What is the maximum speed of water?

Question

Is there a maximum speed that water can be pushed through a pipe or hose such that it can't move any faster? What happens to water when it gets beyond that speed? Does it just not move faster or does it turn to steam?

Answer 1

Pipes have pressure on the upstream side, and less pressure on the downstream side. This pressure drives the flowrate, using math analogous to Ohm's law (Voltage = Current * Resistance ==> pressure = flowrate * resistance

Consider a garden hose. At the faucet, typically 60 to 90 psi. At the outlet, where it jets into the air, atmospheric pressure, i.e. no gauge pressure. So, the pressure drops along the length of the hose. If the hose is large (low resistance), lots of flow. For a tiny hose, small flow.

Most piping folks believe the limit is 2 to 5 ft/sec. I approved an application with 35 ft/sec that worked fine. I later found a high powered system that goes up to 50 ft/sec.

The speed of sound of water is about 3x faster than in air. This could be considered to be limiting, however, supersonic flows are theoretically possible, if you had enough pressure and flow capability (and money!). Water would not form steam except at very high temperatures, as it would be under pressure to achieve high flowrates.

Limits are normally considered to be abrasion of the pipe wall or collapse of liners in the pipe. As a practical matter, flows are limited to rates similar to the typical 2 to 5 ft/sec by the huge pressure drops needed to get above those sorts of flows. When high pressure pumps are needed for pressure drops in the pipelines, generally more economical to use larger pipes.