Flowmeter Question

[sheiko]

Dear members,

Probably an easy question for some of you but, as I am not an instrument technician, I would like to have your views.

The issue is that for several orifice meters in my plant the range is not the same in the instrument specification sheets and in the DCS.

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

So my questions are:

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

2/ Is the range displayed in DCS equal to the transmitter range? If not, how to know the transmistter range?

"We don't believe things because they are true, things are true because we believe them."

 

[zdas04]

There is nothing called a "flow meter" in inputs most of the time, so it is probably OK. For inputs it is critical that the device range be correct. If a static pressure instrument has a 4-20 mA range of 0-1000 kPa, then telling the DCS that 20 mA is 1200 kPa would result in calculated volume being wrong. As long as all three dynamic inputs match between the instrument's calibrated range and the DCS then the flow should be computed properly.

On the other hand, if you are calculating flow rate in an auxiliary device and sending flow to the DCS (e.g., some 3-in-1 transmitters have a calculation routine built in and can send flow rate over a 4-20 mA input signal) then you have a huge problem if the end-device calls 20 mA something different than the DCS does.

David

 

[jmw]

The calibrated range for the flowmeter is dependent on the flowmeter.
It is usually sized according to the process conditions. It will not usually be an exact match.
The transmitter output may be calibrated via span and bias settings to represent a particular flow range. This is not necessarily the calibrated range.

The DCS input may have an alternative setting for the span and bias.
For example:
Process flow range: 25 to 75 m3/hr
Sensor calibrated range: 15 - 85m3/hr (closest match to process)
Sensor 4-20mA output: 20 to 80m3/hr (maximum output resolution but with allowance for excursions)
DCS display: 0 to 100m3/hr (most convenient display resolution. But you have to know that any reading outside of the input signal range (20-80) is junk.

It could be that the sensor output is set as 10-80 where anything between 10 and 15 is indicative of some sort of flow behaviour but not necessarily to be treated with any confidence. All a signal in this range says is that your flow is below the calibrated minimum range.

The main issue is to be sure that when the DCS says 62m3/hr that the flow meter is measuring 62m3/hr.
A mismatch between any of these settings can result in problems. Most usually because different people set up different parts of the system from the flowmeter through to the DCS and haven't been made aware of the actual settings of some other part of the system and thinks that 4-20mA input is equivalent to a different flow range than the one it was configured as.

There is often a lot of confusion between the sensor's calibrated range and the 4-20mA span and bias settings. And, not surprisingly, when it comes to telling the DCS what the 4mA and 20mA values are equivalent to is another matter again.


JMW

http://www.ViscoAnalyser.com

 

[hacksaw]

welcome to the club, you'll hav to dig through you plant record books, talk to thw instrument shop (they have current cal data), and coordinate the collection of facts you have then talk to operations to see what they are doing, ultimately you get to the material and energy balances for the plant, so even the plant manager will have some say...

 

[danw2]

The DP transmitter must be configured to match the primary flow element (orifice plate, averaging pitot tube, whatever).

Somewhere in the plant, there is a sizing sheet for the primary flow element for which 'design' conditions were used to calculate what the DP is at a given flow rate.

Not matching the primary flow element to the setup of the DP transmitter creates garbage right at the beginning of the data stream, rather than further downstream, like at the DCS.

Applying scaling factors in the DCS can be very valid, as described above, assuming that the transmitter is configured for the flow from its specific primary flow element.