Low Sulfur Fuels.

Low sulfur fuels have a low viscosity and reduced lubricating properties. Screw pumps need some lubricity of the pumped medium to reduce friction, especially at high pressures. Too low lubrication threatens a failure of the fuel pump and thus repair costs and downtime.

What is mixed friction?
Technically, situations might occur in which it comes to solid friction (friction of metal against metal) instead of liquid friction (friction of metal against the liquid). Usually this transition proceeds smoothly and an intermediate state is established. Technically spoken: It comes to mixed friction.

Abrasive and adhesive wear.
The mixed friction conditions within the system between screw and pumped liquid as well as enclosing pump housing lead after a short period of time to predominantly abrasive and in the worst case to adhesive wear. This manifests itself mainly in the wear forms scratches, grooves and scoring and in the formation of crests up to material transfer.

When the adhesive wear has reached a certain extent, the pump drive can no longer move the screw against the surface of the pump housing. The pump is blocking.

Abrasive wear, however, leads to material removal mainly in the screw bores. This increases the internal backflow and the pump can no longer maintain the required system pressure. The pump is running, but flow rate significantly decreases.

Increasing the surface hardness.
To prevent this, the surface hardness of the pump components must be increased. Due to the high process temperatures, certain hardening processes can lead to distortion on the components, which is impossible to correct and render the components unusable.

A binding coating is applied to the metal screws used in KRAL pumps. Through this coating the screws receive a strongly hardened surface with much less friction.