Pressure Requirements for the Fuel Transfer System{1256} Caterpillar

Engine:

3406C (S/N: 4ZR1-UP; 1LS1-UP)

3412 (S/N: 81Z1-14500)

3412C (S/N: 81Z14501-UP)

3412C (S/N: BAK1-UP)

3508 (S/N: 23Z1-UP)

3508B (S/N: 6PN1-UP)

3512 (S/N: 24Z1-UP)

3512B (S/N: 8RM1-UP)

3516 (S/N: 5SJ1-UP; 25Z1-UP)

3516B (S/N: 7RN1-UP; 1NW1-UP)

Power Module:

PM3412 (S/N: 5FL1-UP; CAJ1-UP)

PM3508 (S/N: BLM1-UP)

PM3512 (S/N: BRF1-UP; BNR1-UP)

PM3516 (S/N: CAC1-UP; BPD1-UP)

Introduction

This Special Instruction describes the method for determining the correct fuel pressure to the fuel transfer system. An external fuel supply line that is set up improperly will cause the fuel transfer pump to lose prime. This will cause the fuel transfer pump to run dry. When this happens, the fuel transfer pump will eventually fail.

Power modules that are equipped with the fuel transfer system require a positive head. Fuel is not drawn into the fuel transfer pump. A positive head (normally atmospheric pressure) is required in order to push the fuel through the suction pipe and into the fuel transfer pump.

The fuel transfer pump requires a specific minimum Net Positive Suction Head Available (NPSHA) to operate properly. The NPSHA can be estimated by adding the Static Suction Head (SSH) and Maximum External Restriction (MER). Then subtract the Dynamic Restriction (DR) in the piping for the external fuel supply that is connected to the power module.

NPSHA = [SSH + MER] - DR

Do not perform any procedure that is outlined in this Special Instruction until the entire Special Instruction has been read and understood.

Requirements

The power module will have one of two different types of fuel transfer systems. One system is configured with the 141-7898 Fuel Transfer Pump As (Illustration 1) and the other system is configured with the 207-4512 Fuel Transfer Pump As (Illustration 2).

Illustration 1	g01020143
Typical example (1) 141-7898 Fuel transfer pump

Illustration 2	g01020199
Typical example (2) 207-4512 Fuel transfer pump

The maximum external restrictions for the fuel transfer systems are listed in Table 1.

Note: All piping for the external fuel supply to the power module must be a minimum of 38.1 mm (1.5 inch) in diameter. This size of piping will optimize the friction losses for the external fuel system.

Follow the steps that are listed below in order to calculate the NPSHA for the external fuel supply for the power module.

Static suction head - This is the pressure difference (difference in vertical height) that is between the external fuel connection of the power module and the lowest possible fuel level in the external fuel supply tank.

The static suction head pressure difference is considered negative if the external fuel connection of the power module is above the fuel level in the external fuel supply tank.

The static suction head pressure is considered positive if the external fuel connection of the power module is below the fuel level in the external fuel supply tank.

1. Determine whether the static suction head is negative or positive. Add the value to the fuel transfer system's external restriction that is listed in Table 1.

   Example from Illustration 3

   -3 ft + 7 ft of #2 Diesel Fuel = 4 ft of #2 Diesel Fuel

2. Determine the dynamic restrictions in the external piping.

   1. Measure the straight lengths of pipe. Add the measurements together for a total of the straight length of pipe.

      Example from Illustration 3

      (2.0 ft + 0.25 ft + 0.25 ft + 2.75 ft) = 5.25 ft

   2. Count the number of pipe fittings that are used in the external piping. Use table 2 in order to obtain the equivalent straight length of pipe for each type of pipe fitting. Add the equivalent straight length of pipe for each type of pipe fitting together for a total equivalent.

      Example from Illustration 3

      3 Standard elbows of 1.5 inches in diameter

      4.1 ft + 4.1 ft + 4.1 ft = 12.3 ft

   3. Total the two straight lengths of pipe from the result of Steps 2.a and 2.b. Multiply the total straight lengths of pipe by 0.12 ft. #2 Diesel Fuel/ft.

      The result is the total dynamic restriction.

      Example from Illustration 3

      5.25 ft + 12.3 ft = 17.55 ft

      17.55 ft × 0.12 ft. #2 Diesel Fuel/ft = 2.106 ft. #2 Diesel Fuel

      Note: 0.12 ft. #2 Diesel Fuel/ft is a constant for the friction loss of the fluid.

3. Subtract the total dynamic restriction in the external piping in Step 2.c from the result of Step 1 in order to determine a final value for NPSHA.

   Example from Illustration 3

   4 ft of #2 Diesel Fuel - 2.106 ft. #2 Diesel Fuel = 1.894 ft. of #2 Diesel Fuel = Properly plumbed

   For the proper operation of the fuel transfer system, the piping for the external fuel supply must provide a NPSHA value that is greater than zero.

Illustration 3	g01026878
Example for the NPSHA calculation with the 141-7898 Fuel Transfer Pump As Because the external fuel connection of the power module is above the fuel level in the external fuel supply tank, the static suction head pressure difference is considered negative.

Refer to Application and Installation Guide, LEBX0030, "Fuel Systems" for additional information.