Centrifugal pumps: startup, key factors and types of failure

In one line

A centrifugal pump is damaged more often by a bad startup or a badly done shutdown than by natural wear — and most of its failures fall into one of a few recognizable categories, if you know where to look.

What they are

Pumps are classified into two large groups by how they deliver energy to the liquid:

• Positive displacement: they add energy discontinuously and raise the liquid's pressure until it is displaced.
• Dynamic (centrifugal): they add energy continuously, increasing the liquid's velocity inside the pump.

Centrifugal pumps dominate industry because of their much lower cost, continuous flow, wide range of models for different services, mechanical simplicity and lower maintenance cost compared with positive-displacement pumps.

How they work

They consist of a rotating part (the impeller), which transfers kinetic energy to the fluid by giving it velocity, inside a static part (the casing), where that velocity is converted into the pressure rise that finally pushes the liquid along.

Startup and shutdown

Before and during startup:

• Check that the pump is full of product
• Check that the cooling valves are open
• Start with the discharge closed and open it slowly afterward
• With very hot products, preheat the pump to avoid thermal shock

After startup:

• Watch the seal's auxiliary systems
• Check the motor's current draw
• Verify that suction and discharge pressures are within range
• Do not let it run at minimum flow or at the edge of its curve
• Watch for proper lubrication and top up the level
• Keep temperatures regulated

The four factors you always need to watch

For this family of equipment, four factors account for most problems when neglected: lubrication, cooling, alignment and sealing. They are not four loose checklist items: they are the four points where a bad startup, a bad shutdown or normal wear end up showing up first.

How failures are classified

The same failure can be classified from three different angles, and each one helps think about the problem in a different way:

By how it shows up:

| Type | Definition | Example | |---|---|---| | Catastrophic | Shows up suddenly and completely | Runaway | | Progressive | From the gradual drift of a design or operating parameter | Shaft failure from fatigue | | Infant | Shows up shortly after startup | Poor design of a lubrication pump's coupling | | Periodic | As a consequence of the end of a life cycle | Shaft failure from fatigue due to a manufacturing defect | | Random | Can show up at any time, from conditions the equipment was not designed for | Casing failure when trying to cool it with external water |

By the equipment's situation:

• In operation: when the equipment is running — these are the most common failures (example: cavitation)
• On standby: the equipment is stopped and must start up for an emergency (example: bearings in a standby gearbox, with contaminated oil, that fail when it is put into service)

By dependency:

• Primary: the failure is due to causes inherent to the equipment itself
• Secondary: the failure is due to the breakdown of another piece of equipment

What this classification is for

Knowing whether a failure is infant, periodic or random points directly to the action needed: an infant failure signals a design or assembly problem that should be fixed before it repeats across the rest of the identical pumps in the plant, while a periodic one signals the expected end of a life cycle and calls for adjusting the preventive maintenance interval, not for finding someone to blame. Telling a primary failure apart from a secondary one also avoids the common mistake of repairing the pump without fixing the other equipment that actually caused it to fail.

Benefits

• The startup and shutdown checklist prevents a good share of "infant" or mishandling failures, which are avoidable and do not depend on the equipment's wear
• Watching the four key factors (lubrication, cooling, alignment, sealing) focuses attention where most problems actually originate
• Classifying the failure before repairing it helps decide whether the problem is in the equipment itself, in another related piece of equipment, or in the original design or assembly

Limitations to keep in mind

• The classification helps think through the problem, but it does not replace a root-cause analysis when a failure keeps repeating
• The startup and shutdown checklist only works if the operator follows it every time, not only when someone is supervising
• It does not cover positive-displacement pumps, which work on different principles and have different failure modes

In summary

Most centrifugal pump failures concentrate in a handful of places: a bad startup or shutdown, or a lapse in lubrication, cooling, alignment or sealing. Classifying the failure by how it shows up, what situation the equipment was in, and whether it depends on another piece of equipment is the first step before deciding what to repair and what to prevent.