What is a pressure surge or water hammer and what causes them?
The pressure surge could happen in a pipeline if there is any change in the rate of flow in a pipeline. So the change in velocity of moving fluid in a pipeline can cause the pressure surge, if the flow change is very rapid then the surge can cause many problems in a pipeline. The surge could be propagated at acoustic speeds and this would vary with materials and the thickness of the pipe.
The surge can be determined by calculating the increase in pressure due to the surge by assuming the instantaneous valve closure. A longer closure time can reduce the surge force but it won’t change the maximum surge pressure. The maximum fluid transient loading will pass through the piping only once and no reflection will be considered. If a valve is closed or a pump is tripped then a positive pressure wave is generated in the upstream direction and a negative pressure wave is generated in the downstream direction. This drop-in pressure in the downstream side can cause cavitation and backflow, and this would create its own surge effect. The pressure surge in the pipeline can cause the failure of the piping system or the equipment and it can also cause the failure of the pipeline, supports, instrumentation, and components.
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What are the factors that could cause water hammer/surge?
- The fluid property must be considered like density, bulk modulus, flow velocity, temperature
- It could be due to the piping layout like plot plan, piping class, isometrics
- Normal/control shutdown, emergency shut down
- It could cause because of the closure of the valve or pump shut off, we must consider the accuracy of the closing time
- It could happen because of the operational changes or because of changes in layout
- Temperature changes in the water can also cause the pressure surge
We must consider the transported medium
If the transported medium is gas then the velocity would be high and the changes in the velocity would be high too. Density would be low and the pressure pulsation is moderate. If the transmitted medium is fluid which would be used for most of the process, velocities are low and the velocity changes are low when compared to the gases. The density will be high and pressure pulsation will be high too.
What are the problems that can be caused by the surges in a pipeline?
- When the pressure is really high, it could lead to the deformation or ruptures in a pipeline and components
- If the pressure is too low then it could cause the collapse of the pipeline, leakage into the line at joints and seals
- The reverse flow could occur and this could cause damage to the seals, brush gears on motors, draining of storage tanks and reservoirs
- Pipeline movement and vibration could also happen because of the surge and this could cause overstressing and failure of supports which would eventually lead to the pipe failure.
- Large unbalanced loads can cause pipe movements
- If the surge pressure is below the operating pressure then it could cause cavitation and also the collapse of the pipeline
How to control the water hammer?
- Use pipelines that have a high-pressure rating
- Slowly closing valves: we must use valves that close slowly like a diaphragm or a gate valve
- Low fluid velocities
- The pipeline control must be properly done like the start-up and shut down process
- Shorter branch pipe length
- Shorter lengths for straight pipe: expansion loops and also elbows, the elbows can reduce the pressure waves
- We can do looped piping, so lower velocity flows from both sides of a loop and can serve a branch
- Flywheel on pump
- Pumping station bypass
- Use air vessels
- We can use surge tanks to prevent the pressure surge, so any water hammer in the pipe would blow into the surge tank and the pressure would be vented safely to the tank void
How to calculate the time of closing the valve in order to avoid the surge pressure?
- Required time for closing the valve-like suddenly or gradually
- The flow velocity must be less than 3m/sec
- Length of the pipe if the pipe is shorter then the chances of water hammer is high
- The elastic property of the pipe and also the elasticity of the fluid
The time is depended on the
- Pipe length
- Speed of pressure wave
- Positive and negative pressure waves
During the fluid flow in the pipe, if the valve closes suddenly and stops the flow, the kinetic energy will be changed into elastic resilience and creates a serial positive and negative pressure wave vibrating back and forth in the pipe till the energy lost by friction. So if the liquid in the valve stops, the kinetic energy of the water will be converted into potential pressure.
Pressure wave travelling time from the valve and back to the valve can be calculated by
t = 2L/c
L= Pipe length in meter, C= Speed of pressure wave (meter/second)
If the valve time of closure TC is greater than 2L/c then the closure is gradual and if the TC is less than or equal to 2L/c then the valve closure is considered as sudden.
What is the need to do the surge analysis?
We must do surge analysis so that the pipeline and the apparatus which is connected to it will be protected. Surge analysis can protect human life and it can reduce the cost. A surge analysis could only be accurate as the system data entered as inputs. Only if the input is accurate and the computation model is a faithful reproduction of the real system conditions, will the analysis yield a high degree of accuracy. We must be very careful while doing a surge analysis if a certain condition exists.
- If the maximum change of fluid velocity exceeds 4 fps
- If the length of the pipeline exceeds 1000 feet
- If there are fast-acting valves in the system
- If there are pumps in the system
What are the documents that is required for the surge analysis?
- Pump datasheet
- Piping drawings inside pumping stations
- All chamber drawings inside the pumping station
- Site layout in order to determine the surge vessel location
- Rising main drawings
- Discharge chamber drawings
- Details of the air valve
- Details of the non-return valve
- Pump operating levels
- The wall thickness, length, and diameter of the pipeline
- Material that is used for the construction of the pipeline, pressure class and design pressure of the pipe, and also how the pipes are mounted like buried or placed on supports
How can we do the surge analysis of a system?
- We must make a surge model of the system and check the steady-state, separate model for each system maximum and minimum pressure
- Analyze upset conditions
- Evaluate transient results: pressure, velocities, and unbalanced forces
- The time of the unbalanced force must be determined and should add mechanical model, time histories per relevant node
- We must make modifications in the system where it is required like, additional supports, axial stops, reinforcement, etc.
How to mitigate pressure surge?
We can use different types of measures to mitigate the pressure surge in a pipeline, but without any analysis, we won’t be able to do this. So just make sure if there is a problem of a surge in our system which is beyond the design pressure. We must make sure that the devices that are used to do the mitigation of surge pressure are maintained properly. So there are many ways to do surge mitigation and some of them are
- Providing relief valves
- Providing surge tanks
- Increase the diameter and wall thickness of the pipeline
- Increase the closure time of the valve.