Metodics of heating elements calculation
The heating elements (ceramic pads) for circumferental welded pipe and tube joint are calculated in the following order
For example, let`s calculate the heating elements for stress relieving PWHT of round circumferental welded joint on a pipeline with a diameter of 219 mm and a wall thickness of 20 mm made of steel 15X5M (Russian mark of steel). ASME analogue is ASTM A-335.
At the first stage, it is necessary to define a uniform heating zone (uniform heated band). To calculate that it is required to multiply 3 times of a thickness of the pipe to every side from the weldede joint axis. In our example, the uniform heating zone must be not less than 20x3=60mm to every side of the welding joint.
The temperature of the heat treatment of the 15X5M (Russian mark) steel ASME analogue ASTM A-335 according to some Russian standards is 720 – 750°С. With one-sided heating of pipelines to temperatures above 700 ° C and when thermal insulation can not be installed from the inside, active heat loss into the pipeline due to thermal radiation starts at high temperatures. In this case, it is necessary that zone A, covered by heating elements, must be at least twice times wider than the zone of uniform heating. So the zone covered by the heating elements must be at least 60 * 2 = 120 mm in each side from the welding joint.
In the next step, it is necessary to define length of a heating belt by circle of a pipe.
In our example, it is equal to (219 + 10) * 3.14 = 719 mm, where 10 mm is the thickness of the ceramic bead insulator (a constant value) of which the heating elements are made. When the width of the zone covered by the heating elements is large enough, it is recommended to split it into several heating zones, which will be located symmetrically both sides from the welded joint. In our case, the zone is divided into two similar zones. Heating elements with a width of at least 120 mm and a length allowing to accommodate an integer number of heaters in the belt without overlapping are selected from the table of catalogue sizes, taking into account the operating voltage used to power the heating elements. The gap between the heating elements must not be bigger than 25 mm. So the best way in our example when using het treatment units with 60 V output voltage is choosing heating elements with a size of 355x150 mm (order code 161407). In this case, in each heating belts will contain 2 heating elements, the gap between the heaters will be (719-355 * 2) /2=4.5 mm.
This variant of the selection will be optimal when the heaters are connected to heat treatment units of 75 and 150 kVA which allows to connect up to 4 heating elements on each channel. It makes possible to perform heat treatment of 6 or 12 welded joints of the mentioned size at the same time. Each channel of the heat treatment unit is controlled by one thermocouple so by setting up a thermocouple at the top of the welded joint, the requirements of OST 36-50-86 fulfilled.
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