Water pump equipment accounts for more than 15% of the total investment in mechanical equipment, and is also the main energy consuming equipment in terms of energy consumption, so it is the main power equipment.

Definition of pump:

A mechanical device that converts mechanical energy into liquid energy and uses it to transport liquids.

2. Classification of water pumps:

Due to differences in the amount of water, distance, head, and medium transported, there are various forms of water pump equipment in sewage treatment plants, mainly divided into blade pumps, positive displacement pumps, and other types of pumps such as screw pumps.

2.1 Vane pump

Blade type water pumps are a large category in water pumps, which rely on the high-speed rotation of the impeller to complete its energy conversion. The different blades in the impeller result in different mass forces acting on the water flow, and the direction of water flow out of the impeller is also different. According to the direction of water flow from the impeller, blade type water pumps can be divided into three types: radial flow, axial flow, and oblique flow. When liquid particles flow in the impeller, they are mainly subjected to centrifugal force, axial lift, and the combined effects of these two forces. Correspondingly, they are called centrifugal pumps, axial flow pumps, and mixed flow pumps.

The main water pump used in wastewater treatment systems is a vane pump. Among them, centrifugal pumps are the most common due to their high efficiency, rapid start-up, stable operation, reliable performance, and easy adjustment. Before starting a centrifugal water pump, both the pump casing and suction pipe must be filled with water. There are also many types of centrifugal pumps, generally classified as follows:

2.2 Positive displacement pump

A positive displacement pump relies on the mechanical motion inside the pump to periodically change the volume of the working chamber, causing suction and compression of the liquid, allowing the liquid to obtain energy, and achieving pressurization and transportation of the liquid. Its forms include: reciprocating (plunger pump and diaphragm pump), rotary (gear pump and screw pump).

2.3 Other types of pumps

Refers to some special types of pumps other than vane pumps and volumetric pumps, such as jet pumps, water hammer pumps, water ring vacuum pumps, etc. The working principles of these pumps are different. For example, jet pumps use gas or liquid to move in a special pipe section (throat), generating negative pressure suction to produce negative pressure and transport liquid; A water hammer pump utilizes the force of water flow released from a high location to generate water hammer pressure when a valve suddenly closes, sending water to a higher position.

Performance parameters of pump 3

According to regulations in our country, the model of a pump generally consists of two parts: numbers and Chinese pinyin. The numbers represent the diameter of the pump's suction port, discharge port, flow rate, head, etc., while the pinyin letters represent the pump's type, structure, etc. The above model parameters are indicated on the equipment nameplate. Due to the fact that most of the pinyin letters for the model, type, structure, etc. of water pumps are determined by the manufacturing enterprises themselves, even if the types of water pumps are the same, the representation methods are not consistent.

The performance of commonly used pumps is generally composed of parameters such as flow rate, head, power, speed, efficiency, suction port diameter, discharge port diameter, and pump impeller diameter. When selecting equipment, the main consideration is the flow rate, head, power, and allowable vacuum suction height of the pump.

3.1 Traffic Q

The amount of water pumped or discharged per unit time, in m3/h or L/s. The flow rate of a vane pump is inversely proportional to the head, while the flow rate of a volumetric pump is independent of the head.

3.2 Head H

The energy increase per unit mass of liquid from the pump inlet to the pump outlet is the pump head, mH2O or Pa.

1atm=1kgf/cm2 (kg)=1bar=760mmHg=10mH2O=105Pa=0.1MPa. 1mH2O=10kPa

3.3 Allow suction of vacuum height Hs

When the pump axis is higher than the liquid level in the water tank, in order to prevent cavitation, the allowable vertical height of the pump axis from the liquid level in the suction tank is the maximum vacuum height allowed at the inlet of the water pump at a standard atmospheric pressure and a water temperature of 20 ℃, in meters. Hs also varies with Q, generally speaking, as Q increases, Hs decreases. When the pump axis is lower than the liquid level in the pool, this item may not be considered.

3.4 Speed z

Refers to the number of revolutions of the pump shaft per unit time, r/min。

3.5 Power and Efficiency

The power of a pump is divided into effective power and shaft power. Effective power, also known as output power, Ne，kW。 Shaft power, also known as input power, N，kW。

The efficiency of a pump is an indicator of the economic efficiency of its operation, also known as the overall efficiency of the pump, η=(Ne/N)*100%。 Efficiency can reflect the utilization of energy in a pump, as there are various energy losses (including mechanical losses, volume losses, hydraulic losses) during operation, and it is impossible to convert all the power input from the prime mover into the effective power of the liquid. The higher the η, the higher the energy utilization rate and the smaller the loss. Generally, η is taken as 0.85-0.9.

4 Centrifugal pumps

4.1 Main components of centrifugal pump

A centrifugal pump is composed of many components. Taking the commonly used single machine single suction centrifugal pump in sewage treatment engineering as an example, the functions, materials, and composition of each component will be explained.

4.1.1 Impeller (also known as working wheel)

The shape and size of the impeller are determined through hydraulic calculations. When selecting impeller materials, mechanical strength, wear resistance, and corrosion resistance should be considered. At present, most impellers are made of cast iron, cast steel, and bronze.

The impeller is divided into single suction and double suction types. Generally, high flow centrifugal pumps use double suction impellers.

Impellers are divided into three types based on their cover plate conditions: closed impellers, open impellers, and semi open impellers. Closed impellers are the most widely used, typically having 6-8 blades, with some up to 12 blades. In general, for sewage pumps containing suspended solids, open or semi open impellers are sometimes used to avoid blockage. These impellers have fewer blades, usually 2-5 blades.

4.1.2 Pump shaft

The pump shaft is used to rotate the impeller, and commonly used materials are carbon steel and stainless steel. The pump shaft requires sufficient torsional strength and stiffness. The impeller and pump shaft are connected by keys. Keys are connecting components between rotating bodies, and flat keys are generally used in centrifugal pumps.

4.1.3 Pump casing

The pump casing of a centrifugal pump is usually cast in the shape of a snail shell, and its water passage part requires good hydraulic conditions.

4.1.4 Pump seat

There are flange holes on the pump seat for fixing with the base plate or foundation. There are screw holes on the top of the pump casing for filling and releasing water, which are used to fill the water and exhaust the air inside the pump casing when the water pump is started. In addition, there are pressure gauge and pressure gauge screw holes, drain screw holes, drain screw holes, etc.

The impeller and pump shaft are the rotating parts of a centrifugal pump, while the pump housing and pump seat are fixed components. There are three connecting parts between the two: the shaft seal device between the pump shaft and pump housing is a packing box, the leakage reduction device at the joint between the impeller and the inner wall of the pump housing is a leakage reduction ring, and the rotating connection device between the pump shaft and pump seat is a bearing seat.

4.1.5 Shaft sealing device

There is a gap between the shaft and the shell, and if no measures are taken, it may lead to liquid leakage (such as single suction centrifugal pumps) or suction into the atmosphere (such as double suction centrifugal pumps). For this purpose, a sealing device called a shaft seal needs to be installed at the gap between the shaft and the shell. At present, the commonly used shaft seal devices include packing seals and mechanical seals.

(1) Packing seal: Packing seal is widely used in centrifugal pumps. In recent years, it has taken many forms, with the most common being the gland packing seal, which consists of five components: shaft sleeve, packing, water seal tube, water seal ring, and gland. Packing, also known as packing, plays a sealing role in the shaft sealing device by blocking water and gas. The commonly used packing is asbestos rope packing soaked in oil or graphite. In recent years, various high-temperature resistant, wear-resistant, and corrosion-resistant packing materials have also emerged, such as fillers woven from carbon fiber, stainless steel fiber, and synthetic resin fiber. The packing is compressed by a pressure cap, and the tightness of the pressure cap should be appropriate. Generally, it is advisable for the water in the water sealed pipe to drip out through the gaps of the packing.

The packing sealing structure is simple and reliable in operation, but its service life is not long, and it cannot guarantee that toxic, corrosive, and valuable liquids will not leak.

(2) Mechanical seals, also known as end face seals, are divided into many types such as unbalanced and balanced types.

4.1.6 Leakage reduction ring

There is a rotating joint between the outer circle of the impeller suction port and the inner wall of the pump casing, which is the interface between high and low pressure and has relative motion, making it prone to leakage. At the same time, wear between the impeller and the pump casing is prone to occur at the gap between the seams. To reduce the backflow of high-pressure water from the pump casing to the suction port and extend the service life of the impeller and pump casing, a metal ring called a leak reducing ring or wear ring is usually embedded at the joint, which is a vulnerable component.

4.1.7 Bearing seat

The bearing seat is used to support the shaft, and the commonly used bearings for water pumps are rolling bearings and sliding bearings. Rolling bearings can be divided into ball bearings and roller bearings according to the load size. According to the load characteristics, it can be divided into radial bearings, thrust bearings, and radial thrust bearings.

4.1.8 Couplings

The output of the electric motor is transmitted to the water pump through a coupling. Couplings, also known as "backrest" wheels, come in two types: rigid and flexible. Rigid couplings are commonly used for connecting small water pump units and vertical pump units. Flexible couplings are generally used for connecting large and medium-sized horizontal pump units.

4.1.9 Axial force balance measures

A single suction centrifugal pump, due to the lack of symmetry in its impeller, exerts unequal pressure on both sides of the impeller during operation. There is an axial force △ p acting on the impeller, which pushes towards the suction port. This axial force, especially for multi-stage single suction centrifugal pumps, is quite large and must be solved by a specialized axial force balancing device. Generally, a balance hole is drilled on the rear cover plate of the impeller and a leakage reducing ring is installed on the rear cover plate. Although this method may reduce the efficiency of the water pump, it is still widely used in general single-stage single suction centrifugal pumps.

4.2 Performance curve of centrifugal pump

For operators of wastewater treatment plants, understanding the performance curve of the water pump is essential for accurate and effective scheduling and management of the pump operation. The performance curve represents the variation relationship between characteristic parameters such as flow rate, head, and power of the water pump. The performance curves of various water pumps can be found in the sample and manual of the pump. After understanding the performance curves, the water pump can be controlled to operate at higher efficiency conditions.

Performance curve drawing of centrifugal pump: Under constant speed n and other conditions, gradually open the pump outlet valve to change the flow rate Q, measure the N, H, and η of the pump at different flow rates Q, and draw the characteristic curve of the pump.

The higher the efficiency η of the water pump, the greater the amount of water pumped under the same power consumption or the lower the power consumption under the same amount of water. Therefore, during operation, while meeting the head requirements, the water pump should be operated within the flow range around the highest efficiency point as much as possible (the operating condition at the highest efficiency point is the optimal operating condition). To achieve this goal, the following measures are often taken: replacing the impeller or cutting the impeller; Change the speed of the water pump; Use valves to regulate water flow. But using the third method will increase the resistance of the suction pipe and waste more energy.

Flow regulation of centrifugal pump:

(1) Change the opening of the outlet valve of the pump: if the valve is opened larger, Q will increase and H will decrease; When the valve is closed, Q decreases and H increases. The characteristic is easy to operate and widely used, but with high energy loss.

(2) Outlet bypass diversion regulation: simple and convenient, but not economical.

(3) Change the pump speed: increase n, and both Q and H will increase. Need to adjust the speed mechanism or replace the motor. (4) Cutting impeller: After the impeller is cut small, both Q and H decrease. But the impeller cannot be restored after cutting.

(5) Series and parallel regulation: Multiple pumps are connected in series, Q remains constant, and H increases (Q total=Q1=Q2, H total=H1+H2); Multiple pumps in parallel, H remains constant, Q increases (H total=H1=H2, Q total=Q1+Q2). In reality, there are losses. Consider the matching of each pump.

4.3 Cavitation and Installation of Centrifugal Pump

The specific reasons for cavitation are as follows:

1) The installation position of the pump is too high above the suction surface, that is, the geometric installation height of the pump is too high;

2) The atmospheric pressure at the installation location of the pump is relatively low, such as in high-altitude areas;

3) The temperature of the liquid conveyed by the pump is too high;

4) The liquid level at the suction port of the collection well is too low, causing both air and water to be sucked in at the same time.

As mentioned above, correctly determining the pressure at the pump suction port (allowing for vacuum suction height) is the key to controlling the pump from cavitation during operation. In practical use, it is necessary to control the relative height Hg between the pump and the suction surface.

4.4 Selection of centrifugal pumps

4.4.1 Types of centrifugal pumps

Clear water pump (IS, D, Sh type): suitable for conveying clean water and cleaning liquids with physical and chemical properties similar to water.

IS type: single-stage single suction cantilever water pump, widely used. Lift 8-98m, flow rate 4.5-360m3/h.

D-type: multi-stage pump, suitable for low flow but requiring high pressure head. Lift 14-351m, flow rate 10.8-850m3/h.

Sh type: Double suction pump, suitable for high flow but requiring low pressure head. Lift 9-140m, flow rate 120-12500m3/h.

(2) Corrosion resistant pump (F-type): suitable for transporting corrosive liquids such as acids and alkalis. The parts in contact with liquids are made of corrosion-resistant materials and generally use mechanical sealing devices. Lift 15-105m, flow rate 2-400m3/h.

(3) Oil pump (Y-shaped): suitable for transporting petroleum products. Special requirements include good sealing performance and a cooling system when transporting high-temperature oil products. Lift 60-600m, flow rate 6.25-500m3/h.

(4) Impurity pump (P-type): suitable for transporting suspensions and slurries, etc. It is also divided into sewage pumps (PW), mud pumps (PN), etc. The special requirements are that the impeller is not easily blocked by impurities, wear-resistant, and easy to disassemble and wash. Therefore, the impeller has a wide channel, fewer blades, and is commonly used in open or semi closed impellers.

4.4.2 Selection of centrifugal pumps

(1) Determine the type of centrifugal pump based on the properties and operating conditions of the transported liquid.

(2) Determine the flow rate and pressure head. After determining the flow rate according to the production task, the Bernoulli equation is used to calculate the required pressure head of the pipeline at the maximum flow rate based on the pipeline arrangement.

(3) Select the pump model. Select the specific model according to (2) and list the various performance parameters of the pump.

(4) Calculate the shaft power. If the density of the conveyed liquid is greater than that of water, the shaft power of the pump should be calculated.

5 submersible pumps

A submersible pump consists of a submersible motor, pump body, outlet bend pipe, and lifting pipe, and is suitable for transporting sewage containing certain impurities. At present, the submersible pumps available in the market are divided into clean water pumps and sewage pumps. Large and medium-sized submersible sewage pumps are commonly used for water inlet lifting and discharge of sludge or residual sludge. Small submersible sewage pumps can be moved and operated at any time, and can discharge various types of accumulated water in tanks.

6 displacement pump

Positive displacement pumps are suitable for working conditions with low flow rate, high head, high impurity content or viscosity of the working medium, as well as for metering and dosing applications.

Diaphragm pumps are divided into two types: pneumatic diaphragm pumps and electric diaphragm pumps.

Metering pumps belong to reciprocating volumetric pumps, which are divided into plunger metering pumps, hydraulic diaphragm metering pumps, and mechanical diaphragm metering pumps.

The characteristics of screw pumps are small fluctuations in flow and pressure, low noise, long service life, self-priming ability, and simple structure. Single, double, triple, and five screw pumps can be designed as needed.