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food Marketing - Technology 4/2022

food Marketing & Technology is the international magazine for executives and specialists in the food industry.

Digital How an

Digital How an Innovative Dynamic Sealing Solution Ensures Minimal Leakage and Trouble-Free Plant Operation As a manufacturer of dynamic seals for reciprocating and rotating applications, STASSKOL is always confronted with customer requirements that place the most diverse demands on the respective sealing system. Sealing of powders and highly viscous liquids: SDF Series Seals of rotating shafts (so-called shaft seals) are subject to special stresses in agitators and conveyors. If solid particles or highly viscous media are processed, the sealing surface may be damaged - the seal becomes contaminated with product particles and may lose its function. In contrast to gaseous media, liquid components or solids can also lift the sealing partners away from each other, causing a large leakage path. Furthermore, an inhomogeneous distribution of the mixed or conveyed product in the process chamber additionally leads to punctual radial loads on the shaft, which can cause misalignment or out-of-round running of the shaft in the sealing area. To meet these special requirements, the SDF Series - STASSKOL DynamicSeal Faceseal - was developed (Figure 1). Here, two sealing discs are pressed axially against opposite housing surfaces by means of a sealing medium. An elastomer sleeve between the sealing washers ensures static sealing between the discs and the shaft. On the other hand, it prevents rotation so that the discs rotate with the shaft without slippage. At the same time, the sealing discs are pressed against the inner surfaces of the housing by the elastomer - supported by sealing medium - so that a high sealing efficiency between sealing Figure 1: Structure of the STASSKOL SDF Series (here: type SDF300) Figure 2: Smallest samples from detectable sealing materials discs and housing is ensured. A special feature here is the torque transmission between the silicone and the sealing discs via a Reuleaux triangle, which ensures a good form fit and thus, in addition to torque transmission, also good sealing efficiency. SDF Series seals are designed to be axially split, allowing easy mounting around the shaft with the machine bearings installed. The jigsaw joint in the sealing elements, in addition to the good mechanical connection and high sealing effect, simultaneously ensures simple and tool-free assembly. The material makes the difference To achieve good tribological performance, i.e. low heat generation and low wear, STASSKOL has always relied on its own materials. Based on the knowhow built up over many decades in the development of high-performance polymers, new recipes were developed for the SDF Series and tested on the in-house tribometer. The focus was on minimizing friction and wear to maximize the service life of the SDF Series. In addition to high wear resistance, special attention was also paid to properties such as Young’s modulus, tensile strength, and elongation at break, to prevent damage to the sealing discs under heavy mechanical stress as far as possible. Especially in the food sector, product contamination must be avoided at all costs. To achieve this reliably, the sealing discs of the SDF Series can also be made of a special material that enables efficient detection of particles. The development of this formulation is based on tests in which tiny samples were introduced into chocolate and identified by X-ray detection. Even the smallest samples could be reliably detected in the product (see Figure 2). This means that the customer can be offered a sealing system which, in addition to an efficient sealing effect, combines maximum wear resistance with high mechanical stability, and whose detectability of the special sealing material guarantees a high level of process reliability, particularly in the food sector. To confirm this in practice, the most promising material combinations were tested in the form of SDF Series prototypes on a shaft seal test rig under near-process operating conditions. When varying pressure and speed, the temperature at the sealing area was recorded in addition to wear and leakage rates (Figure 3). The test results showed that the material SK933 is the ideal solution for the sealing discs, as it simultaneously offers the best tribological and thermal performance with high strength. Parallel to the experimental tests, numerical investigations were carried out on the gasket. The results obtained food Marketing & Technology • August 2022

Digital while at the same time accepting the most difficult tasks, such as the efficient sealing of shafts with high eccentricity. The optimized materials are designed for long service life, and special formulations enable detectability of any contamination, especially in the food sector - a feature that was not previously available in shaft seals. Figure 3: Temperature profile at the sealing surfaces during test bench runs from the tests could be used to validate the computational model. The simulation allows a more detailed look at the mechanical and thermal loading of the sealing elements and a size scaling without further experimental tests (Fig4). The practical test at the customer’s site shows: The sealing concept reduces product leakage to zero To be able to make a final evaluation of the performance of the seal, a field test was carried out on a twin-shaft mixer. In the industrial plant, 3.5 to 5 tons of cereal meal at a time are mixed with minerals, stones up to 4 mm in size, and oil in batch operation to produce poultry feed. The mixing process takes about 4 to 5 minutes for one batch and is carried out 24 hours a day, 5 days a week. The two shafts are each 200 mm in diameter and rotate at 40 rpm. The previous sealing solution, a carbon ring seal, was no longer a suitable solution for the customer because product leakage of up to 25 kg per seal per day was occurring and there was a high consumption of air as a sealing medium. With the goal of saving resources through lower energy consumption for the sealing air and reduced feed loss due to product leakage, the twin-shaft mixer was equipped with the flagship of the SDF Series, the SDF300 seal. Immediately after replacing the seals, the improvements in the standard process conditions became apparent. Sealing gas pressure and consumption were reduced and no more product leakage was detected (Figure 5 right). Seal run-in behavior was also improved by changing the system to the SDF300. The optimum tribological properties in the form of low wear and low friction also guarantee low temperatures in the contact zone between the sealing partners of the SDF300 during the first hours of operation. This prevents sticking as well as possible thermal damage to the product. The conclusion: The SDF Series - a seal for all cases The new SDF Series sealing concept from STASSKOL combines high sealing efficiency and long service life with easy handling, low cleaning requirements, and the possibility of mounting on an installed shaft without dismantling the equipment. The design of the SDF Series is kept simple to minimize costs, Figure 4: Temperature distribution in the gasket determined with FEM simulation Practical use at the customer’s site shows that the universal and versatile SDF Series in the form of the SDF300 masters the challenges and quickly pays for itself by reducing leakage and energy consumption. This saves resources and increases the reliability and availability of industrial plants. fmt Figure 5: Agitator before (above) and after (below) fitting with the STASSKOL SDF Series shaft seal (type SDF300) food Marketing & Technology • August 2022

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