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Why does the orifice plate have a 3:1 turndown ratio in practice?
Using a flow orifice plate as a flow element in differential type flow meters, it is a common practice to limit use to a 3:1 to 4:1 turndown ratio. Where does this common practice come from?
Orifice plate turndown ratio
The limit of DP type flow meters using orifice does not come from the beta ratio limit of the flow orifice plate nor any other flow orifice plate dimensions. The limitation comes from reading error due to the non-linear relationship between the differential pressure in the flow orifice plate and the calculated flow rate.
In simple concept we can calculate orifice flow rate:
Qm = C x (Differential Pressure)^0.5
Qm: Flow Rate
C: meter constant
Differential pressure: Differential pressure in the flow orifice plate
From the above equation, we know that the flow rate is directly proportional to the square root of the differential pressure. This non-linear relationship produces a significant error in the lower current when the turndown ratio is greater than 3:1.
Basically the differential pressure transmitter can itself have a turndown ratio of up to 100:1 but the square root extraction limits its capability only in the upper limit.
See the calculation example below which shows an increase in the error reading when the turndown ratio is below 3:1.
Orifice Flow Meter Error Reading Calculation Example
From the above example, we know that the % error reading (after calculating the square root above) is increasing as the flow rate decreases. The % error reading is greater than 0.5% when the flow rate is less than 2500 l/m.
Set this flow rate as our minimum flow rate because the flow rate measurement has high error below this point. If we look further, 2500 l/m min flow and 7500 l/m max flow correspond to 3:1 turndown ratio. This is where the 3:1 turndown ratio comes into play in our common practices.
If we put thrust at a minimum flow, say 1000 l/m, we will have a turndown ratio of 7.5:1. This will lead us to an error reading of 2.8% which means very poor measurement accuracy.
Thus some engineers usually use a 3:1 turndown ratio as their rule of thumb to decide whether or not they will use a DP type flow orifice plate flow meter.
If the required turn down is more than 3:1, it is better to use another type of flow meter even though such a high turn down ratio is possible by installing a parallel differential pressure type flow orifice plate flow meter. Split range configuration.
I have no reason about it, except that the parallel configuration would give us a higher cost of material procurement.
Advantages of orifice plates
- Low cost
- Can be used in wide range of pipe sizes (3.175 to1828.8mm)
- Can be used with differential pressure devices
- Well known and predictable characteristics.
- Available in many materials.
Disadvantages and limitations of Orifice plates
- Cause relatively high permanent pressure loss.
- Tend to clog, thus reducing use in slurry service.
- Have square root characteristics.
- Accuracy dependent on care during installation.
- Changing characteristics because of erosion, corrosion and scaling.