What Safety Factor (2.0 vs 3.0) Is Appropriate When Calculating the Burst Pressure of a Titanium Heater for a Hydraulic Control System?

When a hydraulic engineer designs a titanium heater for a high-pressure hydraulic control system, such as those in aircraft actuators, subsea equipment, or heavy machinery, the calculation of the burst pressure must include a safety factor (SF) to account for variations in material, manufacturing defects, and service conditions. The burst pressure ( P burst ) of a thin-walled titanium tube is given by P burst = SF(2σ ult t/D), where σ ult is the ultimate tensile strength (Grade 2: 345 MPa), t is wall thickness and D is outer diameter. For non-critical systems that are inspected on a regular basis, choosing SF=2.0 gives you a burst pressure that is twice the maximum operating pressure. For safety-critical systems or systems with vibration, pressure spikes or corrosive conditions, the threefold buffer is necessary. Choosing SF=3.0 permits this. The suitable SF relies on the consequences of failure and the operating environment.

Choice of safety factors for titanium pressure components
The safety factor for burst pressure includes six factors not included in the simple hoop stress calculation: 1) variation in tube wall thickness (usually +/- 5-10% of the nominal), 2) ovality (out-of-roundness reduces burst pressure by 10-25%), 3) surface defects (scratches reduce local strength by 20-50%), 4) temperature derating (strength decreases 10-20% at 100°C vs. room temperature), 5) fatigue (cyclic pressure reduces life), and 6) corrosion allowance (wall thinning over time). With a safety factor of 2.0 (normal for non-critical hydraulic systems) you presume these variables are well regulated and that the system is routinely inspected. A safety factor of 3.0 (as is customary for pressure vessels per ASME Section VIII) assumes worst-case combinations of all factors and no access for inspection.

SF=2.0 versus SF=3.0 Burst Pressure Quantitation for Typical Tube Sizes
Tube OD (mm) | Wall Thickness (mm) | Max Operating Pressure (bar) at SF=2.0 | Max Operating Pressure (bar) at SF=3.0 |Recommended Application 12.7 1.0 108 72 Low pressure, non-critical 12.7 1.5 163 108 Moderate pressure
16.0 1.5 130 86 General hydraulic 16.0 2.0 173 115 Higher pressure 20.0 1.5 104 69 Medium pressure, bigger flow
20.0 2.0 138 92 General industrial 20.0 2.5 173 115 High-pressure hydraulic 25.4 2.0 109 73 Subsea moderate pressure
25.4 2.5 136 91 Deep subsea, SF=3.0 25.4 3.0 163 109 Subsea, SF=3.0 necessary
Guide to Choosing Safety Factor Based on Scenario
Application & Consequences of FailureTypical working pressure (bar)Recommended SF Reasoning
Aircraft hydraulic line heater (safety crucial) 210-350 3.0-4.0 Failure causes loss of control. Must have high SF.
Subsea control system (1500 m depth, 150 bar external) 200 (internal) 3.0 External pressure may cause collapse. 3.0 minimum API 17E SF=
Industrial hydraulic press (non-critical) 150-250 2.0 Periodic inspection. Failure leads to downtime, not safety hazard.
Agricultural equipment hydraulic heater 100-200 2.0 Moderate consequence. SF=2.0 criteria.
Mobile machinery (excavator, loader) 200-300 2.5 Vibration and shock loads need intermediate SF .
Test stand heater (intermittent, supervised) Up to 400 2.0 Supervised immediate shutdown if leak detected.
Seawater hydraulic system (corrosive) 100-150 3.0 Corrosion allowance needed. SF=3.0 was due to wall thinning.
High-cycle hydraulic system (>1 million cycles/year)Any 3.0+ fatigue analysis SF=3.0 not good for heavy cycles. Fatigue calculation of S-N curve required.
Engineering Factors Increasing Required Safety Factor
Increase the safety factor to 3.0 or greater where any of the following situations exist. Welded tube (recommended for hydraulic service) reduces burst pressure by 20-40% at the weld seam. Due to the reduction in titanium strength by 10-20% above 80C operating temperature, the SF should be adjusted based on temperature derating factor. Pressure cycling (greater than 10,000 cycles/year) necessitates fatigue analysis. SF=3.0 might not be enough. In corrosive environment (seawater, acids), the wall thins with time and the SF has to take into consideration the expected deterioration. Dynamic burst calculations must take account of additional stresses due to vibration and mechanical shock. For any combination of these parameters, a design review with full finite element analysis (FEA) and fatigue life prediction is recommended rather than relying on safety factor alone.

Conclusion: SF=3.0 for Safety-Critical, SF=2.0 for Hydraulic Service Non-Critical
A safety factor of 2.0 is ideal for non-critical applications where heater failure would merely lead to down time and the hydraulic control system is routinely monitored, e.g. the burst pressure of a titanium heater in a hydraulic control system. Safety factor of 3.0 necessary for safety-critical (aircraft, submarine, high-pressure) or corrosive conditions, vibration, or pressure cycling. There is a large difference in operating pressure between SF=2.0 and SF=3.0. For example, a titanium tube of 20 mm x 2.0 mm diameter can be operated at 138 bar with SF=2.0, but only at 92 bar with SF=3.0, a difference of 33%. When specifying a titanium heater for any hydraulic application, the maximum operating pressure, operating temperature, cyclic duty and the consequences of failure should be provided to the supplier to allow the correct safety factor to be selected and the correct wall thickness specified to meet the safety and performance requirements.