Centrifugal Pumps: Basic Concepts of Operation, Maintenance, and Troubleshooting

In Part I of the article, two basic requirements for trouble free operation and longer service life of centrifugal pumps are mentioned in brief.

1. PREVENT CAVITATION
Cavitation of the pump should not occur throughout its operating capacity range.

2. MINIMIZE LOW FLOW OPERATION

Continuous operation of centrifugal pumps at low flows i.e. reduced capacities, leads to a number of unfavorable conditions. These include reduced motor efficiency, excessive radial thrusts, excessive temperature rise in the pumping fluid, internal re-circulation, etc. A certain minimum continuous flow (MCF) should be maintained during the pump operation.

Operating a pump under the condition of cavitation for even a short period of time can have damaging consequences for both the equipment and the process. Operating a pump at low flow conditions for an extended duration may also have damaging consequences for the equipment.

The condition of cavitation is essentially an indication of an abnormality in the pump suction system, whereas the condition of low flow indicates an abnormality in the entire pumping system or process. The two conditions are also interlinked such that a low flow situation can also induce cavitation.

The concept of cavitation is explored in detail under following topics:

1. Meaning of the term ‘cavitation’ in the context of centrifugal pumps.
2. Important definitions: Static pressure, Dynamic pressure, Total pressure, Static pressure head, Velocity Head, Vapor pressure.
3. Mechanism of cavitation.
4. General symptoms of cavitation and its effects on pump performance and pump parts.
5. Types of cavitation:
1. Vaporous cavitation

i. Classic cavitation

ii. Internal re-circulation cavitation

2. Gaseous cavitation

i. Air ingestion induced cavitation
6. Methods to prevent cavitation

The topics 1 to 4 are covered in detail in this part of the article. The topics 5 to 6 shall be explored in next part of the article.

Readers of Part I showed keen interest and appreciation about the approach in which the topic of Centrifugal Pumps has been discussed. The same enthusiasm, response and feedback are solicited from the readers.

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Centrifugal Pumps: Basic Concepts of Operation, Maintenance, and Troubleshooting (Part- I)

The operating manual of any centrifugal pump often starts with a general statement, “Your centrifugal pump will give you completely trouble free and satisfactory service only on the condition that it is installed and operated with due care and is properly maintained.”

Despite all the care in operation and maintenance, engineers often face the statement “the pump has failed i.e. it can no longer be kept in service”. Inability to deliver the desired flow and head is just one of the most common conditions for taking a pump out of service. There are other many conditions in which a pump, despite suffering no loss in flow or head, is considered to have failed and has to be pulled out of service as soon as possible. These include seal related problems (leakages, loss of flushing, cooling, quenching systems, etc), pump and motor bearings related problems (loss of lubrication, cooling, contamination of oil, abnormal noise, etc), leakages from pump casing, very high noise and vibration levels, or driver (motor or turbine) related problems.

The list of pump failure conditions mentioned above is neither exhaustive nor are the conditions mutually exclusive. Often the root causes of failure are the same but the symptoms are different. A little care when first symptoms of a problem appear can save the pumps from permanent failures. Thus the most important task in such situations is to find out whether the pump has failed mechanically or if there is some process deficiency, or both. Many times when the pumps are sent to the workshop, the maintenance people do not find anything wrong on disassembling it. Thus the decision to pull a pump out of service for maintenance / repair should be made after a detailed analysis of the symptoms and root causes of the pump failure. Also, in case of any mechanical failure or physical damage of pump internals, the operating engineer should be able to relate the failure to the process unit’s operating problems.

Any operating engineer, who typically has a chemical engineering background and who desires to protect his pumps from frequent failures must develop not only a good understanding of the process but also thorough knowledge of the mechanics of the pump. Effective troubleshooting requires an ability to observe changes in performance over time, and in the event of a failure, the capacity to thoroughly investigate the cause of the failure and take measures to prevent the problem from re-occurring.

The fact of the matter is that there are three types of problems mostly encountered with centrifugal pumps:

· design errors

· poor operation

· poor maintenance practices

The present article is being presented in three parts, covering all aspects of operation, maintenance, and troubleshooting of centrifugal pumps. The article has been written keeping in mind the level and interests of students and the beginners in operation. Any comments or queries are most welcome.

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