Method of monitoring a pumping limit of a multistage turbocompressor with intermediate cooling

A method of operating a multistage turbocompressor system in which at an intake temperature T.sub.s and pressure p.sub.1 a gas is drawn into a first turbocompressor, gas compressed in the first turbocompressor is subject to intermediate cooling, cooled gas is compressed in subsequent turbocompressors in cascade with subsequent cooling to temperatures T.sub.ri where i is a number which represents the number of the cooling stage in succession at which said cooling occurs, and the gas after a last turbocompressor and cooling stage of the cascade has a discharge pressure p.sub.2 for a volume rate of flow F through the cascade, said method comprising the steps of:

• (a) driving said cascade and measuring said intake temperature T.sub.s, each temperature T.sub.ri, temperature differences dTs and dT.sub.ri of the temperatures T.sub.s and T.sub.ri and a predetermined reference temperatures Tref, the pressures p.sub.1 and p.sub.2 and the volume rate of flow F;
• (b) describing a pump limit function Y=m.F+b relating said pressures p.sub.1 and p.sub.2 and said rate of flow F in a stored compressor performance graph, where Y is selected from one of a pressure difference and a ratio of the discharge pressure p.sub.2 and the intake pressure p.sub.1, and m and b are coefficients which are linear functions of the intake temperature T.sub.s and temperatures T.sub.ri and the temperature differences dTs, dT.sub.ri where:
• m=m.sub.0 +m.sub.1.dT.sub.s +.SIGMA.m.sub.2i.dT.sub.ri and
• b=b.sub.0 +b.sub.1.dT.sub.s +.SIGMA.b.sub.2i.dT.sub.ri
• where
• m.sub.0 and, b.sub.0 are empirically determined constants and temperature coefficients m.sub.1, b1, m2i, b2.sub.i are determined by thermodynamic calculations with variations of the intake and cooling temperatures and are stored in a control unit;
• (c) operating the turbocompressor cascade at its pumping limit for the purpose of calibrating the pump limit function to yield pressure, temperature and volume values from which the coefficients m.sub.0, b.sub.0 for the pump limit function are determined and providing the coefficients m.sub.0, b.sub.0 as inputs to the control unit;
• d storing said pump limit function;
• e comparing measured values of the volume rate of flow F and the pressures p.sub.1 and p.sub.2 of step (a) during driving of said cascade with corresponding values in said compressor storage graph;
• and
• f generating a control signal upon operation of said cascade with said measured values falling below a predetermined minimum value of a distance from said stored pump limit function.