A feed pump doesn't run where its data sheet says. It runs where its curve crosses your system's curve — and if you never draw that intersection, you are guessing whether your cyclones get their design head. Total dynamic head is how you find the point the pump actually lives at.
What total dynamic head really is
Total dynamic head (TDH) is the total energy a pump must add to move fluid through your piping at your flow rate. It is the sum of three things: the static head (the elevation difference the fluid has to be lifted), the friction head (everything lost to pipe and fitting resistance), and the velocity head (the kinetic energy of the moving fluid), plus any pressure the destination requires. For a solids-control feed pump the destination pressure is the cyclone's feed-head requirement — the residual head the cones need at the manifold to make their cut.
Two of those terms behave very differently. Static head is fixed — it exists at zero flow and never changes. Friction head grows roughly with the square of flow rate. Velocity head is usually small and often cancels when the suction and discharge lines are the same size. Getting TDH right is mostly about getting the friction and the required cyclone head right at the flow you actually run.
The system curve and where the pump lives
Plot TDH against flow and you get the system head curve: it starts at the static head at zero flow and sweeps upward as friction climbs with the square of flow — the classic H = Hstatic + kQ² shape. On the same axes, the pump has its own head-capacity curve that falls as flow rises. Where the two cross is the operating point — the condition of service, the flow and head the pump will actually deliver in your system.
This is the tool that answers the real questions. Add a cone bank and the system curve steepens, the operating point slides left, and each cone may drop below its design head — which is why an under-pumped desilter ropes. Foul a line or pinch a valve and friction rises, the curve lifts, and flow falls. You want the operating point to sit at your design flow and head, and ideally near the pump's best efficiency point (BEP), where it runs smoothest and cheapest.
Reading it in mud units
One conversion keeps you honest on a rig: head in feet and pressure in psi are not the same number, and the bridge is density. Head (ft) equals pressure (psi) times 2.31 divided by specific gravity. The same manifold pressure means less head as the mud gets heavier, so a feed pump that held its cyclones in spray on light mud can fall short of head as you weight up — even though the gauge reads the same psi. TDH is in feet; convert the cyclone's psi requirement to feet at the current mud weight before you compare it to the pump curve.
The equation
Total dynamic head:
TDH = Hstatic + Hfriction + Hvelocity + Hrequired
System curve (friction grows with the square of flow):
Hsystem = Hstatic + k·Q² · operating point = pump curve ∩ system curve
Convert psi to feet at the mud weight: Head (ft) = psi × 2.31 ÷ SG.
Draw the intersection, don’t trust the data sheet. The pump runs where its curve meets your system curve, not at its rated point. If cyclones lose spray after you add cones or weight up, the operating point has moved — recompute TDH at the real flow and mud weight before you blame the cones.
Common questions
What is total dynamic head (TDH)?
It is the total head a pump must add to move fluid through your system at your flow rate: static head plus friction head plus velocity head, plus any pressure the destination requires. For a feed pump the destination requirement is the cyclone's feed head.
What is the pump operating point?
It is where the pump's head-capacity curve crosses the system head curve — the actual flow and head the pump delivers in your system, also called the condition of service. Aim for it to sit at your design flow near the pump's best efficiency point.
Why do cyclones lose head when I weight up?
Because head in feet equals psi times 2.31 divided by specific gravity, so the same manifold pressure is less head in heavier mud. As you weight up, the cyclone's head requirement in psi rises to hold the same feet of head — check TDH at the current mud weight.
