Abstract
Recent studies have proved that the utilization of polyethylene (PE) short-section or penstock is a promising water hammer control tool. However, the interplay between the magnitude attenuation and the phase offset of pressure-wave oscillations remains challenging. This study aimed at inspecting the capacity of a dual PE penstock/short-section-based control technique, with regard to the aforementioned interplay. In this technique, a PE penstock was lumped to the transient initiating zone of the main pipe and a short-section of the counter extremity of the pipe was replaced with PE. The transient pressure-wave behavior in a gravitational viscoelastic pipe involving cavitation was described by the extended 1D water hammer equations embedding the Vitkovsky and Kelvin–Voigt add-ons. The numerical solution was performed by the fixed grid method of characteristics. The high- (HDPE) and low-density (LDPE) were demonstrated in this study. Analysis revealed that upgrading techniques based on LDPE enabled a desirable tradeoff between the magnitude attenuation and the phase offset of pressure-wave oscillations. Particularly, the dual penstock/short-section specific upgrading technique allowed a more important attenuation magnitude of pressure peak (or crest), and led to a similar expansion of the wave oscillation period. Furthermore, results evidenced that the proposed technique outperformed the renewal of the original piping system.
HIGHLIGHTS
A dual polyethylene (PE) penstock upstream and a PE inline short-section downstream are addressed to palliate cavitation.
The rheological mechanical behavior of PE is modeled referring to the generalized linear Kelvin–Voigt approach.
High- and low-density PE are utilized for the short-section or penstock.
The tradeoff between the wave magnitude attenuation and the wave period relaxation is inspected.
INTRODUCTION
Currently, polyethylene (PE) materials are increasingly used for hydraulic piping systems thanks to their low cost, easy processing, lightweight, and high corrosion resistance. In addition, these materials have the potential capacity of dampening high- (and low-) pressure loads. The physical explanation for this property relies on the viscoelastic behavior of these materials which has dissipative and dispersive effects on the pressure wave (Ferry 1970; Güney 1983; Brinson & Brinson 2008; Tjuatja et al. 2023). These effects, which are similar to unsteady friction losses, result in pressure-wave oscillation pattern characterized by a small magnitude and a high oscillation period. Thereupon, based on the viscoelastic property of PE, several design measures were addressed in recent literature as alternatives to classical ones.
Within this framework, this paper initially gives a brief overview of the fixed grid-based method of characteristics (FG-MOC) used for the numerical discretization of the extended one-dimensional water hammer equations (E-1D-WH Eqs), then the proposed control design tool is studied and analyzed for a test case involving cavitation and, finally, a summary and conclusions are provided.
METHODOLOGY
It is worth noting that the one-phase water hammer solution (9) is re-established whenever the cavity collapses (i.e. ).
Finally, the stability of the numerical solution (Equation (9)) is ensured according to the Courant–Frederic–Lewy rule: (Wylie & Streeter 1993).
It should be pointed out, herein, that the suitability and accuracy of the MOC-FG model were previously validated by the authors against the experiment of Covas et al. (2004) (e.g. Triki 2018a, 2018b).
The next section is dedicated to the examination of the key advantage of the proposed dual branching/inline technique-based upgrading design strategy for the attenuation of pressure-surge-wave magnitude, and the expansion of the pressure-wave oscillation period.
CASE STUDY
For such a scenario, the dual penstock and short-section technique-based upgraded system setup consists in lumping a PE penstock upstream of the main piping system and substituting a short-section downstream of the main pipe with PE material (Figure 5(b)). For comparison purposes, the results related to the conventional and dual techniques built upon the inline or branching strategies are also presented (Triki 2017, 2018a; Trabelsi & Triki 2020c). The (dual sub-)short-section or (dual sub-)penstock lengths are: or ), respectively, and the diameters of the dual short-sections or dual penstocks are: .
The PE penstock and short-section types utilized in this study are the low-density PE (LDPE) and high-density PE (HDPE) whose creep compliance and retardation time coefficients embedded in the Kelvin–Voigt formula are: and , respectively (Keramat & Haghighi 2014). Besides, the elastic creep compliance of steel material is , which involves a wave speed value: .
The transient pressure-surge-wave responses are predicted using a MOC-FG-based algorithm for a specified time step input: .
Hereafter, the interpretation of the results uses the following rules:
It is interesting to highlight, herein, that the last rule (Equation (18)) is used to evaluate the tradeoff between the attenuation effect of the magnitude of up- or down-surge and the expansion effect of the period of pressure-wave oscillations.
System layouts . | Materials . | T1 (s) . | Δhup-surge (m) . | Δhdown-surge (m) . |
---|---|---|---|---|
Original main piping system | Steel | 0.472 | 41.8 | 32.6 |
HDPE | 2.321 | 5.8 | 8.5 | |
LDPE | 4.351 | 2.6 | 5.2 | |
Inline technique | HDPE short-section | 0.798 | 27.9 | 32.6 |
LDPE short-section | 1.31 | 16.5 | 19.6 | |
Dual HDPE sub-short-sections | 0.817 | 27.5 | 32.6 | |
Dual LDPE sub-short-sections | 1.235 | 17.7 | 20.7 | |
Branching technique | HDPE penstock | 0.986 | 27.9 | 22.6 |
LDPE penstock | 1.875 | 16.5 | 13.2 | |
Dual HDPE sub-penstocks | 0.817 | 27.5 | 32.6 | |
Dual LDPE sub-penstocks | 1.26 | 17.7 | 20.7 | |
Dual branching and inline techniques | Dual HDPE penstock/short-section | 0.836 | 27.2 | 32.5 |
Dual LDPE penstock/short-section | 1.273 | 17.6 | 20.7 |
System layouts . | Materials . | T1 (s) . | Δhup-surge (m) . | Δhdown-surge (m) . |
---|---|---|---|---|
Original main piping system | Steel | 0.472 | 41.8 | 32.6 |
HDPE | 2.321 | 5.8 | 8.5 | |
LDPE | 4.351 | 2.6 | 5.2 | |
Inline technique | HDPE short-section | 0.798 | 27.9 | 32.6 |
LDPE short-section | 1.31 | 16.5 | 19.6 | |
Dual HDPE sub-short-sections | 0.817 | 27.5 | 32.6 | |
Dual LDPE sub-short-sections | 1.235 | 17.7 | 20.7 | |
Branching technique | HDPE penstock | 0.986 | 27.9 | 22.6 |
LDPE penstock | 1.875 | 16.5 | 13.2 | |
Dual HDPE sub-penstocks | 0.817 | 27.5 | 32.6 | |
Dual LDPE sub-penstocks | 1.26 | 17.7 | 20.7 | |
Dual branching and inline techniques | Dual HDPE penstock/short-section | 0.836 | 27.2 | 32.5 |
Dual LDPE penstock/short-section | 1.273 | 17.6 | 20.7 |
As can be seen from Figure 6, two issues arise during the mechanism of surge formation and propagation across the original system case utilizing the steel material for the main piping system: an excessive pressure head drop appears and the cavitation phenomenon develops as a result of the valve operation, which leads to severe pulsating water level oscillation. In this regard, the pressure head firstly drops to the saturated pressure head value of the liquid (i.e. ), and then rises sharply to the 1st pressure head peak , associated with the compression of the air pockets. This leads to down- and up-surge magnitudes of: and , respectively.
Figure 6 further illustrates that the pressure-wave curves associated with the upgraded systems employing HDPE unique or dual short-sections, or HDPE upstream short-penstock and downstream short-section are almost undistinguishable during the 1st cycle of pressure-wave oscillations. In addition, a perusal of Figure 6 reveals a slight attenuation of pressure-wave crest allowed by these upgraded system layouts. Consequently, these layouts are ruled out from further interpretation.
Unlike the steel material-based main piping system, a close examination of Figures 6 and 7 confirms that the cavitation phenomenon is mitigated in the renewed system cases utilizing HDPE or LDPE material for the main piping system. Specifically, the preceding system setups lead to a substantial attenuation of 1st pressure-wave magnitude. In this respect, the attenuation capacities of 1st pressure-surge crest and peak involved by an HDPE or LDPE main piping system are: { and } or { and }, respectively. Nonetheless, the foregoing system cases lead to a considerable relaxation of the 1st period of pressure-wave oscillations. Specifically, the phase shifts between the HDPE or LDPE type-based main pipe system layouts and their counterpart involved by the steel material-based pipe are: or , respectively. This leads to these ratios of 1st pressure-surge crest (or peak) and phase shift: { or }, and { or }, respectively.
Likewise, as per Figures 6 and 7, the cavitation is also palliated in the controlled system cases involving an HDPE or LDPE penstock, namely, as per Table 1, the attenuations of 1st pressure-surge crest (and peak) corresponding to upgraded systems made of an HDPE or LDPE penstock are: { and } or { and }, respectively. Conjointly, these upgraded system layouts lead to a significant relaxation of the 1st period of pressure-wave oscillations. Specifically, the phase shifts between HDPE or (LDPE) penstock-based controlled layouts and their respective case corresponding to the steel main pipe are: or , respectively. This, in turn, involves the ratios of the 1st pressure-surge crest (or peak) and phase shift: { or }, or { or }, respectively (Table 1).
On the other hand, Figures 6 and 7 illustrate practically undistinguishable pressure-wave curves for the upgraded system layouts based upon LDPE dual sub-short-sections, LDPE sub-penstocks, and LDPE upstream penstock and downstream short-section. Specifically, as per Table 1, these setups allow similar values of up- and down-surge magnitudes: and , respectively. However, these setups involve different period values of the 1st cycle of pressure-wave oscillations: , , and (Table 1). In other words, these upgraded system setups allow the attenuation capacity values: { and }, and phase shift values , , and , respectively. Consequently, they allow the magnitude ratios of the up- or down-surge to the 1st phase shift: { or }, { or }, and { or }, respectively.
A priori, this wave group allows an acceptable tradeoff between the 1st pressure-magnitude attenuation and the 1st cycle of pressure-wave oscillation period. Perhaps, the specific setup designed upon an LDPE upstream penstock and downstream short-section slightly outperforms other setups concerning the compromise between the two design criteria (i.e. magnitude attenuation and relaxation of oscillation period).
CONCLUSION
Based on the simulation results, the findings proved that the dual technique utilizing a PE penstock upstream and an inline short-section downstream could improve the steel material-based main piping system capacity, while providing a satisfactory tradeoff between the attenuation magnitude of pressure head, on the one hand, and the relaxation of the 1st period of pressure-wave oscillations, on the other hand. Precisely, the comparison between the different upgrading techniques or the renewal of the piping system and the original piping system suggests that the specific layout of the dual technique utilizing an LDPE penstock upstream and an inline short-section downstream is superior to other upgraded or renewed systems.
Additionally, the key advantage of the proposed technique relies on the easy and practical implementation merit as compared with other conventional water hammer control devices. For example, the replacement of burst pipe sections of an existing steel piping system with their respective ones composed of HDPE or LDPE types constitutes a straightforward application of this control measure.
The above-mentioned conclusions can provide some insights into developing practical design measures within the hydraulic network framework.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.