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Transcript of Problem Solving - Centrifugal Pumps · PDF filethe pump to the discharge side of the pump....

  • Problem Solving - Centrifugal Pumps

  • The information provided in this document is given in good faith, but Alfa Laval is not able to accept any responsibility for the accuracy of its content, or any consequences

    that may arise from the use of the information supplied or materials described.

    Inside View This document has been produced to support pump users at all levels, providing an invaluable reference tool. It includes information on general Centrifugal pump problems and their effect on Centrifugal pumps with suggestions as to probable causes and solutions. Main sections are as follows:

    1. Introduction 2. Centrifugal Pump Problems 3. Alfa Laval Solutions to Specific Centrifugal Pump Problems 4. How correct System Design and Installation can avoid potential problems

    5. Problem Solving Table

  • Contents Page

    Section 1.0: Introduction 3 Introduction to problem solving Centrifugal pumps.

    Section 2.0: Centrifugal Pump Problems 5

    Overview of pump problems typically found on Centrifugal pumps. 2.1 No or low flow 7 2.2 No or low pressure 14 2.3 Excessive power consumption 15 2.4 Excessive noise or vibration 16 2.5 Seal leakage 20

    Section 3.0: Alfa Laval Solutions to Specific Centrifugal Pump Problems 31 How Alfa Laval can provide solutions to specific pump problems. 3.1 Ability to self-prime 31 3.2 Ability to handle high inlet pressures 32 3.3 Rouging 33

    Section 4.0: How correct System Design and Installation can avoid 35

    potential problems Advises guidelines relating to correct pump installation, system design and pipework layout.

    4.1 Pipework 35 4.2 Protection 35 4.3 Operation 36 4.4 Pre-start up Checks 36

    Section 5.0: Problem Solving Table 37 Provides summary of probable causes and solutions to the most common problems.

  • 3

    1.0 Introduction In most pumping systems, the pump is likely to be one of the most vulnerable components and systems frequently show the pump to be at fault regardless of what may be wrong. Upon investigation the likely causes of the problem are inadequate control of the pumped fluid or a change in operating requirements of which the system or pump is not capable of handling.

    Pumps that are correctly installed and operated within their design parameters will give typically long trouble-free service of >10 years, unless on very arduous duties. In order to correctly identify the problem it is important to gather as much information relating to the process as follows:

    Reconfirm original duty requirements and/or system design. Check for any process changes i.e. pressure, temperature, fluid viscosity etc.

    Check whether the system was undergoing routine maintenance. How long did the pump operate before the problem.

    Check the appearance and condition of the pump internal components.

    Check when the pump was last serviced.

    Check for any changes in pump noise or vibration.

    This will save considerable time and effort in leading to the most appropriate solution. The most common problems associated with Centrifugal pumps are shown herein with particular effects on our range of Centrifugal pumps.

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    2.0 Centrifugal Pump Problems Typical pump problems include:

    No or low flow No or low pressure Excessive power consumption Excessive noise or vibration Seal leakage

    These are common for all Centrifugal pump types shown as follows:

    Pumps

    Positive Displacement

    Rotodynamic

    Rotor Reciprocating Multi-Stage Single Stage

    Multi-Rotor Single Rotor Diaphragm Plunger End Suction Double Entry

    Screw

    Gear - Internal

    Circumferential Piston

    Rotary Lobe

    Archimedian Screw

    Flexible Member

    Peristaltic

    Vane

    Progressing Cavity

    Piston Simplex

    Multiplex

    Process

    Rubber Lined

    Submersible

    General Gear - External

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    Typical causes are as follows: No or low flow

    Pump is not primed The motor is turning pump in the wrong direction Valves are closed or there is an obstruction in the suction or discharge pipework The end of the suction pipework is not submerged A strainer or filter is clogged Insufficient Net Positive Suction Head available (NPSHa) Air leak in the suction pipe No power to the pump Pump speed too low Pumped media viscosity is higher than expected Pressure is higher than calculated Flow switch faulty

    No or low pressure

    Valves are closed or there is an obstruction in the suction or discharge pipework A strainer or filter is clogged on the inlet Flow is higher than calculated The motor is turning pump in the wrong direction Insufficient Net Positive Suction Head available (NPSHa) No power to the pump Pump speed too low Pumped media viscosity is higher than expected

    Excessive power consumption

    Flow is higher than calculated with low outlet pressure Viscosity too high Mechanical contact in the pumphead

    Excessive noise or vibration

    Pipework is not properly supported Pump is cavitating Impeller contact with casing/backplate Gas entrainment Loss of shaft support (bearing failure in motor) Pumped media contains unexpected abrasive particles Poor pipework / system design Dry running of seal

    Seal leakage

    Pumped media contains unexpected solids Chemical corrosion / attack Pump is cavitating Too high discharge pressure Too high temperature Pump / shaft vibration Incorrect fitting Incorrect selection of seal materials Pump allowed to run dry Insufficient or no auxiliary flushing services

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    2.1 No or low flow 2.1.1 Pump is not primed

    Centrifugal pumps are capable of evacuating (pumping) a modest amount of air from the suction side of the pump to the discharge side of the pump. Filling the inlet system with fluid or at least filling the pump (wetted pumping elements) will make a major improvement in the pumps priming capability.

    2.1.2 The motor is turning pump in the wrong direction Centrifugal pumps should never be allowed to operate in the wrong direction of rotation. If the pump is not fitted with an impeller screw the impeller may unscrew from the shaft making contact with the casing, thereby causing seizure.

    To ensure correct direction of rotation the motor can be started and stopped momentarily to check the motor fan is rotating clockwise as viewed from the rear end of the motor.

    Impeller damage

    Fluid Out

    Fluid In View from rear end

    Correct rotation

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    If an impeller screw is fitted, the pump is still capable of achieving low flow with the motor turning in wrong direction.

    If pump is operating in reverse rotation, use motor manufacturers instructions to ensure appropriate direction.

    2.1.3 Valves are closed or there is an obstruction in the suction or discharge pipework Any obstruction in the pipework and/or valves being closed can significantly increase pressure. An increase in pressure on the outlet will have the effect of a decrease in flow, highlighted in curve below:

    Impeller screw

    Pump curve

    Duty point

    Increase in pressure

    Resultant decrease in flow

  • 9

    Pressure action on Surface of liquid (Pa)

    Pressure drop (hfs)

    The following symptons may occur: Low outlet pressure (see 2.2.1) Seal dry running (see 2.5.4) Increase in noise/vibration (see 2.4) Reduced NPSHa (see 2.1.6)

    2.1.4 The end of the suction pipework is not submerged

    In this situation no fluid will be allowed to enter the pump, cavitation may occur (see 2.1.6 below) and potential for dry running of seal (see 2.5.4).

    2.1.5 A strainer or filter is clogged

    If a strainer or filter is clogged in the suction pipework, this will have the effect of increasing pressure loss thereby decreasing flow as described in 2.1.6 below. If a strainer or filter is clogged in the discharge pipework, this will have the effect of increasing pressure loss thereby decreasing flow as described in 2.1.3 above.

    2.1.6 Insufficient Net Positive Suction Head available (NPSHa)

    For satisfactory pump operation the condition at the inlet of a pump is critical. The system on the inlet side of the pump must allow a smooth flow of fluid to enter the pump at a sufficiently high pressure to avoid cavitation. This is called the Net Positive Suction Head, generally abbreviated NPSH. It is critical that the net positive suction head available (NPSHa) in the system is greater than the net positive suction head required (NPSHr) by the pump. The value of NPSHa in the system is dependent upon the characteristic of the fluid being pumped, inlet piping, the location of the suction vessel, and the pressure applied to the fluid in the suction vessel. This is the actual pressure seen at the pump inlet. It is important to note, it is the inlet system that sets the inlet condition and not the pump. NPSHa is calculated as follows: NPSHa (metres) =

    _ _ +ve -ve +ve -ve

    Where: Pa = Pressure absolute above fluid level (bar) hs = Static suction head (m) hfs = Pressure drop in suction line (m) Pvp = Vapour pressure (bar a)

    Static suction head (hs)

    Vapour pressure (Pvp)

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    The term cavitation is derived from the word cavity, meaning a hollow space. In pump terminology, cavitation is an undesirable vacuous space in the inlet port of the pump normally occupied by fluid. The lowest pressure