ARTICLE #43: LIFTING WATER WHAT ARE YOUR PUMP OPTIONS?
Consider a typical sewage collection system. At the initial point of discharge, water first flows (by gravity) into a network of (sloped down) drain pipes, which gradually intercept to a larger main pipe. Eventually, all this water needs to be lifted to a sewage wastewater processing plant. The lift varies from several feet to hundreds of feet in some cases. To accomplish this eventual lift, several ways can be considered.
1. Archimedes screw is the oldest known pumping method, and it is still used today, although rarely. It has obvious benefits of being simple, with seals or packing to worry about. However, it requires significant space to accommodate the necessity of the low angle of incline, which also increases weight substantially. Typically, only one bearing, near the discharge (drive) side is provided, and thus the entire rotor sits cantilever at a tight clearance, required to keep water from flowing back. Eventually, rotor sags through the clearance and wears the bottom trough, requiring frequent, and rather expensive, repairs. When a second (lower) bearing is provided, it is lubricated by grease, which, due to submergence, gets eventually washed out and is difficult to reduplicate. Removal of the unit for maintenance is also difficult due to weight and accessibility.
2. Self-priming centrifugal pumps are common, mostly for relatively low flow applications, under 1,000 gpm or so. Like any other pump, it has pluses and minuses. Maintenance is easy, as the pump sits on the surface, and trash can be removed by the access ports at the side of the casing. Priming is achieved by either:
a. foot valve
b. check valve
c. auxiliary vacuum pump evacuation of the inlet air
Both (a) and (b) options are the Achilles hill of the method. Vacuum pump adds complexity to the system. However, if these issues are understood and pumps regularly and properly maintained, a reliable operation results. If an install-and-forget maintenance is practiced, these applications become a problem after 3-5 years, when wear opens clearances and priming no longer looks, nor works, as good as it did on a glossy paper brochure.
3. Vertical sump design solves priming problems by submersing the pumping element under water. A long shaft connects it to a surface-mounted electric motor, keeping it from getting wet. Maintenance of packings or seals is easy, sine the pump does not need to be pulled for repacking service. However, with a long shaft comes the alignment problem, unbalance, or wear of the line bushings. Lubrication of bushings can be problematic, due to plugging or breaking of the long (and often flimsy) grease tubing. When tubing, bushings or impeller require maintenance, the entire unit needs to be pulled, which can be an issue for hard-to-access places.
4. Submersible (wet) pumps have an electric motor directly coupled to a pump, making an entire unit compact and relatively light, which makes them well accepted for relatively low depth wet well (10-20 feet) at horsepower typically under 30 hp. Submergence of the motor, while a benefit on one hand, can also be a cause of its trouble on another hand. To make sure water does not get to the motor windings, a double mechanical seal, filled with oil, is installed. As impeller wears out (wastewater applications can be nasty!), unbalance and vibrations eventually tend to deflect a cantilever shaft, and fail the seal. Knowing that to be an issue, submersible pump motors are typically made with better quality stator windings as compared to dry, surface application, motors, and even after being flooded, the windings continue to function without short, for some time. Moisture sensors are provided to detect, warn and alarm, but unfortunately, many operators do not have these connected, or disconnect them on purpose, to avoid nuisance alarms, and thus setting the units on a road of eventual undetected failure.
Even for a perfectly maintained submersible pumps, with no impeller wear and resultant unbalance to consider, a mechanical seal life has a final life span. While a secondary seal sees a better environment (clean oil), a primary seal is in direct contact with dirty sewage, and thus eventually wears. While the exact value of such seal life depends on the application, it is likely not to significantly exceed 5 years on the average, from the practical consideration, and thus a pump can not be viewed as install-and-forget, neither.
5) Submersible (dry) pumps are similar to the wet submersible, except that they are installed in dry well, and connected to wet well via suction piping. Servicing and pulling such pump for maintenance is easer with simpler and obviously cleaner access. The issue of a mechanical seal life, however, remains the same as for the wet submersibles. Also, cooling of the motor is no longer done by it being submersed, and thus dry submersibles require circulation of a portion of the pumpage thru the cooling passages of the motor housing, and thus plugging of these passages by the dirty pumpage is frequently an issue, followed by motor overheating.
6. Dry well sewage pumps the expense of constructing of a dry well next to a wet well is often justified by eliminating a long shaft (such as in (3)), or dangers of flooding of the motor windings (such as in (4) and (5)). Such installation looks no differently then any other surface mounted pumps with vertically oriented shafts coupled to the motor shafts. Packing or mechanical seals are a matter of choice and preference, with decisions on that very similar to regular surface mounted pumps as well. The main concern is a potential of flowing of the entire pumping station, in which case a dry-designed motors fail quickly, making it then very difficult to take any corrective action, until the entire station gets pumped out on emergency service.
7. Dry well U-Jointed shafting pumps solves the concern of possible station flooding. However, all issues of longer shafting come into consideration. Typically, 2, 3 or even more segments of the pump-to-motor shafting are present, with pillow blocks guiding the shafting along the way. Alignment of such shafting is critical. Just is critical is a need to balance the shafts, and preferably the entire shafting train, -a difficult or expensive process, with balancing machines of special design, to accommodate very long shafts. Lubrication of the bearings of the U-Joints as well as pillow blocks is also critical, and needs to be followed by the proper preventative maintenance procedure, and, if neglected, high vibrations and failures would be a norm, not an isolated event.
8. Other Methods (comments from our readers)
I use eductors regularly in the emergency dewatering of sunken vessels. I do not use a reduction in throat area to obtain suction (negative pressure) as this would reduce my desired effect of using the eductor. By not reducing the size of the discharge pipe, I am able to pass materials encountered in a sunken boat (paper towels, rags, mud, etc.) that would normally stop up the suction hose of the dewatering pump. I obtain the negative pressure to lift water via a high pressure pump clean water discharge (50-90 psi) directly into the flow. By actual field use, I have found:
a) lift is proportional to eductor input pressure
b) about 5 feet lift is max. with 3 eductor @ 50 psi input from a 5.5 hp air cooled motor/pacer pump (rated 1280 gpm)
c) an 11 hp hale fire pump with 90 psi +/- on a 2-1/2 fire hose into a 3 eductor will discharge water about 20 distance from an elevation of 5 to 0
d) I have been on job with a 6 cyl. gas fire pump hooked to a 6 eductor and the effect is impressive
Sorry I do not have psi as no pitot was available during emergency operations. I will obtain one in future and put together some numbers and share with you.
James CATFISH Younce
CATFISH DIVING AND SALVAGE COMPANY, INC.
b. Dry-sub: separately coupled pump to motor
Our company provides a design solution to the problems you described at the above alternatives.
Bringing the motor close to the pump is obviously a desirable approach, as it eliminates the long connecting shafting,
as well as loads impacted on the pump bearings due to inherent misalignment of the segmental shafts leading up.
As you explained very well, the desire to locate the standard motor on the surface is to eliminate any possibility of it getting
flooded, as such are a common fear of any municipal wastewater processing plants at their lift stations.
The standard submersible motors, even dry, still suffer form an issue of eventual failure of the mechanical seal, which then
allows water to enter the motor windings. Also, most submersible motors are typically not available in stainless
materials, which is sometimes required for pumping water contaminated with chemicals, hot streams, or sea, salt, brine,
or similar applications. In our design, both motor and the pump wet end are made form stainless steel, or even better
materials, as might be required, such as hastelloy, titanium, composites, etc.
The key to our design is separation of the pump from the motor, as shown below. There is a coupling between the motor and
the pump (not shown), plus a close-coupled arrangement bracket, eliminating the need for alignment. That way,
the unit is compact, with motor in close proximity to the pump, yet no seal separating the wet end from the
motor windings. If the entire station gets flooded, the unit will continue to operate. Should design like this was available before, Katrina might never have happened.
We custom build such design, attaching the motor to any type of a pump: elbow, horizontal or vertical centrifugals,
wet or dry pit submersibles, axial flow, vertical turbine, and other types. Examples what we can do:
+ = (shown conceptually)
Ricky Mathis, Design Manager
Pumping Machinery, LLC
Conclusions are recommendations
There are several methods to lift water to the surface, and each has pluses as well as drawbacks. Each option should be treated on its merit, and with respect. None allow a install-and-forget attitude. The modes of failure, critical path to failure, and root cause for each of these is different, and by understanding of the fundamental principles and applying proactive maintenance and operating strategies, failures can be prevented, or significantly reduced. Such methods are further discussed at our Specific Comprehensive Operation of Pumping Equipment (SCOPETM), as presented at www.pumpingmachinery.com/consulting/SCOPE/scope.htm
Which of these methods works best for you? What are the issues you may have had and overcame by implementing SCOPE methodology? Let us know. Pumps & Systems and Pumping Machinery present to you these series to help build awareness and knowledge of how these alternatives work and when. Do not be scared of the pitfalls instead, understand the potential issues and apply the options best suited for your reality.
As always, a parting Quiz! which other method of water lifting is common and what are the benefits and drawback of it? The first three answers will get you a free pass to the Pump School session, per schedule posted at our web site: www.pumpingmachinery.com/pump_school/pump_school.htm
Dr. Lev Nelik, P.E., APICS
Pumping Machinery, LLC
Pump School Training Services