Process for controlling the flow of a fluid in a valve and apparatus for carrying out said process

Claim 1

A process for controlling the flow at a sonic and subsonic rate of a fluid through a valve of given section having a movable valve member, said process comprising initially calculating a plurality of correction coefficients in dependence on corresponding selected values of a ratio of the pressure of the fluid upstream of the valve P upstream and downstream of the valve P downstream , each of the coefficients being indicative of a flow Q real of the fluid for the respective ratio, the selected values of the ratio corresponding to respective subsonic rates of the flow Q real , and initially storing the plurality of correction coefficients in a memory having a pluralaity of memory locations having respective addresses, respective ones of said correction coefficients being stored in respective ones of said memory locations, measuring the upstream pressure P upstream and the downstream pressure P downstream and the position d of the valve member, calculating the flow Q real of the fluid and comparing it with a set value Q O , the difference signal Q real -Q O commanding the displacement of the valve member in the desired direction, the flow Q real being calculated by first of all effecting the product Q sonic =k×P upstream ×d representing the real flow of the fluid when the flow rate of the fluid is sonic, k representing a constant which is a function of the temperature of the fluid and of the section of the valve, then comparing the upstream pressure P upstream with the downstream pressure P downstream so as to determine whether the flow of the fluid is at a sonic or subsonic rate, the flow Q sonic representing the real flow Q real when the flow rate is sonic, accessing the memory when the flow rate is subsonic to read out therefrom the correction coefficient corresponding to a ratio of the measured upstream pressure P upstream and the measured downstream pressure P downstream , and multiplying the value Q sonic by the read-out correction coefficient only when the flow rate is subsonic so as to obtain the value Q real of the real flow, and controlling said valve in response to the value Q real of the real flow for both sonic and subsonic flow.