Valves, Actuators & Pump Control


Valve technology without stress

February 2017 Valves, Actuators & Pump Control

Maintenance can be defined as the degradation management of engineered materials (equipment and systems) to retain their performance within their designed operating parameters.

Just as stress can accelerate deterioration of metals in a corrosive environment, operational stress moves equipment and systems toward failure. Limiting stresses within the operating environment maintains reliability. The elements of the maintenance which are relevant are:

1. Protecting components from stress.

2. Monitoring their condition.

3. Undertaking component replacement prior to the failure threshold level caused by stress excesses.

The impact of maintenance costs over time can, depending on the application, easily exceed the savings made on the initial investment when purchasing inappropriate valve technology, or when choosing solutions not using state-of-the-art technology. The aim of this article is to show, using the example of industrial (butterfly) valves, how quality of components can effectively contribute to the cost efficiency of industrial production plants.

Many industrial processes use flow control technology based on valves. But, valve technology often plays a secondary role in contrast to other equipment, for instance filters, energy recovery systems, crushers, pumps and conveyors. As a general rule, valves contribute 1-3% of the total investment costs of industrial production plants.

Therefore valves often receive a lack of consideration, even though one valve failure can lead to serious problems, in some cases, even a a plant shutdown. Hence, it is important to choose the most suitable flow control technology for the application in terms of an adequate valve type and the definition of the appropriate materials to be used. Once the valves have been installed, and the plant put into service, operational and maintenance costs start accruing.

Capex vs Opex

Quality aspects of the valves have a considerable impact on the operational cost, for instance, the effect on pumping capacity, power demand of actuators and the energy efficiency of the entire plant. Corrosion is one of the major threats in industrial processes, therefore it is important to design the valves using the appropriate materials to prevent corrosion damages and thus additional costs.

Safety, product quality and plant as availability must be maximised, while operating and maintenance costs are kept as low as possible, which often causes a dilemma between the aspects of Capex and Opex. As an example, when using components having low average material life, savings on the Capex side can be made, but on the other hand, Opex will be higher due to a more frequent replacement. Vice versa, attempts to reduce specific costs of maintenance or repair, most often result in higher Capex.

On average, 1-3% of the investments costs of an industrial plant are spent on low and medium pressure valves, while maintenance costs including replacement and repair of such valves are estimated to be in the range of 4-7% of the Opex.

The butterfly valve example

The life expectancy of valves used in production processes is generally in the range of 5-10 years, depending on the quality of the valve, the working conditions, the frequency of operation and quality of maintenance. A standard valve is expected to provide 10 years of trouble-free service in these applications, while speciality valves can give reliable service for time spans up to 15 years or more.

When looking, e.g. at butterfly valves, various aspects affecting the service life or average material life time (AMLT).

One of the most important aspects of valve malfunctioning is the design and quality of the sealing element. The life span and reliability of elastomeric seated butterfly valves is largely dependent on the valve liner, which is the heart of the valve. Careful evaluation of this seemingly simple element should go beyond the material grade alone. The liner is responsible for containing the media within the valve body, providing shaft sealing and valve to flange sealing. When this primary seal fails, shaft and body will be in contact with media and leaking of the valve will begin to occur.

Interference between seal and the disc determines the pressure rating and operating torque. Consistent, repeatable and accurate manufacture of the seal results in reliable operation, whereas compound mix determines physical properties such as hardness, chemical resistance (e.g. volume change or material degradation), tear strength and age hardening of the elastomeric material. The better the finishing or polishing of metallic discs at all sealing points, the lower the torque and internal tightness will be.

Design of the liner and selection of the most suitable rubber determines the stability and the reliability of actuation of the valve. Larger diameter butterfly valves (>DN300) have historically suffered from stalling, hesitation, and subsequent uncontrollable opening rates when coupled to an actuator. This usually happens as a consequence of alteration of the elastomeric liner material, and due to poor valve design resulting in flange compression increasing the valve break torque.

Butterfly valves that use metal pins to transmit the rotation power to the valve disc suffer the potential danger of media penetrating to the interior of the shaft, where it then has an excellent point for galvanic corrosion due to the dissimilar metals of the shaft, the body and the disc.

Coated discs (e.g. Halar, Epoxy, Rilsan) generally having a coating thickness of 0,3 – 0,6 mm, which in theory appears very good. However, a coated disc is ultimately fully reliant on the total and complete encapsulation of the disc. Any breach of surface continuity will result in heavy corrosion and ultimate breakdown.

Specific costs of valve replacement

Depending on the quality of an installed valve, the life expectancy and the maintenance cost can vary significantly.

In a comparison between the TCO of a low-cost commodity valve that must be replaced once a year, versus a standard branded valve with a life expectancy of 10 years, savings of approximately 30% can be recorded when using the quality technology.

Investing in quality technology also has a direct impact on the total costs, resulting in a positive return on investment. Consequently, Capex expenditures create future benefits. Even though capital costs of industrial plants have decreased significantly over the last 10 years, pressure still remains on costs of operation, maintenance and repair. A valve is not just an irrelevant component in a process. Rather, it is an element that has a significant impact. Therefore, it is worth a closer look when designing, building, operating and maintaining an industrial production plant.

Gemü Valves Africa is dedicated to understanding its customer’s processes and then recommending the most suitable valve technology in terms of safe and cost optimised solutions.

For more information contact Claudio Darpin, Gemü Valves Africa, +27 (0)11 462 7795, claudio.darpin@gemue.co.za, www.gemu-group.com





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