In this blog we will discuss Velocity Head as it is the final component left of our 6 part NPSH blogs and the one we have avoided addressing previously.
Reference books would describe velocity head as a basic concept of fluid engineering that represents the bulk motion, i.e. the kinetic energy, of the fluid. It can also be converted into the pressure that the fluid would gain were it to be stopped without any energy loss.
Physics books would quantify it as velocity squared over 2 x Gravity.
Unfortunately, or fortunately depending on how you look at it, most people didn’t take physics in school and/or have not opened a reference book in a few years so what does velocity head mean to them?
Here is my guess: imagine a stream of liquid in a pipe entering a pump as if it were a train moving along a set of railroad tracks entering a station. If the locomotive driver is in neutral at a velocity of 20 miles per hour, as he enters the station and hits a single stationary train car, that single car would easily be pushed out of the way by the train. The push that the train had on the single stationary car as it entered the station is similar to the push a moving liquid has as it enters the suction eye of a pump. In the pump industry that push or pressure is referred to as “Velocity Head”.
Did the train analogy leave you with unanswered questions? Still unclear about the concept of velocity head? Well then lets approach the subject from a different angle.
The drawing below illustrates a pipe with two independent pressure taps. The tube to the left will measure the static pressure in the pipe. Any instrumentation attached to this tube will register a consistent reading whether the fluid is moving down the pipe or not. The tube to the right has an opening facing the direction of flow. This tube will be subject to a ram effect and register an increase in pressure as the velocity of the fluid increases.
The difference in pressure readings between the left and the right tube is the head resulting from the velocity of the fluid, otherwise referred to as “Velocity Head”.
Looking back at the formula used in previous blogs to calculate net positive suction head available, we see how Velocity Head (Hv) fits into the overall NPSHA formula.
NPSHA = Ha +/- Hs – Hf + Hv – Hvp
With Velocity Head being the last and final factor of NPSHA, this concludes our 6 blog segment on NPSH. If you missed the previous blogs, they are available in our Tech Talk section of our Blog pages. In the next blogs we will be starting a segment that investigates the connection between NPSH and cavitation.
Until Next Time,