Optimizing Pneumatic Components in Automotive Plants
The fundamental question this paper poses is, “Are factories happy with vendors of manufacturing equipment (using pneumatic components) dictating their energy footprint requirements?” Compressed air is a significant energy consumer in every plant and to fully understand the ramifications of the imbedded misconceptions with respect to compressed air supply, one must take into consideration the actual point of use needs for compressed air.
One of the largest categories of compressed air uses, in automotive manufacturing plants, is the actuation of pneumatic cylinders. These cylinders are found throughout plants in the form of rotary valve actuators, slide gate actuators, and in the internal operating components for production and packaging machinery, material handling services, and a myriad of other devices. Supporting these actuators are a “rogues gallery” of pneumatic components creating unnecessarily high energy costs in the compressed air system. Remember the rule of thumb that for every 2 psig increase in system pressure, air compressors are forced into a one percent increase in energy consumption. Consistently restricting air flow, these pneumatic components force compressed air system pressures to be significantly higher than necessary.
Pneumatic Components Restrict Air Flow and Force
It has long been the habit of industry to perceive cylinder actuation timing adjustment to be a function of pressure, thereby requiring the installation of the ubiquitous pressure regulator in the supply line.
It should be recognized that, with the exception of those cylinder applications and services where small and incremental changes in down/up-force for nip rolls, press rolls, lift applications where down/up thrust or tensioning must be carefully controlled, and on some valve operators for positioning I/P devices, most pressure regulators are misused and misapplied as de facto flow controllers. The vast majority of the balance of the cylinder applications are not, in fact, pressure dependent.
The thrust of this portion of the report is to focus factory staff on the reality that cylinder actuation is a function of the time rate of change (recharge) initiated by adding the specific volume of air needed to properly actuate the device as the air is gated into the internal volume of the cylinder. Stated in another and more simplistic fashion, cylinder actuation time is a function of mass flow and time of the rate of recharge, not pressure.
It is the restriction created by the pressure regulator, along with the other parallel restrictions in tubing, fittings, filters, oilers, solenoid valves and other components controlling the flow of the volume of air into the cylinder over a chosen period of time. Unfortunately, at the same time, this type of assembly creates the perception of the need to artificially raise pressure (add energy) in the plant to overcome the restriction these devices create.
The Improper Use of Pressure Regulators as a Faux Flow Control Device
Plant staff should seriously consider an incremental project to first educate all concerned personnel on this issue. Subsequent to the education effort, steps should be taken to identify point of use applications involving cylinders which are amenable to change and begin removing filters, lubricators, and regulators, along with concurrent undersized tubing and fittings, upsizing of feed tubing, and installing common bar stock needle flow control valves. The net result will be more accurate control of stroke times and a commensurate reduction in the pressure needed to do the job. The reader should note that care should be taken to ensure the elastomers in the cylinder do nor require lubrication and consider a retrofit to new elastomers if they do.
This in turn will result in allowing staff to lower the pressure in the overhead transmission piping of the compressed air system.
Remember, the force applied on a cylinder is directly proportional to the cross sectional area of the cylinder ram times the net article pressure of air in PSIG applied to that ram. The hidden factor that is generally overlooked is the time domain of the stroke. If one is desirous of sizing the approach pipe/tubing to adequately feed a sufficient volume (NOT PRESSURE) of air to do the task in the time allotted, one must realize that the time frame of one minute is the common denominator in calculating flow in cubic feet per MINUTE.
If a cylinder is designed to stroke in a two second time domain, one must take into account that there are 30 two-second increments in every minute. If you know the cylinder diameter and stroke length, you have a volume. In this example, in order to delineate the exact flow, you must multiply the volume times 30. That number is the actual cubic feet per minute equivalent but taken in two seconds. This scenario is described as a “Sudden Event Demand” and must be properly dealt with.
Please understand that this is not a full essay on all the specific factors surrounding compressed air supply to cylinders. The intent is to alert the reader to the specific issue of the improper use of pressure regulators as a faux flow control device. This is the preeminent issue that must be understood fully in order to create an open pathway for beneficial change.
Flow Restrictions in a ¼” NPT Regulator
Image 1: The Components of a ¼” Regulator
Image 1 shows a disassembled standard ¼” NPT industrial air pressure regulator. The left portion is the main body. The next item to the right is the diaphragm and the follower plate with an integral flow port. The next item to the right is the compression spring and follower cup. The final item to the right is the bonnet with an integral threaded compression rod.
A close view of the body of the pressure regulator shows a small raised rod in the center as the spring-loaded pilot poppet assembly. The hole above the poppet contains a .060” drilled hole to allow air from the input from the adjacent pipe nipple to flow up into the visible cavity area.
Image 2: The Diaphragm and Follower Plate
Image 2 shows the diaphragm and follower plate with a center orifice port through which the air is supposed to flow on overpressure. The hole in the orifice centers directly on the spring-loaded pilot rod in the main body. When a worker tightens down the thread compression rod into the follower cup, it compresses the spring against the diaphragm. This in turn presses the pilot hole in the diaphragm more tightly against the spring-loaded poppet.
For the sake of comparison, this photograph superimposes the point of a .7 mm lead pencil immediately adjacent to the pilot hole. In this way the viewer can easily see the very small diameter of the pilot hole in relation to the pencil point . Taken in a common sense view, the tighter one compresses the spring against the diaphragm follower plate, the tighter the compressive pressure pushes the orifice against the pilot rod. This in turn creates an inordinately high restriction which prevents mass flow of air from passing rapidly through the orifice, a higher bleed off pressure.
Naturally the adjacent pressure gauge reflects a higher pressure. The physical laws of air dictate that with reduced flow, pressure must rise until it mirrors the highest supply pressure. Given the linear characteristics of this standard compression spring, the more the rod is turned down into the bonnet, that tighter the orifice plate is pushed against the pilot rod.
Image 3: Inside the Regulator Housing
Image 3 shows the regulator housing with the pin (actually a poppet) removed from the .060” hole. The air comes in from the nipple at top and rises through the .060” hole in the larger hole to the left of the hole the poppet came out of. Note the poppet has a tapered shoulder which seats in the cavity below the hole as shown in next view.
The housing has a seat where the tapered poppet sits in the center hole, pin up through the hole in the center. The cavity to the right is the air exit out of the regulator through the elbow. The post protruding from the bottom is the stop for the poppet bottom which has a small spring for purported flow control and to keep the poppet from being blown out of its hole. This view shows the poppet on its spring and follower cup to keep things centered.
All being said, the air in must flow through the .060” inlet hole, through the cavity area, and around the shaft of the poppet, a passage considerable smaller than the .060” hole, into the lower cavity, and out through the elbow. It is easily understood that this mechanism creates extremely high levels of restriction against the flow of air while at the same time establishing a barrier that allows the adjacent pressure gauge to read a higher pressure.
It would be nice if someone could logically explain how an orifice this small can pass a sufficient volume of air through it to successfully actuate a large cylinder in an acceptable short time frame while using a minimum of energy to do so. Unfortunately, the laws of Physics militate against the possibility.
Image 4: A ¼” NPT “Mini-Regulator”
Image 4 shows the so-called mini ¼” NPT regulator found in great abundance throughout industry. The assembly is very similar to the larger standard unit. For the sake of comparison, however, a common sewing needle has been inserted through the pilot hole in the diaphragm, the same size as the internal flow port under the white seat area in the lower body, to substantiate the .035 inch hole through which the air must pass.
Image 5: A 1 ½” NPT Pressure Regulator
Image 5 shows the configuration of a 1 1/2” NPT Regulator is essentially the same as the others shown in the article. The hole in the center of the body is 1/8”. The poppet is .220 inches in diameter. The air cross over port seen in the poppet stem leads to a hole in the bottom of the poppet seat that is slightly less than .080” in diameter. However, please take notice of the poppet.
Image 6: A poppet orifice measuring .060” in a 1 ½” NPT Regulator
Image 6 shows the passage in the poppet stem is the path through which the exit air flows out of the regulator, to the left past the 0 ring seal which separates the top chamber from the bottom one, through the center of the stem. Image 7 reveals the determinant port through which the air finally passes to exit the regulator. It measures .060 “ which actually limits flow to 11.5 cfm at 90 psig.
Image 7: The 1 ½” NPT Housing
In Image 7 we can see how compressed air exits through the small tube seen on the side of the housing, NOT a 1 ½” port, but the reduced passage from which the flow through the poppet exits the regulator. The folly of installing this type of regulator is immediately obvious.
The perception that higher pressure settings in a regulator allow for more air flow are totally erroneous and flies in the face of common sense and reason, not to mention the laws of physics. With this firmly in mind, it now becomes abundantly clear that any facility wishing to improve the actuation of pneumatic cylinders and the equipment on which they work, should seek an alternative to the standard practice of using regulators. It is far more reasonable to utilize a common bar stock precision linear needle flow control valve to control the rate of recharge of the cylinder volume.
Efforts must also be made to increase the diameter of pneumatic approach tubing, solenoid valves and their internal porting, and other component issues. This will reduce restrictions to flow in the most expedient and cost-effective manner. If pressure must, by nature of a service requirement be regulated, the use of a low differential, high flow, piloted regulator should be a point of focus.
Along with the pressure versus flow issue we must simultaneously consider the supply control issue. The usual means of supply control is a solenoid valve which opens when demand is calling for supply. Research on this subject demonstrates that the same restriction issue is found to be prevalent in most solenoid valve applications. Image 8 depicts a common manifold mounted solenoid found on a myriad of production and packaging machinery.
The orifice, in Image 8, was gauged with the help of the small nail which was then measured at .060 inches with the calipers. Banks of these types of valves are found throughout American industry where overhead transmission pipe compressed air pressures are typically 85 PSIG or greater.
Image 8: A Common Manifold Mounted Solenoid
We can further follow this logic and common sense train of thought by observing, in Image 9, a common ¼” NPT solenoid valve found on all too many point of use applications throughout industry. Please take careful note of the fact that while the pipe connections are 1/4 “ NPT, internal to that connection the actual flow ports are less than one half that size. This demonstrates the forcing factor requiring application of larger amounts of energy at the compressor to attempt, albeit all too often unsuccessfully, to overcome restriction through higher compressed air pressures at unnecessarily higher energy costs.
Image 9: A Common ¼” Solenoid Valve With Small Flow Ports
Quick Disconnect Hose Fittings
Image 10: Quick Disconnect Hose Fittings
The last in our rogue’s gallery is the ubiquitous quick disconnect hose fittings (Image 10) found everywhere in industry. If an interested person were to look carefully inside these devices, they would quickly see the very small air passages which are the reason these devices typically have a minimum of 7 PSIG differentials across them. Some are even higher going to as much as 12 PSIG differential.
When you look at these pneumatic components closely, it is easy to see why pressure differentials in the range of 40 PSIG are not uncommon. The higher pressures are obviously governed by the inability of the small diameter openings in these types of valves, taken in combination with restrictions in FRL sets, to pass sufficient volumes of air at lower pressures to accomplish the task assigned the mechanism at a faster rate and lower energy applied in the compressor room.
The consequence is an overall higher cost of operation, quality excursions, and reduced rates of product throughputs. It is long past time for industry to wake up and take a look at how they specify and purchase pneumatic equipment.
For more information please contact Peter Stern, Quality Maintenance Services, tel: 828-349-3007
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