Pressure test of pipelines
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Before conducting the pipeline hydrostatic test, exhaust the air from the pipeline and fill it with water using a pump. Water can be considered incompressible. If a pipe is filled with water, then when the make-up water pump operates, water should not be able to enter the pipe. So why does the pressure in the pipe still increase? Moreover, since there is no flow of water, how can the pump operate? Does the impeller not rotate? As long as the impeller is rotating, water is flowing through, which indicates that water is entering the pipeline. But wasn’t the pipe filled with water from the beginning? Moreover, water cannot be compressed; so I don’t understand how water can still flow into the pipes after the pump starts operating. I’m seeking help from experts to clarify this.
According to Bernoulli’s equation, the total pressure of water can be divided into three parts. They are respectively the pressure generated by height, the dynamic pressure generated by flow, and the static pressure within the liquid. A pressure gauge measures static pressure. Water is incompressible, but it can transmit pressure. In a closed piping system, by using a pressure pump to apply pressure inside the pipes, water may not flow in, but the static pressure increases. In a less precise terms, it can be understood as the pressure pump doing work inside the piping system; the dynamic pressure of the fluid flowing in the pressure pump is converted into static pressure within the piping system. Note that the water here is driven by the impeller to flow inside the pressure pump, generating dynamic pressure, rather than flowing into the piping system.
The test pressure for the water pressure in the water pipes comes from the pump; it can be understood as the condition where the outlet valve of the pump is closed when the pump is started