Rangeability problem

[jotake]

I need to measure the nitrogen production after the generation package. The minimum flow is 64 Nm3/h, the normal flow is 85 Nm3/h and the maximum flow is 1350 Nm3/h. The N2 generation pressure is 10 Kg/cm2 g and the temperature is 38ºC. The line size is 6”.

An oriffice plate is not suitable because of rangeability problem. Which other type of flow measurement instrument can I use. We have thougth about a vortex meter but the size recommended by the vendor is 1” vortex. With this size the pressure drop for the maximum flow is over the allowable one.

Thank you for your help!

 

[JLSeagull]

Several technologies can do 100:1 turndown. A Coriolis meter is a good example.

 

[ScottyUK]

Orifice / DP cells in a split range configuration should achieve that turn down ratio at a lower cost than a Coriolis meter, although without the level of precision offered by the Coriolis meter. Both DP cell capsules need to be rated to withstand the highest DP present, but that is not usually a problem in practice. The ranges should overlap so the low range transmitter covers (say) 0 - 11% of flow and the high range transmitter covers 9 - 100% of flow to ensure a definite crossover.


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Sometimes I only open my mouth to swap feet...

 

[JimCasey]

Inertial-type flowmeters are rangeability-limited. Orifice plates, pitot tubes, all read a signal that varies with the square of the velocity. So a 100:1 signal only gives you 10:1 rangeability.

Vortex meters read frequency proportional to velocity so they have inherently greater rangeability. You are looking for 22:1, and that would be easy to do in a vortex meter. If your application has too much DP at max flow for the 1" meter, go to a 1 1/2" meter section. Follow or exceed manufacturer's suggestion for straight pipe runs upstream and downstream, just as you would with an orifice.

Coriolis meters are also inherently proprtional to flow, and it IS a true Mass-flow measurement but measuring a gas you are looking at such a tiny signal that it can get swamped by line vibration. Also they are expensive. Much better signal-to-noise ratio with liquids because they are denser.

Turbines, paddle wheels, etc. have mechanical bearings that may be damaged by an overspeed or just by accumulated time. Cheap, but not as durable, accurate, or stable. Also these work beter on liquids because the liquids can both lubricate and pressure-balance the bearings.

Positive displacement meters are accurate*, but they also have bearings. *accuracy is relative to a particular pressure and temperature, so you wind up having to use additional compensation instrumentation to compensate. PD meters work much better on liquids for that reason. Also, if somebody opens a valve downstream of a PD meter and it loses its backpressure, it can overspeed and be junked in a New York Second.

Hot Wire anemometer is promising for your application. The amount of heat pulled off the hot wire is proportional to the mass flow rate. As long as the service is clean and there is nothing in it to foul the wire, it would be a practical choice.

I still think I'd use the vortex meter.

 

[jotake]

First of all, thank you very much for your answers.

JLSegull coriolis meter would be a really good option but the the price is completely unacceptable in the current shedule of project.

ScottyUK I do not understand your solution. An orifice could not be use because of beta problems. The orifice for measuring the minimum flow would be so small that it would act as a restriction oriffice for the maximum flow. What do you mean with orifice/dp cell in split range? Two transmiters with only 1 orifice plate? Two orifice plates in series? It would be great if you can explain a little bit more.

JimCasey your explanation was great. After today's meetings in the office, a vortex meter seems to be the most probable option we will follow. As you propose, we are thinking about a 1 1/2" diameter vortex meter. The only problem with this size is that the minimum flow that it can be measured is 84 Nm3/h instead of the real 64 Nm3/h. We are asking the instrument supplier about the error in the measuring of 64 Nm3/h. If this error is acceptable we will follow this solution. What do you think?

Let me thank you all, once again!

 

[Ashereng]

jotake,

Two orifice meter run. One for the low flow range, one for the high.

Or, if you prefer, you can use a senior orifice meter:

http://www.emersonprocess.com/daniel/products/gas/Orifice/Senior/Productdetail.htm

which allows you to switch plates under full line pressure.


With regards to the coriolis meter. How much is too much?

Personally, I would look into either a turbine or vortex, for the type of application you described, initially.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[jotake]

Ashereng,

This is the first time I use Eng-tips forum. What do you mean with "Have you read FAQ731-376 to make the best use of Eng-Tips Forums?"? I have just read it and I do not know what I have done wrongly. Could you explain it more to me in order to not repeat my faults? I find this forum really interesting and useful and because of this I would like to follow the rules as much as possible.

Thank you!

 

[ScottyUK]

jotake,

The split range system uses a orifice sized for maximum flow and two transmitters. We use this on our export steam instrumentation where we are required to maintain accurate control from a maximum flow of approx 600Te/hour down to perhaps 30Te/hour at minimum. A single orifice plate and DP cell would be useless at the bottom end of this range, but with a second DP cell ranged to suit measurement at the bottom end we can maintain control.

Don't worry about the FAQ comment at the end of AsherEng's post - it's a hint to all posters to keep their posts to a reasonable standard. It wasn't directed to you.


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Sometimes I only open my mouth to swap feet...

 

[jotake]

ScottyUK,

One last question about the split range system. If the orifice plate is sized for the maximum flow, when the minimum flow is circulating the pressure drop originated in the orifice will be almost negligible. Will be the low range transmiter capable of reading this small pressure drop?

Sorry for so much question and thank you for your help.

 

[ScottyUK]

It's not my design of system - I specialise in electrical and control and stray over into instrumentation rather than being a true instrument designer - but yes, the DP at low flow appears to be adequate for our purposes. I'll try to catch one of the instrument guys and get a few more details of the configuration.


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Sometimes I only open my mouth to swap feet...

 

[JLSeagull]

You can buy a pressure differential transmitter with a range below one-inch water differential. I would not want to go that low but 10-inch water column would be common for a low range. That same transmitter with a 1-inch water differential would not be damaged by the high rate differential.

The dual chamber orifice assembly and two transmitters could exceed the cost of a Coriolis meter. A turbine meter could be desirable but the signal interface is not as simple as the dp transmitter, etc.

 

[Ashereng]

jotake,

You did everything right. The FAQ is part of my signoff - as ScottyUK said, it is a reminder to all posters.

With regards to the range of flow, my suggestion is still to change plates. Trying to measure a couple of inchs water column is not the way to go. Two plates (with a changer) or two meter runs will give better results (depending on what you need out of the measurement of course).

I am not quite sure what JL meant when he said the interface is not as simple on a turbine meter. We use lots of tubine meters, and the interface is a 4-20mA signal HART. They work well enough.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[JLSeagull]

Ashereng,
Frankly I have not purchased a turbine meter in the last 10 years. That client wanted Halliburton. However, they took exception to a specification written around their own product and were eliminated. Some of the manufacturers, perhaps still including Halliburton just cannot get to 4-20 mA HART. That is the issue. I do not want to deal with the pre-amplifiers and other obsolete crap that some manufacturers required.

 

[jotake]

Once again, thank you all for your help.

JL, the orifice plate with 2 transmiters seems to be a good solution. The only doubt I have is if with a 1 in water column range transmiter I would be able to read the pressure drop generated by the minimim flow (64 Nm3/h) in the orifice sized for the maximum flow (1344 Nm3/h). I think that the pressure drop generated will be almost negligible but the transmiter vendor could help me with this.

Ashereng, the solution you are talking about (two different plates with a changer) is automatic or does it need the manual action of a worker?

Finally, between the following option, which do you think is the most economical:

1) A single orifice plate with two transmiters.
2) The two orifices plates with a changer (senior orifice meter)
3) A vortex meter.

Sorry for so many questions, but I think I am near the final solution.

Thank you very much!

 

[dcasto]

How about an ultra sonic meter?

 

[JLSeagull]

Size the orifice plate for the maximum flow and high differential case. You could use about 150-inch water diffential for 150 psig. Then run the calculations with the selected bore and low flow rate (100% on the low dp meter) to determine the differential for the low dp transmitter. Adjust the flow rate for the low range until the dp is acceptable.

 

[dcasto]

The BLM now accepts Waffer V-cone meter.

http://www.blm.gov/nhp/efoia/wo/fy07/im2007-022.htm

 

[Ashereng]

The senior is a manual operation - guy goes out and does the change.



"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[ScottyUK]

jotake,

I don't think you have posted your requirements in terms of accuracy and repeatability for this measurement?



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Sometimes I only open my mouth to swap feet...