Thermal Growth
What is the difference between OLTR and thermal growth?
OLTR (Offline-to-Running Temperature) is the ΔT used in the growth formula; thermal growth is the resulting dimensional change in inches or millimeters.
How do you measure thermal growth in the field?
Perform a cold check and a hot check of shaft alignment; the difference in centerline position approximates the growth, though dynamic loads may alter results.
What is the C-value for stainless steel shafts?
0.0000074 in./in./°F; for a 10-inch stainless shaft heating from 70°F to 130°F, growth = 60 × 10 × 0.0000074 = 0.00444 inches.
Why is API 686 relevant to thermal growth?
API 686 mandates applying thermal offsets as Step 4 in its shaft alignment process, ensuring reliability by aligning for hot conditions.
Can thermal growth cause horizontal misalignment?
Yes; expansion occurs in vertical, axial, and horizontal directions, altering both vertical and horizontal alignment between connected shafts.
What happens if thermal growth is ignored in a high-temperature pump?
The pump casing, shaft, and supports expand unevenly, causing misalignment, bent shafts, seal failures, and premature bearing wear.
Is thermal growth predictable?
Yes; it can be calculated using OEM data or measured via temperature logging and alignment checks, then used to set intentional cold offsets.
What is the most critical thermal growth scenario?
Differential growth, where connected machines (e.g., pump and motor) expand differently due to material or temperature disparities.
Thermal growth is the physical expansion of rotating machine components (shafts, casings, bases, supports) caused by temperature increases from cold to operating conditions, resulting in shaft centerline movement that induces misalignment if uncompensated. It is calculated as the product of the material's coefficient of linear thermal expansion, the temperature differential, and the linear dimension from the machine base to the shaft centerline.
In shop floor maintenance, thermal growth is critical for aligning high-temperature pumps, turbines, and motors handling steam or hot oil. Maintenance personnel perform 'hot alignment' by intentionally offsetting machines during cold alignment so they achieve precise alignment at operating temperature. This compensates for shaft centerline movement due to expansion, preventing misalignment, excessive vibration, and bearing failures. Proper thermal growth compensation ensures reliability and longevity of rotating equipment in industrial settings.
Cold-only alignment: Aligning machines only at standstill without compensating for thermal offsets, leading to hot misalignment, excessive vibration, and bearing failures.
Ignoring differential growth: Failing to account when the motor and pump expand at different rates due to different materials or temperatures, causing differential misalignment and shaft bending.
Soft foot or base issues: Performing alignment without first eliminating soft foot or verifying foundation integrity, causing false alignment readings that worsen under thermal growth.