7 replies to this topic

[sheiko]

Dear folks,

Probably an easy question for some of you but, as I am not an instrument guy nor an expert, I would like to have your views.

The issue is that for several orifice flowmeters in my plant the range is not the same in the instrument specification sheets and in the DCS (I have not yet checked locally, at the intrument level).

For example, ranges are:

Case n°1 (range displayed in the DCS is higher than in the specification sheet):
- Specification sheet: 0 - 500 kg/h
- DCS: 0 - 600 kg/h

Case n°2 (range is higher in the specification sheet):
- Specification sheet: 0 - 400 kg/h
- DCS: 0 - 300 kg/h

Are such deviations acceptable? I believe not, but what are the real consequences on the measurement accuracy in each case?

Edited by sheiko, 14 April 2012 - 01:25 AM.

[Technical Bard]

This is a serious problem, in my opinion, unless you have a very "smart system".

A typical flowmeter (orifice type) is generating a differential pressure with a defined range (say 0-100 inWC). This range corresponds to some flow range (say 0-500 litre/h). This is converted to a curent signal (4-20 mA), which goes to the DCS. The DCS uses it's range (say 0-600 litre/h) to convert the signal into a reading on the operators' screen and for storage in the historian database (assuming you have one).

Therefore, any flow reading on the DCS will be 20% higher than the actual flow passing through the meter.

Now, if you have smart transmitters that send a data signal with flow information, you might be ok. But I'm not a controls guy and don't know the details of how these modern systems work - so hopefully someone with more knowledge than I will help you out.

[breizh]

Sheiko.

My first thought is that you have to compare DCS range and Field instruments .
Hopefully it will be "only" paper work.

Breizh

[sheiko]

@Technical Board,
If I understand well, the DCS will underestimate the flowrate by 25%, in the case 2. Right?
I will ask the control system guys about the type of signal transmitted to the DCS.

@Breizh,
Do you know what should I exactly check in the field?
I believe there will be some sort of digital panel next to the orifice flowmeter where local setting of the instrument can be done, but I have not checked yet. Is that what you meant?

Edited by sheiko, 14 April 2012 - 01:55 PM.

[breizh]

Sheiko ,
You should ask your E&I technician to control the chain from field ( sensor , transmitter , wiring ) till DCS and simulate a 4-20 mA (guess) to ensure you get the right info back to your system and it is covering the full range . Normally this is a routine test .

Note : You mentioned (Kg/h) , is it a gas or liquid flowrate ? From your meter you should get m3/h and then you have to measure somewhere the density of the flow and this may introduce some errors as well .


Breizh

Edited by breizh, 15 April 2012 - 01:16 AM.

[sheiko]

Note : You mentioned (Kg/h) , is it a gas or liquid flowrate ? From your meter you should get m3/h and then you have to measure somewhere the density of the flow and this may introduce some errors as well .


Breizh

Both services: gas and liquid.
There were no units displayed in the DCS, the technician told me the numbers were in kg/h. But I have to check indeed.
Thanks for pointing out.

Edited by sheiko, 15 April 2012 - 10:18 PM.

[pavanayi]

Sheiko,
For any flow transmitting instrument (or for that matter pressure transmitter etc), there is a maximum range that it can be used to measure. A flow meter, when procured, might have a maximum range of 0-2000 m³/hr. This normally corresponds to 4-20 mA from the FT.

But as with other equipments, the range that you need in your process will not always match the standard equipment ranges sold by different manufacturers. For example, the above transmitter might have been procured to measure a flow in the range of 0-800 m³/hr.

Normally, before shipping, the vendor would have re-ranged the instrument as per the datasheet sent to him (0-800 m³/hr). Or otherwise, it would be re-ranged by plant instrument guys when it is installed first time around. The re-ranging is a straight forward procedure using industry standard communication methods like HART. This means the transmitter will send 4-20 mA in the range of 0-800 m³/hr In the DCS too, the display would have been configured to show 0-100% as 0-800 m³/hr. And usually in most cases, the specification sheet would show the same value.

Note now that there are three ranges:
1. The maximum range the instrument can physically measure (0-2000 m³/hr)
2. The re-ranged instrument on installation (0-800 m³/hr)
3. The DCS range (0-800 m³/hr)

Ideally the installed range on instrument and DCS range should match.
Sometimes, due to many reasons, the range required by process will change. It might be during commissioning or after de-bottlenecking or plant revamp etc. In that case, the instrument will neeed to be re-ranged for the new required flow. The process department will send the request to plant instrument guys. For example, the new required flow might be 0-1000 m³/hr.
This can still be measured by the same instrument, but would only need re-ranging in the field and re-ranging in the DCS. If the changes are not done in both places, you have problems. Sometimes (as is the usual case), changes would have been done in plant and DCS, but the paperwork would have gone missing, misplaced etc. The file might still show the earlier range.

Hope you have understood the bigger picture.
Check with the instrument manufacturer's website with the model number what is the absolute maximum range of the transmitter. Check with the instrument department what is the actual range of the instrument in the field and what is that in DCS. If all are matching, check what happend to the paperwork

Again this is a chemical engineer's understanding of how these things work. So take it in that light!!!

Edited by pavanayi, 16 April 2012 - 03:46 AM.

[sheiko]

Thank you pavayani.
I agree and believe that the first thing to do is indeed to check that the 0 and 20 mA signals correspond to the same flowrates in both the transmitter and the DCS.

FYI, I have asked the same question in a Instrument Forum. Here is the link:
http://www.eng-tips.....cfm?qid=320158

Edited by sheiko, 17 April 2012 - 07:05 PM.