Water Pumps: Types and Selection

INTRODUCTION

A water pump is a machine that converts energy (electrical or otherwise) into hydraulic energy. In doing so, it generates pressure that allows a liquid to be moved from an origin point to a destination, overcoming gravity, pipe friction, and any required delivery pressure.

There are two main operating principles:

• Centrifugal Motion: A rotating impeller throws the liquid outward by centrifugal force, generating velocity that is converted into pressure. These are the most common in domestic and industrial use.

• Positive Displacement: They trap a fixed volume of liquid and mechanically push it. Ideal when very high pressure or precise flow rates are required (diaphragm, piston, gear pumps).

TYPES OF WATER PUMPS

Each application requires a particular type of pump. Knowing them allows for the correct decision to be made from the outset. Below are some common pump types:

Single-Stage Centrifugal

A single impeller. Ideal for medium flow rates with moderate pressures. Widely used in simple domestic and industrial systems.

Multi-Stage Centrifugal

Several impellers in series. Generates high pressures with relatively low flow rates. Perfect for multi-story buildings and high-pressure hydropneumatic systems.

Jet

Uses an ejector to lift water from shallow wells (less than 7 m). Very common in homes with a well or underground cistern.

Peripheral

High pressure, low flow. Suitable for small domestic applications where space and cost are critical factors.

Submersible

Operates completely underwater. Ideal for deep wells, basement drainage, wastewater pumping, and swimming pools.

Twin Impeller

Greater efficiency at high flow rates. Common in municipal systems, agricultural irrigation, and large-scale industrial applications.

Diaphragm

Pumps liquids with suspended solids or aggressive substances. It has no mechanical seals in contact with the liquid.

KEY CONCEPTS YOU SHOULD KNOW

TOTAL DYNAMIC HEAD (TDH)

TDH (Total Dynamic Head) is the total head that the liquid must overcome to reach a specific point. It is calculated with Bernoulli's equation:

It is measured in meters of water column (m.w.c.), psi, bar, or kPa. A practical estimate for simple domestic installations is to sum the height to be overcome plus 10% for friction losses, plus the required delivery pressure.

NPSH – NET POSITIVE SUCTION HEAD

NPSH (Net Positive Suction Head) describes the minimum pressure available at the pump inlet to prevent cavitation.

• Required NPSH: Established by the manufacturer. Minimum pressure the liquid must have when entering the impeller.
• Available NPSH: Calculated by the designer based on the installation. Depends on: liquid type, temperature, altitude, suction height, and suction losses.

    Available NPSH = Pa – Pv + H_suction – Hf

CAVITATION

Occurs when the pressure at the pump inlet drops below the liquid's vapor pressure, forming bubbles that violently implode. It produces noise (as if pumping stones), vibration, loss of performance, and severe damage to the impeller and volute.

FLOW RATE AND SIMULTANEITY FACTOR

Flow rate (volumetric flow) is the amount of liquid passing through a pipe section per unit of time. It is expressed in lpm, gpm, m³/h, etc.

In installations with multiple outlets, not all operate at the same time. The simultaneity factor corrects for this statistical excess:

    Q = Q_max / √(N – 1)

Where:

    Q_max = sum of flow rates of all appliances (lpm)

    N     = total number of appliances or taps

 

PERFORMANCE CURVE

Every manufacturer provides an H-Q curve (pressure vs. flow rate). The higher the flow rate, the lower the pressure. The operating point is the intersection between the pump curve and the system curve. Selecting a pump whose operating point is close to its highest efficiency zone reduces electricity consumption and extends its useful life.

HOW TO SELECT THE CORRECT PUMP

QUESTIONS YOU MUST ANSWER FIRST

✓ What is the application? (filling tank, pressurizing network, draining wastewater, swimming pool)

✓ What type of water will be pumped? (clean, wastewater, with solids, seawater, hot)

✓ Where does the water come from? (positive suction from elevated tank, or negative from well)

✓ What is the height difference between the source and the highest point of use?

✓ How much flow rate do I need? (number and type of appliances that will be operating)

✓ What delivery pressure do I require at the points of use?

✓ What electrical power is available? (voltage, number of phases)

EXAMPLE: PUMP FOR A TWO-STORY HOUSE

Appliance allowance:

A pump is sought in the catalog that, at 13 gpm, delivers at least 26.6 m of pressure.

Do you need advice for your project?

At HIDROTECO, we have specialists in water systems, pumping, and treatment for residential, commercial, and industrial projects in Costa Rica.