The Magazine For Pump Users Worldwide Reprinted from November 2007 pump zone com Materials of Construction
The Magazine For Pump Users Worldwide  Reprinted from November 2007  pump-zone.com  Materials of Construction
Materials of Construction Building a Better Boiler Feed Pump Robert Aronen Boulden International Dennis Plaizier and Dave Sinclair Hydro Scotford Inc Say goodbye to concerns about pump seizure By properly applying composite materials to a troublesome boiler feed water pump this large process facility in Alberta can now run one pump during full rate operation with very low vibration and higher efficiency T he reliability and efficiency impacts of close clearance wear components wear rings inter stage bushings throat bushings and pressure reducing bushings have been well documented In 1985 Bloch and Geitner cited the following potential problems associated with excessive wear ring clearance efficiency losses loss of rotor stability shaft breakage driver overloading bearing overheating or failure unequal load sharing in parallel pump operation noise and damage typically associated with cavitation and possible total pump destruction p 31 36 One application for which the wear components are particularly important is boiler feed water In the typical process plant the boiler feed water pump is a multistage horizontally split between bearings design that is heavily dependant upon the wear components for rotor stability Material selection is very important to ensure long term reliability in this service The value of proper application and installation of modern composites is demonstrated by the results from a 9 stage boiler feed water pump from a large process facility in Central Alberta Why Wear Components are Important The close clearance wear components in a centrifugal pump perform similar functions They separate high pressure areas within the pump from lower 2 NOVEMBER 2007 pressure areas via a minimal clearance between a rotating and stationary member Due to the differential pressure across these components there is substantial flow from the high pressure to lower pressure regions of the pump recirculation flow If inadequate clearance lies between two metal components the rotating and stationary elements could possibly seize and lead to substantial pump damage Bloch 1988 Conversely as the clearance between rotating and stationary components increases the recirculation flow within the pump increases and efficiency drops Bloch and Geitner 1985 Over time this will become evident to the pump operators as recirculation flow increases to the point where the pump can no longer operate at design capacity What the operators may not notice is that the wear components also contribute substantially to rotor stability Lobanoff and Ross 1992 p 440 451 Increased clearance at the wear components can lead to higher vibration shorter bearing life and the potential for high energy failure modes such as shaft breakage Furthermore recirculation flow at the first stage impel Diameter of Wear Rings API 610 Clearance for metal wear rings Typical Clearance for Composite Wear Rings 4 001 5 000 in 102 127 mm 0 015 in 0 38 mm 0 007 in 0 18 mm 5 001 6 000 in 127 152 mm 0 017 in 0 43 mm 0 008 in 0 20 mm 6 001 7 000 in 152 178 mm 0 018 in 0 46 mm 0 009 in 0 23 mm 7 001 8 000 in 178 203 mm 0 019 in 0 48 mm 0 010 in 0 25 mm Table 1 Typical clearance values for metal and composite wear rings www pump zone com a PUMPS SYSTEMS reprint
Materials of Construction  Building a Better Boiler Feed Pump Robert Aronen, Boulden International  Dennis Plaizier and Da...
Property English Units Thermal Expansion Coefficient X Y plane 1 8 X 10 6 in in F Thermal Expansion Coefficient Z direction to 300 F 180 X 10 6 in in F Dynamic Coefficient of Friction 25 ft min 0 20 Limiting PV 155 000 ft min psi Hardness 75 80 Shore D Water Absorption
Property  English Units  Thermal Expansion Coefficient  X-Y plane   1.8 X 10-6 in in   F  Thermal Expansion Coefficient  Z...
Materials of Construction Temperature 240 F 120 C Suction Pressure 26 psig 182 kPa Discharge Pressure 1151 psig 7936 kPa Rated Power 317 hp 237 kW Speed 3550 rpm Table 3 Process conditions for boiler feed pump This design includes a double suction first stage impeller with wear rings an eye side wear ring and inter stage bushing at each of the other eight stages two throat bushings and a pressure reducing bushing between the 5th and 9th stages The impellers are 9 125 in 232 mm diameter with 5 in 127 mm diameter wear rings This pump design creates unique challenges which are particularly suited to the application of composite wear components The rotor configuration a long thin shaft see Figure 2 suspended between bearings relies heavily upon the wear rings for rotor stability While the pump is idle there is substantial rotor sag about 0 008 in 0 2 mm at the center bushing The rotor sag goes away only when the pump is running and hydrodynamic forces at the wear rings are established During alignment of the pump to the driver slow roll if coupled to a steam turbine driver or at start up there will be contact at the center stage bushing and other wear components During full speed operation off design conditions like low flow operation dry running or cavitation can also cause contact between rotating and stationary components potentially leading to seizure The service conditions shown in Table 3 were typical for a process plant boiler feed pump Pump Retrofit Details To optimize the performance of this pump during the available repair time the repair facility selected a composite material for all of the stationary wear components The existing 12 percent chrome metal rotating components were used in combination with the composite material Running clearance between rotating and stationary components was reduced to 0 008 in 0 203 mm 50 percent of the original design values Instead of fabricating new components with complex geometry for the stationary composite components the existing components were used as holders for composite inserts The existing metal bores were increased and thin radial wall composite inserts were manufactured and pressed into the metal holders Additional design detail was needed for the split interstage bushings as shown in Figure 3 Using the existing components as holders for the composite reduced the required machining time eliminating the need for milling slots and other details on the outside diameter of the components 4 NOVEMBER 2007 Figure 3 Composite insert in metal case ring holder To prevent axial movement of the composite the metal holders incorporated a small shoulder on the low pressure side There were no pins or screws used for anti rotation purposes the composite relying upon a heavy interference fit for antirotation With a very low coefficient of friction and other properties that reduce risk of seizure this method of installation has proven effective with this composite during the past ten years of field experience Turning the Rotor Retrofitting a pump with composites is a fairly straightforward proposition However as the clearance at the wear components decreases rotor concentricity becomes more important To ensure success with the new tighter clearance the repair facility must pay attention to a few fundamental details Before the rotor is installed the pump should be fully inspected The split line should be checked to ensure it is true the line bore and seal chamber bore should be checked for concentricity see Figure 4 and the rotating element should have limited run out typically 0 002 in max www pump zone com Figure 4 Line boring the pump case a PUMPS SYSTEMS reprint
Materials of Construction  Temperature  240  F  120  C   Suction Pressure  26 psig  182 kPa   Discharge Pressure  1151 psi...
Individual components should be checked for condition and concentricity For example if using the existing case ring as a holder for a composite insert the metal ring should fit properly within the pump case Loose or distorted components should be repaired or replaced as needed Once the rotor is in place the case bolted together and the bearing housings doweled and bolted in place one final check is needed The rotor should turn freely which requires a reasonable understanding of the pump internals Due to the aforementioned rotor sag along with friction from the mechanical seal faces and bearings there will be contact between rotating and stationary components However the resistance should not be excessive which would indicate mechanical interference between rotating and stationary parts Once the rotor is turning a competent repair facility will be capable of determining whether the rotor is turning freely or is inhibited by mechanical interference which could cause premature failure of the pump Composite wear materials reduce the risk of pump seizure and can be installed with tighter clearances than those listed in the API 610 standard Results of Retrofit The boiler feed water pump in Alberta has been running since June 2007 and the retrofit has delivered everything plant personnel wanted One pump now meets full load production demands Risk of metal to metal seizure is dramatically reduced The pump is running with very low vibration compared to previous levels As an added bonus amperage draw on the motor has fallen by 5 percent Using the rated power of 317 hp 237 kW and assuming a power cost of 0 06 kWh a typical price in North America this is worth a lot of money For this one pump the plant will save roughly 6 200 for every year of operation a PUMPS SYSTEMS reprint www pump zone com NOVEMBER 2007 5
Individual components should be checked for condition and concentricity. For example, if using the existing case ring as a...
Materials of Construction Conclusions The benefits of running a pump with reduced clearance at the wear rings and similar components are well documented by field studies and pump design texts Composite materials allow pump repair facilities to reduce the clearance at the wear components to values substantially less than the recommended minimums for metal components as listed in API 610 Furthermore the latest generation composites have material properties which can stand up to tough services process upsets and off design operation reducing the risk of pump failure Quality pump overhaul practices help deliver the full benefit of composite materials with reduced clearance Thorough component inspection verification of rotor concentricity within the case and proper composite installation methods contributed to the success of this application P S References 1 Aronen R Boulden B and Russek M 2007 Driving Pump Reliability Forward with Advanced Composite Wear Rings Proceedings of the 23rd International Pump Users Symposium Turbomachinery Laboratory Texas A M University College Station Texas pp 15 19 2 Bloch H P 1988 Practical Machinery Management for Process Plants Volume 1 Improving Machinery Reliability Second Edition Houston Texas Gulf Publishing Company 3 Bloch H P and Geitner F K 1985 Practical Machinery Management for Process Plants Volume 4 Major Process Equipment Maintenance and Repair Houston Texas Gulf Publishing Company 4 Komin R P 1990 Improving Pump Reliability in Light Hydrocarbon and Condensate Service With Metal Filled Graphite Wear Parts Proceedings of the Seventh International Pump Users Symposium Turbomachinery Laboratory Texas A M University College Station Texas pp 49 54 5 Lobanoff V S and Ross R R 1992 Centrifugal Pumps Design and Application Second Edition Houston Texas Gulf Publishing Company 6 Pledger J P 2001 Improving Pump Performance Efficiency with Composite Wear Components World Pumps Number 420 Robert Aronen is the managing director of Boulden International Brussels Belgium 32 498 56 20 53 robert bouldencompany com www bouldencompany com Dave Sinclair is the general manager and Dennis Plaizier is the sales manager at Hydro Scotford Inc Range Road 215 Highway 15 Box 3298 Fort Saskatchewan Alberta T8L2T3 780 992 0100 Fax 780 992 0200 dsinclair hydroscotford com dplaizier hydroscotford com www hydroscotford com 6 NOVEMBER 2007 www pump zone com a PUMPS SYSTEMS reprint
Materials of Construction Conclusions The benefits of running a pump with reduced clearance at the wear rings and similar ...