Quality Magazine

Quality test & inspection: Choose The Right Seal

April 18, 2006


The seal in any connector is critical to the reliable long-term function of the connector. Seals that wear or degrade quickly need to be changed frequently, adding to the cost of the connector. Several factors contribute to how the seals perform in the connector, including material, size, seal style, sealing surface and loads applied on the seal.

The selection of seal material can be a trade off between chemical compatibility, durability, temperature resistance and seal-ability. Though, for most applications there is an elastomer that performs well across a variety of parameters.

When choosing a connector, compare the size and style of seals. In general, the larger the seal's cross section, the longer the seal will last, providing a more reliable seal. A larger seal will be more tolerant of surface imperfections.

A seal under complex loading will wear sooner than a seal only under compression loads. Twist-on connectors apply both compression and shear loads to seals. If a twist-on connector is the best choice for the application, use one that minimizes the complex loads placed on the seal.

To extend the life of the seal, the connector should allow the seal to be fully relaxed when not connected. Maintaining a preload will cause the seal to take a set and will decrease the ability of the seal to function properly. Fully relaxed seals tend to have more clearance and make it easier to connect and disconnect without damaging the seal.

Connecting over burrs, sharp edges, flash, threads or rough surfaces can cut or abrade the seal-decreasing the life of the seal. For instance, when connecting to tubes, the sealing surface may be smooth but the seal must pass over the sharp edge of the tube. A fully relaxed seal will have a larger diameter and will pass easily over the sharp edges, minimizing cuts and abrasions. Even if the edge of the tube is not sharp, it might roll the seal out of position, causing it to be unable to make a seal. A fully relaxed seal provides the greatest clearance and therefore makes it less likely for the seal to be damaged during connection and disconnection.

Often one of the easiest ways to increase the life of the seals is to lubricate them. The lubrication reduces abrasion and the tendency of the seal to move, extrude or pull off during connection and disconnection.



The largest seal possible is necessary on as-cast or threaded parts. Source: FasTest Inc.

Seal Material

A variety of seal types and materials are available. Nitrile (Buna-N) is generally used for seals internal to the connector. Mating seals are commonly Neoprene due to the material's balance of seal-ability, media compatibility and toughness.

Polyurethane seals also can be used as mating seals due to their resistance to cutting, abrasion and extrusion, and are generally compatible with the same media used with Nitrile. Polyurethane is not recommended for use with hot water, steam or applications above 200 F. Other seal materials are fluorocarbon, silicone, fluorosilicon and ethylene propylene.



Seal Size

In general, the best possible scenario is to incorporate the largest seal possible given the application. When sealing on threads or when sealing an as-cast or other less-than-ideal sealing surface, seals must be oversized to minimize premature wear. Special seals also are needed to seal on irregular shapes.

On certain connectors, when they are not actuated, the seals are unstressed and are larger in diameter than the tube. When the connector is placed on the tube, the tendency for the seal to roll or be cut by the tube end is dramatically reduced because of this larger size. The seal is not pressed against the tube until the connector is actuated.



Seals come in a variety of sizes and materials. Source: FasTest Inc.

Seal Stress

Connectors should be engineered to minimize complex stresses, such as compression plus shear, on the seals to promote long seal life. For example, by limiting the torque-to-finger tight, the complex stresses on the seal are dramatically reduced. The seal is primarily in compression and shear stress is kept to a minimum. In addition, because the pressure of the test media is used to make the seal and not the torque on the connector, the installation is fast, easy, repeatable and it improves the life of the seal.

When the connector is disconnected the seal should return to a fully relaxed position. No preload is kept on the seal, which extends the life of the seal and makes the connectors easier to use. A fully relaxed seal will have a larger diameter and greater clearance, to fit over the ends of tubes. In addition, the seal is less likely to be nicked, cut or rolled during installation.



A larger seal minimizes the likelihood of rolling or being cut by the tube. Source: FasTest Inc.

Seal Pressure Assisted

In addition to the initial seal, dynamic or pressure assisted seals can add to the reliability of a connector. Seals can be stretched, compressed or moved to provide pressure-assisted sealing. Pressure-assisted sealing means that as the test pressure increases, the sealing ability of the connector increases. Using pressure-assisted seals allow the seal to be completely relaxed in the disconnected state, reducing the stress on the seal and increasing the life of the seal. Fully relaxed seals make it easier to connect and disconnect without damaging the seal. Pressure-assisted seals can mean there is less sliding of the seals and therefore the seals will last longer.

Sealing surface and internal vs. External Connections

Often what seems like the most obvious sealing surface is not the best choice. If the surface is not smooth or is subject to nicks and scratches, the part should be evaluated to see if another sealing surface can be used.

All other things being equal, pick a sealing surface that corresponds to a standard size connector. It is also important that the sealing surface hold to tolerances appropriate for the connector. For example, it may be necessary to seal on the inside surface of an expanded tube, because the outside diameter of the end of the tube has tolerances that are outside of the requirements of the desired connector. However, the inside diameter of the neck of an expanded tube may allow the tolerance to be held permitting consistent sealing by the connector.

Face seals on tubes should be avoided. In this application the seals tend to have an unacceptably short life. The thin surface area and sharp edges tend to over-stress the seals or even cut them. The thin area of the tube also makes this type of connection more vulnerable to failures due to side loads. Look for connectors that seal on the outside or inside surface of the tube.



Complex stresses must be minimized to promote long seal life. Source: FasTest Inc.

Choosing the Best Connector

Choose connectors with seals that incorporate the following qualities:

  • Optional materials for chemical exposure
  • Optional materials for durability
  • Largest sealing surface
  • Largest cross section to resist applied loads and to be more tolerant of surface imperfections
  • Connectors that allow the seal to fully relax when not in use
  • Lubrication options

The seal is the backbone of any connector. Careful consideration must be made to determine the specifications for your connector, especially the stress and environment the seals will encounter. Determine any chemical interaction, temperature variation and connection type before investigating connectors. These are the major parameters that need to be known before selecting the proper connector.



For answers to specific seals or connector questions, visit www.fastestinc.com and go to "Support/Ask an Engineer." For more information, call (800) 206-0316 or e-mail customproducts@fastestinc.com.

Sidebar: Tech Tips

  • If a twist-on connector is the best choice for the application, use one that minimizes the complex loads placed on the seal.
  • A fully relaxed seal provides the greatest clearance and therefore makes it less likely for the seal to be damaged during connection and disconnection.
  • Connectors should be engineered to minimize complex stresses, such as compression plus shear, on the seals to promote long seal life.
  • In addition to the initial seal, dynamic or pressure-assisted seals can add to the reliability of a connector.


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