Chemical Liquid Filter – Part 4: Industry Applications & Common Selection Mistakes

Author: James Sang

Introduction

A Self-priming pump is an improved type of centrifugal pump. Unlike a standard centrifugal pump, which must be completely filled with liquid before starting, a self-priming pump has a built-in liquid storage chamber that keeps a certain amount of liquid inside the pump.

After the first filling, the pump can automatically remove air from the suction pipe and lift liquid from a lower-level tank without using a foot valve or manual priming.

This design makes self-priming pumps ideal for applications where the liquid level is below the pump installation position, such as chemical transfer, wastewater handling, plating systems, and industrial liquid circulation.



1. Basic Working Principle

A normal centrifugal pump can only move liquid when the pump chamber and suction line are already filled with liquid.

A self-priming pump solves this problem by using:

· A built-in liquid storage chamber 

· A gas-liquid separation area 

· A special internal flow design 

When the pump starts, it mixes the stored liquid with the air inside the suction pipe. The pump separates the air and liquid automatically:

· Air is pushed out through the discharge side.

· Liquid returns inside the pump and continues the circulation process.

After the air is completely removed, the pump works like a normal centrifugal pump and continuously transfers liquid.



2. Key Components That Make Self-Priming Possible

1) Liquid Storage Chamber (Priming Chamber)

The enlarged pump body contains a small amount of liquid after shutdown.

Its main functions:

· Stores liquid for the next startup

· Prevents the pump from losing prime

· Allows automatic suction without manual filling

This is the most important part of the self-priming function.



2) Gas-Liquid Separation Chamber

Located above the pump chamber, this area separates air from liquid.

During operation:

· Air rises because it is lighter.

· Liquid falls back because it is heavier.

This continuous separation allows the pump to remove air from the suction pipe.



3) Check Valve (One-Way Valve)

The check valve is installed at the suction side.

Its functions:

· Keeps liquid inside the pump after shutdown

· Prevents liquid from flowing back into the tank

· Allows faster restarting

Without this function, the pump would lose its stored liquid and need to be manually filled again.



4) Impeller and Volute Casing

The impeller and volute work similarly to a standard centrifugal pump.

Their functions:

· Rotate at high speed

· Create centrifugal force

· Mix liquid with air during the priming process

· Provide pressure for liquid transfer



5) Air Discharge Passage

During self-priming:

· Air separated from the liquid is discharged through the outlet.

· Liquid remains inside the pump and returns to the circulation path.

This process repeats until the suction pipe is fully filled with liquid.



3. Step-by-Step Self-Priming Process

Step 1: Initial Liquid Filling

Before the first operation:

1. Add liquid into the pump through the filling port.

2. Close the filling cover.

3. The check valve keeps the liquid inside the pump chamber.

The pump now has enough stored liquid for self-priming.



Step 2: Air and Liquid Mixing Process

When the motor starts:

· The impeller begins rotating.

· The stored liquid is pushed into the volute casing.

· A low-pressure area is created at the suction side.

· Air from the suction pipe is pulled into the pump.

Inside the pump:

· Air mixes with liquid.

· A gas-liquid mixture is created.

· The impeller continues circulating this mixture.



Step 3: Automatic Air Separation

The gas-liquid mixture enters the separation chamber.

Because air and liquid have different densities:

· Air moves upward and exits through the discharge pipe.

· Liquid falls back into the pump chamber.

The liquid then returns to the impeller and repeats the cycle.



Step 4: Complete Priming and Normal Pumping

After several cycles:

· Air inside the suction pipe is completely removed.

· The suction pipe becomes full of liquid.

· The pump changes from self-priming mode to normal centrifugal pumping mode.

At this stage:

· Flow becomes stable.

· Pressure reaches normal operating conditions.

· The pump continuously transfers liquid.



Step 5: Shutdown and Liquid Retention

After stopping:

· The check valve closes.

· Liquid remains inside the pump chamber.

· The pump keeps its prime.

During the next startup:

· No manual filling is required.

· The pump can restart and automatically lift liquid again.



4. Special Features of Plastic Chemical Self-Priming Pumps

For chemical applications, many self-priming pumps use PP or PVDF plastic materials.

Common applications include:

· Electroplating systems

· Chemical processing

· Acid and alkali transfer

· Waste liquid treatment

· Industrial circulation systems

Main Advantages

Corrosion Resistance

The wetted parts are made from chemical-resistant plastics:

· PP (Polypropylene)
Suitable for many acids, alkalis, and general chemical liquids.

· PVDF (Polyvinylidene Fluoride)
Provides higher chemical resistance and better temperature performance.



Typical Self-Priming Height

Most industrial plastic self-priming pumps can achieve:

4–6 meters self-priming height

However, actual performance depends on:

· Liquid temperature

· Liquid viscosity

· Suction pipe length

· Pipe diameter

· Installation conditions

Higher temperature or higher viscosity liquids will reduce self-priming ability.



Avoid Long-Time Dry Running

Although self-priming pumps can run briefly during the air-removal process, they should not operate without liquid for a long time.

Long-term dry running may cause:

· Mechanical seal damage

· Overheating

· Pump failure

The pump should always have enough liquid inside before operation.



5. Simple Explanation

A self-priming pump works like this:

Keep some liquid inside → Start the pump → Mix liquid with air → Separate and remove air → Fill the suction pipe with liquid → Pump normally

The built-in storage chamber and gas-liquid separation design allow the pump to remove air automatically and start pumping without repeated manual filling.

Because of this advantage, self-priming pumps are widely used for:

· Tank unloading

· Chemical transfer

· Waste liquid collection

· Plating systems

· Applications with unstable liquid levels

A self-priming pump is the ideal solution when the liquid source is lower than the pump installation position and reliable automatic suction is required.