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Three hydraulic tubing questions ... ratings, size and type .....

Three hydraulic tubing questions ... ratings, size and type .....

Three hydraulic tubing questions ... ratings, size and type .....

I am building my first hydraulic project ( or at least my first in many years ). Started the project 6 months ago ... finally getting around to finishing it.

I have a pump and cylinder ( salvaged from a forklift ). My closest "educated guess" is that the pump puts out 8 GPM (it took about 15 seconds to fully rise and the cylinder has a 2 gal volume). The relief valve was set to about 2500 PSI (600 - 3600 spring in it).

Now I need the controls.

I have picked valves ... checked the flow ( pressure drop ) and pressure to make sure they are OK.

So, now the interconnecting tubing.

The valve bodies (using cartridge type solenoid valves) for the most part are #8 (1/2") SAE ports. The cylinder has a 1/2" NPT connection on it.

At first I made the assumption that I would use 1/2" tubing (.065 wall). And I started running into problems. The first chart I looked at said this tubing was good for 3650 psi working pressure ( either welded or seamless 1010 ... J524 or J525). Then the next chart I looked at said this is only good for 2500 psi working pressure (McMaster Carr)!

I think the explanation may be in the safety factor. I found a note in one chart saying "piping rated at 6:1 safety factor .... less shock can be used at 4:1 ... severe service 8:1 ...).

So, first question ... am I right? Although the McMaster Carr tubing is only rated at 2500 psi, can I use it for 3650 if my application has "minimal shock"? Will all tubing meeting J524 or J525 have the same rating?

I should note that I have a three port flow control valve ( third port is a return port ) so I can slowly accelerate and decelerate the cylinder ... minimal shock on the system ... starts and stops moving on full dump on return port. I have been experimenting with this valve and have it working off a PLC to control the cylinder movement so that I get a "soft" start and stop of the lift.

Then I started checking fluid velocities. I found two charts. One said: Pressure lines 25 ft/s, Return 10 ft/s and Suction 4 ft/2 ... A second recommended 19, 9 and 3.5 respectively.

From the chart, this converts ( for 8 GPM flow ) to tubing diameters of: Pressure .362", Return .571" and Suction .905"

The cylinder will lift and lower a load. It is a single acting cylinder. It will lower by gravity.

As such, I am now thinking all lines must be considered "return" lines while lowering. I am making this assumption because at times the lift may be lowered with no load no it. With no pump or heavy load "pushing" it down, I "think" I want minimal restriction in the lines or the lift may come down very slowly.

Are these assumptions correct and I go up to say a 3/4" line ( 3/4" dia with .083 wall .584 I.D. .... 3050 psi or 2300 psi depending on the chart )?

Finally, what is the difference in Seamless and Welded tubing? I know the obvious difference in the manufacturing but after reading three articles, they all say with the quality of welding and the possibility of defects (scratches) when drawing seamless tubing, there is no real difference in the two.

On the one site, welded pipe was $2.00 per foot while seamless was $6.50 per foot.

I don't mind buying seamless if there is a good reason but I don't want to waist money if there is no need to.

Thanks so much ...... Mike

RE: Three hydraulic tubing questions ... ratings, size and type .....

Whether seamless or welded dom, if it satisfies the mechanical requirements use it.

Keeping velocities low keeps flow losses low, reducing pressure required to move fluid through the plumbing. The velocity fiqures you are quoting are maximum recommended velocities for reasonable pressure drop. There is nothing wrong designing for lower velocities.


RE: Three hydraulic tubing questions ... ratings, size and type .....

Thanks! Funny, the more I look at hydraulics the more I see the parallels with electrical engineering.

So the only question left is the tubing rating. Is there a "standard" rating for tubing?

As I said, it seems different distributors list what seems to be the same tubing with different ratings.

Is this just because of the safety factor they put on it (I "think" some are listing their tubing at 4:1 while others use 6:1 ... if I multiply the lower pressure pipe by 6 or the higher pressure pipe by 4 they come out very close to the burst pressure) or is the working pressure rating dependent on the manufactures rating?

For example if we take two tube from different manufactures, 3/4" tube with a .083 wall in steel ( 1010 ) and both meet J524 then will they have the same burst rating?

If so then it would be up to me to determine if I want a 4:1 or 6:1 or 8:1 safety ratio.

Thanks so much again!!!!!!!!!!


RE: Three hydraulic tubing questions ... ratings, size and type .....

You get to chose the factor of safety. The working pressure will then be related to burst pressure by your chosen FS. Burst pressure is a calculated figure based on material properties.



RE: Three hydraulic tubing questions ... ratings, size and type .....

Thanks again!

Exactly what I needed to know!!!!


RE: Three hydraulic tubing questions ... ratings, size and type .....

WOW ... Thanks so much Ted!

I just had time to finally got through the entire Womack Machine site. It is awesome! Amazing amount of information there!!!!

I have referred to it a couple of times in the last few day.

Thanks so much .... Mike

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