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Rotary pump drives
S PECIAL
FEATURE
Rotary p u m p drives The m o s t c o m m o n d r i v e f o r r o t a r y p u m p s is the AC i n d u c t i o n motor. It is, by f a r , the w o r k h o r s e of m o d e r n i n d u s t r y . H o w e v e r , it is o n l y o n e of a g r e a t v a r i e t y o f d r i v e types a n d styles. This a r t i c l e will f a m i l i a r i z e the r e a d e r with s o m e a l t e r n a t i v e s t h a t h a v e been u s e d with success. These a l t e r n a t i v e s o f t e n p r o v i d e greater versatility and practicality u n d e r less t h a n i d e a l c i r c u m s t a n c e s .
Fig. 1. Top: Horizontal, motor driven pump. Middle: Double extended motor driving two pumps (opposite rotations); Bottom: Face mounted pump and motor. (right) Conventional horizontal motor driven pump.
WORLD PUMPS DECEMBER 1995
I By James R. Brennan, Imo Industries Inc., Monroe, NC, USA Electric motors Figure 1 shows b o t h a single e x t e n d e d electric m o t o r direct driving a p u m p a n d a double e x t e n d e d version, a c o m m o n a r r a n g e m e n t for p u m p s providing fluid power to c o m p l e x hydraulic machinery. P u m p s can be foot m o u n t e d or flange m o u n t e d ( b o t t o m view). When flange m o u n t e d , the p u m p - t o - m o t o r bracket is usually m a c h i n e d to tolerances t h a t allow the m o t o r a n d p u m p aligning pilot d i a m e t e r s to engage the b r a c k e t with sufficient accuracy to provide good shaft-to-shaft alignment. Note t h a t the two p u m p s m u s t be p r o v i d e d for o p p o s i t e directions of rotation; one is clockwise while t h e o t h e r is counterclockwise. Both designs have b e e n u s e d b o t h h o r i z o n t a l l y a n d with the drive shafts vertical. For vertical configurat i o n s , t h e flange m o u n t i n g a r r a n g e m e n t is more readily serviceable a n d is frequently found in engine room services a b o a r d ships where horizontal space is at a p r e m i u m . In the vertical a r r a n g e m e n t , if the p u m p is on top, provisions should be m a d e to detect shaft seal leakage which can e n t e r into the electric motor. Hollow shaft electric motors are often used for vertical, deep well style p u m p s where relatively long drive shafts are needed. The uppermost shaft is e x t e n d e d t h r o u g h the m o t o r ' s hollow shaft, keyed in place for r o t a t i o n a n d a d j u s t e d in height with an a d j u s t i n g n u t a r r a n g e m e n t , figure 2, left view. This allows p o s i t i o n i n g the p u m p ' s rotor w i t h i n the p u m p casing from the drive end, even w h e n the p u m p is 65 or more feet (20 or more meters) below the motor. Hollow shaft right angle bevel gear boxes are also used in this a r r a n g e m e n t w h e n the p r i m e mover is to be a horizontal shaft machine, right view, figure 2. In both cases, the dead weight of t h e vertical shafting as well as m o d e s t a m o u n t s of a x i a l h y d r a u l i c loads from the p u m p are absorbed in the m o t o r or gear box b e a r i n g system. Synchronous AC electric m o t o r speeds are a f u n c t i o n of the n u m b e r of "poles" or w i n d i n g s in t h e m o t o r s t a t o r a n d the frequency of the a l t e r n a t i n g c u r r e n t supplied. C o m m o n motors available include:
((} 1995, Elsevier Science Ltd 0262 1762/95/$7.00
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PECIAL FEATURE
Synchronous RPM AT Poles 2 4 6 8 10 12
50 HZ
60 HZ
3000 1500 1000 750 600 500
3600 1800 1200 900 720 600
The full load speed of these m o t o r s will d e p e n d on t h e i r size a n d p e r c e n t load b u t it is typically 95 to 99 p e r c e n t of s y n c h r o n o u s speed a n d t h u s a c c o u n t s for p u m p r a t e d s p e e d s n o r m a l l y being shown as 1750 rpm, 2900 r p m a n d the like. Larger a n d / o r lightly loaded m o t o r s will o p e r a t e closest to t h e i r s y n c h r o n o u s speed. At four or more poles, m o t o r size a n d cost are inversly p r o p o r t i o n a l to the n u m b e r of poles. AC m o t o r s can also be provided in multiple, fixed speed designs u s i n g one or more windings. Single w i n d i n g , m u l t i s p e e d AC m o t o r s will o p e r a t e at two speeds in the ratio of 2:1, for e x a m p l e 1800 a n d 900 rpm. These m o t o r s allow for slow speed at startup, p e r h a p s for a cold, very Viscous liquid c o n d i t i o n a n d can be switched to high speed o p e r a t i o n w h e n desired. Multiple w i n d i n g s can allow four or more selectable fixed speeds as a f u n c t i o n of the frequency of the c u r r e n t a p p l i e d a n d the w i n d i n g s chosen to be energized. While n e i t h e r c o m m o n n o r i n e x p e n sive, m u l t i s p e e d m o t o r s can save s u b s t a n t i a l energy costs if wide v a r i a t i o n s in p u m p flow are n o r m a l l y needed for the application. They are generally less costly t h a n full variable speed electric drive systems. Motor designs are available in with c o n s t a n t t o r q u e or c o n s t a n t power capability. Most AC electric m o t o r s are bidirectional. The direction of r o t a t i o n can be reversed by reversing two of the t h r e e power connections. An u n u s u a l electric m o t o r drive in very c o m m o n b u t h i d d e n use is t h e s u b m e r s e d electric motor. Many low rise p a s s e n g e r a n d freight elevators are hydraulic r a t h e r t h a n cable operated. The electric m o t o r a n d hydraulic p u m p are m o u n t e d inside the hydraulic oil t a n k with the p u m p a n d m o t o r completely s u b m e r g e d in oil. This saves space as well as m o t o r a n d p u m p complexity, figure 3. The p u m p shaft plugs into the m o t o r shaft. The p u m p shaft end b e a r i n g also acts as the m o t o r shaft end bearing. Drive speed is typically 2 pole (50 or 60 HZ) a n d full load speed is in the o r d e r of 92 to 95 p e r c e n t of s y n c h r o n o u s speed (3400 r p m would be n o m i n a l for a 60 hz, 2-pole drive). It is a lower speed t h a n c o n v e n t i o n a l electric m o t o r s d u e to the friction drag of the m o t o r r o t a t i n g in oil r a t h e r t h a n air. This m o t o r style is n o t sealed b u t fully open a n d
MOTOR GEAR
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PUMP \ J e x p o s e d to the liquid e n v i r o n m e n t . Reliability has proven excellent. Before leaving electric motors, suffice it to say t h a t there are variable frequency m o t o r controls allowing variable flow rates. With a p p r o p r i a t e c o n t r o l systems, t h e s e c a n allow p u m p s to provide m e t e r e d flow, c o n s t a n t p r e s s u r e or a n y m a n n e r of p u m p i n g characteristics to suit the designer's purposes. DC m o t o r s can also provide v a r i a b l e s p e e d for p u m p c o n t r o l a n d are f r e q u e n t l y used as emergency b a c k u p p u m p drives tied to a b a t t e r y s u p p l y for power. The b a t t e r y packs allow p u m p o p e r a t i o n for sufficient time to safely s h u t down the system served
WORLD PUMPS DECEMBER 1995
Fig. 2. Left: Vertical, motor driven deep well pump; right: Deep well pump with right angle gear on top.
Deep well screw pump for ship and barge cargo tanks.
S PECIAL FEATURE
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Fig. 3. Submersible pump/motor for hydraulic elevator service.
by the p u m p . An e x a m p l e of such a n arrangem e n t is a l u b r i c a t i n g oil p u m p drive which services a large m o t o r - g e n e r a t o r set. Several m i n u t e s are required to b r i n g the motor-gene r a t o r set to a stop in a n emergency s h u t d o w n mode. The DC m o t o r drive lube p u m p reliably provides l u b r i c a t i o n to the system b e a r i n g s while the m a c h i n e r y coasts to a stop.
Turbines
T u r b i n e s are the n e x t m a j o r class of drives c o m m o n l y used for rotary p u m p s . They are found in a variety of industries. The s t e a m turbine, like m o s t k i n d s of turbines, is a high speed driver. Even large t u r b i n e s are rarely o p e r a t e d below a b o u t 1500 r p m due to efficiency c o n s i d e r a t i o n s as well as size a n d cost. For high speed p u m p s , direct drive speeds u s i n g t u r b i n e s are typically 3000 to 3600 rpm. Speeds to 5000 r p m are n o t u n c o m m o n a n d small d i s p l a c e m e n t p u m p ope r a t i o n beyond 12,000 r p m has been successfully achieved. Steam t u r b i n e drivers are f o u n d in refinery service as well as o t h e r process industries where s t e a m is g e n e r a t e d for process heat. It c a n be the p r i m a r y power s u p p l y for driving p u m p s or, sometimes, the auxiliary power supply acting as b a c k u p to a n electric m o t o r drive. Figure 4, top, is a typical h o r i z o n t a l s t e a m t u r b i n e drive. On critical services, a t u r b i n e drive a n d a m o t o r drive p u m p will o p e r a t e in m a i n a n d s t a n d b y m o d e s side by side. Occasionally, the through-drive a r r a n g e m e n t shown is used; the o u t e r m o s t driver being s t a n d b y a n d with a n over
Fig. 4. Top: Steam turbine through driving pump via double extended motor; Bottom: Motor through driving pump via double extended pump.
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WORLD PUMPS DECEMBER 1995
S r u n n i n g c l u t c h c o u p l i n g i n s t a l le d b e t w e e n t h e drivers. Alternatively, t a n d e m p u m p s , lower view, are s o m e t i m e s used for s e p a r a t e f u n c t io n s b u t driven from a c o m m o n shaft. Fo r rotary p u m p s serving n a t u r a l gas p i p e l i n e centrifugal c o m p r e s s o r s , s m a l l s t e a m t u r b i n e s are s o m e t i m e s used as gas e x p a n s i o n t u r b i n e s driving e m e r g e n c y p u m p s . In t h e e v e n t of a c o m p l e t e p o w e r failure, c o m p r e s s o r s d i s c h a r g e gas is a l l o w e d t o flow t h r o u g h a t u r b i n e e x h a u s t i n g to c o m p r e s s o r inlet or flare a n d p r o v i d e s t h e m o t i v e force to keep l u b r i c a t i n g oil p u m p s o p e r a t i n g while t h e c o m p r e s s o r t r a i n is c o a s t i n g to a stop. Most r o t a r y p u m p s are in t h e 1000 kw r a n g e or less an d so t h e very large kinds of drivers are u n c o m m o n . However, c o m b u s t i o n gas t u r b i n e s have been a p p l i e d as r o t a r y p u m p drivers. Figure 5 shows such an a r r a n g e m e n t w h e r e b y a h e a v y c rud e oil l o ad i n g p u m p is driven t h r o u g h a r e d u c t i o n gear by a c o m b u s t i o n gas t u r b i n e (left side of p h o t o ) . Again, this kind of driver is i n h e r e n t l y a high s p e e d m a c h i n e a n d will often use a r e d u c t i o n gear, as shown in t h e photo, to p r o d u c e an o u t p u t s p e e d suitable for a r o t a r y p u m p drive. U n c o m m o n also, b u t n o t u n h e a r d of, are w a t e r t u r b i n e drives such as t h e Pelton w h e e l design used for relatively low p o w e r a p p l i c a t i o n s w h e r e p r e s s u r i z e d w a t e r is an available p o w e r source. Oil spill c l e a n u p r o t a r y p u m p s have been driven by such t u r b i n e s on m a r i n e a p p l i c a t i o n s using t h e vessel's fire p u m p system to s u p p l y t h e w a t e r power.
PECIAL FEATURE
Fig. 5. Crude oil pump with combustion gas turbine/gear drive.
Virtually all t u r b i n e s m u s t be built for a specific d i r e c t i o n of r o t a t i o n . They ar e n o t b i d i r e c t i o n a l so be sure to specify correctly.
Engines R e c i p r o c a t i n g diesel a n d gasoline engines are c o m m o n p u m p drives in r e m o t e regions w h e r e on-site p o w e r m u s t be self contained. R e m o t e cr u d e oil p i p e l i n e p u m p s are very typical of t h i s service. Like t u r b i n es, e n g i n e s m u s t be
Fig. 6. Diesel/reduction gear driven pipeline pump.
WORLD PUMPS DECEMBER 1995
S PECIAL FEATURE
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Fig. 7. Hydraulic motor or air motor driven pump.
b u i l t for a specific rotation; u s u a l l y counterclockwise as you face the engine o u t p u t shaft. A m a n u a l l y engagable clutch is often p a r t of the diesel package allowing engine s t a r t a n d o p e r a t i o n w i t h o u t driving the p u m p . A t o r s i o n a l analysis is u s u a l l y performed on the drive t r a i n to d e t e r m i n e if a n adverse t o r s i o n a l v i b r a t i o n mode will exist. Drive t r a i n modifications may t h e n be m a d e before c o n s t r u c t i o n to avoid t o r s i o n a l v i b r a t i o n problems. Figure 6 shows a
Fig. 8. Vertical, hydraulic motor drive lubricating pump.
diesel e n g i n e w i t h s e p a r a t e r e d u c t i o n gear driving a pipeline screw p u m p . Some on a n d off road hydraulic m a c h i n e r y will use one or more hydraulic p u m p s driven from a n e n g i n e m o u n t e d p o w e r take-off. A power take-off is a gear b o x with m u l t i p l e o u t p u t drives a n d m o u n t i n g pads. The m o u n t i n g p a d s will accept SAE two or four bolt flange m o u n t e d pumps. The p u m p shafts are n o r m a l l y m a l e s p l i n e s t h a t e n g a g e female s p i n e s i n the power take-off.
[m Rotary m o t o r s The last group of i n d e p e n d e n t drivers found in i n d u s t r i a l p u m p a p p l i c a t i o n s are, themselves, rotary positive d i s p l a c e m e n t machines: air motors a n d hydraulic motors. Note t h a t b o t h are i n h e r e n t l y e x p l o s i o n proof if u s i n g air or oil as the power t r a n s f e r m e d i u m . Air motors, v a n e a n d axial p i s t o n being the m o s t c o m m o n designs, are also u s e d for e m e r g e n c y drives b a c k i n g u p electric m o t o r driven p u m p s as well as for some small, p o r t a b l e p u m p i n g applications. Occasionally, they are used with n a t u r a l gas as the power m e d i u m b u t m o r e often it is low p r e s s u r e (100 psi) compressed air. Hydraulic m o t o r s are u s e d w h e r e h i g h e r power levels are n e e d e d a n d where electricity m a y n o t be available or convenient, figure 7. They are also used to acheive variable speed for variable flow r e q u i r e m e n t s . Figure 8 shows a hydraulic motor (top) driving a vertical l u b r i c a t i n g oil p u m p . The h y d r a u l i c s u p p l y p u m p , n o t shown, is driven from a ship propulsion c o m b u s t i o n gas turbine. The lube p u m p drive is, essentially, the p r o p u l s i o n p l a n t t h a t it serves. System reliability is e n h a n c e d at no cost p r e m i u m .
WORLD PUMPS DECEMBER 1995
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PECIAL FEATURE
Fig. 9. Lubricating pump driven from machine served.
Machine driven A u x i l i a r y p u m p s are o f te n d r i v e n f r o m t h e m a c h i n e r y t h e y serve. T h e r e is no m o r e reliable p o w e r source t h a n t h e i n e r t i a of t h e served e q u i p m e n t . P u m p d i r e c t i o n of r o t a t i o n a n d drive s p e e d m u s t m a t c h t h a t available from t h e served m a c h i n e . Figure 9 shows a r o t a r y p u m p driven from a barrel type boiler feed w a t e r p u m p at 3505 rpm. It p r o v i d e s l u b r i c a t i n g oil to t h e m a c h i n e bearings. Frequently, t h e p u m p tak es s u ct i o n from t h e lube oil r e s e r v o i r w hi c h is, necessarily, l o c a t e d below t h e served m a c h i n e c e n t e r l i n e in o r d e r to gravity d r a i n t h e bearings. Thus, such p u m p s will n o r m a l l y be p r o v i d e d with an inlet p r e s s u r e lower t h a n a m b i e n t a t m o s p h e r i c p r e s s u r e . This i m p o s e s s o m e i n h e r e n t limits on p u m p d i s p l a c e m e n t a nd speed unless a p u m p b o o s t i n g a r r a n g e m e n t is used.
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Fig. 10. Magnetic drive, seal-less three screw pump.
Fig. 11. Motor driven pump via parallel shaft gear reducer.
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Magnetic drives
Magnetic drives have been in service on centrifugal p u m p s for m o r e t h a 20 years a n d are now available for m a n y r o t a r y p u m p s , figure 10. They e l i m i n a t e t h e n e e d for a p u m p s h a f t seal, p e r h a p s t h e w e a k e s t c o m p o n e n t in any p u m p .
WORLD PUMPS DECEMBER 1995
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PECIAL FEATURE
m a g n e t i c m a t e r i a l on its outside diameter. The m a g n e t i c fields i n t e r a c t allowing torque t r a n s m i s s i o n t h r o u g h the b a r r i e r can to the actual p u m p i n g shaft or rotor. P u m p s of this design are used to h a n d l e toxic or d a n g e r o u s liquids as well as liquids t h a t t e n d to crystallize, destroying c o n v e n t i o n a l m e c h a n i c a l seals.
I n d i r e c t drives: gears and belts J I
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J Fig. 12. Top: Belt drive, adjacent pump and motor; Bottom: Belt drive, opposed pump and motor.
The a c t u a l p u m p driver is u s u a l l y a c o n v e n t i o n a l motor. It is flexible coupled to the p u m p shaft. The p u m p shaft drives a coupling h u b with the h u b inside d i a m e t e r lined with very powerful rare e a r t h m a g n e t i c material. This h u b r o t a t e s a r o u n d a can or t h i n walled c o n t a i n e r inside of which is a n o t h e r p u m p shaft m o u n t e d h u b with
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WORLD PUMPS DECEMBER 1995
Reduction gears or belts are s o m e t i m e s used either because the p u m p design requires low drive speeds, to tailor the p u m p o u t p u t flow to more exactly m a t c h the a p p l i c a t i o n required flow, or to acheive very low i n l e t p r e s s u r e operation. Occasionally, belt drives are also used to allow periodic speed changes to change p u m p o u t p u t flow rate, for example, where there are seasonal v a r i a t i o n s in liquid p r o d u c t d e m a n d . The use of r e d u c t i o n gears, figures 6 a n d 11, is straight forward, the only c a u t i o n being to check the driver a n d p u m p directions of r o t a t i o n a n d to specify the duty severity of the gear correctly. If gears or pulleys for belt drive are to be m o u n t e d directly on the p u m p shaft, t h e n the side load due to gear reaction loads or belt t e n s i o n loads m u s t be accommodated. Unless designed for it, m o s t p u m p shafting will n o t a c c o m m o d a t e any significant side loads w i t h o u t risk of a l i g n m e n t damage, shaft breakage or p r e m a t u r e shaft fatigue failure. Directly m o u n t e d g e a r s or pulleys, figure 12, top, m u s t be accurately aligned with t h e i r c o r r e s p o n d i n g drive gear or drive pulley. It is good practice to m i n i m i z e whatever side loads are i n h e r e n t in such a r r a n g e m e n t s to m a x i m i z e p u m p life. Where p u m p shafts are n o t designed or capable of the required side load, a j a c k shaft t h a t carries the gear or pulley between its own bearings can be used, figure 12, bottom. In this a r r a n g e m e n t , t h e p u m p shaft is flexibly c o u p l e d in t h e c o n v e n t i o n a l m a n n e r to the jack shaft. Note t h a t t i m i n g belts (toothed belts) require less t e n s i o n to effectively t r a n s m i t t o r q u e a n d m a y be preferable u n d e r otherwise m a r g i n a l conditions.
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Other drive c o n s i d e r a t i o n s
The use of a fluid coupling between p u m p a n d drive t r a i n will allow infinately variable speed a n d t h u s variable flow over the coupling's speed range, figure 13. The fluid coupling m o u n t s m u c h like a concentric shaft speed reducer. Cooling w a t e r is n o r m a l l y required to remove some of the h e a t due to drive inefficiency w h e n o p e r a t i n g at higher speed r e d u c t i o n ratios. Fluid drives allow
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Fig. 13. Twin screw pump motor driven through fluid coupling.
"soft" s t a r t u p of p u m p s a n d o t h e r e q u i p m e n t in the drive t r a i n r e d u c i n g or e l i m i n a t i n g h a r d m e c h a n i c a l s t a r t u p shocks t h a t can result from electric m o t o r drives. Flexible couplings should n o t be overlooked w h e n a p p l i e d to p u m p drives. Some of these couplings can w i t h s t a n d m i s a l i g n m e n t s m a n y t i m e s w h a t can be tolerated by the drive or driven e q u i p m e n t . Since the coupling costs are b u t a small fraction of a driver or p u m p cost, it is i m p o r t a n t to align c o u p l e d shafts w i t h i n the tighest specification from the driver, gear or p u m p supplier. In the a b s e n c e of a specific r e c o m m e n d a t i o n , t o t a l dial i n d i c a t o r r e a d i n g s of 0.003 inches (0.076 m m ) a n g u l a r a n d parallel are r e a s o n a b l e u p to 3600 r p m drive speeds for m o s t e q u i p m e n t . Do n o t u s e t h e c o u p l i n g m i s a l i g n m e n t capability as your a l i g n m e n t criteria. Some flexible c o u p l i n g designs will impose axial forces on the coupled e q u i p m e n t shafts, p a r t i c u l a r l y couplings with axially r e s t r a i n e d e l a s t o m e r i c c e n t e r m e m b e r s . Be s u r e b o t h coupled machine's bearings can absorb the r e s u l t a n t a x i a l l o a d i n g s or use a l t e r n a t i v e coupling designs. Some flexible couplings are available with integral overload p r o t e c t i o n capabilities. These
i n c l u d e slip clutch as well as s h e a r p i n designs. Such couplings offer a d d i t i o n a l p r o t e c t i o n to the p u m p a n d its drive t r a i n should severe j a m m i n g of the p u m p take place as m i g h t be expected w h e n h a n d l i n g r a n d o m high solids c o n t e n t flow. Be sure t h a t ALL r o t a t i n g shafts, couplings, etc. are properly g u a r d e d to p r e v e n t p e r s o n a l injury. W i t h o u t doubt, o t h e r drive a r r a n g e m e n t s will find t h e i r way into use in the future, the only l i m i t a t i o n s being the designer's creativity, econ o m i c s a n d the user's willingness to try somet h i n g new. The e n g i n e e r i n g p r i n c i p l e s to apply drives are basic a n d f u n d a m e n t a l . Carefully t h o u g h t out systems comprised of a d e q u a t e l y rated c o m p o n e n t s will deliver reliable, efficient service over long periods of time. •
James R. Brennan, currently group m a n a g e r for three pump divisions of Imo Industries specializing in crude oil transport pumps, is a 1973 MIE graduate of Drexel University, Philadelphia, PA, USA. He has over 25 years experience with screw pumps at Imo Industries, is a m e m b e r of the Society of Petrol e u m E n g i n e e r s , and w a s e n g i n e e r i n g m a n a g e r of a pump division for five years. He has authored many papers and articles as well as spoken at a number of industry conferences.
WORLD PUMPS DECEMBER 1995
PECIAL FEATURE
FEATURE
Rotary p u m p drives The m o s t c o m m o n d r i v e f o r r o t a r y p u m p s is the AC i n d u c t i o n motor. It is, by f a r , the w o r k h o r s e of m o d e r n i n d u s t r y . H o w e v e r , it is o n l y o n e of a g r e a t v a r i e t y o f d r i v e types a n d styles. This a r t i c l e will f a m i l i a r i z e the r e a d e r with s o m e a l t e r n a t i v e s t h a t h a v e been u s e d with success. These a l t e r n a t i v e s o f t e n p r o v i d e greater versatility and practicality u n d e r less t h a n i d e a l c i r c u m s t a n c e s .
Fig. 1. Top: Horizontal, motor driven pump. Middle: Double extended motor driving two pumps (opposite rotations); Bottom: Face mounted pump and motor. (right) Conventional horizontal motor driven pump.
WORLD PUMPS DECEMBER 1995
I By James R. Brennan, Imo Industries Inc., Monroe, NC, USA Electric motors Figure 1 shows b o t h a single e x t e n d e d electric m o t o r direct driving a p u m p a n d a double e x t e n d e d version, a c o m m o n a r r a n g e m e n t for p u m p s providing fluid power to c o m p l e x hydraulic machinery. P u m p s can be foot m o u n t e d or flange m o u n t e d ( b o t t o m view). When flange m o u n t e d , the p u m p - t o - m o t o r bracket is usually m a c h i n e d to tolerances t h a t allow the m o t o r a n d p u m p aligning pilot d i a m e t e r s to engage the b r a c k e t with sufficient accuracy to provide good shaft-to-shaft alignment. Note t h a t the two p u m p s m u s t be p r o v i d e d for o p p o s i t e directions of rotation; one is clockwise while t h e o t h e r is counterclockwise. Both designs have b e e n u s e d b o t h h o r i z o n t a l l y a n d with the drive shafts vertical. For vertical configurat i o n s , t h e flange m o u n t i n g a r r a n g e m e n t is more readily serviceable a n d is frequently found in engine room services a b o a r d ships where horizontal space is at a p r e m i u m . In the vertical a r r a n g e m e n t , if the p u m p is on top, provisions should be m a d e to detect shaft seal leakage which can e n t e r into the electric motor. Hollow shaft electric motors are often used for vertical, deep well style p u m p s where relatively long drive shafts are needed. The uppermost shaft is e x t e n d e d t h r o u g h the m o t o r ' s hollow shaft, keyed in place for r o t a t i o n a n d a d j u s t e d in height with an a d j u s t i n g n u t a r r a n g e m e n t , figure 2, left view. This allows p o s i t i o n i n g the p u m p ' s rotor w i t h i n the p u m p casing from the drive end, even w h e n the p u m p is 65 or more feet (20 or more meters) below the motor. Hollow shaft right angle bevel gear boxes are also used in this a r r a n g e m e n t w h e n the p r i m e mover is to be a horizontal shaft machine, right view, figure 2. In both cases, the dead weight of t h e vertical shafting as well as m o d e s t a m o u n t s of a x i a l h y d r a u l i c loads from the p u m p are absorbed in the m o t o r or gear box b e a r i n g system. Synchronous AC electric m o t o r speeds are a f u n c t i o n of the n u m b e r of "poles" or w i n d i n g s in t h e m o t o r s t a t o r a n d the frequency of the a l t e r n a t i n g c u r r e n t supplied. C o m m o n motors available include:
((} 1995, Elsevier Science Ltd 0262 1762/95/$7.00
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PECIAL FEATURE
Synchronous RPM AT Poles 2 4 6 8 10 12
50 HZ
60 HZ
3000 1500 1000 750 600 500
3600 1800 1200 900 720 600
The full load speed of these m o t o r s will d e p e n d on t h e i r size a n d p e r c e n t load b u t it is typically 95 to 99 p e r c e n t of s y n c h r o n o u s speed a n d t h u s a c c o u n t s for p u m p r a t e d s p e e d s n o r m a l l y being shown as 1750 rpm, 2900 r p m a n d the like. Larger a n d / o r lightly loaded m o t o r s will o p e r a t e closest to t h e i r s y n c h r o n o u s speed. At four or more poles, m o t o r size a n d cost are inversly p r o p o r t i o n a l to the n u m b e r of poles. AC m o t o r s can also be provided in multiple, fixed speed designs u s i n g one or more windings. Single w i n d i n g , m u l t i s p e e d AC m o t o r s will o p e r a t e at two speeds in the ratio of 2:1, for e x a m p l e 1800 a n d 900 rpm. These m o t o r s allow for slow speed at startup, p e r h a p s for a cold, very Viscous liquid c o n d i t i o n a n d can be switched to high speed o p e r a t i o n w h e n desired. Multiple w i n d i n g s can allow four or more selectable fixed speeds as a f u n c t i o n of the frequency of the c u r r e n t a p p l i e d a n d the w i n d i n g s chosen to be energized. While n e i t h e r c o m m o n n o r i n e x p e n sive, m u l t i s p e e d m o t o r s can save s u b s t a n t i a l energy costs if wide v a r i a t i o n s in p u m p flow are n o r m a l l y needed for the application. They are generally less costly t h a n full variable speed electric drive systems. Motor designs are available in with c o n s t a n t t o r q u e or c o n s t a n t power capability. Most AC electric m o t o r s are bidirectional. The direction of r o t a t i o n can be reversed by reversing two of the t h r e e power connections. An u n u s u a l electric m o t o r drive in very c o m m o n b u t h i d d e n use is t h e s u b m e r s e d electric motor. Many low rise p a s s e n g e r a n d freight elevators are hydraulic r a t h e r t h a n cable operated. The electric m o t o r a n d hydraulic p u m p are m o u n t e d inside the hydraulic oil t a n k with the p u m p a n d m o t o r completely s u b m e r g e d in oil. This saves space as well as m o t o r a n d p u m p complexity, figure 3. The p u m p shaft plugs into the m o t o r shaft. The p u m p shaft end b e a r i n g also acts as the m o t o r shaft end bearing. Drive speed is typically 2 pole (50 or 60 HZ) a n d full load speed is in the o r d e r of 92 to 95 p e r c e n t of s y n c h r o n o u s speed (3400 r p m would be n o m i n a l for a 60 hz, 2-pole drive). It is a lower speed t h a n c o n v e n t i o n a l electric m o t o r s d u e to the friction drag of the m o t o r r o t a t i n g in oil r a t h e r t h a n air. This m o t o r style is n o t sealed b u t fully open a n d
MOTOR GEAR
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PUMP \ J e x p o s e d to the liquid e n v i r o n m e n t . Reliability has proven excellent. Before leaving electric motors, suffice it to say t h a t there are variable frequency m o t o r controls allowing variable flow rates. With a p p r o p r i a t e c o n t r o l systems, t h e s e c a n allow p u m p s to provide m e t e r e d flow, c o n s t a n t p r e s s u r e or a n y m a n n e r of p u m p i n g characteristics to suit the designer's purposes. DC m o t o r s can also provide v a r i a b l e s p e e d for p u m p c o n t r o l a n d are f r e q u e n t l y used as emergency b a c k u p p u m p drives tied to a b a t t e r y s u p p l y for power. The b a t t e r y packs allow p u m p o p e r a t i o n for sufficient time to safely s h u t down the system served
WORLD PUMPS DECEMBER 1995
Fig. 2. Left: Vertical, motor driven deep well pump; right: Deep well pump with right angle gear on top.
Deep well screw pump for ship and barge cargo tanks.
S PECIAL FEATURE
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Fig. 3. Submersible pump/motor for hydraulic elevator service.
by the p u m p . An e x a m p l e of such a n arrangem e n t is a l u b r i c a t i n g oil p u m p drive which services a large m o t o r - g e n e r a t o r set. Several m i n u t e s are required to b r i n g the motor-gene r a t o r set to a stop in a n emergency s h u t d o w n mode. The DC m o t o r drive lube p u m p reliably provides l u b r i c a t i o n to the system b e a r i n g s while the m a c h i n e r y coasts to a stop.
Turbines
T u r b i n e s are the n e x t m a j o r class of drives c o m m o n l y used for rotary p u m p s . They are found in a variety of industries. The s t e a m turbine, like m o s t k i n d s of turbines, is a high speed driver. Even large t u r b i n e s are rarely o p e r a t e d below a b o u t 1500 r p m due to efficiency c o n s i d e r a t i o n s as well as size a n d cost. For high speed p u m p s , direct drive speeds u s i n g t u r b i n e s are typically 3000 to 3600 rpm. Speeds to 5000 r p m are n o t u n c o m m o n a n d small d i s p l a c e m e n t p u m p ope r a t i o n beyond 12,000 r p m has been successfully achieved. Steam t u r b i n e drivers are f o u n d in refinery service as well as o t h e r process industries where s t e a m is g e n e r a t e d for process heat. It c a n be the p r i m a r y power s u p p l y for driving p u m p s or, sometimes, the auxiliary power supply acting as b a c k u p to a n electric m o t o r drive. Figure 4, top, is a typical h o r i z o n t a l s t e a m t u r b i n e drive. On critical services, a t u r b i n e drive a n d a m o t o r drive p u m p will o p e r a t e in m a i n a n d s t a n d b y m o d e s side by side. Occasionally, the through-drive a r r a n g e m e n t shown is used; the o u t e r m o s t driver being s t a n d b y a n d with a n over
Fig. 4. Top: Steam turbine through driving pump via double extended motor; Bottom: Motor through driving pump via double extended pump.
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WORLD PUMPS DECEMBER 1995
S r u n n i n g c l u t c h c o u p l i n g i n s t a l le d b e t w e e n t h e drivers. Alternatively, t a n d e m p u m p s , lower view, are s o m e t i m e s used for s e p a r a t e f u n c t io n s b u t driven from a c o m m o n shaft. Fo r rotary p u m p s serving n a t u r a l gas p i p e l i n e centrifugal c o m p r e s s o r s , s m a l l s t e a m t u r b i n e s are s o m e t i m e s used as gas e x p a n s i o n t u r b i n e s driving e m e r g e n c y p u m p s . In t h e e v e n t of a c o m p l e t e p o w e r failure, c o m p r e s s o r s d i s c h a r g e gas is a l l o w e d t o flow t h r o u g h a t u r b i n e e x h a u s t i n g to c o m p r e s s o r inlet or flare a n d p r o v i d e s t h e m o t i v e force to keep l u b r i c a t i n g oil p u m p s o p e r a t i n g while t h e c o m p r e s s o r t r a i n is c o a s t i n g to a stop. Most r o t a r y p u m p s are in t h e 1000 kw r a n g e or less an d so t h e very large kinds of drivers are u n c o m m o n . However, c o m b u s t i o n gas t u r b i n e s have been a p p l i e d as r o t a r y p u m p drivers. Figure 5 shows such an a r r a n g e m e n t w h e r e b y a h e a v y c rud e oil l o ad i n g p u m p is driven t h r o u g h a r e d u c t i o n gear by a c o m b u s t i o n gas t u r b i n e (left side of p h o t o ) . Again, this kind of driver is i n h e r e n t l y a high s p e e d m a c h i n e a n d will often use a r e d u c t i o n gear, as shown in t h e photo, to p r o d u c e an o u t p u t s p e e d suitable for a r o t a r y p u m p drive. U n c o m m o n also, b u t n o t u n h e a r d of, are w a t e r t u r b i n e drives such as t h e Pelton w h e e l design used for relatively low p o w e r a p p l i c a t i o n s w h e r e p r e s s u r i z e d w a t e r is an available p o w e r source. Oil spill c l e a n u p r o t a r y p u m p s have been driven by such t u r b i n e s on m a r i n e a p p l i c a t i o n s using t h e vessel's fire p u m p system to s u p p l y t h e w a t e r power.
PECIAL FEATURE
Fig. 5. Crude oil pump with combustion gas turbine/gear drive.
Virtually all t u r b i n e s m u s t be built for a specific d i r e c t i o n of r o t a t i o n . They ar e n o t b i d i r e c t i o n a l so be sure to specify correctly.
Engines R e c i p r o c a t i n g diesel a n d gasoline engines are c o m m o n p u m p drives in r e m o t e regions w h e r e on-site p o w e r m u s t be self contained. R e m o t e cr u d e oil p i p e l i n e p u m p s are very typical of t h i s service. Like t u r b i n es, e n g i n e s m u s t be
Fig. 6. Diesel/reduction gear driven pipeline pump.
WORLD PUMPS DECEMBER 1995
S PECIAL FEATURE
m
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Fig. 7. Hydraulic motor or air motor driven pump.
b u i l t for a specific rotation; u s u a l l y counterclockwise as you face the engine o u t p u t shaft. A m a n u a l l y engagable clutch is often p a r t of the diesel package allowing engine s t a r t a n d o p e r a t i o n w i t h o u t driving the p u m p . A t o r s i o n a l analysis is u s u a l l y performed on the drive t r a i n to d e t e r m i n e if a n adverse t o r s i o n a l v i b r a t i o n mode will exist. Drive t r a i n modifications may t h e n be m a d e before c o n s t r u c t i o n to avoid t o r s i o n a l v i b r a t i o n problems. Figure 6 shows a
Fig. 8. Vertical, hydraulic motor drive lubricating pump.
diesel e n g i n e w i t h s e p a r a t e r e d u c t i o n gear driving a pipeline screw p u m p . Some on a n d off road hydraulic m a c h i n e r y will use one or more hydraulic p u m p s driven from a n e n g i n e m o u n t e d p o w e r take-off. A power take-off is a gear b o x with m u l t i p l e o u t p u t drives a n d m o u n t i n g pads. The m o u n t i n g p a d s will accept SAE two or four bolt flange m o u n t e d pumps. The p u m p shafts are n o r m a l l y m a l e s p l i n e s t h a t e n g a g e female s p i n e s i n the power take-off.
[m Rotary m o t o r s The last group of i n d e p e n d e n t drivers found in i n d u s t r i a l p u m p a p p l i c a t i o n s are, themselves, rotary positive d i s p l a c e m e n t machines: air motors a n d hydraulic motors. Note t h a t b o t h are i n h e r e n t l y e x p l o s i o n proof if u s i n g air or oil as the power t r a n s f e r m e d i u m . Air motors, v a n e a n d axial p i s t o n being the m o s t c o m m o n designs, are also u s e d for e m e r g e n c y drives b a c k i n g u p electric m o t o r driven p u m p s as well as for some small, p o r t a b l e p u m p i n g applications. Occasionally, they are used with n a t u r a l gas as the power m e d i u m b u t m o r e often it is low p r e s s u r e (100 psi) compressed air. Hydraulic m o t o r s are u s e d w h e r e h i g h e r power levels are n e e d e d a n d where electricity m a y n o t be available or convenient, figure 7. They are also used to acheive variable speed for variable flow r e q u i r e m e n t s . Figure 8 shows a hydraulic motor (top) driving a vertical l u b r i c a t i n g oil p u m p . The h y d r a u l i c s u p p l y p u m p , n o t shown, is driven from a ship propulsion c o m b u s t i o n gas turbine. The lube p u m p drive is, essentially, the p r o p u l s i o n p l a n t t h a t it serves. System reliability is e n h a n c e d at no cost p r e m i u m .
WORLD PUMPS DECEMBER 1995
S
PECIAL FEATURE
Fig. 9. Lubricating pump driven from machine served.
Machine driven A u x i l i a r y p u m p s are o f te n d r i v e n f r o m t h e m a c h i n e r y t h e y serve. T h e r e is no m o r e reliable p o w e r source t h a n t h e i n e r t i a of t h e served e q u i p m e n t . P u m p d i r e c t i o n of r o t a t i o n a n d drive s p e e d m u s t m a t c h t h a t available from t h e served m a c h i n e . Figure 9 shows a r o t a r y p u m p driven from a barrel type boiler feed w a t e r p u m p at 3505 rpm. It p r o v i d e s l u b r i c a t i n g oil to t h e m a c h i n e bearings. Frequently, t h e p u m p tak es s u ct i o n from t h e lube oil r e s e r v o i r w hi c h is, necessarily, l o c a t e d below t h e served m a c h i n e c e n t e r l i n e in o r d e r to gravity d r a i n t h e bearings. Thus, such p u m p s will n o r m a l l y be p r o v i d e d with an inlet p r e s s u r e lower t h a n a m b i e n t a t m o s p h e r i c p r e s s u r e . This i m p o s e s s o m e i n h e r e n t limits on p u m p d i s p l a c e m e n t a nd speed unless a p u m p b o o s t i n g a r r a n g e m e n t is used.
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Fig. 10. Magnetic drive, seal-less three screw pump.
Fig. 11. Motor driven pump via parallel shaft gear reducer.
1
Magnetic drives
Magnetic drives have been in service on centrifugal p u m p s for m o r e t h a 20 years a n d are now available for m a n y r o t a r y p u m p s , figure 10. They e l i m i n a t e t h e n e e d for a p u m p s h a f t seal, p e r h a p s t h e w e a k e s t c o m p o n e n t in any p u m p .
WORLD PUMPS DECEMBER 1995
S
PECIAL FEATURE
m a g n e t i c m a t e r i a l on its outside diameter. The m a g n e t i c fields i n t e r a c t allowing torque t r a n s m i s s i o n t h r o u g h the b a r r i e r can to the actual p u m p i n g shaft or rotor. P u m p s of this design are used to h a n d l e toxic or d a n g e r o u s liquids as well as liquids t h a t t e n d to crystallize, destroying c o n v e n t i o n a l m e c h a n i c a l seals.
I n d i r e c t drives: gears and belts J I
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J Fig. 12. Top: Belt drive, adjacent pump and motor; Bottom: Belt drive, opposed pump and motor.
The a c t u a l p u m p driver is u s u a l l y a c o n v e n t i o n a l motor. It is flexible coupled to the p u m p shaft. The p u m p shaft drives a coupling h u b with the h u b inside d i a m e t e r lined with very powerful rare e a r t h m a g n e t i c material. This h u b r o t a t e s a r o u n d a can or t h i n walled c o n t a i n e r inside of which is a n o t h e r p u m p shaft m o u n t e d h u b with
Ii
WORLD PUMPS DECEMBER 1995
Reduction gears or belts are s o m e t i m e s used either because the p u m p design requires low drive speeds, to tailor the p u m p o u t p u t flow to more exactly m a t c h the a p p l i c a t i o n required flow, or to acheive very low i n l e t p r e s s u r e operation. Occasionally, belt drives are also used to allow periodic speed changes to change p u m p o u t p u t flow rate, for example, where there are seasonal v a r i a t i o n s in liquid p r o d u c t d e m a n d . The use of r e d u c t i o n gears, figures 6 a n d 11, is straight forward, the only c a u t i o n being to check the driver a n d p u m p directions of r o t a t i o n a n d to specify the duty severity of the gear correctly. If gears or pulleys for belt drive are to be m o u n t e d directly on the p u m p shaft, t h e n the side load due to gear reaction loads or belt t e n s i o n loads m u s t be accommodated. Unless designed for it, m o s t p u m p shafting will n o t a c c o m m o d a t e any significant side loads w i t h o u t risk of a l i g n m e n t damage, shaft breakage or p r e m a t u r e shaft fatigue failure. Directly m o u n t e d g e a r s or pulleys, figure 12, top, m u s t be accurately aligned with t h e i r c o r r e s p o n d i n g drive gear or drive pulley. It is good practice to m i n i m i z e whatever side loads are i n h e r e n t in such a r r a n g e m e n t s to m a x i m i z e p u m p life. Where p u m p shafts are n o t designed or capable of the required side load, a j a c k shaft t h a t carries the gear or pulley between its own bearings can be used, figure 12, bottom. In this a r r a n g e m e n t , t h e p u m p shaft is flexibly c o u p l e d in t h e c o n v e n t i o n a l m a n n e r to the jack shaft. Note t h a t t i m i n g belts (toothed belts) require less t e n s i o n to effectively t r a n s m i t t o r q u e a n d m a y be preferable u n d e r otherwise m a r g i n a l conditions.
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Other drive c o n s i d e r a t i o n s
The use of a fluid coupling between p u m p a n d drive t r a i n will allow infinately variable speed a n d t h u s variable flow over the coupling's speed range, figure 13. The fluid coupling m o u n t s m u c h like a concentric shaft speed reducer. Cooling w a t e r is n o r m a l l y required to remove some of the h e a t due to drive inefficiency w h e n o p e r a t i n g at higher speed r e d u c t i o n ratios. Fluid drives allow
S
Fig. 13. Twin screw pump motor driven through fluid coupling.
"soft" s t a r t u p of p u m p s a n d o t h e r e q u i p m e n t in the drive t r a i n r e d u c i n g or e l i m i n a t i n g h a r d m e c h a n i c a l s t a r t u p shocks t h a t can result from electric m o t o r drives. Flexible couplings should n o t be overlooked w h e n a p p l i e d to p u m p drives. Some of these couplings can w i t h s t a n d m i s a l i g n m e n t s m a n y t i m e s w h a t can be tolerated by the drive or driven e q u i p m e n t . Since the coupling costs are b u t a small fraction of a driver or p u m p cost, it is i m p o r t a n t to align c o u p l e d shafts w i t h i n the tighest specification from the driver, gear or p u m p supplier. In the a b s e n c e of a specific r e c o m m e n d a t i o n , t o t a l dial i n d i c a t o r r e a d i n g s of 0.003 inches (0.076 m m ) a n g u l a r a n d parallel are r e a s o n a b l e u p to 3600 r p m drive speeds for m o s t e q u i p m e n t . Do n o t u s e t h e c o u p l i n g m i s a l i g n m e n t capability as your a l i g n m e n t criteria. Some flexible c o u p l i n g designs will impose axial forces on the coupled e q u i p m e n t shafts, p a r t i c u l a r l y couplings with axially r e s t r a i n e d e l a s t o m e r i c c e n t e r m e m b e r s . Be s u r e b o t h coupled machine's bearings can absorb the r e s u l t a n t a x i a l l o a d i n g s or use a l t e r n a t i v e coupling designs. Some flexible couplings are available with integral overload p r o t e c t i o n capabilities. These
i n c l u d e slip clutch as well as s h e a r p i n designs. Such couplings offer a d d i t i o n a l p r o t e c t i o n to the p u m p a n d its drive t r a i n should severe j a m m i n g of the p u m p take place as m i g h t be expected w h e n h a n d l i n g r a n d o m high solids c o n t e n t flow. Be sure t h a t ALL r o t a t i n g shafts, couplings, etc. are properly g u a r d e d to p r e v e n t p e r s o n a l injury. W i t h o u t doubt, o t h e r drive a r r a n g e m e n t s will find t h e i r way into use in the future, the only l i m i t a t i o n s being the designer's creativity, econ o m i c s a n d the user's willingness to try somet h i n g new. The e n g i n e e r i n g p r i n c i p l e s to apply drives are basic a n d f u n d a m e n t a l . Carefully t h o u g h t out systems comprised of a d e q u a t e l y rated c o m p o n e n t s will deliver reliable, efficient service over long periods of time. •
James R. Brennan, currently group m a n a g e r for three pump divisions of Imo Industries specializing in crude oil transport pumps, is a 1973 MIE graduate of Drexel University, Philadelphia, PA, USA. He has over 25 years experience with screw pumps at Imo Industries, is a m e m b e r of the Society of Petrol e u m E n g i n e e r s , and w a s e n g i n e e r i n g m a n a g e r of a pump division for five years. He has authored many papers and articles as well as spoken at a number of industry conferences.
WORLD PUMPS DECEMBER 1995
PECIAL FEATURE