Design of a pilot scale Water hummer system

Project Overview

Most engineers involved in the planning of pumping systems are familiar with the terms “hydraulic transient”, “surge pressure” or, in water applications, “water hammer”. Pressure transients are also referred to as surge pressure or, if referring to water systems, water hammer. The latter term suitably reflects the harmful effects that the hammer-like blows accompanying the pressure surges can have on pipes and system components. Water hammer causes piping, valves, pipe fixtures, supports, system components, etc. to suffer the added strain of dynamic loads. The term “water hammer” is used to describe the phenomenon occurring in a closed conduit when there is either an acceleration or retardation of the flow. In contrast to a force, pressure is non-directional; i.e. it does not have a vector. Not until a hydrostatic pressure starts acting on a limiting area, is a force exerted in the direction of the area normal. As it is not possible to altogether avoid pressure transients when operating a piping system, the art lies in keeping the pressure transients within controllable limits. What makes matters even more complex is the fact that the damage caused by impermissibly high surge pressures is not always visible. Often the consequences do not become apparent until long after the event, for example a pipe rupture, lose or disconnected flanges. The root cause of damage then tends to remain in the dark. The question as to whether a transient flow or surge analysis is necessary during the planning phase or not is less readily answered. Under unfavorable circumstances, damage due to water hammer may occur in pipelines measuring more than one hundred meters and conveying only several tenths of a liter per second. But even very short, unsupported pipelines in pumping stations can be damaged by resonant vibrations if they are not properly anchored. By contrast, the phenomenon is not very common in building services systems, e.g. in heating and drinking water supply pipelines, which typically are short in length and have a small cross-section. The owners or operators of systems affected by water hammer are usually reluctant to pass on information about any surge damage suffered. But studying the photos taken of some “accidents” (Figs. 1,2,3) one thing is clear: the damage caused by water hammer by far exceeds the cost of preventive analysis and surge control measures. The ability to provide reliably designed surge control equipment, such as an air vessel or accumulator1, flywheel and air valve, has long been state of the art. The technical instruction leaflet W 303 “Dynamic Pressure Changes in Water Supply Systems” published by the German Association of the Gas and Water Sector clearly states that pressure transients must be considered when designing and operating water supply systems, because they can cause extensive damage. This means that a surge analysis to industry standards must be performed for every hydraulic piping system at risk from water hammer. Dedicated software is available for this purpose – an important tool for the specialist surge analyst to use.

Project Objectives

Consultants and system designers are faced with the following questions, which we hope to answer in this project:
  • How can we know whether there is a risk of water hammer or not?
  • How significant are approximation formulas for calculating water hammer?
  • Which parameters are required for a surge analysis?
  • What does a surge analysis cost?
  • How reliable is the surge control equipment available and how much does it cost to operate it?
  • How reliable is a computerized analysis?

System designer and surge analyst must work together closely to save time and money. Water hammer is a complex phenomenon; the purpose of this brochure is to impart a basic knowledge of its many aspects without oversimplifying them.