Centrifugal pumps cannot operate reliably without being filled with liquid before startup. If the suction line or casing contains air, the impeller will only spin the air instead of moving liquid. This process of filling the pump and suction line with liquid is known as priming.
Proper priming is critical to prevent dry running, seal failure, and cavitation damage. Depending on the application and pump design, several priming methods are used.
Manual Priming
Process
- The pump casing and suction line are manually filled with liquid through a fill connection
- A vent valve at the pump discharge allows trapped air to escape during filling
Advantages
- Simple, no additional equipment required
- Low cost
Limitations
- Labour-intensive — not practical for large systems
- Risk of incomplete priming if air is trapped
- Unsuitable for deep suction lifts
Best Used For: Small portable pumps, domestic systems, or situations with short suction lines.
Foot Valve (Retained Priming)
Process
- A foot valve (check valve with strainer) is installed at the end of the suction pipe
- After initial priming, the foot valve holds liquid in the suction line and pump casing, preventing it from draining back when the pump stops
Advantages
- Maintains prime between operations
- Simple, low maintenance if clean water is pumped
Limitations
- Foot valve failures cause pump to lose prime
- Additional suction head losses (affecting NPSHa)
- Not reliable in sandy, silty, or debris-laden water
Best Used For: Agricultural irrigation, shallow well pumping, clean water systems.
Self-Priming Pumps
Process
- Pump casing is designed with a large reservoir that retains liquid after shutdown
- During startup, the impeller churns air and liquid, creating a vacuum that draws water up the suction pipe until fully primed
Advantages
- Fully automatic — no external equipment required
- Can re-prime even if suction line drains
- Widely used in portable and trailer-mounted units
Limitations
- Less efficient than standard end-suction pumps
- Limited to moderate suction lifts (< 8 m)
Best Used For: Construction dewatering, sewage bypass, portable site pumps.
Vacuum-Assisted Priming
Process
- A separate vacuum pump or compressor system evacuates air from the suction line and pump casing
- Once primed, the vacuum system disengages and the centrifugal pump runs normally
Advantages
- Fast, reliable priming even with long suction lines
- Can handle large diameter suction hoses and deep lifts
- Fully automatic with continuous priming capability
Limitations
- Higher capital cost (additional priming system)
- Requires more complex controls and maintenance
Best Used For: Large-scale dewatering, flood control, mine dewatering, sewer bypass pumping.
Jet Ejector Priming
Process
- A small auxiliary pump or pressurised water supply drives a jet ejector
- The ejector creates a vacuum, pulling air out of the main pump suction
- Once liquid fills the suction, the main pump starts operating
Advantages
- No moving parts in the ejector (long service life)
- Works reliably in remote or hazardous environments
Limitations
- Requires auxiliary water supply
- Less common due to availability of compact vacuum-assisted pumps
Best Used For: Specialised industrial systems, remote installations, hazardous liquids.
Priming Tanks & Gravity Priming
Process
- A dedicated priming tank or overhead reservoir is connected to the pump suction
- Gravity keeps the pump casing and suction line filled before startup
Advantages
- Very reliable — as long as tank is maintained
- No moving parts or external priming equipment
Limitations
- Requires space and infrastructure to install priming tank
- Not practical for mobile pumping systems
Best Used For: Permanent industrial or municipal installations.
Comparative Summary
| Method | Automation | Cost | Reliability | Best Applications |
|---|---|---|---|---|
| Manual Priming | Low | Low | Low | Small portable pumps |
| Foot Valve | Medium | Low | Medium | Irrigation, shallow wells |
| Self-Priming Pump | High | Medium | High | Site dewatering, sewage bypass |
| Vacuum-Assisted | Very High | High | Very High | Mining, flood control, sewer bypass |
| Jet Ejector | Medium | Medium | Medium | Industrial, hazardous liquids |
| Priming Tank | High | Medium | Very High | Municipal, industrial plants |
Summary
The choice of priming method depends on system size, mobility, and required reliability. For small domestic or agricultural systems, manual priming or foot valves may suffice. For portable dewatering and sewer bypass, self-priming or vacuum-assisted priming is standard. In large permanent installations, gravity-fed priming tanks offer simple and robust solutions.