Pressure Compensation for a Backup Well Pump

Primary and secondary pump assemblies are mounted to and supported by a supporting device in a well. The supporting device has a valve that has a first position allowing flow from the primary pump assembly while the secondary pump assembly is in a storage mode, and a second position allowing flow from the secondary pump assembly. A barrier in the intake of the secondary pump assembly blocks entry of well fluid into the secondary pump assembly while the valve is in the first position. The secondary pump is filled with a buffer fluid. A pressure compensator mounted to the secondary pump assembly has a movable element that moves in response to a difference between well fluid pressure on an exterior of the secondary pump assembly and the pressure of the buffer fluid contained in the secondary pump to reduce a pressure differential.