Water Hammer Report

Faculty of engineering

Mechanical Engineering Department

Water Hammer Report

2

The report was written by :

Ahmed Mohi Eldin Fahmy

Ahmed Mustafa Mohamed Youssef

Ahmed Ragab Fathy Ibrahim

Ahmed Mohamed Abdel Rahman Mohamed

Ahmed Mahmoud Abass Ashry

3

Contents

1

• Introduction

2

• Definition of water hammer

3

• Casues

4

• Features

5

• Damage caused by water hammer

6

• Equation of water hammer pressuare

7

• Instantanous valve closure

8

• Methods to reduce or eliminate water hammer

9

• Visualization

10

• CONCLUSION

4

Water hammer refers to fluctuations caused by a sudden increase or decrease in flow velocity. These pressure fluctuations can be severe enough to rupture a water main. Potential water hammer

problems should be considered when pipeline design is evaluated, and a thorough surge analysis should be undertaken, in many instances, to avoid costly malfunctions in a distribution system. Every major system design change or operation change such as the demand for higher flow rates should include consideration of potential water hammer problems. This phenomenon and its significance to both the design and operation of water systems is not widely understood, as evidenced by the number and frequency of failures caused by water hammer .

Water Hammer is a term used to define the destructive forces, pounding noises, and vibrations which develop in a piping system when a column of non-compressible liquid flowing through a pipe line at a given pressure and velocity is stopped abruptly.

The tremendous forces generated at the point of impact or stoppage can be compared in effect to that of an explosion .

A water transport system’s operating conditions are almost never at a steady state.

Pressures and flows change continually as pumps start and stop, demand fluctuates, and tank levels change. In addition to these normal events, unforeseen events, such as power outages and equipment malfunctions, can sharply change the operating conditions of a system. Any change in liquid flow rate, regardless of the rate or magnitude of change, requires that the liquid be accelerated or

decelerated from its initial flow velocity. Rapid changes in flow rate require large forces that are seen as large pressures, which cause water hammer.

Introduction

Definition of water hammer

5

Water flowing through a pipe has a definite amount of energy of flow This is known as kinetic energy and can be calculated by using the formula :

K.E : kinetic energy

M : mass of water which is flowing V : velocitry of flow

g : acceleration due to gravity

When the flow of water in a system is abruptly stopped, this kinetic energy must be absorbed. In an unprotected piping system this energy is dissipated by straining and expanding the piping and various components in the system and is accompanied by a dangerous pressure rise in the system . Entrained air or temperature changes of the water also can cause excess pressure in the water lines. Air trapped in the line will compress and will exert extra pressure on the water. Temperature changes will actually cause the water to expand or contract, also affecting pressure. The maximum pressures experienced in a piping system are frequently the result of vapor column separation, which is caused by the formation of void packets of vapor when pressure drops so low that the liquid boils or

vaporizes. Damaging pressures can occur when these cavities collapse. In conclusion most of the causes might be :

1. pump startup or shutdown; Pump startup can induce the rapid collapse of a void space that exists downstream from a starting pump. This generates high pressures , Pump power failure can create a rapid change in flow, which causes a pressure upsurge on the suction side and a pressure down surge on the discharge side. The down surge is usually the major problem. The pressure on the discharge side reaches vapor pressure, resulting in vapor column separation , and the formed vaccum may result in the pipe deformation.

2. valve opening or closing variation in cross-sectional flow. Valve opening and closing is

fundamental to safe pipeline operation. Closing a valve at the downstream end of a pipeline creates a pressure wave that moves toward the reservoir. Closing a valve in less time than it takes for the pressure surge to travel to the end of the pipeline and back is called sudden valve closure. Sudden valve closure will change velocity quickly and can result in a pressure surge.

3. changes in boundary pressures , such as losing overhead storage tank, adjustments in the water level at reservoirs, pressure changes in tanks .

4. rapid changes in demand conditions ,such as hydrant flushing .

6

6. pipe filling or draining ,as when air release from pipes 7. check valve or regulator valve action.

8-Improper operation or incorporation of surge protection devices can do more harm than good. An example is over sizing the surge relief valve or improperly selecting the vacuum breaker-air relief valve.

1-The undertaking of a water hammer analysis, and selection of protection measures, should be an integral part during the design phase. There are now propriety water hammer programs available, which can assist designers in identifying potential water hammer problems and help in the selection protection measures. The use of these programs should be limited to experienced designers with intimate knowledge of water distribution systems.

2-The magnitude of transient pressures (or water hammer) and the time duration of the transient condition depends on the flow rate velocity, pipeline material and the system boundary conditions such as tanks, pumps, air valves, control valves and changes in pipeline diameter.

4- Steel pipe has pressure wave speed of 1000 m/s compared to 250 m/s for polyethylene pipe. The sudden closing of a valve with a pipe flow velocity of 1.0 m/s would generate a pressure change of 100 m head in the steel pipe compared to 25 m head in the polyethylene.

There are many factors that influence transient pressure surges, they include:

 Pipeline profile (particularly high points)

 Pipeline anchorage

 Type of pipe material (and presence of linings)

 Location of storage’s

 Type of check valves (some are vulnerable to valve slamming)

 Pump performance curves and operating speeds

 Rotational moment of inertia’s for pump/motor assemblies

 Pump station configurations

 Location and type of air valves

 Protection devices installed

 Configuration of piping network

 Valve types, sizes and their opening/closing speed

Features

Factors affecting the pressure

surge(Water Hammer)

7

Damage to pipes, fittings, and valves, causing leaks and shortening the life of the system. Neither the pipe nor the water will compress to absorb the shock.

The main damages are :

 Pipeline bursts and leaks

• Pipelines can fail by buckling resulting from excessive vacuum during transient conditions in the case of thin walled large diameter steel pipe, low pressure rating plastic pipe and plastic pipes exposed to high temperatures.

• Cement lining in steel pipes has spalled off the pipeline in situations where the pipeline is subjected to vacuum conditions accompanied by large pressure fluctuations. The exposed metal surface

corrodes resulting in accelerated pipeline failure.

• Asbestos Cement rubber ring joints have failed from vacuum pressures resulting from pump

stoppages. The vacuum pressures have allowed air to enter the pipeline via the rubber ring joints and the joints have failed with time exposure.

 Damaged Equipment

• This may occur due to the violent movement of mechanical parts. Examples of these are check valves slamming shut following pump stoppages at multiple pump stations and the sudden closure of large orifice air valves when filling pipelines.

8

Let "dv" is change in velocity in time "dt" as the valve is closed abruptly. In time dt an element of liquid, of length "Cdt" is brought to rest. The mass of the liquid compressed against the valve and comes to rest in time "dt" will be .

Multiplying both sides by ' a '

( )

Ph : Water Hammer pressuare

ρ

C : speed of sound in the fluid V : Velocity of fluid

9

If the time for closing the valve "T" is assumed to zero, the valve closure is called Instantaneous valve closure. T is the time for closing the valve. Consider a pipe of length "L" leading from a reservoir and terminating in a valve at its downstream. When the valve is Instantaneous closed a pressure of magnitude "Ph" is formed and moves up with velocity "C". The wave undergoes

reflections at the reservoir end as well as at the valve .

For t = 0, the pressure profile is steady, which is shown by the pressure head curve running horizontally because of the

assumed lack of friction.Under steady-state conditions, the flow velocity is v0.

The sudden closure of the gate valve at the down stream end of the pipeline causes a pulse of high pressure∆h

The pressure wave generated runs in the opposite direction to the steady-state direction of the flow at the speed of sound and is accompanied by a reduction of the flow velocity to v = 0 in the high pres-sure zone. The process takes place in a period of time0 < t < Tr, where Tris the amount of time needed by the pressure wave to travel up and down

the entire length of the pipeline.

At t = Trthe pressure wave has arrived at the reservoir. As the

reservoir pressure p =constant, there is an unbalanced condition at this point.With a change of sign, the pressure wave is

reflected in the opposite direction. The flow velocity changes sign and is now headed in the direction of the reservoir.

A relief wave with a head of -∆h travels down stream towards the gate valve and reaches it at a time t = Tr. It is accompanied

by a change of velocity to the value -v0.

10

Upon arrival at the closed gate valve, the velocity changes from -v0to v = 0.This causes a sudden negative change in pressure of

-∆h.

The low pressure wave -∆h travels upstream to the reservoir in a time Tr< t < Tr , and at the same time, v adopts the value

v = 0.

The reservoir is reached in a time t = Tr, and the pressure

resumes the reservoir’s pressure head.

In a period of time Tr< t <2Tr, the wave of increased pressure

originating from the reservoir runs back to the gate valve and v once again adopts the value v0.

At t = 2Tr, conditions are exactly the same as at the instant of

11

Valve Opening and Closing Times

If the time of closure of a valve is less than the time a pressure wave to travel from its point of initiation to the end of a pipeline and return then the valve is described as having a rapid closure. Extending the closure times is often restricted to short pipelines. Some facilities employ the two stage closing process whereby the valve is closed to a 15-20% open position rapidly and then the last closure is over an extended period. Similarly the valve opening is a two stage process.

Lower fluid velocities

To keep water hammer low, pipe-sizing charts for some applications recommend flow velocity at or below 5 ft/s (1.5m/s).

Shorter lengths of straight pipe

Add elbows, expansion loops. Water hammer is related to the speed of sound in the fluid, and elbows reduce the influences of pressure waves.

Flow Control Valve

Flow control valves can provide a means of changing the hydraulic grade line to reduce the potential for column separation. In a pipeline with varying slopes that may include ascending and descending gradients there is a great potential for column separation. This is particularly so if the HGL is below any of the peaks in the pipeline profile during one or more of the operating scenarios. It is a common solution to provide a flow control valve at the end of the pipeline to ensure that the HGL remains above the profile.

12

safety valves :

Safety valves are another device used for protecting a pipe-line against water hammer . They permanently close an opening in an operating pipe-line. When the pressure in the pipe-line rises to a predetermined limit, the safety valve opens; when the pressure drops, it closes automatically. Sometimes, the closing process is retarded so as to avoid changes in pressure induced by it. The valve is kept in closed position by means of a weight or a spring. The limiting pressure at which the safety valve opens, should not be exceeded at the point where it is installed. During very abrupt changes in pressure, such as may occur during water hammer, the safety valve sometimes does not react quickly enough due to its inertia.

Air/Vacuum Release Valves

It's often used to remediate low pressures at high points in the pipeline. Though effective, sometimes large numbers of air valves need be installed.

Although in practice the admission of air is not without problems, most of the problems are found during the release of air, sometimes resulting in pressures even higher than if air valves were not installed.

Relief Valves

Entrained air or temperature changes of the water can be controlled by pressure relief valves, which are set to open with excess pressure in the line and then closed when pressure drops. Relief valves are commonly used in pump stations to control pressure surges and to protect the pump station. These valves can be an effective method of controlling transients. However, they must be properly sized and selected to perform the task for which they are intended without producing side effects. To be effective against shock waves a pressure relief valve must be placed as close as practicable to the main pipe which is being protected. If a valve is located on a branch pipe the shock wave will have passed the branch by a distance of about twice the branch length before the reflected wave from the relief valve gets back to the pipe. junction as a reduced pressure wave.

Bypass Valves

Bypass valves take the form of a valve in parallel to the pumps. The concept is that on loss of power there is still a reduced flow into the pipeline via this valve. This prevents the column separation occurring immediately downstream of the pump discharge check valve.

13

Automatically-controlled valves

Water hammer often damages centrifugal pumps when electrical power fails. In this situation, the best form of prevention is to have automatically-controlled valves, which close slowly. (These valves do the job without electricity or batteries. The direction of the flow controls them.) Closing the valve slowly can moderate the rise in the pressure when the downsurge wave resulting from the valve closing returns from the reservoir.

Pump

Pump startup problems can usually be avoided by increasing the flow slowly to collapse or flush out the voids gently. Also, a simple means of reducing hydraulic surge pressure is to keep pipeline velocities low. This not only results in lower surge pressures, but results in lower drive horsepower and, thus, maximum operating economy .

Surge Tank

A surge tank is a reservoir with a free fluid level which is attached to the pipe-line which has to be protected against the effects of water hammer. The level in the reservoir

corresponds to the pressure in the pipe-line at the steady state. In the course of water hammer, the reservoir fills and empties. A pressure roughly corresponding to the instantaneous level in the reservoir is ensured in the pipe-line close to the surge tank. With a correctly

designed surge tank, the variations in pressure are much smaller than in pipe-line without surge tank. The required dimensions of the surge tank have to be determined from the calculated water hammer.

In some cases, such as that of the protected penstock of a governor-controlled turbine, for example, undamped fluctuations of the level in the surge tank may occur. Attempts to eliminate such phenomena may affect the required dimensions of the surge tank. The reservoir of a surge tank may have either a constant cross-section or a variety of shapes (refer to Fig. 12.1). Various shapes of the reservoir are designed mainly to reduce the required volume of the surge tank, to achieve the highest damping effect and to ensure stability of the level in the reservoir .

Surge tanks are not very suitable for protecting hydraulic systems operated under higher pressures, because they would have to be very deep. In such cases, other solutions an air chamber, for example. may be used.

14

One Way Surge Tank

A one-way surge tank is a reservoir with a free fluid level which is attached to the pipe-line trough a non-return flap valve. As long as the pressure in the pipe-line is higher than that corresponding to the level in the reservoir, the flow in the pipe-line is not affected in any way. When the pressure in the pipe-line drops below this value, the flap valve opens, the liquid from the reservoir flows into the pipe-line, which prevents any further drop in pressure. When the

pressure rises, the valve closes again.

Air Vessel

A pressure vessel containing air and water. It is a very effective device for controlling both positive and negative pressure surges and is often used as a last resort because of high capital costs.

Flywheel :

An effective device attached to pumps for generally shorter pipeline lengths. They help to dampen surges by slowly decelerating the pump speed on pump stoppage.

15

A very effective method to visually demonstrate water hammer in a system is to use animation enhancement. The animation program shows a time-based simulation of the changing hydraulic levels and velocity profiles changing along the pipeline profile during a transient condition. It shows the interaction between the travelling pressure surge wave, changing hydraulic level and velocity profiles and the impact of boundary conditions such as storage tanks, check valve, pumps, changes in pipe diameter, etc. The program is particularly useful when demonstrating water hammer to personnel with limited experience in the subject.

 pipeline velocities must be low to reduce the effects of water hammer  Careful must be taken on closing or opening of valves

 the noise and knock in a system is an indicator of the magnitude of a surge

 surge protection devices should be installed on an experimental basis because it is not possible to accurately determine the magnitude of surges

A water hammer investigation should be an integral part during the design phase for a new project, and if potential water hammer problems are identified, then the most effective selection of protection devices should be installed for that system

Visualization

16

1-Technical report ,Water Hammer by Z. Michael Lahlou, Ph.D., Technical Assistance Consultant Pictures of accidents

2-WATER HAMMER – A CONSULTANTS EXPERIENCE/Jim Gugich, Director, Gugich & Associates Pty Ltd Water Supply Consulting Engineers.

3 – jay R smith mfg co. Engineering handbook of water hammer arrestors 4 – slideshows at www.scribd.com

5 - Water hammer in pipe line systems by j.zaruba