Transient Pressure Loss in Compressor Station Piping Systems

“Dynamic pressure loss” is often used to describe the added loss associated with the time varying components of an unsteady flow through a piping system in centrifugal and reciprocating compressor stations. Conventionally, dynamic pressure losses are determined by assuming a periodically pulsating 1-D flow profile and calculating the transient pipe friction losses by multiplying a friction factor by the average flow dynamic pressure component. In reality, the dynamic pressure loss is more complex and is not a single component but consists of several different physical effects, which are affected by the piping arrangement, structural supports, piping diameter, and the level of unsteadiness in the flow stream. The pressure losses due to fluid-structure interactions represent one of these physical loss mechanisms and are presently the most misrepresented loss term. The dynamic pressure losses, dominated at times by the fluid-structure interactions, have not been previously quantified for transient flows in compressor piping systems. A number of experiments were performed by SwRI utilizing an instrumented piping system in a compressor closed loop facility to determine this loss component. Steady and dynamic pressure transducers and on-pipe accelerometers were utilized to study the dynamic pressure loss. This paper describes findings from reciprocating compressor experiments and the various fluid modeling studies undertaken for the same piping system. The objective of the research was to quantitatively assess the individual pressure loss components which contribute to dynamic pressure (non-steady) loss based on their physical basis as described by the momentum equation. Results from these experiments were compared to steady state and dynamic pressure loss predictions from 1-D and 3-D fluid models (utilizing both steady and transient flow conditions to quantify the associated loss terms). Comparisons between the fluid model predictions and experiments revealed that pressure losses associated with the piping fluid-structure interactions can be significant and may be unaccounted for by advanced 3-D fluid models. These fluid-to-structure losses should not be ignored when predicting dynamic pressure loss. The results also indicated the ability of an advanced 1-D Navier Stokes solution at predicting inertial momentum losses. Correspondingly, the three-dimensional fluid models were able to capture boundary layer losses affected by 3-D geometries.