The phenomenon of different steam enthalpy increment of pipes in parallel is called thermal deviation, these pipes are called deviation pipes.

The causes of thermal deviation are two aspects as follows. 

Uneven lateral thermal (heat) : the parallel pipes of superheater pipes bundle are distributed evenly along the width of hearth, and the flame center of the hearth radiates heat to the surrounding and transfer heat to the water-cooled wall, therefore the temperature of flue gas near the boiler wall is lower than the flame center. After flue gas leaves from the hearth and enters to the convective flue, it still remains the characteristics with uneven temperature above mentioned. Therefore, the pipes in the middle of the flue are heated stronger, and the pipes at two sides of the flue are heated weaker, forming uneven heating. 

When the combustion inside the boiler is not well organized, the flame center deflects, the burner load is inconsistent, part of water-cooled wall of the hearth slags, and pulverized coal re-burns at local part of horizontal flue of the hearth, which will cause the uneven temperature of flue gas inside the boiler and the uneven temperature will continue in the convective flue in different degree, thus causing unevenly heating of the superheater. 

Uneven hydraulic(flow) at steam side:  

When the steam flow in the pipes in parallel, the heat absorption of steam/kg in the pipes with large flow is small, namely the enthalpy decrement is small, then the steam temperature inside the pipe and the pipe wall temperature is lower. In the pipe with small flow , the heat absorption of steam/kg is big, namely the enthalpy small, per kg of steam heat absorption capacity is big, the enthalpy increment is big, then the steam temperature inside the pipe and the pipe wall temperature is bigger. So, the uneven steam flow will also generate thermal deviation.