VENA Contracta

It’s amazing to learn that small changes could bring the big differences. I am talking about science, in other words engineering and one would know it better through experience only.

I have been working on a project related to pressure relief valves which requires an approval from National Board, and should be in compliance with the ASME standards and codes. The National board approval is possible only if we could achieve a certain amount discharge of the medium obeying all other operational requirements. We could able to follow all operational requirements but achieving required discharge resulted in futile effort. Everyday, we tried with new spring and new set pressure values but the result was same. No matter how many changes we made in springs, the results always backfired at us. Then we started digging into our basics and I believe, that was the best step of the whole project.

In our valve, we have a nozzle which supposed to have an angular step on the pathway of medium. But unfortunately the step, which was supposed to be a smooth transition, was a sharp step (like a chamfer), causing an obstacle in the flow path. This obstacle may seem to be a tiny one, but the theory behind the low discharge was outstanding. This small step caused the flow to form 'vena contracta'.

At the 'vena contracta' pressure drop occurs. Even though it gains some pressure after passing through the vena contracta, it would always be less then the initial pressure. Even though the valve opens at the rated set pressure (as per the spring force), the drop in pressure causes the valve to close very early which would eliminate the option of reaching the full capacity at required/predefined pressure value. Hence, the low discharge. The slight inaccuracy in transition (dimension of the transition 0.025”) resulted in deficient discharges (almost 200 lb/hr less), isn’t it amazing??? I am not quite sure if this was the main cause for the low discharge, but I can say that it was one of the definite reasons for the failure.

Even though, we all read about the vena contracta, but we would understand its affect only when we face this kind of problem. In fact, not only this, but all the theories.
(Picture taken from Spirax-Sarco Ltd)