When a desilter loses its spray and the centrifuge feed gets erratic, the reflex is to blame the cones or the bowl. Often the real culprit is upstream: a feed pump quietly cavitating, delivering less head and less flow than the gauge suggests. NPSH is the number that tells you whether your pump is being starved — before it eats its own impeller.
What cavitation actually does
A centrifugal pump lowers the pressure at its suction eye to draw fluid in. If that pressure falls to the fluid's vapour pressure, the liquid flashes to vapour, forming bubbles that collapse violently as they reach the higher-pressure impeller. The result is lost head and flow, vibration, a gravelly noise, pitted impellers, and a cyclone manifold that never gets the feed head it needs. On a rig, cavitation shows up as cones dropping out of spray and a centrifuge feed that surges — problems that no amount of cone or bowl adjustment will fix.
NPSH available vs required
Every pump has a required NPSH (NPSHr) from its curve — the suction head it needs to avoid cavitation at a given flow. Against that you compute the available NPSH (NPSHa) — what your piping and fluid actually deliver at the suction. The rule is simple and unforgiving: keep NPSHa comfortably above NPSHr, with margin. When NPSHa falls to NPSHr, the pump cavitates.
NPSHa is built from atmospheric pressure pushing on the tank, minus the fluid's vapour pressure, plus or minus the static height of fluid above the pump, minus the friction losses in the suction line. Everything that lowers the first two or raises the last erodes your margin.
The traps that catch people out
Three field conditions quietly destroy NPSH margin. Gas-cut mud raises the effective vapour pressure — entrained gas flashes easily, so a pump that was fine on clean mud cavitates on gas-cut returns (which is also why a degasser upstream protects your feed pumps). Hot mud raises vapour pressure directly. And a starved or restricted suction — a long line, a partly-closed valve, a plugged suction screen, or a tank level that dropped — raises friction loss and lowers static head. Altitude lowers atmospheric pressure and hurts too. Any of these can pull NPSHa below NPSHr without a single thing changing at the cone or bowl.
The equation
Net positive suction head available, in feet of head (field units):
NPSHa = (Patm − Pvap) × 2.31 ÷ SG + hs − hf
Patm, Pvap = atmospheric and fluid vapour pressure (psi); SG = fluid specific gravity; hs = static suction head (+ if the tank level is above the pump); hf = suction friction loss (ft). Cavitation occurs when:
NPSHa ≤ NPSHr (NPSHr from the pump curve)
Diagnose cyclone and centrifuge feed problems at the pump before you touch the equipment. A gravelly pump, dropping cone spray and a surging centrifuge feed together are the signature of cavitation. Protect NPSH margin: degas the mud, keep suction lines short and clear, and don’t run the feed tank low.
Common questions
What is NPSH in simple terms?
It is the suction-side pressure margin a pump has above the point where the fluid would boil at the impeller eye. NPSH available is what your system delivers; NPSH required is what the pump needs. Keep available above required, with margin, or the pump cavitates.
Why does gas-cut mud cause pump cavitation?
Entrained gas raises the fluid's effective vapour pressure and compressibility, so it flashes at the suction more easily. That lowers NPSH available and can push a previously healthy pump into cavitation — which is one reason a degasser upstream protects your feed pumps.
How do I stop a feed pump cavitating?
Restore NPSH margin: degas the mud, shorten and clear the suction line, open restricted valves, keep the feed tank level up, and avoid running unnecessarily hot. If the pump is simply undersized for the duty, its NPSH-required may be too high for your suction.
