【正文】 given pipeline geometry) describing the pressure at either ends of the pipeline, and the friction losses, respectively(Ref 2: Kurz, R., M. Lubomirsky, 2021). Among other issues, this means that for a pressor station within a pipeline system, the head for a required flow is prescribed by the pipeline system (Figure 2). In particular, this characteristic requires the capability for the pressors to allow a reduction in head with reduced flow, and vice versa, in a prescribed fashion. The pipeline will therefore not require a change in flow at constant head (or pressure ratio). Figure 2: Stafion HeadFlow relationship based on Eq. 4. In transient situations (for example during line packing), the operating conditions follow initially a constant power distribution, . the head flow relationship follows: cons tHP ss ??? ? ?m （ ） Qcons tH ss 1??? ?? and will asymptotically approach the steady state relationship (Ref 3: Ohanian, S., , 2021). Based on the requirements above, the pressor output must be controlled to match the system demand. This system demand is characterized by a strong relationship between system flow and system head or pressure the large variations in operating conditions experienced by pipeline pressors, an important question is how to adjust the pressor to the varying conditions, and, in particular, how does this influence the efficiency. Centrinagal pressors tend to have rather flat head vs. flow characteristic. This means that changes in pressure ratio have a significant effect on the actual flow through the machine (Ref 4:Kurz, R., 2021). For a centrifugal pressor operating at a constant speed, the head or pressure ratio is reduced with increasing flow. Controlling the flow through the pressor can be acplished by varying the operating speed of the pressor This is the preferred method of controlling centrifugal pressors. Two shaft gas turbines and variable speed electric motors allow for speed variations over a wide range (usually from 4050% to 100% of maximum speed or more).It should be noted, that the controlled value is usually not speed, but the speed is indirectly the result of balancing the power generated by the power turbine (which is controlled by the fuel flow into the gas turbine) and the absorbed power of the pressor. Virtually any centrifugal pressor installed in the past 15 years in pipeline service is driven by a variable speed driver, usually a twoshaft gas turbine. Older installations and installations in other than pipeline service sometimes use singleshaft gas turbines (which allow a speed variation from about 90100% speed) and constant speed electric motors. In these installations, suction throttling or variable inlet guide vanes are used to Drovide means of control. Figure 3: Typical pipeline operating points plotted into a typical centrifugal pressor performance map. The operating envelope of a centrifugal pressor is limited by the maximum allowable speed, the minimum flow (surge flow),and the maximum flow (choke or stonewall)(Figure 3). Another limiting factor may be the available driver power. Only the minimum flow requires special attention, because it is defined by an aerodynamic stability limit of the pressor Crossing this limit to lower flows will cause a flow reversal in the pressor, which can damage the pressor. Modem control systems prevent this situation by automatically opening a recycle valve. For this reason, virtually all modern pressor installations use a recycle line with control valve that allows the increase of the flow through the pressor if it es near the stability limit. The control systems constantly monitor the operating point of the pressor in relation to its surge line,and automatically open or close the recycle valve if necessary. For most applications, the operating mode with an open, or partially open recycle valve is only used for startup and shutdown, or for brief periods during upset operating conditions. Assuming the pipeline characteristic derived in Eq. 4, the pressor impellers will be selected to operate at or near its best efficiency for the entire range of head and flow conditions imposed by the pipeline. This is possible with a speed (N) controlled pressor, because the best efficiency points of a pressor are connected b