Process flow pump issues

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Hello everyone, I have a question I’d like to ask: In the chemical process design manual, I found a classification of pumps, with some of them classified according to the process flow, as shown in the table below! Could an expert please help me explain this? The feed pump (why is it called a feed pump, and where is it mainly used, such as for water injection or oil transfer); the return pump (in what situations is it used, and why is it called a return pump); the circulation pump; the product pump; and the injection pump (in what processes is it used?) For example, an injection pump can be used to inject chemical agents! ) Please help me explain it clearly; I really don’t quite understand it. Table of Characteristics and Selection Requirements for Typical Chemical Process Pumps
Pump Name, Characteristics, Selection Requirements

Feed Pumps (including raw material pumps and intermediate feed pumps)
1. Stable flow rate
2. Generally high head
3. Some raw materials have high viscosity or contain solid particles
4. The inlet temperature of the pump is usually at room temperature, but the inlet temperature of some intermediate feed pumps can exceed 100°C
5. The pump must not stop operating during use
1. Centrifugal pumps are generally chosen
2. For higher heads, positive displacement pumps or high-speed pumps can be considered
3. The pump should have a 100% redundancy rate

Return Pumps (including top, intermediate, and bottom of tower return pumps)
1. Large variation in flow rate, relatively low head
2. The inlet temperature of the pump is not high, usually between 30–60°C
3. High reliability is required
1. Single-stage centrifugal pumps are generally used
2. The pump should have a redundancy rate of 50%–100%

Bottom of Tower Pumps
1. Large variation in flow rate (flow rate is usually controlled by level)
2. High flow rate
3. High inlet temperature, generally above 100°C
4. The liquid is usually in a two-phase state (liquid and vapor), with low NPSHa
5. High reliability is required
6. Harsh operating conditions, with possible sedimentation of dirt
1. Single-stage centrifugal pumps are generally chosen; for high flow rates, double-suction pumps can be used
2. Pumps with low net positive suction head are selected, along with appropriate priming height
3. The pump should have a 100% redundancy rate

Circulation Pumps
1. Stable flow rate, relatively low head
2. A wide variety of media
1. Single-stage centrifugal pumps are chosen
2. The type and material of the pump depend on the nature of the medium
3. The pump should have a redundancy rate of 50%–100%

Product Pumps
1. Low flow rate
2. Low head
3. Low inlet temperature (product at the top of the tower is usually at room temperature; product taken from intermediate stages and from the bottom of the tower has slightly higher temperatures)
4. Some product pumps operate intermittently
1. Single-stage centrifugal pumps are suitable
2. For products with high purity or high value, reliable sealing is required; the pump should have a 100% redundancy rate ； Yes, for ordinary products, the spare rate is 50% to 100%. For pumps that operate intermittently, pumps are generally not used for this purpose. Pump 1: 1. The flow rate is very low, and precise metering is required. 2. It operates at normal temperatures. 3. The discharge pressure is high. 4. The medium injected is a chemical substance, which is often corrosive. 1. Plunger or diaphragm metering pumps should be selected. 2. For corrosive media, the flow-through components of the pump are usually made of corrosion-resistant materials. Sewage pumps: 1. The flow rate is low, and the head is low. 2. Sewage often contains corrosive substances and abrasive particles. 3. Flow rate control is required during continuous operation. 1. Sewage pumps or slurry pumps should be selected. 2. The backup rate for these pumps is 100%. 3. Corrosion-resistant materials are usually required. Fuel oil pumps: 1. The flow rate is low, and the pressure at the pump outlet remains stable (usually 1.0–1.2 MPa). 2. The viscosity is high. 3. The temperature at the pump inlet is generally not high. 1. Rotary pumps or centrifugal pumps can generally be used. 2. Due to the high viscosity, heating is usually necessary for transportation. 3. The backup rate for these pumps is 100%. Lubricating oil pumps and seal fluid pumps: 1. The pressure of lubricating oil is usually 0.1–0.2 MPa. 2. The pressure of the seal fluid in mechanical seals is generally 0.05–0.15 MPa higher than the pressure in the sealing chamber. 1. These pumps are usually supplied together with the main machine. 2. Screw pumps and gear pumps are commonly used, but centrifugal pumps are often used for centralized oil supply in centrifugal compressors