Experimental validation of Steam Turbine control oil actuation systems transient behavior

A. Tradii, S. Rossin

GE Nuovo Pignone


A safety critical circuit, such as the overspeed trip protection dedicated to the steam valve fast closure, cannot be designed without a verification of the entire oil circuit including the complex valve dynamics.
Such systems are widely used in steam turbines design from GE Oil & Gas Nuovo Pignone both in generator drive and mechanical drive application when connected to centrifugal compressors. Steam turbines manufactured by GE Oil & Gas plant in Florence, cover a range in power from 2 MW to 100 MW, and they play a strategic role in the company's growth.
In the construction of rotating machinery, the hydraulic systems are of fundamental importance for a reliable and safe operation of the machines as well as to guarantee proper plant availability. To date, the hydraulic systems used for control valves and trip valves are partly designed according to the plant needs, and made out of standardized components which have been tested over time. With such approach, the first opportunity we have to link them together is during the machine installation at the customer's site. Up to now this is the only moment for the circuit to be optimized, in order to comply with the trip valve closure time, and meet the international regulations requirements. With such approach the oil circuit optimization will occur only through a number of subsequent approximation steps, acting on the hydraulic circuit parameters that influence the closure time of the valve, until getting the desired result. The more trip valves installed in the same steam turbine, the more complex this optimization process will be, introducing concerns in the machine reliability.
A complex multi physics parametric Flowmaster model has been built to simulate the trip valve dynamics as a function of the hydraulic circuit; this allows GE O&G to study the transient dynamic behavior during the steam turbine trip event as a result of sudden changes in pressure in the hydraulic circuit. Computational results have been compared with experimental tests carried out on the valve in several different configurations to identify the valve dynamic domain space. The trip valve has a very complex design, composed by several parts independently moving within the valve body as result from oil pressure change. A thorough data acquisition allowed to firmly validating the Flowmaster results, both in terms of valve displacement and pressure decay evolution through time, as result of back pressures generated by the piston motion.