Proper differential pressure measurement

This is my first column for 2009, but rather than begin the year by discussing some new or innovative flow measurement device, I am going to revisit an old concept that still seems to be misunderstood by some – the differential pressure measurement for orifice flow meters. Differential pressure across the orifice plate is one of several measured parameters used in the equation for calculating volumetric flow rate. If the differential pressure is not measured correctly, a bias error will result in the calculated flow rate. Following are some of the potential pitfalls to be concerned about.

Orifice flow meter basics

Orifice flow meters used for custody transfer applications are typically constructed in accordance with dimensional specifications outlined in one of several applicable industry standards, such as Chapter 14.3 of the American Petroleum Institute Manual of Petroleum Measurement Standards or American Gas Association (AGA) Report No. 3. It is assumed that if an orifice meter meets all of the dimensional specifications, it will correctly measure flow rate. However, that is not always the case.

One of the parameters in the equation used to calculate flow rate is the discharge coefficient, Cd, which is an experimentally determined value. Thousands of flow tests were run over a range of operating conditions on a number of different orifice meters to establish the discharge coefficient value. As long as an orifice meter is operated within the range of operating conditions used to determine the discharge coefficient, it should provide a reasonably accurate measure of flow rate. If operated outside those bounds, the meter may produce measurement errors. Differential pressure is one of the parameters that must be within acceptable bounds if the meter is to produce accurate results. If the differential pressure is too low or too high, it will introduce an error in the flow rate measurement. Too high and the aerodynamic load on the orifice plate can cause it to deform, changing the bore dimension and shape, which will cause an error in the flow rate determination. There is an AGA Engineering Technical Note that provides detailed guidance on dealing with high differential pressure (i.e., over 100 inches of water column differential). Too low and the flow field dynamics near the orifice plate (e.g., turbulence structure, recirculation zones, etc.) change significantly, altering the relationship between the differential pressure and the flow rate, which, too, will cause an error in the flow rate determination.

Low differential pressure concerns

The performance of today’s state of the art differential pressure transducers is generally quite good. Performance varies from brand to brand and model to model, with some transducers having the ability to measure accurately a differential pressure as low as an inch of water column or, in some instances, even less. However, just because a transducer can accurately measure a differential pressure on the order of an inch of water column does not mean that an orifice meter differential pressure of this magnitude will yield an accurate flow rate. In fact, past research (by George and Morrow) suggests that in order to achieve an accurate flow rate measurement from an orifice meter, the differential pressure may need to be at least 15 to 20 inches of water column. More definitive research on this subject is currently ongoing in a project funded by the Pipeline Research Council International. It is important to note that nearly all of the flow tests run to experimentally determine the orifice discharge coefficient had a differential pressure of over 10 inches of water column to avoid the problems associated with low differential pressure.

Proper pressure transducer calibration

One lesser-known operational characteristic of most differential pressure transducers that is critical to their proper use with orifice flow meters is the effect of line pressure on the transducer calibration. This effect is often referred to as the common mode pressure effect. The calibration of most differential pressure transducers changes as the line pressure imposed on the transducer changes. That is, if a transducer is calibrated at a line pressure of zero gauge (i.e., atmospheric pressure) and then placed on an orifice meter on a pipeline operating at, say, 500 psig, the calibration of the differential pressure transducer is likely to be different or is said to have “shifted.” If not accounted for properly, this shift in transducer calibration will introduce a bias error in the flow rate.