Ø DEFINITION
Water Hammer
· Is a term used to define the destructive forces, pounding noises, and vibrations which develop in a piping system.
· Is usually recognized by the banging or thumping Noise that is heard when valves are shut off. Although this is an easy way to recognize the problem, water hammer doesn’t always make these telltale noises.
“YOU CAN’T ALWAYS “HEAR”
WATER HAMMER “
· Occurs when the flow of moving water is suddenly stopped by a closing valve.
This sudden stop causes the whole column of water behind the valve to slam into the valve, and itself, like a freight train crashing into a wall.
The tremendous spike of pressure that is caused is called water hammer, and it not only acts like a tiny explosion inside pipes, it can be just as destructive.
· Uncontrolled water hammer on a water line of just 50 PSIG will commonly result in pressure spikes of 250 to 400 PSIG.
· This pressure spike is not just at the valve, it reverberates backwards from the valve throughout the plumbing system, rattling and shaking pipes, until it is absorbed.
· If there are no measures taken to control it, water hammer will accelerate the failure of water heaters, valves, backflow preventers, washing machines hoses, pipe, fittings, etc.
Illustrating…Sketch
Fig. 1 illustrates the sequence of flow when a valve or quick closing is suddenly closed.
(Unprotected System)
Fig.1 (a) depicts water discharging freely through
A Quick closing valve.
Fig. 1 (b) shows the piping after quick closure.
Water, being non-compressible, piles up against the seat of the quick closing
Valve and a shock wave is created which rebounds back and forth in
The piping system.
Fig. 1 (c) indicates the shock wave rebounding all the way to the main or some point of relief.
At the point of relief there is a reversal in pressure wave and it travels back toward
The point of closure.
This sequence of pressure wave generation continues until it is dampened out and the energy is dissipated.
Ø SHOCK INTENSITY
Quick valve closure is defined as the closure time equal to
Or less than (2L/a) seconds. This will cause maximum pressure rise. This pressure rise can be calculated by using
Joukowsky’s formula.
Also, Joukowsky’s formula.
In (S.I Units)
Where △ H (max) = change in pressure in meter head
a = celerity or the speed of the surge wave
Through the liquid in the pipeline in m/s
Vo = change in velocity (m/s)
g = acceleration due to gravity as (9.8 m/s²)
The wave celerity c of a circular pipe is:
A general solution for the celerity of long gravitational waves in a stratified liquid (Johns Hopkins University. Dept. of Civil Engineering. Technical report)
Is the density of the fluid (water: 1000 kg/m3), K Is the compression modulus of the fluid (water: 2.19 109 Pa),
D Is the average diameter of the pipe [m]
E Is the elasticity modulus of the pipe material [Pa],
e is wall thickness [m]
= 1 if the pipe can move freely in the length direction = 
if the pipe is restrained, and
if the pipe is restrained, and 
Is the Poisson’s ratio for the pipe material.
· The wave celerity (another word for velocity) in pipelines will most often be more than 200 – 300 m/s, but it will always be lower than the wave celerity of sound in the fluid (in water the celerity of sound is close to 1450 m/s)
· Quick closure can produce an approximate pressure rise of 60 times the velocity of flow.
· The formula provides an initial first step guide as to what likely pressures could be developed during a transient condition.
· It also demonstrates the impact of different pipe materials. Steel pipe has celerity 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.
Reflection of water hammer Figure: illustrates the variations in pressure and how the flow propagates forwards and backwards in the pipeline after the closure of the valve. For simplicity any friction is neglected. The time it takes for the wave to move through the pipe is
Where L is the length of the pipeline and a is the wave celerity.
Investigating the Causes of Water Hammer- 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.
- 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.
Transients occur at pump start-up and shut-down. Running centrifugal pumps have a significant damping effect on transients. Therefore the starting of pumps in general cause only minor problems compared to the shutting down. Also, the fact that most pumping mains have non-return valves installed in the pump stations contribute to the larger problems with transients at pump shut-down.
Hence the causes of water hammer are varied. There are, however, four common events that typically induce large changes in pressure:
1. 1- Pump startup can induce the rapid collapse of a void space that exists downstream from a starting pump. This generates high pressures.
2. 2- 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.
3. 3-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. The pressure surge resulting from a sudden valve opening is usually not as excessive.
4. 4-Improper operation or incorporation of surge protection devices can do more harm than good. An example is oversizing the surge relief valve or improperly selecting the vacuum breaker-air relief valve. Another example is to try to incorporate some means of preventing water hammer when it may not be a problem.
Water hammer and cavitation in pumped mains Water hammer (or transient) is a wave phenomenon that occurs in the liquid in a pipeline when a pump starts or stops or when a valve closes or opens.
In principle any change in flow velocity can cause water hammer. In some cases the effect results in a hammering noise from the pipeline, which is why it is called water hammer. Often water hammer is the strongest physical load a pipeline is exposed to. Typical damages are breaks in the pipes over shorter or longer distances. Numerable examples of this kind of damages can be given.
· Water hammer :
The description of water hammer from a physical point of view takes its starting point in Newton's second law, which expresses that force equals mass times acceleration. This means that severe water hammer occurs where large masses of fluid are given high accelerations.
Water hammer is closely connected with other wave phenomena, for example the propagation of sound waves. An essential component of water hammer is the reflection of waves at the end of the pipeline regardless of whether the end is open or closed. A negative pressure wave, which occurs for example when a pump shuts down, will be reflected at the end of the pipeline and return as a positive wave.
I. Pump start-up :
- Transients at pump start-up occur when the start-up lasts for a shorter period than the acceleration of the water column in the pipe.
- A rough estimate for the time of acceleration is the reflection time Tf = 2 L/a
- General experience is that a pipeline attains its steady state flow in less than one or two times Tf
- A distinction between short and long pipes is useful in this context. A short pipe is a pipe where the acceleration period is shorter than the start-up of the pump. A long pipe is a pipe where the period of acceleration is considerably longer than the start-up of the pump. By definition short pipes do not cause water hammer problems.
II. Pump shut-down :
- Transients at pump shut-down are normally much more complicated and critical than at pump start-up.
- Most cases of pipe bursts occur at pump shut-down.
- The pump shut-down lasts from a fraction of a second to a few seconds depending on the size and speed of the pump and the load on it.
- A steel pipe with a length of 10 m is a short pipe, because a pressure wave will return to the pump with a negative sign after less than 2 milliseconds, whereas a pressure wave in a 3-kilometer long plastic pipe will first return after around 20 seconds.
- When a pump shuts down the rotation reduces rapidly, but it is important to understand that a stopped pump does not imply that the flow also stops.
- Most pumps have a considerably open passage, which allows a continuation of the flow if a pressure drop over the pump occurs.
- The non-return valve prevents the flow going backwards through the pump.
- Whether the flow continues or not after pump shut-down can in principle be estimated from Figure
· Cavitation :
If no specific precautions are taken, the water hammer caused by pump start-up or shut-down may cause the pressure to fall below the vapor pressure of the fluid. If this happens the continuity of the fluid is broken, or in other words - the fluid is boiling. This is cavitation
(Table): shows the vapor pressure of water as a function of the temperature. It shows that the vapor pressure in pumped mains is negligible compared to the normal barometric pressure of 10.13 mWc for temperatures below 200C.
Water is most often highly saturated with atmospheric air. If the pressure reduces because of water hammer, the fluid can become supersaturated with air, which is then released as small bubbles. The small bubbles then accumulate and form large bubbles. Since such accumulations cannot be dissolved as soon as they are released, the effect of water hammer can result in a net creation of air or gas pockets. Pumping of fluids with a high content of dissolved gasses, for example carbonated beverages, requires special analysis.
Example of measurement of water hammer (Figure1): shows the length profile of an approximately 3 km long sewer pressure main in western Denmark.
Measurements of pressure were taken close to the pump station as indicated.
Figure 1
(Figure2): shows the pressure head as a function of time
Over a period of 210 seconds (3½ minutes).
Figure 2 clearly illustrates how the water hammer is more damped when the pump is running compared to when the pump is turned off.
- At the start (t = 0) both pumps are turned off. Then one pump is turned on and shortly after the second is turned on too. The figure shows that the pressure is considerably higher than the steady state pressure during the acceleration period due to a reduced pump performance.
- Both pumps are shut down approximately 1 minute after start-up, and the pressure drops to a negative value of approximately - 2 mWc. This negative pressure has the effect that a certain flow continues through the pump and that the non-return valves stay open. About 0.5 minutes after pump shut-down the pressure rises sharply to a high of approximately 75 mWc. After this follows several repeats of the drop-rise sequence, but with decreasing amplitude.
- From these measurements it is seen that the pressure in the pipeline varies by almost 80 mWc, which is much more than acceptable for the actual uPVC pipe being used, with respect to the risk of failure because of fatigue. Based on the results of a subsequent computer simulation an air chamber was installed at the pump station. The air chamber reduced the pressure fluctuation to an acceptable level.
- The measurements also showed some short periodic pressure fluctuations (between 12 and 30 seconds), which indicated that cavitation occurred somewhere in the pipeline. This assumption was supported by a strong noise at the end of the pipeline near the reservoir. Computer simulation later confirmed that cavitation occurred near the first high point in the pipeline.
The influence of water hammer on the design of pipelines Because water hammer creates the most severe forces on the pipelines, the selection of pipe material and the determination of the thickness of the pipe wall depend primarily on the stresses from the transients.
However, it is rare that internal water pressure is the one and only factor in the dimensioning.
Other factors often play an important role, for example external water and soil pressure including traffic load.
Ø SAFE LIMITS OF PRESSURE RISE
SAFE LIMITS OF PRESSURE RISEMost plumbing valves and fittings are designed for 150 pound maximum working pressure;
Therefore, it is desirable that the pressure rise due to quick closure be kept under the 150 pound per square inch.
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ONTROL OF WATER HAMMER Precautions and Devices
1- Revolution control of pumps
- Revolution (or speed) control of the pump motor is the most efficient and flexible method for reducing water hammer.
- The standard method is the use of a so-called ramp by which the revolution of the pump varies linearly with time (the speed is ramped-down). As the length (in time) of the ramp can usually be fully adjusted, an almost total elimination of the water hammer is possible.
- It should be mentioned that a linear shut-down of the pump is not optimal from a theoretical point of view, because the pump head varies quadraticly to pump speed Thus, a shut-down ramp where the speed follows the square root of the time can reduce the necessary ramp length.
- The only serious weakness of the use of speed control is the problem with power failure. For most countries statistics of the so-called LOLP (Loss-of-Load-Probability) for the power network are available. In Denmark the LOLP on the 10–20 kV network is about 0,5 to 1 per year, but for a local network the value is higher.
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.
2- Flywheel (In Pump)
- The pump inertia can have a significant effect on the water hammer especially in the case of short pipelines.
- The moment of inertia can be increased by installing a flywheel, which in many ways is an excellent way of reducing water hammer. However, nowadays flywheels are uncommon because they take space and are expensive. Also, the start-up procedure of the motor is more complicated when a flywheel is involved.
-
3- Air chamber (or air vessels)
- Air chambers are frequently used to reduce water hammer caused by pumps and valves
- The basic principle is that the compressed air in the air chamber acts as a kind of pump the first seconds after pump shut-down. The compressed air slows down the loss of velocity in the main pipe, which reduces water hammer.
Disadvantages:
1- The pressure is kept high on the upstream side of the pump, forming a pressure gradient backwards through the pump, which will force the non-return valve to close faster and this can sometimes damage the valve.
2- A larger air chamber needs a compressor to keep a constant air content in the tank.
3- Air chambers are pressure tanks usually made of steel, and they have to comply with strict safety requirements.
4- Surge tanks (or surge tower)
A surge tank (or surge tower) is in principle an air chamber open to the atmosphere. Thus, the chamber should be higher than the working head of the pump plus the maximum head variations caused by pump run-up and shut-down. Surge tanks are mostly used for large pipelines with low geometric heads.
Because surge tanks are open chambers, the safety conditions are less critical compared to air chambers. Surge tanks are therefore often built in reinforced concrete.
5- One-way surge tanks
If the connection between the main pipeline and the surge tank has a non-return valve that only allows outflow from the tank, this is a one-way surge tank. Because of the non-return valve this type of tank does not need the same height as the normal surge tank, and it can be used in systems where pump shut-down is rare (but critical), for example in connection with loss of power. A separate system for filling the tank has to be included.
6- Valves
Gradually closing control valves can be an efficient way of reducing water hammer. In principle the valves can be placed anywhere in the pipeline, although a placing in the pump station is most obvious.
A control valve can be driven by compressed air. In this way it will also work as a non-return valve in case of loss of power.
7- Bypass around the pump
A pipe directly from the pump sump to the main pipeline (with a non return valve included) can fill the main pipe almost unhindered and in this way reduce water hammer. The principle is only useful in pipelines with low geometric head
The principle may seem promising but the gain is often negligible because most pumps already have a certain free opening through the pump wheel. This free opening will often be sufficient for filling the pipe.
8- Air valves
An air valve opens whenthe pressure in the pipe falls below zero. Air valves are efficient to avoid negative pressures in pipelines, which will reduce the high water hammer pressure peaks after reflection.
9- Non-return valves and water hammer
Most often pipelines are equipped with non-return valves to avoid return flow and flooding of the pump station when the pump is off. Non-return valves are usually placed just downstream the pump. Various principles are used in the design of non-return valves. Most often a mechanical spring (or gravity) constantly presses a valve or a ball towards a valve seat. The valve is kept open by the flow as long as the pump is running. Therefore a non-return valve inevitably causes a certain loss of energy.
When the power to the pump is cut-off, the rotation will slow down fast. The duration of the shut-down period will last from a few fractions of a second to several seconds. If several pumps run in parallel and only one pump is stopped (or if an air chamber is present), the shut-down will proceed more rapidly because a full counter pressure is maintained during the shut-down. This situation can be damaging to the non-return valve if the valve is unable to close before a return flow through the pump and the valve has developed. The impact can be a mechanical blow when the valve hit the valve seat, or it can be strong water hammer in the fluid because of the sudden interruption of the flow. Damages of non-return valves are often connected with this situation. Non-return valves in larger installations like power stations, district heating systems, etc. are often provided with dampening to avoid such damages.
The ideal non-return valve is a valve that closes exactly at the moment when the flow changes direction. In practice the valve closes an instant later because of the inertia of the valve. For this reason the valve is actuated of a force from the flow towards the valve seat with which the mechanical impact is intensified. In addition an internal water hammer is generated with high pressure on the upstream side and low pressure on the downstream side (in respect to normal flow direction).
Figure shows the flow velocity through the non-return valve during pump shut-down. Experience has shown that the maximum return velocity VRmax attained in a given valve is a function of the acceleration dV/dt at the time when the flow direction changes.  |
A capped stand pipe or air chamber is not an effective solution to controlling water hammer. Since nothing separates the air from the water within an air chamber, it only takes a few short weeks before the air is absorbed into the water, leaving the
air chamber waterlogged and completely ineffective. Laboratory tests confirm that the air is depleted by simple air permeation and by interaction between static pressure and flow pressure.
As Illustrated in Fig.
11- An Engineered, Mechanical
WATER HAMMER ARRESTER
The most effective means of controlling water hammer is a measured, compressible cushion of air that is permanently separated from the water system, that is, an engineered water hammer arrester. Arresters employ a pressurized cushion of air and a dual o-ring piston, in a sealed seamless chamber, which permanently separates this air cushion from the
Water in the system. When a valve closes the water column is diverted into the arrester thus pushing the piston up the arrester chamber against the pressurized cushion of air. The air cushion in the arrester reacts instantly, preventing the pressure spike that causes water hammer. The piston then returns to its original position after the shock is absorbed, ready for the next occurrence.
12- HYDROTROLS
- It’s a type Of Engineered, Mechanical WATER HAMMER ARRESTER.
- Uses heavy duty balanced expansion bellows to internally absorb the hydrostatic shock pressure occurring in water lines. These bellows are both pneumatically and hydraulically controlled in a pressurized expansion chamber so that they never come into metal to metal contact with other parts of the unit, and cannot be subjected to excessive stresses or strains which might cause metal fatigue and bellows failure.
- •Compact in size
- •Big in performance
- •Maximum capacity
- •Light-weight - needs no support straps
- •Requires no service or maintenance
- •Extremely durable - May be installed in concealed areas
- With HYDROTROL'S in-line design, expansion bellows are an integral part of the waterline, so that they respond instantaneously in absorbing and controlling hydrostatic shock.
- A pressurized compression chamber provides a pneumatic cushion that governs the bellows' expansion under normal waterline pressure; so that the full bellows expansion capacity is available for controlling hydrostatic shock.
- The bellows are of balanced design and construction with heavier and stronger convolutions positioned in the bellows assembly to insure each convolution expanding evenly and equally, thereby providing the maximum surface area for absorbing and dissipating the shock pressure into the pneumatic cushion.
- As hydrostatic shock occurs, these pressures cause the bellows to expand into the pneumatic cushion of the compression chamber. This expanding movement of the bellows provides the displacement required to absorb and control the shock pressure generated in the line. The force of the shock expanding the bellows creates a self-energizing pneumatic pressure, which prevents the bellows from over-expanding and coming into contact with the top of the compression chamber.
- The combined cushioning effect of both the pneumatic and hydraulic pressures governs the bellows action, so that shock waves do not bounce back into the piping system and acts to quickly stabilize the water and piping system.
