When an R717 pump repeatedly fails, replacing the pump or mechanical seal without checking the operating conditions can simply result in the same failure happening again. The first step is to establish what the pump is actually experiencing while running.

1. Start with the actual operating conditions

Record suction pressure, discharge pressure, ammonia temperature, flow, pump speed and the liquid level or vessel pressure feeding the pump. Compare these values with the pump's specified duty and operating limits.

2. Check the suction conditions

Ammonia is particularly sensitive to pressure and temperature because the liquid can approach its saturation condition. Restrictive suction piping, a blocked strainer, excessive suction lift, low vessel level or an unsuitable valve arrangement can reduce the pressure available at the pump inlet.

If pressure falls sufficiently relative to the ammonia saturation pressure, vapor can form. That can lead to unstable operation, loss of performance and potentially damaging cavitation or flashing conditions.

3. Check for flashing and vapor at the suction

Flashing is a thermodynamic condition in which part of the liquid changes phase because of pressure and temperature conditions. The suction system therefore needs to be evaluated together with the pump.

4. Look at the operating point

Compare measured duty with the manufacturer's performance information rather than judging the pump only by the discharge gauge. Operation away from the intended duty can change hydraulic loads and internal pressure conditions.

5. Investigate seal and lubrication conditions

Repeated seal damage should be investigated for heat, dry running, pressure conditions, incorrect seal materials, shaft or sleeve condition, alignment and the condition of any seal lubrication or barrier system.

6. Check the complete installation

Pipe strain, poor support, restrictive fittings, badly positioned elbows, incorrect valve configuration and insufficient instrumentation can all contribute to pump problems. A pump should be treated as part of a system, not as an isolated component.

A practical troubleshooting sequence

• Confirm actual ammonia temperature and suction pressure.
• Confirm discharge pressure and approximate flow.
• Check suction piping, valves and strainers for restriction.
• Compare the operating point with the pump curve and NPSH information.
• Inspect the seal, shaft, bearings and coupling for evidence of heat, wear or misalignment.
• Check whether the failure repeats under the same operating conditions.

The most useful question is often not “Which part failed?” but “What operating condition caused that part to fail?”

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In centrifugal pumps, the lowest pressure commonly occurs near the impeller eye. If the pressure there becomes too low for the liquid's temperature, vapor bubbles can form. Their subsequent collapse can produce noise, vibration, loss of head and, when severe or persistent, erosion and damage to hydraulic surfaces.

What causes cavitation?

• Insufficient NPSH available at the pump suction.
• Excessive suction-pipe friction or restrictive fittings.
• A blocked or undersized suction strainer.
• Low liquid level or excessive suction lift.
• High liquid temperature, which increases vapor pressure.
• Operating where the pump's NPSH requirement is higher.
• Unexpected pressure losses or changes in the suction system.

Typical warning signs

Cavitation may be associated with a crackling or gravel-like sound, fluctuating pressure, vibration, reduced head and unstable flow. These symptoms are not unique to cavitation, so measurements should be used to confirm the cause.

Cavitation and NPSH

The practical investigation compares NPSHa, the NPSH available from the system, with NPSHr, the NPSH required by the pump at the operating condition. Pump manufacturers publish NPSHr data, while NPSHa depends on the actual installation and fluid conditions.

How can cavitation be reduced?

Improve suction conditions: reduce unnecessary suction losses, keep suction piping appropriately sized, maintain adequate liquid level, reduce excessive temperature where the process allows it, remove unnecessary restrictions and ensure the pump operates at an appropriate duty point.

Cavitation is therefore not simply a “bad pump” problem. It is often a system condition created by the relationship between pressure, temperature, flow and the pump's hydraulic requirements.

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What is NPSHa?

NPSHa means Net Positive Suction Head available. It is determined by the actual system and fluid conditions at the pump suction. It is influenced by pressure above the liquid, static liquid level, suction-line losses and the liquid's vapor pressure at operating temperature.

For a typical open-tank arrangement, available suction head can be represented conceptually as pressure head plus static head, minus suction losses and vapor-pressure head. For closed systems, vessel pressure must also be included.

What is NPSHr?

NPSHr means Net Positive Suction Head required. It is a characteristic of the pump and is normally supplied by the manufacturer as a function of flow. It represents the suction condition required by the pump under the specified test or performance criterion.

Why does NPSHr change with flow?

As flow changes, velocity and internal hydraulic losses change. The pressure distribution at the impeller inlet also changes, so the pump's NPSH requirement is not one constant value for every operating condition.

The basic relationship

For a given operating point, NPSHa needs to be greater than NPSHr with an appropriate engineering margin. The exact margin should be established using the pump manufacturer's guidance and the application; it should not be treated as one universal number for every pump.

What can reduce NPSHa?

• Lower liquid level in the supply vessel.
• Higher liquid temperature and vapor pressure.
• Long or undersized suction piping.
• Additional elbows, valves or strainers.
• A partially blocked suction strainer.
• Lower pressure in a closed vessel.
• Higher flow through the suction line.

Why this matters in troubleshooting

If a pump works when the vessel is full but becomes noisy or loses performance as the level drops, the change in NPSHa should be investigated. Likewise, a temperature increase can reduce suction margin by increasing vapor pressure.

Always compare NPSHa and NPSHr at the actual flow, speed, liquid and temperature rather than using values from a different operating condition.

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Viscosity is one of the most important fluid properties to establish before selecting a pump. A liquid that looks similar to water can behave very differently once its viscosity increases, particularly at lower temperatures.

The effect depends on the pump type. Centrifugal pumps can experience reduced flow, reduced head and efficiency as viscosity increases, while positive displacement pumps are often well suited to thicker liquids. However, higher viscosity can also increase torque and therefore the power required.

Why temperature must be included

Viscosity is usually temperature-dependent. Oils, syrups, polymers, chocolate, adhesives and other viscous products may become substantially thicker when cooled. Therefore, the viscosity used for pump selection should be the viscosity at the actual operating temperature, not simply a value from a product data sheet at another temperature.

What can happen when viscosity is underestimated?

• The pump may deliver less flow than expected.
• Efficiency can fall and energy consumption can increase.
• Motor or shaft torque can become higher than expected.
• A centrifugal pump may move away from its expected operating point.
• Pressure losses in suction and discharge piping can increase.

What information should be supplied?

When specifying a pump, provide the liquid viscosity, operating temperature, required flow, required pressure or head, density and whether the viscosity changes significantly during the process. For non-Newtonian liquids, also provide the relevant rheological information because a single viscosity value may not describe the fluid adequately.

Good pump selection starts with the liquid, not just the pipe size. Knowing the actual viscosity allows the pump type, speed, motor and materials to be evaluated against the real duty.

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Density is the mass of liquid contained in a given volume. In pump engineering it matters because a pump normally produces head, while pressure gauges display pressure. The relationship between the two depends on the liquid density.

Head is not the same as pressure

For a given pump head, a denser liquid produces a higher pressure difference than a less dense liquid. A useful relationship is:

Pressure = Density × Gravity × Head

This is why a pump's head rating can remain the same while the pressure observed on a gauge changes when the pumped liquid changes.

Density and motor power

Hydraulic power is also affected by density. For the same flow and head, increasing density increases the hydraulic power required. If the motor was selected using a lower-density liquid and the application changes to a substantially denser product, motor loading should be checked.

Where density becomes especially important

• Fuel and oil transfer.
• Chemical processing.
• Slurries and liquids containing suspended solids.
• Food products with changing concentration.
• Applications where the product concentration changes during production.

Do not confuse density with viscosity

Density describes how much mass is contained in a given volume. Viscosity describes resistance to flow. They are different properties and both may be important for pump selection.

When requesting a pump selection, provide the actual or expected operating density together with viscosity, temperature, flow and required head or pressure. This gives the pump supplier enough information to evaluate hydraulic performance and motor loading correctly.

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Liquid temperature is often treated as a simple process specification, but it can affect almost every part of a pumping system. The same liquid at 20°C and 120°C may require very different pump arrangements, materials, seals and suction conditions.

Temperature changes viscosity

Many liquids become less viscous as temperature increases and more viscous as temperature decreases. This can change the hydraulic performance of the pump and the losses in the pipework. Oils and other viscous products can be particularly sensitive to temperature.

Temperature changes vapor pressure

As temperature rises, a liquid's vapor pressure generally increases. This reduces the pressure margin available at the pump suction and can increase the risk of flashing or cavitation if the suction conditions are not adequate.

Temperature affects materials and seals

Pump casing materials, impellers, mechanical seals, O-rings, gaskets, lubricants and coatings all have operating limits. A material that is suitable at ambient temperature may not be suitable at a significantly higher or lower temperature.

Temperature can also affect the process itself

• Product viscosity may change.
• Density may change.
• Vapor pressure may increase or decrease.
• Elastomers can harden, soften or lose compatibility.
• Lubricants can change viscosity and film strength.
• Thermal expansion can introduce mechanical loads into piping and equipment.

What should be specified?

Provide the minimum, normal and maximum liquid temperature where possible. Also state whether the pump starts with a cold product and then reaches operating temperature, because start-up conditions can be significantly different from steady-state conditions.

Temperature should be considered as part of the complete pump duty, not as an isolated number. It interacts with viscosity, vapor pressure, materials and suction conditions.

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A pump curve is one of the most useful engineering documents when selecting or troubleshooting a pump. It shows how the pump is expected to perform at a particular speed, impeller diameter or operating configuration.

What does a typical pump curve show?

• Flow: the volume of liquid moved per unit of time.
• Head: the energy added to the liquid, normally expressed in metres of liquid.
• Efficiency: how effectively the pump converts input power into hydraulic output.
• Power: the shaft or motor power required at different operating points.
• NPSHr: the pump's stated net positive suction head requirement under defined conditions, where provided.

The operating point matters

The pump does not simply operate at the maximum flow printed on the curve. The actual operating point results from the interaction between the pump and the system. The pump curve describes what the pump can produce; the system determines how much head is required at each flow.

Why selecting by pressure alone can be misleading

A requirement such as “10 bar” does not provide enough information to select a pump. The required flow, static head, friction losses, liquid properties, temperature, suction conditions and operating speed must also be established.

What to check on a pump curve

1. Confirm the required flow rate.
2. Calculate the required total dynamic head.
3. Locate the duty point on the pump curve.
4. Check efficiency and power at that point.
5. Check NPSHr against the available suction conditions.
6. Confirm the liquid properties and temperature used for the selection.
7. Check that the pump is not being selected far outside its intended operating range.

A pump curve is therefore more than a sales chart. It is a technical tool for matching a pump to a real system and for diagnosing why a pump may not deliver the expected flow or pressure.

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Every liquid has a vapor pressure that changes with temperature. In pumping systems, this matters because the pressure at the pump suction must remain sufficiently above the liquid's vapor pressure to avoid unwanted vapor formation.

Why vapor pressure matters in a pump

If the local absolute pressure in the pump or suction line approaches the liquid's vapor pressure, part of the liquid can vaporise. This can lead to flashing or, depending on where the vapor forms and collapses, cavitation-related problems.

Temperature has a major influence

As liquid temperature increases, vapor pressure generally increases. This means that heating a liquid can reduce the available suction margin even if the pipework and pump remain unchanged.

Vapor pressure and NPSH

Vapor pressure is one of the inputs used when determining NPSH available. A higher vapor pressure means a larger portion of the available absolute pressure is consumed by the liquid's tendency to vaporise, leaving less margin at the pump suction.

Why this is especially important for refrigeration

Refrigerants such as ammonia (R717) can operate close to saturation conditions. Small changes in pressure or temperature can therefore have a significant effect on whether the fluid remains liquid at the pump suction. This is why suction pressure, liquid temperature and vessel conditions need to be considered together when troubleshooting an ammonia pump.

Typical signs that vapor pressure may be relevant

• Noise or vibration that changes with liquid temperature.
• Loss of pump capacity as temperature increases.
• Flashing observed in a suction line or sight glass.
• Repeated cavitation-type damage.
• A pump that performs correctly under one operating condition but not another.

Vapor pressure is not simply a laboratory property. It is directly connected to the real suction conditions of the pump and should be considered whenever temperature, pressure or phase change is important.

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Why are suction strainers used?

Strainers may prevent debris, welding scale, rust or process solids from entering a pump. This can be particularly useful during commissioning or where contamination is possible.

The problem with restriction

The suction side is normally where pressure margin is most critical. A strainer that is too small, has an unsuitable mesh or becomes partially blocked can increase pressure loss and reduce NPSHa.

As the strainer becomes dirty, pressure drop across it can increase. A pump that initially operated correctly can therefore develop suction problems later without any change to the pump itself.

How should a suction strainer be selected?

• Size it for the required flow rather than simply matching the pump nozzle.
• Consider screen open area, not only connection diameter.
• Select a mesh appropriate for the solids that actually need to be removed.
• Consider pressure drop at clean and dirty conditions.
• Provide access for inspection and cleaning.
• Check the effect on the application's NPSHa.

Do not automatically make the mesh finer

A finer mesh can capture smaller particles, but it can also create greater resistance and clog more quickly. The correct choice balances equipment protection with acceptable suction losses.

What should you check when a pump starts cavitating?

If a pump develops noise, vibration or loss of performance, inspect the suction strainer before assuming the pump itself has failed. Check the pressure conditions and compare the current installation with the original design.

For critical suction applications, instrumentation across the strainer can provide useful information about increasing differential pressure.

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What causes pipe strain?

• Piping that does not naturally line up with the pump nozzles.
• Insufficient or incorrectly positioned pipe supports.
• Thermal expansion or contraction.
• Heavy valves or fittings supported by the pump nozzle.
• Forcing pipework into position during installation.
• Changes to the piping system after alignment.

Why is pipe strain a pump problem?

The pump casing and nozzles are not intended to act as structural supports for an incorrectly installed pipe system. External loads can distort the equipment or shift alignment between connected rotating components.

That can contribute to coupling problems, vibration, bearing loading and mechanical seal issues. Thermal movement can make the situation more complicated because an installation that appears acceptable when cold may experience different loads at operating temperature.

Check the piping before commissioning

Pipework should be independently supported and should line up with the pump connections without excessive force. Check the installation against the pump manufacturer's allowable nozzle loads where those limits are specified.

Pipe strain and alignment

Alignment should not be treated as a one-time adjustment made before the pipework is completed. If connecting the piping moves the pump or changes shaft alignment, the system needs to be corrected before commissioning.

A practical check

With piping correctly positioned and supported, verify that pump connections do not need to be pulled, pushed or twisted into alignment. If loosening a connection causes the pipe to move noticeably, investigate the support and alignment arrangement rather than forcing the connection back together.

For larger installations, a formal piping-load assessment may be appropriate. Always follow the pump manufacturer's installation requirements.

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Elbows are often treated as simple fittings, but their location can matter when designing pump suction piping. A sharp change in direction close to the suction nozzle can create disturbed or uneven flow entering the pump.

Why suction elbows deserve attention

A pump performs best when the liquid approaches the suction nozzle with a reasonably uniform velocity profile. An elbow immediately before the pump can introduce swirl and uneven velocity distribution. The effect can become more significant when the pump has a relatively sensitive suction arrangement or when multiple fittings are installed close together.

Keep the suction approach simple

Where practical, provide a straight section of suction pipe between the last major disturbance and the pump inlet. The exact arrangement should follow the pump manufacturer's installation requirements and the piping geometry of the application.

Things to consider

• Number of elbows close to the pump suction.
• Elbow orientation and whether the flow is forced to turn immediately before entering the nozzle.
• Reducers, valves, strainers and other fittings near the suction.
• Pipe diameter and resulting liquid velocity.
• Total suction pressure loss and its effect on NPSHa.

Discharge piping is different

Elbows on the discharge side also create pressure loss, but the suction side deserves particular attention because losses reduce the pressure available at the pump inlet. A poor suction arrangement can therefore contribute to cavitation or unstable pump operation.

Good suction piping is usually simple, adequately sized and arranged to give the pump a clean approach to the liquid.

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When a pump is installed above the liquid level, the pump must draw the liquid upward through the suction pipe. This arrangement is commonly described as suction lift. It is important to understand that the pump does not literally “pull” a liquid upward by creating an unlimited vacuum; atmospheric or vessel pressure, vapor pressure and system losses determine the actual suction conditions.

Static lift is only part of the calculation

The vertical difference between the liquid surface and the pump centreline contributes to the suction condition. Friction losses in the suction pipe, fittings, valves and strainers must also be considered. The resulting absolute pressure at the pump inlet must remain suitable for the liquid and pump.

Why suction lift can become difficult

• Long or undersized suction pipe increases friction loss.
• Too many fittings add additional pressure loss.
• A blocked strainer can reduce suction pressure further.
• Higher liquid temperature can increase vapor pressure.
• Low atmospheric or vessel pressure reduces the available suction margin.
• Air leaks can cause loss of prime and unstable operation.

Priming matters

A conventional centrifugal pump generally needs the casing and suction arrangement correctly filled before it can operate as intended. If the pump repeatedly loses prime, the cause should be investigated rather than simply repriming it each time.

Suction lift and NPSH

Suction lift is directly connected to NPSHa. As the pump is positioned higher above the liquid surface, the available suction pressure generally decreases. The system must therefore be checked against the pump's NPSHr at the actual operating flow.

A pump installation should be evaluated using the complete suction system—not vertical height alone.

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The liquid level in a tank, vessel or surge drum is not simply an operating convenience. It forms part of the pump's suction conditions and can directly affect whether the pump receives a stable supply of liquid.

Static head changes with level

When the liquid level rises above the pump suction, the hydrostatic head available at the pump inlet generally increases. When the level falls, that available head decreases. In systems with a large change in operating level, the lowest expected liquid level should be considered during pump selection and troubleshooting.

Low level can create several problems

• Reduced suction pressure and reduced NPSHa.
• Vortex formation at the tank outlet.
• Air entrainment into the suction line.
• Unstable flow and vibration.
• Loss of prime in systems using suction lift.

Vortexing is not always obvious

A tank may appear to contain plenty of liquid while the outlet still experiences poor suction conditions. The inlet geometry, flow rate, liquid level, tank dimensions and nearby internal flow patterns all influence the risk of vortex formation.

For refrigeration and closed vessels

In closed systems, the pressure above the liquid surface is also important. The relevant suction condition is therefore not determined by liquid level alone. Vessel pressure, liquid temperature and pressure losses through the suction piping should be considered together.

Always evaluate the minimum operating liquid level when checking pump suction performance. The worst suction condition often occurs when the vessel level is low and the pump is still expected to deliver its required flow.

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Pressure gauges are simple instruments, but they provide some of the most useful information available when diagnosing pump performance. A single discharge pressure reading rarely tells the complete story. Suction and discharge measurements together are much more informative.

Why measure suction and discharge?

The difference between suction and discharge pressure gives an indication of the pressure increase produced by the pump. When the liquid density is known, these pressure measurements can be related to pump head.

Gauge position matters

Ideally, pressure measurements should represent the conditions close to the pump suction and discharge nozzles while avoiding locations where fittings or excessive turbulence can distort the reading. The connection arrangement should also avoid trapped gas or liquid where that could affect the measurement.

What gauges can help identify

• Unexpectedly high suction pressure loss.
• Blocked or restricted suction strainers.
• Changes in system resistance.
• Operating away from the expected pump duty point.
• Changes in pump differential pressure.
• Possible flashing or unstable suction conditions.

Pressure is not the same as flow

A pressure gauge cannot by itself confirm the pump flow rate. A pump can show pressure while delivering very little flow, particularly if a discharge valve is closed or the system resistance is high. Flow should therefore be measured or independently established where accurate troubleshooting is required.

Use gauges as diagnostic instruments

Record suction pressure, discharge pressure, liquid temperature, pump speed and flow whenever possible. Comparing these readings with the original commissioning data or the pump curve can reveal changes that may otherwise be missed.

A gauge is most valuable when its reading is recorded under known operating conditions and compared with other system measurements.

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“The pump won't prime” is a common complaint, but priming failure is a symptom rather than a diagnosis. The first step is to establish what type of pump is installed and whether the pump is designed to self-prime under the actual conditions.

Common causes

• Air entering through a suction flange, gasket, fitting or valve stem.
• Suction piping not filled correctly.
• Incorrect or leaking foot valve or check valve.
• Liquid level too low for the installation.
• Blocked suction strainer or restricted suction pipe.
• Suction pipe diameter or arrangement creating excessive losses.
• Pump casing not properly filled before start-up.
• Incorrect pump rotation or mechanical condition.

Check for air leaks

Air can enter a suction system without producing an obvious liquid leak. A suction-side connection can therefore appear dry while still allowing air into the system. Inspect flanges, threaded joints, seals, valves and other connections carefully.

Check the liquid supply

Confirm that the pump actually has an adequate liquid supply under the operating condition. For suction-lift systems, check the liquid level, suction height and total suction losses. For flooded suction systems, check vessel pressure and the pressure available at the pump inlet.

Check the pump itself

If the suction system is correct, inspect the pump for internal wear, damaged components, incorrect rotation, seal problems or other mechanical issues. A worn pump may be unable to establish the expected pressure difference even when the suction line is correctly filled.

Avoid repeatedly restarting without finding the cause

Repeated dry running or operation with inadequate liquid can damage seals, bearings or internal components depending on the pump design. The correct approach is to identify why the pump is losing or failing to establish prime.

Start with the suction system, verify the liquid supply, then inspect the pump. A structured check is usually faster than repeatedly priming and restarting the equipment.

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A bare shaft pump is supplied without a directly mounted motor, allowing the pump to be coupled to a separately selected driver. A closed-coupled arrangement mounts the pump and motor together as an integrated package. Both arrangements can be technically suitable; the correct choice depends on the application and installation requirements.

Bare shaft arrangement

With a bare shaft pump, the motor, coupling, baseplate and guarding are selected as part of the complete installation. This gives greater flexibility when the driver type, speed or motor size needs to be matched specifically to the application.

• Flexible choice of motor and driver.
• Useful where different speeds or drive arrangements are required.
• Can be well suited to larger industrial installations.
• Requires correct coupling alignment and suitable baseplate/support design.

Closed-coupled arrangement

A closed-coupled pump typically has the motor and pump assembled into a compact package. This can simplify installation and reduce the footprint, while eliminating some of the alignment work associated with a conventional shaft-and-coupling arrangement.

• Compact installation.
• Generally simpler installation.
• No conventional flexible coupling alignment between a separate motor and pump.
• Motor and pump selection are more closely tied together.

Maintenance considerations

Neither arrangement should automatically be considered maintenance-free. Bearings, seals, couplings, motors and other components still require inspection according to the pump design. Bare shaft systems place greater emphasis on shaft alignment, soft foot, base condition and coupling condition.

Which should be selected?

Start with the duty rather than the construction style. Establish the required flow, head, liquid properties, temperature, speed, motor power, installation arrangement and maintenance requirements. Then select the pump package that best satisfies those requirements.

The pump duty determines the equipment; the installation determines which configuration makes the most practical sense.

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Abrasive liquids contain particles that can wear surfaces as the liquid moves through the pump. Examples can include mineral slurries, sand-bearing water, certain process slurries and other liquids containing hard suspended solids.

What causes abrasive wear?

Particles moving at high velocity can impact and slide against wetted components. Wear can be influenced by particle hardness, particle size, concentration, shape, velocity and the material of the pump components.

Components commonly affected

• Impellers.
• Wear plates or wear rings where fitted.
• Pump casing and liners.
• Mechanical seal components.
• Valve seats and other downstream components.

Pump speed matters

Higher liquid velocity can increase wear in many abrasive applications. Pump speed should therefore be considered together with required flow and head rather than simply choosing the highest available speed.

Material selection matters

The appropriate construction depends on the solids and the application. Depending on the duty, hardened metals, elastomers, ceramics, rubber-lined components or other wear-resistant materials may be considered. Compatibility must be evaluated for the complete liquid, not only its abrasive fraction.

Do not ignore the suction system

Large or heavy particles can settle in poorly designed suction piping. This can restrict the pump inlet and create additional suction losses. Strainers should also be selected carefully because an overly fine or undersized strainer can become a significant restriction.

Abrasive service is a wear-management problem as much as it is a pump-selection problem. The objective is to achieve the required duty while controlling velocity, materials, solids handling and maintenance intervals.

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Corrosion can damage a pump even when its hydraulic performance is correct. Selecting a pump for a corrosive liquid therefore requires more than choosing a material based on the liquid's name.

Concentration and temperature matter

The corrosivity of a chemical can change with concentration and temperature. A material that performs well at one concentration or temperature may not provide the same service life under another condition.

Look beyond the pump casing

The wetted material specification should include all relevant components, not just the casing. Depending on the pump design, this may include:

• Impeller or rotor.
• Shaft or shaft sleeve.
• Mechanical seal faces.
• O-rings and elastomers.
• Gaskets.
• Wear rings, bushings and internal components.

Mechanical seals need special attention

A seal has several material combinations working together. The face materials, elastomers, springs and secondary seals must all be compatible with the liquid and operating temperature. A chemically compatible casing does not automatically mean the complete pump is chemically compatible.

Ask for actual operating conditions

For reliable selection, provide the chemical identity, concentration range, minimum and maximum temperature, density, viscosity, solids content if applicable, required flow and required pressure or head.

Corrosion can also change during operation

Mixing, dilution, contamination or concentration changes can alter the fluid. If the process can vary, the pump should be checked against the full expected operating envelope rather than one nominal condition.

Corrosion resistance is a system-level compatibility question. The correct pump is one where the hydraulic design and every wetted component are suitable for the actual process conditions.

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High-temperature liquid service can create challenges that are not obvious from the pump's flow and head requirement. The temperature can change the fluid properties while simultaneously placing additional demands on the pump materials and sealing system.

Vapor pressure is critical

As liquid temperature rises, vapor pressure generally increases. This can reduce the suction pressure margin and increase the risk of flashing or cavitation. A system that works correctly when cold may therefore behave differently once the liquid reaches process temperature.

Viscosity can change

Many liquids become less viscous when heated. This can change pump performance and pipe friction. The pump should be evaluated using the properties at the actual operating temperature rather than assuming water-like behavior.

Seals and elastomers

Mechanical seal faces, elastomers, gaskets and other components have temperature limits. The temperature at the seal may also differ from the bulk liquid temperature depending on the pump design, circulation arrangement and heat generation.

Thermal expansion

Piping and pump components expand as temperature changes. Poorly supported or constrained pipework can transfer forces into the pump casing and contribute to alignment or mechanical problems.

Before selecting a pump

• Specify normal and maximum liquid temperature.
• Determine vapor pressure at operating temperature.
• Confirm viscosity and density at operating temperature.
• Check NPSHa against NPSHr.
• Confirm casing, shaft, seal and elastomer compatibility.
• Consider thermal expansion and piping support.
• Check the motor and bearing operating limits.

High-temperature pumping is not simply “normal pumping at a hotter temperature.” Temperature can change both the fluid and the equipment conditions at the same time.

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Shear-sensitive liquids contain structures or components that can be damaged or changed by excessive mechanical shear. Depending on the product, this can affect texture, viscosity, particle size, emulsion stability or other quality characteristics.

Where shear can occur

Shear can be introduced by high-speed rotating components, narrow clearances, valves, restrictions and high-velocity flow. The amount of shear experienced by a product depends on the complete pumping system rather than the pump alone.

Pump selection matters

Different pump designs expose liquids to different flow patterns and mechanical conditions. Positive displacement pumps, centrifugal pumps and specialty sanitary pumps can behave differently with the same product. The appropriate design depends on the product and the required duty.

Speed is important

Reducing pump speed can reduce the severity of some shear mechanisms, but it does not automatically make a pump suitable for a shear-sensitive product. The pump's internal geometry, clearances and operating point also matter.

Examples of products where shear may matter

• Emulsions.
• Some dairy and food products.
• Products containing fragile particles or cells.
• Polymers and certain chemical formulations.
• Products where texture or structure is part of the finished specification.

What should be discussed with the pump supplier?

Describe the product, its viscosity range, temperature, solids or particle content, acceptable product degradation, required flow, pressure and expected operating cycle. If laboratory or production testing is available, actual product behaviour should be used where possible.

For shear-sensitive products, the question is not only “Can the pump move it?” but also “Can the pump move it while maintaining the required product quality?”

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In a centrifugal pump, the impeller transfers energy to the liquid. Changing the effective impeller diameter changes the hydraulic performance of the pump, so impeller trim is an important part of pump selection and performance adjustment.

What does “trim” mean?

For many centrifugal pumps, the impeller can be supplied or machined to different diameters within the manufacturer's allowable range. A smaller impeller generally produces less head at a given flow than a larger impeller operating at the same speed.

Why impeller trim is useful

A pump may be capable of producing more head than the system actually requires. Selecting or trimming the impeller can allow the pump to operate closer to the required duty instead of relying on excessive throttling to control the system.

Trim is not a universal “make the pump smaller” solution

The effect of changing diameter depends on the pump design and operating speed. The manufacturer's pump curves should be used for the actual impeller diameter rather than relying only on simplified affinity-law estimates.

What should be checked?

• Required flow and total dynamic head.
• Available impeller diameters for the specific pump.
• Efficiency at the required duty point.
• Motor power and operating margin.
• NPSHr at the operating point.
• Whether the pump remains within its recommended operating range.

Impeller trimming during repairs

Removing material from an impeller during a repair should not be treated as a simple machining operation. The resulting diameter, balance, clearances and hydraulic performance need to remain suitable for the intended duty.

Impeller trim should be selected from the actual pump performance requirements, not simply chosen by eye or by diameter alone.

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Positive displacement pumps, including many gear pumps, move a defined volume of liquid for each revolution. Unlike a centrifugal pump, closing or severely restricting the discharge does not simply make the pump stop producing flow. Pressure can continue to rise until something in the system relieves it or fails.

What does a PRV do?

A pressure relief valve (PRV) provides a controlled path for liquid when the pressure reaches its set point. On a properly designed system, the valve protects the pump and associated equipment from excessive pressure.

The relief path must be correctly arranged

For many positive displacement pump applications, the relief arrangement needs to protect the pump itself. The exact configuration depends on the pump, liquid and system design. A relief valve located somewhere else in the plant may not provide adequate protection for every possible blocked-flow condition.

Important considerations

• Relief valve set pressure.
• Maximum allowable working pressure of the pump and pipework.
• Liquid temperature and viscosity.
• Relief flow capacity.
• Whether the liquid can safely return to the suction side.
• Potential heat generation if liquid is recirculated for an extended period.

A PRV is not normally a flow-control valve

A relief valve is intended to protect the system from excessive pressure. It should not automatically be treated as the normal method of controlling pump flow unless the system has specifically been designed for that operating mode.

Never assume the pump is protected

When commissioning or modifying a gear-pump installation, identify all possible blocked-discharge conditions and verify that the protection arrangement responds at an appropriate pressure. The pump, relief device and downstream equipment should be evaluated as one pressure-containing system.

With a positive displacement pump, a blocked discharge can become a pressure problem very quickly. Correct relief protection is therefore a fundamental part of the installation.

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Pump seals are often treated as a replacement part, but the sealing system is an engineered component of the pump. A seal that is correct for one liquid and operating condition may be unsuitable for another.

Common sealing approaches

• Mechanical seals: use controlled contact between seal faces and are widely used across industrial pump applications.
• Packing: uses compressed packing material around the shaft or sleeve and may be suitable for certain services where controlled leakage is acceptable.
• Magnetic or canned arrangements: can provide sealless containment for selected applications.

What determines seal selection?

Seal selection should consider liquid chemistry, temperature, pressure, shaft speed, shaft diameter, seal chamber conditions, solids content, lubrication and the required leakage performance.

Mechanical seal faces

Mechanical seals commonly use two mating faces with carefully controlled flatness and surface finish. The combination of materials is important because one face must operate against the other under the actual liquid and temperature conditions.

Elastomers are just as important

O-rings and secondary seals must be chemically and thermally compatible. A face material can be suitable while an elastomer fails because of temperature, chemical attack or swelling.

Why seals fail

• Dry running or inadequate lubrication.
• Excessive temperature.
• Pressure outside the seal's intended range.
• Incorrect face or elastomer material.
• Shaft movement, runout or poor alignment.
• Contamination or abrasive solids.
• Incorrect installation or damaged components.

Investigate the failure, don't only replace the seal

A failed seal contains useful evidence. Face appearance, wear patterns, deposits, heat damage, elastomer condition and shaft condition can help identify the actual failure mechanism. Simply installing another seal without correcting the underlying condition can lead to another failure.

Seal life is strongly influenced by the operating environment. The best seal is the one selected for the complete application—not merely the one that fits the shaft.

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In a pump set with a separate motor and coupling, correct alignment is essential. Even a newly installed pump can develop alignment problems if the base, pipework, temperature or mounting conditions change after installation.

Two important alignment conditions

Angular misalignment occurs when the shafts are not pointing in the same direction. Parallel or offset misalignment occurs when the shaft centrelines are displaced from one another. Real installations can have both at the same time.

Soft foot

Soft foot occurs when one or more motor feet do not sit correctly on the mounting surface. Tightening the hold-down bolts can distort the motor frame and change the shaft position, making alignment measurements misleading.

Pipe strain can change alignment

A pump may be aligned correctly before the piping is connected and then move when the pipework is forced into position. Thermal expansion can also change equipment position during operation. Alignment should therefore be checked under the actual installation conditions.

Symptoms of alignment problems

• Repeated coupling failures.
• Excessive vibration.
• Premature bearing wear.
• Mechanical seal leakage or shortened seal life.
• Abnormal shaft or coupling temperatures.
• Unexpected movement or noise.

Use the correct measurement method

Dial indicators, laser alignment systems and other suitable methods can be used depending on the installation. The important point is to follow the equipment manufacturer's tolerances and account for any required thermal growth or operating offsets.

Alignment is not a one-time event

After installation, maintenance work, bearing replacement, coupling changes, piping modifications or significant thermal changes, alignment should be considered again. Base condition and pipe supports should also be checked if alignment repeatedly changes.

Good alignment starts with a good base, correct pipework and a controlled measurement process.

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“The pump is not performing” can describe many different problems: low flow, low pressure, excessive power, vibration, unstable operation or failure to reach the expected duty point. A structured investigation prevents unrelated symptoms from being treated as the cause.

Start with measurements

Record flow, suction pressure, discharge pressure, liquid temperature, pump speed and motor current where practical. Compare these values with the pump's selected duty and the manufacturer's performance data.

Check the suction side

• Is the suction valve fully open?
• Is the strainer clean?
• Is the liquid level adequate?
• Is there excessive suction piping loss?
• Could air or vapor be entering the pump?
• Is the liquid temperature different from the original design condition?

Check the discharge system

Confirm that valves are in the correct position and that the discharge line has not become restricted. A changed downstream process, blocked filter, control valve problem or modified pipework can move the system operating point.

Check the pump itself

If the system conditions are correct, inspect the pump for wear, damaged components, incorrect rotation, internal blockage, excessive clearances or other mechanical problems. For centrifugal pumps, impeller condition and diameter can have a major influence on performance.

Check the driver

Confirm the actual pump speed. A motor running below the intended speed, a variable-speed drive setting or an incorrect motor connection can change the pump's performance. Motor current can also provide useful information, but current alone does not prove the hydraulic condition of the pump.

Compare with the pump curve

Plot or estimate the measured duty against the pump curve where possible. If the pump can produce the required head only at a different flow than expected, the system resistance or pump selection may need further investigation.

Don't troubleshoot a pump using pressure alone. Flow, head, speed, liquid properties and suction conditions must be considered together.

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A running motor does not automatically mean that the pump is producing the expected hydraulic performance. If the discharge pressure is low or absent, the investigation should start by confirming what the pump is actually doing and then working systematically through the suction, pump and discharge system.

Start with the basics

• Confirm the pump is rotating in the correct direction.
• Confirm the actual pump speed.
• Check that the pump is correctly primed where priming is required.
• Confirm the suction valve and discharge valve positions.
• Confirm that the pump has an adequate liquid supply.

Check the suction side

A pump cannot produce the expected discharge condition if liquid is not reaching the pump correctly. Inspect the suction pipe, valves, strainers, liquid level and possible air ingress. In systems operating close to vapor-pressure conditions, check for flashing or vapor formation as well.

Check the pump

Internal wear, damaged or incorrectly installed components, excessive clearances, a damaged impeller or other mechanical problems can reduce the pressure the pump develops. A centrifugal pump with an incorrect impeller diameter can also have a significantly different performance from the original selection.

Check the discharge system

Verify the pressure gauge and its location before assuming the pump has no head. Check for open bypasses, incorrect valve positions, a ruptured or bypassed control arrangement, or a system configuration that is allowing the liquid to return without building the expected pressure.

Pressure alone is not enough

Measure flow as well as suction and discharge pressure where possible. A pump operating at zero flow can still develop pressure, while a pump with a system problem may show a pressure reading that does not represent the actual pump duty.

Compare with the original duty

Use the pump curve and commissioning information to compare the actual operating point with the expected one. If the pump is rotating correctly and the suction conditions are good but the measured head remains substantially below the expected value, further inspection of the pump may be required.

A pump that is running but not producing pressure should be diagnosed from measurements—not from the motor running status alone.

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Ammonia (R717) is commonly pumped in refrigeration systems under conditions where pressure and temperature are closely related. Because ammonia can be near its saturation condition, a pressure drop in the suction system can cause some of the liquid to vaporise.

Flashing versus cavitation

Flashing and cavitation are related but should not automatically be treated as identical. Flashing refers to the liquid changing into vapour when the local pressure reaches conditions that permit vapour formation. Cavitation generally describes vapour bubbles forming and subsequently collapsing as pressure conditions change through the pump.

Where can flashing occur?

• Across a restriction or partially closed valve.
• Through an undersized or blocked suction strainer.
• In an undersized suction pipe.
• Where excessive suction-line friction reduces pressure.
• When liquid temperature is too high for the available pressure.
• When vessel or receiver conditions provide insufficient suction pressure.

Why flashing is a problem for a pump

A centrifugal or other liquid pump is intended to receive a suitable continuous liquid supply. Vapour entering the pump can reduce the effective liquid flow, cause unstable operation and contribute to noise, vibration and loss of hydraulic performance.

What should be measured?

For an R717 troubleshooting investigation, record suction pressure, discharge pressure, liquid temperature, pump speed, flow where available, vessel or receiver conditions and the condition of the suction piping. These measurements should be considered together rather than interpreting pressure or temperature in isolation.

How can the risk be reduced?

• Minimise unnecessary suction pressure losses.
• Keep suction piping appropriately sized.
• Keep strainers clean and correctly selected.
• Maintain suitable liquid level and vessel conditions.
• Check liquid temperature against the actual pressure condition.
• Verify NPSHa against the pump's NPSHr where applicable.
• Investigate restrictions before changing pump components unnecessarily.

For ammonia pumps, pressure and temperature must always be considered together. A suction condition that appears acceptable from one measurement may still be close to a phase-change condition when the complete system is evaluated.

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Impellers rotate at significant speed and must be manufactured, repaired and installed with appropriate attention to balance. Even a relatively small mass imbalance can create a much larger centrifugal force as rotational speed increases.

What causes imbalance?

• Uneven machining or casting.
• Material removed from one side during repair.
• Corrosion or erosion.
• Deposits or product build-up.
• Incorrect machining of the impeller.
• Damage from foreign objects.

Why speed matters

The centrifugal force associated with an imbalance increases strongly with rotational speed. This is why balance becomes particularly important for high-speed rotating equipment and why an impeller that appears acceptable visually should not automatically be assumed to be dynamically satisfactory.

What can excessive imbalance cause?

• Increased vibration.
• Additional bearing loading.
• Shaft deflection or stress.
• Coupling problems.
• Mechanical seal movement or shortened seal life.
• Premature wear of other rotating components.

Balancing after repair

If material has been removed or significant repairs have been carried out on an impeller, the component should be evaluated according to the applicable balancing requirements for the equipment. The required balance quality depends on the rotor, speed and applicable engineering standard or manufacturer specification.

Balance is only one part of vibration control

A balanced impeller cannot compensate for poor alignment, bent shafts, bearing damage, pipe strain, resonance, hydraulic instability or other mechanical problems. Vibration should therefore be investigated as a complete system condition.

Correct impeller balance protects the rotating assembly, but good vibration performance requires the entire pump system to be mechanically and hydraulically sound.

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