Does size really matter?

[PAB_OH]

Mad,
Not to be nit-picky, I just thought I might shed some light on your question pertaining to how flow in ft/sec relates to PSI. In short, it doesn't. What those numbers represent is ideal conditions conducive to laminar flow, if I recall correctly. It has been a while since I took Fluid Power in college but I believe that is what those numbers represent.

In all reality there is minimal pressure present in a line when there is flow, it is when you stop the flow that you begin to build pressure. A pump doesn't create pressure, it merely creates flow. When that flow is impeded, you get pressure. So the size of the line up until that point has almost nothing to do with pressure just how "good" the fluid flows through it.

Now, if the line is too small and your pump/motor output is high you will get turbulent flow in that line, which is inefficient at best and can lead to high temps and premature breakdown of the fluid at worst. By going to the next size up line you ensure that if you have a high output pump you are taking full advantage of the speed afforded you while not creating flow problems.

Once the pressure starts to build it is actually sometime better to be using a smaller diameter line/hose as the hoop stress is not as great (smaller diameter = smaller surface area for the liquid to act on inside the line/hose).

If'n you already knew this, sorry. Don't mean to preach to the choir. Also, if my "technical" jargon is off, again sorry. It's beena few years since I worked on hydraulics, I'm a bit rusty (no pun intended!) /forums/images/graemlins/grin.gif.

Hope this helps.

 

[PAB_OH]

Mad,
Not to be nit-picky, I just thought I might shed some light on your question pertaining to how flow in ft/sec relates to PSI. In short, it doesn't. What those numbers represent is ideal conditions conducive to laminar flow, if I recall correctly. It has been a while since I took Fluid Power in college but I believe that is what those numbers represent.

In all reality there is minimal pressure present in a line when there is flow, it is when you stop the flow that you begin to build pressure. A pump doesn't create pressure, it merely creates flow. When that flow is impeded, you get pressure. So the size of the line up until that point has almost nothing to do with pressure just how "good" the fluid flows through it.

Now, if the line is too small and your pump/motor output is high you will get turbulent flow in that line, which is inefficient at best and can lead to high temps and premature breakdown of the fluid at worst. By going to the next size up line you ensure that if you have a high output pump you are taking full advantage of the speed afforded you while not creating flow problems.

Once the pressure starts to build it is actually sometime better to be using a smaller diameter line/hose as the hoop stress is not as great (smaller diameter = smaller surface area for the liquid to act on inside the line/hose).

If'n you already knew this, sorry. Don't mean to preach to the choir. Also, if my "technical" jargon is off, again sorry. It's beena few years since I worked on hydraulics, I'm a bit rusty (no pun intended!) /forums/images/graemlins/grin.gif.

Hope this helps.

 

[MadReferee]

Thanks, no problem. I understand the physics, it's just that I didn't quite understand the reasoning for the rating in ft/sec.

 

[MadReferee]

Thanks, no problem. I understand the physics, it's just that I didn't quite understand the reasoning for the rating in ft/sec.

 

[Zoombah]

PAB_OH,

I'm impressed. Theoretical question for you. Same cylinder, same load, same 12.5 gpm pump, same max pressure 2,850 psi -- does the 1/2" flex-line extend the cylinder faster than the 3/8" flex-line? Or asked another way -- does the 3/8" flex-line restrict the flow enough or cause enough turbulence that the cylinder takes longer to extend than if a 1/2" flex-line is used? I think the answers to these questions are what I'm after.

The level of knowledge available on forums like TBN never ceases to amaze me.

Thanks
Zoom

 

[Zoombah]

PAB_OH,

I'm impressed. Theoretical question for you. Same cylinder, same load, same 12.5 gpm pump, same max pressure 2,850 psi -- does the 1/2" flex-line extend the cylinder faster than the 3/8" flex-line? Or asked another way -- does the 3/8" flex-line restrict the flow enough or cause enough turbulence that the cylinder takes longer to extend than if a 1/2" flex-line is used? I think the answers to these questions are what I'm after.

The level of knowledge available on forums like TBN never ceases to amaze me.

Thanks
Zoom

 

[PAB_OH]

ugggh!
Off the top of my head, ALL things being equal, the answer is no. I mean all things! The coefficient of flow for alll the fittings would have to be the same etc...

Obviously if you have a 1/2" flex line then you have a 1/2" fitting for it coming off your ridgid line which is probably 3/8", I'm assuming. I think the "K" factor for a 1/2" fitting is probably higher (or lower, can't rememeber which is better right now) than that of a 3/8" fitting. Basically using the bigger fitting and line will only HELP to ensure you don't create turbulent flow.

In reality, in a system this small (almost all CUT's) I would THINK that the any gain seen in speed would be minimal and is largely based more on the GPM of the pump. The big thing to avoid is long runs of narrow line, especially flex line. The ridgid line (SS tube for instance) is very smooth inside relative to hose and in addition hose can and will be bent, causing directional changes that will induce turbulence.

I hope I'm helping and not making it more confusing. Also, if anyone catches errors correct me. I feel like I'm skating on thin ice. It really has been some time since I've worked through one of these problems let alone "thunk on" the physics behind it.

 

[PAB_OH]

ugggh!
Off the top of my head, ALL things being equal, the answer is no. I mean all things! The coefficient of flow for alll the fittings would have to be the same etc...

Obviously if you have a 1/2" flex line then you have a 1/2" fitting for it coming off your ridgid line which is probably 3/8", I'm assuming. I think the "K" factor for a 1/2" fitting is probably higher (or lower, can't rememeber which is better right now) than that of a 3/8" fitting. Basically using the bigger fitting and line will only HELP to ensure you don't create turbulent flow.

In reality, in a system this small (almost all CUT's) I would THINK that the any gain seen in speed would be minimal and is largely based more on the GPM of the pump. The big thing to avoid is long runs of narrow line, especially flex line. The ridgid line (SS tube for instance) is very smooth inside relative to hose and in addition hose can and will be bent, causing directional changes that will induce turbulence.

I hope I'm helping and not making it more confusing. Also, if anyone catches errors correct me. I feel like I'm skating on thin ice. It really has been some time since I've worked through one of these problems let alone "thunk on" the physics behind it.

 

[MadReferee]

</font><font color="blue" class="small">( does the 3/8" flex-line restrict the flow enough or cause enough turbulence that the cylinder takes longer to extend than if a 1/2" flex-line is used?)</font>

I can't speak for 1/2" to 3/8" but some of us typically use 1/4" hose for controlling the top link. The 1/4" hose provides sufficient restriction to slow down the cylinder movement to allow for feathering. Using 3/8" hose will definitely make the cylinder move significantly faster.

Based on this actual application, I would have to say that a smaller hose would slow down the movement of the cylinder, given all else was equal.

 

[MadReferee]

</font><font color="blue" class="small">( does the 3/8" flex-line restrict the flow enough or cause enough turbulence that the cylinder takes longer to extend than if a 1/2" flex-line is used?)</font>

I can't speak for 1/2" to 3/8" but some of us typically use 1/4" hose for controlling the top link. The 1/4" hose provides sufficient restriction to slow down the cylinder movement to allow for feathering. Using 3/8" hose will definitely make the cylinder move significantly faster.

Based on this actual application, I would have to say that a smaller hose would slow down the movement of the cylinder, given all else was equal.