Thermal Stress on Vacuum Insulated Tubing (VIT)
The total stress on the Vacuum Insulated Tubing (VIT) joint is a function of the differential temperature between the inner and outer tubes. The Multilayered High Vacuum Insulation Tubing (VIT) is very efficient in controlling heat flux from the inner tube to the wellbore annulus & casing therefore using calculations and field experience a 650°F steam injection temperature (inner tube temperature) will yield an average outer tube temperature of approximately 100°F for a temperature differential of 550ºF. To obtain the thermal load on the joint the “temperature differential” is multiplied by the “coefficient of thermal expansion for steel” and the “Young Modulus of Elasticity for Metals” as follows:
(550ºF) (8.64 x 10-6 in/in-°F) (32,000,000 lb-ft/in²)
(550) (8.64) (10-6) (32,000,000)
= 152,064 psi
Total stress of 152,064 psi is therefore attempting to simultaneously compress the inner tube and stretch the outer tube. The outer and inner tubes being welded together at both ends this force of 152,064 psi stress is applied inversely proportional to the respective tubes areas.
Cross Sectional Area
|Cubic Inches||% of Total||
85,156 psi (comp)
66,908 psi (tension)
The thermal expansion (growth) of the VIT joint is from stress applied to the outer tube minus the prestress:
Net Outer Tube Stress = (66,908 psi tension – 23,620 psi prestress) = 43,288 psi
Thermal Expansion = (43.288) / (32,000,000) x 100-feet x 12 inch/foot = 1.63 inch per 100 feet
Proof of weld integrity:
It is convenient and worthwhile at this juncture to emphasizing an advantage of the OTSI ThermalTube 3H (VIT) compared to the Chinese Vacuum Insulated Tubing from Shengli Petroleum (Freet) relative to proof of weld integrity OTSI gets from the Bakeout process. The 700°F high temperature Bakeout Process and Controlled Getter activation reaches is a substantial event and upon removal of VIT joints from the oven inner and outer tube differential temperatures are approximately 550°F. See thermal stress values above. This thermal event (Bakeout) causes thermal stresses on welds connecting inner and outer tubes high enough to be considered a test of weld integrity. Thermal stresses are calculated follow:
- Weld area (based on fillet weld at face of the inner tube being about 3/8-inch high) and an approximate OD of 4.5” and ID of 3.915” is 3.86 in2.
- Force on Welds = Cross sectional area of the Inner Tube x Compressive Stress on the Inner Tube (psi).
- Weld Stress = Force on Welds x Cross Sectional Area
Force on Welds = (2.59 in2) (66,908 psi) = 173,292 pounds
Weld Stress = 173,292 pounds/3.86 in2 = 44,894 psi
The 173,292 pound force and 44,894 psi weld stress thermally induced into each weld with the subsequent cooling to atmospheric conditions is a near-destructive test of each weld. If any weld suffered from manufacturing or materials issue it should fail during this Bakeout or cool-down cycle with the resulting immediate loss of vacuum.
OTSI compared to Chinese VIT from Freet:
Because each VIT joint receives a K-Value test after Bakeout proving thermal performance, the test simultaneously gives proof of weld integrity. It is known the Shengli Petroleum (Freet) VIT product from China does not incorporate a Bakeout nor Getter activation (Getter activation being dependent on the first steam cycle), this difference represents a substantial differential in quality control and expected thermal performance between OTSI and Freet.