NPSH, cavitation and recirculation - Impact and remedies

Name: NPSH, cavitation and recirculation - Impact and remedies
Rating: 4.3 (4 reviews)

How to effectively overcome the detrimental affects of cavitation and recirculation.

Created byGhazanfar Khan

Last updated 2/2021

English

What you'll learn

Meaning of NPSH3, NPSHR and NPSHA in practical terms
Relationship between NPSHR, cavitation and recirculation
Factors which lead to cavitation and recirculation in pumps and pumping systems
Pumping system reliability versus NPSH margin - for pumps for different industry and applications
How to differentiate cavitation and recirculation damage and identify the root causes
Factory testing of NPSH3/ NPSHR
How to prevent cavitation or recirculation damage in new installations and pumps already installed

Course content

1 section • 19 lectures • 2h 26m total length

Introduction6:26
Specific speed13:07
Specific speed is used to describe the shape of the impeller. Likewise the shape of characteristic curves of the pump are also defined by the specific speed including the efficiency range which the pump can achieve. A good understanding of the specific speed is essential for an indepth understanding the pumps and pumping systems.
Exercise on Specific Speed2:38
Calculate the specific speed for a double-suction pump using Q-H curves, NPSH, efficiency, and power curves, yielding 23.8 rpm (1228 rpm US) and 63 rpm (3250 rpm US) across cases.
NPSH14:00
The acronym NPSH has many different meaning when mentioned as NPSHA, NPSHR, NPSH3, NPSHi etc. A clear understanding of all these expressions is developed in this lecture. Besides the students will be able to calculate the NPSH available for a system and the NPSH curve of a system. This will help in selecting a new pumping system and will provide insight into resolving cavitation related problems from industry.
Exercise NPSHA calculation2:47
Cavitation9:57
In this lecture the cavitation phenomenon and its damage on the pump impeller is explained. After this lecture the attendees will be able to decipher the types and the cause of cavitation by looking at the damaged parts.
Suction specific speed12:48
Understanding suction specific speed and its impact on the behavior of the pump will help those selecting new pumping systems to make a correct and reliable selection. With the knowledge presented in this lecture it will be possible to assess some of the damaged pump parts for the causes and corresponding remedies.
Exercise Suction Specific Speed1:49
Recirculation6:59
Recirculation and cavitation signs and manifestations are confusing unless these are carefully examined. This lecture explains the phenomenon of recirculation, its causes and corrective actions necessary to prevent this failure mode.
Net Positive Suction Head margin12:02
NPSH margins are generally taken lightly and a ballpark figure is used for the NPSH margin across the board. However, selecting the right NPSH margin for a given application is essential. This lecture provides a sound basis and guidelines from the hydraulic institute to select the right NPSH margin to ensure long and trouble free operation.
Exercise on NPSH margin2:28
NPSH Testing19:08
Exercise on NPSH testing of pumps3:38
Reliability and NPSH margin8:30
There is a strong relationship between the pump reliability and NPSH margin. The concept of Suction Energy is provided in this module and how to calculate the same. Next the practical recommendations are included on the NPSH margin for different categories of pumps vis-a-vis the Suction Energy.
Exercise on Reliability and NPSH margin4:35
Detecting cavitation8:50
Cavitation has tell-tale signs which one must learn to decipher when the pumps are in service. detecting cavitation while the pumps are operating or installed provides a head start before the problem assumes undesirable proportions.
Overcoming cavitation and its affects7:10
Many times the available NPSH is enough for a given application but the factors which negatively impact the NPSH available are so overwhelming that the good advantage already available is lost. Learn the hidden impact of dissolved gas, poor intake structure, speed changes and how to overcome these factors.
Increase NPSH available4:34
Improving the NPSH available can save money and improve the performance and reliability of the pumping system. We will learn how to apply these techniques to real life scenarios.
Reduce NPSH required5:05
In real life sometimes it is not possible to increase the NPSH available. In such a situation the next option is to reduce the NPSH required. You will learn different techniques and approaches to reduce the NPSH requirement for a situation.

Requirements

Must be familiar with the basics of rotodynamic pumps
Engineering knowledge and field experience is desirable
Knowledge related to rotary machines and overall maintenance is helpful

Description

This is a rigorous course on Net Positive Suction Head, cavitation and recirculation. The course material has been compiled based on information from several handbooks, manufacturers recommendations, industry best-practice, guidelines given in norms and standards and contents of research papers and conference presentations. All the above has been blended in the light of the authors field experience.

The course focuses on the concepts and practical implications related to

NPSHi, NPSH3, NPSHR (required) and NPSHA (available)
Cavitation
Recirculation
on pumping system’s life and performance. And to learn how to

Measure NPSH – NPSH testing, permissible tolerances
Provide adequate NPSH margin as per industry guidelines
Improve the NPSHA and reduce NPSHR
Mitigate the adverse effects of cavitation, recirculation

The course comprises of 13 lecture and 6 exercises covering the topic in detail. It covers the following:

Definition and calculation of specific speed and the corresponding shapes of the pump characteristic curves,
Definition and calculation of suction specific speed and its impact on cavitation and recirculation,
Detailed explanation of NPSH, cavitation and recirculation phenomenon,
Suction Specific speed and reliability,
NPSH margin and NPSH margin ratio to avoid cavitation and recirculation,
NPSH testing of pumps,
Methods to improve the NPSH available,
Methods to decrease the NPSH required,
How to detect cavitation in the field,
Identify the operational problems through examination of the damage pattern to the pump parts,
How to prevent or minimize the cavitation and recirculation damage in the field.

Who this course is for:

Process designers and process engineers
Persons responsible for selection, and testing and approval of equipment
Engineers and managers responsible for trouble shooting and corrective actions