Useful information on pump stationary seals in sealless and hermetically sealed pumps
What is a pump stationary seal?
Unlike shaft seals, which prevent liquid escaping from a rotating or reciprocating shaft, stationary seals do not have to seal around any moving parts. They include O-rings, gaskets, and diaphragms. Whereas shaft seals are expected to leak to some extent (this is important for lubrication and cooling purposes), stationary seals are defined as hermetic as there should be zero leakage - except in the case of a pump failure.
What is a gasket?
A gasket is usually designed in the form of a flat ring or sheet but can be made in complex shapes to match the opposing faces of the components to be sealed. Gaskets rely on compression to prevent leaks and are made from a softer material than the components to be sealed. Typically, gaskets are formed from thermoplastics or elastomers but can also be made partly or wholly from soft metals such as aluminium or copper. Some gaskets require a sealant to be added to the gasket surface.
What is an O-ring?
O-rings are circular, moulded seals, with a circular cross-section. They are normally installed in a groove on the face of one or both opposing faces of two components. As the components are fastened together, the O-ring is compressed and slightly deformed between the opposing faces to create a hermetic seal.
It is important to select an O-ring suitable for the environment and operating conditions it will experience. For example, O-rings can be prone to swelling, through absorption of chemicals, and thermal expansion. The increased volume from either of these processes can cause an O-ring to extrude beyond its groove, leading to a reduced sealing ability. O-rings can also be damaged by incorrect installation.
What materials are used for making stationary seals?
Gaskets and O-rings are usually manufactured from synthetic rubbers, called elastomers, or thermoplastics.
Thermoplastics, such as PTFE and Nylon, are characterised by an ability to be softened by heating and then cast into complex shapes. Some thermoplastics have excellent impact, abrasion and tear resistance. PTFE (Teflon® is a common tradename) is used for many pump components including gaskets, seals and diaphragms because of its excellent corrosion resistance and chemical inertness. PTFE also has a wide operating temperature range (up to 250°C).
O-rings are often made from elastomers (synthetic rubbers). These can be manufactured to have specific physical and chemical properties by the addition of fillers and other compounds and through special vulcanisation processes. Compounds are generally known by the tradenames of their manufacturers.
Table 1. Elastomers and thermoplastics used for gasket, O-ring and diaphragm manufacture
| Material | Tradename | Advantages | Disadvantages | Typical operating temperature range |
|---|---|---|---|---|
| Nitrile rubber (NBR) | Buna N | Resistant to oil and low cost | Deteriorates when exposed to air and sunlight | -40 °C to 100°C |
| Butyl rubber (IIR) | Exxon Butyl® Polysar Butyl® Esso Butyl® |
Resistant to dilute acids and alkalis | Not suitable for use in mineral or petroleum based fluids. | -50 °C to 100°C |
| Epichlorohydrin (CO, ECO) | Hydrin® | Resistant to acids and alkalis | Poor compression properties Corrosive effect on metals. |
-35 °C to 125°C |
| Polyacrylate rubber (ACM) | HyTemp Acralen |
Good resistance to hot oil | poor resistance to water, acids and alkalis | -10 °C to 120°C |
| Chloroprene rubber | Neoprene CR |
Good chemical resistance and mechanical properties over a wide temperature range | Not suitable for aromatic or chlorinated hydrocarbons | -40°C to 120°C |
| Styrene butadiene rubber (SBR) | Buna S® Perbunan® |
Not resistant to oil or fuel | -25°C to 100°C | |
| Ethylene propylene diene rubber (EPDM) | Dutral® Vistalon® |
Suitable for polar fluids. | Not suitable for petroleum-based fluids and di-ester lubricants | -50 °C to 130°C |
| Hydrogenated nitrile rubber (HNBR) | Better oil and chemical resistance than nitrile rubber (NBR) | not suitable for aromatic oils and polar organic solvents | -40 °C to 150°C | |
| Fluorosilicone rubber (FVQM) | Resistant to di-ester lubricants | not suitable for use with phosphate esters | -60 °C to 200°C | |
| Silicone rubber (VMQ) | Good for food/pharmaceutical applications | Not suitable for use with superheated steam or petroleum fluids | -60 °C to 200°C | |
| Fluoroelastomer (FKM) | Viton® Fluorel® |
Resistant to oils and most chemicals | -10 °C to 200°C | |
| Perfluorinated elastomer (FFKM) | Kalrez® Chemraz® |
Excellent chemical resistance. Expensive | -10 °C to 250°C | |
| PTFE | Teflon® | Resistant to all chemicals, low coefficient of friction Requires no lubrication |
Not elastic – cannot be resealed | -180 °C to 250°C |
What is a ‘Sealless’ Pump or a Hermetically Sealed Pump?
In applications involving the pumping of extremely hazardous fluids, where any leakage cannot be tolerated, so-called ‘sealless’ pumps should be considered. Although such designs do not feature a shaft seal, some form of stationary seal is always necessary so they are more accurately described as hermetically sealed pumps.
Magnetic Drive Pumps
In a magnetically driven pump, the drive shaft from the motor rotates an assembly of magnets on the outside of the pump housing or barrier. Opposing this, on the inside of the barrier, is a matching ring of magnets on a shaft attached to the pumping element. Torque is transferred through the barrier as a result of the coupled magnets so no shaft seal is required. However an effective O-ring or gasket is required to seal between the barrier and the body of the pump.
Canned Motor Pumps
Another way of eliminating the shaft seal and reducing the danger of leaks is by encapsulating the motor’s rotor winding so that the windings and entire drive shaft is immersed in the pumped fluid. Again, these systems are not completely ‘sealless’ as some form of O-ring or gasket is needed to seal the containing can to the body of the pump. Canned motor pumps have fewer bearings, can be more efficient and tend to be smaller than magnetic drives. The main disadvantage of canned motor pumps is that a motor failure requires replacement of the whole unit.
Diaphragm Pumps
Diaphragm pumps are a type of positive displacement pump that uses a flexible membrane instead of a piston or plunger to move fluid. The edges of the diaphragm are clamped between two separate housings. By expanding the diaphragm, the volume of the pumping chamber is increased and fluid is drawn into the pump. Compressing the diaphragm decreases the volume and expels some fluid. In these systems, the edge of the diaphragm acts as the stationary seal and the choice of material is important for performance, chemical resistance and lifetime. Diaphragms are typically manufactured from elastomers such as EPDM rubber, Buna N, Neoprene, Viton or PTFE and should be replaced regularly to eliminate the risk of leakage.
Peristaltic Pumps
Peristaltic pumps operate by means of rollers acting directly on a hose containing the liquid. Peristaltic pumps are used extensively as metering pumps in laboratory and medical applications but advances in materials technology have led to heavy duty pumps capable of dealing with highly viscous fluids and slurries in a wide range of industrial applications.
The advantage of peristaltic pumps is that the pumped fluid is always contained and never in contact with the pump mechanism – the tube is effectively a stationary seal. These systems are ideal when hygiene is important, for example, in medical applications and food processing. Peristaltic pumps can also handle solids. Wear of the flexible elements can be an issue, and these should be replaced regularly to avoid the possibility of failure.
Summary
Stationary seals, or hermetic seals, are used to seal non-moving pump components. So-called ‘sealless’ pumping systems will always feature stationary seals such as O-rings, gaskets, or diaphragms. A gasket is a flat ring or sheet, often made in complex shapes to match the opposing faces of the components to be sealed. O-rings are circular, moulded seals, with a circular cross-section. They are normally installed in a groove on the face of one or both opposing faces of two components. When compressed, O-rings and gaskets deform slightly between the opposing faces to create a hermetic seal.
When choosing a stationary seal, it is important to select a material suitable for the environment and operating conditions it will experience.
