Meter Max For Orifice

<meter max>

The maximum differential pressure at the orifice? It determines the maximum flowrate that the orifice flowmeter can measure.

<why we are designing orifice at meter max?>

Because flowrates over 100% will simply indicate 100%; there is no more range available for the meter to indicate flows above 100% of its rated range.

We don't usually know the maximum possible flow in the system. In order to guarantee that our design will deliver the highest flow we want, we have a margin of safety that will usually translate into extra capacity, especially when a piping system is new.

Typically the margin between the maximum process flow and the meter maximum is between 10 and 25%, depending on how accurately you can predict the flowrate. You don't want to have the meter normally operating at the upper limit of its range because any variation in the flow on the high side "pegs" the meter and you have no indication of the flowrate.

Before the advent of flow computers, the engineer designed an orifice meter based on maximum flow that could be developed. Then rounded it up to an even or decimal value. For example if the maximum rate were 12650 GPM, then the maximum flow would be set rounded at 15000 GPM. Then they would size the orifice meter that would generate this flow at the Dp transmitters maximum, say 100 in H2O. The plate might end up as a 6.1057 Inch in a 10" line. Now then the primative electronics only had to take the squareroot of the Dp times 1500 to get the GPM (square root of 100 is 10, 10 times 1500 is 15000 GPM.

Today with computers behind the scenes you can put in a 6 inch plate and span the Dp to 0 to 150 in H2O and the computer will take care of it all.

An Orifice Plate is a primary element metering device. Depending on what you're metering they are designed to match your full spectrum of meter capacity.