Water Hammer (Part1)

Most engineers involved in the planning of pumping systems are familiar with the terms “hydraulic transient”, “surge pressure” or, in water applications, “water hammer”. 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-a, 1-b, 1-c) 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 accumulator , 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 have to be considered when designing and operating water supply systems, because they can cause extensive damage. This means that a surge analysis to industry standards has to 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. Consultants and system designers are faced with the following questions.

• How can we know whether there is a risk of water hammer or not?
• How significant are approximation formulas for calculating water hammer?
• Can the surge analysis of one piping system be used as a basis for drawing conclusions for similar systems?
• 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 have to 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.

Steady and unsteady flow in a pipeline

When discussing the pressure of a fluid, a distinction has to be made between pressure above atmospheric [p bar], absolute pressure [p bar(a)] and pressure head h [m]. Pressure head h denotes the height of a homogeneous liquid column which generates a certain pressure p. Values for “h” are always referred to a datum, (e.g. mean sea level, axial centerline of pipe and pipe crown etc.).

As a rule, system designers start by determining the steady-state operating pressures and volume rates of flow. In this context, the term steady2 means that volume rates of flow, pressures and pump speeds do not change with time. Fig. 2.1-a shows a typical steady flow profile:

With a constant pipe diameter and a constant surface roughness of the pipe’s inner walls, the pressure head curve will be a straight line. In simple cases, a pump’s steady-state operating point can be determined graphically. This is done by determining the point where the pump curve intersects the piping characteristic.

A pumping system can never be operated in steady-state condition all the time, since starting up and stopping the pump alone will change the duty conditions. Generally speaking, every change in operating conditions and every disturbance cause pressure and flow variations or, put differently, cause the flow conditions to change with time. Flow conditions of this kind are commonly referred to as unsteady or transient. Referring specifically to pressures, they are sometimes called dynamic pressure changes or pressure transients. The main causes of transient flow conditions are:

• Pump trip as a result of switching off the power supply or a power failure.
• Starting or stopping up one or more pumps whilst other pumps are in operation.
• Closing or opening of shut-off valves in the piping system.
• Excitation of resonant vibrations by pumps with an unstable H/Q curve.
• Variations of the inlet water level.

Fig. 2.1-b may serve as a representative example showing the pressure envelope3 with and without an air vessel following pump trip.

h_steady in Fig. 2.1-b is the SteadyState pressure head curve. Pressure head envelopes h_minWK and h_maxWK were obtained from an installation with, hmin and hmax from an installation without air vessel. Whereas h_minWK and h_maxWK are within the permissible pressure range, hmin gives evidence of vapour pressure (macro cavitation) over a pipe distance from 0 m to approximately 800 m. Almost across the entire length of the pipe, the value of h_max exceeds the maximum permissible nominal pressure of the pipe PN 16 (curve marked “PN pipe“) and is, therefore, inadmissibly high. As a rule, vapor pressure is a most undesirable phenomenon. It can have the following harmful effects:

• Dents in or buckling of thinwalled steel pipes and plastic tubes.
• Disintegration of the pipe’s cement lining.
• Dirty water being drawn into drinking water pipelines through leaking connecting sockets.

We will continue this technical posts and also come back to the subject of macro-cavitation, i.e. liquid column separation, in following parts.

REF: KSB Know-how, Volume 1