Will a Venturi flow tube in a filter deliver accurate flow under negative head conditions?
Will a Venturi flow tube in a filter deliver accurate flow under negative head conditions?
(OP)
I have noticed that the gravity flow filters in our water treatment plant have been allowed to run off the scale on loss-of-head gauges and I am wondering if a venturi is accurate if the flow is actually being pulled by vacuum (or very little head)rather than a pressure flow. If it does cause flow to be wrong what would you predict the outcome, an error high or low.





RE: Will a Venturi flow tube in a filter deliver accurate flow under negative head conditions?
RE: Will a Venturi flow tube in a filter deliver accurate flow under negative head conditions?
Depending on the precise shape of the venturi, it may have a more rapid reduction in size in one direction when compared to the other; most I have seen have a quicker reduction from pipe diameter upstream with a more gradual transition back to the original pipe diameter in the downstream direction, so it may or may not give equivalent readings for equivalent velocities in opposite directions, but I would not expect them to be equivalent without a calibration to specifically verify that supposition.
Independent events are seldomly independent.
RE: Will a Venturi flow tube in a filter deliver accurate flow under negative head conditions?
RE: Will a Venturi flow tube in a filter deliver accurate flow under negative head conditions?
Your basic understanding of fluid flow is somewhat off as well. No fluid is " pulled by vacuum (or very little head)rather than a pressure flow." as you contend. All fluid motion is by higher energy (static head plus velocity head) on one side, lower on the other side. If you convert all pressures and velocities to absolute pressures and heads, obtain the EGL (energy grade line), you will ALWAYS see higher energy levels on one side of a fluid in motion, lower pressure to where it is going. The venturi doesn't care how flow is generated, "pressure or vacuum" is the same in absolute terms. Vacuum pressures do not convert to negative absolute pressures. Within the variation caused by velocity head, flow is always from higher pressure to lower in absolute pressure terms.
You may need to pay more attention to what your operation crew is telling you, rather than what you want to tell them. Listen a bit closer. Tell us what "loss of head" means in real terms to the venturi gauge location and come back with any more questions.
Independent events are seldomly independent.