Hi All,

I need to fill 1000 ltr of water to water tank and when it was reached to 1000 ltr, the water pump which has flow 3 m3/h should stop and to make sure the water is 1000 ltr, I must measure it with digital display and should read 1000 ltr of water on display. At present I use the system of flow as below,

* Burkert Paddle Wheel Flow Transmitter Model 8025, Size 1" *
Burkert Batch Controller Model 8600 with operator unit Model 1077-1
which installed and connected directly to solenoid valve (plug in direct to solenoid valve)
* Burkert 2/2 way solenoid valve Model 406 with diameter 1"
which will shut if the volume of water is reached to 1000 ltr and the display controller read 1000

The problem I face is the volume of water is not meet with the display when I measure it. For example, I set the controller 100 but the water I measured is not 100 ltr but 101,3 ltr and when I set to 1000 the water I measured is 1030 ltr not 100 ltr and be advised the accuracy I need should be less than 1%. If anybody knows, how to solve it please let me know or perhaps I must change the brand. Looking forward to hearing from you soon and thank you.

Sorry for my poor english.

Kindly regards,
Harijanto

 

Fisher-Rosemount/Micromotion has flow transmitters which are accurate and reliable but you do pay for these qualities. But if it is imperative that you are near exact on your measurement then this should work for you.

 

Not sure of how the brands of equipment work, but here are some general questions:

1. How far away is the valve from the tank? Is the pipe draining after you close the valve?

2. You mentioned 1% accuracy. Your test of entering 100 ltr and getting 101,3 ltr is nearly 1% already (1,3% to be exact). If you go for 1% at 1000 ltr you are saying it is ok in a range of 990 to 1010 ltr.

3. I don't know if you will ever get it to stop at EXACTLY 1000. if it is that critical, you may want to consider a 2-stage system, where you fill with the big pipe up to 970 or so, then switch to a slower flowrate to 'trickle' up to 1000. This assumes that time is less of an issue than accuracy.

Just my thoughts on the subject....

--Joe Jansen

 

Hi,

I am not familiar with the equipment that you are using, but I have used other flow meter / display combo before, and the accuracy that you have is pretty much what you can expect from these kind of setup.

The display that I used has scaling and offset settings that you can tweek to bring the results even closer to the actual. Does yours have these settings?

I am assuming that you are filling something that you can't insert a lever switch... otherwise you can place a secondary device that will measure the level.

Dan B.

 

As long as you will have imagination, you might improve the volume measured. But you will never have your litres as desired. Why?
Fist you should open the water valve slowly (to the minimum flow the meter is accurate) then stats closing the valve at say 950 mesured
volumeto the minimum flow again. This will reduce the the start/stop error of the flow meter: it is not a bullet smashing on rock !!!
Also, stirred water increases volume, Yes.
Then how to do ?
First work with mass of water, not volume and mesure the weight of water near the end and few more drips. Use load cells. It will be difficult calibrating.
One more detail: the weight density of water reduces as temperature increases.
If you are interested in the formulation, let me know, I will supply the formulas up to the critical point.
[email protected]

 

Harijanto

From your description, I concluded that your problem can be related to how fast you can interrupt the flow after it reaches the desired value.

If this is the problem, there are techniques developed exactly for this purpose.

One of them is the so called Bang-bang control.
That is an dual stage on-off control, where a larger valve, such as your present 1" solenoid operates untill you aproach the desired total flow, lets say for instance 970 liters. At that point it is shut off, while a much smaller flow continues through a smaller valve, possibly about 1/4" untill you reach the 1000 liters desired.

You will need of course of a second smaller solenoid valve for the "fine" slower filling and your controler needs to have two outputs, with
different set points, or else you need two controlers and valves for the same total flow control loop.

If your problem is not that, it may be on the accuracy of the transmitter. Is it possible to calibrate it ? Positive displacements and similars
such as paddle wheel transmitters sometime are not user calibrated. Do you have any instructions on calibration for it ?

Vitor

 

Where is the flowmeter installed ?

Is it 5 feet from the outlet, is the flow off the first reading or all readings......in other words is the liquid counted but not in the tank yet ........

How quickly does the valve shut off .....is there a delay in shut off. Is there a tight shut off or does the liquid drain back......this will
usually result in a short count though.....

Is the paddle wheel the proper size for the piping and pressure.....

Does the controller quit counting when it reaches the preset desired even though there is some flow left or does it continue to count actual flow.....example, I have seen PLC's programmed to go until a counter has counted x number of pulses and then shut off the valve and this happens, but an extra couple gallons actually flow into the
tank........

Just some things I have run into in the past.....

Dave Ferguson
UPM-Kymmene
Blandin Paper Company
DAVCO Automation

 

the use of weighing tanks for flowmeter calibration is used in national calibration facilities---i have read of this in the metrology
journals. but it is important to have the instant stop and start of the flow and this is done with rotating sleeve kind of valve. imagine sleeve which which fits over the pipe, the end of the pipe is sealed tight. there are holes in the sleeve and the pipe. when the holes are lined up the water flows. when there is small rotation
of few degrees the holes are not aligned and the flow stops. i think this can be done in milliseconds without much force and water hammer.

rgds
leelock

 

Calibration systems also use knife valves especially if they need to control the flow to minute quantities.

CEESI has such a device.

Walt Boyes

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Branom Instrument Co.-- P. O. Box 80307-- 5500 4th Ave. So.
Seattle, WA 98108-0307
Phone: 1-206-762-6050 ext. 310 -- Fax: 1-206-767-5669
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If you can not slow down the pump, you may program it in a way that after filling 90% or 95% of the setpoint it should pulsate. Say 1sec ON 0.5 sec OFF this way you can achieve better accuracy. I did this in a pharmaceutical syrup manufacturing system and was very successful.

Tanweer Ahmed
Phone: +9221-636 5519
Fax: +9221-568 2972
E-mail: [email protected]
URL: www.maxpages.com/tanweer

 

The commonest cause of water batching system error is conflicting demands on the distributed water service. This causes fluctuations in pressure and flow rate.
This is ok if you don't mind batch duration times varying but it is critical to your choice of flowmeter.
In potable or domestic water supply systems you may be using a low cost system set which is ok but when you choose an inferential volumetric meter such as a paddle wheel you must be aware of its "linearity".
Good accuracy depends on good repeatability. Even quite simple meters will have sufficient repeatability for you to get a 1% batch accuracy (even though this may be better than the meters claimed "accuracy"). Much of the error at any given flowrate is a repeatable offset (systematic)error which you measure and compensate for. If you don't control flowrate then you must depend on the linearity of the meter i.e. what the systematic error is at different flowrates; and unless you can linearise (measure and correct for the offset errors at different flowrates)then instead of working at the repeatability limits of the meter you are restricted to the linearity limit.
A feature of some inferential meters is that they are not particularly linear and quote accuracy (when they mean linearity) as %fsd. This means you will see a increasing error as your flow rate reduces.
When batching you also have an error associated with the time it takes the valve to close. Assume you have a constant flowrate (e.g.30l/min in a 1" line) Then at 0.5l/sec a solenoid valve closing in less than 1sec will only contribute about +0.5l or +0.5% to you 100litre batch error. But this is an over run error. You compensate for this by initiating closure early i.e. at the 99.5litre point. This should leave a residual error of +/-0.1l or so. Even if flow rate varies, the magnitude of this error will be small. But you must be sure this valve is well maintained, shuts tight and does not stick. In general, fast closing valves cause water hammer problems but solenoids are commonly used in water lines (you have two on your washing machine) so fast closing isn't the major problem it is on larger lines or pumped services. (if pumped, you do need to worry more about this and to make sure you trip the pump when you close the valve).
At a 1000l/batch the error due to the valve is now around 0.01% and negligable.
Thus the major error source, assuming your meter is in good order, is due to the non linearity of the meter. Test this by measuring how the time taken to deliver a 100 litre batch varies during the working day and how this relates to the error in batch delivered (unless your batch controller gives flowrate information).
You can compensate this by ensuring that the flow rate is constant. One simple option is to fit a downstream pressure regulator before the meter and set it to a pressure below the normal working pressure (above it) such that most of the time, variations in supply pressure do not go below this value. This will help ensure constant flow rate. Now you can identify the repeatable offset (systematic) errors in your system and compensate for them by calibration of the meter and pre-close of the valve.