Hammer Effect in Pipelines: Packing, Attenuation & Flow

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In summary: Now, consider the situation where one of the valves is opened. In this case, the fluid will start flowing through the open valve and the two neighboring pistons at the ends of the cylinder will start to move away from each other. In addition, the two pistons at the middle of the cylinder will start to move towards each other. The net result is that the overall velocity of the fluid will decrease, and the pressure will also decrease.This is analogous to the situation where the viscous dissipation occurs in the real world. The compression and expansion of the fluid damps out the waves, and the pressure eventually returns to its original value.
  • #1
Mikealvarado100
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Assume a pipe which can be closed rapidly simultaneously from both sides and just one wave can propagates along it. (This model is completely subjective and can not be done actually. Because if both sides of a pipe close rapidly and simultaneously, then two waves propagate not one). The pipeline has friction throughout.

  1. Is any flow in this pipeline after closing both sides?
  2. Do Packing and Attenuation occur for this pipeline?
  3. Thank you; Mohsen
 
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  • #2
Mikealvarado100 said:
Assume a pipe which can be closed rapidly simultaneously from both sides and just one wave can propagates along it. (This model is completely subjective and can not be done actually. Because if both sides of a pipe close rapidly and simultaneously, then two waves propagate not one). The pipeline has friction throughout.

  1. Is any flow in this pipeline after closing both sides?
  2. Do Packing and Attenuation occur for this pipeline?
  3. Thank you; Mohsen
What are your thoughts on this?
 
  • #3
Hi
Actually I think there is no any flow in the pipe (like what was before closing the valves), but compression of water (due to hammer phenomenon) creates flow in the closed system (which can make Packing and Attenuation). It means that friction can dampen the wave after a while.
Am I right?
 
  • #4
Mikealvarado100 said:
Hi
Actually I think there is no any flow in the pipe (like what was before closing the valves), but compression of water (due to hammer phenomenon) creates flow in the closed system (which can make Packing and Attenuation). It means that friction can dampen the wave after a while.
Am I right?
Yes. Exactly.
 
  • #5
Hi
Let me use this opportunity to ask about 'Attenuation' phenomenon. Does Attenuation is called to the headloss of flow due to compression of fluid (regarding to my previous explanation)? I know what Packing is. But about Attenuation, am I right?
 
  • #6
It means that the magnitude of the pressure variations is decreasing with time as s result of viscous dissipation.
 
  • #7
Thank you very much.
Attenuation is decreasing in head (pressure) which is due to viscosity OR Decreasing in head (pressure) which is due to density change (which results flow)? Which one?
explain it more please.
Mohsen
 
  • #8
After the valves are closed, the original kinetic energy of the fluid will be converted into compressional and expansion energy of the fluid, and will also be partially present as kinetic energy. The compressional and expansion energy will exchange with the kinetic energy as time progresses, as the expansion and compression waves pass up and down the fluid in the pipe. However, if there is no viscous dissipation, then the total of all these energies will never change, and the exchange will continue forever. Viscous dissipation damps out all this mechanical energy and converts it into internal energy of the fluid. This will result in and increase in internal energy of the fluid and an accompanying slightly higher temperature of the fluid (if the system is adiabatic). And, in the end, the pressure will be uniform, the density will be uniform, and the velocity will be zero everywhere.
 
  • #9
Dear Chestermiller
You are right, But I can not still comprehend why you focus on Viscosity not Density. Compressional and expansion, changes Density of the fluid. Why you focus on Viscosity? This is my question exactly. What is the effect of decreasing and increasing of pressure (Compressional and expansion) on viscosity and density separately?
This phrase made me confused: 'if there is no viscous dissipation, then the total of all these energies will never change, and the exchange will continue forever'. Would you explain more about it? Do you mean pressure increasing makes higher viscosity and pressure decreasing makes lower viscosity? If so, what is the relation between this and attenuation?
Thank you very much for your answer in advance.
 
  • #10
I'm afraid I haven't explained this very well, so I've gotten you terribly confused. So I'm going to try an entirely different approach. I'm going to introduce an analog system to our fluid in a tube that exhibits qualitatively all the key features of our fluid, including mass (inertia), compression/expansion, and viscous dissipation.

Consider a horizontal tube, closed at both ends, with a series of n equally spaced frictionless pistons situated along the cylinder axis, each with mass m. Each piston is connected to its two immediately neighboring pistons by a massless spring of spring constant k. The two pistons at the very ends of the cylinder are connected to the closed ends of the cylinder by a spring of spring constant k. This is case A. The pistons of mass m are analogous to the distributed mass of the fluid. The springs are analogous to the distributed compression/expansion behavior of the fluid. Now, at time zero, each of the pistons is given an initial velocity of v0. This corresponds to the initial velocity of the fluid at the time that the valves are closed. The total kinetic energy of the pistons at this time is ##nm\frac{v^2}{2}##. As time progresses, the total elastic energy stored in the springs plus the sum of the potential energies of the n pistons will be equal to this same value, and it will never change. The way that the velocities of the pistons and the elastic energy of the springs is distributed at any time is immaterial. This corresponds to the case where the fluid viscosity is zero.

Case B: Now consider the same model as above, but instead of just a spring being attached between each pair of pistons, there is also a damper. There are also dampers connected between the two pistons at the very ends and the closed ends of the cylinder. In this case, as time progresses, the viscous response of the dampers dissipates the sum of the kinetic energies of the pistons and the elastic energies stored in the springs. So, even though the total mechanical energy still starts out as ##nm\frac{v^2}{2}##, as time progresses the total mechanical energy decays to zero. At infinite time, the springs are at their unextended length, the velocities of all the pistons is zero, and the pistons are all equally spaced.

During the deformations in these two models, there will be regions where some of the springs are compressed and some of the springs are extended. This corresponds respectively to increased density and decreased density. But, in the end, the density is again uniform.

Are you able to understand these two cases and how they relate to your fluid problem?

Chet
 
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  • #12

1. What is the "hammer effect" in pipelines?

The "hammer effect" in pipelines, also known as water hammer, is a hydraulic phenomenon that occurs when there is a sudden change in fluid flow or pressure in a pipeline. This can result in a sudden increase in pressure, causing a shock wave that travels through the pipeline and can cause damage or failure if not properly controlled.

2. How does packing affect the hammer effect in pipelines?

Packing, or the material used to seal the pipeline joints, can have a significant impact on the occurrence and severity of the hammer effect. If the packing material is too stiff or rigid, it can increase the intensity of the shock wave. On the other hand, if the packing is too loose or weak, it may not effectively absorb the shock wave, leading to potential damage.

3. What is attenuation in relation to the hammer effect?

Attenuation refers to the reduction in intensity of the shock wave as it travels through the pipeline. This can be achieved through various methods such as using flexible pipe materials, installing pressure relief valves, or incorporating dampeners. Proper attenuation can help minimize the impact of the hammer effect and prevent damage to the pipeline.

4. How does flow rate affect the hammer effect in pipelines?

The flow rate of fluid through a pipeline can greatly influence the occurrence and severity of the hammer effect. Higher flow rates can lead to faster pressure changes and increase the likelihood of water hammer. Properly sizing the pipeline and controlling the flow rate can help mitigate the hammer effect.

5. What measures can be taken to prevent the hammer effect in pipelines?

There are several measures that can be taken to prevent the hammer effect in pipelines, including installing check valves to prevent reverse flow, using flexible pipe materials, and incorporating pressure relief valves and dampeners. Regular maintenance and inspection of the pipeline system can also help identify and address potential issues before they lead to water hammer.

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