There is no formal service definition. It can be considered to refer to the high cost of replacing the valve or the working conditions that reduce the processing capacity.
The global need to reduce process production costs in order to improve the profitability of all sectors involved in harsh service conditions. These range from oil and gas, petrochemicals to nuclear power and power generation, mineral processing and mining.
Designers and engineers are trying to achieve this goal in different ways. The most appropriate method is to increase uptime and efficiency by effectively controlling process parameters (such as effective shutdown and optimized flow control).
Safety optimization also plays a vital role, because reducing the number of replacements can lead to a safer production environment. In addition, the company is working to reduce equipment (including pumps and valves) inventory and required disposal. At the same time, facility owners expect huge turnover from their assets. Therefore, increased processing capacity will result in fewer (but larger diameter) pipes and equipment and fewer instruments for the same product stream.
This shows that, in addition to having to use larger individual system components for wider pipe diameters, it is also necessary to endure prolonged exposure to harsh environments to reduce in-service maintenance and replacement requirements.
Components including valves and valve balls need to be robust to suit the desired application, but they may also extend their life. However, the main problem with most applications is that metal parts have reached their performance limits. This indicates that designers may find alternatives to non-metallic materials in demanding applications, especially ceramic materials.
Typical parameters required to operate components under harsh conditions include thermal shock resistance, corrosion resistance, fatigue resistance, hardness, strength, and toughness.
Resilience is a key parameter, because components that are less resilient can fail catastrophically. The toughness of ceramic materials is defined as the resistance to crack propagation. In some cases, it can be measured using the indentation method to obtain artificially high value. The use of a single-side incision beam can provide accurate measurement results.
Strength is related to toughness, but refers to a single point where a material is catastrophically damaged when stress is applied. It is commonly referred to as the “modulus of rupture” and is obtained by measuring the three-point or four-point bending strength on a test rod. The value of the three-point test is 1% higher than the value of the four-point test.
Although many scales including Rockwell hardness tester and Vickers hardness tester can be used to measure hardness, the Vickers microhardness scale is very suitable for advanced ceramic materials. The hardness changes in proportion to the wear resistance of the material.
In valves operating in a cyclic manner, fatigue is the main concern due to the continuous opening and closing of the valve. Fatigue is the threshold of strength. Beyond this threshold, the material tends to fail below its normal bending strength.
Corrosion resistance depends on the operating environment and the medium containing the material. In addition to “hydrothermal degradation”, many advanced ceramic materials are superior to metals in this field, and certain zirconia-based materials will undergo “hydrothermal degradation” after being exposed to high-temperature steam.
The geometry, thermal expansion coefficient, thermal conductivity, toughness and strength of the components are affected by thermal shock. This area is conducive to high thermal conductivity and toughness, so the metal components can function effectively. However, advancements in ceramic materials now provide acceptable levels of thermal shock resistance.
Advanced ceramics have been used for many years and are popular among reliability engineers, plant engineers and valve designers who require high performance and high value. According to specific application requirements, it is suitable for different formulations in a variety of industries. However, four advanced ceramics are of great significance in the field of rigorous maintenance of valves, including silicon carbide (SiC), silicon nitride (Si3N4), alumina and zirconia. The materials of the valve and valve ball are selected according to the specific application requirements.
The valve uses two main forms of zirconia, which have the same thermal expansion coefficient and stiffness as steel. Magnesium oxide partially stabilized zirconia (Mg-PSZ) has the highest thermal shock resistance and toughness, while yttria tetragonal zirconia polycrystalline (Y-TZP) is harder, but is susceptible to hydrothermal degradation.
Silicon nitride (Si3N4) has different formulations. Gas pressure sintered silicon nitride (GPPSN) is the most commonly used material for valves and valve components. In addition to its average toughness, it also has high hardness and strength, excellent thermal shock resistance and thermal stability. In addition, in high-temperature steam environments, Si3N4 can replace zirconia to prevent hydrothermal degradation.
With a stricter budget, the concentrator can choose from SiC or alumina. Both materials have high hardness, but are not harder than zirconia or silicon nitride. This shows that the material is very suitable for static component applications, such as valve liners and valve seats, rather than valve balls or discs that are subject to higher stress.
Compared with the metal materials used in demanding valve applications (including ferrochrome (CrFe), tungsten carbide, Hastelloy and Stellite), advanced ceramic materials have lower toughness and similar strength.
Demanding service applications involve the use of rotary valves, such as butterfly valves, trunnions, floating ball valves and springs. In such applications, Si3N4 and zirconia have thermal shock resistance, toughness and strength, and can adapt to the most demanding environments. Due to the hardness and corrosion resistance of the material, the service life of the component is several times that of the metal component. Other benefits include performance characteristics over the life of the valve, especially in areas where cut-off and control capabilities are maintained.
This was demonstrated in the case of a 65mm (2.6 inch) valve kynar/RTFE ball and liner exposed to 98% sulfuric acid plus ilmenite, the ilmenite being converted to titanium oxide pigment. The corrosive nature of the media means that the life of these components can be as long as six weeks. However, the use of spherical valve trim (a proprietary magnesium oxide partially stabilized zirconia (Mg-PSZ)) manufactured by Nilcra!" (Figure 1) has excellent hardness and corrosion resistance and has been provided for three years. Intermittent service, without any detectable wear and tear.
In linear valves (including angle valves, throttle valves or globe valves), due to the “hard seat” characteristics of these products, zirconia and silicon nitride are suitable for both valve plugs and valve seats. Similarly, alumina can be used in certain linings and cages. Through the matching ball on the seat ring, a high degree of sealing can be achieved.
For the valve core, including spool valve, inlet and outlet or valve body bushing, any one of the four main ceramic materials can be used according to the application requirements. The high hardness and corrosion resistance of the material have proven to be beneficial in terms of product performance and service life.
Take the DN150 butterfly valve used in the Australian bauxite refinery as an example. The high silica content in the medium causes high levels of wear on the valve bushings. The liner and valve disc used initially were made of 28% CrFe alloy and lasted only eight to ten weeks. However, due to the introduction of valves made of Nilcra!" zirconia (Figure 2), the service life has been increased to 70 weeks.
Due to its toughness and strength, ceramics work well in most valve applications. However, it is their hardness and corrosion resistance that help extend the life of the valve. In turn, this reduces the cost of the entire life cycle by reducing downtime for replacement parts, reduced working capital and inventory, minimal manual handling, and improved safety through reduced leakage.
For a long time, the application of ceramic materials in high-pressure valves has been one of the main concerns, because these valves are subject to high axial or torsional loads. However, major players in this field are developing valve ball designs that improve the survivability of the actuation torque.
The other major limitation is size. The size of the largest valve seat and largest valve ball (Figure 3) produced by magnesia partially stabilized zirconia are DN500 and DN250, respectively. However, most current specifiers prefer to use ceramics to make parts whose dimensions do not exceed these dimensions.
Although ceramic materials have now been proven to be a suitable choice, there are still some simple guidelines that need to be followed to maximize their performance. Ceramic materials should be used first only if there is a need to reduce costs. Both inside and outside should avoid sharp corners and stress concentration.
Any potential thermal expansion mismatch must be considered during the design phase. In order to reduce the hoop stress, it is necessary to keep the ceramic outside rather than inside. Finally, the need for geometric tolerances and surface finishing should be carefully considered, as these tolerances may significantly increase unnecessary costs.
By following these guidelines and best practices in selecting materials and coordinating with suppliers from the beginning of the project, an ideal solution can be achieved for each demanding service application.
This information has been obtained, reviewed and adapted from materials provided by Morgan Advanced Materials.
Morgan Advanced Materials-Technical Ceramics. (November 28, 2019). Advanced ceramic materials suitable for serious service applications. AZoM. Retrieved from https://www.azom.com/article.aspx?ArticleID=12305 on May 26, 2021.
Morgan Advanced Materials-Technical Ceramics. “Advanced ceramic materials for serious service applications”. AZoM. May 26, 2021 .
Morgan Advanced Materials-Technical Ceramics. “Advanced ceramic materials for serious service applications”. AZoM. https://www.azom.com/article.aspx?ArticleID=12305. (Accessed on May 26, 2021).
Morgan Advanced Materials-Technical Ceramics. 2019. Advanced ceramic materials suitable for serious service applications. AZoM, viewed on May 26, 2021, https://www.azom.com/article.aspx? ArticleID = 12305.
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Post time: May-26-2021
