Seal variants for twin screw pumps from Jung Process Systems
Twin screw pumps are versatile and can be used in countless applications, including hygienic applications in the food, cosmetics, pharmaceutical, and chemical industries. In order to reliably perform pumping of the desired fluid, the pump shafts must be sealed accordingly. This is necessary to keep the fluid in the pump and prevent leakage into the environment and to prevent foreign bodies or contaminants from entering the pump. Strict regulations and requirements apply, particularly in the food industry. Jung Process Systems twin screw pumps can be equipped with a wide variety of seal options. The modular system and uniform installation space also allow the sealing system to be retrofitted, for example, to respond easily to product changes or process modifications.
The single-acting mechanical seal consists of a rotating counter ring and a stationary sliding ring. Each of this rings is sealed against the shaft and the housing by means of elastomer rings. The sliding surfaces are lubricated by the pumped medium. It is suitable for clean media that do not tend to stick or harden and are not harmful to the environment. Its simple design makes it cost-effective and low-maintenance.
The difference to the regular, single-acting mechanical seal is that this seal can be used for sticky and/or hardening media. In terms of design, this variant differs in that the sliding ring and counter ring are made of tungsten carbide (WC), a much less brittle and break-sensitive hard metal. In addition, the counter ring is equipped with a significantly reduced sliding surface, the so-called knife edge. This allows it to be used even with very high-viscosity and/or media that tend to stick or harden.
The double-acting and flushed mechanical seal consists, strictly speaking, of a product-side and an atmospheric-side mechanical seal. The space between these back-to-back mechanical seals is flushed with a carrier medium, which lubricates the sliding surfaces and dissipates the frictional heat. This also makes it possible to use this seal variant for higher-viscosity media, where simple mechanical seals would otherwise fail.
The lip seal does not belong to the family of mechanical seals. Nevertheless, this type of seal is often used, especially for simpler and less critical applications. The lip seal is a radial shaft seal in which an elastic, spring-supported sealing lip seals against the shaft, thus preventing the medium from escaping from the pump. A shaft protection sleeve is used to protect the shaft journal from wear. If the stationary sealing lip runs into the rotating shaft, this design offers the advantage that only the inexpensive shaft protection sleeve needs to be replaced and not the entire shaft.
The main components of the single mechanical seal
Two high-precision ground surfaces made of materials such as silicon carbide (SiC) or tungsten carbide (WC) seal rotating and stationary parts.
Task:
Separation of product and environment by means of a minimal fluid film
Requirement:
Highest surface quality, perfect plane parallelism
The operating principle is based on two plane-parallel ground rings:
- Rotating sliding ring – this sits on the shaft
- Stationary counter ring – this sits firmly in the housing
A microscopically thin fluid film forms between these two sliding surfaces, which lubricates the sliding surfaces and also dissipates the frictional heat. This creates a virtually leak-free and reliable seal, even under demanding operating conditions.
For use in the food, beverage, or pharmaceutical industries, the situation is further complicated by the fact that the seal must also meet the highest hygiene requirements.
The proven solution: the mechanical seal
It ensures that the product does not escape from the pump via the shaft and that no air or foreign substances enter the pump – even with changing pressures and temperatures.
In the HYGHSPIN twin screw pump, the shaft seals of the two shafts are located in the main product flow and are constantly flushed with fresh product. Unlike other pump types, this means that no "forced flushing" of the seal chamber from the pressure side in accordance with API Plan 11 is required.
Advantage:
The elastomers are highly resistant to cleaning processes (CIP/SIP), are suitable for high temperatures, and meet hygiene requirements.
Seal carrier: Semi-cartridge unit
The entire seal is often pre-assembled as a cartridge seal. The advantage of this design is its easy installation/removal. Unlike component seals, this allows for quick and easy maintenance. In food and beverage production, strict requirements apply to hygienic design, cleanability, and material selection:
- Arrangement of the shaft seal in the main flow
- Stainless steel and certified materials for all parts in contact with media
- Self-emptying pump head
CIP/SIP capability for safe and residue-free cleaning
Optional: Double-acting seals with barrier fluid for maximum product safety
* For secondary seals in hygienic applications, molded rings made of EU1935/2004 or FDA-compliant elastomers such as EPDM, FKM, HNBR, or PTFE are used. These ensure radial sealing between the sliding surfaces and the shaft journal or housing.
In hygienic applications, the defined compression and deformation of the molded rings offers the advantage over O-rings that a flush-fitting surface is created and no product residues can accumulate in dead spaces or cavities.
The space between the two seals is flushed or blocked with a backing or barrier fluid.
This fluid has several functions:
- Building up pressure against the product to prevent leakage to the outside
- Lubricating and cooling sliding surfaces
- Removal of wear particles
- Optional: Barrier against germs in hygienic processes
Typical barrier media, depending on the application, include water, glycol-water mixtures, alcohols, or food-grade oils.
To maintain the required barrier pressure, the seal is connected to a barrier system or flushing system.
This system regulates:
- Pressure and temperature of the barrier fluid
- Volume flow and replenishment
- Monitoring of leaks and the operating status of the mechanical seals
In hygienic applications, so-called loss flushing is preferred. This involves a small amount of water flowing continuously through the flushing chambers of the mechanical seals. Since this flushing flow does not circulate in the circuit but is discharged, no product can accumulate in the flushing medium even if product leaks into the flushing chambers. This reliably prevents any contamination of the pumped medium.
The single-acting mechanical seal consists of a rotating counter ring and a stationary sliding ring, each of which is sealed against the shaft and the housing by means of elastomer rings. The sliding surfaces are lubricated by the pumped medium. It is suitable for clean media that do not tend to stick or harden and are not harmful to the environment. Its simple design makes it cost-effective and low-maintenance.
As with the single-acting mechanical seal, elastomer molded rings provide radial sealing between the sliding/counter rings and the shaft and housing. A spring assembly ensures constant contact pressure between the product-side and atmosphere-side sliding surfaces, regardless of the operating condition or pressure in the pump.
Double, locked/flushed mechanical seal
When absolute product safety, high operational reliability, or the handling of critical media are required, a double-acting and locked mechanical seal is used. This shaft seal design is considered technically leak-proof, making it particularly suitable for critical or toxic applications where no product leakage into the environment is permitted.
The operating principle – double sealing provides better protection
In contrast to the single-acting version, the double-acting mechanical seal has two sealing systems arranged one behind the other:
- an inner seal separates the pumped medium from the barrier or flushing fluid
- an outer seal separates the barrier or flushing fluid from the environment.
Between these two seals is a barrier or flushing chamber filled with a suitable barrier or flushing fluid.
The result:
When a suitable sealing system is used, a secure and hermetic seal between the product chamber and the environment can be achieved while ensuring maximum operational reliability.
