One of the most common issues in building water supply systems and industrial facilities is a pump running without discharging or pumping water. In many cases, despite the motor running, no water flows from the taps, or the pump operates with abnormal noise and severe vibration. The primary cause of this problem is air entering and becoming trapped inside the pump casing and suction lines.

Pump air priming is a simple yet vital process that bleeds trapped air from the system and completely fills the housing interior with water. In this specialized article from Fidar Energy, we provide a step-by-step guide to bleeding air from various types of residential water pumps and industrial pumping systems.

Why Does Air Get Trapped in a Water Pump?

Centrifugal pumps are designed to move liquids and lack the ability to create a vacuum to displace gases. When the water level in the storage tank drops, the check valve malfunctions, or a minor leak exists in the inlet piping, air gets sucked into the housing.

This air accumulation causes a destructive phenomenon known as cavitation. In fluid engineering, to prevent fluid vaporization and the formation of air bubbles at the suction inlet, a key parameter called Net Positive Suction Head (NPSH) is calculated.

4 Key Signs That Your Pump Needs Air Priming

Before taking action, you can identify air binding in the system by checking for the following symptoms:

• No Water Discharge: The electric motor turns on and rotates, but water pressure is zero and there is no outflow.
• Abnormal Noise: A rattling sound similar to pebbles or crushing nuts inside the housing (directly linked to cavitation).
• Pressure Gauge Fluctuations: The pressure gauge needle fluctuates erratically at very low values or remains fixed at zero.
• Pump Casing Overheating: Continuous pump operation without water flow prevents impeller cooling, causing severe casing overheating.

Step-by-Step Guide to Bleeding Residential Water Pumps

To prime residential pumps (jet, centrifugal, peripheral, etc.), follow these steps carefully and in order:

Complete Power Disconnection: Unplug the pump or turn off the miniature circuit breaker for the pump circuit. Never attempt to loosen the bleed screw while the pump is connected to power.

Relieve Residual Line Pressure: Open one cold water tap in the building—preferably the closest one to the pump or on the lowest floor—to completely drain trapped pressure and water from the lines.

Loosen the Bleed Screw: On top of the pump casing, there is a small hexagonal or brass screw. Carefully loosen this screw using an appropriate wrench.

Fill the Pump Casing with Water: Using a pitcher or funnel, slowly pour clean water into the housing through the bleed screw port. Continue until air bubbles stop escaping and water completely overflows.

Tighten the Screw and Initial Startup: Wrap a small amount of Teflon tape around the bleed screw threads and tighten it securely. Then, reconnect the power supply and test the pump's operation.

How to Bleed Industrial Pumps and Booster Sets

In large-scale water supply systems, industrial facilities, and booster pump sets, the air priming process differs due to system scale and fluid types. Automatic air release valves are typically used in these lines; however, manual bleeding remains necessary during initial startup or after major overhauls.

1. Industrial Systems with Standard Electric Motors

In cooling water transfer lines or chiller systems where the pump is coupled to a standard three-phase electric motor, air bleeding is performed via needle valves located on the volute casing. By slightly cracking open the inlet valve while keeping the air release valve open, the system’s hydrostatic pressure completely purges the trapped air.

2. Critical Line Pumps in Oil and Gas Industries

In refinery and petrochemical units handling chemical, flammable, or volatile fluids, the pump is strictly coupled with an explosion-proof electric motor. In these setups, air bleeding is never vented into the open atmosphere; instead, the air release valve outlets are piped directly to flare systems or recovery tanks to ensure complete environmental safety.

Summary

Timely pump priming is the simplest and most effective way to prevent mechanical seal failure and avoid motor winding overheating. If the pump continues to trap air after following these steps, be sure to inspect the check valve condition and check for suction pipe leaks.

The engineering team at Fidar Energy, supplier of industrial and building pumps, is ready to provide technical consultation and answer your queries.