A performance map is the chart that describes what a centrifugal compressor can do: how much head or pressure ratio it produces at a given flow and speed. Reading it is essential to running the machine safely, because the map's edges mark the surge and choke boundaries the operating point must stay between. This guide explains how to read a centrifugal compressor map, how the operating point moves, and how SCADA plots the live point against it.
Performance Map in one line: A centrifugal compressor performance map plots the head (or pressure ratio) the machine develops against its inlet volumetric flow, drawn as a family of curves for different running speeds. Each speed line shows how head falls as flow rises at that speed. The usable region is bounded on the left by the surge line, where flow is too low and the compressor becomes unstable, and on the right by the stonewall or choke line, where flow can rise no further. The operating point is where the current speed line meets the system resistance.
The vertical axis of a centrifugal compressor map is head - the energy the machine adds per unit mass of gas, often shown instead as pressure ratio - and the horizontal axis is inlet volumetric flow. For a single running speed, the machine does not produce one fixed head; it produces a curve. At low flow it develops high head, and as flow increases the head it can produce falls, tracing a line that slopes down to the right. That single line is the compressor's behaviour at that one speed, and it captures the fundamental trade-off of a centrifugal machine: you can have more head or more flow, but not the maximum of both at once.
A real map is a family of these lines, one for each speed the machine can run at. Higher speeds sit higher and further right on the chart - more speed produces more head and shifts the whole curve up and out - while lower speeds nest below. Threaded across the speed lines are efficiency islands, contours of constant efficiency that show where the machine runs most economically, with the best-efficiency region in the middle of the map. Reading the map means picking the speed line for the current speed, finding where the required flow lands on it, and reading off the head the machine will produce there - and noting how close that point sits to the efficiency sweet spot.
The map's two vertical boundaries are the operating limits, and they matter far more than any point in the middle. On the left is the surge line, connecting the low-flow end of each speed curve. If flow drops too low for the current speed, the compressor can no longer maintain stable forward flow against the discharge pressure, and it surges - flow briefly reverses, then re-establishes, then reverses again in a violent, damaging oscillation that can wreck bearings, seals, and the rotor. Surge is the most dangerous thing a centrifugal compressor can do, so operation must stay to the right of the surge line with margin, which is exactly what an anti-surge control system enforces by recycling or blowing off gas to keep flow up.
On the right is the stonewall, or choke, line, marking the high-flow end of each speed curve. As flow rises, gas velocity through the impeller and diffuser passages climbs until it reaches sonic conditions somewhere in the flow path; at that point the machine chokes and simply cannot pass any more flow no matter how much the downstream pressure is dropped - the speed curve plunges nearly vertically. Choke is not as immediately destructive as surge, but it caps output and runs the machine at poor efficiency and high stress. The usable operating window is the region between the surge line on the left and the stonewall on the right, and staying inside it across all speeds is the core of operating a centrifugal compressor safely.
Where the machine is actually running is the operating point - the intersection of the current speed line and the resistance of the system it is pushing against. Two things move it. Changing speed moves the machine to a different speed line, raising or lowering both head and flow, which is the normal way of controlling a variable-speed centrifugal compressor. Changing the gas also moves the point: molecular weight matters because a lighter gas produces less pressure rise for the same head, so a swing in gas composition shifts where the point sits and effectively reshapes the map in pressure terms, which is why maps are referenced to a design gas.
A cloud SCADA such as Merobix makes this live. By taking in inlet flow, suction and discharge pressure, temperature, and speed, it can compute the current head and flow and plot the live operating point on the machine's performance map, moving in real time as conditions change. That gives the control room an immediate, visual sense of where the machine is relative to its surge line and stonewall - how much surge margin is left, whether it is drifting toward choke, and whether it is sitting near best efficiency or off in a corner. Trending the point over time reveals slow shifts from fouling or changing gas, and alarming as it approaches the surge line backs up the anti-surge controller. For a remotely operated compressor, seeing the live point against the map turns a set of raw numbers into an at-a-glance answer to the only question that really matters: is the machine running safely and efficiently right now.
It plots the head (or pressure ratio) the compressor produces against inlet volumetric flow, as a family of curves for different running speeds. Each speed line shows head falling as flow rises, and efficiency contours show where the machine runs most economically. The usable region is bounded by the surge line on the left and the stonewall or choke line on the right.
The surge line is the left boundary, marking the lowest flow at which the compressor stays stable at each speed; below it the machine surges, with damaging flow reversals. The stonewall or choke line is the right boundary, where flow reaches sonic conditions and cannot increase further. Safe operation stays in the window between them, with margin from the surge line in particular.
A performance map is referenced to head, which is independent of gas, but the pressure rise the machine delivers for a given head depends on molecular weight. A lighter gas produces less pressure rise for the same head, so a change in gas composition shifts where the operating point sits in pressure terms and effectively reshapes the map. That is why maps specify a design gas and why SCADA computes head from live conditions.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.