4.3 Preventive Maintenance & Troubleshooting

The Importance of Maintenance

In the water distribution industry, running equipment until it breaks is a dangerous and expensive strategy. Preventive maintenance (PM) involves scheduled tasks designed to keep equipment running smoothly, while predictive maintenance (PdM) uses technology to monitor the health of equipment and predict failures before they happen.

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Pump Sealing Systems

Where a pump shaft enters the pump casing, there is a gap that water would naturally pour out of. To prevent this, pumps use sealing systems, primarily packing rings or mechanical seals.

Packing Glands

Packing consists of braided rings of synthetic fibers (often infused with graphite or Teflon) stuffed into the stuffing box around the shaft. A metal plate called a gland is tightened down to compress the packing against the shaft.

Critical Rule of Packing: Packing must NEVER be tightened so much that it completely stops leaking. The friction between the rotating shaft and the stationary packing generates immense heat. A controlled amount of water leakage—typically 20 to 60 drops per minute—is absolutely necessary to lubricate and cool the shaft. If an operator over-tightens the gland to stop the drip, the packing will burn, scorch the shaft sleeve, and eventually seize the pump.

When adjusting packing, operators should tighten the gland nuts only a fraction of a turn at a time, wait 10-15 minutes for the temperature to stabilize, and check the drip rate again.

Mechanical Seals

Mechanical seals consist of two extremely flat, highly polished surfaces (often made of ceramic, carbon, or silicon carbide). One face is stationary, and the other rotates with the shaft. They are pressed together by a spring.

• Advantage: They do not leak at all under normal operation, keeping the pump room dry and saving water.
• Disadvantage: They are delicate. Because the faces rely on a microscopic film of water for lubrication, running a pump dry for even a few seconds will destroy a mechanical seal. They are also easily damaged by abrasive sand or grit in the water.

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Lubrication and Bearings

Bearings support the rotating shaft of a pump and motor, reducing friction. They require proper lubrication (oil or grease) to survive.

The Dangers of Over-Greasing

A common mistake among inexperienced operators is over-greasing motor bearings. Pumping too much grease into a bearing housing causes several severe problems:

1. Heat Buildup: The churning of excess grease generates massive friction and heat, destroying the bearing.
2. Blown Seals: The pressure from the grease gun can blow out the bearing seals, allowing dirt in.
3. Motor Winding Damage: Excess grease can be forced past the seals directly into the motor windings. The grease degrades the electrical insulation, eventually causing a short circuit and motor failure.

Always follow the manufacturer's guidelines for the exact type of grease, the required volume, and the frequency of application. When greasing, the drain plug on the bottom of the bearing housing should be removed to allow old grease and excess pressure to escape.

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Shaft Alignment

For equipment where a motor and a pump are separate units coupled together (like an end-suction centrifugal pump), alignment is critical.

The centerline of the motor shaft must perfectly match the centerline of the pump shaft. Even a misalignment of a few thousandths of an inch will place severe radial loads on the shafts. This causes the shafts to flex slightly on every rotation, leading to:

• Rapid bearing failure.
• Premature destruction of the mechanical seal or packing.
• High vibration levels.
• Eventual shaft snapping due to metal fatigue.

Alignment is traditionally done with dial indicators, but modern maintenance relies heavily on laser alignment tools which are highly accurate and easier to use.

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Troubleshooting Common Problems

When a pump or motor fails to operate correctly, operators must methodically diagnose the issue.

1. Pump is Running but Delivering No Water

• Loss of Prime: Centrifugal pumps cannot pump air. If the casing is filled with air instead of water, the impeller will just spin. Ensure the pump is fully primed and the suction line is flooded.
• Closed Valve: Check that all suction and discharge valves are actually open.
• Clogged Impeller: Rags or debris may be completely blocking the eye of the impeller.

2. Pump Sounds Like It's Pumping Gravel

This is the classic symptom of cavitation. Cavitation occurs when the pressure on the suction side of the pump drops below the vapor pressure of water. The water literally boils at ambient temperature, forming vapor bubbles. When these bubbles pass into the high-pressure side of the impeller, they collapse violently. This collapse sends microscopic shockwaves that chip away at the metal, eventually destroying the impeller.

• Solution: Increase suction pressure (ensure suction valves are fully open, clean suction screens) or decrease the flow rate by throttling the discharge valve slightly.

3. Excessive Vibration

Vibration is the enemy of rotating equipment. Causes include:

• Misalignment between the pump and motor.
• Imbalance in the impeller (due to wear or lodged debris).
• Failing bearings (often indicated by a high-pitched whine or grinding noise).
• Bent shaft.

4. Motor Trips Immediately on Startup

If the motor overload or breaker trips within seconds of pressing start:

• Electrical Short: There may be a short to ground in the motor windings.
• Mechanical Binding: The pump shaft may be physically locked (seized bearings, jammed impeller). An operator can verify this by locking out the power and attempting to turn the pump shaft by hand.
• Phase Loss: The phase monitor may be preventing the start due to a blown fuse on one leg of the power supply.