The most costly decision you can make when faced with a malfunctioning pump is to start disassembling it. In the vast majority of on-site malfunctions, the fault does not lie with the pump itself: it lies in the suction line, the alignment, the operating conditions, or the fluid itself. The pump is simply the component that signals this in the loudest way.
This guide covers the diagnostics to perform before starting the pump: what vibration indicates, how misalignment shortens bearing life based on mathematical principles, how to distinguish between cavitation and recirculation, and which symptoms are specific to which pump types.
First, this question: the pump or the system?
A pump operates at the point where it is located within the system to which it is connected. If the valve on the discharge line is partially closed, the filter is clogged, or the plumbing has changed from the original design, the pump is not “faulty”—it is simply operating at the wrong point. A pump operating at the wrong point will vibrate, overheat, wear out its mechanical seal, and quickly wear out its bearing.
That is why the diagnosis begins with three measurements, none of which require removing the pump:
- Suction and discharge pressure. The difference between the two gives the actual discharge head produced by the pump. When you compare this to the curve, you can see where the pump is operating.
- Motor current. Its position relative to the rated value indicates whether the pump is under- or overloaded.
- Vibration. A measurement taken from the bearing housing indicates internal damage without having to disassemble the unit.
Any intervention carried out without these three pieces of data is merely a guess. If all three are available, the location of the fault can often be narrowed down on the very first attempt.
What Vibration Tells Us: ISO 20816
The measurement and evaluation of machine vibration are standardized by the ISO 20816 series. ISO 20816-1 specifies the general requirements and procedures; measurements are performed on the machine’s rotating, non-rotating, and non-piston parts. Most pumps fall under the scope of ISO 20816-3: industrial machines with a power rating above 15 kW and operating at speeds between 120 and 30,000 rpm.
ISO 10816, which is still commonly heard in the field, is the previous name for this series; you may come across it in older specifications and device menus. They describe the same thing, but the current reference is ISO 20816.
Absolute value or change?
This is the most commonly overlooked aspect of the standard. The evaluation criteria in ISO 20816-1 are based on both vibration magnitude and changes in vibration. In other words, the question “How many mm/s?” isn’t enough on its own; the question “How much has it changed compared to last month?” is just as informative.
In practice, this means that a vibration that has doubled but remains below the threshold value is a more urgent warning than a vibration that is close to the threshold but has remained stable for years. For this reason, the first measurement taken during commissioning (the reference signature) forms the basis for all subsequent diagnostics. In a facility without a reference, vibration measurement is merely a rough threshold check.
The standard defines the same criteria for both operational monitoring and acceptance testing. Requesting a vibration measurement when commissioning a new pump prevents any potential disputes from arising later on.
An alarm is not a diagnosis
A rising vibration value doesn’t mean “this part is broken”; it means “it’s worth checking out.” To find the cause, you look at the frequency spectrum—imbalances, misalignment, bearing damage, and looseness all manifest at different frequencies. Directly interpreting a general threshold exceedance as a call for part replacement results in replacing sound parts while leaving the actual fault unaddressed.
Alignment and bearing life: the cube law
Coupling alignment is not just a "nice-to-have" adjustment; it directly and significantly determines bearing life. The reason for this is laid out in the mathematics of bearing life.
ISO 281 defines the dynamic load ratings of bearings and the calculation of their basic life. Basic life (L10) is the life corresponding to 90% reliability—that is, the time that nine out of ten bearings operating under the same conditions will last without sustaining fatigue damage. The calculation is based on a power of the ratio of the dynamic load rating (C) to the equivalent load (P): a power of 3 for ball bearings and a power of 10/3 for roller bearings.
This force alone explains why alignment is so important. An alignment error places an additional load on the bearings, and their service life decreases in inverse proportion to the cube of that load:
| Load capacity | Ball bearing life (force 3) |
|---|---|
| Design load | 100% (reference) |
| 25% higher | approximately 51% — nearly half |
| 50% higher | about 30% |
| Twice as much | about 12% — one-eighth |
The values in the table are calculated based on the power of force as defined in ISO 281. The lesson to be learned is this: an alignment error that increases the bearing load by a quarter will cause a bearing expected to last two years to fail in just one year. The statement, “The coupling is a little off but it’s working,” is a decision whose consequences will be felt a year later.
The same principle applies to pipeline stress. A pipe that is suspended from the pump—whose weight is not supported or whose thermal expansion has not been accounted for—stresses the casing and places a constant load on the bearing. This effect is even more pronounced in hot-service pumps—which is why our hot oil pump guide addresses thermal expansion and a rear-removable design.
Cavitation or recirculation?
Both produce noise and vibration, and both wear down the impeller, but the causes are opposite, and so are the solutions.
- Cavitation occurs when there is insufficient pressure on the suction side. The fluid vaporizes at the pump inlet; as the pressure rises, the bubbles collapse and begin to erode the metal. It sounds like gravel. The solution lies in the suction line: level, line loss, temperature, and vapor pressure. The calculation is provided with examples in our NPSH and cavitation guide.
- Recirculation, on the other hand, occurs when the pump is operating at very low flow rates. The flow pattern inside the impeller is disrupted, and the fluid recirculates. Throttling the valve to “protect the pump” actually has the opposite effect in this case.
The most practical way to distinguish between the two is to change the flow rate. If the noise decreases when you open the valve slightly, the problem is low flow rate; if it gets worse, check the suction side. In a hot fluid, the cavitation threshold is lower because the vapor pressure is high.
The situation is different for vacuum machines and blowers: the overheating and strain encountered there are mostly caused by the pressure differential exceeding the machine’s continuous operating limit—these limits are discussed in detail in our blower guide and vacuum pump guide.
Common Failures by Pump Type
The same symptom can indicate different issues depending on the pump type. The table below shows which guide to consult for a detailed diagnosis.
| Pump Type | Typical failure | Detailed Diagnosis |
|---|---|---|
| Air-operated double-diaphragm (AODD) | Diaphragm fatigue, air valve sticking, increased air consumption | AODD Guide |
| Dosing (solenoid / mechanical / peristaltic) | Gas lock, siphoning, calibration drift, hose fatigue | Dosing Guide |
| Magnetic coupling | Decoupling, vortex heating in the containment shell, dry running | Magnetic coupling guide |
| Gear / lobe / progressing cavity pump | Increased internal leakage, flow rate drop due to wear, stator damage during dry running | Viscous transfer guide |
| Vacuum (liquid-ring / oil-sealed vane) | Inability to reach target pressure, oil contamination, rise in water temperature | Vacuum guide |
| Blower (lobe / side-channel) | Rotor contact due to thermal expansion, pressure differential overload | Blower Guide |
| Pool pump | Clogged pre-filter, air leakage at the suction inlet, prolonged turnover time | pool pump guide |
| Hot oil / hot service | Thermal fatigue of the mechanical seal, excessive heating in the bearing area | Hot oil guide |
When the casing, gasket, and elastomer cannot withstand the fluid, the failure is due to material properties rather than mechanical causes; we address this distinction in our material selection guide.
Symptom → cause → initial examination
| Symptom | Possible cause | Initial Check |
|---|---|---|
| Gravel noise, low suction pressure | Cavitation | Suction line loss, tank level, fluid temperature |
| Noise is present but decreases when the valve is opened | Recirculation at low flow rate | Compare the operating point to the curve |
| Vibration is at the same level but has doubled in the last month | Developing damage (balance, bearing, looseness) | Compare with the reference signature; frequency content |
| Bearing life is ending prematurely on a recurring basis | Alignment error or pipe stress | Coupling alignment; pipe hangers and expansion |
| Mechanical seals leak within a short time | Dry running, improper flushing, shaft runout | Flushing line; shaft axial/radial runout |
| Motor current exceeds the rating | Excessive flow rate, high density/viscosity, or mechanical friction | Fluid properties; can the shaft be turned by hand? |
| Motor current is well below expected | Pump is running dry—not drawing in fluid or suction line is closed | Is the suction side primed? Check the discharge valve position |
| Flow rate has decreased over time, but pressure remains the same | Increased internal clearance due to wear (PD pump) or impeller wear | Compare with data sheet flow rate; check speed |
| The coupling rubber frequently breaks down | Alignment error or torsional shock | Alignment; startup conditions and load shock |
| Casing is overheating, no flow | Operating in a closed loop—energy is being converted to heat | Bypass/safety valve; minimum flow rate requirement |
Maintenance plan: what and how often
Specific intervals vary depending on the machine, the fluid, and operating hours; the manufacturer’s data sheet is the definitive guide. However, the basic framework of the schedule is the same at every facility:
- During commissioning: the vibration reference signature, suction/discharge pressure, motor current, and alignment data are recorded. Without these four data points, none of the subsequent measurements can be interpreted.
- Shift/daily: visual leak inspection, abnormal noise, bearing temperature, and pressure gauge readings.
- Periodic: vibration measurement and comparison with the reference, filter and strainer cleaning, oil level and condition.
- During planned downtime: alignment check (including thermal expansion), coupling components, mechanical seals, and pipe hangers.
- Recording: Each measurement is recorded with the date. Since significant changes in vibration are critical, even the standard is useless without proper recording.
Repair or renovation?
The decision is not emotional but calculated. Three questions are enough:
- Is the malfunction recurring? If the same part is failing for the third time, the problem lies not with the part itself, but with the operating conditions or the installation. Installing a new part resets the counter but does not resolve the underlying cause.
- Is the pump still the right one? The facility may have expanded, the fluid may have changed, or the flow rate requirement may have shifted. A pump operating at the edge of its curve will continue to operate at that same point even after it has been repaired.
- Is a replacement available? Securing a replacement part for a standby pump on a critical line is part of the decision to repair.
We also supply replacement parts for both domestic and imported pumps; if you provide us with the make and model of your pump, we can work with you to identify the appropriate part.
Frequently Asked Questions
What should the pump vibration be in mm/s?
There is no single figure; the limit varies depending on the machine’s power, speed, and installation configuration. Most pumps fall under the scope of ISO 20816-3 (over 15 kW, 120–30,000 rpm). More importantly, the standard provides criteria based on both magnitude and change. A vibration that was below the limit but has doubled in the last month is more urgent than a consistently high value.
ISO 10816 or ISO 20816?
ISO 20816 is the current series; ISO 10816 is its previous designation. ISO 10816 is still used in older specifications, measurement device menus, and field practice. If you are drafting a new specification, refer to ISO 20816.
Does alignment really affect bearing life that much?
Yes, and the mathematics is outlined in ISO 281. The basic life calculation is based on a power of the load factor; for ball bearings, this power is 3. An alignment error that increases the bearing load by 25% reduces the service life by approximately half, while an error that doubles the load reduces it to one-eighth.
The pump is getting hot, but the flow rate is normal. What's the problem?
First, separate the bearing casing from the pump body. If the pump body is overheating, the pump is most likely operating at a very low flow rate or with a blocked line: since the energy supplied cannot be carried away by the fluid, it is converted into heat. If the bearing area is overheating, check the alignment, lubrication, and axial load. Recording the measurements is the quickest way to determine which factor has increased.
How do I distinguish cavitation from recirculation?
Adjust the flow rate. If the noise decreases when you open the discharge valve slightly, the pump was operating at a very low flow rate (recirculation). If the noise increases, the suction side is insufficient (cavitation), because as the flow rate increases, the suction line head loss increases and the available suction lift decreases even further.
The same part keeps breaking—why?
This is because what is being changed is the symptom, not the cause. A recurring failure almost always points to one of three causes: the pump is operating at the wrong duty point, there is stress in the installation (misalignment, pipe load), or the fluid is not suitable for the selected material. After the third occurrence, stop replacing parts and measure these three factors instead.
Is an expensive device necessary for vibration measurement?
Frequency analysis requires advanced equipment, but even a simple general-level measurement is sufficient for tracking trends—as long as the measurements are taken and recorded at the same point and under the same conditions. Since the change in the value is more important than the value itself, regular and comparable measurements take precedence over expensive equipment.
Summary
When a pump fails, the quickest solution is not to remove the pump, but to check three parameters: suction/discharge pressure, motor current, and vibration. When it comes to vibration, it’s the change—rather than the absolute value—that matters; therefore, the reference signature recorded during commissioning serves as the basis for all subsequent diagnostics. Alignment, meanwhile, is not merely an aesthetic adjustment but a parameter that significantly determines bearing life.
If there is a recurring failure, replacing the part will reset the counter but will not resolve the underlying cause: the failure will not be resolved unless the operating point, installation stress, and material compatibility are measured.
If you share the brand, model, and symptoms of the pump you have, we can help with the diagnosis and, if necessary, with sourcing parts. You can browse our selection of domestic and imported pump replacement parts or get in touch with us for technical support.