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Sewage Bypass Fundamentals – Self-Priming Centrifugals versus Submersibles Pumps

Water & Wastewater
A Photo of a bypass pump

Sewage Bypass Fundamentals – Self-Priming Centrifugals versus Submersibles Pumps

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 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 disadvantages; Suction lift limitations. Main advantages:Suction piping/hoses are accessible through most openings. Can be used in parallel for larger flows and in series for higher heads.

When sizing centrifugals for your bypass pumping job it will be necessary to know the following:

1) Desired capacity (peak flow rate) in Gallons Per Minute (Gpm). Normally this is a given factor that is predetermined or measured in the field.

2) 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 “Net Positive Suction Head”. This is the single most common mistake when choosing/selecting a self-priming centrifugal pump is neglecting the “Net Positive Suction Head Requirements” – NPSHR of the pump. A function of the pump design requires part of the 33.9 feet (14.7 inches) available.

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 very important to the successful operation of the bypass pump operation.

NPSHA: There is 14.7 inches (or 33.9 feet) 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 (28 inches) you can lift water no more than 34 feet therefore, if you stood on top of a 4 story building and you had a straw long enough to reach your glass of water it would rise no further than 34 feet up. But as far as pumps are concerned practical suction lifts should not exceed 22 feet at sea level.

Notes: As you go up in elevation there is less atmospheric pressure, so you will loose approximately 1 foot of NPSH for every 1000 feet of elevation; therefore, in Denver for example the NPSHA would be approximately 29 feet. Therefore, you will not be able to lift fluids as high. Cavitation, a complex phenomenon, occurs when the suction lift is so great, or exceeded that the liquid will vaporize, and the flowing stream will consist of liquid plus pockets of vapor, creating a noise that sounds like you are pumping rocks.

NPSHR: The single most common mistake when choosing/selecting a self-priming centrifugal pump is neglecting the “Net Positive Suction Head Requirement” – NPSHR of the pump. A function of the pump design requires part of the 33.9 feet (14.7 inches) available.

3) Static discharge head: 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 over come by the pump and is added the total head.

4) Size, type and length of piping and fittings. Friction/Velocity Tables are used to factor this into the total head: Theseuseful tables show the different size conduits by diameter (be it hose, and the various types of pipe) 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 over 10 fps will mean excessive Hp loss.

5) Pressure at the discharge point if any. If there is pressure it is factored into the dynamic head calculations by converting the existing pressure into feet by multiplying times 2.31’ and adding the ?? to the total head. So that the existing pressure can be over come by the pump.

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 1200 Gpm. The depth of the manhole that we will be pumping from upstream of the rupture is 20 feet deep and the effluent can only rise 5 feet 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 feet between the 2 manholes.

Parameter: 8” Standard Steel Pipe – Friction Loss in feet 4.17 (1200 Gpm, velocity 7.66 fps) per 100 feet.

The dynamic discharge head for this example would be calculated as follows:

Static Suction Lift (20’ depth minus 5’  15.0’ of retention)

Suction pipe length 20.0

Equivalent pipe length to allow for 15.0 fittings/bar strainer

35’/100 x 4.17’ per 100 feet =   1.46

Total Suction Lift 16.46’

 

Static Discharge Head 10.0’

Discharge pipe length         800.0

Equivalent pipe length to allow for 96.0 fittings

896’/100 x 4.17’ per 100 feet = 37.36

Total Suction Lift 47.36’

 Adding the two totals:

16.46’

47.36

 

You would get 63.82 feet for the total dynamic head.

In evaluating two different pumps to be used (Curves A & B below) we see that both pumps have the capacity and head to bypass our example. However, the NPSHR for Pump A is 19 feet and that for Pump B 10 feet at the required flow of 1200 Gpm.

Now lets check 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 feet below the rim of the manhole.

Pump A: We subtract from the NPSHA of 33.9 feet the total suction lift requirements of 16.46 feet which leaves us 17.44 feet. This is the amount of      atmospheric pressure left to overcome the NPSHR by the pump, 19 feet. 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 feet 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 feet the total suction lift requirements of 16.46 feet which leaves us 17.44 feet. This is the amount of      atmospheric pressure left 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 28 inches, 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.

Versatile centrifugal pumps are the most commonly used pumps for bypasses over 90% of the time because they are capable of handling large amounts of liquids and solids. 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  They can come with or without an enclosure for sound attenuation.

Submersibles are centrifugal pumps with a motor directly attached to it in a common housing. 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. Main advantages:Instant priming. No suction lift limitations, however, there are minimum submergence requirements. 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 centrifugals.

When considering the use of submersible pumps for your bypass pumping job it will be necessary to know the following:

1) Desired capacity (peak flow rate) in Gallons Per Minute (Gpm)

2) Static discharge head: The vertical distance in feet from the eye of the impeller to the discharge point.

3) Size, type and length of piping and fittings.

As illustrated for the centrifugal pump example above the dynamic discharge head for a submersible would be calculated in the same manor but for Static Discharge Head only.

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 15 Mgd (10,417 Gpm) which is more than what a single 12-inch pump can handle. Therefore multiple pumps in parallel would be required. Discharge heads can exceed 90 feet which would require pumps in series.

Pumps in Parallel: Pumping in parallel is the use of 2 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. Common suction lines will require pumps with vacuum assisted priming devices.

Pumps in Series: Pumping in series makes use of 2 or more pumps; the first pumps discharge is connected to the second pumps suction. The result is the production of twice as much head. 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 way.

Standby Pumps: When planning your bypass make an allowance for having a minimum backup capacity of 50% of the anticipated peak flow rate; however, 100% backup is the norm.

Your Bypass Plan should include the following:

1. BYPASS SYSTEM STARTUP AND OPERATION PROCEDURES

2. A BYPASS MONITORING PROGRAM

3. A SEWAGE SPILL & RESPONSE PLAN

4. A MONITORING LOG

5. QUALIFIED OPERATORS, INSTALLATION AND TRAINING LIST

The operation procedure should include the cleaning out twice daily minimum of the primary pumps impellers of any debris caught up in them. See pictures 1 & 2, as you can see what can get caught up in the bypass pumps impellers. 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, both types 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.

Written By Jose Somera – Project Manager for Griffin Dewatering

If you would like more infromation, please email: gpe@griffinpump.com

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