Nothing builds character like fixing the same pump leak for the third time. It’s a frustrating experience, and a sign that the real issue hasn’t been identified yet.

All mechanical seals on centrifugal pumps rely on fluid to maintain a film over the mechanical seal face. This is done intentionally to lubricate and cool the seal. On a flushed seal, this would come from an external source (usually water, though not always). However, on a single mechanical seal (internal or external), it is the fluid in the system that lubricates the seals. If the seal is working correctly, this fluid evaporates off, and you don’t ever see it. However, over time as the seal wears, the fluid over the face increases until you see a drip or two or more.

Excessive leakage should not be ignored and is a sign of failure in the mechanical seal or the internal bearing. These parts can commonly become worn or misaligned from incorrect installation and internal stress. Leaks result in lost production time, contamination risks, and product loss.

In this post, we will discuss where leaks occur, what causes them, and the best practices and solutions for fixing them.

How to Diagnose a Centrifugal Pump Leak?

It’s easiest to spot visible leaks, like dripping or spraying around the shaft, seal housing, casing, or flange joints. Sometimes leaks aren’t as obvious, especially if there is no air gap between the pump head and the motor.

Questions to help diagnose the problem:

1. Where is the visible fluid loss coming from?

• Mechanical seal face.
• Casing.
• Connections.

1. Is vibration or noise present?

Check for:

• Misaligned parts.
• A loose impeller.
• Cavitation.
• Reverse motor operation.
• Insufficient NPSH.
• Restricted suction flow.
• Operation away from Best Efficiency Point (BEP).
• Pressure spikes.

1. When does the leak occur?

At startup:

• Mechanical seal is not properly lubricated (momentary dry running).
• Incorrect pump rotation during startup.
• Initial pressure spike (short vibration surge).

During operation:

• Internal component wear (seal, shaft/sleeve, bearing).
• Misalignment (seal, backplate, impeller) is causing uneven seal seating.
• Improper or not-yet-established water flush to stuffing box.
• Cavitation or vibration.
• Deadheading.
• Reverse motor operation.

During shutdown or after stopping:

• Backflow or reverse pressure.
• Loss of hydraulic pressure on the seal.

1. Is it constant or intermittent?

Constant:

• Wear, damage, or poor sealing.

Intermittent:

• Fluctuations in pressure, temperature, or vibration.

1. Does the leak change with operating conditions?

Higher pressure:

• The sealing system is being overloaded.

Temperature changes:

• Thermal expansion or material compatibility issues in the sealing system.

Speed changes:

• Dynamic effects from shaft behavior and seal hydrodynamics.

1. Has anything changed lately?

Check for:

• Maintenance.
• Process conditions (flow pressure, temperature).
• Alignment or piping modifications.

1. Is the pump operating within its design parameters?

Check for:

• Running too far off BEP.
• Deadheaded or low-flow operation.
• Excessive suction or discharge pressure.
• Reverse operation.

After answering these questions, find the most likely cause of the failure listed below.

Common Causes of Centrifugal Pump Leaks

1. Mechanical Seal Failure

Mechanical seals are the most common leak point in a pump.

A mechanical seal’s primary role is to keep your pump leak-free. They are designed to prevent the pumped product from escaping from the rotating shaft. When they fail, the product will leak from the seal.

This is why it’s important to have an open yoke between the pump housing and the motor. With an open yoke design, if the seal leaks, the leakage will drain onto the floor, which is obvious and easy to clean up. It's recommended to hose down this area during clean-in-place procedures to keep it clean. In the case of a closed yoke pump, if the seal leaks, the space between the pump and the motor fills with the product, which can then spoil and contaminate your product. This is why closed yoke pumps aren’t used in the food and beverage industry.

Let’s discuss the most common causes of mechanical seal failure and solutions.

A. Wear

Over time, wear on your parts naturally occurs, and internal tolerances change. This causes misalignment, cracks, and leaks.

Two overlooked wear points in a centrifugal pump are the shaft and the impeller gaskets. If the shaft is scratched or nicked during cleaning or maintenance, the mechanical seal O-ring may leak.

On locked impeller pumps, gaskets seal the impeller. Some of these gaskets are designed to leak if they fail, warning you of gasket failure to avoid contamination.

B. Misalignment

Misalignment of the mechanical seal or backplate during installation creates uneven pressure on the seal face, preventing proper seal seating. This can sometimes create an unusual noise or vibration but will definitely cause leakage.

Be careful when installing these parts to make sure they are perfectly aligned. Also, replacing the spring is advised when changing the seal.

C. Residue Buildup

Seal faces are precision-machined flat surfaces that require careful handling. Abrasive particles can cause wear on the seal face over time. This creates uneven contact pressure and can shift the seal’s alignment, leading to leakage paths.

When installing a mechanical seal, it’s important to avoid touching the seal face. Doing so can add grit, contaminate the product, and affect its sealing capabilities. Your fingerprint is capable of introducing microscopic contamination that is large enough to disrupt the precision surface of the seal.

As we mentioned, a small amount of the pumped fluid seeps onto the seal, lubricating the surfaces before it dries or drips off. If you’re pumping high Brix fluids, this can lead to crystallization, and those abrasive sugar crystals can wear down the seal face over time. Also, hot and sticky fluids (applications like wort or syrups) are tough on seals. This is why we recommend hosing down the external mechanical seal area during clean-in-place procedures. Another option is switching to internal, DG, or flush seals. These seals have a higher resistance to abrasive particles. A DG seal has a hardened seal seat, and the internal and flushed seals have liquid to wash away the abrasive particles.

It’s important to regularly clean and inspect your seals. Check out the manufacturer’s guidelines for cleaning your specific mechanical seal.

D. Dry Running

The pump must be primed prior to startup, including some self-priming pumps (which require an initial prime). This means that all of the air is removed from the pump and suction line, and that only liquid is present in the system.

If the seal fluid film (lubrication) is compromised, dry running can occur. In most mechanical seals, lubrication isn’t applied externally. Instead, a microscopic fluid film at the interface of the seal faces is what provides lubrication. This fluid film can come from the product being pumped or from an external source supplied through a seal support system, like a flush seal. Dry running causes excessive friction and heat, which will wear, warp and crack the seal face, leading to leakage pathways.

To prevent this from happening, make sure you properly prime the centrifugal pump before starting. You can also change, depending on your pump type, to self-lubricating seals. Also, check for sufficient flow and contamination.

E. Cavitation

Often described as a crackling sound, cavitation is one of the leading causes of centrifugal pump failure.

Cavitation occurs when fluctuations in the pressure of a liquid cause it to drop below and then rise above its vapor pressure, resulting in the formation and collapse of gaseous vapor bubbles. This results in microjets of pressure blasting from the bubbles with enough pressure to damage steel. The constant pressure fluctuations and increased vibrations place stress on the seal housing, leading to premature wear and eventual failure. When this happens, it’s not a good day to be an internal pump part, including a mechanical seal.

To avoid centrifugal pump cavitation, check suction conditions, NPSH, and avoid pumping liquids at boiling temperatures. Read more here.

F. Incompatible Fluid

If the product, cleaning agents, temperature, or pressure are not suited for that seal material, your seal will quickly deteriorate.

Take the time to check if the product and the (CIP) procedure cleaning agents you're pumping are compatible with the seal material. Also, check that the temperature and pressure range are rated for that seal material. That is why we recommend EPDM elastomers over Buna when possible. EPDM withstands heat and caustics better.

Another key point is picking the right seal type for your product. As we mentioned, if you're pumping sugary products, this can lead to crystallization, and those abrasive sugar crystals can wear down the seal face over time.

Take time to complete routine inspections and planned part replacement.

2. Improper Water Flush

The stuffing box sits around the pump shaft and holds the E-style mechanical seal.

It’s designed to be full of flush liquid (usually water) and have a very small flow to lubricate, clean and cool the seal. This stops the seal from leaking. Without proper flushing, the stuffing box will fill with the pumped liquid which is a leakage and contamination problem.

If the water flush is not being utilized, the stuffing box should be removed, and the seal should be converted to a D or DG style seal.

External Causes of Centrifugal Pump Leaks

Not all leaks are seal failures. A lot of leaks get misdiagnosed as seal failure when they’re external failures or system driven failure.

1. Casing and Tri-Clamp Misalignment

Installing the pump casing or connections incorrectly will lead to persistent leaking.

Uneven torque on the pump casing will create uneven gasket compression. This will cause leaks or product spraying around the casing joint. The bolts should be tightened evenly in a cross-pattern to ensure uniform load distribution across the gasket.

The same applies to Tri-Clamp connections. These fittings require precise alignment and even gasket compression to maintain a seal. If they become misaligned, the gasket will not seat properly, causing leaks.

Another cause of misalignment is excessive vibration. Excessive vibration is often caused by pressure spikes (water hammer) or cavitation, which leads to gasket creep, extrusion, and ferrule movement.

This is why proper installation is crucial. Take the time to regularly inspect gaskets and Tri-Clamps for misalignment, as well as checking for excessive vibration during operation.

Operational Errors

Operational errors happen to all of us (we’re only human after all). Here are some common errors that can lead to system failure.

1. Wrong rotation during startup

Running a centrifugal pump in reverse is a fast way to cause damage to your pump. When the shaft spins opposite its intended direction, internal hydraulic performance is compromised, and internal components may become stressed beyond their design limits.

Reverse rotation commonly occurs during installation or maintenance (ex. Replacing a motor, modifying a starter, or adding a variable frequency drive). When electrical connections are configured incorrectly, the motor will rotate in the wrong direction and drive the shaft and impeller backwards.

While the pump is rotating in the wrong direction, it will still move fluid. However, it won’t be able to generate proper pressure or flow, resulting in poor system performance. Reverse operation will also lead to vibration, excessive noise, and cavitation.

After you install or service a motor, it’s important to check the motor’s rotation. A “bump test” (briefly energizing a motor) will give you enough time to verify that the shaft is turning in the right direction.

2. Deadheading

Deadheading is when a pump continues operating without flow. This can be caused by a closed discharge valve, a blockage in the line, or a check valve that fails to open.

Running a centrifugal pump against a closed discharge valve quickly leads to the pump overheating, seal failure, and catastrophic pump damage. Even when properly primed, operating at little to no flow removes the cooling and lubrication that the mechanical seal depends on, which can rapidly lead to seal failure.

The energy from the impeller rotating will begin to heat the recirculating fluid, eventually turning it into vapor. This creates unstable conditions that will damage internal components, such as bushings and mechanical seals.

To prevent deadheading, use a recirculation line from the pump discharge line upstream of the discharge valve back to the pump supply source. Installing flow switches, temperature monitors, or automatic bypass systems can also help protect the pump from deadheading.

We hope this helps explain common causes and reasons why your centrifugal pump may be leaking.

If you have a topic you'd like us to cover, please let us know in the comments!