A method for preventing damage to a downhole pump impeller of a downhole pump includes setting a rotatable part of a downhole pump to a selected depth, monitoring the depth of the rotatable part, determining that the depth of the rotatable part has significantly changed, and taking a corrective action to return the depth of the rotatable part to said selected depth in order to prevent damage to an impeller that is liable to be caused by a change in depth of the rotatable part. The downhole pump further has a line shaft, an impeller engaged with the line shaft for pressurizing fluid to be extracted from a well, and monitoring apparatus for monitoring the depth of a distal end of the line shaft in order to prevent damage to the impeller that is liable to be caused by a change in depth of the distal end.