By: Jose Somera
Whether it's for new construction, lift station rehabilitation or malfunction, a broken gravity line, force main rupture, tie-ins, or any combination thereof, most bypasses are accomplished using centrifugal and submersible pumps.
Centrifugal pumps move liquid via rotating impellers that increase the pressure of the fluid. They can be used in parallel to achieve greater flows and in series to achieve higher head pressure; suction piping and hoses are accessible through most openings. Their main disadvantage is suction lift limitation.
When sizing centrifugals, make sure to know the following:
1) Desired capacity (peak flow rate) in gallons per minute (gpm), which is predetermined or measured in the field.
2) Static suction lift is the vertical distance in feet from the eye of the impeller to the fluid level; and two types of net positive suction head — available and required — are involved.
The net positive suction head available (NPSHA): At sea level, 14.7 psi is the atmospheric pressure that can lift water approximately 34 feet. As this pressure pushes down on the fluid, the vacuum within the pump's suction conduit pushes the fluid up and into the eye of the pump's impeller. The centrifugal force creates pressure and slings the discharge out of the pump.
The process is analogous to drinking water through a straw. The atmospheric pressure on the fluid in the glass combined with the negative pressure created in the straw causes the fluid to rise. If you stood on a four-story building and had a straw long enough to reach a glass on the ground, the water would rise no further than 34 feet. For pumps, practical suction lifts should not exceed 22 feet at sea level.
Atmospheric pressure decreases as elevation increases, losing approximately 1 foot of net positive suction head for every 1,000 feet of elevation. For example, in Denver, where the elevation is 5,281 feet, the NPSHA would be approximately 29 feet. Therefore, it's not possible to lift fluids as high as at sea level.
If the suction lift is excessive, the liquid will vaporize, resulting in cavitation. In this situation, pockets of vapor in the flowing stream create a noise that sounds like pumping rocks. Because it's damaging, cavitation is detrimental to performance.
To perform properly, pumps have a net positive suction head requirement (NPSHR) as a function of their design. That is, the pump requires part of the 14.7 psi available. The work that can be done, therefore, on the suction side of the pump is limited.
When selecting a centrifugal pump, the first consideration should be its NPSHR. The single most common mistake when selecting a self-priming centrifugal pump is neglecting its NPSHR.
3) The static discharge head is the vertical distance in feet from the eye of the impeller to the discharge point. The vertical distance is head, in feet, that must be overcome by the pump and is added to the total head.
4) Friction loss depends on the size, type, and length of piping and fittings. Friction/velocity tables are used to factor friction loss into the total dynamic head (TDH). These useful tables show the different size conduits (pipe or hose) by diameter, indicating the head loss (resistance) in feet when various flow rates in gpm are passed through the conduits. These tables also show the velocity of the effluent as it passes through the conduit in feet per second (fps). When sizing conduit for a bypass, as a rule of thumb the velocity should not exceed 10 fps. Velocities over 10 fps will result in excessive horsepower loss.
5) Pressure at the discharge point, if any, is factored into the dynamic head calculations so the pump can overcome existing pressure. The existing pressure is converted into feet by multiplying the psi by 2.31 and adding the result to the total head.
Vacuum-assisted non-clog centrifugal pumps are the most commonly used pumps for bypasses because they can handle large amounts of liquids and solids and have air-handling capability. Diesel-powered units flex-coupled to the pump can be disconnected and replaced with a horizontal motor and paired with a variable frequency drive (VFD) control for long-term bypasses at an existing pump station. These pumps are available with an enclosure for sound attenuation.
Submersibles are centrifugal pumps with a motor directly attached in a common housing. Their size and weight prohibits use through most access openings, they must be pulled out to clean out debris lodged in the impeller, and they require a generator if there's no electrical power source nearby.
On the plus side, submersibles offer instant priming. They have no suction lift limitations, but there are minimum submergence requirements. Though the information may not be published on the pump curves, submersibles require a specific amount of submergence over the volute to operate properly and for motor cooling characteristics. Therefore, submersible pumps also need NPSHA, but it's not as critical as for aboveground centrifugals.
When considering a submersible pump, know the following:
The dynamic discharge head is calculated in the same manner as shown in the sidebar on page 119 for centrifugals, but for static discharge head only.
Submersible pumps have their place in bypass work. Usually they're electrically driven, but some are hydraulically driven with a power pack that can be either diesel- or motor-driven.
PLANNING THE BYPASS
Peak flows can easily exceed the capacity of any single pump, requiring pumping in parallel: the installation of two or more pumps with common or separate suction lines connected to a single conduit of fluid. The flow is multiplied by the number of pumps combined in parallel.
Discharge heads of more than 90 feet require pumping in series: arranging two or more pumps so each pump's discharge is connected to the next pump's suction. The result is the production of the additive head pressure generated by each pump.
This setup isn't advised for sewage bypass applications because it's impossible to discharge to the designated point if one of the pumps should fail. Submersibles aren't commonly used in this manner.
The bypass plan should include the following:
— Jose Somera is location manager for Griffin Dewatering Farwest.
Putting theory into action
To illustrate the selection of a centrifugal pump for a specific application, suppose a ruptured sewer line has an estimated flow rate of 1,200 gpm. Pumping will be from a 20-foot-deep manhole upstream of the rupture. The effluent can rise only 5 feet from the bottom of the manhole before it starts to backup into residences. The effluent will be discharged 800 linear feet to a manhole on a trunk line one street over, and there's an elevation rise of 10 feet between the two manholes.
Parameters: 8-inch standard steel pipe
Friction loss = 4.17 per 100 feet
Flow rate = 1,200 gpm
Velocity = 7.66 fps
The dynamic discharge head would be calculated as follows:
1) Static suction lift (20-foot depth minus 5 feet of retention) = 15 feet
2) Static discharge head = 10 feet
3) Adding the two totals:
Many pumps would work here, but don't look just at flow versus head curve. Factor in net positive suction head requirement (NPSHR) to narrow the search. A well-designed pump has not only good head and flow characteristics, but minimal NPSHR as well.