Experimental analysis of viscoelastic transmission pipe system under transient flow

Transient flow in a pressurized pipe system is an intermediate state flow that arises between one constant flow and another. In other words, whenever the flow conditions change from a steady-state due to any deliberate or accidental disturbance, a transient flow is created in the pipeline (Chaudhry, 2014). This phenomenon is one of the most severe cases of damage in pressurized pipelines.In many previous studies related to transient flow analysis, the pipe wall has been made of metal and concrete materials with elastic mechanical behavior. In recent years, the increasing use of plastic pipes (such as polyethylene and PVC) has led to the development of mechanical models of transient flow, taking into account the viscoelastic behavior of these materials.In recent years, polymer pipes such as polyethylene and PVC, due to their technical and economic advantages over other pipes such as steel, cast iron, concrete, and asbestos, have increased day by day. This makes the need to understand the structural behavior and hydraulic performance of polymer pipes even more urgent. The modeling method of polymer pipes for transient flows analysis has several fundamental differences from non-polymer pipes. These differences are mainly related to the interaction of fluid fluctuations with the characteristics of pipe wall structures. Polymers generally exhibit viscoelastic mechanical behavior that affects the intensity, formation, and damping of pressure fluctuations in transient currents. In these equations, it is usually assumed that the pipe wall is made of concrete and metal and has a linear elastic behavior. In comparison, polymer pipes have inelastic behavior.The present study aims to investigate the pressure response of viscoelastic pipeline under transient flow. For this purpose, first, the pressure signal's initial peak and the effects of line packing are investigated. The effect of the transient valve's closing time in different flows on the pressure signal is investigated in the following. Another important issue is the overpressure. In this research, laboratory values of overpressure are compared with the theoretically calculated values.

The laboratory model of this research was designed and built in the hydraulic laboratory of the Faculty of Water Engineering, Shahid Chamran University of Ahvaz, to evaluate the response of the viscoelastic pipeline system under transient flow. The pipes are high-density polyethylene (HDPE) (SDR11, PE100, NP16) with a length of 158 meters, an inner diameter of 5.05 cm, and a thickness of 6.5 mm. According to the four stages of waterhammer, if the constant pressure of the pipeline is low, the pressure signal in the third stage, after returning from the tank and reaching the transient flow valve, enters pressures less than the vapor pressure of the fluid, and due to the column separation. This phenomenon reduces the pressure signal capability to detect other system malfunctions. To avoid this problem, a pressurized reservoir was used as the upstream boundary condition at the pipe system's upper boundary.

The initial peak pressure due to the effects of friction and fluid inertia and the delayed deformation of the pipe wall is completely weakened in the first period of the pressure wave and does not exist in subsequent periods. The transient flow signal analysis showed that the classical waterhammer equation could not predict the observed maximum transient pressure of fast transient in polyethylene pipe. The calculation of the wave speed in polyethylene pipes based on the modulus of static elasticity is significantly less than that of elasticity's dynamic modulus. As the valve's closing time increases, their maximum pressure peaks and pressure drop gradients decrease, and this peak gradually weakens and disappears. In addition, depending on the time difference between closing the valve, the pressure wave will have a time delay. The results showed that a significant energy drop with phase change in the pressure wave is observed in all measurement locations.

After quickly closing the transient valve, a significant pressure peak is observed, followed by a sudden drop in the pressure signal. At a constant flow, the initial peak pressure decreases with increasing valve closing time. Suppose the calculations are based on the modulus of elasticity provided by the factory. Even if the elastic tube is assumed, the amount of overpressure is 16.6 to 37.65% less than the actual value.This is an important reason to consider polymer pipes' viscoelastic behavior by precision transient flow hydraulic models or to consider the dynamic modulus of elasticity, sometimes up to twice the static modulus of elasticity proposed by manufacturers, in the design phase.