Diagnostics and Troubleshooting Guide: Centrifugal Pumps Decreasing Performance or No Discharge

1. Description of the problem and scope of application

This manual is intended for system diagnostics and troubleshooting related to reduced supply or complete lack of discharge of centrifugal pumps, which are critical disturbances in industrial processes. Such malfunctions lead to production stoppages, reduced efficiency and increased operational costs. The guide covers a wide range of centrifugal pumps used in industries such as chemical, food, water and wastewater, oil and gas and general industrial applications.

Problems with reduced supply or lack of injection can have varying degrees of severity:

• Critical: Complete lack of supply, which leads to an immediate stop of the technological process and significant financial losses.
• Significant: A significant reduction in feed that affects product quality, process efficiency and may cause other equipment to fail.
• Minor: A small but steady decrease in supply, which leads to increased energy consumption and a reduction in the period between repairs.

The purpose of the manual is to provide engineering personnel, mechanics, and operators with the tools to quickly and accurately identify the root causes of malfunctions and effectively restore equipment to service.

2. Precautions

WARNING! SAFETY! Before starting any diagnostic or repair work on pumping equipment, ensure that all safety standards and procedures are followed. Failure to follow these precautions could result in personal injury, equipment damage, or death.

• Blocking/Tagging (LOTO): Be sure to perform the blocking and tagging procedure in accordance with DSTU EN 1037:2018 "Safety of machines. Prevention of unexpected start". Make sure the power source is disconnected and locked, and that any moving parts cannot be accidentally started.
• Personal Protective Equipment (PPE): Always use the required PPE: safety glasses (EN 166), safety gloves (EN 388), safety shoes (EN ISO 20345), helmet and, if necessary, protective clothing.
• Stored Energy: Ensure that all stored energy (system pressure, electrical charge, potential energy from weight) is safely discharged or blocked. Drain fluid from pump and piping before disassembly.
• Hot surfaces and fluids: The pump and fluid may be hot. Allow the equipment to cool to a safe temperature, or use heat-resistant PPE.
• Hazardous liquids: When working with aggressive, toxic or flammable liquids, follow special safety procedures, use appropriate personal protective equipment (eg, chemical resistant gloves, respirators) and ensure adequate ventilation.
• Rotating parts: Never work near rotating parts without proper protection and power off.

3. Necessary diagnostic tools

For effective diagnostics, the following set of tools that meet the relevant standards is required:

4. Initial evaluation checklist

Before starting a detailed diagnosis, it is necessary to conduct a preliminary assessment in order to collect basic information and narrow down the search for a malfunction:

5. Systematic flow of diagnostics

This section presents a solution tree to help systematically identify the root cause of a problem.

1. SYMPTOM: Reduced supply or no boost.
   • STEP 1: Check initial conditions and visual signs (See p.4).
     • IF the pump is running but there is no or minimal supply and the fluid level in the reservoir is low or absent.
       • THEN PROBABLE CAUSE: Insufficient fluid flow to suction / Empty tank.
       • CHECK: Inflow to the tank, blockage of the suction pipeline, clogging of the suction filter.
       • GO TO: item 8 (Problems in the suction pipeline).
     • IF the pump is running, but there is a loud noise, crackling, gurgling, similar to the sound of gravel.
       • THEN PROBABLE CAUSE: Cavitation or airlock.
       • CHECK: Suction pressure with vacuum gauge/manometer. Use an ultrasonic tester to confirm cavitation/air leaks.
       • GO TO: STEP 2.
     • IF the pump is running relatively quietly, but the delivery is significantly reduced and the discharge pressure is below normal with normal suction pressure.
       • THEN PROBABLE CAUSE: Impeller wear, internal leaks, or changes in the injection system.
       • CHECK: Motor current consumption (current-measuring clamps). If the current is significantly reduced at reduced feed, this may indicate wear on the impeller.
       • GO TO: STEP 3.
     • IF the pump is operating normally, but the delivery is reduced and the discharge pressure does not meet the process requirements, while the suction pressure is normal, and there are no visible signs of wear or cavitation.
       • THEN PROBABLE CAUSE: System curve mismatch or pump selection incorrect.
       • CHECK: System curve analysis.
       • GO TO: STEP 4.
   • STEP 2: Diagnose cavitation and airlocks.
     • IF the suction vacuum is high (close to the saturated vapor pressure of the liquid at the current temperature) or the suction pressure is very low.
       • THEN PROBABLE CAUSE: Cavitation caused by insufficient NPSHa.
       • CHECK: Clogged suction filter, valves partially closed, suction pipe length/diameter too long, fluid temperature.
       • GO TO: item 7 (Cavitation).
     • IF there is gurgling, flow noise, and boost pressure is erratic or absent, especially after starting or idling.
       • THEN PROBABLE CAUSE: Airlock in pump housing or air leak in suction line.
       • CHECK: Airtightness of the suction pipeline (visually, with a soapy solution, with an ultrasonic tester). Fill valve condition.
       • GO TO: Item 7 (Air Jam / Filling Problems).
   • STEP 3: Diagnose mechanical wear.
     • IF motor current consumption is lower than rated or significantly reduced compared to normal operating conditions at reduced feed.
       • THEN PROBABLE CAUSE: Significant wear on the impeller.
       • CHECK: Visual inspection of the impeller and internal surfaces of the housing (after disassembly).
       • GO TO: item 7 (Wearing of impeller and housing).
     • IF the vibration exceeds the permissible limits according to ISO 10816 (for example, for class II pumps, the vibration is greater than 4.5 mm/s RMS for the rated speed).
       • THEN THEN PROBABLE CAUSE: Bearing wear, misalignment, imbalance, or internal damage (such as a part of the impeller breaking off).
       • CHECK: Detailed vibration analysis, thermal imaging of bearings, misalignment check.
       • GO TO: item 7 (Impeller and housing wear), also consider bearings and seals.
   • STEP 4: System curve analysis.
     • IF all pump parameters (suction pressure, no cavitation, normal vibration, no wear) are normal, but the actual delivery remains low and the discharge pressure corresponds to a "new" duty point on the pump characteristic that does not meet the process requirements.
       • THEN PROBABLE CAUSE: A change in the hydraulic characteristics of the system (system curve) or the initial wrong selection of the pump.
       • CHECK: Recalculation of the system curve taking into account new operating conditions or pipeline modifications. Comparison with the operating curve of the pump.
       • GO TO: p. 7 (Inconsistency of the system curve).

6. Matrix of malfunctions and causes

The following chart will help you quickly navigate through the symptoms, most likely causes, diagnostic tests, and expected results.

Symptom	Probable Causes (ranked by probability)	Diagnostic Test	Expected Result if the cause is confirmed
No feed / Very low feed	Air in the pump (High) No fluid at the suction (High) Clogged suction (Medium) Wrong direction of rotation (Low)	Opening the air valve; Checking the level in the tank; Visual inspection of the filter; Checking the motor connection.	Air outlet, supply recovery; The tank is empty; The filter is clogged; The pump does not prime, despite rotation.
Low supply, noise, vibration, drop in discharge pressure, pulsations	Cavitation (High) Clogged suction line (Medium) Liquid temperature too high (Medium)	Pressure measurements on suction and discharge; Ultrasonic tester; Thermal imager; Checking filters.	The suction pressure is close to the saturated steam pressure; Characteristic Related Articles Diagnostic Guide for Hydraulic Pump Cavitation: Inlet Restriction, Reservoir Level, Fluid Viscosity, and Intake Line Air Leak This guide covers cavitation in hydraulic pumps, a critical phenomenon that can lead to mechanical degradation, loss of performance and… Diagnosis and Resolution Guide: Oscillation and Hunting of Industrial Control Valves 1. Description of the Problem and Purpose This guide was developed for maintenance technicians, reliability engineers, and plant managers working… Fault diagnosis: Cavitation on hydraulic pumps – suction side check and leak location 1. Problem Description & ScopeThis diagnostic guide covers the systematic identification, root cause analysis, and resolution of cavitation and air… This website uses functional cookies to ensure proper operation. No tracking or advertising cookies are used. Cookie Policy | Privacy Policy Accept All Necessary Only