Help in understanding pressure regulator flow rate chart

[pietro82]

Hi all,

I'm looking for some pneumatic pressure regulator datasheet and there are some pressure - flow rate that are a bit strange, therefore I'm asking you for some help.

I usually have seen charts like this one (Camozzi ER104 valve)

http://s2.postimg.org/vwftwxzqh/chart_3.jpg

Where the envelope curve is quite similar to a pressure - flow rate curve of a flow through a nozzle.

In a Festo catalogue there is this kind of chart (Valve Festo MMPE range: 0-1 bar):

http://s27.postimg.org/ax4rqhvur/chart_2.jpg

Look at the dotted area: as you can see to increase the flow rate the outlet pressure must be increased. Why? The pressure differential between the inlet and the outlet is decreased, therefore the pressure should decrease, right?

There are also other charts like the following one (Valve Festo MMPE range: 0-10 bar):
:

http://s27.postimg.org/fhqy5ffk3/chart1.png

It's surprising that the flow rate at 2bar is lower than 4 bar, even if is the line with the highest pressure differential between the inlet and the outlet, why?

I would believe the two festo valves have a kind of special feature that changes its bahaviour or maybe I'm a bit mislead. Any help is appreciated.

Thanks


Cheers

 

[Compositepro]

All curves are the result of the specific design of each regulator. Look at the design drawings. Many regulators have a pitot tube in the outlet that causes the valve to open more at high flow rates to compensate for droop due to spring compression.

 

[pietro82]

Hi,

Thanks for your reply, I didn't know the design with the pitot tube and this will explain me the behaviour of the second chart. I still don't understand which design brings the one of the third chart.
There is also something else I haven't understood about datasheet parameters of pressure regulators: there is the step response at zero volume. I would believe it's not phycally measured, right? How is it calculated? Is there any standard about it?

thanks

cheers

 

[gerhardl]

I am notquite sure about your questions, but:

If you think about a pressure regulator mechanically it is often constructed as a membrane or disc beeing mechanically adjusted to lower or higher opening (eg. flow) and balanced mechanically as several forces together act under and over to balance the disc. The forces are a composite of mechanical forces (often comnined with springs) and pressure over and under pr. area unit times exposed area.

This will explain why mechanically regulation of opening can expose different areas, different actions of springs and different reactions on areas from flow/pressure conditions.

Any valve starting opening will at crack opening level give turbulent, even sometimes 'cavitation' conditions briefly, flow is impossible to measure accurately before the flow stabilizes until furter opening is given (often to one to three percent of full flow, depending of valve type).

Sometimes this is simplified in charts, and the laminar flow at say, 2 %, is shown as the 'true' value down to opening moment. This is of course not true, no valve will go from zero opening to accurate flow conditions instantly.

 

[pietro82]

Well I don't really understand the behaviour of the valve festo VPPE. Here it's steady state pressure-flow rate chart.

Link

it seems he valve flow area increases with the pressure and it's suprising to me. In the pressure regulators I know the valve area is proportional to the pressure differential between the inlet and the outlet. In the valve datasheet it's written: "Pilot-actuated diaphragm regulator", is it the following kind of valve?

http://s30.postimg.org/vh2e1moyp/pilot_valve.png

Thanks

 

[Compositepro]

What is the point of speculation? If you are curious why a valve curve looks a certain way, then look at the drawing for that specific valve. Design possibilities are infinite and some are really fascinating. That is why there are many manufacturers of pressure regulators. I am amazed by instrument grade regulators that contain pneumatic amplifiers and can regulate pressure to a fraction of an inch of water column.

 

[pietro82]

Hi Compositepro,

I'm sorry for the delayed reply, I have found some useful information and it tooks a while to meditate about them. I'm trying to model a pneumatic pressure regulator in order to perform some simulations of a pneumatic system. I don't have to reproduce precisely all effects of a specific pressure regulator, but only the curve I find in the regulator datasheet. Here the flow rate - pressure curve of the pressure regulator I'm trying to model:



The thing I really don't understand is: Why the valve opening area decreases with increasing of the outlet pressure?
I supposed it was due to force pressure unbalance to the poppet, so I derived the poppet force balance equation in static condition and from my calculation the area have to decrease with increasing of the outlet pressure. Is there any other effect? I don't understand.

Here the valve technical drawing of the valve of before mentioned curves:


I admit I didn't understand so much the drawing: are the things I typed correct? What is the function of the ball valve? I believed its position set the valve opening area, but it seems the spring is not connected with the ball.

Thanks for your helps

cheers