Day Tank Pump Sizing Guidelines
Design Considerations

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The proper sizing and selection of a fuel oil pump set for filling a day tank does not have to be a complex problem.  This paper is a general guide to help the design engineer determine the three main areas of concern: pump lift, pump head, and pump prime.  In critical or borderline applications, then an experienced hydraulic engineer should always be consulted.

Pump Lift

A rotary gear pump will lift fuel by displacing air from the suction line to the discharge line.  The rotary gear pump creates low pressure in the suction line which allows the higher atmospheric pressure (14.7 PSI at sea level) to lift liquid into this vacuum.  If a perfect vacuum could be created and maintained, then fuel could theoretically be lifted to 34 feet.  However, a perfect vacuum cannot be created and the vapor pressure of fuel oil is such that a high vacuum cannot be achieved without causing the fuel to vaporize. Therefore, pump lift is normally 50% of theoretical lift or 17 ft. (7.4 PSI). The Tuthill pump curves are based on 15"HG (17 ft.1).

To determine the total available lift, the following factors need to be considered:

1.

The vertical distance from the tank to pump inlet: this is the main factor in lifting capabilities.  This measurement should be taken from the bottom of the main tank to the pump's inlet port.

2.

The total length of pipe:  The total length of vertical and horizontal pipe and its size is important due to internal friction.  This added friction will reduce lift and must be considered.  Calculations are based on 60F and frictional resistance will increase as temperature decreases.

3.

The fittings in the line:  Elbows, tee's, foot valves, etc. will disrupt flow and create added resistance.

4.

The elevation above sea level:  As the elevation increases above sea level, the atmospheric pressure is reduced and the pump's vacuum is reduced thereby reducing overall lift.

Pump Head

The pump's head is the theoretical vertical distance a pump will push fuel.  When lifting fuel from a buried tank, the maximum pressure that can be obtained when using #2 fuel is 100 PSI.

To calculate the total head (pressure) required, there are three factors that need to be taken into consideration:

1.

The vertical distance from pump to day tank:  this measurement should be taken from the outport port on the pump to the day tank's upper most piping connection.

2.

Total length of pipe and its size:  the length and size of pipe adds resistance and this added resistance needs to be added to total pump head.

3.

The fittings in the line:  valves, checks, elbows, tee's , etc., adds extra resistance and this added resistance needs to be added to total pump head.

Pump Prime

Maintaining the prime on a pump is of critical importance.  Fuel must be present at the pump suction with no air pockets.  Foot valves in the main tank or check valves at the pump set can be used to prevent fuel from flowing back to the main tank and thereby losing prime.

Pump cavitation is the inability of a pump to discharge fuel properly and can occur for any number of reasons:

1.

The total lift is too high for the pump.

2.

The total combination of lift and suction pipe resistance is greater than 15"HG.

3.

Restrictions in suction line such as a clogged filter or blocked line.

4.

Air leaks in suction line.

5.

An improperly plumbed system.

Cavitation can occur gradually and will eventually ruin a pump.

A fuel strainer is highly recommended on the inlet side of the pump because rotary gear pumps can be greatly affected by foreign particles causing the pump to wear or seize.

Notes:

1 PSI = 2.31 feet of head is the conversion factor for water.  As a general rule this is a safe conversion for #2 diesel fuel.  To be precise use the following:

Specific gravity of #2 = 0.88 @ 60F

Weight of #2 = 7.3 lbs/gallon

Viscosity of #2 diesel fuel = 

35 SSU @ 100 F

40 SSU @ 70 F

60 SSU @ 20 F

80 SSU @ 0 F

Table #1:  Frictional head loss (in ft.) for 100 ft. of standard weight pipe at 60 F and at sea level - #2 fuel.

  Pipe Size
3/8" 1/2" 3/4" 1" 1" 1" 2"
GPM 1 5.5 1.9 .45 .13      
2 15.2 5.5 1.1 .5      
4 55.5 20.3 5.1 1.4 .5 .2  
7   61 15.3 4.6 1.2 .5  
10     26.3 8.5 2.5 .9 .2
19       28.5 7.5 3.5 1.2

Table #2:  Frictional loss in pipe fittings in terms of equivalent feet of straight pipe.

Size Ball Valve 45 Elbow 90 Elbow Std Tee Check Valve Angle Valve Glove Valve
3/8" 0.28 0.70 1.4 2.6 3.6 8.6 16.5
1/2" 0.35 0.78 1.7 3.3 4.3 9.3 18.6
3/4" 0.44 0.97 2.1 4.2 5.3 11.5 23.1
1" 0.56 1.23 2.6 5.3 6.8 14.7 29.4
1" 0.74 1.60 3.5 0.7 8.9 19.3 38.6
1" 0.86 1.80 4.1 8.1 10.4 22.6 45.2
2" 1.10 2.40 5.2 10.4 13.4 29.0 58.0

Table #3: Lifting capacities at various elevations.

Elevation Atmospheric Pressure Available Lift
Sea Level 14.7 psi 17 feet
1000 feet 14.2 16
2000 13.6 15.5
3000 13.1 15
4000 12.6 14.5
5000 12.1 14
6000 11.7 13.5

Table #4: Oil flow capacity of pipes.

NPT Size 2 FPS 4 FPS 10 FPS 15 FPS 20 FPS 30 FPS
1/8" .36 GPM .72 GPM 1.8 GPM 2.70 GPM 3.6 GPM 5.4 GPM
1/4" .60 1.2 3.0 4.5 6.0 9.0
3/8" 1.2 2.4 6.0 9.0 12.0 18.0
1/2" 1.92 3.8 9.5 14.0 19.0 29.0
3/4" 3.36 6.6 16.0 25.0 33.0 50.0
1" 5.50 11.0 27.0 41.0 55.0 83.0
1" 9.4 19.0 47.0 70.0 94.0 140
1" 13.0 26.0 65.0 95.0 130 190
2" 21.0 42.0 105 156 210 312

Notes:

1.

Pipe size should be selected on the basis of oil flow velocity.  Under sizing results in a high pressure drops and power loss.  Over sizing may reduce the pressure and power loss but may be expensive to plumb.

2.

Pump Suction Lines:  As a general rule select suction lines to keep oil velocity within the range of 2 to 4 feet per second.  Lower velocities may be used in order to keep total lift resistance under 12" HG.

3.

OIL RETURN LINES:  As a general rule select oil return lines to keep oil velocity within the range of 10-15 ft. per second.

Pump Suction Lift Worksheet.

Please fill out the following before starting the worksheet:

Vertical pipe length: _______ Pipe diameter: _______ Elevation Above Sea Level: _______
Horizontal pipe length: _______ Pump GPM: _______ Fitting types in line: _______

Notes:

Please reference data tables for items listed below.

1. Total vertical length of pipe (pump inlet to bottom of tank): _______ft
2. Total length of pipe (vertical & horizontal): _______ft
3. Additional length due to fittings in line (Table #2): _______ft
4. Add results of #2 & #3: _______ft
5. Divide the result of #4 by 100: _______cu ft
6. Pipe size diameter: _______in
7. Pump Capacity: _______GPM
8. Frictional head loss (Table #1): _______Per 100 ft (horiz)
9. Additional head loss - multiply results of #5 by #8: _______ft
10. Repeat steps in items #2 thru #9 for each pipe size used in line: _______ft
11. Total lifting capacity needed (add the results of #1, #9, & #10): _______ft
12. Elevation above sea level: _______ft
13. Available pump suction lift (Table #3): _______ft
14. Subtract results of item #11 from item #13: _______ft
__ If results of item #14 are positive, then system is properly sized.
__ If results of item #14 are negative, then system is beyond a safe lifting capacity.
__ If results of item #1 are less than results of item #13, then increase pipe size.

Pump Head Worksheet.

Please fill out the following before starting the worksheet:

Vertical pipe length: _______ Pipe diameter: _______ Elevation Above Sea Level: _______
Horizontal pipe length: _______ GPM: _____ HP: _____ Fitting in line: _______

Notes:

Please reference data tables for items listed below.

1. Total vertical height from pump to day tank inlet:  __ft x .433 = _______PSIG
2. Total length of vertical and horizontal pipe: _______ft
3. Additional length due to fittings and valves (Table #2): _______ft
4. Total of #2 + #3: _______cu ft
5. Divide result of #4 by 100: _______ft
6.  Pipe size: _______in
7. Pump capacity: _______GPM
8. Frictional head loss (Table #1): _______Per 100 ft
9. Additional head: multiply #5 x #8 = __ft x .433 =  _______PSIG
10. Pressure required at day tank: _______PSIG
11. Total pressure required : Add #1 + #9 + #10 = _______PSIG
__ If pressure required is less than 100 PSIG, then Tuthill pump is suitable.
__ If pressure required is more than 100 PSIG, then contact your factory representative for assistance.

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