Pneumatics Speed Distance of Air Line

[Chris Elston]

I've got a pneumatic question for everyone. I've a project where I have a spec to install valves in an enclosure. Typically we mount valves out on the system, and typically the valves are pretty close to the cylinder, within 3 feet or usually not longer than 36 long air lines. I understand the length of line effects flow to a cylinder, but I wasn't sure about the speed or reaction time of a cylinder with longer air lines. Since I've got to remotely mount my air valves in a NEMA 12 enclosure about 8-10 feet away, I didn't know if I would see a lag in my cylinders. In some cases, I will be controlling 5/32 port grippers, smaller cylinders with 8-10 feet of air line before I get to the valve. My initial reaction is that I'll probably have a speed issue, and would prefer to mount the valve close to the cylinder to get the best results. However I am “game” to try anything once, and it would make for a cleaner installation…..I guess….if one could argue. I’ve done this one other time, see picture here: http://forums.mrplc.com/index.php?act=modu...y&cmd=si&img=57 But the airlines weren’t no more than 3 feet away in that picture. Has anyone had any real life experiences with lengthen your air line runs from the control valve? My machine will have a 1.5 to 2.0 second cycle time. Crazy eh??? Oh did I mention that I will be controlling that valve with profibus? Shouldn't matter, but thought I'd mention it. Key point: Will moving the cylinder further away from the valve cause a cylinder to act sluggish and slow? Or will that only degrade the FLOW? In this case, running BIGGER lines to the cylinder should offset the flow (sCFM) to the device? Could I run say a 1/4 line to the device, then reduce it down to 5/32 size like a nipple at the cylinder port to get maximum flow? I can't purchase 1/4 NPT ports or these smaller grippers, they only come with 5/32 NPT. Need a little help brushing up on my pneumatics theory….

[Chris Elston]

EDIT………. In running some numbers... I found out that this could possibly be ok. I plugged some numbers into my air spreadsheet, and with 120 inches of air line, with a 1/4 second cycle (extend and retract) using 5/32 line, the CV required was pretty small. I will be using SMC VQ2000 valves, which can flow 0.4 CV. This should be adequate enough to flow the air to the gripper cylinder at the 1/4 cycle time I need. It appears initially by these finding, it will work.

[ECSI]

The last project that I worked on used pneumatic cylinders to rotate a 300 pound drum. Speed was a big issue because the SV's were so far from the cylinders. I wasn't involved in the pneumatic design, but what they did was put check valves on each port of the cylinders so that the air was locked in each air line right up to the cylinder. So, when the cylinder had to extend, the retract side would bleed to atmosphere right at the port, while the retract air line remained charged for when it was called to duty. Vice versa for retraction. Adding these check valves increased our speed dramatically.

[PdL]

On a plant we have FESTO cilinders in Devicenet remote panels and the air lines length are up to about 50 ft. Mostly controlling valves in a piping system, not time critical anyway. No problems in the valves control though.

[Chris Elston]

OT: I've had about six PMs about the air spreadsheet.... I just updated it, because I added a few more things, so you might want to download it again if you already have it. It's always been available for download here: http://forums.mrplc.com/index.php?autocom=downloads&showfile=133 Enjoy!

[larry818]

Just a note... if you're gonna be putting air stuff in a sealed enclosure, but sure there is adequate ventilation to prevent the enclosure from becoming a bomb.

[jstolaruk]

In my experience, yes. I try to minimze the length of airline from the valve to the actuator and wherever possible hard pipe it It looks better when the customer comes into see it and I don't have any issues with pneumatic movements exceeding their allotted cycle time. Customers like the hard pipe better than the plastic because instead of replacing it after five years, they can go ten (at least by their maintenance standards). If I do go with the plastic I spec. Parkers Paraflex (sp?).

[Gerry]

Try to avoid using a lot of elbows in the line as each one will have a flow restricting effect. The longer path will probably require some - just try to plan the path with a minimum no. of elbows.

[OkiePC]

I think the lag time you will add will be measured in milliseconds with only eight feet of extra air line. In my experience using the smallest possible diameter will help (reduce the compressible volume), up to the point you are limiting airflow required by the device. We have two machines with valve manifolds inside panels. I bitched and moaned at first for the same reasons, but now I am a believer. It tends to keep operators from jacking with regulators and manual overrides. The panels will get oily inside if you use any air line lube, so keep wiring up high, and use good labelling systems. We drilled holes to let excess oil drip out and keep pilot exhausts vented. Our panels use quick connect poly tubing, with bulkheads on the panels, so changing an air line is very easy. It is also easy to trouble shoot, with labels at each connector on the panel. You can swap lines around or check for the presence of pressure very easily. Another advantage is having an individual ball valve to isolate each section of the machine. Our panels have a ball valve entering low on one side making LOTO a cinch. Just the fact that you are opening a panel reminds you to lock out the energy. JMHO Paul

[Peter Nachtwey]

I do a lot of hydraulic ( fluid power ) control. Think of the piston as representing a mass equal to the mass of the actuator piston and rod plus the load. On either side is a spring. The larger the trapped volume of air the lower the spring constant. Putting a mass on a slinky is a good way to describe your system. Basically, if you quadruple the trap volume of air you will halve the natural freqnecy of your system. Fortunately you do not have to control the position.

[gravitar]

There's all sorts of "tricks" that a skilled fluid power designer can use to speed up the reaction time of a cylinder. You can mount a check-valve-like device called a "quick dump" out on the pneumatic device that is a closed passage when air flows in one direction, but vents to atmosphere in the other. This way, the air doesn't have to return all the way back to the valve and out the muffler. You could probably put one of these on each end of the cylinder to shorten the exhaust path on both strokes. There's probably several other ways of accomplishing the same thing, too..

[OkiePC]

While it's true that a quick exhaust valve will speed up the motion of the cylinder, it cannot help with the lag time between turning on a solenoid, and a pressure change in the cylinder sufficient to start its motion. That delay is what will increase with an increase in air supply volume between the valve and the actuator. If you want higher flow to make the cylinder travel faster, then quick exhaust valves will do it. If you need adjustable speed, put the flow control between the cylinder port and the Q/E valve. Of course, always control speed by controlling the flow of the cylinder exhaust as close to the cylinder as possible. Use a check/valve bypass type flow control to only limit the exhaust, so you have independent control of the extend rate and the retract rate. Manifold based flow controls are "springy", I don't like them. You probably already know all this stuff, but someone else may benefit from it and I'm bored...Paul

[fluidpower1]

One thing no one has addressed about long lines is the amount of extra compressor volume required to fill these lines each cycle. Unlike lines filled with oil the air in each line to an actuator must be filled and exhausted each cycle to make the actuator move. Doesn't sound like much but on small cylinders it can be two or three times what it takes to run it. Mounting valves directly to the actuator not only reduces response time but saves air and keeps from over working the compressor or having to add more compressors to keep up with the wasted energy. Another response reducer is valves with Solenoid/Pilot operators. A solenoid valve receives the signal from the Control Circuit and shifts quickly. However, nothing is happening at the actuator until the Pilot Operated portion of the valve shifts to redirect air flow. This action is normally in miliseconds but always adds cycle time to the operation. Direct Solenoid Operated Valves alway respond faster but are no match for long air lines. I have found that using a regulated supply to cylinders that have a heavy load in one direction and are driven by that load when reversing can reduce cycle time and make the load forced portion of the cycle much smoother. It's in the book.