Flow and Pressures Through Parallel Connected Pipes

"No results found for http://imageshack.us/photo/my-images/835/problemub.jpg/".

Use the electrical circuit analogy, and apply Kirchoff's Law and Ohm's Law.

here's the image:

As there is only one pipe, the question must now be asked, "what is the original poster going on about", perhaps.

There is no flow through the PRV until the source pressure is >= 500kPa.

The problem (see image) states that the source is able to maintain flow at x m³/h to keep pressure at the PRV at 600 kPa.

You ask if " does this mean the pressure drop to atmospheric across the PRV is greater than the pressure drop through to the actuator? ".

By the phrase 'pressure drop through to the actuator', I'm assume you mean the pressure acting on the actuator. Is that correct?

I also assume this is compressible fluid (air) since you state venting to atmospheric. While you may mean that, it's entirely possible to vent to a reservoir of hydraulic fluid that is atmospheric pressure (dumping to the top of the tank).

Is that correct?

I should've been clearer in the OP, say you apply a flow which results in 500kPA at the PRV, at the instant of the pressure at this point reaching >500kPa, the PRV opens since the spring is tuned to open at 500kPa. At this point, the PRV vents to atmosphere, if it does open 100% at 501kPa (ignore the original diagram), what does this mean for pressure across the ball valve? Will it only ever see 480kPa no matter what the upstream pressure is so long as its >500kPa?

ignore that, let me re-iterate, if the flow when the pressure 500kPa at the PRV is 50m3/h, the flow increases to 70m3/h, the pressure at the relief valve becomes 501kPa, the relief valve opens completely, is the pressure drop across the ball valve/pipe/actuator 501kPa or 500kPa?

Please read this #8 at least two times!

Then read it again.

And again.

thanks for GREAT help. This is not a homework question otherwise I wouldn't be here. I'm an electrical engineer trying to broaden my horizons on instrumentation. Don't refer me to my instructor, I don't have one. If you plan to refer me somewhere else, then don't bother replying and leave the question unanswered, save us all the energy in having to read useless responses.

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Since you are more familiar with electricity I shall try to explain with an electric similitude.

Let assume you have a current source with which you supply to a resistor. You want to maintain the voltage drop under a value but either your source is not constant or your resistor is variable . You can use a transistor with a voltage divider and a series resistor to derive some of the current so that the voltage will never go over the set threshold. The source of current is to be assimilated to the pump which delivers a flow of incompressible fluid. The varible resistor is the load to be assimilated with the actuator and the pressure relief valve is the equivalent of the transistor combination which is blocked as long as the pressure (voltage) does not rise over the set value by the volatge divider.

Let aside all complications about how valves work and all other comments which only make you dizzy.

It si VERY easy to make similitudes between hydraulics, pneumatics and electric systems. Easier for hydraulics due to incompressibility (technically speeking and only for low frequencies) than for pneumatics but it works well for both.

I'm not sure I follow you completely.

Let's first start with some basics. Is the fluid compressible (gas) or incompressible (liquid)?

What is the load on the actuator? Is it moving a load against gravity or just moving a mass?

Thanks very much Anthony, that is very helpful!

May I add in electrical terms, it will be the equivalent mechanical hysteresis of the PRV (between its sensitivity to flow fluctuations and its mechanical opening/closing response time) that will be the determining factor. Which if not properly set or spaced apart, will lead to a more sustained type of oscillatory condition.

This allegedly simple system is actually rather complex, depending on valve and actuator specifics. To add a bit to Anthony's GA:

Relief valves (to say nothing of rupture disk relief devices) come in various designs. Some of them more or less blow wide open, and may or may not reseat. Others will crack open slightly at the set point, and open progressively wider as the upstream pressure increases. A common arrangement is that the valve attains its rated capacity when the upstream pressure accumulates to 110% of the cracking pressure. In the present example, this would mean that the upstream pressure would build to 550 before the relief valve reaches its rated flow.

Then you would need to determine how the downstream ball valve and actuator will respond. A key concept here is the flow coefficient (Cv or Kv) of various valves at various fractions of full opening.