Poor plumbing practices can permanently cripple a fluid power circuit even if it was designed with the best engineering practices and assembled with the most up-to-date components. Undersized lines, elbows instead of bends, incorrect component placement, and long piping runs are a few of the items that strangle fluid flow.
Other problems, such as using tapered pipe threads or lines with thin walls, can make a circuit a maintenance nightmare that requires daily attention. Fortunately, there are numerous publications that assist in specifying correct line size and conductor thickness to give low pressure drop and safe working-pressure limits.
Because pneumatic circuits are less complicated and operate at lower pressures than hydraulic systems do, they are not as vulnerable to plumbing problems. One very important aspect that often is overlooked is the length and size of lines between the valves and actuators. Piping between the valve and actuator should be as short as possible and of the minimum diameter to carry the required flow. The reason for this is that all the air in the pipes between the actuator and valve is wasted every cycle. These runs must be filled to make the device move but the air it takes to fill them does no work. During each cycle, air in the actuator lines exhausts to atmosphere without helping cycle time or force. For this reason, always mount the valve close to the actuator ports.
Another aspect of plumbing a pneumatic system is the in-plant pipe installation procedure. To get the required amount of compressed air to the point of usage requires some planning — or the site may be starved at times.
Pipe materials and size: Air systems are normally plumbed with Schedule 40 black iron pipe. (Galvanized pipe is not recommended because some galvanizing material may flake off and get into moving parts.) Several other available plumbing materials could be used for air piping because pressure is relatively low. Some mechanics recommend plastic pipe, but be aware a few synthetic compressor lubricants attack plastic and cause it to lose strength. This type of damage weakens the plastic until it can burst, sending shards of plastic flying everywhere in the plant. Never use any piping material not specifically designated by code.
To help select pipe size, the chart in Figure 3-1 shows flow (in cfm) down the left-hand side, length of run (in feet) across the top, and minimum Schedule 40 pipe size in the body at the intersection of these two.
This chart is based on a 1-psi pressure drop for the run lengths given. The right-hand column shows approximate compressor horsepower for the flow figures on the left. Using larger than specified pipe is of little help in reducing pressure drop, but provides more storage volume to handle short brief-flow needs. This chart does not consider fittings and valves, but they also must be considered when figuring the length of a run. Add 5 to 7 ft of pipe length for each fitting or valve — to be on the safe side.
Not having enough air to run the equipment is expensive, so never try to save a few cents at installation by skimping on pipe size. One or two pipe sizes over minimum add little to cost up front, but can make a big difference later. It is less expensive to run oversize pipe initially than to have to add a line later.
There are three basic compressed-air piping layouts that meet the requirements of most industrial plants. Some facilities may have two or more of these systems to handle special needs. In general, smaller plants use a modified grid system, especially when the facility is growing. A unit distribution system offers flexibility, but can be expensive up front. A loop system is best suited to new construction; it provides extra storage capacity and dual supply for short bursts of high flow.
Figure 3-2 shows a typical grid-system layout using a centrally located air compressor. All air from the receiver goes to a large header pipe that runs down the center of the plant or department. Branch lines from the header go to separate areas where working drops come down to specific machines. With preplanning for future working drops, this arrangement is very flexible.
Figure 3-3 shows a typical loop piping system for compressed air. Again, the compressor and receiver are at a central location. The oversized loop around the periphery of the plant — or department — adds storage and allows flow with low pressure drop. It also allows for short bursts of high-volume flow to any section because flow in the loop is bi-directional. (Another way to get short high-volume flows with any of these piping systems is to install extra receiver tanks at or near areas that need such flow.)
Figure 3-4 illustrates a unit distribution layout that works well in plants that run departments on different days or shifts — or plants that started out small and added compressors as business grew. It is the most expensive configuration of the three for a new installation, so is not often used there. One advantage of the multiple compressors is that there is always backup air available for critical operations should a single compressor fail. The disadvantage — besides higher price — is that some compressors might be neglected by maintenance personnel because they are spread throughout the facility.
Figure 3-5 shows a typical pipe run layout for optimum performance from a compressed air system. Strict attention to the details shown here assures a smooth-operating and trouble-free air system.
Machines can be plumbed with any of the materials recommended for plant piping. However, because piping at the machine is usually much smaller, polyethylene, nylon, or vinyl tubing with push-to-connect fittings will work very well. Such tubing and fittings come in a variety of sizes (and colors) and require only a few tools to install. The push-to-connect fittings also release easily for troubleshooting checks or rework.