For many years, the natural gas industry has used transmitters to measure differential pressure (DP) in conjunction with electronic orifice meters. Inherently, these devices may not maintain a stable zero and can transmit false readings as actual flow. The "no-flow cutoff" setting in a flow computer can eliminate this effect by zeroing all readings below the cutoff value.
Careful consideration must be taken when setting the no-flow cutoff to avoid causing additional measurement error. False flow will be recorded if the value is set too low and real flow can inadvertently be zeroed out if it is set too high. Further, these measurement errors are magnified when combined with an oversized orifice plate.
In 2013, the API MPMS (Manual of Petroleum Measurement Standards), Chapter 21.1 ("Flow Measurement Using Electronic Flow Metering Systems – Electronic Gas Measurement"), was updated for its recommendations related to setting the no-flow cutoff. By this standard, the cutoff value should be set to 0.25% of the DP transmitter’s calibrated range, not to exceed 0.5 inches. However, it is crucial to also consider the relevant measurement conditions since the optimal cutoff value may vary from this recommended setting. The measurement conditions may include:
- Free liquids in the meter tube;
- Low flowing wells; and
- Well operations with plunger lifts or intermitters.
Regardless of the measurement conditions, a cutoff value of zero will potentially result in recorded false flow and inaccurate flow times.
While a no-flow cutoff setting can prevent errors related to transmitter-drift by zeroing readings below the cutoff value, careful consideration must be given when setting the cutoff value to prevent additional measurement error from being introduced.
Determining where to set the cutoff value can be difficult, especially since it can change over time. Industry standards and measurement conditions should both be considered when establishing the optimal setting. For additional information or guidance on setting your no-flow cutoff, be sure to consult with your company’s measurement experts, whether in-house or third-party.
"Sometimes you simply need to drift aimlessly, in order to reach the destination of aim."
~ Lionel Suggs, American author