M Tech, Chartered Engg.
Just imagine thousands of people running a marathon race and all are passing through a big gate. Someone suddenly closes the gate. What will happen? Those who are near to the get will hit badly on the doors, the people who are away from gate do not know what has happened and continued running. They will collapse on each other creating a huge chunk of people in small area. Slowly when they realize that the door is closed they would turn back and move in opposite direction. But still people far away are running towards gate. Somewhere at mid-way there will be a big chaos.
1. Sudden valve closures and openings
2. Pump Start-up / trip
3. Column separation / vapor Collapse
Valve actuation (closure / opening) is one of the primary causes of surge events. Rapid valve closures can introduce surge into the lines through two events.
On the upstream side of the valve a compression wave will be created due to the change in momentum of the fluid. This wave will then travel back to the pumping source and on the downstream side of the valve the momentum of the fluid will carry it away from a now fixed wall. This will result in a rapid pressure decrease near the valve that can lead to the formation of a vapor pocket.
Because of their quick acting nature, the effect of the valve actuation should always be considered appropriately. Actuation of the main inline valves can cause distinct pressure changes in lines as the mass flow rate is either increase or decrease significantly. For these reasons, the logic behind valve actuation should be considered and appropriate cases should be developed to determine the maximum line pressures that can occur during valve actuation events.
Also, while evaluating the Surge, elevation changes through the pipeline run must also be considered as they will cause pressure changes due to the change in fluid head. During transient events, the changes in pressure due to varying flow rate as well as differential head plays a vital role in defining the pressure profile.
Additionally, it is to be noted that the surge pressure is directly proportional to the wave speed and the velocity in the fluid. Thus, the surge pressure will be higher with higher wave speed and higher velocity. Whereas, the wave speed is a function of liquid density and other parameters such as Young’s modulus of elasticity of pipe material, bulk modulus, pipe outside diameter and pipe wall thickness.
Other major factor contributing to Surge pressure is the closing time of the valve. An extremely short closing time leads to high surge pressure and thus high surge force.