A slurry pump that used to hit its duty point and now doesn’t is one of the most common complaints on site. Flow drops, discharge pressure drops, or both, and the pump is still running. Before anyone reaches for a spanner, the fault needs isolating to one of a handful of likely causes.
This guide works through those causes in order of likelihood, starting with the checks that don’t require opening the pump.
Before you start: isolate and lock out the pump according to site procedure before any physical inspection. Don’t open a pressurised slurry pump casing without confirming it’s been drained and de-energised.
The symptom: flow or head has dropped, but the pump still runs
This guide covers a pump that starts, runs, and doesn’t trip, but no longer delivers the flow rate or discharge head it used to. It doesn’t cover a pump that won’t start, trips on overload, or has failed completely. Those point to different causes and need separate diagnosis.
Most likely causes, in order
| Likely cause | What to check | What it tells you |
|---|---|---|
| Wear ring or impeller clearance has opened up | Measure clearance against the manufacturer’s original spec. Compare to the last recorded measurement if one exists. | Internal recirculation increases as clearance grows, cutting head at the same speed. This is the most common cause on a centrifugal slurry pump with meaningful running hours. |
| Impeller or liner wear from abrasion | Inspect impeller vanes and casing liner for material loss, especially at the cutwater and vane tips. | Worn vane profiles can’t generate the same head even with clearances intact. Common in high-silica or high-hardness slurries. |
| Partial blockage in suction line or impeller | Check suction strainer, foot valve, and impeller eye for packed solids or debris. | A partial blockage restricts flow before it reaches the impeller. Often intermittent, so check while running if it’s safe to do so. |
| Suction starvation (NPSH shortfall) | Check sump level, suction lift, and NPSH available (NPSHa) against the pump’s required NPSH (NPSHr). | If NPSHa has dropped below NPSHr, the pump cavitates and loses head. Common after a sump level change or a longer suction line. |
| Slurry density or concentration has changed | Check actual specific gravity and solids concentration against the design duty point. | A pump sized for one slurry density performs differently on a heavier or more concentrated mix. This isn’t a fault, but it looks like one. |
| System resistance has increased | Check for a partially closed valve, scaled or narrowed discharge pipe, or increased static lift. | A pump curve intersects a system curve at one point. If the system curve has shifted, the duty point moves even though the pump is fine. |
| Speed reduction (VFD or drive issue) | Confirm actual pump speed against the design speed, especially on a VFD or belt-driven unit. | Centrifugal pump output falls sharply with speed. A small speed drop produces a larger drop in head. |
Work through the list in order
Start with wear ring clearance and impeller condition. On a slurry pump doing real duty hours, wear is the default explanation until something else stands out. Checking clearance is quick and doesn’t require dismantling beyond removing an inspection cover in most designs.
If clearances and wear are within spec, move to suction-side checks. A partial blockage or NPSH shortfall both starve the impeller of the flow it needs, and both can look identical to wear from the discharge gauge alone.
Only after ruling out the pump itself should you look at the system: resistance changes, density changes, and speed. These aren’t pump faults, but they’re mistaken for one often enough to justify checking before opening the casing.
Common misdiagnosis
Teams often replace an impeller or reline a casing before confirming the slurry’s actual specific gravity and solids concentration. If the slurry has changed since the pump was selected, no amount of wear ring adjustment will restore the original duty point. Confirm the process conditions first.
When to stop and escalate
Stop and consult the manufacturer or a qualified pump engineer if:
- Wear has progressed enough that structural integrity is in question
- The cause can’t be isolated after working through this list
- Correcting the identified cause would mean operating outside the pump’s original design conditions
- The application involves a hazardous or high-consequence fluid
