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waterhammer

  • A Proposed Guideline For Applying Waterhammer Predictions Under Transient Cavitation Conditions Part 1: Pressures and Part 2: Imbalanced Forces

    Matthew Stewart P.E., AECOM Management Services; Trey Walters, P.E., Applied Flow Technology; Greg Wunderlich P.E., AECOM Management Services; Erin A. Onat Applied Flow Technology  - Presented at the 2018 ASME PVP Conference July 16, 2018

    Part 1: Pressures

    Waterhammer analysis (herein referred to as Hydraulic Transient Analysis or simply “HTA”) becomes more complicated when transient cavitation occurs (also known as liquid column separation). While standard HTA transient cavitation models used with analysis based on the Method of Characteristics show good correlation when compared to known test/field data, the great majority of test/field data are for simple systems experiencing a single transient. Transient cavitation in more complicated systems or from two or more independently initiated transients have not been validated against data.

    Part 1 of this paper describes the various safety factors already provided by ASME B31.3 for pressure containment, provides criteria for accepting the results of HTA calculations that show the presence of transient cavitation, and makes recommendations where the user should include additional safety factors based on the transient cavitation results.

    Situations are discussed where waterhammer abatement is recommended to reduce hydraulic transient pressures and forces, and for increasing confidence in HTA results in specific cases. The result is a proposed comprehensive and pragmatic guideline which practicing engineers can use to perform waterhammer analysis and apply pressure predictions to pipe stress analysis.

    Part 2: Imbalanced Forces

    Waterhammer analysis (herein referred to as Hydraulic Transient Analysis or simply “HTA”) becomes more complicated when transient cavitation occurs (also known as liquid column separation). This complication is exacerbated when trying to predict imbalanced forces as this often involves comparing pressure times area (“PxA”) forces at two locations (for example at elbow pairs). Whereas the pressure at each elbow location has increased uncertainty because of transient cavitation, the difference in PxA forces at elbow pairs involves subtracting one potentially uncertain pressure from another uncertain pressure. Exacerbating this uncertainty yet further, the existence of vapor in a liquid system can dramatically affect the fluid wavespeed and, hence, the timing of the pressure wave travel between two locations such as elbow pairs; so the pressure calculated at each location would not actually occur at exactly the same time.

    This Part 2 discusses methods of accounting for uncertainty in HTA imbalanced force predictions due to cavitation. The criteria in this paper assume that cavitation in the HTA has been assessed and accepted per the criteria in Part 1 of this paper.

    A guideline is proposed for accepting and applying such results and, in particular, makes recommendations on safety factors to use in pipe stress analysis for different cases. The specific recommendations depend on numerous factors including:

    • Presence or absence of cavitation in hydraulically connected or isolated parts of the system
    • If cavitation occurs, whether the peak forces occur before or after cavitation first occurs
    • Size of the cavitation vapor volumes with respect to the computing volumes
    • Use of point forces as a conservative substitute in place of potentially less certain elbow pair forces or the manual assessment of maximum envelope values for the force.

    Situations are discussed where waterhammer abatement is recommended to reduce hydraulic transient forces, and for increasing confidence in HTA results in specific cases. The result is a proposed comprehensive and pragmatic guideline which practicing engineers can use to perform waterhammer analysis and apply imbalanced force predictions to pipe stress analysis.

     

     

     

     

  • Addressing Low Pressure Transients

    Addressing Low Pressure Transients

    Amy Marroquin, BLACOH Surge Control; Scott Lang, Applied Flow Technology, Presented at the ASME 2020 Pressure Vessels and Piping Conference (Virtual)

    Low transient pressures in piping systems are different in many ways to high transient pressures. While high pressures can obviously burst pipes or damage components, low pressures can collapse pipes, pull in environmental contaminants, bring components out of solution, or induce transient cavitation, a particular concern for hydrocarbon liquids. This paper will use examples of computer modeling to reveal how common system events such as pump trips or valve closures induce low-pressure transient waves that have potential to be just as destructive as more intuitive high-pressure waves.

    Fluid transient studies and literature often focus on high pressures, or do not clearly demonstrate how liquids with low vapor pressures (such as many hydrocarbons) can be affected. Even discerning a pipe’s negative pressure rating through codes and standards can be a challenge. It is shown that low-pressure transients are a potential issue in any liquid system. It is further demonstrated that “Rule of Thumb” or typical simplified calculations are not sufficient to capture these effects, and cannot be used to properly locate and size equipment.

    Download the AFT Technical Paper  

     

     

     

     

     

     

     

     

  • AFT Impulse 7 – Visualize More for Faster Solutions

    Applied Flow Technology Releases New Version of Their Waterhammer and Surge Analysis Software

    Bordeaux, France - November 14, 2018 – Applied Flow Technology President, Trey Walters, P.E., announced the release of AFT Impulse™ 7 to help engineers visualize more at the prestigious Pressure Surge Conference in Bordeaux, France where AFT joins top authorities to discuss global advancements on waterhammer and surge.

  • AFT to Present at Prestigious Pressure Surges Conference

    COLORADO SPRINGS, Colo., USA, October 12, 2018 -- Applied Flow Technology's (AFT) very own Trey Walters, P.E. and Purple Mountain Technical Group's (PMTG), Dylan Witte, will join approximately one-hundred of the world's leaders in Bordeaux, France this November for the 2018 Pressure Surges Conference

  • Pump Specific Speed And Four Quadrant Data In Waterhammer Simulation – Taking Another Look

    Pump Specific Speed And Four Quadrant Data In Waterhammer Simulation

    Trey Walters, P.E., Applied Flow Technology, Trygve Dahl, Ph.D., P.E., Intelliquip, David C. Rogers, P.E., Rogers Engineering Hydraulics, Presented at the ASME 2020 Pressure Vessels and Piping Conference (Virtual)

    In some situations, it is possible for flow to go backwards through a pump during a transient waterhammer event. Sustained reverse flow will lead to reverse rotation. Understanding and predicting the pump behavior during waterhammer under these conditions is typically accomplished using previously published four-quadrant pump data. Historically, the selection of which data to use is based on the similarity of pump specific speed. The weaknesses of using specific speed are described and an improved method of selecting appropriate four-quadrant data is given based on fundamental curve shapes for head and power in the normal operating zone.

    Download the AFT Technical Paper  Download Data File

     

     

     

     

     

     

     

     

  • When the Joukowsky Equation Does Not Predict Maximum Water Hammer Pressures

    Trey Walters, P.E., Applied Flow Technology; Robert A. Leishear, Ph. D., P. E., Leishear Engineering, LLC 

    The Joukowsky equation has been used as a first approximation for more than a century to estimate water hammer pressure surges. However, this practice may provide incorrect, non-conservative, pressure calculations under several conditions. These conditions are typically described throughout fluid transient text books, but a consolidation of these issues in a brief paper seems warranted to prevent calculation errors in practice and to also provide a brief understanding of the limits and complexities of water hammer equations.

    To this end, various issues are discussed here that result in the calculation of pressures greater than those predicted by the Joukowsky equation. These conditions include reflected waves at tees, changes in piping diameter, and changes in pipe wall material, as well as frictional effects referred to as line pack, and the effects due to the collapse of vapor pockets. In short, the fundamental goal here is to alert practicing engineers of the cautions that should be applied when using the Joukowsky equation as a first approximation of fluid transient pressures.

     

    ASME Journal of Pressure Vessel Technology,
    December 2019, Vol. 141 / 060801-1.  View the Journal Paper  Button

    Presented at the 2018 ASME PVP Conference July 16, 2018View the ASME Conference Paper

     

     

     

     

     

     

     

     

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