Pipe Flow Conditions

The most important-and most difficult to measure-aspects of flow measurement are flow conditions within a pipe upstream of a meter. Flow conditions refer to: the gas velocity profile, irregularities in the profile, varying turbulence levels within the velocity or turbulence intensity profile, swirl and any other fluid flow characteristics which will cause the meter to register flow different than that expected. This will cause the meter to differ from the original Calibration State referred to as reference conditions that are free of installation effects.

Installation effects which cause flow conditions within the pipe to vary from reference conditions are: insufficient straight pipe, exceptional pipe roughness or smoothness, elbows, valves, tees and reducers, just to name a few. Certainly, a common understanding of how these installation effects impact the meter is important since devices which create upstream installation effects are common components of any standard metering design. Flow Conditioning refers to the process of artificially generating a reference, fully-developed flow profile and is essential to enable accurate measurement while maintaining a cost-competitive meter standard design.

Industry-accepted nomenclature and discussions are presented which explain commonly referred to flow conditions.

The most commonly used description of flow conditions within the pipe is the velocity flow profile. For general fluid dynamic background Miller (1996) offers a thorough textbook description of velocity profiles and distortions of the profile due to upstream piping effects.
Equation 1 describes the shape of the velocity flow profile. The value of n determines the shape of the velocity flow profile. Karnik (1993) and others use Equation I to determine the flow profile's shape within the pipe by fitting a curve to experimentally measured velocity data. Karnik (1993) was the first to actually measure transverse velocities within the high-pressure natural gas environment using hot wire technology to accomplish the data fit.
A fully developed flow profile is used as the Reference State for meter calibration and determination of Coefficient of Discharge (Cd). For Reynolds Number 105 to 106 n is approximately 7.5; for Re of 106, n is
approximately 10.0 where a fully developed profile in a smooth pipe is assumed.

Since n is a function of Reynolds Number and friction factor, more accurate values of n can be estimated by using
where f is the friction factor. It is not the intent here to provide detailed instructions for determining friction factors. The Colebrook (1939) equation or Moody (1944) diagram can be utilized as illustrated and detailed by Karnik (1993).

A good estimate of a fully developed velocity flow profile can be used for those without adequate equipment to actually measure the velocities within the pipe. White (1986) and Karnik (1993) utilize the following straightpipe-equivalent length to ensure a fully developed flow profile exists.