Sewer Bypass Fundamentals
Typically bypass pumping will occur because of new construction, lift station rehabilitation, lift station malfunctioning, broken gravity line, force main rupture, tie-ins or a combination of these factors. Almost all bypass work can be accomplished with the use of centrifugal and submersible trash pumps.
The following are some recommended sewage bypass fundamentals, which should be considered in evaluating all bypass projects. These will also apply to the bypassing of channel flow, storm water etc., as well as sewage. Applying the following simple techniques will make things a lot easier for your bypass operation.
Most bypass work is accomplished using centrifugal pumps. Main advantages: Suction piping/hoses are accessible through most openings; can be used in parallel for larger flows and in series for higher heads; main disadvantages and suction lift limitations.
When sizing centrifugals for your bypass pumping job it will be necessary to know the following:
- Desired capacity (peak flow rate) in gpm. Normally this is a given factor that is predetermined or measured in the field.
- Static suction lift: The vertical distance in feet from the eye of the impeller to the fluid level. Net Positive Suction Head — NPSH plays an important role when selecting the right pump.
When selecting a centrifugal pump for bypass work, the first consideration should be NPSHR by the pump. Neglecting the NPSHR of the pump is the single most common mistake when choosing a self-priming or priming assisted pump. A function of the pump design requires part of the 33.9 ft (14.7 in.) available as further defined in this article.
A centrifugal, and in a way submersibles, require part of the “Net Positive Suction Head Available” — NPSHA. The work that can be done therefore, on the suction side of the pump is limited, so NPSH becomes important to the successful operation of the bypass pump operation.
NPSHA: There is 14.7 in. (or 33.9 ft) of atmospheric pressure available at sea level. To put it simply, it is atmospheric pressure pushing down on the fluid (as the pump creates a vacuum within the suction conduit — negative atmosphere) that pushes it up and into the eye of the pump impeller, the centrifugal force then creates pressure and then slings out the discharge of the pump. Just like when you are drinking water through a straw you create a vacuum within the straw, the fluid rises up into the straw due to atmospheric pressure. Under a perfect vacuum (29.92 in. of mercury Hg) you can lift water no more than 33.9 ft therefore, if you stood on top of a four-story building and you had a straw long enough to reach your glass of water it would rise no further than 33.9 ft up. Due to limiting factors such as pump efficiencies, pump suction lifts are limited to approximately 28 ft total suction lift, but for practical purposes, suction lifts should be limited to 25 ft or less.
NPSHR: In order to do the work, a pump has a net positive suction head requirement. The single most common mistake when choosing/selecting a self-priming centrifugal pump is neglecting the NPSHR of the pump (this bears repeating). A function of the pump design requires part of the 33.9 ft (14.7 in.) available.
- Static Discharge Head: The vertical distance in feet from the eye of the impeller to the discharge point. The vertical distance is head pressure, in feet, that must be over come by the pump and is added the total head.
- Friction Loss: Size, Type and Length of Piping and Fittings. Friction/Velocity Tables are used to factor this into the total dynamic head (TDH). These useful tables show the different size conduits by diameter indicating the head loss (resistance) in feet when various flow rates in gpm are passed through the different sized conduits. These tables also show the velocity of the effluent as it passes through the conduit in feet per second (fps). When sizing your conduit for a bypass, as a rule of thumb, the velocity should not exceed 10 fps. Velocities more than 10 fps will mean excessive Hp loss.
- Pressure at the Discharge Point, If Any. If there is pressure, it is added into the dynamic head calculations by converting the existing line pressure into feet by multiplying the pressure (psi) times 2.31 and adding the result to the total head.
The following is an example of the selection of a centrifugal pump for a specific application. For example there’s a ruptured sewer line in need of repair and the estimated flow rate in the line is 1,200 gpm. The depth of the manhole that we will be pumping from upstream of the rupture is 20 ft deep and the effluent can only rise 5 ft from the bottom of the manhole before it starts to back up into some homes. The effluent will be discharged 800 lf to a manhole on a trunk line one street over and there is an elevation difference (rise) of 10 ft between the two manholes.
Parameter: 8-in. Standard Steel Pipe – Friction Loss in feet 4.17 (1,200 gpm, velocity 7.66 fps) per 100 ft.
The dynamic discharge head for this example would be calculated as follows:
| Static Suction Lift (20 ft depth minus 5 ft of retention) | 15.0 ft |
| Pipe Friction Loss: Suction pipe length | 20.0 ft |
| Equivalent pipe length to allow for fittings/bar strainer | 15.0 ft |
| 35 ft/100 x 4.17 ft per 100 ft = | 1.46 ft |
| Total Suction Lift | 16.46 ft |
| Static Discharge Head | 10.0 ft |
| Discharge pipe length | 800.0 ft |
| Equivalent pipe length to allow for fittings | 96.0 ft |
| 896 ft/100 x 4.17 ft per 100 ft = | 37.36 ft |
| Total Discharge Head | 47.36 ft |
| Adding the two totals: | 16.46 ft |
| 47.36 ft |
You would get 63.82 ft for the total dynamic head (TDH).
In evaluating two different pumps to be used (Curves A & B below) we see that both pumps have the flow capacity and head pressure to bypass our example. However, the NPSHR for Pump A is 19 ft and that for Pump B 10 ft at the required flow of 1,200 gpm.
It is necessary to verify which pump is capable of pumping (lifting) the required flow rate at the given head and at the same time keep the sewage within the manhole 15 ft below the rim of the manhole.
Pump A: We subtract from the NPSHA of 33.9 ft the total suction lift requirements of 16.46 ft, which leaves us 17.44 ft. This is the amount of atmospheric pressure available to overcome the NPSHR by the pump, 19 ft. However since the NPSHR of the pump is greater than what is available this pump would not be able to keep up with the flow and keep the effluent 15 ft below the manhole rim. The pump would still pump part of the total flow but because the suction lift is too great the sewage would eventually back up into the homes.
Pump B: We subtract from the NPSHA of 33.9 ft the total suction lift requirements of 16.46 ft which leaves us 17.44 ft. This is the amount of atmospheric pressure available to overcome the NPSHR by the pump, 10 feet. The NPSHA is greater than what is required by this pump and would therefore be able to keep up with the flow. Most pumps cannot produce a perfect vacuum of 29.92 in. of mercury, normal wear being one factor; therefore, there should be a minimum of 4 feet (more of NPSHA) difference to the NPSHR of the pump. Note using these same parameters this pump would also work in Denver at the higher altitude.
Vacuum-assisted, non-clog centrifugal pumps are the most commonly used pumps for bypasses, because they are capable of handling large amounts of liquids and solids and have an air-handling capability. Diesel powered units flexed coupled to the pump can be disconnected and replaced with a horizontal motor and paired up with a Variable Frequency Drive (VFD) control for those long term bypasses at an existing pump station, where electrical power is available. They can come with or without an enclosure for sound attenuation.
Submersible Pumps
Submersibles are centrifugal pumps with a motor directly attached to it in a common housing and are submerged in the effluent. Main advantages: Instant priming. No suction lift limitations, however, there are minimum submergence requirements. Their main disadvantages: Physical size and weight prohibits use through most access openings. Must be removed (pulled up) to clean out debris (blockage) lodged in the impeller. If electrical power is not available close by you will need a generator. Though not published on most pump manufacturer’s curves, submersibles require a specific amount of submergence over the volute in order to operate properly and for motor cooling characteristics. Therefore submersible pumps also need NPSHA, but it is not as critical as it is for above-ground centrifugals.
When considering the use of submersible pumps for your bypass pumping job, it will be necessary to know the following:
- Desired Capacity (peak flow rate) in GPM.
- Static Discharge Head: The vertical distance in feet from the fluid level to the discharge point.
- Size, Type and Length of Piping and Fittings.
Submersible pumps have their place in bypass work; they commonly come electrically driven, but some are hydraulically driven with a power pack that can be either diesel or motor driven.
Finally, peak flows can easily exceed the capacity of any single pump, therefore multiple pumps in parallel would be required and often designed to allow use of less pumps in low flow times. Discharge heads can also exceed the capacity of a single pump which would require pumps in series.
Pumps in Parallel: Pumping in parallel is the use of two or more pumps with common or separate suction lines connected to common single conduit of fluid. The flow is multiplied by the number combined in parallel.
Pumps in Series: Pumping in series makes use of two or more pumps. The first pumps discharge is connected to the second pumps suction. The result is the production of the additive head pressure generated by each pump. This is not commonly done when bypassing sewage because should one of the pumps fail, you would not be able to discharge to the designated point. Submersibles are not commonly used this manner.
Standby Pumps: When planning your bypass, make an allowance for having a minimum backup capacity of 50 percent of the anticipated peak flow rate; however, 100 percent backup is the normal requirement by most municipalities.
Your bypass plan should include the following:
- Bypass System Startup and Operation Proceedures
- A Bypass Monitoring Program
- A Sewage Spill and Response Plan
- A Monitoring Log
- Qualified Operators, Installation and Training List
The operation procedure should include the cleaning out of the primary pumps impellers of any debris caught up in them, as needed. If left un-cleaned, the pumps become less efficient and eventually won’t pump at all.
Whatever you use, a centrifugal or submersible pump for your bypass job, either type of pumps have their advantages and disadvantages. Whichever kind of pump you use, remember that flow capacity is only one of the fundamental parameters to be considered.
Jose Somera is California location manager for Griffin Dewatering.
