What Is the Opening Force of a Mechanical Seal

Opening force is a critical parameter in the design and selection of mechanical seals. This blog post will provide an in-depth look at the factors that influence opening force and walk through the calculations needed to determine the total opening force for a mechanical seal.

We will explore the four main components that contribute to opening force: spring force, hydraulic force, friction force, and adhesion force. For each factor, we will discuss the underlying principles and provide detailed formulas and example calculations to illustrate how to quantify their impact on the total opening force.

What Is Opening Force

Opening force, also known as lift-off force, is a critical parameter in mechanical seal design and operation. It refers to the force required to overcome the closing forces acting on the seal faces, allowing them to separate and create a fluid film between the faces. This fluid film is essential for proper seal operation, as it prevents direct contact between the faces, minimizes wear, and helps dissipate heat.

Factors Affecting Opening Force

Spring Force

Spring force is one of the primary components of the opening force in a mechanical seal. Mechanical seals typically employ springs, such as coil springs or bellows, to provide a constant closing force on the seal faces. The spring force helps maintain contact between the faces during operation and compensates for any wear or thermal expansion.

The magnitude of the spring force depends on the spring design, material, and compression. Seal manufacturers often provide spring force data for their products, which can be used in the opening force calculation.

Hydraulic Force

Hydraulic force, also known as fluid pressure force, is another significant contributor to the opening force. This force is generated by the fluid pressure acting on the seal faces. The hydraulic force tends to push the faces apart, counteracting the closing forces and aiding in the formation of the fluid film.

The magnitude of the hydraulic force depends on the fluid pressure, the geometry of the seal faces, and the balance ratio of the seal. The balance ratio is a design parameter that determines the proportion of the seal face area subjected to the fluid pressure.

Friction Force

Friction force is a resistive force that opposes the relative motion between the seal faces. In mechanical seals, friction force arises from the contact between the rotating and stationary seal faces. This force depends on the surface finish, material properties, and the contact pressure between the faces.

Friction force contributes to the closing force in a mechanical seal, and its magnitude must be overcome by the opening force to achieve proper seal operation.

Adhesion Force

Adhesion force, also called stiction force, is an attractive force that occurs between the seal faces when they are in close contact. This force is a result of molecular interactions, such as van der Waals forces, and is influenced by the surface properties and the presence of any contaminants or fluid films.

Adhesion force can be significant in certain applications, particularly when the seal faces are highly polished or when dealing with viscous fluids.

Calculating Opening Force

Spring Force Calculation

The spring force (F_s) is calculated using Hooke’s law:

F_s = k × x

Where:

• k is the spring constant (N/m)
• x is the spring compression (m)

Example:

• Spring constant (k) = 10,000 N/m
• Spring compression (x) = 0.005 m

F_s = 10,000 N/m × 0.005 m = 50 N

Hydraulic Force Calculation

The hydraulic force (F_h) is calculated using the following formula:

F_h = P × A × B

Where:

• P is the fluid pressure (Pa)
• A is the seal face area (m²)
• B is the balance ratio (unitless)

Example:

• Fluid pressure (P) = 1,000,000 Pa (10 bar)
• Seal face area (A) = 0.0001 m²
• Balance ratio (B) = 0.8

F_h = 1,000,000 Pa × 0.0001 m² × 0.8 = 80 N

Friction Force Calculation

The friction force (F_f) is calculated using the following formula:

F_f = μ × F_c

Where:

• μ is the coefficient of friction (unitless)
• F_c is the closing force (N), which includes the spring force and any other closing forces

Example:

• Coefficient of friction (μ) = 0.1
• Closing force (F_c) = 100 N (including the spring force calculated earlier)

F_f = 0.1 × 100 N = 10 N

Adhesion Force Estimation

Adhesion force (F_a) is difficult to calculate precisely, as it depends on various factors such as surface properties, fluid properties, and environmental conditions. In practice, adhesion force is often estimated based on experimental data or empirical formulas.

For this example, let’s assume an estimated adhesion force of 5 N.

F_a = 5 N

Total Opening Force

The total opening force (F_o) is the sum of the hydraulic force and the adhesion force, minus the spring force and the friction force:

F_o = F_h + F_a – F_s – F_f

Using the values from the previous examples:

• Hydraulic force (F_h) = 80 N
• Adhesion force (F_a) = 5 N
• Spring force (F_s) = 50 N
• Friction force (F_f) = 10 N

F_o = 80 N + 5 N – 50 N – 10 N = 25 N

In this example, the total opening force is 25 N. This value indicates the minimum force required to overcome the closing forces and create a fluid film between the seal faces.