Hydraulic Transients Research Articles

A new method to calculate hydraulic transients in HDPE pipes using the standard solid model to represent the HDPE viscoelastic behaviour

In this paper, a method for the calculation of hydraulic transients for pressurized flow in high density polyethylene (HDPE) pipes is developed. The proposed method incorporates the Standard Solid Model in the Method of Characteristics (MOC) to represent the viscoelastic behaviour of the pipe material. The Standard Solid Model is able to represent both a short-term elastic response and a long-term viscoelastic response. This model only requires the calibration of two parameters in a test facility, what is an advantage over other methods reported in the literature. The paper present how these parameters are calibrated. Keeping constant the wave velocity and the friction factor, the method estimates with high precision both the extreme values and the damping time of the pressure due to a water hammer in HDPE pipes. The results are compared against the data gathered over tubes with PPI 4710 resin (equivalent to PE100) in the test facility of a HDPE pipe producing plant, located in San Luis Potosí, México. The same procedure can be used to collect data of pipes made with other viscoelastic material. The main contribution of the paper is to avoid the need to perform calibration measurements during the start-up of the hydraulic systems.

Numerical investigation of filling transients in small-scale pipelines with submerged outlet

An elastic model that combines the dead-end and the orifice at the downstream end is proposed in order to study the hydraulic transients during the rapid filling in small-scale pipelines with submerged outlet. In this model, the unsteady water flow is solved by the method of characteristics, the water-air interface is traced by a shock fitting method, and the air phase is controlled by the polytropic law. The model is validated by a filling test in a pipeline with orifice. Then the proposed model is applied to the filling transient analysis in a small-scale pipeline with submerged outlet. Finally, the effects of the submerged depth of the downstream reservoir on the filling transients are analyzed. This model provides a reasonable approximation of the submerged outlet boundary condition in small-scale pipelines.

A novel demonstration of adhered material mobilisation by hydraulic transients

Laboratory experiments have shown that pipe-wall adhered material can be mobilised by transients, where steady-state hydraulic conditions cannot. This process is critical to managing water quality risks in drinking water distribution systems.

SIMULATION OF HYDROCARBON PIPELINE LEAK DETECTION USING THEORETICALLY APPROXIMATED MODEL ON A TRANSIENT-BASED METHOD

Detection and localization of small leaks in pipelines were early developed using a nonlinear adaptive state observer and a special correlation technique, based on pressure and flow measurements at the pipeline inlet and outlet. Simulations and experiments show the results for a gas and a liquid pipeline. This paper x-ray pipeline leak Identification using a transient-based method on hydraulic transients of a brake system with oil at two different temperatures, Leak effects at different frequency response were Simulated, measured and leak detection experiment with MATLAB code were also highlighted. The result obtained show that in certain circumstances the brake response is slow due to the temperature and suggests an appropriate pipe diameter to overcome this problem of low flow rate, the transfer function obtained using the proposed method were able to measure the presence of a leak on the pipe system. Simulations, measurements and leak experiments have shown that the early detection and localization of small leaks in liquid and gas pipelines can be considerably improved using theoretically approximated model as the required computational effort is relatively small, as compare to microcomputers which can be used for leak detection including monitoring of the inner pipeline state.

GPU Acceleration of Hydraulic Transient Simulations of Large-Scale Water Supply Systems

Simulating hydraulic transients in ultra-long water (oil, gas) transmission or large-scale distribution systems are time-consuming, and exploring ways to improve the simulation efficiency is an essential research direction. The parallel implementation of the method of characteristics (MOC) on graphics processing unit (GPU) chips is a promising approach for accelerating the simulations, because GPU has a great parallelization ability for massive but simple computations, and the explicit and local features of MOC meet the features of GPU quite well. In this paper, we propose and verify a GPU implementation of MOC on a single chip for more efficient simulations of hydraulic transients. Details of GPU-MOC parallel strategies are introduced, and the accuracy and efficiency of the proposed method are verified by simulating the benchmark single pipe water hammer problem. The transient processes of a large scale water distribution system and a long-distance water transmission system are simulated to investigate the computing capability of the proposed method. The results show that GPU-MOC method can achieve significant performance gains, and the speedup ratios are up to hundreds compared to the traditional method. This preliminary work demonstrates that GPU-MOC parallel computing has great prospects in practical applications with large computing load.

Formulation of consistent finite volume schemes for hydraulic transients

ABSTRACTIntentionally generated transient waves are used in the detection of defects in pipelines. Experience to date in these applications shows that the wider the band of injected wave frequencies the better is the detection effectiveness. Thus, as these technologies develop further, models that can handle high frequency waves will be required. Finite volume (FV) methods are well suited for high frequency wave phenomena, but their use in the water hammer field has been limited to date compared with other fields such as open channel flow. A major reason for this is that FV methods are formulated for simple boundary conditions such as no-flux or no-slip, but have not been well developed for the typically more complex boundary conditions found in pressurized pipeline systems (e.g. junctions, control valves, orifices, tanks and reservoirs). In those few instances in the literature where FV has been applied to water hammer problems, the approach has been to use FV for internal sections and method of characteristics (MOC) at the boundaries. The global order of accuracy of FV–MOC is governed by the MOC solution. To our knowledge, this paper constitutes a first attempt at handling boundary conditions within the FV framework. The approach places the boundary element within a FV to enforce mass and momentum conservation within this volume. The fluxes between the FV and the adjacent elements are then formulated in the usual manner. The approach is illustrated for the case of a valve, a reservoir and a junction. The finite volume method used is the Boltzmann-type scheme. The accuracy and efficiency of all schemes with the proposed non-iterative FV formulation of the boundary conditions are demonstrated through the following test cases: (i) water hammer wave interactions with a junction boundary characterized by a geometric discontinuity, (ii) water hammer wave interactions with a junction boundary characterized by a discontinuity in the value of wave speed, and (iii) water hammer wave interactions with a junction characterized by a flow rate discontinuity. The pure FV formulation guarantees mass and momentum conservation. The Boltzmann-based FV schemes capture discontinuity as well as wave interactions with boundary elements accurately. The stability of the proposed FV schemes is satisfied when .

Implementation and Validation of a Free Open Source 1D Water Hammer Code

This paper presents a free code for calculating 1D hydraulic transients in liquid-filled piping. The transient of focus is the Water Hammer phenomenon which may arise due to e.g., sudden valve closure, pump start/stop etc. The method of solution of the system of partial differential equations given by the continuity and momentum balance is the Method of Characteristics (MOC). Various friction models ranging from steady-state and quasi steady-state to unsteady friction models including Convolution Based models (CB) as well as an Instantaneous Acceleration Based (IAB) model are implemented. Furthermore, two different models for modelling cavitation/column separation are implemented. Column separation may occur during low pressure pulses if the pressure decreases below the vapour pressure of the fluid. The code implementing the various models are compared to experiments from the literature. All experiments consist of an upstream reservoir, a straight pipe and a downstream valve.

Wave tracking method of hydraulic transients in pipe systems with pump shut-off under simultaneous closing of spherical valves

Abstract The limited wave propagation speed in water permits the propagation of pressure surges in hydraulic transients to be tracked in the time series. This occurs by tracking the primary (F) and the reflected (f) waves. The wave tracking method has been demonstrated to be applicable to highly complex hydraulic systems. The wave propagation induced by passing through a T-junction in the flow complies with the wave conservation laws which concern both the wave reflection and the transmission. It has been shown that all computations can be simply implemented in a computation tool like MS Excel. This considerably ensures both the accuracy and the reliability of computations. At a hydraulic system of Pelton turbines, excellent computational results have been achieved. The application to a highly complex hydraulic system with pump shut-off and the simultaneous closing of a spherical valve also showed excellent results, when compared with the field measurements. The key point in the presented computations is to unify the characteristics of the pump and the spherical valve. The entire computation covers the flow rate in the system, the reverse flow through the pump, the pressure response, the deceleration of the rotational speed and the system oscillations.

Unsteady Pressures Influenced by Trapped Air Pockets in Water-Filled Pipelines

Trapped air pockets may cause severe operational problems in water-filled pipelines. This paper investigates the dynamic behaviour of a single trapped air pocket. A single air pocket creates distinct changes of amplitude, shape and timing of unsteady flow pressure waves when it is located at some point in a pipeline. The severity of the resulting hydraulic transients depends on the size, pressure and position of the trapped air pocket. In this paper, the air pocket is incorporated as a boundary condition in the discrete gas cavity model (DGCM) that also considers the effects of unsteady skin friction. Two distinct case studies are presented: (1) start-up test case (flow starting from rest) and (2) shut-down test case (flow stoppage). The start-up test case has been performed in the University of Montenegro pipeline apparatus (length 55 m, internal diameter 18 mm). A trapped air pocket is confined at the downstream end of the pipeline. The transient event is initiated by rapid opening of a valve positioned at the initial air/water interface. The shut-down test case has been carried out in the University of Adelaide laboratory apparatus (length 37 m, internal diameter 22 mm). A trapped gas pocket is maintained near the midpoint of the pipeline. The shut-down event is initiated by rapid closure of the downstream-end valve. Results of numerical simulations and laboratory investigations are presented and they show profound effects of unsteady skin friction on pressure histories.

1-D MOC simulation software for hydraulic transients: TOPsys

The development of hydropower in China has entered the gold period nowadays. Numerous challenging hydropower projects have been presented to researchers and scholars. In order to adapt to this trend and the need of the nation, TOPsys, an one dimensional simulation software based on the method of characteristics (MOC) for hydraulic transients, has been developed in State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University recently. This software has been employed for transient flow simulations of more than thirty hydropower stations in China and overseas. The whole paper is organized as follows: first, basic mathematical theory of the software was introduced. Subsequently, several typical engineering applications, including conventional power stations and pumped storage power stations, were given for validation. Finally, the versatility as well as the expandability of this software was shown with several special engineering examples.

Numerical analysis of the hydraulic transient in a closed pipe in presence of a bypass valve

ABSTRACT Hydraulic transients generated when sudden changes in flow rates occur in a pressurized pipeline system were and are still today a crucial technical aspect of their operation. This is a common concern of hydraulic engineers aiming to reduce pressure waves in order to limit their related effects onto the hydraulic system components. From this perspective, the presence of a bypass system is recognized as useful to reduce the maximum water hammer overpressure during a pipeline closure; however, the sizing of bypass system is usually heuristic. This paper investigates the sizing procedure for a bypass system (i.e. pipe and valve) mainly considering the overpressure reduction values as dependent on some typical parameters for such pipe systems. A numerical model is used to investigate the efficiency of a bypass system in reducing the maximum water hammer overpressure, showing that an optimally designed bypass system allows to completely close a pipe in a lower time simultaneously saving the integrity of existing devices. Results show that the optimal diameters for a bypass valve is comprised in the range of 20%–40% of the main pipe diameter. In this way, a bypass system allows an overpressure reduction higher than 70%, which is important for faster manoeuvres.

Numerical methods for hydraulic transients in visco-elastic pipes

In technical applications involving transient fluid flows in pipes, the convective terms of the corresponding governing equations are generally negligible. Typically, under this condition, these governing equations are efficiently discretized by the Method of Characteristics. Only in the last years the availability of very efficient and robust numerical schemes for the complete system of equations, such as recent Finite Volume Methods, has encouraged the simulation of transient fluid flows with numerical schemes different from the Method of Characteristics, allowing a better representation of the physics of the phenomena.In this work, a wide and critical comparison of the capability of Method of Characteristics, Explicit Path-Conservative (DOT solver) Finite Volume Method and Semi-Implicit Staggered Finite Volume Method is presented and discussed, in terms of accuracy and efficiency. To perform the analysis in a framework that properly represents real-world engineering applications, the visco-elastic behaviour of the pipe wall, the effects of the unsteadiness of the flow on the friction losses, cavitation and cross-sectional changes are taken into account. Analyses are performed comparing numerical solutions obtained using the three models against experimental data and analytical solutions. In particular, water hammer studies in high density polyethylene pipes, for which laboratory data have been provided, are used as test cases. Considering the visco-elastic mechanical behaviour of plastic materials, 3-parameter and multi-parameter linear visco-elastic rheological models are adopted and implemented in each numerical scheme. Original extensions of existing techniques for the numerical treatment of such visco-elastic models are introduced in this work for the first time. After a focused calibration of the visco-elastic parameters, the different performance of the numerical models is investigated. A comparison of the results is presented taking into account the unsteady wall-shear stress, with a new approach proposed for turbulent flows, or simply considering a quasi-steady friction model. A predominance of the damping effect due to visco-elasticity with respect to the damping effect related to the unsteady friction is confirmed in these contexts. Moreover, all the numerical methods show a good agreement with the experimental data and a high efficiency of the Method of Characteristics in standard configuration is observed. Finally, three Riemann Problems are chosen and run to stress the numerical methods, taking into account cross-sectional changes, more flexible materials and cavitation cases. In these demanding scenarios, the weak spots of the Method of Characteristics are depicted.

Joint-analysis of single and cross-borehole hydraulic pressure transients to characterize the hydraulic properties and connectivity of fractured networks

Joint-analysis of single and cross-borehole hydraulic pressure transients to characterize the hydraulic properties and connectivity of fractured networks

Prediction Method for the Complete Characteristic Curves of a Francis Pump-Turbine

Complete characteristic curves of a pump-turbine are essential for simulating the hydraulic transients and designing pumped storage power plants but are often unavailable in the preliminary design stage. To solve this issue, a prediction method for the complete characteristics of a Francis pump-turbine was proposed. First, based on Euler equations and the velocity triangles at the runners, a mathematical model describing the complete characteristics of a Francis pump-turbine was derived. According to multiple sets of measured complete characteristic curves, explicit expressions for the characteristic parameters of characteristic operating point sets (COPs), as functions of a specific speed and guide vane opening, were then developed to determine the undetermined coefficients in the mathematical model. Ultimately, by combining the mathematical model with the regression analysis of COPs, the complete characteristic curves for an arbitrary specific speed were predicted. Moreover, a case study shows that the predicted characteristic curves are in good agreement with the measured data. The results obtained by 1D numerical simulation of the hydraulic transient process using the predicted characteristics deviate little from the measured characteristics. This method is effective and sufficient for a priori simulations before obtaining the measured characteristics and provides important support for the preliminary design of pumped storage power plants.

Development of Multiple Leakage Detection Method for a Reservoir Pipeline Valve System

The detection of multiple leakages in pipeline systems has been one of the challenging issues for the control of water loss in water distribution systems. Inverse transient analysis can be a useful principle for predicting leakage through the calibration of location and leakage quantity, based on the pressure reflection that originates from an abnormal boundary condition. In this study, an innovative leakage detection method is proposed to address unknown conditions on multiple leakage dimensions through introduction of revised leakage expressions based on a frequency domain approximation. A multiple leakage function was modified for an efficient representation of multiple abnormalities at a reservoir pipeline valve system. An iterative metaheuristic scheme (IMS) was designed to handle an optimization scheme for multiple leakages using a pressure response for a discharge impulse introduced through value manipulation. In order to address unsteady friction in hydraulic transients combined with multiple leakages, both one-dimensional and two-dimensional models were used to derive leakage expressions for turbulent and laminar flow conditions. An isolated multiple leakage function (IMLF) was proposed to exclusively encapsulate the impact of leakages and unsteady fiction. Considering uncertainties in the hydraulic transient propagation, data noise, and multiple local optima issues in large parameter calibrations, three advanced schemes were modularized to improve detectability of IMS. Several hypothetical examples were presented to show the potential of IMS, validity of three advanced schemes, and robustness in multiple leakage prediction compared to existing approaches.

Formulation of a fully implicit numerical scheme for simulation of two-phase flow in a vertical channel using the Drift-Flux Model

This paper introduces a new fully implicit numerical scheme to simulate two-phase flow in a vertical channel. Two-phase flow is widely associated with the nuclear systems and therefore accurate prediction of the void fraction as a key thermo-hydraulic parameter is crucial in operational and safety analysis. The Drift-Flux Model (DFM) is applicable to a wide range of thermal-hydraulic phenomena in an integral pressurized and boiling water reactors. However, to obtain an accurate and stable solution is a challenge task for successful application of the model. The trend toward the more accurate simulation requires advanced numerical methods. The efficiency of the recently develpoed Jacobian-free Newton-Krylov (JFNK) method as an advanced fully implicit solver have been demonstrated in the published literature. However, there are still unsolved problems with this method. Therefore, a much simpler, more reliable and a fast fully implicit model is developed as a variant of Newton method. The objective of the present study is to investigate the validity, accuracy and CPU usage of the proposed model. Also, good agreement was found between our results and those of the experimental data and the RELAP5 code. Comparison of our results with those of JFNK proved viable for handling thermo hydraulic transients without stability constraints. It is simpler, more accurate and faster than the JFNK method.

Experimental and Numerical Analysis of Hydraulic Transients in the Presence of Air Valve

This paper focuses on the hydrodynamic behavior of liquid pipelines in the presence of air valves. The aim of installing such valves on pipeline systems is twofold: on one hand, they release the accumulated air content and on the other hand, they allow the outer air to entrain the system when the internal pressure falls beneath ambient pressure (i.e. vacuum formation). We study the hydraulic transients in the presence of such air valve by means of experimental and numerical methods. The experiments were performed using a special test section where the pipeline close to the air valve was built from plexiglas allowing visual access. Furthermore, pressure signals were recorded at several locations of the pipeline. Numerical analysis was also performed using the commercially available CFD software (ANSYS CFX). It was revealed that the entrained air separates well from the primary liquid and the mixture behaves as two distinct phases.

Characterizing the impact of model error in hydrologic time series recovery inverse problems

Hydrogeologic models are commonly over-smoothed relative to reality, owing to the difficulty of obtaining accurate high-resolution information about the subsurface. When used in an inversion context, such models may introduce systematic biases which cannot be encapsulated by an unbiased "observation noise" term of the type assumed by standard regularization theory and typical Bayesian formulations. Despite its importance, model error is difficult to encapsulate systematically and is often neglected. Here, model error is considered for a hydrogeologically important class of inverse problems that includes interpretation of hydraulic transients and contaminant source history inference: reconstruction of a time series that has been convolved against a transfer function (i.e., impulse response) that is only approximately known. Using established harmonic theory along with two results established here regarding triangular Toeplitz matrices, upper and lower error bounds are derived for the effect of systematic model error on time series recovery for both well-determined and over-determined inverse problems. A Monte Carlo study of a realistic hydraulic reconstruction problem is presented, and the lower error bound is seen informative about expected behavior. A possible diagnostic criterion for blind transfer function characterization is also uncovered.

Mechanism of air-trapped vertical vortices in long-corridor-shaped surge tank of hydropower station and their elimination

The air-trapped vertical vortices (ATVVs) are easy to form above the throttled orifices in the widely used long-corridor-shaped surge tanks (LCSSTs), when the tank water level decreases rapidly during hydraulic transients. These ATVVs may jeopardize the operation safety of the hydropower stations and should be avoided. This study elucidates the formation mechanism of the ATVVs and proposes some simple measures to eliminate them. The 3-D CFD model for predicting the ATVVs is validated first by physical model tests in a model tailrace LCSST, and then the formation mechanism is analyzed based on the numerical results. It is shown that the main influence factor for the ATVVs is the critical submergence, which can be reduced by minimizing the velocity circulation around the throttled orifices. Two practical ATVV elimination measures through suppressing the velocity circulation are compared and verified, and the optimized one is recommended.

Theoretical Analysis of Gas Release during Transient Process of Hydraulic Transients

In the process of hydraulic transient, Cavities water hammer is more serious than the traditional water hammer, the gas content and gas release in the water is very important to establish the simulation model. Through the theoretical analysis of the gas release during the transient process of the hydraulic transient, there are two important conclusions are drawn: 1) the critical point is the boundary of the existence of bubbles. 2) the critical pressure p* is always less than the water vaporization pressure PV, and the bubble radius of RO is greater, the critical pressure is closer to the vaporization pressure of water. Put the water vaporization pressure constant value for the constant pressure of the drying cavity is consistent with the actual, served as a moment of containing gas rate calculation method. Finally it puts forward the consideration of gas release water hammer mathematic model.