Advanced Ceramic Materials for Harsh Service Applications

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Serious service has no official definition.It can be thought of as referring to operating conditions where valve replacement is costly or reduces process capability.
There is a global need to reduce process production costs to improve profitability in all industries involving harsh service conditions.These range from oil and gas and petrochemicals to nuclear and power generation, mineral processing and mining.
Designers and engineers are working to achieve this in different ways.The most appropriate approach is to increase uptime and efficiency through effective control of process parameters such as effective shutdown and optimized flow control.
Safety optimization also plays a vital role, as fewer replacements can lead to a safer production environment.In addition, the company is trying to minimize the inventory of equipment, including pumps and valves, and required handling.At the same time, facility owners expect huge turnover on their assets.As a result, the increased processing capacity results in fewer (but larger diameter) pipes and equipment for the same product flow and fewer meters.
This suggests that in addition to having to be larger for wider pipe diameters, individual system components need to withstand prolonged exposure to harsh environments to reduce the need for in-service maintenance and replacement.
Components, including valves and balls, need to be robust to suit the desired application, but also provide extended service life.However, a major problem with most applications is that metal components have reached the limits of their performance capabilities.This suggests that designers may find alternatives to non-metallic materials, especially ceramic materials, for demanding service applications.
Typical parameters required to operate components under severe service conditions include thermal shock resistance, corrosion resistance, fatigue resistance, hardness, strength and toughness.
Resilience is a key parameter as less resilient components can fail catastrophically.The toughness of a ceramic material is defined as the resistance to crack propagation.In some cases it can be measured using the indentation method, resulting in an artificially high value.Using a single-sided notch beam provides accurate measurements.
Strength is related to toughness, but refers to the single point at which a material fails catastrophically when stress is applied.It is commonly referred to as the “modulus of rupture” and is measured by taking a three-point or four-point flexural strength measurement on a test bar.The three-point test provides 1% higher values ​​than the four-point test.
While hardness can be measured on a variety of scales including Rockwell and Vickers, the Vickers microhardness scale is well suited for advanced ceramic materials.Hardness varies in proportion to the wear resistance of the material.
In valves that operate in a cyclic manner, fatigue is a major problem due to the continuous opening and closing of the valve.Fatigue is the threshold of strength beyond which a material tends to fail below its normal flexural strength.
Corrosion resistance depends on the operating environment and the media containing the material.Many advanced ceramic materials outperform metals in this area, with the exception of some zirconia-based materials that “hydrothermally degrade” when exposed to high-temperature steam.
Part geometry, coefficient of thermal expansion, thermal conductivity, toughness, and strength are all affected by thermal shock.This is an area that promotes high thermal conductivity and toughness, and therefore, the metal parts function effectively.However, advances 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 demand high performance and value.Depending on the specific application requirements, there are different individual formulations suitable for various industries.However, four advanced ceramics are of significance in the field of severe service valves, and they include silicon carbide (SiC), silicon nitride (Si3N4), alumina and zirconia.Valve and valve ball materials are selected based on specific application requirements.
There are two main forms of zirconia used in valves that have the same coefficient of thermal expansion and stiffness as steel.Magnesia partially stabilized zirconia (Mg-PSZ) has the highest thermal shock resistance and toughness, while yttria tetragonal zirconia polycrystalline (Y-TZP) is harder but prone to hydrothermal degradation.
Silicon Nitride (Si3N4) is available in different formulations.Gas Pressure Sintered Silicon Nitride (GPPSN) is the most commonly used material for valves and valve components, offering high hardness and strength, excellent thermal shock resistance and thermal stability in addition to average toughness.In addition, Si3N4 provides a suitable substitute for zirconia in high temperature steam environments, preventing hydrothermal degradation.
Due to tight budgets, specifiers can choose from SiC or Alumina.Both materials have high hardness, but are not stronger than zirconia or silicon nitride.This shows that these materials are well suited for static component applications such as valve bushings and seats, rather than higher stress balls or discs.
Advanced ceramic materials have lower toughness and similar strength than metallic materials used in severe service valve applications, including chromium iron (CrFe), tungsten carbide, Hastelloy and Stellite.
Harsh service applications involve the use of rotary valves such as butterfly valves, trunnions, floating ball valves and springs.In such applications, Si3N4 and zirconia provide thermal shock resistance, toughness and strength to withstand the harshest environments.Due to the hardness and corrosion resistance of the material, the service life of the components is several times higher than that of metal components.Other benefits include the performance characteristics of the valve over its useful life, especially in areas where closing ability and control are maintained.
This is illustrated in the application of a 65 mm (2.6 in) valve kynar/RTFE ball and liner exposed to 98% sulfuric acid and ilmenite, which is being converted to titanium oxide pigment.The aggressive nature of the medium means that these components can last up to six weeks.However, using ball valve trim (Figure 1) manufactured from Nilcra™, a proprietary magnesia partially stabilized zirconia (Mg-PSZ) that provides excellent hardness and corrosion resistance, provides a three-year Uninterrupted service without any detectable wear.
In linear valves including angle, throttle or globe valves, zirconia and silicon nitride are suitable for both plug and seat due to the “hard seat” nature of these products.Likewise, aluminum oxide can be used in some liners and cages.A high degree of sealing can be achieved by matching grinding balls on the valve seat.
For valve bushings, including valve plug, inlet and outlet, or body bushings, any of the four main ceramic materials can be used depending on the application requirements.The high hardness and corrosion resistance of the material prove to be beneficial to the performance and service life of the product.
Take, for example, a DN150 butterfly valve used in an Australian bauxite refinery.The high silica content of the media can cause high levels of wear on valve bushings.The original liners and discs were made of 28% CrFe alloy and were only used for 8 to 10 weeks.However, with valves made of Nilcra™ Zirconia (Figure 2), the service life increased to 70 weeks.
Because of their toughness and strength, ceramics work well in most valve applications.However, it is their hardness and corrosion resistance that contribute to the longevity of the valve.This in turn reduces overall lifecycle costs by reducing downtime for replacement parts, lowering working capital and inventory, reducing manual handling, and improving safety through fewer leaks.
The use of ceramic materials in high-pressure valves has long been one of the major concerns because these valves are subject to high axial or torsional loads.However, major players in the field are now developing valve ball designs to improve drive torque survivability.
Another major limitation is size.The largest seat and largest ball (Figure 3) produced from magnesia partially stabilized zirconia are DN500 and DN250, respectively.However, most specifiers currently prefer ceramics for components of these sizes.
Although ceramic materials have now proven to be a suitable choice, some simple guidelines need to be followed to maximize their performance.Ceramic materials should only be used first when it is necessary to minimize cost.Sharp corners and stress concentrations should be avoided both internally and externally.
Any potential thermal expansion mismatch must be considered during the design phase.To reduce hoop stress, it is necessary to keep the ceramic on the outside, not the inside.Finally, the need for geometric tolerances and surface finishing should be carefully considered, as these can add significant and unnecessarily cost.
By following these guidelines and best practices for selecting materials and coordinating with suppliers from the start of a project, an ideal solution can be achieved for every serious service application.
This information is derived from material, reviews and adaptations provided by Morgan Advanced Materials.
Morgan Advanced Materials – Technical Ceramics.(November 28, 2019).Advanced ceramic materials for demanding service applications.AZOM.Retrieved January 14, 2022 from https://www.azom.com/article.aspx?ArticleID=12305.
Morgan Advanced Materials – Technical Ceramics.”Advanced Ceramic Materials for Harsh Service Applications”.AZOM.January 14, 2022..
Morgan Advanced Materials – Technical Ceramics.”Advanced Ceramic Materials for Harsh Service Applications”.AZOM.https://www.azom.com/article.aspx?ArticleID=12305.(Accessed 14 January 2022).
Morgan Advanced Materials – Technical Ceramics.2019. Advanced Ceramic Materials for Harsh Service Applications.AZoM, accessed 14 January 2022, https://www.azom.com/article.aspx?ArticleID=12305.
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