Mechanical Seal Failures In Sewage Water Pumps Engineering Essay

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Abstract

Generally, a mechanical seal is designed to reach the highest level of performance and life if it is operated well in its design configurations. A mechanical seal is likely to fail only when the leakage overlaps the limits set for environmental or plant site operations. The failure in these seals may take place prior to or after the seal has successfully reached its specified life span. In addition to this, a failure detected in a mechanical seal can act as a primary contributor to failures and downtime in rotary equipments. An in-depth study of the various modes of seal failures may result into extending the overall life of rotary equipments mainly by advancing the seal design and selection of material, installation and operating processes, and lastly, the environmental factors.

This paper exclusively deals with the main causes and various modes by which failures are observed in the operations of mechanical seals. As per the introduction, mechanical seals are industrial equipments used in pumps, compressors as well as several other applications for offering leak-proof seal between various component parts. The research study emphasizes on the different stages of designs of a mechanical seal to be applied for specific applications. It also outlines the major causes of why mechanical seals tend to wear out, as well the different mode of their failures. Apart from this, the paper covers applications of mechanical seals in process plants. Along with providing an in-depth background of these applications, the current research project attempts to offer solutions and schemes of restricting the leakages in mechanical seals.

Some of the major part of the paper highlights the techniques of improving the reliability and duration of mechanical seals. This also gives evidences to reasons why failures are encountered in the operations of mechanical seals.

Briefly, the paper provides a profound understanding and background of mechanical seals, and their basic operations. However, most of the research study covers the reasons and causes behind failures of these seals along with their modes of failure. Additionally, various approaches to avoid such failures have also been outlined as a supportive mechanism to provide an in-depth study of the research topic.

Introduction

A centrifugal as well as rotary displacement pump need to control the pumped fluid that need to exit from the stuffing box, which is the region in which the pump shaft goes into the pump fluid side. Pressure, greater than the surrounding environment pressure, is observed while running the pump fluid inside the stuffing box well on static lifting applications. However the stuffing box will observe a pressure lower than the atmospheric pressure only during the priming stage, that is, a vacuum is created during the priming cycle. Under any of these operating conditions, the seal is likely to virtually stop the release of the pumpage or leakage into the surrounding along with the entry of air into the stuffing box while in the vacuum state (Tyldesly 2003).

A simplest form of mechanical seal can be defined as regular device consisting mainly of a rotary seal face and a driving mechanism that drives the seal as the same speed as that of the pump shaft, a stationary seal face that works in conjunction with the rotary and can be held back with the help of a gland or some stuffing box, a composite form of tension responsible for keeping the rotary face tightly in place against the fixed face to prevent any leakage while the pump in not running, and lastly, static sealing sockets and elastometers.

Because the seal face is the only vulnerable element of the mechanical seal, a damaged or worn out seal can be detected as the one having no carbon nose piece remaining at the instant it began to leak. However, a seal is said to be failed if it has significant amount of carbon left when it started to leak (Tyldesly 2003) (Johnson et al. 1997).

As aforementioned, mechanical seals are extensively used in prevention of leakage of fluids across rotating shafts which is a key function of the seal at right angles to the axis of rotation occurring between a fixed ring and a rotating or moving ring. Additionally, it may also be designed to avoid leakage to take place between a moving shaft and its housing while subjected to extreme pressure, temperature and shaft speed. Therefore, a mechanical seal is either a dynamic design mounted with spring components across the rotating part of the seal assembly, or a fixed seal having spring elements housed in the fixed portion of the seal assembly in order to compensate for misplacement of the shaft and seal (Huebner 2005).

Failure modes of mechanical seals may be seen due to three major causes of failure. They are:

Temperature

Velocity

Pressure

Failures also cause due to the combination of these three factors.

While this illustration is simple in form, it is yet a substantial engineering challenge. Today’s consumer expectations and environmental challenges and regulations do not need a visible leakage for most applications (Huebner 2005). For most chemicals that are toxic and dangerous, the mechanical seals and systems need to be designed in such a way that they are not vulnerable to atmospheric leakages in any conditions. Among the several underlying concepts of mechanical seals, one states that seals are engineered to generate a very small separation between the dynamic and fixed seal faces. This layer must be thin enough to monitor leakage but not permit extreme face contact. Across typical seal applications, real fluid layers range around 0.5 mm. And any condition that may threaten this layer will greatly affect seal performance (Andrews 2005).

The pump, pipes, base-plate, mechanical seal, driver, and the seal assembly are all essential components of the entire pumping system. Damage to or failure of any of these components will tend to affect the performance of the entire system and may cause major failures in its operations. At any operating stage, misapplication on any of these parts can reflect or cause failure in other component. In short, mechanical seals typically depict a barometer for integrity of the system. Strain in piping, cavitation in pumps or coupling misalignment may initially indicate a seal failure. Since seals are usually seen as the problem instead of the symptom of another problem, users having good equipment reliability vary from the ones with unreasonable and poor reliability. This becomes the ability to study and investigate the factors affecting performance (Johnson et al. 1997). The figure below illustrates the distribution of causes of failure.

Seal Failure Cause Distribution

Statement of the Problem

As aforementioned, the main objective for the research study dissertation is to address the problems faced in the working of industrial mechanical seals.

Since shaft misalignment has been stated as a common cause of premature failure in seals, causes of misalignment could be either static or dynamic in nature. The paper attempts to investigate the various modes of failure in the operation of mechanical seals and address these issues by outlining solutions for the same. The study analyzes causes and modes of failures of mechanical seals and how these failures have an adverse effect on the pump efficiency. Additionally, the study deals with the selection of mechanical seals for the better functionality and performance of sewage water pumps. In this issue, sources of static misalignment are discussed, together with corrective measures to be adopted for improving pump reliability(Andrews 2005).

Background of the Study:

Mechanical seals are stated to provide a significant and crucial contribution to the functioning of several types of industries. These omnipresent machineries are seen in centrifugal pumps, reactors, industrial compressors, blowers, etc. along with certain positive-displacement pumps. In spite of their far flung use though, mechanical seals are given relatively lesser consideration. An in-depth understanding of seals, their selection, materials, and operational prerequisites, help users to greatly enhance equipment reliability and eliminate operating cost (Huebner 2005) (Skewis).

Several chemical processes demand variable conditions for operating or batch processing. Variations in pressure, temperature or similar factors, the introduction of intermediate compounds, and hiccups in operations are very common and usual. Machinery may need regular cleaning, steaming and sterilization among batches. These extreme variations of fluctuations result into user over-sizing equipments to permit changes or further increases in capacity. This in turn leads to pumps operating way farther from their best efficiency points. As a result of this, pump reliability, life, and mechanical seal performance are greatly compromised (Tyldesly 2003).

By far, one of the most challenging aspects of seal selection stem from the nature of fluids themselves. Most of the fluids within a refinery are good lubricants. However, very few fluids in chemical process plants have this property in common. In essence, process fluids tend to polymerize across the seal parts or begin to crystallizing on exposure to the surrounding atmosphere. In addition, there could be wide differences in fluid characteristics during a batch or among processes. Therefore, the mechanical seal as well as the system must be so designed to withstand all of these (Andrews 2005) (Johnson et al. 1997). The figure below depicts the typical reasons for failures in mechanical seals.

Methods and Procedures

The research paper is a dissertation that holds its relevancy to the field of mechanical engineering domain. This dissertation conducts an in-depth research study of the various modes of failures in mechanical seals and the major causes behind such failures. It investigates and describes how selection of a mechanical seal is a driving factor for the overall performance of the system and the mechanical seal operation. It also focuses on the operating requirements (Huebner 2005) (Skewis).

Data collection for this dissertation is done through combination of journal articles, websites, text books, and interviews. Also, this project indicates the model or design procedure being used. The research question will be addressed by including profound background knowledge of mechanical seals, their basic operations, problems encountered during its operations and its solutions, modes and causes of failures and lastly, how seal performance will depend on the selection of mechanical seal.

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