Presentation:Water Hammer and Surge Control

in Pumping & Piping Systems

Dr. Mohsen SolimanMechanical Power Eng. Dept

And with

American University at Cairo - AUC

Engineering Services

Industrial Pumps: Selection, operation, and Maintenance

Introduction: What is Water Hammer?: Generally Water hammer is series of pressure pulsationsof varying magnitude, above & below normal pressure of water in the pipe of length L. Amplitude and period of pulsation depend on velocity, size, length and pipe material.

The wave-damping is due to fluid viscosity and the elasticity of pipe-walls.

Head

or

pressure

Valve closing

Valve opening

Wave-length

Time(L/a)

L: pipe lengtha: wave speedis nearly equal to the speed of sound in fluid

What is Water Hammer (in liquid flow): sudden hydraulic shock waves (very high and very low pressure waves) transmitted through pipe-line at very high speed (close to sonic flow). The large amplitude pressure waves travel back & forth within the pipe until the kinetic energy is dissipated by viscous shear & the pipe-elasticity. Water Hammer cause (if pipe is not protected)destructive forces, pounding noises & vibration in a piping system when a column of incompressible liquid flowing through a pipe line is stopped abruptly (sudden change in flow velocity or flow inertia) as with the sudden change in opening or closing of a valve in the pipe line. .

Relationship Between Valve-Closing and Water Hammer (or Pressure Surge)

Ideal (at the valve)

Can water hammer cause damage?Yes. Destructive forces/hammer-blow sounds, associated with Water Hammer can cause annoyances, inconvenience & costly damages. These shock waves impose undesirable stress on piping (unless controlled). It can exist with or without noise. But the high noise is like an alarm warning alerts to water hammer problems and damages that may happen in piping systems. Water hammer can cause pipes or valves to burst if weakness exists in pipe work if pipes are corroded or if the piping have weak joints. Water hammer can cause pipe to move due to vibrations if they are not well secured - resulting in fatigue cracks or friction holes. -To prevent potential damage, the shock waves related to water hammer need to be eliminated. We must use surge tanks or some water hammer protection devices.

What other main causes of Water Hammer (sudden change of liquid velocity)- Sudden starting and stopping of pumps; -Sudden compression/condensation of large air and steam buckets in liquid pipes; -plumbing design faults; - Sudden operation of automatic valves on sprinkler systems;

How to protect a pipe line from water hammer?To reduce shock pressure and confine its action to the section of piping in which it occurs, a suitable control must be provided to absorb and dissipate the energy causing the shock. Air or gas (usually N2) is the most effective medium to be used for this purpose since it is highly compressible. A water hammer suppression device usually refers to Hammer Arrester, Air Chamber, Accumulator or Surge Tank. Water hammer arresters prolong the service life of piping, valves, fittings, and other devices which are part of a water distribution system.

Accumulator

Surge Tank

Surge control valve Surge control valve

-If condensate stands or collected in steam system and is then picked up by high velocity steam to form slug of water propelled down steam main at steam velocity (25m/s).Water Hammer is the noise and movement of pipe work due to slugs of condensatecolliding at high velocity into pipe work, fittings & equipment. This can result in fracture of steam line or fittings leading to hazardous conditions, loss of steam, and downtime. It is important, to reduce the risk of condensate build-up through using good engineering practices of steam & condensate system.

Water Hammer is also in Steam Piping Systems!

Potential sources of water hammer

Reverse gradienton steam main

Steam off-take

Any change in the liquid flow rate (closing/opening a valve, regardless of rate or magnitude of change, requires that the liquid be decelerated/acceleratedfrom its initial flow velocity Vo. A quick process ofclosing/opening of valve must lead to sudden-very large deceleration/acceleration of an incompressible fluid. A pressure wave is generated (called Water Hammer). Very high/low pressure due to a Water Hammer may cause great damages to the pipeline.

Theoretical Modeling and Analysis of Water Hammer in Liquids (The Tank-Pipe-Valve Model)

Consider an ideal ( frictionless flow) tank-pipe-valve system in which a steady flow with velocity V0 is stopped by an instantaneous valve closure. Elasticity of walls is neglected. The valve closure generates pressure wave travels at the wave speed or celerity, a, towards the tank at distance, L. Amplitude, P, of pressure wave is given by Joukowsky formula P=(aV0) where is the density of the fluid. Note that if we consider elasticity of walls, the wave speed (celerity) is less than sonic velocity, a.

How we do a simple analysis for Water Hammer?

Closing a valve at downstream end of pipe line creates pressure wave/surge (sudden increase or decrease in P). For rigid-wall system, pressure wave moves toward tank at sound velocity in the liquid, a (for water a1500m/s). It takes time tc=(2L/a) for pressure waves to go from the valve to the tank and get back to the valve. The time tc called critical valve closure time. Closing valve in less than tc, is called sudden valve closure. To avoid water hammer, valve closure time should be t >(2L/a). Sudden valve closure will change velocity quickly and can result in water hammer or large pressure surges. In fact, water hammer is nothing more than the free vibration of the liquid column. The natural frequency of the vibration is a/(4L) for the open-closed system of Fig.

What is sudden valve closure?

This Process of the oscillating wave propagation continues (with the frequency t= 4L/a) until the viscous effects act to damp out the motion to zero velocity.

Effect of pipe-wall elasticity on Pressure Wave of Water Hammer

Ideal (at the valve)

Wave speed for different pipe wall materials

Values of E & K for different materials

Wave Speed, a

Propagation velocity of a pressure wave in pipeline for varying air content.

Water Hammer in Pumping Systems (also called Surges or Hydraulic Transients)

Practical Examples for Water-Hammer (Surge) Control in Pumping Systems:

Causes & Effects: Sudden changes in flow velocity due to: rapid valve closure, pump starts & stops, and improper line filling as air expelled rapidly through manual vent or throttled valve followed by water at high velocity which is restricted by the outlet causing a surge.

Controlling Surges in Pumping systems

In Vertical Pumps: we are Using Well Service Air Valves

Using Check Valves

Using Fast Closing Check Valves

Using Pump Surge Control Automatic Valves

Example for Pump Control Valve Operation

Additional Surge Relief Equipments

Stand-Pipes and Surge Tanks

Example of Surge Relief Valves

Example of Anti-Slam Combination Air Valves

Example of Vacuum Breaker Valves

Some Practical Types of Automatic Control Valves (for Surge Control):The following types of automatic control valves are used to protect against the Water

Hemmer effects in a pipeline .- It is a fact that uncontrolled water hammer in a pipeline can explode pipes, bust pumps, and blow pivots apart. Many operators will not start their system without

manually controlling a valve in an attempt to minimize water hammer. - Water hammer pressure adds to the normal operating pressure of the system without proper precautions. Pressure pulsations due to water hammer can exceed 400% of the normal operating pressure of the system and can easily exceed the pressure rating of the pipe. Note that water hammer produces large negative pressure pulses which can

collapse pipelines.

Many factors can affect the strength of water hammer pulses:- short pipelines have less water hammer potential than longer pipelines.- other factors include: the pipe size, pipe material, the flow rate, the fluid velocity in the line, and wethere there is trapped air in the pipe.Air pockets in a pipeline can cause water hammer when this air compresses and moves along the line.Pipelines that have set idle for even a few minutes will often have air in them due to vacuum relief valves at high points. It is very important to have air release valves where needed along the pipeline to safely discharge air. -It is recommended that water velocity to be less than one foot per second when filling pipelines. - Maximum velocity once the line is full should be less than five feet per second.Water hammer control can be designed for almost any system. Two general solutions are available: various automatic valves or motor speed controllers.

Automatic solenoid controlled Valve:

A Roto-valve type:

Electronic control Valve:

Electronic control Valve:

Deep Well Pump Control Valve:

Control Valve Applications:

Pump Check Valve:

Advantages of Pump Check Valves:

Application of Check Valves:

Surge Relief Valves:

Types of Surge Relief Valves: Globe surge Relief Valves:

DANFLO Liquid Surge Relief Valves

CavitationWhen the pressure at any point inside a centrifugal pump drops below the vapor pressure of the liquid, vapor bubbles will form, creating cavities in the liquid flow. These bubbles are carried along with the flow until they reach a region of higher pressure where they collapse, producing a shock wave. This phenomenon is called cavitation. When the bubbles are carried onto the surface of the impeller and collapse there, the impact of the liquid suddenly filling the void and hitting the metal will damage the surface by gouging out small pieces. When this action is repeated in rapid succession, it produces noisy operation and vibration. Prolonged operation under these conditions may result in mechanical damage to the pumps impeller and/or casing.Cavitation should not be confused with possible inclusion of air bubbles in the liquid flow. Although air bubbles are not desirable in liquid flow, they do not have the same destructive characteristics as do vapor voids. Air bubbles become smaller when they are compressed, but vapor bubbles totally collapse, producing the shock wave referred to earlier.

VAPOUR PRESSUREAll liquids form vapors at their free surface, creating a pressure that is known as vapor pressure. This pressure increases in value as the temperature of the liquid increases. In a closed system completely filled with liquid, no vapors will form as long as the liquid is subjected to a pressure that is greater than the vapor of the liquid at the existing temperature. However, should the pressure exerted on the liquid be allowed to drop below the vapor pressure, some of the liquid will flash into vapor.This is an important factor to consider in a pumping system where the pressure on the suction side of the pump could drop below the vapor pressure of the liquid. This drop may be caused by insufficient head, high suction lift, excessive friction head or high liquid temperature. If the liquid flashes, the formed vapor can partially or completely stop liquid flow into the pump. The pump is then said to be vapor-bound or vapor-locked.

Cavitation (phenomenon in liquids only)

Cavitation is caused by:Suction velocity too high. / Too many sharp changes in direction of suction line. / Temperature of pumped liquid too high./ Suction lift too high.

The 5 conditions to produce Cavitation:

Plug damaged by cavitation

CavitationIndix, Kc :