A marine pump refers to a machine on a ship that is used to increase the pressure or potential energy of liquids and liquid materials to flow. On ships, they are often used to transport various liquids such as seawater, fresh water, sewage, lubricating oil, and fuel oil.
Pumps are generally required to transport liquids to places with higher positions, longer distances, and higher pressures, and pumps can also be used to generate high-pressure liquids for hydraulic transmission. The pump is driven by an electric motor or other prime movers.
The characteristics of marine pumps are:
when the pump is swayed and tilted, cavitation cannot occur due to fluctuations in the suction liquid level; in order to reduce the occupied area of the pump and facilitate maintenance, a vertical structure is generally used; the flow parts are made of bronze, Made of materials such as brass or stainless steel to minimize corrosion.
Marine pumps are widely used in modern ships. Generally speaking, a diesel-powered cargo ship needs 36 to 50 pumps of various types, accounting for 20% to 30% of the total ship machinery and equipment.
1. Marine pumps have a very wide range of applications on modern ships. According to their different uses, they can be divided into:
(1) Pumps for marine power plants.
There are fuel pumps, lubricating oil pumps, seawater pumps, freshwater pumps, hydraulic pumps for steering gear or other deck machinery, feedwater pumps for boiler plants, cooling water pumps for refrigeration plants, seawater pumps and condensate pumps for seawater desalination plants, etc.
(2) General-purpose pumps for ships.
There are bilge pumps, ballast pumps, fire pumps, daily fresh water pumps, daily seawater pumps, and hot water circulating pumps; there are also general-purpose pumps that double as ballast, firefighting, and bilge pumps.
(3) Special pumps for special ships.
Some special-purpose ships are also equipped with special pumps for their special operating requirements, such as cargo oil pumps for oil tankers, mud pumps for dredgers, salvage pumps on salvage ships, and water jet propulsion pumps on water jet propulsion ships, Fishing pumps on netless fishing boats, etc.
2. According to the different working principles of the pump, marine pumps mainly have the following categories:
(1) Positive displacement pump
The positive displacement pump sucks and discharges liquid by periodically increasing or decreasing the working volume due to the movement of the working parts. When the working volume increases and the pressure decreases, the liquid is sucked in. When the working volume decreases and the pressure increases, the liquid is discharged. The pressure can directly increase the pressure of the liquid. According to the different movement modes of the moving parts, it can be divided into two categories: reciprocating pumps and rotary pumps. The former has a piston pump and plunger pump; the latter has a gear pump, screw pump, vane pump, and so on.
(2) Impeller pump
The impeller pump relies on the impeller to drive the liquid to rotate at a high speed, which continuously generates suction and discharge and transfers the mechanical energy to the liquid to be conveyed so that the pressure of the liquid can be increased, and the purpose of conveying the liquid is achieved. According to the different structural characteristics of the impeller and flow channel of the pump, it can be divided into the centrifugal pump and vortex pump.
(3) jet pump
The jet pump relies on a working fluid with a certain pressure to generate a high-speed jet in the nozzle to eject the fluid, and then increase the energy of the ejected fluid through momentum exchange. According to the working fluid used, it can be divided into water jet pump, steam jet pump, and air jet pump.
In addition to being classified according to different working principles, pumps can also be divided into vertical pumps and horizontal pumps according to the position of the pump shaft; single-suction pumps and double-suction pumps according to the number of suction ports; electric pumps according to the prime mover driving the pump, steam pumps and diesel pumps. Generally, the pumps are named according to their functions on ships, such as the main engine seawater cooling pump, the main engine lubricating oil pump, etc.
In order to indicate the performance of the pump, the following performance parameters are usually given on the nameplate and manual of the pump for easy selection and comparison.
1. Displacement, also known as flow. Refers to the amount of liquid that the pump can deliver per unit time.
What is expressed by volume is called volume flow, usually expressed by Q, and the unit is m³/s or m³/h.
What is expressed by mass is called mass flow, usually expressed by G, and the unit is kg/s, or t/h.
The displacement marked on the nameplate is the displacement of the pump underrated working conditions, which is called the theoretical displacement. In practice, the amount of fluid discharged by the pump is always less than the theoretical displacement.
2. Head, also known as head. Refers to the energy delivered by the pump to the unit weight of the liquid, usually expressed by H, and the unit meters. If the head is all used to increase the potential energy of the liquid, and assuming that there is no loss of pipeline resistance, the head is the height that the pump can make the liquid rise.
3. Power is the amount of work done per unit time, in watts. The power of the pump is divided into output power and input power.
The output power (effective power) of the pump refers to the energy that the pump actually transfers to the liquid per unit of time.
The input power of the pump, also known as the shaft power, refers to the power transmitted by the prime mover to the pump, represented by P. The power indicated on the pump nameplate refers to the input power of the pump. Since the pump has various energy losses in actual work, the effective power of the pump is always less than the shaft power.
4. Efficiency refers to the ratio of the effective power of the pump to the shaft power, expressed as a percentage.
5. Speed refers to the rated revolutions per minute of the pump shaft. It is usually expressed by n, and the unit is rpm. The speed of the reciprocating pump is often expressed in the number of double strokes of the piston per minute, and the unit is times/min. Most pumps are directly driven by the prime mover, and both rotate at the same speed. However, the electric reciprocating pump generally needs to be decelerated, so the speed of the pump shaft is lower than that of the prime mover.