When I first started working with SF6 equipment, the most confusing thing for me to understand was the fittings. The endless list of names, sizes, acronyms, sealants… it can be a bit overwhelming for someone with no prior experience. The purpose of this guide is to provide a detailed framework for working with and identifying SF6 gas fittings, written specifically for someone with little to no knowledge to begin with.
Tongue and Groove (DN8, DN20, etc.)
These utilize metal to metal contact to seal when tightened. These types of fittings do well when vibration is present, under vacuum or positive pressure, and can remain tight under a large range of temperatures. The nice thing about these is that they cannot be over tightened, but the sealing surfaces must be kept clean. The most common sizes for SF6 switchgear are DN8 and DN20, however other less common sizes include DN6, DN12, and DN40. DN stands for diametre nominal, which denotes the size of a pipe (specifically, its inside diameter) in millimeters (mm). For all you cowboys on the imperial system, “DN8″ is .31” inside diameter, “DN20″ would be .79”.
NPT (National Pipe Thread)
Also referred to as tapered thread, this fitting establishes a seal through the metal-to-metal wedging that occurs when this thread is tightened. These connections require a liquid thread sealant or Teflon tape to seal properly.
Parallel Thread
These threads are sealed either at the “face” (top of the threaded portion below the hex bolt) or at the “nose” (bottom of the threads opposite hex bolt). This is accomplished using an O-ring or bonded washer which can be replaced over time. A Teflon based thread sealant can be used as a means of lubrication if the fitting is difficult to tighten, but ensure that the sealant doesn’t end up on the actual sealing surfaces.
Flare Fittings
These compression type fittings are sealed through direct metal to metal contact or with the use of a copper gasket between the two opposing flared surfaces. These are commonly preferred over the NPT because no additional thread sealer is required. Keep in mind that these fittings are available in the JIC (37°) and SAE (45°) versions that are not interchangeable.
Compression Fittings
Composed of an outer compression nut and an inner compression ring or ferrule, these fittings seal by applying a pre-determined amount of torque to the compression ring. The ends of the ferrule are clamped around the pipe and the middle of the ferrule bows away from the pipe, making the ferrule effectively thicker. The result is that the ferrule seals the space between the pipe, nut, and receiving fitting, thereby forming a tight joint. Overtightening can cause damage and leaks, so make sure to follow the correct torque specifications. Parker® and Swagelok® are two common examples.
Malmquist
Commonly found on ABB breakers, the male version of this fitting includes two nitrile O rings and a connection nut. More often than not, a Malmquist connection will be pointing straight down instead of out, so many adapters will have a 90° bend to offset this angle and make it easier for the operator to access.
SF6 Analyzer Fittings
Most of the SF6 analyzers on the market use the same two quick connect fittings for their gas connection ports. For the port that the sample hose connects to, the fitting is called a “Rectus Type 21”. The port that auxiliary storage hose (gas bag, extra cylinder) connects to is called a “Rectus Type 20”. These are self-sealing fittings that are spring loaded and can be operated by hand. Always avoid using any thread sealant when working with these types of fittings, or analyzers in general. The off gassing from the sealant has been known to contaminate gas samples and mess with readings. Use Teflon tape instead as needed.
SF6 Cylinder & Regulator Connections
This will be dependent on what region you are in, but for North America, CGA 590 is the standard connection type for SF6 cylinders and regulators. The SF6 cylinders will come with a valve stem that has a CGA 590 female, and an SF6 regulator will have a CGA 590 male (nipple and nut). You can tell from the picture below that the nut has small notches on the outside of the nut, indicating that this nut has left-handed threads.
So that means Lefty-Tighty, Righty-Loosey. This variation of a CGA connection is referred to as a “Bullet Nose”, which indicates that the seal is made on the metal-to-metal surfaces, not from the threads. A common mistake people make is applying Teflon tape to the nut to obtain a seal. Actually, the tape will cause a leak and typically not prevent a leak. When tape is applied to the CGA nut, it prevents the nose of the bullet-nose CGA to fully contact the mating surface inside the cylinder valve. See below chart for recommended torque values.
When it comes to installation, always follow the manufacturers guidelines. For tongue and groove style fittings, hand tightening (or lightly snugging with a crescent wrench) is all you need. When it comes to any of the other style fittings, I normally refer people to the Adaptall Installation Guide because it goes very in depth and covers a wide variety of connection types.
Tongue and Groove (DN8, DN20, etc.)
Because tongue and grove fittings seal on a metal-to-metal surface, leakage is generally caused by impurities found between the two sealing areas or physical damage. Should leakage occur, uncouple the fittings, and thoroughly clean sealing surfaces (tongue and groove). Check and clean the remainder of the fitting & threads. If needed, before reassembling, apply a non-oil-based lubricant to the threads. Make sure not to get any lubricant on the sealing surfaces, as it is a surface contaminant and can cause leakage.
Flare Fittings:
When leakage occurs, a common cause is surface contamination (particulates/dirt) between the two conical sealing surfaces. To fix the leak, first turn off any SF6 flow that might be running through the fittings. Then, disassemble the two mating fittings. Clean the sealing surfaces and reassemble. Should you find physical damage during inspection, such as a concave in the sealing surface, replace the damaged parts. After cleaning or replacement, (re)tighten the flare nut with a torque wrench set to the manufactures specified torque and check for leaks.
NPT Fittings:
When leakage occurs, a common cause is surface contamination (particulates/dirt) between the two conical sealing surfaces. To fix the leak, first turn off any SF6 flow that might be running through the fittings. Then, disassemble the two mating fittings. Clean the sealing surfaces and reassemble. Should you find physical damage during inspection, such as a concave in the sealing surface, replace the damaged parts. After cleaning or replacement, (re)tighten the flare nut with a torque wrench set to the manufactures specified torque and check for leaks.
Parallel Thread Fittings:
To repair a straight thread (BSP) connection, first turn of any SF6 flow that might be running through the connection point. Unscrew the two mating fittings and inspect the threads on both male and female sides for physical damage. Next, inspect the sealing gasket or O-ring for damage. If there are any signs of cracks or imperfections, replacement is required. Before installing the seal, thoroughly clean off the flat sealing surfaces.
Compression Fittings
These will leak for four main reasons –
If a compression type fitting is leaking, first check to make sure the nut has been tightened past finger tightness. If it has, try tightening the connection ¼ turn to see if the problem is fixed. All Swagelok tube fittings should be tightened 1 ¼” turns past finger tight, so there’s a chance someone did not follow this guideline to begin with. If your fitting is still leaking, you will need to replace both the tube and the nut & ferrule to fix the leak.
*When applying Teflon tape, face the threaded male end of the fitting towards you and wrap in a clockwise direction.
*Be very careful not to cross thread connections. This is more likely to happen when a softer metal (such as brass) is screwing into a harder metal (such as steel).
If you got something from this guide, please share it with someone. Questions about SF6 gas fittings, hoses, or equipment? Reach out at sales@gasquip.com
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