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Wet well mounted pump station: a viable alternative to submersible pump stations
S
PECIAL FEATURE
Wet well mounted pump station: a viable alternative to submersible pump stations I n this article, G o r d o n Flack, Pr¢~ d u c t M a n a g e r , a n d M a r v i n Wood, Manager, Design Engineering, at Smith & Loveless, Inc. describe their c o m p a n y "s w e t w e l l m o u n t e d p u m p s t a t i o n s (WWMPS), which have been p r o v e d in the United States o f America as a cosbeffective a p p r o a c h f o r w a s t e w a t e r p u m p i n g . This t e c h n o ~ ogy is n o w b e i n g i n t r o d u c e d t o o t h e r countries.
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Introduction
The WWMPS was p r i m a r i l y e n g i n e e r e d as a high p u m p efficiency, low m a i n t e n a n c e a l t e r n a t i v e to the concrete, built-in-place type units. Today,
they are also a n alternative to t r a d i t i o n a l 4 inch (100 m m ) , 6 i n c h (150 m m ) a n d 8 i n c h (200 m m ) s u b m e r s i b l e w a s t e w a t e r p u m p s . P e r h a p s the m o s t n o t a b l e characteristic of the WWMPS versus s u b m e r s i b l e p u m p technology, is the location of the m e c h a n i c a l e q u i p m e n t . All p u m p s , motors, piping, valves a n d controls are h o u s e d outside of the wet well u n d e r a fiberglass hood. Inspection, o p e r a t i o n a n d m a i n t e n a n c e are a c c o m p l i s h e d with ease by the operator. Because the s t a t i o n is housed o n a b a s e p l a t e on top of the wet well, the o p e r a t o r needs only to open t h e fiberglass hood to check o n the p u m p station, t h u s d r a m a t i c a l l y r e d u c i n g costs for routine maintenance. There is also a recessed version of the WWMPS for cold climates - the e a r t h i n s u l a t e d low profile of the recessed WWMPS m a k e s it ideal for these h a r s h e r climates. J u s t like t h e above-grade model, its e q u i p m e n t c h a m b e r also keeps p u m p s a n d m o t o r s away from the wet well in a regularly v e n t i l a t e d e n v i r o n m e n t . This semi-buried version also allows for p u m p i n g from deeper gravity m a i n s for wider p u m p i n g applications. Both the above g r o u n d a n d recessed s t a t i o n s can be designed as a two-pump, t h r e e - p u m p or series c o n n e c t e d WWMPS with b u i l t in fail-safe c o m p o n e n t s with the following flows a n d total d y n a m i c head (TDH).
2-Pmnp Flow: 50 GPM to 2500 GPM (3.2 1/s to 157.7 l/s) TDH: Up to 160 feet (up to 48.8 m) 3-Pump Flow: 50 GPM to 5000 GPM (3.2 1/s to 315.4 l/s) TDH: Up to 160 feet (up to 48.8 m)
Series Flow: 50 GPM to 2500 GPM (3.2 l/s to 157.7 l/s) TDH: Up to 375 feet ( u p to 112.5 m)
Two-pump wet well mounted pumping station.
WORLD PUMPS APRIL 1996
Copyright © 1996, Elsevier Science Ltd. All rights reserved. 0262 1762/96/$15.00
S PECIAL FEATURE
Flow schemes The WWMPS was e n g i n e e r e d to d i s t r i b u t e wear o n the p u m p s by a u t o m a t i c a l t e r a t i o n of p u m p s , each eight h o u r s on d u p l e x s t a t i o n s or six h o u r s on t r i p l e x stations. D e p e n d i n g o n a c o n s u m e r ' s needs, a WWMPS model is selected a n d t h e n customized, b u t all of the WWMPS's follow t h e same flow scheme which is detailed below. P r i m i n g the Lead P u m p -- The liquid level rises in the wet well a n d tilts the low level "on" d i s p l a c e m e n t switch, which activates the v a c u u m p u m p a n d p r i m e s the lead p u m p . When the liquid level reaches t h e level-sensing p r o b e in the p u m p , t h e p u m p is primed. The v a c u u m p r i m i n g s h u t s off, a n d the lead p u m p starts. The p u m p is self-priming p u m p s . Thus, there is less chance of designed to r e m a i n p r i m e d from cycle to cycle. p u m p failure due to lack of prime. For m o s t S t a n d b y P u m p -- If the inflow c o n t i n u e s to rise, the high level "on" d i s p l a c e m e n t switch is a p p l i c a t i o n s the p u m p r e m a i n s p r i m e d between tilted, which activates the p r i m i n g a n d p u m p i n g p u m p i n g cycles. This e l i m i n a t e s wear a n d t e a r on the system a n d increases p u m p dependability. cycle of the s t a n d b y p u m p . Transferring Liquid -- The liquid is d r a w n u p Vacuum p r i m i n g systems c o n t a i n a few, simple t h r o u g h the s u c t i o n pipes to t h e centrifugal c o m p o n e n t s t h a t are easily m a i n t a i n e d or p u m p s a n d forced out t h r o u g h the check valves replaced if necessary. A t r a n s p a r e n t p r i m i n g dome allows for observation of the p u m p p r i m e a n d plug valves into the force main. a n d visual i n s p e c t i o n of the electrodes. Any S h u t t i n g Off P u m p s -- The p u m p lowers the a c c u m u l a t i o n on the electrode can be removed wet well level u n t i l the p u m p low level "off v by simply removing the plastic d o m e from the d i s p l a c e m e n t switch tilts a n d s h u t s off t h e p u m p . Wet wells are dark, d a n g e r o u s a n d u s u a l l y p u m p . The p u m p s r e m a i n primed; the v a c u u m are confined e n t r y spaces. There is no need to p u m p will n o t come on u n l e s s the liquid level e n t e r the wet well to m a i n t a i n or r e p a i r the has fallen below the level s e n s i n g probe, a n d the p u m p or the v a c u u m p r i m i n g system. Vacuum low level "on" d i s p l a c e m e n t switch is tilted. p r i m e d p u m p s are designed to be long life [ ] The h e a r t o f t h e e q u i p m e n t : installation, t w e n t y five years or longer. Some p u m p i n g systems are looked u p o n as t e m p o r a r y the pump a n d it is u n d e r s t o o d t h a t complete p u m p overh a u l may be required every five years or less. V a c u u m p r i m e d p u m p s are proven. Over The WWMPS was specifically designed for wastew a t e r p u m p i n g . It features t h e v a c u u m p r i m e d p u m p , which Smith & Loveless believes was the first p u m p specially designed for v a c u u m priming. The p u m p s t a t i o n is factory b u i l t to each c u s t o m e r ' s specifications in a n e n v i r o n m e n t a l l y controlled facility p r i o r to j o b site delivery. This m i n i m i z e s field l a b o u r a n d a s s e m b l y problems. Several factors i n d i c a t e t h a t the WWMPS is a viable a l t e r n a t i v e for s u b m e r s i b l e p u m p s . Here are its advantages: The v a c u u m p r i m i n g s y s t e m allows t h e m a c h i n e r y c h a m b e r to be physically s e p a r a t e d from the wet well. Thus, if the wet well is classified as a confined space, the m a c h i n e r y area remains unclassified. Vacuum priming allows the use o f s t a n d a r d S&L sewage p u m p impellers a n d volutes. Consequently, the p u m p s are of high efficiency a n d the impellers can be t r i m m e d to m e e t the design conditions. The v a c u u m p r i m i n g system is less c o m p l i c a t e d a n d more d e p e n d a b l e t h a n o t h e r systems, such as
WORLD PUMPS APRIL 1996
Recessed twopump wet well mounted pump station.
Close up of WWMPS.
g
S
PECIAL FEATURE
HIGH HEAD LOW HEAD
it
in
SUBMERSIBLE AT lESS THAN B.E.P. Fig. 1. Insufficient head in a smaller pump causes a rapid fall off in efficiency.
20 000 S&L p u m p s have b e e n p l a c e d in service over t h e l a s t fifty years. T h r o u g h t h i s experience, a n d by d e c i d i n g early on t h a t a WWMPS is to be t h e choice, t h e m a x i m u m cost efficiency can be obtained.
WWMPS vs. s u b m e r s i b l e s high head p u m p i n g
Fig. 2. Higher head pumps give smaller size force mains.
It is i m p o r t a n t to i n c l u d e p u m p / p u m p s t a t i o n m a n u f a c t u r e r s a t t h e b e g i n n i n g of a project. This i n v o l v e m e n t can l e a d to c a p i t a l a n d o p e r a t i n g cost savings by c o n s i d e r i n g high h e a d sewage pumps over low head submersible sewage p u m p s . The t i m e to m a k e t h i s decision is before the hydraulic parameters and pump station n e e d s a r e e s t a b l i s h e d . High h e a d centrifugal sewage p u m p s have a d i s t i n c t a d v a n t a g e in h e a d a n d efficiency over c o n v e n t i o n a l low h e a d subm e r s i b l e sewage p u m p s . C o m p e t i t i v e b i d d i n g requires quoting the smallest pump at the h i g h e s t speed. W h e n t h e s m a l l e r size p u m p is deficient in head, t h i s forces t h e b i d d e r to offer a l a r g e r p u m p n e a r e r to s h u t off. This r e s u l t s in a
L~RGE ~
|
~
WORLD PUMPS APRIL 1996
F.IL
,
d r a m a t i c r e d u c t i o n in efficiency a s t h e y move away from t h e b e s t efficiency p o i n t (Figure 1). Historically, E u r o p e a n - d e v e l o p e d s u b m e r s i b l e sewage p u m p s have t e n d e d to be low in b o t h h e a d a n d efficiency. When you c o n s i d e r t h e a d v a n t a g e of high h e a d centrifugal p u m p s , t h e efficiency difference is greatly amplified. Within e c o n o m i c a l force m a i n velocities, say 2 to 6 1/2 feet p e r s e c o n d (0.6 - 2 m / s ) , h i g h e r h e a d p u m p s p r o v i d e for s m a l l e r size force m a i n s ( F i g u r e 2). The d r a m a t i c difference in efficiency ( u s i n g h i g h h e a d , h i g h efficiency p u m p s ) will f r e q u e n t l y m o r e t h a n offset t h e difference in o p e r a t i n g cost between t h e two different size mains. High h e a d p u m p s have an u n q u e s t i o n a b l e a d v a n t a g e in hilly t e r r a i n s with higher static h e a d s (Figure 3). High h e a d p u m p s m u s t be considered at t h e very beginning of a project when t h e flow p a r a m e t e r s a n d the location of the p u m p i n g s t a t i o n ( s ) are being reviewed. Major c a p i t a l costs a n d / o r significant o p e r a t i n g cost r e d u c t i o n s are t h e key. These relate to higher efficiency, fewer p u m p i n g s t a t i o n s being required, increasing t h e capacity of existing force mains, r e d u c i n g t h e risk to a p p l i c a t i o n e r r o r when p u m p s are to o p e r a t e n e a r s h u t off, a n d reducing engineering time. A few p r a c t i c a l e x a m p l e s will serve to verify this. These are a c t u a l c o m p a r i s o n s b e t w e e n a l e a d i n g high h e a d p u m p m a n u f a c t u r e r a n d a leading submersible pump manufacturer.
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E x a m p l e 1: Long force m a i n
A large r e s i d e n t i a l a r e a in a m e d i u m - s i z e d city is served by septic tanks. I n c r e a s i n g p r o b l e m s with i m p r o p e r d r a i n a g e a n d i n c r e a s i n g n i t r a t e levels in t h e g r o u n d w a t e r p r o m p t e d t h e city to p l a n sewers. The sewage m u s t be p u m p e d u p a n d over a g r a d u a l l y rising ridge to t h e sewage t r e a t m e n t p l a n t in a n a d j a c e n t valley. As usual, t h e city w a n t s to m i n i m i z e c a p i t a l c o s t s d u e to l i m i t a t i o n in t h e i r b o n d i n g capacity. The s t a t i c h e a d is 70 feet (21.3 m ) , a n d t h e l e n g t h of t h e force m a i n t o t h e t o p o f t h e h i l l is 6200 f e e t (1900 m ) . The flow, w h i c h is n o t e x p e c t e d to increase, is 410 GPM (25.9 1/sec). A 6 i n c h (150 m m ) ID force m a i n w a s s e l e c t e d with 4.65 ft./sec. (1.42 m / s ) velocity. A long term, Williams a n d Hazen, coefficient of friction, C, of 120 was c h o s e n b a s e d on t h e e x p e c t e d relative r o u g h n e s s of t h e p i p e w h e n c o a t e d w i t h sewage slime. The r e s u l t i n g hydraulic conditions, i n c l u d i n g m a n i f o l d losses, were e s t a b l i s h e d a s 410 GPM a t 174 f e e t TDH (25.9 1/sec, 53 m TDH). C o m p a r i s o n s were m a d e b e t w e e n high h e a d centrifugal sewage p u m p s a n d a l e a d i n g sub-
S PECIAL FEATURE
mersible pump manufacturer offering sewage pumps for installation in the field. High h e a d pumps: (2) 4 inch (100 ram) Size, 40 HP (30 kW), 1760 RPM 50% Pump Efficiency Submersible: (2) 6 inch (150 mm) Size, 88 HP (65 kW), 1770 RPM 37% Pump Efficiency
A comparison between capital costs and operating costs over the expected lives of the two different installations showed the submersible installation to be the higher cost option.
E x a m p l e 2: Very high static head A new development, including a new high school, is located in the foothills of a mountain range. A peak flow of 580 GPM (36.6 l/s) must be pumped over a steep ridge to intersect an existing gravity sewer flowing down an adjacent valley. Static head is 210 feet (64 m), and the required length of force main is 1800 feet (549 m). Hydraulic c o n d i t i o n s , b a s e d on 6 inch (150 ram) piping, are 580 GPM at 270 feet TDH (36.6 l/s at 82 m TDH). Pump comparisons were made between the high head specialist and the submersible pump manufacturer. High h e a d pumps: (2) 4 inch (100 mm) Size, Series Connected Pump sets, 100% Standby 65% Pump Efficiency 40 HP (30 kW), 1760 RPM Motors S u b m e r s i b l e pumps: Could not meet this duty without resorting to additional expensive pumping stations in mountainous terrain near hydraulic centers. This example also shows that high head p u m p s can be connected in series. Series connected pumps often operate at higher efficiencies. The conventional submersible pump conformation does not provide for series connected operation.
E x a m p l e 3: E x p a n d i n g f l o w into an e x i s t i n g force main An existing two-pump, concrete built-in-place station serving the southeast side of a medium
STATIC
!
I Q
sized city was installed 15 years ago. It pumps 300 GPM (18.9 l/s) through 3580 feet (1091 m) of existing 6 inch (150 mm) force main. The southeast side has experienced greater than expected residential growth, and the pumps can no longer handle the flow. A substantial portion of the force main was installed under a busy commercial avenue. The city wants to replace the aging station, which has experienced high maintenance costs, but can't afford the cost of a new force main and the disruption of the commercial area. The new peak flow requirement is 700 GPM (44.2 l/s). The static head from the low water level in the wet well to the outfall of the force main is 26 feet (7.9 m). Peak to average flow rates, coupled with friction losses in the force main, dictate the need for a two-speed station in order to reduce energy costs. Primary and secondary hydraulic conditions were calculated as 700 GPM at 190 feet TDH (44.2 1/s at 57.9 m TDH) and 400 GPM at 85 feet TDH (25.2 1/sec at 25.9 m TDH) respectively. A comparison was made based on these capacities between the high head and submersible pumps.
Fig. 3. High head pumps have an advantage with higher static heads.
High head pumps: (2) 4 inch (100 ram) Size, 60/27 HP (44.7/20.1 kW), 1760/1170 RPM 63/63% Pump efficiency 100% standby Submersible: Alternate A: Four (4) pumps, single speed (2) 6 inch (150 mm) Size, 88 HP (65 kW), 1770 RPM 45% Pump Efficiency (2) 4 inch (100 mm) Size, 20 HP (14.9 kW), 1750 RPM 52% Pump Efficiency 100% Standby
WORLDPUMPS APRIL 1996
(
S
PECIAL FEATURE
A l t e r n a t e B: Three (3) p u m p s -- s t e p flow, single s p e e d (3) 6 inch (150 m m ) , 88 HP (65 kW), 1770 RPM 35% P u m p Efficiency 50% S t a n d b y Each pump approximately 350 GPM a t 190 feet TDH (22 l / s a t 57.9 m TDH) B o t h s u b m e r s i b l e a l t e r n a t i v e s were r e j e c t e d d u e to p o o r efficiencies a n d high o p e r a t i n g costs a s s o c i a t e d with friction losses in t h e e x i s t i n g 6 inch (150 m m ) force main. The h i g h e r efficiencies o b t a i n a b l e with t h e h i g h e r h e a d solids h a n d l i n g p u m p s a n d t h e a d v a n t a g e of h i g h e r p r e s s u r e w i t h o u t a signific a n t i n c r e a s e in p o w e r cost m a k e it w o r t h w h i l e to b r i n g t h e p u m p / p u m p s t a t i o n m a n u f a c t u r e r in a t t h e b e g i n n i n g of t h e p r o j e c t before t h e pipeline hydraulic parameters and the pump s t a t i o n n e e d s are e s t a b l i s h e d . Both c a p i t a l cost a n d o p e r a t i n g cost can be greatly affected.
Example 4: Cost effectiveness A simple cost comparison shows that WWMPSs s o l d to m u n i c i p a l i t i e s r e q u i r e less
Cutaway drawing of WWMPS.
Diagram showing the WWMPS layout.
m a i n t e n a n c e , fewer p a r t s r e p l a c e m e n t a n d less overall service t h a n s u b m e r s i b l e p u m p stations. A c o u n t y in West Virginia, for e x a m p l e , h a s 52 s u b m e r s i b l e p u m p s t a t i o n s a n d s p e n d s a p p r o x i m a t e l y $36,000 p e r y e a r on p a r t s , which e q u a t e s to $692 p e r s t a t i o n , p e r year. This h i g h c o s t m a i n t e n a n c e is a financial b u r d e n for a n y municipality. A town in North Carolina has 12 submersible stations a n d s p e n d s a p p r o x i m a t e l y $8400 p e r y e a r for parts, equating to $700 p e r station, p e r year. In c o m p a r i s o n , a c o u n t y in Virginia h a s 42 p u m p stations, r a n g i n g in age from one to 36 years, from S m i t h & Loveless. This c o u n t y s p e n d s a p p r o x i m a t e l y $1800 p e r y e a r in parts: t h i s e q u a t e s to $43 p e r station, p e r year. The savings a r e s u b s t a n t i a l .
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Conclusion
Wet well m o u n t e d p u m p s t a t i o n s have been successfully a p p l i e d in t h e U.S. a n d p r o v i d e an a l t e r n a t i v e to s u b m e r s i b l e p u m p s . Economically, long life a n d s i m p l e r e p a i r s save on l o n g - t e r m costs. Mechanically, t h e above g r o u n d e q u i p m e n t p r o v i d e s ease in i n s p e c t i o n a n d repair, while i m p r o v i n g safety. These benefits along with t h e v e r s a t i l i t y of t h i s t e c h n o l o g y m a k e it a viable s o l u t i o n to w a s t e w a t e r p u m p i n g p r o b l e m s . •
For more information, contact: Jodel Wickham Chen, Marketing Communications Manager, Smith & Loveless, Inc. 14040 Santa Fe Trail Drive, Lenexa, Kansas 66215-1284, USA. Tel: +1 913 888 5201; Fax: +1 913 888 2173.
(
WORLD PUMPS APRIL 1996
PECIAL FEATURE
Wet well mounted pump station: a viable alternative to submersible pump stations I n this article, G o r d o n Flack, Pr¢~ d u c t M a n a g e r , a n d M a r v i n Wood, Manager, Design Engineering, at Smith & Loveless, Inc. describe their c o m p a n y "s w e t w e l l m o u n t e d p u m p s t a t i o n s (WWMPS), which have been p r o v e d in the United States o f America as a cosbeffective a p p r o a c h f o r w a s t e w a t e r p u m p i n g . This t e c h n o ~ ogy is n o w b e i n g i n t r o d u c e d t o o t h e r countries.
]]
Introduction
The WWMPS was p r i m a r i l y e n g i n e e r e d as a high p u m p efficiency, low m a i n t e n a n c e a l t e r n a t i v e to the concrete, built-in-place type units. Today,
they are also a n alternative to t r a d i t i o n a l 4 inch (100 m m ) , 6 i n c h (150 m m ) a n d 8 i n c h (200 m m ) s u b m e r s i b l e w a s t e w a t e r p u m p s . P e r h a p s the m o s t n o t a b l e characteristic of the WWMPS versus s u b m e r s i b l e p u m p technology, is the location of the m e c h a n i c a l e q u i p m e n t . All p u m p s , motors, piping, valves a n d controls are h o u s e d outside of the wet well u n d e r a fiberglass hood. Inspection, o p e r a t i o n a n d m a i n t e n a n c e are a c c o m p l i s h e d with ease by the operator. Because the s t a t i o n is housed o n a b a s e p l a t e on top of the wet well, the o p e r a t o r needs only to open t h e fiberglass hood to check o n the p u m p station, t h u s d r a m a t i c a l l y r e d u c i n g costs for routine maintenance. There is also a recessed version of the WWMPS for cold climates - the e a r t h i n s u l a t e d low profile of the recessed WWMPS m a k e s it ideal for these h a r s h e r climates. J u s t like t h e above-grade model, its e q u i p m e n t c h a m b e r also keeps p u m p s a n d m o t o r s away from the wet well in a regularly v e n t i l a t e d e n v i r o n m e n t . This semi-buried version also allows for p u m p i n g from deeper gravity m a i n s for wider p u m p i n g applications. Both the above g r o u n d a n d recessed s t a t i o n s can be designed as a two-pump, t h r e e - p u m p or series c o n n e c t e d WWMPS with b u i l t in fail-safe c o m p o n e n t s with the following flows a n d total d y n a m i c head (TDH).
2-Pmnp Flow: 50 GPM to 2500 GPM (3.2 1/s to 157.7 l/s) TDH: Up to 160 feet (up to 48.8 m) 3-Pump Flow: 50 GPM to 5000 GPM (3.2 1/s to 315.4 l/s) TDH: Up to 160 feet (up to 48.8 m)
Series Flow: 50 GPM to 2500 GPM (3.2 l/s to 157.7 l/s) TDH: Up to 375 feet ( u p to 112.5 m)
Two-pump wet well mounted pumping station.
WORLD PUMPS APRIL 1996
Copyright © 1996, Elsevier Science Ltd. All rights reserved. 0262 1762/96/$15.00
S PECIAL FEATURE
Flow schemes The WWMPS was e n g i n e e r e d to d i s t r i b u t e wear o n the p u m p s by a u t o m a t i c a l t e r a t i o n of p u m p s , each eight h o u r s on d u p l e x s t a t i o n s or six h o u r s on t r i p l e x stations. D e p e n d i n g o n a c o n s u m e r ' s needs, a WWMPS model is selected a n d t h e n customized, b u t all of the WWMPS's follow t h e same flow scheme which is detailed below. P r i m i n g the Lead P u m p -- The liquid level rises in the wet well a n d tilts the low level "on" d i s p l a c e m e n t switch, which activates the v a c u u m p u m p a n d p r i m e s the lead p u m p . When the liquid level reaches t h e level-sensing p r o b e in the p u m p , t h e p u m p is primed. The v a c u u m p r i m i n g s h u t s off, a n d the lead p u m p starts. The p u m p is self-priming p u m p s . Thus, there is less chance of designed to r e m a i n p r i m e d from cycle to cycle. p u m p failure due to lack of prime. For m o s t S t a n d b y P u m p -- If the inflow c o n t i n u e s to rise, the high level "on" d i s p l a c e m e n t switch is a p p l i c a t i o n s the p u m p r e m a i n s p r i m e d between tilted, which activates the p r i m i n g a n d p u m p i n g p u m p i n g cycles. This e l i m i n a t e s wear a n d t e a r on the system a n d increases p u m p dependability. cycle of the s t a n d b y p u m p . Transferring Liquid -- The liquid is d r a w n u p Vacuum p r i m i n g systems c o n t a i n a few, simple t h r o u g h the s u c t i o n pipes to t h e centrifugal c o m p o n e n t s t h a t are easily m a i n t a i n e d or p u m p s a n d forced out t h r o u g h the check valves replaced if necessary. A t r a n s p a r e n t p r i m i n g dome allows for observation of the p u m p p r i m e a n d plug valves into the force main. a n d visual i n s p e c t i o n of the electrodes. Any S h u t t i n g Off P u m p s -- The p u m p lowers the a c c u m u l a t i o n on the electrode can be removed wet well level u n t i l the p u m p low level "off v by simply removing the plastic d o m e from the d i s p l a c e m e n t switch tilts a n d s h u t s off t h e p u m p . Wet wells are dark, d a n g e r o u s a n d u s u a l l y p u m p . The p u m p s r e m a i n primed; the v a c u u m are confined e n t r y spaces. There is no need to p u m p will n o t come on u n l e s s the liquid level e n t e r the wet well to m a i n t a i n or r e p a i r the has fallen below the level s e n s i n g probe, a n d the p u m p or the v a c u u m p r i m i n g system. Vacuum low level "on" d i s p l a c e m e n t switch is tilted. p r i m e d p u m p s are designed to be long life [ ] The h e a r t o f t h e e q u i p m e n t : installation, t w e n t y five years or longer. Some p u m p i n g systems are looked u p o n as t e m p o r a r y the pump a n d it is u n d e r s t o o d t h a t complete p u m p overh a u l may be required every five years or less. V a c u u m p r i m e d p u m p s are proven. Over The WWMPS was specifically designed for wastew a t e r p u m p i n g . It features t h e v a c u u m p r i m e d p u m p , which Smith & Loveless believes was the first p u m p specially designed for v a c u u m priming. The p u m p s t a t i o n is factory b u i l t to each c u s t o m e r ' s specifications in a n e n v i r o n m e n t a l l y controlled facility p r i o r to j o b site delivery. This m i n i m i z e s field l a b o u r a n d a s s e m b l y problems. Several factors i n d i c a t e t h a t the WWMPS is a viable a l t e r n a t i v e for s u b m e r s i b l e p u m p s . Here are its advantages: The v a c u u m p r i m i n g s y s t e m allows t h e m a c h i n e r y c h a m b e r to be physically s e p a r a t e d from the wet well. Thus, if the wet well is classified as a confined space, the m a c h i n e r y area remains unclassified. Vacuum priming allows the use o f s t a n d a r d S&L sewage p u m p impellers a n d volutes. Consequently, the p u m p s are of high efficiency a n d the impellers can be t r i m m e d to m e e t the design conditions. The v a c u u m p r i m i n g system is less c o m p l i c a t e d a n d more d e p e n d a b l e t h a n o t h e r systems, such as
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Recessed twopump wet well mounted pump station.
Close up of WWMPS.
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HIGH HEAD LOW HEAD
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SUBMERSIBLE AT lESS THAN B.E.P. Fig. 1. Insufficient head in a smaller pump causes a rapid fall off in efficiency.
20 000 S&L p u m p s have b e e n p l a c e d in service over t h e l a s t fifty years. T h r o u g h t h i s experience, a n d by d e c i d i n g early on t h a t a WWMPS is to be t h e choice, t h e m a x i m u m cost efficiency can be obtained.
WWMPS vs. s u b m e r s i b l e s high head p u m p i n g
Fig. 2. Higher head pumps give smaller size force mains.
It is i m p o r t a n t to i n c l u d e p u m p / p u m p s t a t i o n m a n u f a c t u r e r s a t t h e b e g i n n i n g of a project. This i n v o l v e m e n t can l e a d to c a p i t a l a n d o p e r a t i n g cost savings by c o n s i d e r i n g high h e a d sewage pumps over low head submersible sewage p u m p s . The t i m e to m a k e t h i s decision is before the hydraulic parameters and pump station n e e d s a r e e s t a b l i s h e d . High h e a d centrifugal sewage p u m p s have a d i s t i n c t a d v a n t a g e in h e a d a n d efficiency over c o n v e n t i o n a l low h e a d subm e r s i b l e sewage p u m p s . C o m p e t i t i v e b i d d i n g requires quoting the smallest pump at the h i g h e s t speed. W h e n t h e s m a l l e r size p u m p is deficient in head, t h i s forces t h e b i d d e r to offer a l a r g e r p u m p n e a r e r to s h u t off. This r e s u l t s in a
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d r a m a t i c r e d u c t i o n in efficiency a s t h e y move away from t h e b e s t efficiency p o i n t (Figure 1). Historically, E u r o p e a n - d e v e l o p e d s u b m e r s i b l e sewage p u m p s have t e n d e d to be low in b o t h h e a d a n d efficiency. When you c o n s i d e r t h e a d v a n t a g e of high h e a d centrifugal p u m p s , t h e efficiency difference is greatly amplified. Within e c o n o m i c a l force m a i n velocities, say 2 to 6 1/2 feet p e r s e c o n d (0.6 - 2 m / s ) , h i g h e r h e a d p u m p s p r o v i d e for s m a l l e r size force m a i n s ( F i g u r e 2). The d r a m a t i c difference in efficiency ( u s i n g h i g h h e a d , h i g h efficiency p u m p s ) will f r e q u e n t l y m o r e t h a n offset t h e difference in o p e r a t i n g cost between t h e two different size mains. High h e a d p u m p s have an u n q u e s t i o n a b l e a d v a n t a g e in hilly t e r r a i n s with higher static h e a d s (Figure 3). High h e a d p u m p s m u s t be considered at t h e very beginning of a project when t h e flow p a r a m e t e r s a n d the location of the p u m p i n g s t a t i o n ( s ) are being reviewed. Major c a p i t a l costs a n d / o r significant o p e r a t i n g cost r e d u c t i o n s are t h e key. These relate to higher efficiency, fewer p u m p i n g s t a t i o n s being required, increasing t h e capacity of existing force mains, r e d u c i n g t h e risk to a p p l i c a t i o n e r r o r when p u m p s are to o p e r a t e n e a r s h u t off, a n d reducing engineering time. A few p r a c t i c a l e x a m p l e s will serve to verify this. These are a c t u a l c o m p a r i s o n s b e t w e e n a l e a d i n g high h e a d p u m p m a n u f a c t u r e r a n d a leading submersible pump manufacturer.
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E x a m p l e 1: Long force m a i n
A large r e s i d e n t i a l a r e a in a m e d i u m - s i z e d city is served by septic tanks. I n c r e a s i n g p r o b l e m s with i m p r o p e r d r a i n a g e a n d i n c r e a s i n g n i t r a t e levels in t h e g r o u n d w a t e r p r o m p t e d t h e city to p l a n sewers. The sewage m u s t be p u m p e d u p a n d over a g r a d u a l l y rising ridge to t h e sewage t r e a t m e n t p l a n t in a n a d j a c e n t valley. As usual, t h e city w a n t s to m i n i m i z e c a p i t a l c o s t s d u e to l i m i t a t i o n in t h e i r b o n d i n g capacity. The s t a t i c h e a d is 70 feet (21.3 m ) , a n d t h e l e n g t h of t h e force m a i n t o t h e t o p o f t h e h i l l is 6200 f e e t (1900 m ) . The flow, w h i c h is n o t e x p e c t e d to increase, is 410 GPM (25.9 1/sec). A 6 i n c h (150 m m ) ID force m a i n w a s s e l e c t e d with 4.65 ft./sec. (1.42 m / s ) velocity. A long term, Williams a n d Hazen, coefficient of friction, C, of 120 was c h o s e n b a s e d on t h e e x p e c t e d relative r o u g h n e s s of t h e p i p e w h e n c o a t e d w i t h sewage slime. The r e s u l t i n g hydraulic conditions, i n c l u d i n g m a n i f o l d losses, were e s t a b l i s h e d a s 410 GPM a t 174 f e e t TDH (25.9 1/sec, 53 m TDH). C o m p a r i s o n s were m a d e b e t w e e n high h e a d centrifugal sewage p u m p s a n d a l e a d i n g sub-
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mersible pump manufacturer offering sewage pumps for installation in the field. High h e a d pumps: (2) 4 inch (100 ram) Size, 40 HP (30 kW), 1760 RPM 50% Pump Efficiency Submersible: (2) 6 inch (150 mm) Size, 88 HP (65 kW), 1770 RPM 37% Pump Efficiency
A comparison between capital costs and operating costs over the expected lives of the two different installations showed the submersible installation to be the higher cost option.
E x a m p l e 2: Very high static head A new development, including a new high school, is located in the foothills of a mountain range. A peak flow of 580 GPM (36.6 l/s) must be pumped over a steep ridge to intersect an existing gravity sewer flowing down an adjacent valley. Static head is 210 feet (64 m), and the required length of force main is 1800 feet (549 m). Hydraulic c o n d i t i o n s , b a s e d on 6 inch (150 ram) piping, are 580 GPM at 270 feet TDH (36.6 l/s at 82 m TDH). Pump comparisons were made between the high head specialist and the submersible pump manufacturer. High h e a d pumps: (2) 4 inch (100 mm) Size, Series Connected Pump sets, 100% Standby 65% Pump Efficiency 40 HP (30 kW), 1760 RPM Motors S u b m e r s i b l e pumps: Could not meet this duty without resorting to additional expensive pumping stations in mountainous terrain near hydraulic centers. This example also shows that high head p u m p s can be connected in series. Series connected pumps often operate at higher efficiencies. The conventional submersible pump conformation does not provide for series connected operation.
E x a m p l e 3: E x p a n d i n g f l o w into an e x i s t i n g force main An existing two-pump, concrete built-in-place station serving the southeast side of a medium
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sized city was installed 15 years ago. It pumps 300 GPM (18.9 l/s) through 3580 feet (1091 m) of existing 6 inch (150 mm) force main. The southeast side has experienced greater than expected residential growth, and the pumps can no longer handle the flow. A substantial portion of the force main was installed under a busy commercial avenue. The city wants to replace the aging station, which has experienced high maintenance costs, but can't afford the cost of a new force main and the disruption of the commercial area. The new peak flow requirement is 700 GPM (44.2 l/s). The static head from the low water level in the wet well to the outfall of the force main is 26 feet (7.9 m). Peak to average flow rates, coupled with friction losses in the force main, dictate the need for a two-speed station in order to reduce energy costs. Primary and secondary hydraulic conditions were calculated as 700 GPM at 190 feet TDH (44.2 1/s at 57.9 m TDH) and 400 GPM at 85 feet TDH (25.2 1/sec at 25.9 m TDH) respectively. A comparison was made based on these capacities between the high head and submersible pumps.
Fig. 3. High head pumps have an advantage with higher static heads.
High head pumps: (2) 4 inch (100 ram) Size, 60/27 HP (44.7/20.1 kW), 1760/1170 RPM 63/63% Pump efficiency 100% standby Submersible: Alternate A: Four (4) pumps, single speed (2) 6 inch (150 mm) Size, 88 HP (65 kW), 1770 RPM 45% Pump Efficiency (2) 4 inch (100 mm) Size, 20 HP (14.9 kW), 1750 RPM 52% Pump Efficiency 100% Standby
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A l t e r n a t e B: Three (3) p u m p s -- s t e p flow, single s p e e d (3) 6 inch (150 m m ) , 88 HP (65 kW), 1770 RPM 35% P u m p Efficiency 50% S t a n d b y Each pump approximately 350 GPM a t 190 feet TDH (22 l / s a t 57.9 m TDH) B o t h s u b m e r s i b l e a l t e r n a t i v e s were r e j e c t e d d u e to p o o r efficiencies a n d high o p e r a t i n g costs a s s o c i a t e d with friction losses in t h e e x i s t i n g 6 inch (150 m m ) force main. The h i g h e r efficiencies o b t a i n a b l e with t h e h i g h e r h e a d solids h a n d l i n g p u m p s a n d t h e a d v a n t a g e of h i g h e r p r e s s u r e w i t h o u t a signific a n t i n c r e a s e in p o w e r cost m a k e it w o r t h w h i l e to b r i n g t h e p u m p / p u m p s t a t i o n m a n u f a c t u r e r in a t t h e b e g i n n i n g of t h e p r o j e c t before t h e pipeline hydraulic parameters and the pump s t a t i o n n e e d s are e s t a b l i s h e d . Both c a p i t a l cost a n d o p e r a t i n g cost can be greatly affected.
Example 4: Cost effectiveness A simple cost comparison shows that WWMPSs s o l d to m u n i c i p a l i t i e s r e q u i r e less
Cutaway drawing of WWMPS.
Diagram showing the WWMPS layout.
m a i n t e n a n c e , fewer p a r t s r e p l a c e m e n t a n d less overall service t h a n s u b m e r s i b l e p u m p stations. A c o u n t y in West Virginia, for e x a m p l e , h a s 52 s u b m e r s i b l e p u m p s t a t i o n s a n d s p e n d s a p p r o x i m a t e l y $36,000 p e r y e a r on p a r t s , which e q u a t e s to $692 p e r s t a t i o n , p e r year. This h i g h c o s t m a i n t e n a n c e is a financial b u r d e n for a n y municipality. A town in North Carolina has 12 submersible stations a n d s p e n d s a p p r o x i m a t e l y $8400 p e r y e a r for parts, equating to $700 p e r station, p e r year. In c o m p a r i s o n , a c o u n t y in Virginia h a s 42 p u m p stations, r a n g i n g in age from one to 36 years, from S m i t h & Loveless. This c o u n t y s p e n d s a p p r o x i m a t e l y $1800 p e r y e a r in parts: t h i s e q u a t e s to $43 p e r station, p e r year. The savings a r e s u b s t a n t i a l .
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Conclusion
Wet well m o u n t e d p u m p s t a t i o n s have been successfully a p p l i e d in t h e U.S. a n d p r o v i d e an a l t e r n a t i v e to s u b m e r s i b l e p u m p s . Economically, long life a n d s i m p l e r e p a i r s save on l o n g - t e r m costs. Mechanically, t h e above g r o u n d e q u i p m e n t p r o v i d e s ease in i n s p e c t i o n a n d repair, while i m p r o v i n g safety. These benefits along with t h e v e r s a t i l i t y of t h i s t e c h n o l o g y m a k e it a viable s o l u t i o n to w a s t e w a t e r p u m p i n g p r o b l e m s . •
For more information, contact: Jodel Wickham Chen, Marketing Communications Manager, Smith & Loveless, Inc. 14040 Santa Fe Trail Drive, Lenexa, Kansas 66215-1284, USA. Tel: +1 913 888 5201; Fax: +1 913 888 2173.
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