This paper handles the problem of how to select the node which runs the reliability operation in the WirelessHART network having a grid topology. The split-merge operation can be placed on any rectangular path to improve the transmission success ratio, dynamically adapting the route according to the current link condition, and thus overcoming the inflexibility of the fixed schedule. Based on the observation that almost every link is used in the sensor-to-controller route and the corresponding slot schedule, the proposed scheme gives precedence to the nodes having a high error level. The node error level can be calculated by averaging the error rate of all links emanating from the node. The performance evaluation result measured by extensive simulation using a discrete event scheduler demonstrates that with 75 % split-merge operations, we can achieve over 90 % success ratio when the grid dimension is 5, compared with full allocation.