HYDRAULIC BALANCING OF CENTRIFUGAL PUMP – BALANCING DRUM

                In this post, the axial thrust forces distribution of the multi stage centrifugal pump and the concept of balancing drum will be discussed in detail. Below figure shows pressure distribution in a multi stage centrifugal pump.

 

To understand the concept of hydraulic balancing in multistage centrifugal pump, first we will consider the resultant pressure acting on the pump impellers can be understood by following cases.

Case I:  without installing any balancing devices.

From the above figure, it is clear that

The net pressure acting towards suction side = P5-P1.

The net pressure acting towards discharge side = P5. (Please not this is net pressure without installing any balancing devices)

To hydraulically balance on both suction side and discharge side, the net pressures acting on suction side and discharge side shall be made equal which can be done by installing balancing device.

Case II:  After installing balancing device

For this case also, the reference can be taken from above figure.

The net pressure acting towards suction side = P5-P1.

The net pressure acting towards discharge side = P5-Pb. (Pb is the balancing chamber pressure or pressure developed due to the design of balancing chamber and installation of the balancing device).

For perfect hydraulic balancing, P1 = Pb.

But in practical, Pb (balancing chamber pressure) shall be maintained in such a way that it is slightly greater than suction pressure as the fluid from balancing chamber will be going back to the suction side of the pump.

The balancing device shall balance this axial thrust and this hydraulic device may be balancing drum, balancing disc or combination of both.

The balancing drum is installed at last stage of the pump and is separated by a small radial clearance from the stationary portion of the balancing device called balancing drum head which is fixed to the pump casing.

Through the radial clearance, there will be fluid leakage and balancing chamber pressure is maintained based on the design parameters (dimensions) of the balancing drum.

The forces acting on the balancing drum are as follows.

1.       Towards the discharge end = Discharge pressure * Front balancing area (B) of the drum.

2.       Towards the suction end = Back pressure in balancing chamber * Back balancing area (C) of the drum.

Balancing drum diameter can be selected to balance the axial thrust completely or 90 to 95% depending on the desirability of carrying any thrust bearing loads.

The main drawback of the balancing drum is it lacks automatic compensation of any changes in axial thrust caused by varying impeller reaction characteristics.

For example, if axial thrust and balancing drum forces becomes unequal, then the rotating part will tend to move in direction of greater force. In this case, the thrust bearing must prevent excessive movement of rotating part.

Balancing drum performs no restoring function until such time as drum force again equals to axial thrust.

This drawback can be avoided using the design of the balancing disc.

 

 

INTRODUCTION TO HYDRAULIC BALANCING OF CENTRIFUGAL PUMP – MULTI STAGE

                Hydraulic axial thrust developed in multi stage pump will be sum of the axial thrust acting on the individual impeller which is very high in magnitude. To balance hydraulically for this high magnitude axial thrust, the special hydraulic balancing device used is balancing drum or balancing disc.  In some applications the combination of two balancing devices is used (balancing drum and balancing disc).

Balancing drum, balancing disc and a combination of both is shown in following figures.

 

                                                                        Fig: Balancing drum

                                                                Fig: Balancing disc

                                  Fig: Combination of both balancing drum and balancing disc

Balancing drum

Balancing drum is separated by a small radial clearance from the stationary portion of the balancing device called balancing drum head which is fixed to pump casing.

Balancing disc

Balancing disc is separated by small axial clearance from balancing disc head which is fixed to casing. It is fixed to shaft and rotates with it.

 Further details regarding balancing drum, balancing disc and axial thrust balancing forces in balancing drum and balancing disc in detail will be discussed later.

HYDRAULIC BALANCING OF CENTRIFUGAL PUMP – SINGLE STAGE

During operation of centrifugal pump, the hydraulic imbalance is caused due to the different values of pressures in the pump like discharge pressure and suction pressure acting on either side of the impeller and around circumferential direction of the impeller.

The following are the two hydraulic forces acting on the impeller.

Ø  Radial Thrust

Ø  Axial Thrust

Bearing life of the centrifugal pump also depends on the above two hydraulic forces.

Radial Thrust

The force generated in radial direction due to unequal pressure generation in volute is called radial thrust.

Radial thrust can be reduced by

ü  Double volute casings

ü  Diffuser type casings

 

 Axial Thrust

The force generated in axial direction due to unequal pressure generation on either side of the impeller is called axial thrust.

Axial thrust can be carried away by thrust bearings.

  Axial thrust can be reduced by

ü  Provision of balancing holes on pump impeller

ü  Provision of back vanes or back plate on the impeller

Provision of balancing holes on pump impeller

·         Drilling hole through impeller back shroud into front portion of impeller. There by pressure distribution is in such a way that axial thrust is balanced up to some extent.

Figure below shows the pressure distribution of the fluid and reduction in axial thrust in the centrifugal pump when balancing hole is provided. 

·         Leakage through holes is directed against the flow in the impeller eye causing disturbances.

·         But balancing by this method will not fully reduce axial thrust.

Provision of back vanes or back plate on the impeller

·         Radial vanes are used on back shroud of the impeller to reduce the pressure in the space between the impeller and pump casing.

·         Radial back vanes provided at back shroud of impellers acts as an auxiliary impeller which restricts the entry of the liquid into the clearance between impeller back shroud and casing cover.

·         Balancing by this method reduces axial thrust in a better way when compared to provision of balancing holes.

Figure below shows the pressure distribution of the fluid and reduction in axial thrust in the centrifugal pump when back vanes are provided. 

Pic credits

Axial Thrust in centrifugal pumps - Experimental analysis (15th international conference on experimental mechanics)

 

CENTRIFUGAL PUMP COMPONENTS

The following are the major components of a centrifugal pump and are shown in following figure.

                    

                     

Pump volute casing

Casing that receives the fluid being pumped by the impeller. The shape of the volute casing is in such a way that the cross sectional area gradually increases towards discharge resulting in the conversion of kinetic energy to the pressure energy at the discharge.

Impeller

Impeller is the main part of a centrifugal pump that pumps the process fluid from suction side to discharge side. It consists of equally spaced vanes around the impeller eye. Impeller eye is the entry point at which fluid enters into the impeller.

Shaft

The main function of the shaft is to transmit the torque and supports the impeller and all other parts of a centrifugal pump.

Key

Key is a metallic part which serves as a locking mechanism between the two rotating parts like impeller and shaft etc. It is installed in a groove called key way.

Shaft sleeve

Shaft sleeve is used to protect the shaft from mechanical damage caused by the seals or bearings and chemical damage caused by the aggressive fluids.

Bearings

Bearings give support to the shaft in rotation and also reduce frictional forces. In centrifugal pump bearings are usually designated as inboard bearing and outboard bearing.

Normally both radial bearings and thrust bearings are used in a pump to compensate for both radial and thrust loads.

Mechanical seal or gland packing

Mechanical seal or gland packing prevents leakage of the process fluid into atmosphere at a point where the pump shaft passes out through the casing.

Considerable minimum leakage is allowed in gland packing.

Approximately zero leakage is allowed in mechanical seal.

Gland packing is used in non hazardous services and mechanical seal is used for hazardous fluids where no fluid is allowed to leak into atmosphere.

Lantern ring

The lubricating or cooling fluid supplied to packing is distributed through lantern ring. It is commonly a brass ring with two holes that allows the lubricating or cooling fluid to easily pass through.

Bearing isolator

The bearing isolator solves the purpose of the following.

ü  To avoid the passage of process fluid to bearing housing.

ü  To avoid oil leakage from oil sump to outside of bearing housing.

ü  To avoid dirt into housing in dirty areas.

Deflector

If mechanical seal leaks then the deflector deflects the process fluid perpendicular to the shaft to avoid mixing with bearings and oil. It is generally made up of rubber or plastic.

Oil ring

Oil ring or oil flinger is used to circulate the oil in the oil sump.

Wear rings

These are placed at impeller and casing as to avoid the damage of the impeller and casing. Impeller wear ring gets damaged before the impeller damage and casing wear ring gets damaged before casing damage. These are easily replaceable parts.

Breather

It is a special part which let out the vapours liberated in the oil sump. Allows the air to enter and exit the lube oil sump without particulate contamination.

Gaskets

Gaskets prevent the leakage of the process fluid from wet parts of the pump to atmosphere.

These are placed mainly at five positions.

1.       Between impeller and impeller nut.

2.       Between impeller and shaft sleeve.

3.       Between back plate and casing.

4.       At drain plug.

5.       Between mechanical seal gland and seal.

The following are the wet parts of the pump.

ü  Volute casing

ü  Impeller nut

ü  Impeller

ü  Wear rings (casing wear ring and impeller wear ring)

ü  Back plate

ü  Shaft sleeve

ü  Mechanical seal or gland packing

ü  Seal gland

Throat bush

It serves as the restriction for the fluid flow between impeller and the mechanical seal area.

Constant level oiler

It acts as a lube oil reservoir to the lube oil sump and used to maintain a required level of oil in the lube oil sump for lubrication.

Bearing Housing

Bearing housing support bearings and protect from contaminants for lubrication.

 

 

CENTRIFUGAL PUMPS

Centrifugal pumps are used to transport fluids by the conversion of rotational kinetic energy developed by the impeller to the hydrodynamic energy.

Classification of centrifugal pumps

                    

                         Classification based on API 610

     

 

 

 

PUMPS

Introduction

Pump – A Mechanical device used for following function.

Function

 Increase the pressure of the fluid (liquids / slurries) and transport from one location to other.

Applications

ü  Process / Oil & Natural Gas Industry

ü  Refinery

ü  Power Generation

ü  Irrigation

ü  Fire Fighting

ü  HVAC (Heating, ventilation, and air conditioning)

Principle

The pump develops the flow by converting energy of an electric motor into kinetic energy of operating fluid and then into pressure energy of a fluid that is being pumped.

Classification of pumps

 

 

SURGE OF A CENTRIFUGAL COMPRESSOR

 

Surge

Surge is defined as a sudden upward movement or a sudden large increase.

Compressor Surge

For compressor surge, the sudden upward movement is the increase in discharge pressure and

Compressor surge can be defined as

ü  The point at which the compressor cannot add enough energy to overcome the system resistance or back pressure.

ü  The unstable phenomenon that occurs at low discharge flow and high discharge pressure.

ü  The phenomenon of momentary flow reversal.

ü  The Surge Point represents the minimum inlet volumetric flow where a compressor can maintain performance.

Compressor surge line is the locus of all the surge points for a compressor operating on different RPMs or different IGV positions and can be shown as follows.

Here Valve Sizing line is also known as surge margin line which is calculated as the safety factor for anti surge operation and discussed in detail under the topic of "Surge Test"

                                    

 Compressor surge cycle

1.       Compressor is working at point D; decreased inlet flow shifts the operating point operating point near to surge point A.  Continued reductions in flow will eventually trigger surge at point A.  

2.       When surge occurs, the flow direction reverses to point B.

3.       The flow reversal causes the inlet and discharge pressures to start equalizing. As pressures equalize, the reversed flow rate decreases to point C.

4.       Now the compressor regains its ability to produce head and returns to its performance curve at point D.

                 

 

Surge cycle features

• Flow reverses in 20 to 50 milliseconds. (From A to B in above graph)
• Flow regains to produce normal head in 20 to 120 milliseconds. (From C to D in above graph)
• Surge cycles (A-B-C-D-A) at a rate of 0.3 s to 3 s per cycle.

         

 Surge results in

• Compressor discharge piping vibration 

•  Rise in compressor discharge temperature 

•  Damage to seals, bearings, impeller and shaft. 

•  "Whooshing noise or "Clanking" noise 

•  Unstable Flow and Pressure & Trips may occur.