Hydraulic plumbing leaks are often considered to be an inherent characteristic of hydraulic
machines. While this may have been true 30 years ago, advances in sealing technology and the
development of reliable connection systems means that today, leak-free hydraulic plumbing is
Leak-free reliability begins at the design stage, when the type of hydraulic fitting is
selected for port, tube-end and hose-end connections.
Ports - Connectors that incorporate an elastomeric seal such as
UNO, BSPP and SAE 4-bolt
flange offer the highest seal reliability. NPT is the least reliable type of connector for
high-pressure hydraulic systems because the thread itself provides a leak path. The threads
are deformed when tightened and as a result, any subsequent loosening or tightening increases
the potential for leaks. In existing systems, pipe thread connections should be replaced with
UNO or BSPP for leak-free reliability.
Tube and Hose Ends - ORFS tube and hose end connections feature the high seal reliability
afforded by an elastomeric seal but, due to its cost, ORFS is
not as widely used as compression fittings and JIC 37-degree flare.
Flared connections have gained widespread acceptance due to their simplicity and low cost.
However, the metal-to-metal seal of the flare means that a permanent, leak-free joint is not
always achieved, particularly in the case of tube-end connections.
Leaking flare joints can be eliminated using a purpose-built seal developed by Flaretite.
The Flaretite seal is a stainless steel stamping shaped like a JIC nose, with concentric ribs
that contain pre-applied Loctite sealant. When tightened, the ribs crush between the two faces
of the joint, eliminating any misalignment and surface imperfections. The combination of the
crush on the ribs and the Loctite ensure that a leak-free joint is achieved.
A common cause of leaks from flare joints is incorrect torque. Insufficient torque results in
inadequate seat contact, while excessive torque can result in damage to the tube and fitting
through cold working. The following is a simple method to ensure flare joints are correctly
- Finger tighten the nut until it bottoms on the seat.
- Using a permanent marker, draw a line lengthwise across the nut and fitting.
- Wrench tighten the nut until it has been rotated the number of hex flats listed in the
|Tube Dash Size||Hex Flats|
|10||1.5 - 2.0|
|16||0.75 - 1.0|
|20||0.75 - 1.0|
|24||0.5 - 0.75|
Vibration can stress plumbing, affecting connector torque and causing fatigue.
Tube is more susceptible than hose. If vibration is excessive, the root cause
should be addressed. Ensure all conductors are adequately supported and if necessary,
replace problematic tubes with hose.
Having outlined the benefits of connectors that incorporate an elastomeric seal, it is
important to note that their reliability is contingent on fluid temperature being maintained
within acceptable limits. A single over-temperature event of sufficient magnitude can damage
all the seals in a hydraulic system, resulting in numerous leaks.
A leak-free hydraulic system should be considered the norm for modern hydraulic machines - not
the exception. But the proper selection, installation and maintenance of hydraulic plumbing are
essential to ensure leak-free reliability.
"This book has the potential to save many
organizations lots of money. It should be on the bookshelf of every engineer, supervisor, planner and
technician that deals with hydraulic equipment... it's worth its weight in gold." Find out more
Alexander (Sandy) Dunn
Plant Maintenance Resource Center