Plot head vs flow speed lines, the surge line, and antisurge control line; read surge margin at any operating point, per API 617 and ASME PTC 10.
Understand compressor performance maps, speed line characteristics, and surge limit curves
API 617 sets a 10% minimum surge margin at every specified operating point, defined as SM = (Qop − Qsurge) / Qop. For variable speed service, 20% is the practical design target to absorb gas composition swings, fouling, and control loop dynamics.
For centrifugal compressors at constant inlet density: flow scales linearly (Q ∝ N), polytropic head scales with the square (H ∝ N²), and shaft power scales with the cube (P ∝ N³). A 10% speed reduction therefore drops flow 10%, head 19%, and power 27% — the basis for VFD energy savings.
Per GPSA Engineering Data Book Section 13, surge typically initiates at 65–75% of design flow at any given speed. Exact location depends on impeller backsweep and diffuser geometry — backward leaning impellers surge later (~70%), radial impellers earlier (~75%).
The surge line is the physical aerodynamic boundary where flow reversal begins. The antisurge control line (SCL) is offset 10–15% to the right and is the setpoint at which the recycle valve opens. The SCL provides time for the controller to react before the operating point reaches actual surge.