Repair and Refurbishment of a Centrifugal Pump
AIMS/OBJECTIVES OF THE STUDY
It was aimed that upon completion of this course, the particular pump will not only be put back in good working condition for future practical purposes, a training certificate will also be received indicating the ability to:
– Identify different types of centrifugal pumps and their parts
– Understand pump case repair methods,
– Develop a pump maintenance program,
– Troubleshoot, install and lubricate bearings,
– Understand shaft alignment techniques,
– Identify different pump drivers and their uses,
– Trouble shoot wetted end pumps,
– Understand vibration measurements,
– Assemble and disassemble an ANSI Pump etc.
Centrifugal pumps are the most commonly used kinetic-energy pump. Centrifugal force pushes the liquid outward from the eye of the impeller where it enters the casing. Differential head can be increased by turning the impeller faster, using a larger impeller, or by increasing the number of impellers. The impeller and the fluid being pumped are isolated from the outside by packing or mechanical seals. Shaft radial and thrust bearings restrict the movement of the shaft and reduce the friction of rotation. A centrifugal pump is a rotodynamic pump that uses a rotating impeller to increase the pressure of a fluid. Centrifugal pumps are commonly used to move liquids through a piping system. The fluid enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward into a diffuser or volute chamber (casing), from where it exits into the downstream piping system. Centrifugal pumps are used for large discharge through smaller heads.
Pump is a mechanical device that applies energy to move liquids from one place to another at increased pressure, flow rate and to an elevated height. The development of pumps has enabled man to move away from his early settlement near rivers, lakes, and springs to develop vast areas of land that was previously uninhabitable. The usefulness of pumps has progressed to a point where it has become a necessity to modern high standard of living. In our everyday life pumps play very important parts. Our domestic water supply systems from water boards and private boreholes make use of pumps to distribute water to different locations. Irrigation, fire fighting services employ pumps of different sizes.
A centrifugal pump is a rotodynamic pump that uses a rotating impeller to increase the pressure of a fluid; the fluid enters the pump near the rotating axis streaming into the rotating impeller. The impeller consists of a rotating disc with several vane attached, the vanes normally slope backwards away from the direction of rotation when the fluids enters the impellers at a certain velocity due to the impeller vanes from the impeller centre eye out-ward, it reaches its maximum velocity at the impeller’s outer diameter and leavethe impeller into a diffuser or volute chamber.
The impeller is the essential parts of a centrifugal pump; the performance of the pump depends on the impeller diameters and design. The pump TDH is basically defined by the impeller’s inner and outer diameter and pump’s capacity is defined by the width of the impeller vanes in general there are three possible types of impellers, open, enclosed and semi open impellers, each suitable for specific application. Standard impellers are made of cast iron or carbon steel, while impeller for aggressive fluid and slurries require high end materials to ensure a long pump life. The open impellers are the simplest type of impellers, they consist of blades attached to the hub, and this type of impeller is lighter than any of the other type at the same diameter. Weight reduction leads to less force applied to the shaft and allows smaller shaft diameters, these results in low costs compared to equivalent shrouded impeller. Typically, open impellers operate at higher efficiency because there is no friction between the shrouds and the pump casing. On the other hand side, open impellers have to be carefully positioned in the casing the gap between the impellers and the surrounding casing should be as small as possible to maximize efficiency, the impeller wears the clearance between the impeller and the front and back walls open up this leads to a dramatic drop in efficiency. Due to the minimized clearance between blades and casing, high velocity fluid is close proximity to the stationary casing establish vortices that increase wear dramatically.
Semi-open impellers can be seen as a compromise between open and enclosed impellers. A semi-open impeller is constructed with only one shroud, usually located at the back of the impeller; it usually operates at a higher efficiency than an equivalent enclosed one due to reduced disc friction as there is only one shroud. The advantage of semi-open impellers compared to open ones is that the impellers axial position can be adjusted to compensate for wear. The problem is that the entire backside of the impellers shroud is under full impeller discharge pressure as the front side is under suction pressure increasing along the impeller radius due to centrifugal force. The differential between these pressures causes an axial thrust imbalance manufacturer try to reduce this effect by applying vane to the backside of the impeller, but the efficiency of the so called “pump vanes” decreases if the impeller is moved forward to compensate for wears. A better option to compensate the loss of efficiency is an adjustable wear plate, so that it clearance adjustments can be made. Enclosed impellers consist of blades covered by a front and back shroud the fluid steams through the impeller without interacting with the stationary pump casing. In a well design enclosed impeller, the relative velocity between the fluid and the impeller walls at any given radius is rather small, the disc friction of the shrouds rotating in close proximity to the pump casing causes a lower efficiency as comparable to semi-open or open impellers.
The performance of diffusers has been an important field of research for many years. Reneau et al found that the performance of 2D diffusers is greatly affected by the inlet conditions. High recovery occurs at high area ratios (up to 5) and the minimum head loss occurs at 2θ<7 deg. Much attention is still devoted to research on radial diffusers Lugovaya et al. Goto and Zangeneh presented a new approach to optimizing a pump diffuser based on a three dimensional inverse design method and computational fluid dynamics (CFD). Other researchers designed a twisted return guide vane for a submersible multistage pump, the results of their work demonstrating that return guide vanes with a twisted inlet can reduce the flow loss in radial diffusers . The results show that the pressure changes little at the helix section along the direction of the flow then increases more and more at the diffuser section and hits its maxi mum at the end of the diffuser section Shi et al. Optimized the guide vane by using an orthogonal test method and found that the wrap angle and inlet angle of the radial diffuser have greater impact on the pump head and efficiency. There suits indicate that the three-dimensional guide vanehasa smaller hydraulic loss and a higher pressure conversion capacity. And at the same time, they studied the performance of the deep-well centrifugal pump with the new designed guide vane.
A shaft is a rotating machine element which is used to transmit power from one place to another. The shaft is the connection between impeller and driver unit which is in most cases is an electric motor but can also be a gas turbine. It is mainly charged by a radial force caused by unbalance pressure forces in the spiral casing and an axial force due to the pressure difference between front and backside of the impeller. Most common pump shaft are made of carbon steel. There are several cranks to support the bearings and seals, a high surface quality and small clearances are required. Especially in the area of the bearing’s, clearance and surface quality is important to ensure right positioning of the shaft in the casing and therefore close positioning clearances of the impeller. At the area of the seals, particularly the surface quality is also very important to ensure an adequate seal life span. In the shaft design it is also important to avoid small radiuses at cranks to minimize stress in these areas which are susceptible for fatigue.
METHOD AND MATERIALS
Most of the information was gotten from the internet from different websites, while others was gotten from the school and National Library, workshop/laboratory and consultations or interviews made.
RESULTS AND DISCUSSION
Post Fabrication: Analyzing Pump Performance
A slide from the pump performance presentation is shown in the figure below. It reveals the pump testing experimental set up used in the study.
An attempt has been made to design, construct and test a single stage centrifugal pump. The test has shown that the construction of single stage centrifugal pump is able to produce a head of about 30m instead of the calculated 27m this represent an increase of about 3m above the intended head H, Furthermore volumetric discharge of 9m3/hr was obtain. The performance of a pump is however better evaluated if tested at the design speed and power with all necessary parameters ready by direct measurement. The pump operated was observed to be very smooth with low vibration and noise level on the pump and motor respectively, this will guarantee the reliability of the pump in service.
The project motivates student learning by requiring students to draw and assemble pump parts using solid modeling software, to render the 3D model of an impeller that they designed on a rapid prototyping machine, to fabricate the pump body through drilling, tapping and assembly operations, and to analyze the performance of the pump using conservation of energy.
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