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There is no official definition of serious service. It can be understood as operating conditions where the valve replacement cost is high or the processing capacity is reduced.
There is a global need to reduce process production costs in order to increase the profitability of all sectors involved in poor service conditions. These range from oil and gas and 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 replacement can lead to a safer production environment. In addition, the company is working to minimize equipment inventory, including pumps and valves, and the required disposal. At the same time, facility owners expect a huge shift in their assets. As a result, increased processing capacity results in fewer pipes and equipment (but larger diameters) and fewer instruments for the same product stream.
This shows that in addition to having to be larger for a wider pipe diameter, a single system component also needs to withstand prolonged exposure to harsh environments to reduce the need for in-service maintenance and replacement.
Components including valves and valve balls need to be robust to suit the desired application, but can also provide a longer service life. However, a major problem with most applications is that metal parts have reached the limit of their performance. This indicates 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, 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, resulting in artificially high values. The use of a single-side incision beam can provide accurate measurements.
Strength is related to toughness, but refers to the single point where a material fails catastrophically when stress is applied. It is commonly referred to as the “modulus of rupture” and is measured by performing a three-point or four-point bending strength measurement on a test rod. The three-point test provides a value that is 1% higher than the four-point test.
Although hardness can be measured with a variety of scales including Rockwell and Vickers, the Vickers microhardness scale is very suitable for advanced ceramic materials. The hardness is directly proportional to the wear resistance of the material.
In a valve operating in a cyclic method, fatigue is a major problem due to the continuous opening and closing of the valve. Fatigue is the strength threshold, beyond which the material will often fail below its normal bending strength.
The corrosion resistance depends on the operating environment and the medium containing the material. In this field, many advanced ceramic materials have advantages over metals, except for “hydrothermal degradation”, which occurs when some zirconia-based materials are exposed to high-temperature steam.
Part geometry, thermal expansion coefficient, thermal conductivity, toughness and strength are affected by thermal shock. This is an area conducive to high thermal conductivity and toughness, so metal parts 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 value. According to specific application requirements, there are different individual formulations suitable for a wide range of industries. However, four advanced ceramics are of great significance in the field of severe service valves. They include 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.
Two main forms of zirconia are used in valves, both of which have the same coefficient of thermal expansion 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 and stronger, 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 provides high hardness and strength, excellent thermal shock resistance and thermal stability. In addition, in high-temperature steam environments, Si3N4 is a suitable substitute for zirconia, which can prevent hydrothermal degradation.
When the budget is tight, the specifier can choose silicon carbide or alumina. Both materials have high hardness, but are not tougher than zirconia or silicon nitride. This shows that the material is very suitable for static component applications, such as valve linings and valve seats, rather than valve balls or discs that are subject to higher stress.
Compared with the metal materials used in harsh service valve applications (including ferrochrome (CrFe), tungsten carbide, Hastelloy and Stellite), advanced ceramic materials have lower toughness and similar strength.
Severe service applications involve the use of rotary valves, such as butterfly valves, trunnions, floating ball valves, and spring valves. In such applications, Si3N4 and zirconia exhibit thermal shock resistance, toughness and strength to adapt to the most demanding environments. Due to the hardness and corrosion resistance of the material, the service life of the parts is increased several times compared with metal parts. Other benefits include the performance characteristics of the valve over its lifetime, especially in areas where it maintains its closing capacity and control.
This is demonstrated in an application where a 65 mm (2.6 in) valve kynar/RTFE ball and liner are exposed to 98% sulfuric acid and ilmenite, which is 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 ball valve trim made by Nilcra!" (Figure 1), which is a proprietary magnesium oxide partially stabilized zirconia (Mg-PSZ), has excellent hardness and corrosion resistance, and can provide three years of uninterrupted Service without any detectable wear and tear.
In linear valves, including angle valves, throttle valves or globe valves, due to the “hard seal” characteristics of these products, zirconia and silicon nitride are suitable for valve plugs and valve seats. Similarly, alumina can be used for some gaskets and cages. By matching grinding balls on the valve seat, a high degree of sealing can be achieved.
For valve lining, including valve core, inlet and outlet or valve body lining, any one of the four main ceramic materials can be used according to application requirements. The high hardness and corrosion resistance of the material proved 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 provides a high level of wear on the valve lining. The gaskets and discs initially used were made of 28% CrFe alloy and lasted only eight to ten weeks. However, with valves made of Nilcra!" zirconia (Figure 2), the service life has 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 increase the service life of the valve. This in turn reduces the cost of the entire life cycle by reducing downtime for replacement parts, reducing working capital and inventory, minimal manual handling, and improving safety by reducing leakage.
For a long time, the application of ceramic materials in high-pressure valves has been one of the main problems, because these valves are subject to high axial or torsional loads. However, major players in this field are now developing valve ball designs to improve the survivability of driving torque.
The other major limitation is scale. The size of the largest valve seat and largest valve ball (Figure 3) produced from partially stabilized zirconia with magnesium oxide is DN500 and DN250, respectively. However, most specifiers currently prefer ceramics for components below these sizes.
Although ceramic materials are 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 costs need to be kept to a minimum. Sharp corners and stress concentration should be avoided both inside and outside.
Any potential thermal expansion mismatch must be considered during the design phase. In order to reduce hoop stress, the ceramic must be kept outside, not inside. Finally, the need for geometric tolerances and surface finishing should be carefully considered, as these will significantly increase unnecessary costs.
By following these guidelines and best practices for selecting materials and coordinating with suppliers from the beginning of the project, an ideal solution can be achieved for every harsh service application.
This information is derived from materials provided by Morgan Advanced Materials and has been reviewed and adapted.
Morgan Advanced Materials-Technical Ceramics. (2019, November 28). Advanced ceramic materials for demanding service applications. AZoM. Retrieved from https://www.azom.com/article.aspx?ArticleID=12305 on July 7, 2021.
Morgan Advanced Materials-Technical Ceramics. “Advanced ceramic materials for demanding service applications”. AZoM. July 7, 2021. .
Morgan Advanced Materials-Technical Ceramics. “Advanced ceramic materials for demanding service applications”. AZoM. https://www.azom.com/article.aspx?ArticleID=12305. (Accessed July 7, 2021).
Morgan Advanced Materials-Technical Ceramics. 2019. Advanced ceramic materials for demanding service applications. AZoM, viewed July 7, 2021, https://www.azom.com/article.aspx?ArticleID=12305.
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Post time: Jul-08-2021
