Gareth L. Forbes

A vibration cavitation sensitivity parameter based on spectral and statistical methods

Octave band analysis and PCA used on RMS velocity to obtain key indicators.Mahalanobis distance is used to set thresholds since data is normally distributed.Pump health separated into no cavitation, incipient, and fully formed condition.Method works on a range of types and sizes of centrifugal pumps. Cavitation is one of the main problems reducing the longevity of centrifugal pumps in industry today. If the pump operation is unable to maintain operating conditions around the best efficiency point, it can be subject to conditions that may lead to vaporisation or flashing in the pipes upstream of the pump. The implosion of these vapour bubbles in the impeller or volute causes damaging effects to the pump. A new method of vibration cavitation detection is proposed in this paper, based on adaptive octave band analysis, principal component analysis and statistical metrics. Full scale industrial pump efficiency testing data was used to determine the initial cavitation parameters for the analysis. The method was then tested using vibration measured from a number of industry pumps used in the water industry. Results were compared to knowledge known about the state of the pump, and the classification of the pump according to ISO 10816.

A Review of Machinery Diagnostics and Prognostics Implemented on a Centrifugal Pump

Centrifugal pumps are a widely used machine found in industries such as water, sewerage, oil, and gas. As a result, it is vital that these pumps are monitored, diagnosed, maintained, or replaced prior to the pump failing to reduce downtime, material, and labour costs. Most companies employ a run-to-fail method or a time-based maintenance strategy to service their pumps, instead of condition based maintenance or a predictive maintenance strategy. This paper reviews the state of art in diagnostics and prognostics pertaining to centrifugal pumps. Attention is given to detailing the methods of application, detection of fault modes and results used by researchers in the main areas of diagnostics and prognostics.

Evaluation of Slug Flow-Induced Flexural Loading in Pipelines Using a Surrogate Model

Slug flow induces vibration in pipelines, which may, in some cases, result in fatigue failure. This can result from dynamic stresses, induced by the deflection and bending moment in the pipe span, growing to levels above the endurance limits of the pipeline material. As such, it is of paramount importance to understand and quantify the size of the pipeline response to slug flow under given speed and damping conditions. This paper utilizes the results of an optimization procedure to devise a surrogate closed-form model, which can be employed to calculate the maximum values of the pipeline loadings at given values of speed and damping parameters. The surrogate model is intended to replace the computationally costly numerical procedure needed for the analysis. The maximum values of the lateral deflection and bending moment, along with their locations, have been calculated using the optimization method of stochastic perturbation and successive approximations (SPSA). The accuracy of the proposed surrogate model will be validated numerically, and the model will be subsequently used in a numerical example to demonstrate its applicability in industrial situations. An accompanying spreadsheet with this worked example is also given.

Influence of Axial Boundary Conditions on Free Spanning Pipeline Natural Frequencies

The effect of axial restraint (boundary conditions) on the natural frequency of a free spanning pipeline is examined in this paper. Theoretical calculation of the natural frequency of a straight pipeline with simple boundary conditions is a trivial task with exact solutions being available. A pipeline lying on the seabed however is neither completely straight and the interaction with the soil at the span shoulders create more complex boundary conditions. DNV-RP-F105 provides guidance on the calculation of free span boundary conditions with these increased complexities. The DNV recommended practice does not however take into account the effect of the axial restraint on the natural frequency. Results are presented in this paper for a range of axial stiffness combined with span out of straightness for a free spanning pipeline. The results presented show that the effect of axial restraint for moderately out of straight free spans can cause significant deviation in the calculation of the span natural frequency.Copyright

A single cavitation indicator based on statistical parameters for a centrifugal pump

Cavitation is one of the major problems associated with the operation of centrifugal pumps. Cavitation occurs when vapor bubbles that are formed due to a drop in pressure in the pipes upstream of the centrifugal pump implode under the added pressure within the volute of the pump. These implosions wear away the impeller, and sometimes the volute itself, which if left unchecked, would render the pump inoperable. Much research has been done in the detection of cavitation through: indicators in certain audible frequencies, drop in the net positive suction head (NPSHa), visual inspection using a transparent casing and a stroboscopic light, paint erosion inside the volute, and on the impeller, changes in pressure within the flow or volute, and vibration within certain frequency ranges. Vibration detection is deemed as one of the more difficult methods due to other structural and environmental factors that may influence which frequencies may be present during the onset of cavitation. Vibration measurement, however, is most easily measured and deployable in an automated condition monitoring scenario. It is proposed that an increasing trend in a set of statistical parameters, rather than a firm threshold of a single parameter, would provide a robust indication for the onset of cavitation. Trends in these statistical parameters were obtained from data collected on a pump forced to cavitate under several different operating conditions. A single cavitation indicator is outlined utilizing these statistical parameters that can quantify the level of cavitation in a centrifugal pump.

Characterization of Dynamic Slug Flow Induced Loads in Pipelines

The flow of a liquid mass, i.e. a ‘slug’, inside thin-walled spanning pipelines, produces a lateral traversing force. This moving force initiates dynamic stresses within the structure and is often critical when assessing structural fatigue.Moving slugs in spanning pipelines may be modeled as either a moving concentrated force or a moving mass when investigating the vibration response of the pipeline under the passage of a slug flow. The moving concentrated force model only yields accurate results when the mass of the slug is small in relation to that of the pipeline; although, the moving mass model should be used instead when the slug’s mass cannot be regarded as small in relation to the mass of the pipeline.The modeling of a moving concentrated force is much more readily implemented than that of a moving mass. Thus, the aim of this paper is to identify those situations where the simplification of considering a moving concentrated force can be made, or indeed if dynamic analysis is even required. Results are given in this paper to quantify when the two modeling techniques begin to differ significantly.It is intended that this paper will assist pipeline engineers discriminate between which appropriate conditions to use for either of the two different models of a traversing concentrated force/mass over a structure.Copyright

Computational Fluid Dynamic Analysis of a Vibrating Turbine Blade

This study presents the numerical fluid-structure interaction (FSI) modelling of a vibrating turbine blade using the commercial software ANSYS-12.1. The study has two major aims: (i) discussion of the current state of the art of modelling FSI in gas turbine engines and (ii) development of a “tuned” one-way FSI model of a vibrating turbine blade to investigate the correlation between the pressure at the turbine casing surface and the vibrating blade motion. Firstly, the feasibility of the complete FSI coupled two-way, three-dimensional modelling of a turbine blade undergoing vibration using current commercial software is discussed. Various modelling simplifications, which reduce the full coupling between the fluid and structural domains, are then presented. The one-way FSI model of the vibrating turbine blade is introduced, which has the computational efficiency of a moving boundary CFD model. This one-way FSI model includes the corrected motion of the vibrating turbine blade under given engine flow conditions. This one-way FSI model is used to interrogate the pressure around a vibrating gas turbine blade. The results obtained show that the pressure distribution at the casing surface does not differ significantly, in its general form, from the pressure at the vibrating rotor blade tip.

Characterisation of Slug Flow Conditions in Pipelines for Fatigue Analysis

Understanding the problem of slug flow induced fatigue damage is of particular importance to the reliable operation of pipelines. Slug flow across unsupported pipeline spans, pipeline crossings or vertical engineered buckle initiators, i.e. sleepers, produces dynamic motion in the pipeline resulting in cyclic fatigue stresses. In some cases, the dynamic effects will cause the pipeline to fail at a point of stress concentration. In other cases, however, these effects may be negligible. The current literature provides no guidance as to when the dynamic effects of slug flow must be considered. This paper gives guidance and describes how fatigue due to slug flow in pipelines, which would normally require dynamic analysis, can be quantified using simplified quasi-static analysis. The paper also presents a design process which could be used by pipeline engineers to determine the level of analysis needed, before embarking on more complex and expensive dynamic finite element.Copyright

Cavitation Sensitivity Parameter Analysis for Centrifugal Pumps Based on Spectral Methods

Cavitation is a major problem facing centrifugal pumps in industry today. Unable to constantly maintain operating conditions around the best efficiency point, centrifugal pumps are subject to conditions that may lead to vaporisation or flashing in the pipes upstream of the pump. The implosion of these vapour bubbles in the impeller or volute causes damaging effects to the pump. A new method of cavitation detection is proposed in this paper based on spectral methods. Data used to determine parameters were obtained under ideal conditions, while the method was tested using industry acquired data. Results were compared to knowledge known about the state of the pump, and the classification of the pump according to ISO 10816.

Experimental Power Harvesting from a Pipe Using a Macro Fiber Composite (MFC)

Piezoelectric material can be used to transform ambient vibration energy into small amounts of electrical power. As piezoelectric materials have been developing, such as lead-zirconate-titanate (PZT), Quick Pack (QP), macro fiber composite (MFC) and polyvinylidineflouride (PVDF), many researchers have been investigating their applications. This research investigates the possibility of electrical energy being generated by a macro fiber composite (MFC) patch mounted onto the surface of a pipe structure. In addition to the application of MFC, this research investigates the effect of varying load impedance to; the resonance frequency, voltage and electrical power generated by the structure. The results presented are for a large structure where most previous research on power harvesting has been undertaken on much smaller structures.