,

35 brix = 1.1529 SG
50 brix = 1.230 SG
40 brix = 1.178 SG
70 brix =1.344 SG

Ratioing any two SG's will provide the ratio of indicated level change for a static level at the 2 SG values.

Is SG compensation through a Brix measurement a possibility or not? How is Brix determined/analyzed now? grab sample? coriolis ? in-line refractometer?

If not, what are range of level is needed? 50mm? 500mm? 5m?

Temperature? pressure?

mounting access from the top?

atmosphere over the syrup/juice? steam? air? vapors? vacuum?

Installation limitations?

Can a contact capacitive probe be used? How badly does the syrup/juice "crud up" or deposit on a vertical probe over time?

Does a level probe have to withstand CIP? Does it have to be 3A?

thank you so much for your concern and interests in these issue. i will gather all the data needed and will come back to submit the necessary.

thanks and much appreciated

An ultrasonic level transmitter sounds like an answer for your problem.They can see thru foam and give an accurate level regardless of specific gravity.Radio frequency types are also available.Both can tolerate high temperature and hostile environments.

hi carl, am planning to install a vortex flowmeter-insertion type for an 8 inches pipe steam pipe, not the usual in-line type. do i still have to consider the piping conditions such as the 10D upstream and 5D upstream requirements?

i just need to be literate with this kind of application.

thanks and best regards

Yes, Vortex is even as sensitive to flow stream perturbations as DP. maybe more so.

Typical recommendations are 20D upstream:

even more is there are upstream bends:

but just 5 downstream:

You don't have to pay attention, but then accuracy is out the window.

Just as important though, when investing this much into a flow meter is whether you've considered line pressure compensated vortex, like the Krohne OptiSwirl. An absolute pressure sensor reads the line pressure and can calculate inferred mass flow rate rather than voumetric flow rate.

The tube that looks like a handle is the line pressure impulse tube running to the absolute sensor:

thanks carl, but the example you provided are for the conventional type vortex. what about the insertion rod type inserted into a pipe, such as the LPIV (Low Profile Insertion Vortex) provided by NICE Instruments.

thanks and best regards

If their stuff is different, you'll have to ask them. Given that all vortex work on the same principle, I can't imagine much difference in installation requirements, but they're the people who should know.

Why not download their manual and read through the installation section?

Two notes on using vortex for steam.

1) The vortex meter is inherently a velocity meter. All vortex meters indicate zero flow at low flow rates up to about 10% (any manufacturer has a sizing program they can use to calculate the zero drop off point for your conditions), because there are so few vortices created at low flow. Low flow drop off directly affects the way vortex works in many steam systems because of the way steam distribution is designed.

2) Many steam systems have a very large 'header', a large diameter pipe, which effectively acts as an accumulator, a resevoir of available steam on demand.

The problem with measuring steam flow in the header is that a header is somewhat like a small diameter tank, rather than a pipe. Any given volume of steam flows more slowly, at lower velocity than it would through a smaller diameter pipe.

I've seen properly sized vortex meters indicate zero flow into the steam flow operating range of the plant because of the effect of the header in slowing down the velocity of the steam through the header. So, be warned about low flow in a header.

thanks carl, hope you can still be of assistance in the future....

much appreciated

It may need to be kept warm, so that the <...sugar juice...> doesn't tend to solidify and block the pipes leading to the <...d/p cell...> .

Consider the use of a radar sensor instead.