Blog 26: Engineering basics: Mechanical Pumps

Introduction:

Pumps or mechanical pumps is a mechanical devices that uses to transfer different fluids from one location to another. This works by mechanical action converted from electrical energy into hydraulic energy. This hydraulic energy is used to lift fluids from low to high levels and move fluids from low to high-pressure areas. The pump also serves as a booster in a piping network system. It can also be seen as a device that expends energy in order to raise, transport, or compress fluids.

The hydraulic pump can also be utilized in processes that require high hydraulic pressure. It can be observed with heavy equipment. In general, heavy equipment needs lower suction pressures and high discharge pressures. The low pressure on the pump’s inlet side causes the liquid to rise from a particular depth and the high pressure on the outlet side pushes the liquid to the desired head.

The earliest produce pumps were used for raising water, such as the Persian and Roman waterwheels. Just as earlier mentioned, pumps operate by some mechanism (typically reciprocating or rotary). It consumes energy to perform mechanical work (moving of fluid).

This system can be designed to operate through many energy sources. This includes manual operation, electricity, wind power, engines, etc. Pumps are available in many sizes, from the microscopic type used for medical applications to large industrial types.

History of Mechanical Pumps:

The first mechanical pumps were described by Archimedes in the 3^rd century BC. This is why it is known as the Archimedes screw pump.

The earliest produce pumps were used for raising water, such as the Persian and Roman waterwheels. Just as earlier mentioned, pumps operate by some mechanism (typically reciprocating or rotary). Mechanical pumps consumes energy to perform mechanical work (moving of fluid).

This system can be designed to operate through many energy sources. This includes manual operation, electricity, wind power, engines, etc. Mechanical pumps are available in many sizes, from the microscopic type used for medical applications to large industrial types

If a pump casing contains only one revolving impeller, it is called a single-stage pump. But if a casing contains two or more revolving impellers, it is known as a double- or multi-stage pump.

Types of Mechanical Pumps:

Mechanical pumps are classified into two major types. This can be understood using the following table

Dynamic Pumps:

The dynamic pump also known as the Kinetic pump is the pump that imparts velocity and pressure in the fluid as it passes through the impeller of the pump. In a dynamic pump, the impeller moves at a very high speed that imparts kinetic energy in the fluid making it move very fast. But this creates pressure in the pipe of are that contains the water as the velocity of the water is low in this area.

The dynamic pump is further divided into two major categories:
1) Centrifugal pump: A centrifugal pump is a rotating machine in which flow and pressure are generated dynamically. The energy changes occur by virtue of two main parts of the pump, the impeller, and the volute or casing. The function of the casing is to collect the liquid discharged by the impeller and to convert some of the kinetic (velocity) energy into pressure energy.

2) Vertical pump: Vertical pumps were originally developed for well pumping. The bore size of the good limits the outside diameter of the pump and so controls the overall pump design.

Some more types of Dynamic pumps are:

• Axial Pump
• Horizontal Centrifugal Pumps
• Submersible Pumps
• Fire Hydrant Systems

Advantages of Main Types of Dynamic Pumps:

• The size of these pumps is tiny.
• For installation, they require a little amount of space.
• They are less expensive.
• In comparison to a positive displacement pump, dynamic pumps are easier to maintain.
• They can handle fluids with low to medium viscosity.
• They’re suited for applications with a low to medium headcount.

Disadvantages of Main Types of Dynamic Pumps:

• Shaft misalignment is a concern with these pumps.
• They’re having problems with impeller damage.
• In a short amount of time, wear the ring of this pump was damaged.
• They have issues with seal ring damage.
• Overshooting causes the bearings of these pumps to wear out fast.

Applications of Main Types of Dynamic Pumps:

• For water supply, dynamic pumps are utilized.
• Pumping crude oil is a common use for these pumps.
• They’re in the chemical business.
• They are used in both business and domestic settings.
• Pumps like this are used in the food sector.
• These pumps are commonly utilized in the fire-fighting industry.
• They are used in the manufacturing of cellulose, petrochemicals, hydrocarbons, paint, and beverages.

Positive Displacement Pumps:

A positive displacement pump pushes a fluid by capturing a certain volume of it and pushing (displacing) it into a discharge pipe or system.

An expanding cavity is used on the suction side of some positive displacement pumps, while on the discharge side, a decreasing cavity is used. As the cavity on the suction side extends, liquid flows into the pump, and as the cavity collapses, liquid flows out of the discharge. Throughout each pumping cycle, the volume remains constant.

Positive displacement pumps do not employ impellers, instead relying on spinning or reciprocating elements to force liquid into an enclosed chamber until enough pressure is built up to push the liquid into the discharge system. The pump does not rely on passing the liquid through the impeller to increase the velocity of the fluid as a centrifugal pump does. As a result, a positive displacement pump’s fluid velocity is substantially lower than a centrifugal pump. For particular applications, such as pumping a fluid containing fragile solids, this is frequently a desirable quality.

Positive displacement pumps can also be divided into two major categories:

1) Reciprocating Positive Displacement Pumps: The repetitive back-and-forth movement (strokes) of a piston, plunger, or diaphragm drives a Reciprocating Positive Displacement pump. Reciprocation is the term for these cycles.

The initial piston stroke generates a vacuum, opens an intake valve, closes the exit valve, and pulls fluid into the piston chamber of a piston pump (the suction phase). The intake valve, which is now under pressure, closes when the piston reverses motion, and the outlet valve opens, allowing the fluid in the piston chamber to be emptied (the compression phase). This process is same for Plunger and Diaphragm type of pump.

2) Rotating Positive Displacement Pumps: Rotating cogs or gears, rather than the backward and forward motion of reciprocating pumps, are used to transport fluids in rotary positive displacement pumps.

Two interlocking gears are supported by independent shafts in an external gear pump (one or both of these shafts may be driven). The fluid is trapped between the teeth as the gears rotate, transporting it from the intake to the discharge and around the casing. Because the gears are interlocked, no fluid is transported back through the center. Close tolerances between the gears and the casing enable the pump to create suction at the intake while preventing fluid leakage from the discharge side. Low viscosity liquids are more prone to leakage or “slippage.”

Application of Positive Displacement pumps:

• Pumping low-viscosity fluids, paint spraying, oil production, and high-force cleaning are all done with piston and plunger pumps.
• Metering, spraying, and treatment of water, oils, and paints may all be done with diaphragm pumps.
• Pumps for high-viscosity fluids are used in the petrochemical, food, paint, and oil sectors, among others.
• Lobe pumps are utilized in the food and chemical industries, as well as in pharmaceutical, biotechnology, and sanitary applications.
• Screw pumps are used for transporting fuel, producing oil, and irrigation, among other things.
• Vane pumps are utilized for low-viscosity liquids, fuel loading, and transmission, among other applications.

Advantages of Positive Displacement Pumps:

• As chambers within such pump designs contain a fixed volume – flow is proportional to speed.
• These designs are unaffected by viscosity, meaning should the viscosity increase with temperature, equipment used with a variety of different fluids, or the liquid being pumped, behave as a non-Newtonian fluid.
• These types of units provide constant pressure at pressures up to several bars higher than the required duty point.
• PD pumps operate at reduced speeds this lower speed enables the handling of abrasive or solid-laden liquids which would wear parts in a centrifugal pump.
• Reducing pump speed enables pumps to be oversized, meaning the flow or pressure requirements of an application are met by a unit operating slowly.
• All pumps have a minimum and maximum viscosity level, which is the viscosity limit they are engineered and designed to handle without any slippage

Disadvantages of Positive Displacement Pumps:

• Flow is restricted due to its internal design on some pumps, meaning flow will be less than a centrifugal pump for low-viscosity fluids.
• Due to their internal design they can be difficult to maintain.
• Due to the tight clearances, demands of applications, contact of internal parts, and abrasive liquids used, service intervals tend to be more frequent than that of centrifugal designs.
• Flow can pulsate meaning pulsation dampeners are required to minimize pulsations.
• Such units are not usually cost-effective for low-viscosity fluids, or fluids that need transferring at low to medium flows where a centrifugal Pump could be used.
• When positive displacement pumps are used, they are usually specified to ensure the consistent metering, transfer, and flow of liquids.
• PD pumps are great at building pressure within discharge pipework, but they can be too effective which can cause other issues.

Some Important Pump calculation formulas: