Performance And Specifications Of Power Pumps Engineering Essay

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Introduction

Background:

The means of pumping is the most important way of fluid transfer for thousands of years. Ancient Egyptians used waterwheels with hoppers for the movement of vehicle marked with water for irrigation purpose. In the third century BC; the Greek scientist Stisebeos Alexandria invents the reciprocating pump to pump water. The about the same time discovered the Greek mathematician Archimedes the screw pump, called the composit of mandolins with a spiral going on inside the cylinder. Till that time the there was no much development occurs until the late seventeenth century, where the French explorer Denis invent a pump with straight blades. Then the British explorer John Oblad invents the centrifugal pump with curved blades in 1851. Thereafter, the first use of compressors with axial-flow turbo jet engines where found in the forties of the twentieth century AD.

Reciprocating Pump

Positive displacement pumps are that type of machines which is common for applications that require a very high Pressures and relatively low flow. In this machine, the liquid flows into a contained space, such as a cylinder, plunger, or rotor. Then a moving piston forces the liquid out of the cylinder, increasing the pressure. The use of positive displacement pumps is common in applications that require high discharge pressure and relatively low flow. The discharge pressure generated by a positive displacement pump is - in theory - infinite. If the pump is dead headed, the pressure generated will increase until either a pump part fails or the driver stalls from lack of power.

The Reciprocating Pump - also known as Power Pump- is one type from the Positive Displacement Pump. In his book, Pollak illustrates that the Reciprocating pump has a ram, plunger, piston or other cylindrical element working backwards and forwards within a cylinder or pump barrel; this motion is usually delivered from a crank revolving uniform speed, and connecting rod. Automatic valve control the flow of liquid into the cylinder, and out again. A piston or a plunger moves back and forth in an enclosed cylinder. A reciprocating pump also has a power end and a liquid end. Most piston pumps are single acting; plunger pumps are double acting. The diameter of the piston, the length of the piston stroke and the velocity of the

Pistons determine the pump capacity.

See fig

Power Pumps:

As mentioned by Igor Karassik in his book that the power pump does not develop pressure; it only produces a flow of fluid. The downstream process or piping system produces a resistance to this flow, thereby generating pressure in the piping system and discharge portion of the pump. The flow fluctuates at a rate proportional to the pump speed and number of cylinders. The amplitude of the fluctuations is a function of the number of cylinders. Karassik added that greater the number of cylinders, the lower the amplitude of the flow variations at a specific rpm.

It is always possible for the power pump to produce different capacity as they are capable of operating over a wide range of speeds. Each pump has maximum suction and discharge pressure limits that; when combined with its maximum speed, determine the pump's power rating. The pump can be applied to power conditions that are less than its maximum rating but at a slight decrease in mechanical efficiency

Operating Principles:

In his paper, Samuel explains the pump operation as follows; the reciprocating pump uses a crankshaft-connecting rod mechanism. The crankshaft-connecting rod mechanism converts the rotary movement of the crankshaft to a reciprocating linear movement of pistons. The piston movement creates volume changes. As a cavity opens when a piston retracts, the fluid is admitted through an inlet check valve. When the piston reverses, the inlet check valve closes, and the cavity reduces when the piston extends. The outlet check valve opens and the fluid is forced out by the piston.

Despite that of the fluid being pumped, the discharge volume is fixed for each crankshaft revolution. Pressure is determined by the system flow resistance and pump construction. The Speed reduction is then needed for decreasing high speed from the driver to low pump shaft speed.

Applications for Power pumps are:

• Oil well mud pumps

• Reverse osmosis charge pumps

• Auxiliary boiler feed pumps

• Pipeline pumps

• Oil field water injection pumps

• Slurry pumps

• Process pumps

Performance and specifications:

In his paper, David parker talks about the specification illustrating that the quality and quantity of information on suction conditions will determine the ultimate success or failure of any pump installation. The majority of pump problems, start at the suction. There must be a minimum amount of absolute pressure available to supply fluid to the pump suction. PD pumps generally require less absolute pressure. Net Positive Inlet Pressure Required (NPIPR), at the pump suction flange, is the rating of total inlet losses within that pump at rated conditions. Units are pressure terms; PSI, Kg/cm2, Bar, KPa. These losses include the fluid friction loss along the internal suction path, the change in elevation from the suction flange to the enclosed volume, the fluid friction loss of entering the enclosed volume, and the acceleration to the velocity of the enclosed volume. For any given size, NPIPR will increase with increased viscosity or flow (increased flow = increased speed). Volumes of gas are usually specified relative to standard temperature and pressure (STP) of 680F and atmospheric pressure; 14.7 psia; 200C, 1.034 Kg/cm2 absolute. By specifying the standard volume of gas and specifying the suction pressure and temperature, the volume of gas present at the pump suction can be calculated. This capacity must be added to the liquid capacity in order to size the pump for the required liquid flow rate. If suction pressure is below atmospheric pressure, even small amounts of entrained gas will expand in volume requiring a larger pump. Capacity should be defined for the rated condition. If there is an acceptable range of capacities, the minimum and maximum acceptable should be stated. This allows pump suppliers to offer standard products without having to modify for specific capacity requirements.

Efficiency:

In the reciprocating pump, only two efficiency losses need to be considered; volumetric and Mechanical.

Volumetric efficiency loss is provoked by slippage through valves, ratio of liquid chamber volume at end of stroke to plunger/piston displacement volume, and liquid compressibility.

Mechanical efficiency loss occurs while overcoming mechanical friction in bearing and speed reduction.

The overall efficiency of a reciprocating pump unit is generally above 85%throughout its full operating range. However, in the reciprocating pump can run over 90% because many pumps and reduction units operate at a mechanical efficiency of 98 percent, and the volumetric efficiency can often be above 95 percent.

Viscosity:

Viscosity of a fluid is the ratio of shear stress to the rate of shear strain. It is a measure of its resistance to flow. High viscosity fluids, like rubber, adhesives, or molasses, are very resistant to forces applied to move them. Low viscosity fluids, like kerosene or water, have very little resistance to force. Viscosity is reduced as temperature is increased; hot fluids flow more readily than cold fluids. Viscosity should always be given at a specified temperature. Typical units for viscosity are centipoises, centistokes, and SSU. Positive Displacement pumps maintain high efficiencies throughout the viscosity range. Entrained gasses can be handled in large quantities by most Positive Displacement pump designs; however care must be taken in specifying quantity of gas entrained and flow required.

Positive Displacement pumps are used to maintain the constant flow rate as nozzle pressures change due to clogging and eroding. Precise control of fuel addition rates increases the operators control over combustion conditions. This in turn leads to reduced air emissions, a very critical concern in a highly regulated industry.

The reciprocating pump provides a nearly constant flow rate over a wider range of pressure; the centrifugal pump gives uniform pressure over a range of flow, then it drops dramatically as the flow rate increases. On a reciprocating pump, fluid viscosity has little effect on the flow rate as the pressure increases. However, fluid viscosity has a big impact on the centrifugal pump's pressure and flow rate. The efficiency also drops substantially.

Advantages:

Efficiency is quite high even though there are changes in the required head. It can be up to 85% to 95% or even more. Only with high speed it tends to decrease slightly.

Reciprocating pumps run at much lower operating speeds than centrifugal pumps and thus is better suited for handling viscose fluid.

For a given speed the flow rate is constant regardless of head, the pump is limited only by the power of the prime mover and the strength of the pump parts.

The fluid flow from the reciprocating pump is considerably high.

It is start automatically. No need to fill the cylinders before starting.

Disadvantages:

There are poorer in handling liquids containing solids that tend to corrode valves and seats.

Because of the pulsating flow and pressure drop throw the valves they require larger suction pressure at the section flange to avoid cavitations.

Due to mechanical vibrations , the pulsating flow require a special attention to section and discharge piping design

Oscillating motion of the nozzle creates disturbances that travel at speed of sound from the pump cylinder piping system. These disturbances cause the pressure level of the system to fluctuate with respect to time.

It is difficult to pump viscous liquid in the reciprocating pump

The cost of producing piston pumps is high. This is due to the very accurate sizes of the cylinders and pistons. Also, the gearing needed to convert the rotation of the drive motor into a reciprocating action involves extra equipment and cost.

Discharge flow troubles:

Maintenance cost a lot considered with its availability, because pulsating flow and large number of moving parts, as the particles can get into the small clearances and cause severe wear. The piston pump therefore, should not be used for slurries.

The pulsating characteristics of the fluid flowing into and out of power pumps are significantly affected by the number of pistons. Discharge flow pulsations are the most critical because of the high energy potential generated when the system resistance reacts with the flow to create pressure. Since the magnitude of the discharge pulsation is mostly affected by the number of cylinders, then we can overcome the pulsation flow by increasing the number of cylinders.

Also, we can reduce the discontinuity by using an accumulator at the end of the nozzle which will provides a continues flow.

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