Heat Tensioning of Pipeline
Heat tensioning of the pipeline is carried out in the course of installation. By preheating to the temperature that is between the minimum and maximum working temperature of the system, e.g.. Tmin = 10 °C, Tmax = 130 °C, then Tp = 60 °C. The actual heat tensioning is carried out on an uncovered pipeline, because the pipeline dilates freely in the course of preheating. At reaching Tp, check of dilatation conditions with calculated value is carried out. The values must be identical.
CA - dilatation arm
DB - dilatation arm at TP - without tension
EF - movement range of the pipeline at Tmax and Tmin
In preheating condition there is made lining of compensating elements with expansion cushions, eventually concrete encasement of the fixed points and the like. At Tp backfill of the pipeline is carried out. The preheating temperature must be maintained in the course of this operation. (In case of concrete encasement of the fixed points the preheating temperature must be maintained till hardening of concrete.)
If the expansion cushions are not used in the preheating state, no movement of heat-carrying pipe occurs. By means of this procedure there are used the steel pipeline properties to absorb tensile stresses (at cold one) and compressive stresses (at higher temperature than Tp) in allowable limits.
Tension in the pipeline is calculated by means of the formula:
σ = α . E . ΔT
||expansion ratio 1,21 . 10-5
||modulus of elasticity 2,1 . 105
THE ALLOWABLE VALUES OF THE STEEL PIPE AXIAL TENSION CANNOT BE EXCEEDED.
In case of utilisation of this method, e.g. water, electric current and the like can be used for preheating of the pipeline.
Analogically, also definite axial compensators may be used for compensation of dilatation movements, these bends are placed into the joint of two pipes in points corresponding to calculated values.
The basic difference in use of this method consists in fact that the routing is coupled after welding of the pipeline and pressure test, and backfilled except for joints, where the compensators are placed. At subsequent preheating of the pipeline to the temperature Tp the compensating element is locked with a welded joint and coupled. The subsequent working temperature changes show in the pipeline as change of axial tension (compressive stress or tensile one). This method allows faster procedure at actual realization of installation, when it is not necessary to leave the pipeline uncovered all the time as at conventional preheating.