2 replies to this topic

[sammy p]

i was wondering. i remember it was explained once to me. if a valve either increases or decreases the area for the fluid to flow then howcome the change in presssure will not just get larger and the speed fastger and the flow rate remain constant?

thanks

[Art Montemayor]

Sammy:

First off, it’s not your English; it’s your explanation that is horrible and I think that is why you are getting your fluid characteristics confused. I’m assuming you are a university-level Chemical Engineering student.

What I think you mean is:
As a fluid valve is either opened or shut, its orifice for flow is getting bigger or smaller. As the valve opens more, the pressure drop across the valve becomes less. As the valve closes more, the pressure drop across the same valve becomes greater.

Under the above conditions, the fluid cannot be maintained at a constant flow rate – using the same valve and pipe size. This is analogous to choking a person around the neck. The more you choke, the less words he/she can speak. The less you choke, the more he/she can say to warn you that it hurts.

As you throttle a valve (close down on it) you increase the pressure drop across it and make it harder for the fluid to flow. The result is less fluid. There is a factor used to measure this effect. It is called the C_V factor and it is used to designate the capacity of various types of valves.

The average velocity flowing through the valve is incidental. It is the average velocity in the associated piping that is used as a guideline – and here, the cross-sectional area is constant and thus the average velocity is kept constant for a given amount of volumetric fluid capacity. Here, I’m assuming that we are discussing a liquid fluid, not a gaseous fluid (otherwise categorized as non-compressible and compressible flow).

[JBradley]

Aww - I think it was a bit harsh calling your explanation horrible!!! I think I understood anyway.

It would depend on how you're pumping the liquid through the valve. If it's a centrifugal pump then, from looking at the pump curve, the volumetric throughput will drop as the pressure is increased (i.e. the valve closed (or constricted I guess I should say)). Which is why the velocity through the valve doesn't just increase.

If you have a positive displacement pump then you'll get just what you said. The valve will close and to conserve the mass balance the liquid velocity will just increase local to the valve. But a flow control valve for a PD pump wouldnt be in-line, you'd put it in a return loop.