In this article, we will learn what is water hammer? Along with we will capture the basics, definition, causes, effects, how to fix or stop, etc.
What is Water Hammer? Definition & Meaning
Water Hammer Basics
Let’s try to understand the basics of water hammer. We all know liquids are barely compressible, this means their volume can’t be changed under the application of pressure.
- This property is used in hydraulics systems but this same property becomes a damaging factor in piping.
- Water is heavy and when thousands of liter of water is moving then it carries a lot of energy.
So when we stop it suddenly pressure waves to get created and it has nowhere to go. This creates big noise, we call it a water hammer. Plumbing is the most under-acknowledged modern marvel that is for sure, we will realize this by this example.
Water Hammer Meaning
If the pipeline is of 1m diameter and its length is 100 km then the mass of water inside is about 80 million kg. If the operator closes the valve fast, it will create a huge spike in pressure which will cause rupture to pipes, which will be as devastating as bombs.
Water travels through pipelines with a velocity that is dependent on,
- pressure head,
- design of pipes,
- material of pipes etc.
Water Hammer Definition
This water moving inside pipes when stopped suddenly by the operator by closing valve or by another cause, there is a sudden rise in pressure of water because the momentum of water got destroyed.
- This sudden stoppage of water creates a pressure wave inside the pipeline.
- This pressure wave travels throughout the water and makes hammering action on the walls of this pipe.
Hence, water hammer is defined as a phenomenon of sudden pressure rise in pipes. It is also called as hammer blow as well.
This pressure rise is dependent on many variables such as
- Operating speed at which valve is closed
- Velocity of flow
- Length of pipe
- Elastic properties material of pipe
- Properties of fluid
This phenomenon is very important because the rise in pressure is so steep that it may lead to the bursting of pipes and may lead to a big accident.
Example & Explanation of Water Hammer
When water is stored at tanks or in dams, it contains some form of energy by virtue of its position which we called as potential energy.
This potential energy is the product of density, gravitational acceleration, and height at which water is stored.
- P.E = ρgh
- P.E = Potential energy
- ρ = Density of water
- g = Gravitational acceleration
- h = height of water
K.E = 1/2 mv2
- K.E = Kinetic energy
- m = Mass of water
- v = Velocity of water
It is dependent on mass (m) and square of velocity (v).
So when water running at such a high speed is stopped abruptly then this energy will get converted into pressure energy which will be dependent on the speed at which the velocity of fluid was reduced.
If it will be sudden then due to sudden change pressure rise will be sudden, if it is gradual pressure rise will be gradual. This all phenomenon happens due to the conversion of one form of energy into another.
Water Hammer Equation & Calculation
Wave will travel from valve to tank and from tank to valve, T
- T = Distance Travelled by Wave / Velocity of Pressure Wave
- T = (L+L) / C
- T = 2L/C
- L = the length of pipe
- C = the velocity of pressure wave
To understand this we need to go through this example
Consider a long pipe is carrying liquid and valve is attached to it at extreme end. On one end there is a tank and on another end, the valve is attached. This valve is being closed slowly.
- A= area of cross section of pipe
- L = length of pipe
- V = velocity of flow of water inside pipe
- t = time taken to close valve which is in seconds
- P = intensity of pressure wave produced
So the mass of liquid in the pipe is m = ρ A L
Let us just assume that closure of valve is being done such that liquid flowing through the pipe is brought to halt with uniform de-acceleration. From initial velocity V to zero in time t.
Condition – when the valve is closed gradually
Retardation of water = (v – 0)/t = v/t
The axial force which is producing retardation, F1
- F1 = mass x retardation
- F1 = ρ A L x v/t
The force created by pressure wave, F2
- F2 = P x A
When we equate these forces,
- F1 = F2
- ρ A L x v/t = P x A
- P = ρ L v/t
Then the head of pressure H,
- H =P/W [Here w was a specific weight]
- H = ρ L v/Wt
- H = ρ L v/ρgt [W = ρg]
- H = L v/gt
So the head of pressure will be, H = L v/gt
The closure of the valve is done very gradually when t > 2L/C
Closure of valve is said to be instantaneous when t < 2L/C
Where C = Velocity of the pressure wave.
What Causes Water Hammer? Sound, Noise
After getting the basic idea about water hammer, let’s check what causes water hammer. So, what are the main causes of water hammer?
The causes of water hammer shall be as follows,
Cause#1 Sudden Valve Closure
Sudden closure of valves, it may create water hammer sound or noise continuously. Think it in a simple example. If you run in a track, suddenly someone stops you! You will feel a shock.
Now, if you run faster, your shock will be more and if you run at a slower speed, your shock will be less.
- In case of water, the philosophy is same and sudden closure will create a shock which creates water hammer.
- This shock is called as hydraulic shock.
- If the water velocity in the pipe is high, hydraulic shock will be high i.e., water hammer will be high.
- If the water velocity in the pipe is low, hydraulic shock will be low i.e., water hammer will be low.
Cause#2 Volume Difference Between Steam & Water & Heat Loss
In case of condensate pipes, due to heat loss, condensate starts to change its phase into water. As steam occupies 1600 times more volume than water, if steam changes its phase into the water, a huge vacuum will be created. Water will try to fill that vacuum and can create a water hammer.
- If the volume of steam condenses is more, vacuum will be more and water velocity will be high. So, water hammer will be high.
- If the volume of steam condenses is less, vacuum will be less and water velocity will be low. So, water hammer will be low.
Cause#3 Velocity Difference Between Steam & Water
In case of condensate pipes, as the velocity of steam is 10 times of water velocity, steam goes faster in the pipe. Now, steam starts to condense, and the amount of condensate increases in the pipe. Many times, this condensate fills the pipe and create a barrier, which causes a huge pressure difference between the two sides of that barrier.
Downstream pressure will be drastically reducing and the water will try to fill that gap & create a water hammer.
Cause#4 Float Valve Stability
If float valves are not stable, and it up and down as well as creating a web into the system,
Cause#5 Blocking of Suction Pipe
Blocking of suction pipe can also be a reason for water hammer.
Cause#6 Fitting Problems
Improper fittings of pipes or between pump and piping may create a water hammer.
Cause#7 Trip of Pump
In many cases, a trip of the pump may be a reason for creating shock in pipes and creates water hammers.
Apart from above, there may some other reasons like,
- In case, valves are worn
- Disconnect the power suddenly, etc.
What are the Bad Effects of Water Hammer?
We have learned what is the cause of water hammer in pipelines? Let’s see the effects of water hammer one by one,
Water can make a total system failure by a sudden surge in the piping system. It does damage to equipment such as flow meters, pressure gauges, etc. because such instruments have a delicate inner structure that is not designed to withstand such high-pressure waves.
As we have learned that shock due to water hammer depends on many factors. A big shock can fail the entire system, however, a continuous small shock can damage the pump, its associated equipment, etc.
Due to this uneven pressure or shock, the associated piping system will be damaged, which can damage the entire system as well.
It will incur leakage in the piping system.
Due to leakage in the piping or pipelines, which will reduce the head requirements, as well as the efficiency of the system.
Electrical System Damage & Safety
Due to the water leakage, running electrical system may come into contact with water and the electrical system will be damaged.
Short circuits may happen and electrical hazards may be encountered, which is totally unsafe for the workers.
Support System Damage
Due to high shock, supports of piping or pipelines may be damaged.
Operation & Maintenance Issues
Due to continuous problems, it increases the maintenance issues.
Water Hammer Noise or Sound
Due to the shock, the water hammer creates sound or noise.
Damage and destruction of piping leading to the industrial accident. Especially steam plants, where condensate travels at slow speed but steam always travels at high speed, in such cases water gets dragged by steam at very high speed which is 10 times speed of the water.
Unanticipated external damages
If the fluid that is being transported is corrosive then it will damage pipelines, external environment. It might pollute the environment. Leakages can be harmful to close by electrical equipment, components, constructed infrastructure. It can also damage the piping support system.
Leakages can cause an impact on the health of maintenance personnel. Slippage, electric shock, or anything can happen. Operating employee’s life will be in constant danger.
Time lost to maintenance
Replacement of pipelines or equipment will be the most time consuming and costly affair. While the system will be down for repairs that will reduce revenue and increase financial losses. Specific point damage in such cases holds the potential to keep the whole system in downtime for maintenance.
If corrosive harmful fluid gets released out, not only it will affect employee and maintenance team but also it will pollute the environment and affect the image of the company.
In such cases, water pressure becomes very high and may incur a big accident.
Water Hammer Phenomenon in Home Plumbing
Many times we hear different noises after we turn of tap very fast as if some ghosts are laughing. These are in fact noises made by shock wave which is traveling from pipes. The reason is the same here as well. When the tap is switched off, incoming water from suddenly stops at the tap and this causes the conversion of energy. This energy travels in form of a shock wave which makes that noise. It is not water as much but the shock wave which is exerting pressure on the pipe is making it noise.
How to solve these home plumbing issues?
These are equipment as the name suggests are vessels that accommodate air inside them. It is like a small water bottle size container that has air inside it. Basically, it works on the principle of compressibility. Water and air are present inside. Air is collected in top parts, while water is at the bottom.
Air is compressible to a large extent while water is not compressible. So when a shock wave comes, this water in the air vessel exerts pressure on the air. As earlier mentioned, the air gets compressed by this force, so the whole intensity of the shock wave gets absorbed in this air vessel making it shock proof.
There is no separation of air and water in a vessel. So if the air is not filled in the vessel periodically, the present air inside it gets absorbed in water which reduces the effectiveness of the device.
If the whole vessel is occupied by water, then the whole purpose of the air vessel gets defeated. Sometimes bacterial or fungal spread is seen inside it. This is because direct point of contact between trapped air and water.
Water hammer arrester
In order to remove the limitations of the earlier devices, these devices are produced. The specialty of this device is air and water don’t make contact. Container separates two by the presence of plunger inside it. The plunger is making air-tight contact with the vessel wall due to the presence of O seal rings (like piston rings).
This allows the plunger to slide up and down in the vessel without leakage of air and water. As soon as the shock wave comes plunger moves up and there by compresses air in the top portion of the vessel. This causes the water in the pipeline to exert pressure created by shock on-air via the plunger.
The intensity of the shock wave gets absorbed and the water hammer effect is removed. This device has one more advantage, fungal or bacterial spread can’t happen in vessels. This is because water and trapped air don’t make contact. These devices are costly but they last for a long time.
Water hammer phenomenon in hydroelectric plants
Just to have a basic understanding, first, let’s understand the basic parts of the plant in one-liner explanation.
- Reservoir – it is a place where water is collected. Height and amount of water it can hold decides capacity of power plant.
- Trash rack – it’s like filter system, it collects big stone and debris.
- Dam – Strongly constructed concrete wall which is making sure reservoir water is collected behind it.
- Sluice gate valve – it is flow control valve for water which flows in penstock.
- Forebay – it is used in when surge tanks are not used. Its basic function is to absorb shock wave.
- Surge tank – it is used to absorb shock wave and reduce effect of water hammer. It protects penstock.
- Pen stock – long pipeline which carries water from reservoir to turbine station.
- Turbine – Electricity gets produced as potential energy of water gets converted into kinetic energy in penstock. From kinetic energy, turbine blade rotate. This energy is converted into electricity later.
- Draft tube – Big diameter size pipe to remove water out from turbine station.
- Tailrace – it is channel which carries water from draft tube.
Hydroelectric power plants are very costly to construct. Hence proper design and maintenance is a must. Here for the concept of water hammer focus on penstock. When a large amount of water is being carried and if the turbine has to be stopped then this can’t be done suddenly.
Even it is done slowly it will have many reactions. Water coming down from very height when stopped will create strong pressure waves. So to absorb energy following devices are used.
It is similar to a surge tank. It is a big tank of water from near the reservoir. When turbine load decreases, and flow control valve which feeds water to the turbine is getting closed, in such instances water which was inside penstock valve will come to rest and kinetic energy will get converted into pressure energy. This shock wave will travel along the whole penstock and can damage it severely.
So this energy gets absorbed by forebay where water level increases and extra energy from system gets removed.
It is located away from the reservoir and its working principle is the same as forebay. It is used mostly when space for forebay is not there. These are open tanks connected to penstock somewhere in middle. As soon as a shock wave is created in penstock, the level in surge tanks increases and sometimes water flies upwards due to the intensity of the shock wave. These devices remove unwanted pressure energy from the system.
It has many types such as
- Simple surge tank
- Restricted orifice surge tank
- Differential surge tank
Water hammer in steam plants
In steam plants, this happens when steam lines are long and the plant was lying idle earlier. So hot steam and cold pipes are the idle conditions for the creation of condensate. It happens most of the time when condensates get mixed with steam.
As soon as steam gets created in the boiler it is carried by long steam lines to its application site that may be a turbine or heat exchanger. As this steam is heated but it starts to lose its heat in this journey loses its enthalpy and gets converted into condensate.
Many conditions play a big role here such as insulation, outside temperature, and the nature of steam. This steam condenses to form a condensate. When the system is started at the beginning, the whole system was cold and idle earlier so until it gets heat up, it absorbs steams heat and this causes a reduction in steam temperature and formation of condensate in form of droplets. This explains why condensate proportion is more at the start.
So when steam travels, these condensate particles/ droplets act like small particles of slug and create hindrance to the flow of steam. Just to clarify here, the slug is part of condensate shaped such that it occupies a whole cross-section of the pipe whilst traveling. So with steam, it is also traveling via steam line.
When sudden stoppage, orifice, bend shows up this slag gets obstructed suddenly. So the amount of kinetic energy it was carried by virtue of its velocity suddenly gets converted into potential energy. The stress created in this situation has to be absorbed by the steam line.
How powerful is impact of this?
Saturated steam travels at speed of 20 to 30 m/s while water travels in pipes at speed of 2 to 3 m/s. when this slow-moving water gets dragged by steam at speed 10 times of original speed, this phenomenon of water hammer occurs.
So in steam plants, a water hammer is associated with steam moving at high velocity.
How to Stop or Fix Water Hammer?
Let’s see how to stop or fix water hammers?
Avoid Water Hammer in Pipelines
- Increase diameter of pipe
- Close valve slowly
- Provide surge tank
- Pipe material should be elastic
- Use of water hammer arrester.
- Use of pressure relief valve
- Use of water towers
Avoid Water Hammer in Steam Plants
It is very difficult to eliminate water hammer from the steam systems, however, we can minimize it with the below steps:
- During starting of plant, condensate must be removed from pipes so that it doesn’t get mixed up with steam.
- Steam lines should be installed such a way that they must gradual slope in same as direction of flow.
- Used of steam traps.
- Bypass lines to remove condensate water.
- Sagging should be avoided in pipes.
- Insulation of pipes should be done in proper way because steam gets condensed in absence of insulation.
Avoid Water Hammer by Air vessels
How to Avoid Water Hammers using Air Vessels?
It is pretty much simple in design. It is made of a big container and it is joined to the pipe. It is filled with air and water. As air is less dense than water it gets trapped by water coming from pipes.
When a shock wave appears in the pipeline it enters this vessel and air gets compressed for a time being due to this pressure. So this extra energy got absorbed.
Limitations of Air Vessels
With time this air gets absorbed in water and water starts to occupy space of air. So whole effectiveness of absorbing shock decreases.
Also, there are chances of the formation of bacteria in such areas.
Avoid Water Hammer by Arresters
They are the same as air vessels. Just they are cleverly designed in such a way that water and air don’t make contact. Air is sealed in this chamber by providing a plunger with tightly sealed O rings.
This plunger is free to rotate up and down. So when a water shock wave comes this plunger moves up and compresses this air inside it. This way shock gets absorbed. It solves both limitations faced by air vessels. As there is no contact here between air and water, no bacterial spread can happen. Also stays there for a long time and doesn’t get absorbed in water.
How to prevent it in industrial systems?
What type of check valve is being used also plays a big role in this issue. For example valves such as swing, tilting disc, or piston style check valves are the ones that are dependent on gravity and flow reversal to push the valve into the closed position.
So when valves close, shock waves come down and slam hardly on this valve. So the seal of the valve and its mechanism both get affected. After that wave travels throughout the pipeline.
Silent or spring-assisted check valves are constructed in a bit different way. Spring is inside which pulls back the valve into a closed position before reversal of flow takes place. This whole process reduces the probability of a water hammer.
Air chambers or vessels as discussed earlier for home plumbing can also be used in industry. It is easy in construction as well as easy to install. It is fitted in tee-fitting. A small size pipe is there, which has an empty air chamber inside. This air is trapped on one side plunger and on another side there is water.
When water expands and exerts pressure, the plunger compresses that air so this reduces the magnitude of hock wave and that unwanted energy gets absorbed instead of damaging the pipeline.
Along with this, the following steps can be taken –
- Either old system should be refitted.
- Pressure regulator and pressure reducer should be used at appropriate places.
- System operating pressure and other variables should be relooked and reduced if possible.
- Use of air chambers should be promoted at places which are more vulnerable to such shocks.
- Selection of proper check valves.
|Phases||Condition of occurrence of water hammer|
|Design||Improper selection of steam traps. Improper place selection of steam trap Improper drain size Construction and material used is not up to standard as decided in the design phase.|
|Operation||Absence or improper standard operating procedure – ex – fast opening of the valve, instead of the use of hand wheel to operate it. Carelessness of operator|
|Maintenance||Damaged component or defective setup. Insulation breakage. Steam traps have been eroded. Collection of foreign particles in a steam trap.|
This phenomenon of Water hammer happens in our life many times and we don’t even notice it sometimes. In our day we use tap or valve many times, we close them very fast due to our constant hurrying nature. This article was just to tell us that our small actions to have some repercussions. Hence we should care about handling same from next time.
Hence, we have learned all about water hammer, definition, meaning, equation, etc.
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