Troubleshooting Pump Clearance Loss on Warman AH Slurry Pumps

The fault looks like this: same rpm, same impeller diameter, and the pump delivers less flow and less head than its curve, while shaft power stays flat or climbs. Efficiency has dropped because part of the impeller's work is going into liquid that leaks from discharge back to suction. On a Warman AH-type end-suction slurry pump, the cause is almost always worn recirculation control, but not every clearance matters equally. This page gives the order of checks, what each reading means, and how to prove the fix.

Know why an open gap drops the H-Q curve

Liquid leaves the impeller at high pressure and flows toward the lowest pressure it can reach, which is the impeller suction. With no restriction between impeller discharge and suction, all flow would recirculate. The pump designer creates the restriction with small running clearances between rotating and stationary parts, kept just large enough to avoid contact.

• Open impeller (no suction-side shroud): the impeller-to-casing (suction liner) clearance is the restriction.
• Enclosed impeller (shrouded both sides of the vanes): the clearance between the impeller wear ring and the suction casing wear ring is the restriction.

Even an as-new pump recirculates some flow. Any clearance beyond design magnifies it. Wear of this recirculation control is the main reason centrifugal pumps fall off their curve. Carried to the extreme, wear produces a recessed-impeller behavior in which the impeller acts as a forced vortex. Those pumps deliver much lower flow and head than an equivalent pump with intact recirculation control.

Your presumption about power is correct in direction: the impeller still handles the recirculated flow, so power per unit of delivered flow rises. Measure it; do not infer it.

Check 1: Rule out speed, trim, rotation and suction conditions

Do this first. It takes minutes and eliminates the obvious suspects before you open the pump.

1. Read actual rpm at the pump shaft (tachometer or drive feedback), not the setpoint.
2. Confirm the installed impeller diameter against the datasheet, and confirm rotation direction.
3. Check suction conditions: blocked or partially plugged suction line, air ingestion, low suction level. These shift the curve without any clearance change.
4. Read discharge pressure at zero flow (dead head) if the process allows it, and compare it with the OEM curve.

Outcome: if speed, diameter, rotation and suction are correct and dead head or flow is still low, go to Check 2. If any of them is off, fix it and re-test before opening the pump.

Check 2: Find which gap is open

Measure the clearances with the pump opened per the OEM manual. Read the design values from the pump's maintenance manual or datasheet; do not use another pump type's numbers. There are two very different gaps, and they do not cost the same performance.

Gap	Effect on H-Q performance	Other consequence
Suction side: impeller to suction liner (open) or impeller wear ring to suction wear ring (enclosed)	Significant loss. Recirculation runs from impeller discharge back to suction.	Head and flow fall, power per unit flow rises.
Rear side: impeller to rear cover plate / seal chamber side	No significant change. The discharge side of the impeller sees discharge pressure, so there is no recirculation path and no performance loss.	If no wear rings are fitted on the seal-chamber side, larger clearance raises seal-chamber pressure and the pressure differential across the seal.

Branch A, suction-side gap is over design: this is your performance loss. Go to Check 4 to size it, then to the restoration procedure.

Branch B, only the rear gap is over design: do not expect this to explain a curve drop. Do not spend time chasing it as the performance cause. Instead check seal-chamber pressure and seal condition, because the seal takes the penalty. Then go to Check 3.

A difference of opinion exists on how critical the suction-side gap is on some pump types; clearance sensitivity varies with pump hydraulics. Do not settle it by argument. Measure your pump against its own curve.

Check 3: Measure impeller axial position against the cutwater

Shimming or adjusting to close a suction clearance moves the impeller axially. Rear-side clearance changes can move it too. If the impeller ends up beyond a reasonable match to the volute cutwater (or diffusers), performance changes regardless of how tight the wear gaps are.

• Check that the impeller vanes line up with the cutwater. An impeller shifted axially puts the cutwater out of line with the impeller.
• Check cutwater-to-impeller radial clearance against the OEM value.
• Re-check alignment after any adjustment that moves the impeller.

Outcome: cutwater misaligned or clearance off means it is a second contributor, or the primary one if the wear gaps measure in tolerance. Correct it before you re-test. If alignment and all gaps are in tolerance and the curve is still low, go back to Check 1 conditions and to the recirculation estimate below.

Check 4: Size the leak with an orifice estimate

Published quantitative data on curve drop versus clearance is scarce, and every pump type differs, so one pump's test does not transfer. An order-of-magnitude estimate is still useful for deciding whether the gap explains what you see. Treat the restriction as an equivalent orifice:

Q_leak = Cd * A_gap * sqrt(2 * g * dH)
A_gap  = flow area at the restriction (m^2), for a radial ring gap approx. pi * D_ring * c
dH     = head difference across the gap (m)
Cd     = discharge coefficient (read from a hydraulics handbook; not given here)

Units are dimensionally consistent: m² times m/s gives m³/s. Two points to use:

• Gap area scales with clearance c for a fixed ring diameter. A gap opened from 0.024 in to 0.100 in has 0.100/0.024 = 4.17 times the flow area, so the leak path passes roughly four times the flow at the same head difference under this model.
• The 0.024 in to 0.100 in change is the range tested on a horizontal split-case double-suction pump (Worthington type), which showed a dramatic performance drop. Use it as a directional warning. It is not a Warman AH figure, and no comparable slurry-pump test data was available for the AH.

Compare the estimated leak flow with the flow shortfall from your test. If the estimate is a small fraction of the shortfall, look harder at Check 1 and Check 3 causes.

Other things that move the H-Q curve and dead head

Beyond rpm and diameter, these change curve shape and dead-head pressure. The first three are backed by the clearance findings above; the rest are general pump behavior to consider once clearances are ruled out.

• Suction-side clearance (recirculation).
• Impeller position relative to volute cutwater or diffusers.
• Advanced wear that approaches recessed-impeller (forced vortex) behavior.
• Suction-side inlet conditions and air entrainment.
• Slurry properties: solids concentration and particle size derate head and efficiency compared with a water curve. Test on clear water when comparing to a water curve.
• Worn or damaged vanes and throatbush/liner geometry that differs from the design profile.

Restore clearance and prove the curve is back

1. Record the as-found suction-side, rear-side and cutwater clearances, and the as-found H-Q points (dead head, at least three flow points) with power at each and shaft speed.
2. Replace or restore worn wear parts on the suction side (liner, or wear rings on enclosed designs) per the OEM procedure, and set the suction clearance to the design value from the maintenance manual.
3. Set impeller axial position and confirm cutwater alignment and radial clearance.
4. Check the rear-side gap and, where no discharge-side wear rings exist, seal-chamber pressure against the seal's rating.
5. Re-test at the same rpm on the same medium. Plot dead-head pressure, flow at the duty point, and power at each point against the as-found data and the OEM curve.

Pass criteria: dead head and duty-point head return to the OEM curve at the same rpm, flow at the duty head rises, and power per unit flow drops relative to the as-found test. Seal-chamber pressure sits within the seal's rating. If head recovers but power per unit flow stays high, re-check for a remaining open gap or misaligned cutwater.

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