What is a seal?

When an element is installed in a filter vessel a seal is created where the element meets the vessel. There are many types of sealing mechanisms used by cartridges and bag filter elements. Depending on the criticality of the application, the seal can be crucial to the filter's performance. A small "pinhole" leak in the seal can cause up to 20% by-pass.

In critical applications a redundant O-ring seal is used. The O-rings fit into a cup and locking mechanisms called "tabs" or "ears" and lock the filter in place.

In less critical applications where a small amount of by-pass is acceptable, paying for a redundant O-ring seal is not required. Here, either a double open-end filter cartridge or a snap ring seal on a bag is sufficient.

DOE, Plain
Many filter cartridges utilize a double open-end plain sealing mechanism. There is no gasket or O-ring on the filter cartridge; instead a knife-edge seal in the filter vessel bites into the end of the filter element to created the seal. In coarse filtration, over 10 microns, this is usually sufficient. To check that the knife-edge seals in the filter vessel are in proper condition look at the ends of spent filter cartridge. There should be a clear indentation at each end indicating a good seal.

DOE Gasketed
Many pleated filter elements use a double open-end seal but add a gasket at each end of the filter to ensure a proper seal. A knife-edge seal in the filter vessel bites into the gaskets at the end of the filter element to created the seal. In coarse filtration, over 1 micron, this is usually sufficient. To check that the knife-edge seals in the filter vessel are in proper condition look at the ends of spent filter cartridge. There should be a clear indentation in each gasket indicating a good seal.

SOE (Hydraulic)
Hydraulic elements with a single open-end seal have either an internal O-ring seal or an external O-ring on one end and the other end of the element is closed and flat.

SOE, 222, Flat
A single open ended, 222, flat cartridge is a cartridge with one end closed and the opposing end opened. Both ends are flat and the end that is open has a double O-ring seal. O-ring seal materials include Buna-N, EPDM, Silicone, Viton, and Teflon.

SOE, 222, w/Fin
A single open ended, 222, with fin cartridge is a cartridge with one end closed by a polymeric fin and the opposing end opened. The end that is open has a double O-ring seal. The materials for the O-ring seal include Buna-N, EPDM, Silicone, Viton, and Teflon.

SOE, 226 Flat
For single open-end cartridges with a size 226 O-ring seal, the O-rings are mounted on the outside of the cartridge nipple, and are retained in O-ring grooves. There are two O-rings used for sealing this cartridge on the open end. There are often locking tabs on the open end fitting that permit the element to lock under tabs in the housing. This locking feature prevents the dislocation of the filter during backflushing or sterilizing. The closed end of this cartridge has a flat end cap.

SOE, 226 w/Fin
Same as a SOE, 226 Flat, except that the closed end fitting contains a pointed plastic cross that helps secure the filter in the housing. This fin prevents the cartridge from becoming misoriented in the housing. If the cartridge were to move from vertical alignment, the seal integrity could be compromised, and bypass could occur.

Threaded, NPT
The cartridge end fitting has a national pipe thread (NPT) thread, machined or molded in to accommodate the housing thread. Most cartridges would have a male NPT thread. This sealing mechanism is common in metal cartridges. No elastomeric seal is used with this connection, although approved thread sealants are often used.

Threaded, SAE
This cartridge end fitting is usually reserved for metal cartridges used in high pressure applications, mostly hydraulic and lubrication. The end fitting is machined with an SAE thread size. Sealing is accomplished with an O-ring.

Extended Core
Some filter vessels require filter cartridges with an extended core. This is an approximately 1 ½" extension that centers the element in the vessel. Most manufacturers offer this as an option with their filter elements. In existing applications where there is an existing filter vessel this should be a consideration when choosing an element. In new applications this should not dictate the element choice.

Compression spring
A compression spring is used on one end of some double open-ended cartridges. When the housing is closed, the compression spring exerts an axial force to force the opposite open end fitting in firm contact with the housing knife-edge seal.

Snap-band Cuff
A snap-band cuff is a flexible plastic ring that is bent then snaps into position to form a seal in the filter vessel. The snap-band style was developed to improve sealing efficiency in dust collection.

Flange Top
Flange Top dust collection bags have a donut-shaped piece of fabric that is sewn to the top of the filter bag and forms a seal when compressed between the tubesheet and the cage top.

Ring Top
Ring Top dust collection bags have rings sewn in the flange.

Sleeve Top
Sleeve Top is a separate piece of fabric sewn onto the dust collection bag body so that it extends beyond the bag body. These are used in lieu of a raw edge to prevent puckering when folding the bag into the cage.

Hem Top
Hem Top woven glass dust collection bags are folded back and sewn to avoid unraveling.

Band
The flexible stainless steel "C" Style band fits Parker bag housings and provides a positive seal for liquid filter bags.

Snap Ring
A snap ring is a carbon steel or stainless steel ring that fits most standard liquid filter bag vessels. The ring can snap into a basket and seal against the housing or fit into a groove and be compressed between the filter body and the lid. This is the most common type of bag seal mechanism.

Ring
A ring is a carbon steel or stainless steel ring that fits most standard liquid filter bag vessels. The ring snaps into a basket and seals against the housing or fits into a groove and is compressed between the filter body and the lid. This is the most common type of bag seal mechanism.

Collar
A plastic collar improves a liquid filter bag's seal with the bag house. Collars have a handle that aids in removing the bag.

Most liquid filter bag manufacturers offer their filter bags with sealing options designed to fit into the existing filter vessel. As a rule of thumb, snap rings, sometimes just referred to as rings, do not offer an exact seal. This is particularly true in multi-bag filter vessels where the seal plate often warps.

Collars improve the seal but are not 100% either. Both sealing arrangements allow for some by-pass. On most sewn felt bags a small "bubble" can be noticed where the seam of the bag is sewn and attached to the ring. This is an area where by-pass almost always occurs.

Over-the-top designed housings improve the seal by forming a cold flow seal.

In recirculating systems this small amount of by-pass may not be a problem since the contaminant may be captured on the next pass. If the application calls for bags and critical (over 99%) efficient filtration, a depth or pleated melt blown bag in an over the top filter vessel should be the choice.