concentration control system 2

[1BT]

We are looking to replace a coriolis mass flowmeter, three way control valve and process controller. The system requires the accurate control and mixing of water and Sodium hydroxide from 47% to 23% for site distribution. has anyone experience of good quality equipment that can be used for this application. Our aim is to obtain a 23% concentration taking into account the temperature of the product.

 

[pmharper]

Dear 1BT

From my experience a static mixer combined with a good quality conductivity meter (eg.

http://www.endress.com/)

is good solution. However it might be a problem finding a CT with a built-in (and temperature corrected) NaOH concentration calculation covering the range up and above 23% - but suppliers should be able to help you.

Best regards

Peter

--
Peter M. Harper
Co-Founder, CapSolva

 

[pmharper]

Dear 1BT

Found this:

http://www.emersonprocess.com/raihome/documents/Liq_AppData_2800-06_200204.pdf

This is more or less what you want right ?

Best regards

Peter

--
Peter M. Harper
Co-Founder, CapSolva

 

[1BT]

Peter,

Not quite what I was looking for, this was probably down to my inaccurate description. the system that we have installed at the moment has the following . In line three way control valve, in line mixer, in line mass flow meter and controller.
The water is pumped to the inlet of the mixer, the 47% sodium hydroxide is pumped to the inlet side of the controll valve these products are mixed via the inline mixer they then pass through the coriolis meter which measures the density, a signal is sent back to the control valve to open/close the valve as required to ensure that the corrct concentration is obtained i.e.23%.

We are looking to replace the coriolis meter, control valve and controller with an accurate upto date system, due to obsolesence issues.

Your information was useful and we may have other applications that this can be used on in the future, but unfortunately not exactly what we are looking for for this application.

Regards

Bill

 

[jmw]

Depends how your Coriolis meter was used and what type it is.
Two approachs:
1) Predictive: batch blend ratio determination
2) Feedback: density measurement.

I have seen installations (in pharmaceutical plant) that used upto three mass flow meters, one used as density, and a storage tank and had to use very high quality raw caustic all swept away and replaced with a single fork or tube density meter and a control valve, and with less expensive raw caustic (because of relaxing the specification).

In one system i saw two mass meters were used predictively i.e. they batched water and strong casutic nto the tank in a proportion based on the target quality and the known quality of the strong caustic.

At the storage tank the mix (exothermic) was recirculated through a third mass meter to look at the density.

This system was replaced with a single density meter.
The two flow streams, each with a flow control valve (to maintain the master stream constant and match the other to it) are pipeline mixed using a static mixer. The density meter is downstream of the static mixer and controls the slave stream control valve.

The density meter has on-node calculation of %concentration.

The calculation steps are:
measure density of the mix and the temperature.
Using matrix-referral (i.e. stored curves of temperature Vs density for different concentrations) the density at 15degC is found.

The next step is to calculate the concentration from the density at 15degC. though some operators simply apply a linear scale to the 4-20mA output, where concetration is not linear with density correction algorithms can be programmed. (rarely necessary with caustic).

A caution, in feedback control using mass meters mass meters are optimised for mass flow determination and exploit the variation in resonant frequency of operation with density to provide a density measurement.

A dedicated density meter is optimised in its design for density measurement. They also have sophisticated algorithms and more extensive calibration.

The diference is in the accuracy. Micro-Motion Elite (Emerson) mass meters claim density accuracy equivalent to the Solartron fiscal density tube (at predetermined conditions?) but most other mass meters fall far short of this performance.

Feedback control allows you to relax the specification for the strong caustic and pay less because any variation in its quality is automatically corrected for. In the batch blend method the blend ratio is predicted assuming a known and constant quality of the raw caustic.

In the single instrument feedback approach you can blend on demand without an intermediate 23% storage tank. This also allows you to change the target quality at will.

You can also duplicate this control at point of use by providing a ring main for raw cautic to citical useage points where point of use on demand blending can be performed.

You must decide the accuracy you need for the concentration measurement and compare options and prices accordingly and decide on the method.

You must then look for appropriate measurement techniques which could be density and compare technologies.


JMW

http://www.viscoanalyser.com

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[1BT]

JMW
Many thanks this is an excellent reply,
I have had conflicting advise from manufacturers, this has made it difficult to select the correct system. The system that I am looking to utilise is very similar to your description in your paragraph five i.e. single density meter, static mixer. The density meter is downstream of the static mixer and controls the slave stream control valve. with one slight difference, we only have one control valve, a three way valve, this valve controls the percentage of sodium hydroxide which is allowed into the system, this is mixed with the water before the in line mixer. The proposed density meter has the ability to carry out the calculations as detailed in your reply. Once again many thanks for this well thought out and understandable reply.

Regars

Bill

 

[jmw]

Thanks Bill.

The performance of the three way valve will be important. Without knowing the type, or conditions, I assume it is sized so that the normal control position gives optimum control. If necessary a manual gate valve or two (or similar)can be used to help balance the flows and bring the control valve to its best control position.
Other than that I can't think of any other concerns.
Good luck.
PS It would be interesting to know what instrument you finally choose.

JMW

http://www.viscoanalyser.com

Eng-Tips: Pro bono publico, by engineers, for engineers.

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Montemayor]

1BT:

I concur with all that jmw has so capably described and commented on. My experience with coriolis mass flow meters has been exactly as he describes. If you are dealing with a MicroMotion model, you're working with a top-of-the-line instrument that has few peers in performance and accuracy. I've used them in fine chemical production and, specifically, in feeding a specific concentration and flow of caustic soda as catalyst to batch reactors and the results have been superlative. A lot depends on the quality of the peripheral equipment you've tied in with your loop and the method of application.

MicroMotion offers a lot of useful help in tech service and advice on how to apply their product and I would strongly urge you to followup on this if you have A Micromotion meter. If not, then try anyway. Coriolis meters are the absolute answer to measuring and controlling flowrates in the process industry. This is one of the best examples of how well the instrumentation industry has improved from about 20 years ago. I would stick with the Coriolis application; but the methodology and other equipment must measure up to the meter's capability.

Good luck.

Art Montemayor
Spring, TX

 

[jmw]

I will add a to the commentary so far.

Mass meters operate at the resonant frequency, at which frequency, a phase shift is measured between different points on the measuring element. From this, the mass flow rate is determined. This is the measurement for which the sensor design has been optimised.

Many mass meter manufacturers recognise that the resonant frequency varies with density and have included simple algorithms to determine the density. They also, as often, used simple calibrations.

There are many factors which affect the measurement of the density, and that are corrected for in a dedicated density meter, that may not be compensated for in a mass meter density signal or which may be more marked in a mass meter than a density meter.

For example, Youngs modulus changes with temperature; there may be a velocity of sound effect, pressure effects on tube stiffness and end effects.

The density is a non-linear function of frequency. The frequency is actually a function of the mass of the system. This is the fixed effective mass of the sensor and the effectivemass of the fluid. The mass of the fluid varies with density.

The early mass meters often assumed a linear relationship and made no addittional corrections simply because density was not the prime measurement and because density variations were often not significant in many applications. Many early applications were where the density was an "added value" function.

In recent years many manufacturers have sought to improve the density capability by addressing some or all of these issues. This is because many are now targeting density only applications where it is the mass flow measurement that is incidental.

The Elite from micromotion certainly has a reputation for being the best at density measurement among mass meters.

A caution: Many instruments appear to perform the same because there is no difference in their stated performance at 20degC. It is necessary to look at how they perform away from the calibration conditions to realise their true performance. i.e. at different densities, temperatures and pressures.

This could be an interesting, exhausting excercise but unnecessary. It is all irrelevant if all can meet your target performance. Your target will have cost implications.

Your prime objective is the concentration of NaOH solution.
Once you know what you need in terms of the accuracy of the concentration measurement, I would suggest asking the suppliers to give you their performance guarantees based on your operating conditions.

One last point, many of the improvements in density meters and mass-meter density performance is less to do with sensor design change than it is to do with software and calibration. Do not neglect to see if there is a software upgrade for you existing mass meter and if a re-calibration is needed.

JMW

http://www.viscoanalyser.com

Eng-Tips: Pro bono publico, by engineers, for engineers.

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Montemayor]

JMW:

I want to take this opportunity to personally thank you for a superb, clear, and professional explanation on the application of coriolis flowmeters for density/flow control. Your three posts within this one thread put a lot of other attempts elsewhere to shame when it comes to explaining the ins and outs of coriolis flowmeter application and shortcomings. You certainly earn a carload of "stars" for this contribution and I would humbly suggest that with a few edits and a little formatting your responses could easily be converted into a great, classic FAQ on "How to apply Coriolis meters for flow and density control".

I consider this subject of great technical worth and value to young engineers involved in plant process control - both in the field and in the design. Once again, Kudos to you and your excellent contribution to the subject! I know everyone who understands the challenges of process control and reads your explanations will also be grateful and indebted to your effort.

Art Montemayor
Spring, TX

Art Montemayor
Spring, TX

 

[jmw]

Art,
Thank you, I really appreciate your comments.

I am going to stop while i am ahead and I may well take some of this and put together an FAQ, but i thought one more observation was in order.

I have to say that in the early days of the "D-type" (I think I missed the single tube pre-cursor) I was never fully convinced by those pictures of a man walking on a gramaphone record that Micromotion used to publish.

One of the best demonstrations of the coriolis effect is to hold a large loop of garden hose in one hand and swing it gently back and forth.

Now you have your assitant turn on the water. The imediate and very pronounced twist is all you need to know that this is a "classic" measurement.

This is such an easy and clear demonstration I would recomend everyone to do this first and then wonder about the physics later. Seeing is believing and this certainly makes the physics more palatable.

My biggest regret is that I was never so keen on watering the plants or washing the car that I might have stumbled on this principal myself. I could be rich!

Thanks again.

JMW

http://www.viscoanalyser.com

Eng-Tips: Pro bono publico, by engineers, for engineers.

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[jmw]

I am going to take Montemayors suggestion and post an FAQ. I have already re-posed my suggested experiment in the measurement forum, which is, I think, the proper home for the FAQ when written.

In fact, I have ampified the experiment because it separates out the notion that frequency somehow has something to do with the coriolis effect, when it doesn't. It is a co-incidental feature of the measurement device.

I would also add that I am very mindful of the casual and misleading way the term "coriolis density" has entered into common parlance and is even used in an ISA instruments handbook to refer to all vibrating element density meters.

Coriolis effect has absolutely nothing to do with the measurement of density. Vibrating element density meters can, some of them, more properly be referred to as "Digital density meters" if we accept the prior API descriptions.

In preparing for this FAQ, I found the comments on this site (

http://www.automationtechies.com/sitepages/pid1353.php)candid

and refreshing. I was greatful to discover the author is quite clear about the reasons for the vibrational mode of operation and for the same reasons I had proposed in my draft article.

In addition this article reminded me of two very important points:
1) the importance of bubbles:

Our own analysis shows that up to 92 percent of all Coriolis measurement problems are due to entrained air or gas, yet in the vast majority of cases two-phase flow is not even recognized as the problem

.
There is at least one solution for density only measurements where there is entrained gas and that is the Solartron EGA tube densitometer where the prolem is resolved by changing to a different harmonic at which velocity of sound effects are very much less significant.

2) frequency of operation. Many mass meters operate at quite low frequencies, 80-150Hz, for example.
There are two things to look for in the measurement as a result; the response time and the resolution.
High accuracy and fast response occur together where there is a higher frequency or operation.

Secondly, these low frequencies can result in a sensor suceptible to interference as it is vey close to the primary frequencies for plant noise, especially pipe born. Higher freqeuncy devices are operating at frequencies away from the main frequencies of interfernce and, at higher frequencies, the moise is attenuated much more quickly.

This is one reason why crosstalk is, or has been, a problem for many mass meters and not for dedicated density meters. The Solartron device, for example, operates at around 1000Hz.

This is not to detract from what we know which is that mass flow meters are excellent devices. But each application must be reviewed to identify and anticipate problems.

In this case it would be very educational to discover what the reasons are that 1BT is changing out his mass meter.

JMW

http://www.viscoanalyser.com

Eng-Tips: Pro bono publico, by engineers, for engineers.

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[1BT]

Many thanks jMW.

In answer to your question the equipment in question is now obsolete. for the sake of brevity I will list the main points that may help.

1) The existing equipment is obsolete
2) We are looking to replace the current system i.e. the coriolis instrument that measures the density /consentration/ the converter/programmer and the three way control valve.
3)Before contacting eng tips I did some research on the subject,I found a very interesting paper that gave the details of a meeting at Oxford university, where 45 experts gathered to discuss Coriolis. One of the speakers from Proctor and Gamble has 10,000 coriolis meters installed. The statement that caught my eye was that the meters do not like slug flow and aereated liquids.
4) We have sucessfully used Coriolis on some applications and we have had some major successes using ultrasonics density meters to measure density when we are checking chemicals for the correct spcification when unloading tankers to plant.
5) I did not want to fall into the trap of finding one ideal instrument for all applications and I am trying to find the correct instruments for this particular application.
6) My problems came when I started to contact manufacturers in this case five which for professioal reasons shall remain name less. I narrowed this down to three, Ultrasonic density meter, coriolis and densitomiter
7) I came to the conclusion that for this application the Solartron densitometer would be the best option, my only concern being that given the flowrate, pipe size and flow speed 4m/s a bypass installation was reccomended. This then raised other concerns, response time and also the need for additioal control i.e. orifice plate or valve .
8) I also realise that all of these instruments will not work correctly where entrained gas is present.
9) your reply gave me aditional information and a better understanding, given that I had been bombarded with technical information from manufacturers, that on occassions contradicted each other.
10)On a personal note, I have a mechanical background and some of these concepts are new to me, I am on a steep learning curve.

Finally when I am looking for advise, I keep one basic principle in mind The best teachers teach the pupil not the subject.You are a good teacher. With repect to others I am not interested in the technical equations, I just want to ensure that the correct equipment is installed on a very critical application.

Regards

Bill

 

[unclesyd]

Here are the websites for the type meter we used to control density on several process.
The sites look the same but I included both.

http://www.ohmart.com/

http://www.ohmartvega.com/

 

[jmw]

I have posted three FAQs in the Measurement and process control intrumentaion forum, which deal with coriolis meters (a simple understanding), density measurement and viscosity measurement.
Any feedback would be appreciated.

PS I hope people took the opportunity to try the experiment.

From the link i posted above, which leads to an article by a Foxboro mass flow specialist you will apreciate the problems with entrained gases.

There have been significant imporvements in this area but i will probably put together a further FAQ on entrained gas because there are diferent problems relating to effect on the sensor and where entrained gas is not a contamination but a part of the product.

I have noted that at least one manufacturer has entrained gas density systems, the trade off is less accuracy (than a gas free fluid) but very stable readings anywhere from 0-100% entrained gas.

I will also try and put together a note on concentration control. The NaOH is a classic example but another common problem is ethylene glycol solution as a refrigerant in phramaceutical plants because this does get air entrained whereas ethylene glycol dilution for radiator filling in car plants or for de-icing aircraft are not subject to entrained air and a different approach is required.

JMW

http://www.viscoanalyser.com

Eng-Tips: Pro bono publico, by engineers, for engineers.

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[jmw]

I just posted an FAQ on NaOh blending, in this forum, which i hope will be useful. Please let me have any feedback.

If it is useful i will add another on Ethylene Glycol belnmding which illustrates the approach with entrained gases.

JMW

http://www.viscoanalyser.com

Eng-Tips: Pro bono publico, by engineers, for engineers.

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[saxon]

1BT, I take it your looking for a simple equipment swap out for the obsolence issue and desire to implement better process control. May I recommend the following:

http://www.emersonprocess.com

http://www.foxboro.com

They should have everything you're looking for and then some.

Hope this helps.
saxon

 

[1BT]

Many thanks Saxon

This was my first port of call, I was looking for a more indipendant view which I have received from JMW.Once again thanks for everyones contributions which have been extremely useful.

Regards

Bill