Is there a way to estimate the performance curve ( GPM vs. Head) of a pump used as hydraulic Power Recovery Turbine under no load condition starting from normal operating GPM vs. Head Curve?
About four years ago a paper was presented at the Pump Users Symposium in Houston that you may want to read. It was based on the use of pump as hydraulic turbine to drive generators. It offers a method to estimate turbine performance based on the characteristics of the pump. I am away from my office and unable to provide a detailed reference.
The paper was titled "Numerical Simulation on a Pump Operating in a Turbine Mode". It was presented in 2007. The authors were Sonia Rawal and J. T. Kshirsagar. I looked at the paper and I believe it would address your question. If you do not have access to the proceedings of this symposium, you can purchase copies from Texas A&M Turbomachinery Laboratory.