Parker's metal seals are used in the most extreme environments that demand extreme temperature and pressure solutions. Metal seal technology is used in cryogenic, high temperature, hard vacuum, high pressure, and corrosive applications. The ability to choose the correct sealing technology as well as the correct part number for the application can be challenging. Metal seals perform differently than polymers in application, so standard polymer practices should not be applied to metal seal design. This guide is intended to be used when you are in the process of selecting the correct part number for your application.
Selecting the correct part number
The simplest part number structure is shown in Figure 1 and is broken down into six segments. The first segment identifies the metal seal type, followed by the seal size, the cross section, material, temper and then the plating.
There are over 73 million possible part number combinations not including the diameter of the seal as shown in Figure 2.
In order to determine the correct part number for an application, follow the steps below:
Step 1. Do you prefer to conduct all of your measurements in English or Metric units? The first letter in the part number will be identified as either an “E” or an “M” to denote the units.
Step 2. The seal type is the second digit and requires the most thought. Figure 3 gives a broad overview of when to use each seal type. A filled in green dot represents the best selection for the sealing requirement. A slashed red dot represents the seal is not fit for the sealing requirement. If you are unsure of your seal selection consult a Metal Seal Market Manager at 203-239-3341.
Step 3. The diameter of the seal is identified numerically in the next six digits. If an English part number is being created, the first three out of six digits will be the whole inch value, and the last three digits will represent the remaining decimal. For a Metric part number, the first four out of six digits will represent the whole mm value, and the last two digits will represent the remaining decimal. Please note that the decimal shown in Figure 4 is imaginary and is not printed in the actual part number. For Internal pressure seals the diameter of the seal is measured off of the outer diameter, and for External pressure seals the diameter of the seals is measured off of the inner diameter.
Step 4. The cross section code is the next selection that needs to be made. For all seal types, the larger the cross section, the larger the cross section code. Most seal types have two or more material thickness options per each nominal cross section. The reason for this is to allow the designer to pick the appropriate seating load and match it with the application conditions. A thicker material will always require a higher seating load. The higher the seating load, the better performance will be achieved from the seal. Though this, the designer must ensure that the application has enough bolting load to provide the prescribed load which targets a seal compression of around 20% of its original free height. An estimated benchmark is 200 lbs. per inch circumference. If the cross section code yields a lower value than 200, then it is assumed that the seal will not achieve better than 1x10^-4 cc/sec. If the cross section code yields greater than 200, then the seal should be able to achieve a seal better than 1x10^-4 cc/sec.
Step 5. Material selection is important in metal sealing because each material type provides different performance at elevated temperatures. The most common and readily available material choice would be Alloy 718 and this is rated to 1200 °F by Parker’s Advanced Products standards. Rene 41 is one of Parker’s highest rated materials at maximum application temperature of 1450 °F. As temperatures increase in application, the overall yield strength of the material decreases. In Figure 5 the performance of materials at elevated temperatures is portrayed graphically. At temperatures below 1200 °F it is evident that Alloy 718 has the highest yield strength. As temperature increases the yield strength of 718 decreases drastically and Rene 41 becomes the material with the highest yield strength.
Step 6. The temper code selection is specifically based on the type of metal seal selected. For most seals manufactured with strip material, such as C, E, and U rings, the recommended temper code is a -6 which is for “Solution Heat Treat and Precipitation Heat Treat". O-Rings are always recommended to remain in the “Work Hardened” state which is denoted as -1. For O-ring applications that desire an increased fatigue and stress relaxation resistance, the “Solution Heat Treat and Precipitation Heat Treat” is available. A solid Wire Ring becomes extremely hard in its manufactured state so the -4 “Anneal” option is preferred to conserve resilience in the metallic seal. Corrosive applications, like those in the Oil and Gas industry, have the option of the -8 temper code which is a heat treat per NACE MR0175 standards.
Step 7. Plating is offered as an optional service for an added layer of protection to ensure a great seal. In the uncoated state, a metal seal against mating metal hardware has a multitude of leak paths. Though it may be difficult to see with the bare eye, under microscope it is evident that the surface finish of an uncoated seal is close to 16 u inch Ra as shown in Figure 7. This means that the seal has a level of roughness where there are micro peaks and valleys in the formed base metal. Similarly, the hardware has the same occurrence. When the two uneven surfaces line up with each other, there will always be small gaps between the two, causing an escape path for the fluid.
The purpose of the various plating options is to provide an added layer of a soft base metal to fill in the uneven imperfections of the seal. When the seal is compressed, the plating will fill in the uneven surface finish of the hardware as depicted in Figure 8.
Step 8. The final digit of the part number that needs to be determined is the thickness of the plating to be applied. The thickness of the plating is directly related to the overall surface finish of the mating hardware. Figure 9 shows a graphical representation of surface finish vs nominal cross section providing the output of plating thickness. It should be known that in order to achieve a “bubble tight” seal of 1x10^-4 cc/sec or better, a surface finish of 32 u inch Ra or better must exist. Though the graph may show that hardware at 250 u inch Ra can be compensated with a plating thickness of “G” for the 0.156” cross section, it will not be able to overcome the roughness to obtain a “bubble tight” seal.
At this point you should have a better understanding of how a Parker Metal Seal part number is generated based on your application conditions. If you need any additional engineering assistance please contact the engineers at Parker's Composite Sealing Systems Division - North Haven.
*For detailed information on Parker Hannifin’s Metal Seal product offerings please click here for the “Metal Seal Design Guide” - CSS 5129.
This article was contributed by Jeffrey Labonte, market manager, Parker Composite Sealing Systems Division.