Seal and Gasket Operating-Life Reliability Calculator

Estimate a circular static seal’s leakage-related failure rate, cycle survival and target inspection interval using the NSWC empirical seal model.

Adjusted failure rate

—

failures per million cycles

Enter valid conditions.

Survival at N—%

Target interval— cycles

Target time— h

Exponential mean life— cycles

Conductance H— in

Leakage contribution— /10⁶ cycles

Failure-rate sensitivity: input −10% / +10%

How to use

Use this model only for a circular, nominally static or semi-static seal with a justified NSWC leakage correlation; it is not a general dynamic-seal life model.
Enter the base failure rate in failures per million cycles and the application’s allowable leakage threshold in in³/min.
Enter absolute upstream and downstream pressures in psi and absolute dynamic viscosity in lb·min/in² at the operating condition.
Enter the circular interface radii in inches, softer-material Meyer hardness (or resilient-material Young’s modulus) and contact stress in psi, and harder-surface finish in microinches.
Enter the evaluated duty in cycles, target survival probability in percent, and average cycles per operating hour. Select Calculate; Reset restores the worked example.
Read the adjusted rate first, then survival at the evaluated duty and the target cycle/time interval. The sensitivity chart shows the adjusted-rate change when each listed input is varied independently by −10% and +10%.
Correct any red warning before using a result. Do not use the projection for sliding/rotary seals, age-dependent elastomer wear-out, unqualified materials, unsupported conditions, or safety-critical release without applicable test data.

Equations used

For the NSWC circular-seal relation, the empirical conductance parameter is

H = 0.23(M/C)^1.5 f^2/3
λ_SE = λ_B + [K₁(P₁² − P₂²)H³/(Q_f μ_a P₁)]·[(r_o + r_i)/(r_o − r_i)]
K1 = 3.27 × 10⁻⁴
R(N) = exp(−λ_SEN/10⁶)
N_target = −10⁶ ln(R_target)/λ_SE
t_target = N_target/ṅ

Symbols and units. H is the conductance parameter in inches; M is Meyer hardness of the softer material, or Young’s modulus for resilient rubber, in psi; C is contact stress in psi; f is harder-surface finish converted from µin to inches. λ_SE, λ_B, and the leakage contribution are failures per million cycles. P₁ and P₂ are absolute upstream and downstream pressure in psi; Q_f is allowable leakage in in³/min; μ_a is absolute dynamic viscosity in lb·min/in²; r_i and r_o are inside and outside seal radii in inches. N and N_target are cycles; R and R_target are dimensionless survival probabilities; ṅ is cycles/h and t_target is hours.

Sign convention and assumptions. Use positive absolute pressures with P₁ > P₂, positive radii with r_o > r_i, and positive material/leakage quantities. The leakage term increases the base rate. The model assumes laminar leakage around a circular static interface and representative constant conditions. The survival and interval equations additionally assume a constant failure rate over cycle count.

Limits. This empirical screening model does not represent changing hardness, finish, compression set, contamination, thermal/chemical degradation, dynamic wear, or a Weibull wear-out process. The handbook notes that hardness and finish can change with time. Use component-specific qualification and life-test data when those effects matter.

References.NSWC-11, Handbook of Reliability Prediction Procedures for Mechanical Equipment, Chapter 3, equations 3-4 and 3-5; Engineers Edge seal operating-life equation background.