Totalzer Gas flow

[edwale]

Hello I have installed a Rosemount 8800D Seroes Vortex Flowmeter for the natural gas. The flow meter is reading the nromal flow and also the totalised flow. The normal flow which is 4-20mA siganl will be sent to the Prificy ME pLC. However the totalised flow is only be a local displayed on the instrument. My challend is to try and get the totalised flow on the SCADA (Wonderware).
The issue is before I purchased the flow meter the client only told me they only interested in the normal flow to be diplayed on the screen but now they also want the totalised flow and since the meter has been built to our specifications it will only give out 4-20mA signal for the normal flow and not a pulse output.

Can anyone help me with the following?
1. How to do the totalised flow calculation in Proficy ME PLC using the 4-20mA signal?

2. Is it possible to split the 4-20mA signal using a spliter so one signal is for the normal and one signal can sent to the high speed pulse generator for the totalise flow computation in the PLC.

regards
edwale

 

[danw2]

I was thinking get a HART to Modbus converter and have Wonderware read the totalizer variable via Modbus which would not sacrifice any accuracy and would eliminate the gripes that the totalizer on the meter doesn't match the total in the HMI.

But, the devil is in the details. The totalizer value is not in the list of assignable HART variables, but it is ni the HART burst mode list. How odd.

There are eight variables that can be mapped to one of the four HART variables, but there are nine variables that are available in HART burst mode

page 54 (pdf)
Selections for the Primary Variable are
1) Mass Flow
2) Volumetric Flow
3) Velocity Flow
4) Process Temperature

Selections for the Second (and 3rd and 4th) Variable(s) include all Variables that can be mapped to PV, and also
5) Shedding Frequency
6) Pulse Frequency
7) Total Process Density
8) Electronics Temperature.

(page 69 pdf)
Burst option
Xmtr Vars–Allows the user to define custom burst variables. Select variables from the list below:

1) Volume Flow
2) Velocity Flow
3) Mass Flow
4) Vortex Frequency
5) Pulse Output Frequency
6) Totalizer Value
7) Process Temperature
8) Calculated Process Density
9) Electronics Temperature

The totalizer can only be read in burst mode, which is part of HART multidrop mode which sacrifices the 4-20mA output for the primary variable.

It's probably worth a call to Rosemount to inquire, why the totalizer value is omitted from the HART variables assignable to the primary, secondary, 3rd or 4th variables. Maybe it's a manual error.

I can't help on the details of totalizing an analog signal on the PLC (my PAC has function that makes it easy), other than you sum flow values over time, and hopefully time base is accurate.

Yes, there are commercial current signal splitters, but I can't see how that would help. High speed pulse counters expect read and count pulses over discrete time frames.

There might be a pulse generator card somewhere, but that would introduce substantial error; to convert analog to pulse and then count the pulses and then feed the pulses back in to counte3
r card.

 

[edwale]

Thanks dawn2. I will have a think about what would be the best approach.

 

[edwale]

Does anyone know what the formula is for the tatalise flow? I would like to do the calculation for the toatlise flow in the PLC using the 4-20mA signal.

 

[danw2]

former sum + (instantaneous flow rate value * [sampling interval time interval / number of sampling intervals in engineering unit time value]) = current sum = total

example: constant 300 gpm flow rate sampled once per second
Former sum is initialized to zero.

1st conversion: 300 * 1/60 + 0 = 5 gallons
2nd conversion: 300 * 1/60 + 5 = 10 gallons
3rd conversion: 300 * 1/60 + 10 = 15 gallons
4th conversion: 300 * 1/60 + 15 = 20 gallons
. . . .
59th conversion: 300 * 1/60 + 290 = 295 gallons
60th conversion: 300 * 1/60 + 295 = 300 gallons

 

[edwale]

Thanks Dawn2

 

[kinser06]

As long as the meter your using is a hart smart device i have used a HIM module made by Moore Industries. Similar to a rosemount tri-loop it would go in your control panel and give you an additional analog out of your preference. They have proven to be super reliable for me.

 

[Macbeth]

1.Set a low Flow cutoff rate that is acceptable. (Eliminate that pesky 4.01ma)
2.Set a timer to execute at an acceptable sample interval. (10sec)
3.Run Calculation when the timer cycles.

T = Timer Accumulator (In seconds)
Q = Flow Rate (In GPM)
A = Accumulated Total (In Gallons)
Calculate A = A + ( Q / 60) * T )

FYI
Divided Q by 60 to convert Gallons/Min to Gallons/Second