3.1 Pumps: Types & Principles

Introduction to Water Distribution Pumps

Pumps are the heart of any water distribution system, providing the necessary energy to move water from sources to treatment facilities, and ultimately to the consumers. Understanding how pumps operate, their components, and their applications is essential for any water distribution operator. There are two primary categories of pumps used in the water industry: centrifugal pumps and positive displacement pumps.

Centrifugal Pumps

Centrifugal pumps are by far the most commonly used pumps in water distribution. They operate on a simple principle: a spinning impeller imparts kinetic energy (velocity) to the water, which is then converted into pressure (head) as the water exits the pump. Because they have few moving parts, centrifugal pumps are highly reliable and can handle large volumes of water.

Key Components of a Centrifugal Pump

1. Impeller: The rotating component that transfers energy from the motor to the water. Impellers can be open, semi-open, or closed. Closed impellers are typically used for clean water because they are the most efficient. Open impellers are more suited for raw water with solids, as they are less likely to clog.
2. Volute: The spiral-shaped casing surrounding the impeller. As water leaves the impeller, it enters the volute, which gradually increases in cross-sectional area. This expansion slows the water down, converting its kinetic energy (velocity) into potential energy (pressure).
3. Shaft: The metal rod that connects the motor to the impeller, transmitting the rotational force.
4. Shaft Sleeve: A replaceable metal tube that slides over the shaft to protect it from wear, especially where the shaft passes through the packing or mechanical seal. Without a shaft sleeve, the constant friction from the packing would quickly destroy the expensive shaft.
5. Packing and Mechanical Seals: These components prevent water from leaking out of the pump casing along the rotating shaft.
   • Packing: Consists of braided rings of material (like graphite or Teflon) compressed around the shaft sleeve by a packing gland. A slight leak (a few drops per minute) is absolutely necessary to lubricate and cool the packing. If tightened too much, the packing will burn, scoring the shaft sleeve.
   • Lantern Ring: Often used in conjunction with packing, a lantern ring (or seal water ring) distributes external seal water evenly around the shaft to provide cooling and prevent air from being sucked into the pump when operating under a suction lift.
   • Mechanical Seals: Two highly polished faces (one stationary, one rotating) that are pressed together by springs. They prevent almost all leakage and require less maintenance than packing but are more expensive to replace. Mechanical seals must never be run dry, as the friction will destroy the faces in seconds.
6. Wear Rings: Replaceable rings installed on the impeller (impeller wear rings) and/or the casing (casing wear rings). They provide a tight clearance (often measured in thousandths of an inch) to minimize water bypassing from the high-pressure discharge side back to the low-pressure suction side. They are designed to act as the sacrificial wear point, protecting the more expensive impeller and casing from internal erosion.
7. Bearings: Support the shaft and reduce friction. They can be lubricated by oil or grease. Over-lubrication is a common operational error that can cause bearings to overheat and fail prematurely.

Positive Displacement Pumps

Unlike centrifugal pumps, which add continuous kinetic energy, positive displacement (PD) pumps capture a specific volume of fluid and force it through the discharge pipe. They deliver a constant flow rate regardless of the system pressure, meaning they must never be operated against a closed discharge valve, as this can easily rupture piping or destroy the pump.

Types of Positive Displacement Pumps

1. Diaphragm Pumps: Use a flexible membrane (diaphragm) that pulses back and forth. They are commonly used as chemical feed pumps (e.g., dosing sodium hypochlorite or fluoride) because they can accurately deliver small, precise volumes of highly corrosive chemicals without exposing metallic pump internals to the fluid.
2. Piston Pumps: Use a piston moving back and forth within a cylinder. As the piston pulls back, it draws water in through a suction valve; as it pushes forward, it forces water out through a discharge valve.
3. Rotary Pumps: Use rotating gears, lobes, or screws to trap water and move it from the suction to the discharge side.

Pump Curves and Efficiency

A pump curve is a graphical representation of a pump's performance provided by the manufacturer. It is a critical tool for operators. It shows the relationship between the pump's flow rate (capacity) on the horizontal axis and the head (pressure) it can overcome on the vertical axis.

• Head-Flow Curve: Typically, as the head (resistance) increases, the flow rate decreases. When a pump is operating against an open pipe with no resistance, it achieves its maximum flow (runout). When operating against a closed valve, it achieves its maximum pressure but zero flow (shutoff head).
• System Curve: A curve plotted on the same graph that represents the actual friction and static head of the piping network at various flow rates. The point where the pump curve and the system curve intersect is the exact operating point of the pump.
• Efficiency Curve: Shows how efficiently the pump converts electrical energy into water movement at different flow rates. The highest point on this curve is the Best Efficiency Point (BEP). Operating a pump at or near its BEP minimizes energy costs, reduces wear, and limits vibration.
• Horsepower Curve: Displays the power required to operate the pump across its range of flows.

Operators must understand pump curves to troubleshoot performance issues. For example, if a pump is delivering less flow than expected but the system head is normal, the impeller or wear rings may be worn. If operators need to permanently change the performance of a pump, they can utilize the Affinity Laws to predict changes when modifying the impeller diameter or motor speed.