Why is the PV value of mechanical seals continuously increasing? ? ? ?

This post was last edited by Shutong on 2022-4-5 at 17:45. What is the meaning of the Pv value? The stress conditions on the seal and the surface pressure alone cannot fully reflect the performance of the seal. The recommended end face specific pressure value is also subject to certain conditions, with the average linear velocity of the sealing end face being below 20 m/s. For high-speed mechanical seals, the end-face specific pressure is relatively low, generally ranging from 0.2 to 0.3 MPa. Conversely, for mechanical seals with lower speeds, the end face specific pressure can be higher. Therefore, when selecting and calculating the specific pressure, the influence of the linear velocity at the sealing end face must also be taken into account. The frictional heat at the sealed interface depends on both pressure and velocity; therefore, in engineering practice, it is expressed as the product of the two, namely the PV value. The working behavior of the sealed end face is similar to that of a bearing; not only the specific pressure but also the pv value must be taken into account. If the PV value of a mechanical seal is too high, a large amount of frictional heat is generated at the seal surfaces. In mild cases, this causes the liquid film to vaporize, leading to seal failure; in severe cases, the seal surfaces suffer severe wear, developing grooves or even cracks. Therefore, the PV value is not only an important parameter for the proper operation of a seal but also a performance indicator of the sealing material. For a certain pair of friction material combinations, there is an allowable PV value in a particular medium; if the actual PV value in use exceeds this allowable value, the material will be damaged. Therefore, the PV value is also one of the important parameters of mechanical seals. In daily work, the PV value of a mechanical seal refers to the product of the face pressure and the circumferential velocity. However, in some places (or among some manufacturers), it has a different meaning, and the values can vary significantly; it is therefore necessary to clarify this in order to ensure correct understanding and application. The Pv value of the work. It is the product of the end-face specific pressure and the average velocity of the sealing end face. As mentioned earlier, the end-face specific pressure – the PV value – is a performance parameter during sealing operations and should be taken into account during design. The allowable PV value must not be exceeded! Some manufacturers boast that the allowable PV value of their products increases as operating conditions become more severe; I’ve never believed that!

In my opinion, the use of certain special designs (such as high-pressure and dry gas seals) along with new materials can improve the PV value of mechanical seals. The PV value is a reference standard for measuring the technical capabilities of a sealing factory.

It can be increased, but it’s not possible to automatically raise the allowable PV of a mechanically sealed system that is already in operation

Yes, once the sealing design is finalized, the allowable PV cannot be changed.

There are various definitions of the PV value in the industry; one is the product of the sealing pressure and the average sliding speed, and this value depends solely on the operating conditions； The other is the product of the contact pressure at the end face and the average sliding speed; this is related to the design and is very useful for comparing the loads on the two materials in a friction pair. However, in practical applications, PV values are often criticized; sometimes, for seals with exactly the same PV value, one has failed while the other is still functioning properly. Therefore, the PV value is important and useful, but it is necessary to be aware of its disadvantages. It is also not a simple formula that must be relied on for a sealed design.

To be precise, the increasing PV values mentioned in the original article should be viewed from the perspective of technological progress; what is the reason behind the rising PV values of mechanical seals? 1. Thanks to advances in end-face materials, historically metals, ordinary graphite, and ordinary tungsten carbide were primarily used. Today, the mainstream options include reaction-sintered silicon carbide, pressureless-sintered silicon carbide, monolithic cemented carbide, and diamond coatings. These materials offer high hardness (resistance to wear), high thermal conductivity (for rapid removal of friction heat), and low friction coefficients (resulting in less heat generation), all of which contribute to more stable sealed operation. 2. Thanks to modern processing methods, different grades of flatness and roughness on the sealed end face can result in various PV values; for example, 1 to 3 light bands, enabling different applications such as mirror finishes and frosted finishes. The higher the grade requirement, the higher the processing cost. 3. Thanks to advancements in various sealing surface technologies, such as waviness, taper, helical grooves, hydraulic pads, and laser-processed surfaces. These sealing techniques result in the hydrodynamic pressure effect, which prevents complete contact between the end surfaces; this in turn reduces the friction coefficient and significantly lowers heat generation. 4. Thanks to the increasingly mature application of API flushing solutions, the temperature of the sealing surface can be controlled, lubrication remains stable, and dry friction is avoided. 5. Benefiting from simulation design, coupled simulations of heat, mechanics, and flow fields enable the prediction of the temperature field, film thickness, thermal deformation, and mechanical deformation at the surface, thus facilitating a transition from empirical design to precise and controllable design. Therefore, \"high-quality end face materials + precise machining methods + hydrodynamic seal surface technology + efficient cooling and lubrication systems + accurate simulation design\" work together to reduce frictional heat generation, improve wear resistance and thermal conductivity, thereby continuously increasing the PV value.