by Josh Cosford, Contributing Editor
Is leakage accepted as a necessary evil, or can we banish it for good?
Picture this: You walk into the lobby of a large, billion dollar, global manufacturing megacorp. You make your way across the floor fashioned from imported marble, past a magnificent indoor waterfall and then up to the front desk where a well-dressed receptionist sits at a grand desk fabricated from exotic burled hardwood. She has you sign in at one of the network computer consoles, and then prints off a bar-coded guest tag for you to wear before she invites you to wait for your host on one of the nappa leather lounge chairs.
When your host arrives, you’re guided into the plant, where pristine, glossy grey floors direct you to various areas of the plant. Your trip to inspect a multi-million dollar injection molding machine is paused momentarily as you wait at a crossing to allow a train of automated guided vehicles to pass, as they shuttle parts throughout the facility. You finally arrive at your destination—a 5,000-ton injection molding machine larger than a luxury mobile home. The scene leaves you taken aback.
There are oil-absorbent socks forming a virtual levee underneath the machine’s power unit to prevent the steady flow of leaking oil from cascading across the floor. As your eyes search eagerly for the drip, you notice the power unit covered with a sheen of oil, as though it was competing in the upcoming Mr. Olympia competition. Some areas of the power unit are caked with a waxy sludge—the tell-tale sign this machine has been leaking hot oil for some time.
When you ask your host about the seemingly discordant mess, he looks slightly confused at first, as if you just asked him what the speed of darkness is, and then replies, “I don’t know … it’s always leaked.”
This situation may be extreme, but you would have much more difficulty finding a hydraulic machine more than a few years old without some degree of fluid leakage than you would immaculate and dry.
Leakage seems to be accepted as a necessary evil in the fluid power industry, which is unfortunate if you’re designing, engineering or selling hydraulic machines or equipment. But why should leakage still be a problem?
If the design of a hydraulic system was left to the engineer alone, leakage would be rare indeed. But because bean counters also have a say, the financial resources to eliminate leakage are not infinite. The tools and systems exist to eliminate leakage, but let’s first look at why leakage still occurs.
External leakage occurs when hydraulic fluid manages to find a path from what should be a contained system to the outside of that system. Internal leakage also occurs, but is harder to detect, and also causes a set of symptoms different from leakage to the machine’s environment, so we’ll leave the internal leakage problem for another day. External leakage occurs as fluid escapes the connection points, or when there is a mechanical or physical failure of a component in the system. The most common point of leakage is at fittings and adapters, although this type of leakage is generally a slow drip rather than a heavy stream. When I see leakage at the hose connection, tee, adaptor or other type of junction, it’s typically always with NPT or JIC fittings.
NPT, or National Pipe Thread (Taper) is an American standard for pipe connection dating back to the middle of the 1800s, which gives you an idea of how shocking it is that we’re still using it. The thread is tapered, like a cone, so that when two fittings are tightened, they will compress flank-to-thread, sealing the fitting. Theoretically, the tighter you make the connection, the better it will seal. There are two problems with these types of connections: they require a thread sealant, such as PTFE tape, to fill the spiral leak path inherent to the fitting; and once highly torqued to prevent leakage, they will often be “stretched,” so that future reassembly absolutely prevents a tight seal.
Typically, an NPT thread will seal well the first time and remain sealed for the life of the connection as long as it’s not disassembled and reassembled. Most often, your best bet is to replace the fittings, but that rarely happens when preventing $2,000 per hour of downtime is more important than running out to pick up a $5 fitting. The act of removal and reinstallation can further deform or damage the threads, which is why leakage is inevitable with NPT.
To help reduce leakage, the NPTF (F stands for “fuel”) was developed. It’s designed to work without thread sealant because the threads crush against each other, taking up the gap and negating the requirement for thread sealant. However, just as with regular NPT, disassembly and reassembly can lead to leaks. To be honest, it’s best to just avoid NPT altogether for hydraulic systems, especially because of the time it takes to pipe components properly.
The JIC connection system uses straight threads with male and female cone-shaped tips to allow the quick and easy assembly of hoses or tubes. It’s an invaluable fitting for something like a test bench, where pumps, valves and cylinders are connected and disconnected multiple times per day. If there is a small leak, it’s no big deal because most test benches have a drain table to collect it. Also, just as with NPT, JIC is fine when installed once and left alone, such as with tube assemblies. The metal-to-metal seal surfaces compressed by the straight threads actually provide a great seal. The problem, once again, is when plumbing is disconnected and reconnected. Dirt, scratches or fatigue can prevent sealing, and small leaks are common. If leaks are not an option on your machine, it’s probably best to leave JIC technology alone as well.
So if NPT and JIC are obsolete, what type of connection can be used to avoid leakage, even if the connection is removed and replaced frequently? The cold fact is that metal against metal is not a reasonable solution to prevent leaks. A soft sealing technology must be used, and that technology is a rubber O-ring. A simple O-ring, typically made from synthetic nitrile rubber or fluorocarbon can crush between two hard surfaces, providing a positive seal with no chance of leaking. The only time a positively sealed connection leaks is when the O-ring is damaged or old enough to become brittle. However, by maintaining a good practice of changing O-rings when fittings are removed and replaced, you can ensure they are neither damaged nor brittle.
The SAE straight thread ORB (O-ring boss) connection replaces NPT in any example where bosses, bungs or ports are concerned. The female boss is straight-threaded and contains an internal chamfer to accept the O-ring, which is at the base of the male fitting’s thread. This positive seal is so effective, one could merely hand-tighten the fitting and it still would not leak. Because the threads are used for torquing down the fitting only, their condition is less important than NPT threads, which seal at the thread itself.
On hose connections, where JIC is currently king, ORF (O-ring face) provides an equally efficient seal, as does ORB. The male side has a groove on the face where an O-ring resides. The female side is flat faced and contains the swivel nut to tighten the two sides together. Once torqued, the positive seal prevents leaks, and typically, leaks only appear when dirt or grime get into the fitting. ORB is popular enough that some manufacturers no longer make NPT ports on their valves or manifolds. However, ORF has had less momentum saturating the market, where it seems to only appear on expensive off-highway equipment.
Leaks are not limited to plumbing connections alone. Hydraulic components themselves can leak, such as through the shaft seal on a pump, the rod seal on a cylinder or between the interface of sectional valves. Spills and messes from pump leaks can be avoided by using submersed-pump power units. This doesn’t guarantee the pump doesn’t leak—only that it won’t leak external to the power unit.
Preventing cylinder-rod seal leakage is more difficult, because there is no other technology I’m aware of to seal the moving rod against the stationary gland and bearing. Regardless, leaks can be remedied quickly when noticed, and they can be reduced by ensuring both the cylinder and oil are kept clean. Contaminated fluid can destroy any dynamic seal, and this is a lesson that applies to pumps, motors, cylinders and even the spools of a valve.
Leakage from stack and modular valves is fairly common, and is probably the number one source of external leakage in industrial environments. Sectional mobile valves are sealed with O-rings as well, but they leak less often because they typically remain assembled once installed. An industrial valve system can easily encompass a dozen valves each joined together will bolts and sealed with O-rings. As stacks get higher, the bolts or tie rods sometimes have difficulty providing enough torque, and if seals are not in acceptable conditions, leaks often occur.
To avoid the leakage occurring with sectional and modular stack valves, a hydraulic circuit can be created using cartridge valves in an integrated manifold. This is my favorite method to create a hydraulic circuit, not only because it reduces leaks by eliminating plumbing, but because of the myriad configurations available with cartridge valve circuitry.
An integrated manifold block can have anywhere from two valves to dozens of valves installed for the same circuit, and the only plumbing required is the pressure, tank and work lines. All port passages and connections between valves are internal to the manifold, and if ORB ports are implemented for fluid connection, the chances of leakage are slim to none. The technological advancements in leak prevention aren’t ground breaking, as all of these systems have been around for years, but only recently have they been utilized to their potential. With the leak-free fittings and design options available, there’s really no reason for leakage to be a problem any more.
Filed Under: Hose & Tubing, Sealing