Why can't the oil pipeline stop suddenly?

When a high-speed car suddenly brakes, passengers often lean forward or even get injured because of inertia. Similarly, when the pipeline valve suddenly closes, the pressure in the pipeline rises sharply because the fluid cannot stop immediately and self-compresses and collides with the pipeline, and the sound of knocking on the metal can often be heard. People call this phenomenon of sudden pressure change (pressurization or decompression) caused by sudden change of flow rate "water hammer". According to the kinetic energy theorem, the reduction of fluid kinetic energy is equal to the work done to the outside world, and the work is equal to the product of impact pressure and displacement. Therefore, when the fluid is easy to compress (such as gas) or the pipe wall is easy to expand (such as rubber pipe), the impact pressure is very small, and vice versa. From the energy point of view, part of the kinetic energy of the fluid is dissipated through frictional heat, and the other part becomes elastic potential energy after the fluid contracts and the pipe wall expands. After that, with the mutual transformation and transfer of elastic potential energy and fluid kinetic energy for many times and the influence of friction resistance, the fluid will flow back and forth along the pipeline, and the impact pressure will also spread back and forth with it, but the amplitude is getting smaller and smaller, just like the situation after the train suddenly braked.

The mutual transformation of kinetic energy and potential energy and the transmission diagram of pressure wave 1: The locomotive suddenly braked but the vehicles behind it ran to the left as usual.

Figure 2: The vehicle stops after compressing the spring and hitting the preceding vehicle and locomotive, and part of the kinetic energy of the vehicle is converted into the potential energy of the spring.

Fig. 3: Under the thrust of the spring, the vehicle runs in reverse to the right in turn, and part of the potential energy of the spring is converted into the kinetic energy of the vehicle.

Figure 4: Due to inertia, the springs extend one after another, and the vehicle stops again, and part of the kinetic energy of the vehicle is converted into spring potential energy again.

Figure 5: Under the action of spring tension, the vehicles move to the left in turn, but at a much slower speed.

The magnitude and direction of vehicle speed and elasticity change periodically, and are static after repeated and attenuated many times.

If the fluid in the pipeline is imagined as many elastic fluid segments in series, it is not difficult to understand that the water hammer process is very similar to the above phenomenon. Starting from a certain point in the pipeline, due to the influence of water hammer, the pressure at this point changes periodically near the normal pressure and continuously decreases. The pressure is higher than the normal pressure in half a cycle and lower than the normal pressure in half a cycle. Seen from the whole pipeline, the attenuated positive pressure wave and negative pressure wave propagate back and forth along the pipeline.

Because the oil is difficult to compress, the oil pipeline is difficult to expand, and the impact pressure is high, which is easy to cause major accidents: the pressurization wave may overpressure the place with higher normal pressure, suppress the pipeline and damage the equipment. Decompression wave may produce "negative pressure" at a lower normal pressure-the pressure is lower than atmospheric pressure, which may lead to the danger of crushing large-diameter pipelines, or the pressure is lower than the saturated vapor pressure of oil products, which will evaporate oil products, form bubbles and cavities, and separate oil columns. When the separated oil columns recombine, a larger surge pressure may be produced.

Obviously, slowly closing the valve is the fundamental way to reduce the surge pressure of the oil pipeline, and various automatic control technologies (such as pressure relief protection and water hammer advance protection) can also avoid the harm of water hammer.