As trivial as a valve may seem, it is often the tiny inconspicuous details that determine human and environmental safety as well as the operating costs incurred and the process yield. One of GEMÜ’s strengths as a valve manufacturer is that it offers customised process solutions which join the valve with the actuator and sensor system as an integral unit.
Considerations
Several criteria are taken into consideration when choosing the best valve technology for a specific process. For example, when choosing the type of valve, we distinguish between rotary and quarter turn valves. The difference is that quarter turn valves can be opened and closed with a single 90° rotation, while rotary valves require several rotary movements to fully open or close them. Imagine a fire extinguisher in a refinery or an airport where it is critical that the water used to put out a fire can get to where it is needed as quickly as possible. In this case, quarter-turn valves, such as butterfly or ball valves, are most likely to be used because, thanks to a single 90° rotation, they open fully in the shortest possible time. Even in processes that require a high switching frequency, quarter-turn valves are definitely an advantage.
Where solids, slurries or media containing solids in suspension are involved, mechanical wear (abrasion) and deposits in the system are often a cause of failure. A full-bore design and, where possible, no moving parts in the valve are important features when considering how best to deal with these types of operating conditions. Suitable valve types to be considered here include diaphragm valves (in the full-bore or step-valve design), knife-gate valves and pinch valves. Ball valves are also sometimes used in these applications, for example, when slurries need to be conveyed over long distances or with high pressure. Common areas of application for valves used with solids include handling cement, ashes and residues from incineration procedures, ores, coal dust and mineral products, such as lime or fertiliser.
However, despite all their features and advantages, with full-bore, the above valve types can quickly reach their limits. As soon as additional requirements such as controllability, versatility in the choice of materials, installation size, space requirements and investment costs of the valve need to be considered, it is often necessary to compromise and use alternative technologies. Diaphragm valves, ball valves and pinch valves are now rarely used above a nominal size of DN 300, either because they are no longer available, or because they are very bulky and expensive. Only knife-gate valves can still keep up here, due to their narrow installation width.
Butterfly valves
Compared to other valve technologies, butterfly valves have key characteristics. Out of all the valve types, they have by far the greatest versatility when it comes to adapting the valve to the operating conditions (materials, connections, operator types, control characteristics, etc.). Another feature is the two-way flow direction of the medium through the obturator.
Due its short construction and the shut-off principle, butterfly valves are a cost-effective and adaptable solution. Advantages such as the small and light design make it easier to install and service, particularly if large nominal sizes are required. Many of the valve types previously mentioned cover the nominal size range of DN 15 to DN 300. In comparison, the largest butterfly valves being made today have already reached a nominal size of DN 4000. Depending on the operating conditions required, we can differentiate between concentric and eccentric butterfly valves.
The sealing principle of concentric butterfly valves is based on the penetration of the disc in a liner that is manufactured from elastomer which has been adapted to the relevant conditions. The most frequently used elastomers are EPDM, NBR and FPM. Various other elastomeric materials are also used, whereby the most suitable material is chosen based on the conditions of use. The elastomer liner, or seat, is the heart of a concentric butterfly valve. Properties such as internal and external tightness, the force required for opening and closing (torque), the service life of the butterfly valve and, last but not least, costs primarily depend on the physical-chemical properties of the elastomer and on the geometry of the seat. The secure anchoring of the elastomer liner in the valve body is also a decisive factor in determining whether it is a high-quality butterfly valve as opposed to a cost-effective valve, which in turn has a direct influence on the service life of the valve.
The nature of the sealing surface of the disc, the design of the pin ends and the way in which the disc geometry matches the liner mould are further criteria which determine the torque and tightness of a butterfly valve. In order to address the specific conditions, such as system pressure – particularly where work is carried out in a vacuum and a high switching frequency at increased temperatures is also required – it is beneficial to adhere the liner to the body or fix it to the body using vulcanisation.
Soft-sealing butterfly valves are used at an operating pressure of up to 25 bar and media temperatures up to 140°C. This is very much dependent on the elastomer material, medium, system pressure, size of the butterfly valve and switching frequency. Where there are additional requirements such as high corrosion resistance, as is often required in the chemical industry, or applications where the elastomeric materials can no longer withstand the current conditions, a combination of the soft-sealing principle coupled with the use of PTFE or copolymers of PTFE (e.g. PFA and TFM) is avoided. In terms of corrosion resistance, PTFE and PFA have practically no limitations. A prime example of this is the extraction of chlorine in electrolytic procedures. In cases such as these, where the requirements relating to corrosion are so high and safety is exceptionally important, PTFE butterfly valves with a titan disc are used. The temperature stability is also far higher than on conventional soft-sealing butterfly valves.
Process conditions
In order to achieve the longest possible service life, we need to know the exact process conditions and define a solution that is optimally adapted to these.
The first decision involves choosing the most suitable type of valve technology, after which the materials to be used and a configuration (disc diameter, closing angle setting, seat loose or fixed to the body) adapted to the prevailing operating conditions must be defined. The required connection standards must, of course, be covered since otherwise the valve cannot be properly installed in the system.
A butterfly valve can be operated in open/closed mode, or as a control valve. This requires a suitable actuator that is adjusted to the operating requirements. Pneumatic actuators can be single or double acting. With double acting actuators, the process for opening and closing pneumatic actuators involves supplying them with compressed air on both sides. Single-acting pneumatic actuators are opened and closed on one side by spring pressure. This can be an important aspect, e.g. if, in the event that the compressed air fails or there is a power failure (which can lead indirectly to a lack of compressed air for operation), the butterfly valve needs to be closed or opened immediately for safety reasons.
Motorised actuators are sometimes used where compressed air is not available, which may occur if the valve is a long way from the plant control system. The power supply to the motor can be a three-phase (400 V), single-phase (230 V) or low-voltage (12-24 VAC or DC) system. Motorised actuators tend to incur higher investment costs and, when using rotary valves in particular, have certain advantages compared with pneumatic actuators, primarily if large nominal sizes are involved.
The sensor system as additional accessory is an important component for balanced process solutions since the valve and the actuator, including the overall measurement and control systems and the additional components, form one unit whereby all the components used must be properly synchronised with each other.
A well-rounded product range tailored to the process requirements is an important basic prerequisite for providing the user with the most cost-effective solution. Customised solutions can lead to an increase in productive output, savings in energy thus higher cost efficiency in production. While on the one hand, and in most applications, standard solutions are used, many existing plants and processes have significant optimisation potential. In this case, a precise analysis of the prevailing operating conditions is a prerequisite for a solution which is optimally designed for the operating conditions.
For more information contact Claudio Darpin, GEMÜ Valves Africa, +27 (0)11 462 7795, [email protected], www.gemu-group.com
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