A Comprehensive Guide to Pump Basics

"The knowledge surrounding pumps is vast," said Han Yuanfu, Chairman of Dayuan Pump Industry, in an interview. Pumps are needed wherever liquid flows; even the pacemaker installed in the human heart is another form of pump. There are nearly 50,000 types of pumps worldwide, and one-third of all energy is consumed by various pumps. Therefore, water pumps are closely related to everyone and every household, making it very necessary for ordinary people to understand the basics of pumps.

I. Basis for Pump Classification

(I) Working Principle

1. By Working Principle: Can be further divided into dynamic pumps, positive displacement pumps, and other types.

   • Dynamic Pumps: Rely on the dynamic action of rotating impellers to continuously transfer energy to the liquid, increasing its kinetic energy (primary) and pressure energy. This energy is then converted into pressure energy in the volute or diffuser. They can be further classified as centrifugal pumps, axial flow pumps, partial flow pumps, and vortex pumps, etc.

   • Positive Displacement Pumps: Rely on the periodic change in the volume of an enclosed space containing the liquid to periodically transfer energy, increasing the liquid's pressure until it is forcibly discharged. Based on the motion of the working element, they can be classified as reciprocating pumps and rotary pumps.

   • Other Types of Pumps: Transfer energy in other forms. For example, jet pumps rely on a high-velocity jet of working fluid to entrain the fluid to be pumped, mixing and transferring energy through momentum exchange; hydraulic ram pumps use water hammer effects to lift part of the water to a certain height, transferring energy; electromagnetic pumps cause通电的液态金属 (lit. electrified liquid metal) to flow under electromagnetic force to achieve transportation. Additionally, pumps can also be classified by the nature of the liquid being handled, drive method, structure, application, etc.

2. By Number of Impellers:

   • Single-Stage Pump: Has only one impeller on the pump shaft.

   • Multi-Stage Pump: Has two or more impellers on the pump shaft. The total head of the pump is the sum of the heads generated by each of the 'n' impellers.

3. By Working Pressure:

   • Low-Pressure Pump: Pressure below 100 meters water column.

   • Medium-Pressure Pump: Pressure between 100 and 650 meters water column.

   • High-Pressure Pump: Pressure above 650 meters water column. (Multi-stage centrifugal pumps can reach up to 2800m)

4. By Impeller Inlet Design:

   • Single Suction Pump: The impeller has only one inlet.

   • Double Suction Pump: The impeller has inlets on both sides. Its flow rate is roughly double that of a single suction pump and can be approximated as two single-suction impellers placed back-to-back.

5. By Casing Split Type:

   • Horizontally Split Pump: The joint is on a horizontal plane passing through the axis of the shaft. (The most common horizontally split pump is the double suction pump).

   • Vertically Split Pump: The joint is perpendicular to the axis of the shaft.

6. By Shaft Orientation:

   • Horizontal Pump: The pump shaft is in a horizontal position.

   • Vertical Pump: The pump shaft is in a vertical position.

7. By Method of Directing Water from Impeller to Discharge Casing:

   • Volute Pump: Water leaving the impeller directly enters a pump casing with a spiral-shaped volute.

   • Diffuser Pump: Water leaving the impeller enters guide vanes set outside it before entering the next stage or the discharge pipe. (Commonly used in multi-stage pumps and axial flow pumps).

(II) Operating Principle (Centrifugal Pump Example)

An impeller composed of several curved blades is housed inside a pump casing with a volute channel. The impeller is secured to the pump shaft, which is connected to an electric motor and can be rotated by it. The suction inlet is located at the center of the pump casing and connects to the suction pipeline, equipped with a foot valve (non-return valve) at the bottom. The discharge outlet is on the side of the pump casing, connected to the discharge pipeline, and equipped with a regulating valve.

The centrifugal pump can transport liquids mainly by relying on the centrifugal force generated by the high-speed rotation of the impeller, hence the name centrifugal pump.

Centrifugal Pump Operation Process:
Before starting the pump, first fill the pump with the liquid to be delivered.
After starting, the pump shaft drives the impeller to rotate at high speed, generating centrifugal force. Under this force, the liquid is thrown from the center of the impeller towards the periphery, its pressure increases, and it flows into the pump casing at high speed. Inside the pump casing, the continuously expanding flow path causes the liquid's velocity to decrease, converting most of the kinetic energy into pressure energy. Finally, the liquid flows into the discharge pipe at a higher static pressure.
After the liquid is thrown out, a vacuum is formed at the center of the impeller. Driven by the pressure difference between the atmospheric pressure and the pressure inside the pump (negative pressure), liquid enters the pump through the suction line to fill the position of the discharged liquid.

If air exists inside the pump casing when starting the centrifugal pump, because the density of air is much lower than that of the liquid, the centrifugal force generated by the impeller rotation is very small, and the low pressure generated at the center of the impeller is insufficient to create the vacuum needed to suck up the liquid. Thus, the centrifugal pump cannot work. To ensure the pump is filled with liquid before starting, a foot valve is installed at the bottom of the suction pipeline. Additionally, a regulating valve is installed on the centrifugal pump's outlet pipeline for start/stop control and flow regulation.

II. Basic Parameters of Pumps

The basic parameters characterizing the main performance of a pump are as follows:

1. Flow Rate (Q)
   Flow rate is the volume or mass of liquid delivered by the pump per unit of time.
   Volumetric flow rate is denoted by Q, with units: m³/s, m³/h, L/s, etc.
   Mass flow rate is denoted by Qm, with units: t/h, kg/s, etc.
   The relationship between mass flow rate and volumetric flow rate is:
   Qm = ρQ
   Where ρ – density of the liquid (kg/m³, t/m³). For water at room temperature, ρ=1000 kg/m³.

2. Head (H)
   Head is the increase in energy per unit weight of liquid from the pump inlet (pump inlet flange) to the pump outlet (pump outlet flange). It is the effective energy acquired by one Newton of liquid passing through the pump. Its unit is N·m/N = m, representing the liquid column height, simply referred to as meters.

3. Rotational Speed (n)
   Rotational speed is the number of revolutions of the pump shaft per unit time, denoted by the symbol n, unit is r/min.

4. Net Positive Suction Head (NPSH)
   Net Positive Suction Head is the main parameter indicating cavitation performance. It was formerly denoted by Δh in China.

5. Power and Efficiency
   The power of a pump usually refers to the input power, i.e., the power transmitted to the pump shaft by the prime mover, also known as shaft power, denoted by P.
   The effective power, also called output power, is denoted by Pe. It is the effective energy obtained by the liquid delivered from the pump per unit time.
   Since head is the effective energy obtained per unit weight of liquid output from the pump, the product of head, mass flow rate, and gravitational acceleration equals the effective energy acquired by the liquid output from the pump per unit time – i.e., the pump's effective power:
   Pe = ρgQH (W) = γQH (W)
   Where:
   ρ – density of the liquid delivered by the pump (kg/m³);
   γ – specific weight of the liquid delivered by the pump (N/m³);
   Q – flow rate of the pump (m³/s);
   H – head of the pump (m);
   g – gravitational acceleration (m/s²).
   The difference between the shaft power P and the effective power Pe is the power loss within the pump, measured by the pump's efficiency. Pump efficiency is the ratio of effective power to shaft power, denoted by η.

Example:
Flow rate 200 L/s, Head 37.5m, Selected pump model ASP200B, Impeller diameter 360mm, Speed 1450 RPM, Efficiency 87%. Operating point shaft power 84.5kW.
If the speed changes to 1000 RPM, what are the flow rate, head, and power at this condition according to the affinity laws?
N1 = 1450 RPM, N2 = 1000 RPM
Q1= 200 L/s => Q2 = Q1 x N2/N1 = 200 × (1000/1450) = 138 L/s
H1 = 37.5m => H2 = H1 x (N2/N1)² = 37.5 × (1000/1450)² = 17.8m
P1 = 84.5kW => P2 = P1 x (N2/N1)³ = 84.5 × (1000/1450)³ = 27.7kW

III. What is Flow Rate? What symbol is used? How is it converted?

The volume of liquid discharged by the pump per unit time is called the flow rate, denoted by Q. Units: cubic meters per hour (m³/h), liters per second (L/s). Conversions: L/s = 3.6 m³/h = 0.06 m³/min = 60 L/min
G = Qρ, where G is weight and ρ is the specific gravity of the liquid.
Example: A pump has a flow rate of 50 m³/h. What is the weight of water pumped per hour? The specific gravity of water ρ is 1000 kilograms per cubic meter.
Solution: G = Qρ = 50 × 1000 (m³/h · kg/ m³) = 50000 kg / h = 50 t/h

IV. What is Head? What symbol is used? What unit is used? How is it converted to pressure and what is the formula?

The energy obtained per unit weight of liquid through the pump is called head. The pump's head includes the suction lift, approximately equal to the pressure difference between the pump outlet and inlet. Head is denoted by H, unit is meter (m). Pump pressure is denoted by P, unit is Mpa (Megapascal). H = P / ρ. If P is 1 kg/cm², then H = (1 kg/cm²) / (1000 kg/m³) => H = (1 kg/cm²) / (1000 kg/m³) = (10000 kg/m²) / 1000 kg/m³ = 10m.
1 Mpa = 10 kg/cm², H = (P2 - P1) / ρ (P2 = outlet pressure, P1 = inlet pressure).

V. What is NPSH? What is Suction Lift? What are their units and symbols?

When the pump is operating, a certain vacuum pressure at the impeller inlet can cause the liquid to vaporize, forming bubbles. These cavitation bubbles, moved by the liquid, cause erosion on metal surfaces like the impeller, damaging them. This vacuum pressure is called the vaporization pressure. Net Positive Suction Head (NPSH) is the excess energy per unit weight of liquid at the pump suction inlet over the vaporization pressure. Unit is meters, denoted by (NPSH)r. Suction lift is the Required NPSH (Δh): the allowable vacuum degree for the pump to suck liquid, i.e., the allowable installation height of the pump, unit is meters.
Suction Lift = Standard Atmospheric Pressure (10.33 meters) - NPSHr - Safety Margin (0.5 meters)
Standard atmospheric pressure can support a vacuum height of 10.33 meters in a pipeline.
Example: A pump has a Required NPSH of 4.0 meters. Find the suction lift Δh?
Solution: Δh = 10.33 - 4.0 - 0.5 = 5.83 meters

VI. What is Pump Cavitation and its Causes?

1. At a certain temperature, when the pressure is reduced to the vaporization pressure at that temperature, the liquid produces vapor bubbles. This phenomenon of bubble formation is called cavitation.

2. Cavitation Collapse
   The bubbles generated by cavitation, when flowing to a high-pressure area, decrease in volume and collapse. This phenomenon of bubbles disappearing in the liquid due to rising pressure is called cavitation collapse.

3. Causes and Hazards of Cavitation
   During pump operation, if the absolute pressure of the pumped liquid in a local area of the overflow components (usually somewhere slightly behind the inlet of the impeller blades) drops to the liquid's vaporization pressure at the current temperature for some reason, the liquid vaporizes there, producing a large amount of vapor and forming bubbles. When the liquid containing numerous bubbles moves forward into the high-pressure area within the impeller, the high-pressure liquid surrounding the bubbles causes them to shrink rapidly and collapse. At the moment the bubbles collapse, liquid particles fill the cavities at very high speeds, generating intense water hammer effects with very high impact frequencies striking the metal surface. The impact stress can reach hundreds to thousands of atmospheres, and the impact frequency can reach tens of thousands of times per second, potentially piercing the wall thickness in severe cases.

4. Cavitation Process
   The process of bubble formation and collapse in a pump, leading to the destruction of the overflow components, is the cavitation process in pumps. Besides damaging overflow components, cavitation also generates noise and vibration, causes performance degradation, and in severe cases, can interrupt the liquid flow, preventing normal operation.

VII. What is the Pump Characteristic Curve?

The curve representing the relationship between the main performance parameters is usually called the centrifugal pump's performance curve or characteristic curve. Essentially, it is the external manifestation of the liquid movement laws inside the pump, obtained through actual measurement. Characteristic curves include: Flow Rate vs. Head curve (Q-H), Flow Rate vs. Efficiency curve (Q-η), Flow Rate vs. Power curve (Q-N), Flow Rate vs. NPSH curve (Q-(NPSH)r). The purpose of the performance curve is that for any flow rate point on the pump, a corresponding set of head, power, efficiency, and NPSH values can be found on the curve. This set of parameters is called the operating condition or operating point. The operating point with the highest efficiency is called the Best Efficiency Point (BEP), which is generally the design point. Usually, the rated parameters of a centrifugal pump coincide with or are very close to the design point and the BEP. Operating within the high-efficiency range on the curve is energy-efficient and ensures normal pump operation, thus understanding pump performance parameters is crucial.

VIII. What is Pump Efficiency? What is the formula?

It refers to the ratio of the pump's effective power to its shaft power. η = Pe / P
The pump's power usually refers to input power, i.e., the power transmitted to the pump shaft by the prime mover, also called shaft power, denoted by P.
Effective power is: the product of the pump's head, mass flow rate, and gravitational acceleration.
Pe = ρg QH (W) or Pe = γQH / 1000 (KW)
ρ: density of the liquid delivered by the pump (kg/m³)
γ: specific weight of the liquid delivered by the pump γ = ρg (N/m³)
g: gravitational acceleration (m/s)
Mass flow rate Qm = ρQ (t/h or kg/s)

IX. What is a Pump Comprehensive Performance Test Bed?

A comprehensive performance test bed is equipment that can accurately test all performance parameters of a pump through precision instruments. The national standard accuracy is Grade B. Flow rate is measured with a precision turbine flow meter, head with a precision pressure gauge, suction lift with a precision vacuum gauge, power with a precision shaft power meter, and speed with a tachometer. Efficiency is calculated based on measured values: n = (Pe / P) * 100%.

About Dayuan Pumps

Zhejiang Dayuan Pump Industry Co., Ltd. (Stock Code: 603757), founded in 1990, is a professional pump manufacturer and water treatment solution provider integrating R&D, manufacturing, sales, and service.

As a pioneer in submersible pumps, Dayuan is also one of the drafters of national and industry standards for pump products and is recognized and designated as a "High-Tech Enterprise" and "Provincial Level Enterprise Technology Center" by the state.

The company holds up to 326 patents and offers 7 major categories, 31 series, and over 2000 product models, widely applicable in agricultural irrigation, domestic water supply, industrial applications, municipal water supply and drainage, wastewater treatment, fire protection, and other industries and fields. In the "Made in China 2025" strategy, Dayuan has demonstrated excellent performance in aerospace, large-scale petrochemical, nuclear power, and other projects. Its proprietary brand "Dayuan" products have passed CE, GS, TUV, ROHS, and other quality and product certifications. It has over 10,000 sales and service outlets nationwide and exports to more than 100 countries and regions including Southeast Asia, Africa, the Middle East, South America, and Europe.