Pump Not Pumping? A Step-by-Step Diagnostic Sequence

Key Formula / Key Facts Box

Overview — What It Is and Why It Matters

A pump that does not develop flow is a symptom of many possible faults: air‑bound suction, mechanical blockage, loss of prime, excessive wear, or a mismatched motor speed. Because pumps are often the heart of fluid‑handling systems, early detection protects capital equipment, maintains product quality, and avoids unnecessary energy consumption. Misdiagnosis can lead to cavitation damage, bearing failure, or catastrophic system shutdown.

The Method — Derivation and Variants

The diagnostic sequence is rooted in the fundamental energy equation for a centrifugal pump:

H = frac{P ; eta}{rho ; g ; Q}

where

• H = total head (m or ft)
• P = shaft power (W or hp)
• η = overall hydraulic efficiency (dimensionless)
• ρ = fluid density (kg/m³ or lb/ft³)
• g = gravitational acceleration (9.81 m/s² or 32.174 ft/s²)
• Q = volumetric flow rate (m³/s or cfs)

In US‑customary units the same relationship becomes

H_{ft}=frac{P_{hp};eta;550}{rho_{lb/ft³};32.174;Q_{cfs}}

Both forms are useful: the SI version aligns with most engineering textbooks, while the US version matches the data sheets supplied by many pump manufacturers. The diagnostic steps are derived by isolating each variable (head, flow, power, NPSH) and comparing the calculated value with field measurements.

Worked Example

Example 1 – US Units

A 5‑hp centrifugal pump is rated for 150 gpm at 100 psi discharge. In service the motor draws 95 % of its rated current, but the flow gauge reads 60 gpm and discharge pressure is only 40 psi. Determine the most likely fault.

1. Assume η≈0.70. Hydraulic power required at the rating: (P_h = frac{Q;Delta P}{1714;eta}=frac{150;text{gpm}times100;text{psi}}{1714times0.70}approx12.4;text{hp}).
2. Measured hydraulic power: (P_{h,meas}=frac{60times40}{1714times0.70}approx2.5;text{hp}).
3. Motor electrical power ≈ 5 hp × 0.95 = 4.75 hp.
4. The large gap between expected (≈12 hp) and measured (≈2.5 hp) indicates a severe suction problem—most often loss of prime or air binding.

Example 2 – SI Units

A 30 kW, 2500 rpm, single‑stage pump is specified for 0.025 m³/s at 500 kPa. On‑site readings are 0.012 m³/s and 180 kPa. Compute NPSH_available and comment on the likely cause.

1. Suction pressure is half of the rated pressure: (P_{s}=frac{500;text{kPa}}{2}=250;text{kPa}). Convert to head: (H_s=frac{250times10^3}{998times9.81}approx25.5;text{m}).
2. Vapor pressure of water at 20 °C ≈ 2.3 kPa → head (H_{vp}=frac{2.3times10^3}{998times9.81}approx0.23;text{m}).
3. NPSH_available = H_s – H_{vp} ≈ 25.3 m. The manufacturer’s NPSH_required is 1.2 m, so cavitation is unlikely.
4. Since flow is only 48 % of design, the probable cause is a mechanical blockage or impeller wear rather than NPSH deficiency.

Calculator

Validate head, flow, and power relationships with an online tool: Pump Total Dynamic Head Calculator

Reference Values & Typical Ranges

• Suction pressure for water‑filled systems: 0.5–2.0 bar (7–30 psi).
• NPSH_available for municipal water service: 1.5–3.0 m (5–10 ft).
• Motor current draw when fully primed: 90–110 % of rated amperage.
• Vibration velocity for healthy centrifugal pumps: < 4 mm/s (ISO 10816‑1).
• Mechanical‑seal leakage rate: < 0.5 L/h.

Application Guidance

During commissioning, ensure the suction line is completely filled and that all inlet screens are clean. Record motor current, vibration, and discharge pressure before load and after start‑up; deviations point to specific fault categories. If flow is low while pressure is near‑nominal, inspect the impeller for wear or fouling. For intermittent failures, schedule weekly priming checks and verify that check valves are not closing on start‑up.

Common Mistakes, Limits & Safety Notes

1. Mixing US and SI units in a single calculation – always convert before substituting.
2. Assuming rated motor current equals actual load; a stalled pump can still draw near‑rated current.
3. Neglecting suction‑line friction losses, which reduces NPSH_available.
4. Skipping visual inspection of inlet strainers – a clogged screen is a frequent cause of low flow.
5. Over‑tightening pump couplings, leading to shaft misalignment and premature bearing wear.
6. Ignoring safety interlocks; a pump that fails to start may still be energized, posing shock hazards.
7. Applying the same sequence to positive‑displacement pumps without accounting for their constant‑flow characteristic.
8. Exceeding the pump’s recommended NPSH margin, which can induce cavitation and rapid erosion.