Bearings in centrifugal pumps - skf.com

1 Bearings in centrifugal pumps Application handbook Table of contents 2 Preface 3 General 8 Pump Bearings 19 Ball Bearings in centrifugal pumps 28 Roller Bearings in centrifugal pumps 33 Bearing technologies for the next generation pump 35 Bearing installation 1. Preface This application handbook is one of a series of application handbooks designed to pro- vide specific application recommendations for SKF customers when used with the SKF General Catalog. It is not possible, in the limited space of this handbook, to present all the informa- tion necessary to cover every application in detail. SKF application engineers should be contacted for specific bearing recommen- dations. The higher the technical demands of an application and the more limited the available experience, the more advisable it is to make use of SKF's engineering service. We hope you find this handbook interest- ing and useful. 2. General Principles of centrifugal pumps A pump is a device for lifting, transferring, of the moving fluid is converted to potential flow based on the direction the fluid enters or moving fluids by suction or pressure energy at higher pressure.

2 As the fluid leaves the inlet (eye) of the impeller, see figure from one position to another. the impeller through the pump discharge, Radial and mixed flow pumps are The centrifugal pump is a type of pump more fluid is drawn into the pump inlet either single or double suction designs. that uses the kinetic energy of a rotating where the pressure is lowest. This fluid A centrifugal pump produces head, H. impeller to impart motion to the fluid, see passes through the impeller as still more as a function of the rate of fluid flow, Q. figure The rotating impeller acceler- fluid enters the impeller. through the impeller, see figure Head ates the fluid through its vanes and into There are three classifications of centrifu- is the energy content in the pumped fluid, the pump casing where the kinetic energy gal pumps : radial flow, mixed flow and axial expressed in meters, m (ft). Figure Figure Classes of centrifugal pumps 'MPX 'MPX. 'MPX. $FOUSJGVHBM QVNQ $FOUSJGVHBM QVNQ.

3 $FOUSJGVHBM QVNQ. " ". ". Radial flow " ". " 3 PUBUJPO 3 PUBUJPO. 3 PUBUJPO. Figure Mixed flow ) 2. )FBE. '. &'. Axial flow 'MPX SBUF #&1. 3. The hydraulic performance of a centrifugal The centrifugal pump impeller is most Pump Operation pump is characterized by the mechanical typically supported on its own shaft and shape and size of the impeller, using an Bearings and driven by an electric motor, A pump is selected for an application to index number called specific speed, ns, see and less often by an engine or a turbine. produce a desired flow and head. The figure The pump shaft is connected to the driver performance curve of a typical radial The specific speed number of a pump is either directly through a flexible coupling centrifugal pump is illustrated in the figure calculated by the following equation: or indirectly by a belt drive. The impeller The curve shows the head, efficiency, n Q1/2 can also be rigidly connected to the motor power requirements, and net Positive Suc- ns = 3/4.

4 H shaft. tion Head required (NPSHr) of the pump where versus the flow. ns = specific speed n = pump rotational speed, r/min Figure Q = pump flow rate, m3/s (US gallons/min). at best efficiency point, BEP. H = pump total head, m(ft) at the BEP. 4 QFDJGJD TQFFE OT. *US units are in parenthesis The characteristics of a pump based on specific speed are approximately as accord- . 4*. ing to the table. 64.. Pump characteristics Specific ns Characteristic speed low 10-35 low flow (500-1750) high head medium 35-85 medium flow (1750-4250) medium head 3 BEJBM 7 BOF 'SBODJT 7 BOF .JYFE GMPX "YJBM GMPX. high 85-160 high flow (4250-8000) low head highest 160-300 maximum flow (8000-15000) minimum head Figure A centrifugal pump consists of a hydrau- lic assembly and a mechanical assembly, see figure The components of the hydraulic assembly are the impeller, casing .FDIBOJDBM )ZESBVMJD Hydraulic assembly (volute), inlet and discharge piping, and BTTFNCMZ BTTFNCMZ Impeller/propeller shaft seal.

5 The components of the mechan- Suction inlet ical assembly are the shaft and Bearings , Volute Seal rings pump frame and housing seals, baseplate, and drive coupling or belt sheaves. Mechanical assembly For petrochemical applications, the Shaft seal Labyrinth pump industry has developed standards for Mechanical the manufacture and supply of centrifugal Packing pumps . Two important standards are the Shaft Bearings ASME/ANSI for chemical process Housing/frame pumps and API 610 for general refinery Drive coupling/sheave service pumps . These standards define the minimum technical requirements for the mechanical design of the pumps and bear- ings etc. Because of the strong American influence on petrochemical plant engi- neering, these standards have worldwide implications. 4. The point of highest pump efficiency is Pump Bearing Loads In single stage end suction pumps , the called the Best Efficiency Point or BEP. magnitude and direction of the net axial This is the pump design point and the load is most influenced by the design of the The pump Bearings support the hydraulic operating point where the flow has the impeller.

6 Four typical impeller designs are loads imposed on the impeller, the mass of least friction and disturbance as it passes illustrated in the figure The semi- impeller and shaft, and the loads due to the through the pump. For lowest power con- open impeller with pump-out vanes and shaft coupling or belt drive. Pump Bearings sumption, the pump is operated between the closed impeller with two wear rings keep the shaft axial end movement and 80 and 100% of BEP. Because of practical and balance holes are most common in lateral deflection within acceptable limits considerations, it is common for a pump to petrochemical and paper mill process ap- for the impeller and shaft seal. The lateral operate in the range of 50 to 120% of BEP. plications. deflection is most influenced by the shaft Pump operation at a flow rate below the stiffness and bearing clearance. BEP causes poor hydraulic performance The hydraulic loads comprise of hydro- Figure and increased hydraulic impeller loads.

7 Static and momentum forces from the fluid. Pump operation above the BEP can result The forces on the impeller are simplified in cavitation and increased vibration. into two components: axial load and radial 'SPOU )VC CBDL. The NPSHr is the head at the pump inlet load. TISPVE TISPVE. (suction) needed for the pump to satisfac- torily draw the fluid into the impeller. If the available head at the pump inlet, called net Axial Load Positive Suction Head available (NPSHa), is The axial hydraulic pressures acting on a less than the pump's NPSHr, cavitation will single stage centrifugal pump are illustrated occur and performance will be reduced. in the figure The axial load is equal to Cavitation is the phenomenon that oc- Figure the sum of the forces: curs when the local pressure of the fluid is 1. the hydrostatic force acting on the less than its vapor pressure and local vapor impeller's front shroud and hub (back). is formed from the fluid. A pump operat- shroud due to the hydraulic pressures ing with insufficient NPSHa, experiencing acting on the surface areas of the cavitation, develops small vapor bubbles shrouds near its inlet that grow in size as they 2.

8 The momentum force due to the change move further into low pressure areas of in direction of the fluid flow through the the impeller. This causes unbalanced flow impeller, and Semi-open impeller and pressure on the impeller. As the vapor 3. the hydrostatic force due to the hydraulic bubbles reenter high pressure areas of pressure acting on the impeller (suction). the impeller, they collapse, exerting forces opening. The hydrostatic forces dominate on the impeller that cause impeller dam- the impeller loading. age, shaft deflection and increased bearing The magnitude and direction of the axial 7 BOF. loading. force may change during the pump start The common nominal pump rotational process owing to varying flow conditions speeds for small and medium size pumps in the side spaces between the impeller are 1,500 and 3,000 r/min at 50 Hz shrouds and casing walls. The changes Semi-open impeller with vanes frequency and 1,800 and 3,600 r/min at in flow conditions and the consequential 60 Hz frequency.

9 Other rotational speeds changes in pressure distributions on the are possible with belt and gear driven impeller shrouds result in changes to the pumps , etc. axial load. 8 FBS SJOH. Figure Closed impeller with one wear ring /14)S. )S. ) 2 /14 .. &GGJDJFODZ .. )FBE.. ' . &'. #BMBODF IPMF. 41 (3 . 108&3 . 108&3 .. Closed impeller with two wear rings 0 'MPX SBUF. #&1 and balance holes 5. In pumps with open and semi-open load than the impeller with balance holes. In a real volute at the BEP, the flow is most impellers, the axial load is normally The magnitude and direction of the axial like that in the theoretical volute except at directed towards the suction side owing load can change from its design value if the cutwater (or tongue) which is needed to the pressure on the large area of the pump-out vane clearance changes due to for the volute construction, see figure hub shroud. Closed pump impellers with wear or is not set within tolerance and if The disturbance of flow at the cutwater wear rings can have near balanced (zero) balance holes become plugged with debris.

10 Causes a non-uniform pressure distribution axial load or more usually low axial load Pump-out vanes and balance holes reduce on the circumference of the impeller result- directed towards the suction. With in- pump efficiency by several percentage ing in a net radial load on the impeller. The creased suction pressures, the axial load points. radial load is minimum when the pump can be directed opposite to the suction. The axial load in double suction impel- is operating at the BEP and is directed Impeller pump-out vanes and balance ler pumps is balanced except for possible towards the cutwater. The radial load holes are employed to balance the axial imbalance in fluid flow through the two increases in magnitude and changes direc- load. impeller halves. tion at flows greater than and less than the Pump-out vanes (also called back vanes) In multistage pumps , impellers are BEP, see figure are small radial vanes on the hub shroud arranged in tandem and back-to-back to used to increase the velocity of the fluid balance the axial load.