Shipyards & Ship Repair · Technical guide
Hydraulic Fitting Sealing: Methods and Installation Mistakes
Quick answer
Hydraulic connections are sealed in four ways: O-ring, bonded seal (Dowty washer), metal cone face and tapered thread. On parallel threads, the seal is made by a seal or a cone; tapered threads such as BSPT and NPT require a sealant on the thread. Most leaks stem from mismatched components, galling on stainless steel threads and misalignment stress; at initial commissioning, pressure is raised gradually while the connections are checked.
Even with the right product, incorrect installation can make a connection leak from day one. A large share of the leaks found in the field are caused not by the product but by a dirty sealing surface, an overtightened cone, an O-ring installed twisted, or a tube forced into place off-axis. In this article we bring together the sealing methods used in hydraulics and the most common installation mistakes made with stainless steel connections.
Four basic sealing methods
| Method | Where is it used? | What to watch for |
|---|---|---|
| Thread sealing | NPT and BSPT tapered threads | Sealant must not spill from the end of the thread into the line; overtightening strains the thread |
| Metal-to-metal cone face | JIC 37°, BSP 60° cone, 24° metric ferrule connection | Surface must be free of scratches; overtightening crushes the cone |
| O-ring | ORFS face, O-ring boss ports, SAE flange, 24° cone with O-ring | Correct size and material; replace every time the connection is taken apart |
| Bonded seal and ED seal | BSPP and metric parallel ports | Seating surface must be flat and clean; do not reuse |
Thread sealing
On tapered threads (NPT, BSPT), the seal is achieved by compressing the thread flanks together and by the sealant filling the gap between them. If PTFE tape is used, leave the first one or two threads bare and wrap the tape clockwise as viewed from the end of the male thread; otherwise, shreds of tape get into the line and can clog valves and filters. When selecting a liquid thread sealant, check its compatibility with the fluid and temperature. Wrapping tape on parallel threads (BSPP, UNF, metric) does not create a seal; on these threads, sealing is provided by a seal, an O-ring or a cone face.
Metal-to-metal cone face
The sealing surface is a 37° cone on JIC connections, a 60° cone on BSP hose ends and a 24° cone on metric ferrule connections. Because these surfaces work without a seal, even the smallest scratch or dent opens up a leak path. An overtightened cone is crushed and will not seal when it is next assembled. Instead of a torque value, the "flats from finger tight" method (counting hex flats after finger-tightening) is also used; on stainless steel, friction in the threads can make torque readings misleading, so this method can give more consistent results.
O-ring
On ORFS connections, the O-ring sits in a groove on the flat face; on O-ring boss ports, it sits on the neck on the hex side of the male fitting. O-ring connections stand up well to vibration and repeated assembly; however, the O-ring must be the correct size, made of a material compatible with the fluid, and undamaged during installation. Before installation, lightly lubricate the O-ring with system fluid and take care not to cut it as it passes over the sharp edge of the thread.
Bonded seal and ED seal
On BSPP and metric parallel ports, sealing is provided either by a bonded seal (a metal washer with a bonded rubber seal) placed under the hex, or by an elastomer seal (ED) on the male fitting. The port face must be flat, burr-free and wide enough for the seal to seat. These seals take a permanent set once compressed; always use a new one when a connection has been taken apart.
Galling on stainless steel and anti-seize compound
The oxide layer that protects stainless steel can tear when two stainless surfaces slide against each other under load. The exposed metal surfaces stick together, thread material tears away and builds up, and the connection seizes. This is known as galling, or cold welding. A galled stainless steel connection often has to be cut off to be removed, and the thread in the mating port is damaged as well.
To reduce the risk:
- Clean the threads before assembly; chips, sand and weld spatter can trigger galling.
- Use a suitable anti-seize compound on stainless steel threads. On food, oxygen or special chemical lines, check that the compound is suitable for the application.
- Tighten slowly and steadily; do not tighten stainless steel connections with high-speed impact tools.
- If the thread starts to bind, do not force it; take it apart, inspect the thread, clean it and try again.
Misalignment: a hidden cause of leaks
A tube or hose should approach the port it connects to along its own axis. Pulling a misaligned tube into place by tightening the nut puts uneven load on the cone faces and the O-ring. The connection may seem leak-free at first, but it starts to leak under vibration and pressure cycling; sometimes the tube cracks from fatigue at the connection point.
- Position the tube with clamps first, and tighten the connection last.
- Connect hoses without twisting them; the layline printed on the hose should stay straight.
- When tightening the nut, hold the body with a second wrench; otherwise the body turns and the connection at the other end loosens.
- Use vibration clamps on long, unsupported tube runs.
Reading the source of a leak correctly
When a leak is spotted, the first reflex is to tighten the nut a little more; but where the leak is coming from tells you what the solution is.
- Between the nut and the body: There may be a scratch, crushing or insufficient tightening on the cone face. An overtightened cone will not be fixed by tightening it further; inspect the surface and replace the part if necessary.
- From the port thread or under the hex: The O-ring, bonded seal or ED seal is damaged, or the seating surface is defective. Replace the seal and inspect the port face.
- Between the hose end and the hose: There is a problem in the crimp area. Replace the hose assembly; do not attempt to re-crimp the end.
- From the tapered thread itself: The sealant is insufficient or the thread is damaged. Take the connection apart and clean it, inspect the thread and prepare it again.
Initial commissioning and leak checks
- Flush the line after installation; assembly debris damages valves and pumps.
- Run the system at low pressure first and bleed the air.
- Raise the pressure gradually to the working value, visually inspecting the connections at each step.
- When looking for leaks, hold a dry cloth or a piece of paper around the connection. Never search for leaks by hand on a pressurized line: a fine jet of oil can penetrate the skin and cause serious injury.
- If there is a leak, stop the system and fully release the pressure before doing any work. Never tighten a connection under pressure.
- Repeat the check after the first warm-up; on some connections, thermal expansion and settling only show up later.
Checklist
- Is the sealing method right for the connection type: tapered thread, cone face, O-ring, seal?
- Are the sealing surfaces clean, scratch-free and burr-free?
- Are the O-rings and seals new, the correct size and compatible with the fluid?
- Was anti-seize compound used on stainless steel threads?
- Were tubes and hoses connected on-axis, without twisting, and properly supported?
- Was tightening done to the manufacturer's torque or flats value?
- Were gradual pressurization and a safe leak check carried out at initial commissioning?
