Hi All,

You might have replied to this subject several times, but please bear with me. In era of modern DP smart transmitters do we still need 2 transmitters for 10:1 turndown?

I have a gas flow application (6" pipe line), which must have turndown of 10:1 (2000 - 20000 Kg/hr) using Orifice plate. I have done the calculation for orifice for 250mbar DP and results shows 0.6 beta.

I have two doubts.

1. As we have high accuracy DP transmitters available with 0.025% of span accuracy (As per vendor claims); do we still require two transmitters to measure the flow with 10:1 turndown? What will be the error for low flow (10% of max flow)?

2. Rosemount has launched new models '3051S ultra for flow' which Rosemount claims accuracy of 0.04% reading!!. They say this model can have 200:1 DP turndown with accuracy may be around 0.06% of reading. I believe with this single transmitter I can measure 10:1 flow without reduced accuracy.

Sorry If I am not clear with the concept of accuracy
Thanks

Dasa

 

The problem you might have to consider is that the orifice characteristics will change at low flows - Reynolds No will be less for one. If you size one transmitter for full flow, and then repeat the calculation for the same orifice at 25 % flow, the DP that results will not necessarily be 6.25 % of the original. Your DP transmitter may give stupendous accuracy, but the overall flow value may not be as good as if you used 2 transmitters.

Bruce.

 

I would say yes provided you are reading the DP from the transmitter with HART of Fieldbus so that you eliminate the errors from 4-20 mA signal

Roy