DP flow, How to setup in DCS

R

Thread Starter

Rajesh Kumar

Dear Friend,

The feed water flow transmitter in the HRSG having 0 to 5000mm H20 range and in DCS it is 267.6 T/H. The field transmitter doesn't have a square root in it. The calculations are done by DCS. I want to know do we need to input the range in DCS logic to calculate flow inside DCS? i mean the transmitter giving only 4-20mA to DCS. So the vendor of HRSG says that the Range is not necessary in DCS and need only 4-20 mA to the DCS. Can you advice?
 
It depends on what the DCS is doing to calculate the flow from this signal. Most DCS will convert the 4-20 ma input signal to some value in some engineering units. If the feedwater flow is not compensated for temperature, and the only DCS calculation is to take a square root of the DP signal it probably doesn't make much difference, as long as you are reading 267.6 T/Hr when the transmitter is at 20 ma. Personally, I always preferred to range the input signal in DP units and then take the square root and multiply by a constant to get the flow units. In your case, with 5000 mm H2O, the constant would be 3.874.

Again, this assumes there is no temperature compensation being applied.
 
The square root extraction calculation is part of the process of signal conditioning. It is not usually provided in pure analog instruments, since they may or may not be used for orifice differential pressure measurement. The function is often provided in Foundation Fieldbus instruments.

The function is part of the signal conditioning configured for the differential pressure input. That configuration needs to know the range for which the 4mA and 20mA has been calibrated in flow terms. Usually the 4mA corresponds to zero flow rate, while the 20mA may correspond to say 100 bpm. The square root function would be selected if this is an orifice flow measurement. All of this is done during configuration of the I/O point in the DCS strategy builder.

Dick Caro
Certified Automation Professional (ISA)
Buy my books at the ISA Bookstore:
Wireless Networks for Industrial Automation
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Consumers Guide to Fieldbus Network Equipment for Process Control
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D

david mertens

Hi,

You should range the analog input from 0 to 1, then execute the square root and subsequently range the result to the calculated or measured 4 and 20mA flow.

Kind regards,
David
 
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