Mechanical Seal Spring Pressure Selection and Calculation: Starting from Precise Control of Face Closing Force

The sealing performance and service life of mechanical seals depend largely on the reasonable design of face pressure. Too small face pressure leads to face opening and leakage, while too large causes intensified friction and wear. In our years of seal design and manufacturing practice, Huahao Sealing Co., Ltd. (霍邱县华豪密封件有限公司) has accumulated rich experience in spring pressure design. This article systematically introduces the calculation methods and selection principles for mechanical seal spring pressure, helping seal engineers achieve precise design.

1. Composition and Calculation of Face Pressure

1.1 Face Pressure Formula

The mechanical seal face pressure Pb consists of spring pressure Ps and effective pressure from media Pp:

Pb = Ps + (β - k) · P

Where:

• Pb: face pressure (MPa)
• Ps: spring pressure (MPa), obtained from spring force Fs divided by seal face area A
• β: balance coefficient, reflecting the ratio of the area of the rotating ring subject to media pressure to the seal face area
• k: film pressure coefficient, reflecting the ratio of the average liquid film pressure between seal faces to the media pressure
• P: media pressure (MPa)

1.2 Spring Pressure Calculation

Spring pressure Ps = Fs / A

Where spring force Fs = n × k_s × x

• n: number of springs (typically 4-12)
• k_s: single spring stiffness (N/mm)
• x: spring compression (mm)

For a Φ50 mm seal face (area 19.63 cm²), if 8 small springs are used with single spring stiffness 0.5 N/mm and compression 5 mm:

• Fs = 8 × 0.5 × 5 = 20 N
• Ps = 20 / 19.63×10⁻⁴ = 0.0102 MPa

It can be seen that spring pressure must be carefully designed — too large or too small affects sealing performance.

2. Recommended Spring Pressure Ranges

2.1 Classification by Mounting Method

1.Internally-mounted mechanical seals: spring pressure recommended at 0.15-0.25 MPa. The rotating ring of internally-mounted seals is subject to media pressure, and the face pressure is mainly provided by media pressure; spring pressure only needs to ensure face closure during startup and shutdown.

2.Externally-mounted mechanical seals: spring pressure recommended at 0.20-0.35 MPa. The rotating ring of externally-mounted seals is not subject to media pressure, and the face pressure is entirely provided by spring force, requiring greater spring pressure.

3.Bellows mechanical seals: spring pressure recommended at 0.10-0.20 MPa. The bellows' own elasticity provides part of the face closure force and compensation capability, allowing spring pressure to be appropriately reduced.

2.2 Classification by Media Viscosity

• Low-viscosity media (<1 cP, such as liquid hydrocarbons, LPG): spring pressure 0.10-0.18 MPa. Low-viscosity media have poor lubricity, and excessive spring pressure leads to dry friction at the face, accelerating wear.

• Medium-viscosity media (1-100 cP, such as water, light oil): spring pressure 0.15-0.25 MPa. This is the most common condition, with moderate spring pressure.

• High-viscosity media (>100 cP, such as heavy oil, syrup): spring pressure 0.25-0.35 MPa. High-viscosity media easily form a thick oil film at the seal face, requiring greater spring pressure to maintain face contact.

2.3 Classification by Media Pressure

• Low pressure (<0.5 MPa): spring pressure takes the upper limit, ensuring face closure
• Medium pressure (0.5-3 MPa): spring pressure takes the middle value
• High pressure (>3 MPa): spring pressure takes the lower limit, mainly relying on media pressure for face pressure

3. Selection of Balance Coefficient β

3.1 Definition of Balance Coefficient

Balance coefficient β = A2 / A1

Where A1 is the seal face area and A2 is the effective area of the rotating ring subject to media pressure.

• β>1: unbalanced type, media pressure increases face pressure
• β=1: balanced type, media pressure does not affect face pressure
• β<1: over-balanced type, media pressure decreases face pressure (rarely used in practice)

3.2 Recommended β Values

• Low-pressure conditions (<0.6 MPa): β=1.0-1.2 (unbalanced type), simple structure
• Medium-pressure conditions (0.6-3 MPa): β=0.75-0.85 (partially balanced type)
• High-pressure conditions (>3 MPa): β=0.65-0.75 (deeply balanced type)

4. Selection of Film Pressure Coefficient k

The film pressure coefficient k reflects the pressure distribution of the liquid film between seal faces. For different seal face material combinations and media, k values differ:

• Ordinary water media: k=0.5
• Light oil media: k=0.5-0.6
• High-viscosity oil media: k=0.6-0.7
• Liquefied gas media: k=0.7-0.85 (easily vaporized, liquid film pressure close to media pressure)

5. Design Example

For a centrifugal pump mechanical seal with media as water at room temperature, pressure 1.5 MPa, seal face Φ50 mm, internally-mounted partially balanced structure, β=0.8, k=0.5:

• Take spring pressure Ps=0.2 MPa
• Face pressure Pb = 0.2 + (0.8-0.5) × 1.5 = 0.2 + 0.45 = 0.65 MPa

Does this value exceed the allowable face pressure of 0.6 MPa for the antimony-impregnated graphite/silicon carbide pair? Adjustment is needed. β can be reduced to 0.75:

Pb = 0.2 + (0.75-0.5) × 1.5 = 0.2 + 0.375 = 0.575 MPa

This meets the requirement.

Conclusion

The design of mechanical seal spring pressure is a key guarantee for sealing performance and life. Huahao Sealing Co., Ltd. provides a full range of carbon graphite seal rings and offers customized spring pressure design and calculation services based on customer operating conditions. Please contact our technical team — we will provide the optimal sealing solution based on professional engineering experience.