What is a Sewage Pump?
, not to be confused with sump pumps
are specific types of pumps and pumping systems used to pump sewage from a bathroom and/or laundry room in your basement or anywhere else that is below your main sewer/septic lines.
These guidelines cover the steps that need to be taken to accurately select the correct sewage pump and applicable systems to use in sewage ejectors. The system is selected first and in sizing a system you need to work through five steps to determine:
- System Capacity (GPM required)
- Total Head
- Pump Selection
- Basin Size
- Simplex/Duplex System
System Capacity refers to the rate of flow in gallons per minute (GPM) necessary to efficiently maintain the system. The "Fixture Unit" method is suggested for determining this figure. This approach assigns a relative value to each fixture or group of
fixtures normally encountered. Determination of the required SYSTEM CAPACITY is as follows:
- List all fixtures involved in the installation and using Figure 1 assign a Fixture Unit value to each. Add to obtain total.
- Referring to Figure 2 locate the total Fixture Unit amount along the horizontal axis of the graph and follow vertically until intersecting the plotted line. Read the SYSTEM CAPACITY in GPM along the vertical axis.
TOTAL HEAD is a combination of two components—Static Head and Friction Head —and is expressed in feet . (Refer to Typical Installation Illustration, Figure 4).
- Static Head is the actual vertical distance measured from the minimum water level in the BASIN to the highest point in the discharge piping.
- Friction Head is the additional head created in the discharge system due to resistance to flow within its components. All straight pipe, fittings, valves, etc. have a friction factor which must be considered. These friction factors are converted to, and expressed as, equivalent feet of straight pipe, which can then be totaled and translated to Friction Head depending on the flow and pipe size. Basically this is reduced to four steps.
- It will be necessary to determine the discharge pipe size. In order to ensure sufficient fluid velocity to carry solids, (generally accepted to be 2 feet per second), flows should be at least:
9 GPM through 1-1/4" pipe
13 GPM through 1-1/2" pipe
21 GPM through 2" pipe
30 GPM through 2-1/2" pipe
46 GPM through 3" pipe
- The length of the discharge piping is measured from the discharge opening of the pump to the point of final discharge, following all contours and bends.
- To determine the equivalent length of discharge piping represented by the various fittings and valves, refer to Figure 6 and total all values. Add this to the measured length of discharge pipe and divide by 100 to determine the number of 100 ft. increments.
- Refer to Figure 5 and find the required Pump Capacity. (determined from Figure 2). Follow gallon per minute to pipe size being used. Multiply this number by the number of 100 foot increments.
- Add the Static Head and Friction Head to determine Total Head.
Every centrifugal pump has a unique performance curve. This curve illustrates the relationship of flow (GPM) to pressure (Total Head) at any point. The pump will operate at any point along this performance curve.
Pump capacity is therefore the flow the pump
will generate at any specific pressure. The object is to select a pump whose performance curve passes either through or close to the design-condition, preferably above.
Start with the smallest pump horsepower size that will pass the required solid size. If the solid size is not required or mentioned, go to Step 2 and check performance.
With the pump selected in Step 1, check performance curve(s) to see that it passes above or through the design condition.
SOLID HANDLINGS requirements may be determined by local codes and/or by the type of application and types of solids anticipated. Unless otherwise stated by codes, a sewage pump should have the capability of handling spherical solids of at least 2" in diameter in installations involving water closet.
BASIN selection is best accomplished by relating to required System Capacity as determined by the Fixture Unit method.
Figure 7 shows recommended basin diameters, assuming the normal pump differential (distance in inches between turn-on and turn-off), and running time ranges from 15 seconds to 4 minutes. Basin depth, however, should be at least 24" below basin inlet for most pumps and deeper where greater pumping differentials are anticipated.
Since basin size is directly related to frequency of pump operation, it is important to select a basin of sufficient size to insure that the pump does not short cycle.
The question of whether to use a SIMPLEX or DUPLEX system depends on the type of installation and/or local codes. Generally, a determination can be made using the following guidelines.
- Domestic — Simplex System is usually adequate.
- Commercial — OPTIONAL — Depending on the type of business and the need for uninterrupted sanitary drainage facilities.
- Public or Industrial — DUPLEX System is essential. While you are sizing the system and before you select the pump, you will need to know — and consider — or make allowances for the following:
- Volts/Phase/Hertz—What is available?
- Will the pump share a circuit?
- Does the home, business, etc. have circuit breakers or fuses?
- What is the breaker or fuse amp rating? Make sure it is enough.
- Check local or state codes for (1) Solid size requirements (2) Amp ratings/circuit cord size/ratings or type (3) Pipe material/size/depth to bury (4) Tank size/location.
- Are there plans for future expansion? As in, adding upstairs bath, basement plumbing, washing machine, etc.
Example Sizing Problem
- What PUMP CAPACITY would be required to handle the drainage from a 4 bathroom home, also including a dishwasher, a washer, a laundry tray, a kitchen sink, water softener, basement shower, a 13,000 gallon pool and a bar sink (1-1/2" trap)?
1. From Figure 1:
2. Refer to Figure 2:
Find 55 Fixture Units on the horizontal axis. Follow vertically until intersecting the line then horizontally to the left. The pump capacity on the vertical axis is 30 GPM.
- Determine the Total Head of the installation illustrated in Figure 4, The Typical Installation Illustration:
1. That Static Head in this instance is 7 feet.
2. Friction Head:
a. Since the required PUMP CAPACITY in this illustration of 30 GPM is less than the 46 GPM necessary to carry solids through 3" pipe, 2" or 2-1/2" pipe should be used. If 3" pipe is preferred or required, a PUMP CAPACITY of at least 46 GPM is required.
b. Measurement of the length of the discharge pipe totals 200'.
c. Refer to Figure 6 and note the friction factor in equivalent feet for each fitting:
3-90 degree elbows, 2" ---- 16 equivalent feet
1-gate valve, 2" --- 1 equivalent feet
1-swing check valve, 2" --- 17 equivalent feet
Totals 34 equivalent feet
Adding 34 feet to the measured pipe length, the total effective pipe length becomes 234 feet or 2.34 100-feet increments.
d. Refer to Figure 5. Find the 30 GPM required PUMP CAPACITY on the left scale and follow over to the 2" PVC pipe size column. Friction Head is 1.8 x 2.34 = 4.2 feet.
3. Total Head Required:
Total Head = Static Head + Friction Head
Total Head = 7 + 4.2
Total Head = 11.2
- Due to the existence of water closets in this installation, a pump with 2" Solids Handling capacity should be used unless otherwise specifically stated by applicable codes. Use Figure 3 to select pump.
- To determine the Basin size, find the Pump Capacity (30GPM) in the column on the left of Figure 7. Any BASIN diameter of 18" or greater is acceptable.
- Since this application is domestic, a Simplex System is sufficient.
- Summary: Recommended selections for this installation would be a Simplex System utilizing an 18" or greater diameter Basin and a 2" Solids Handling pump capable of delivering at least 30 GPM at 11".
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