Report Material Selection - Tap Valve

Case study

Tap valves include ceramic parts: a) identify the properties according to which ceramics are more interesting than any other materials class; b) select the best material according to a penalty or performance function, attributing reasonable weight factors to all significant properties.

Discussion

Taps drip because the rubber washer is worn or the brass seat is pitted by corrosion, or both. Ceramics have good wear resistance, and they have excellent corrosion resistance in both pure and saltwater. Many households tap now use ceramic valves. The sketch shows how they work.

A ceramic valve consists of two disks mounted one above the other, spring-loaded so that their faces are in contact. In order to seal well, the mating surfaces of the two disks must be flat and smooth, requiring high levels of precision and surface finish; typically, tolerance < 0.02mm and surface roughness < 0.1 μm. The outer face of each has a slot that registers it and allows the upper disc to be rotated through 90° (a quarter-turn). In the off position the holes in the upper disc are blanked off by the solid part of the lower one; in the on position the holes are aligned. This configuration permitted to achieve a less wearable condition compared with the older ones.

First approach

First approach is finding which are the most important characteristics, those chosen for primary selection. The choice fell on hardness (Hardness-Vickers) and Price per Unit Volume. To get an idea of which materials we must consider, we initially made a Penalty Function at level 2 between the two characteristics mentioned above according to the relationship: Z = C/C₀ + α HV₀/HV, as we want to minimize costs and maximize hardness.

The relative values were calculated and reported in the graph by considering standard values: C/C₀ = σ_f0/ρ₀c_m0 ρ/c_m ρ/σ_f. HV₀/HV = σ_f0/σ ρ₀/ρ.

Figure 1 - Ceramic Valve Components.

Case study

Tap valves include ceramic parts: a) identify the properties according to which ceramics are more interesting than any other materials class; b) select the best material according to a penalty or performance function, attributing reasonable weight factors to all significant properties.

Discussion

Taps drip because the rubber washer is worn or the brass seat is pitted by corrosion, or both. Ceramics have good wear resistance, and they have excellent corrosion resistance in both pure and saltwater. Many households tap now use ceramic valves. The sketch shows how they work.

A ceramic valve consists of two disks mounted one above the other, spring-loaded so that their faces are in contact. In order to seal well, the mating surfaces of the two disks must be flat and smooth, requiring high levels of precision and surface finish; typically, tolerance < 0.02mm and surface roughness < 0.1 µm. The outer face of each has a slot that registers it and allows the upper disc to be rotated through 90° (a quarter-turn). In the off position the holes in the upper disc are blanked off by the solid part of the lower one; in the on position the holes are aligned. This configuration permitted to achieve a less wearable condition compared with the older ones.

Figure 1 - Ceramic Valve Components.

Penalty function

First approach is finding which are the most important characteristics, those chosen for primary selection. The choice fell on hardness (Hardness-Vickers) and Price per Unit Volume. To get an idea of which materials we must consider, we initially made a Penalty Function at level 2 between the two characteristics mentioned above according to the relationship: Z = C₀ + α HV₀/HV, as we want to minimize costs and maximize hardness.

The relative values were calculated and reported in the graph by considering standard values: C/C₀ = σ_f0/ρ_0Cm0 ρ_Cm/σ_f. HV₀/HV = HV₀/σ_f0 ρ₀/σ_f/HV_ρ.

Figure 2 - Penalty Function Diagram, Level 2.

From the diagram at level 2 it appears that ceramics, as said before, are the most interesting. So we then continued the selection by evaluating the same diagram at level 3 with the same X and Y axes values, from which we obtained the main materials of our study; in particular, favouring technical ceramic materials, obtained by sintering and with lower cost and maximum hardness characteristics (in a range of values we put as maximum or minimum limits). We also gave a high value of α since we wanted to maximize the HV and minimize the Cost.

Figure 3 - Penalty Function Diagram, Level 3.

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Selected materials

Materials came out from this diagram are:

• Alumina (94%) (cold pressed and sintered)
• Alumina (96%) (pressed and sintered)
• Alumina (97%) (cold pressed and sintered)
• Alumina (99%) (cold pressed and sintered)
• Boron carbide (hot pressed) (commercial purity)
• Silicon carbide (sintered)
• Silicon nitride (sintered)
• Zirconia (Y2O3 stabilized, transformation toughened)

Other characteristics and weight factors

Other characteristics were subsequently evaluated, in particular Wear Resistance, Friction Coefficient and Corrosion Resistance. Through an enhanced digital logic comparison table, we found the relative weight factors using "Degree of Importance":

• A value of 1 for the Pi property and a value of 1 for the Pj property if of equal importance;
• A value of 2 for the Pi property and a value of 1 for the Pj property if Pi is more important than Pj;
• A value of 3 for the Pi property and a value of 1 for the Pj property if Pi is much more important than Pj.

We considered:

• Price per Unit Volume and Hardness of equal importance.
• Price per Unit Volume and Hardness more important than Friction Coefficient.
• Price per Unit Volume and Hardness much more important than Corrosion Resistance and Wear Resistance.
• Friction Coefficient more important than Corrosion Resistance and Wear Resistance.
• Corrosion Resistance and Wear Resistance of equal importance.

For what concerns the main characteristics, they were divided into decreasing and increasing properties, in particular the Wear and Corrosion Resistance values have been evaluated on a scale from 1 to 10, where 10 is excellent and 1 is unacceptable. 8 is equivalent to more than acceptable in our case.

The decreasing properties were normalized inversely and the increasing properties directly.

We then passed to the calculation of the Performance Function by the formula:

Since Wear resistance and Corrosion Resistance are respectively Very Good and Excellent for all the materials considered, they don’t make a real change in the final result, but we considered as well in the performance function.

“Alumina (94%) (cold pressed and sintered)” is the best material to choose according to this case study.

Conclusion

The results show how alumina is the best material for the field considered. It shows the best properties in terms of hardness, resistance to corrosion and wear if compared with the other materials we considered. It has a low coefficient of friction and the low cost per volume unit and this makes it suitable for many fields of application like electrical insulators and connector bodies, substrates, high temperature components, water faucet valves, mechanical seals, vacuum chambers and vessels, centrifuge linings, spur gears, fuse bodies, heating elements, plain bearings and other wear