- Generator Set:
- C175-16 (S/N: WYB1-UP)
Introduction
Do not perform any procedure that is outlined in this Special Instruction until the entire Special Instruction has been read and understood.
This Special Instruction provides calculations for determining the minimum inlet pressure and maximum back pressure of the fuel return on C175 Engines.
Calculations for the Inlet Fuel Pressure
Caterpillar provides the C175 Engine with the following items: primary fuel filter/water separators, the fuel transfer pump and the secondary fuel filters. The customer must provide fuel with an ISO cleanliness of 18/16/13 at the pressures and flows that are provided in Technical Marketing Information (TMI).
Refer to Table 1 in order to estimate the minimum pressure of the inlet fuel with a day tank or standby application.
| Minimum Pressure of the Inlet Fuel | ||||||||
| Diameter of Pipe, Fittings, and Valves (Inches) | Length of Pipe (Meters) | Number of Elbows | Number of Tees | Number of Ball Valves | Number of Double Poppet Foot Valves | Number of Solenoid Valves CV>30 | Minimum Height of Fuel in Tank (Meters) (1) | Minimum Inlet Pressure (kPa) (2) |
| 1.5 | 4 | 4 | 1 | 1 | 1 | 1 | 0.5 | -6.7 |
| 1.5 | 6 | 5 | 1 | 1 | 1 | 1 | 0.5 | -7.7 |
| 1.5 | 8 | 5 | 1 | 1 | 1 | 1 | 0.5 | -8.2 |
| 2.0 | 10 | 5 | 1 | 1 | 1 | 1 | 0.0 | -5.6 |
| 2.0 | 15 | 6 | 2 | 1 | 1 | 1 | 0.0 | -6.5 |
| 2.0 | 20 | 7 | 2 | 1 | 1 | 1 | 0.0 | -7.0 |
| ( 1 ) | The minimum height of fuel in the tank is the height of the fuel level that is above the inlet port for the engine. This height is measured at the lowest fuel level in the tank. |
| ( 2 ) | Inlet pressure is an estimate that is good for flows up to 41 liter per minute with standard No. 2 diesel. |
Use the equations below in order to estimate the minimum inlet pressure if the configuration of the fuel delivery is not listed above.
Note: All distances that are located below are in meters.
| Equation for Pipe with 1.5 In Inside Diameter | |
| PINLET = 8.829(z2 - z1) + [- 0.242(L + 2.3E + 3.0T + 0.6BV)] - 5FV - 2SV | |
| L = 2.0" Diameter Straight Length (m) | E = Number of 1.5 In Elbows |
| T = Number of 1.5 InTees | BV = Number of 1.5 In Ball Valves |
| FV = Number of 1.5 In Foot Valves | SV = Number of Solenoid Valves (CV>30 |
| z1 = Height of fuel inlet port (3) | z2 = Height of minimum fuel level (3) |
| ( 3 ) | This measurement is measured from a common plane of reference. Refer to the example below. |
| Equation for Pipe with 2 In Inside Diameter | |
| PINLET =8.829(z2 - z1) + [- 0.075(L + 2.6E + 4.0T + 0.8BV)] - 1.5FV - 2SV | |
| L = 2" Diameter Straight Length (m) | E = Number of 2 In Elbows |
| T = Number of 2 In Tees | BV = Number of 2 In Ball Valves |
| FV = Number of 2 In Foot Valves | SV = Number of Solenoid Valves (CV>30 |
| z1 = Height of fuel inlet port (3) | z2 = Height of minimum fuel level (3) |
| ( 3 ) | This measurement is measured from a common plane of reference. Refer to the example below. |
Calculations for the Back Pressure of the Fuel Return
The back pressure of the fuel return on the C175 Engine is equally important and critical to maintaining proper system pressure. Maintain the back pressure of the fuel return below 60 kPa (9 psi). Refer to Table 4 in order to estimate the back pressure of the fuel return for systems that are using a day tank or standby application.
If a fuel cooler is needed, the return line must be a minimum of 1 and 1/2 in inside diameter. This diameter must be considered in the calculations of the back pressure. Table 4 calculates the maximum back pressure of the fuel return that is using a Caterpillar supplied fuel cooler.
In some installations with the engine fuel supply from a day tank, the excess engine fuel return drains into the main tank. If the day tank is higher, the pressure can cause fuel from the engine and the day tank to drain into the main tank. The draining fuel could lead to a loss of prime and to hard starting. A check valve in the return line with 35 ± 20 kPa (5 ± 3 psi) of cracking pressure ( 398-1860 Check Valve Gp ) will prevent leakage without creating a significant amount of restriction. For an example of the installation, refer to Illustration 1.
 | |
| Illustration 1 | g02548176 |
(A) 1.524 m (5 ft) (1) Day tank (2) Ball valve (3) C175 Engine (4) Fuel return (5) Check valve (6) Fuel supply (7) Fuel pump for the day tank supply (8) Main fuel tank (B) 9.144 m (30 ft) | |
| Maximum Back Pressure of the Fuel Return | |||||||
| Diameter of Pipe, Fittings, and Valves (Inches) | Length of Pipe (Meters) | Number of Elbows | Number of Tees | Number of Ball Valves | Fuel Cooler | Maximum Height of Fuel in Tank (Meters) (4) | Maximum Back Pressure of Fuel Return (kPa) (5) |
| 1.0 | 4 | 4 | 1 | 1 | No | 1.5 | 42.9 |
| 1.0 | 6 | 5 | 1 | 1 | No | 1.5 | 50.5 |
| 1.0 | 8 | 5 | 1 | 1 | No | 1.5 | 54.1 |
| 1.5 | 10 | 5 | 1 | 1 | Yes | 2.5 | 53.4 |
| 1.5 | 15 | 6 | 2 | 1 | Yes | 2.5 | 56.0 |
| 1.5 | 20 | 7 | 2 | 1 | Yes | 2.5 | 57.7 |
| ( 4 ) | Maximum height of the fuel in the tank is top of the fuel tank above the inlet port to the engine. |
| ( 5 ) | Back pressure is an estimate that is good for flows up to 39 liter per minute with standard No. 2 diesel. |
Use the equations below in order to estimate the maximum back pressure of the fuel return. Use the equations below if the configuration for the return fuel is not listed above.
Note: All distances that are located below are in meters.
| Equation for Pipe with 1 In Inside Diameter | |
| PBACKPRESSURE=8.829(z3 - z1) + 1.768(L + 1.6E + 2.0T + 0.5BV) | |
| L = 1 In Diameter Straight Length (m) | E = Number of 1 In Elbows |
| T = Number of 1 In Tees | BV = Number of 1 In Ball Valves |
| z1 = Height of fuel inlet port (3) | z3 = Height of top fuel tank (3) |
| ( 3 ) | This measurement is measured from a common plane of reference. Refer to the example below. |
| Equation for Pipe with 1.5 In Inside Diameter | |
| PBACKPRESSURE=8.829(z3 - z1) + 0.226(L + 2.3E + 3.0T + 0.6BV) + 25FC | |
| L = 1.5 In Diameter Straight Length (m) | E = Number of 1.5 In Elbows |
| T = Number of 1.5 In Tees | BV = Number of 1.5 In Ball Valves |
| z1 = Height of fuel inlet port (3) | z3 = Height of top fuel tank (3) |
| FC = The number of Caterpillar fuel coolers (6) | |
| ( 3 ) | This measurement is measured from a common plane of reference. Refer to the example below. |
| ( 6 ) | The pressure drop across the fuel cooler is for a fuel cooler that is supplied by Caterpillar. Other restrictions of fuel coolers may vary considerably. Contact the supplier of the fuel cooler for the data on the pressure drop at your site. |
Schematic
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| Illustration 2 | g01953442 |
(A) Z3 (B) Z2 (C) Z1 (1) Fuel tank (2) Foot valve (3) Ball valve (4) Solenoid valve (5) Optional fuel cooler (6) Common plane of reference (7) Flexible lines | |
Examples
| Example of Calculation for Inlet Pressure | |
| Pipe with 1.5 In Inside Diameter | |
| L = 10 m | E = 8 |
| T = 2 | BV = 1 |
| FV = 1 | SV = 1 |
| z1 = 1 m | z2 = 2.0 m |
- PINLET = 8.829(z2- z1) + [{- 0.242(L + 2.3E + 3.0T + 0.6BV)} - 5FV - 2SV]
- PINLET = 8.829(2.0 - 1.0) + [{- 0.242(10 + 2.3(8) + 3.0(2) + 0.6(1))} - 5(1) - 2(1)]
- PINLET = 8.829(1.0) + [{-0.242(35)} - 5 - 2]
- PINLET = 8.829(1.0) + [{-8.47} - 5 - 2]
- PINLET = 8.829(1.0) + [-15.47]
- PINLET = -6.641 kPa
-6.64 > -10 kPa = Inlet pressure is OK
| Example of Calculation for the Back Pressure of the Fuel Return | |
| Pipe with 1 In Inside Diameter | |
| L = 10 m | E = 6 |
| T = 1 | BV = 1 |
| z1 = 1 m | z3 = 3 m |
- PBACKPRESSURE = 8.829(z3- z1) + 1.768(L + 1.6E + 2.0T + 0.5BV)
- PBACKPRESSURE = 8.829(3 - 1) + 1.768(10 + 1.6(6) + 2.0(1) + 0.5(1))
- PBACKPRESSURE = 8.829(2) + 1.768(22.1)
- PBACKPRESSURE = 56.7 kPa
56.7 < 60 kPa = Back pressure is OK
| Example of Calculation for the Back Pressure of the Fuel Return with a Cat fuel cooler | |
| Pipe with 1 and 1/2 In Inside Diameter | |
| L = 10 m | E = 6 |
| T = 1 | BV = 1 |
| z1 = 1 m | z3 = 3 m |
| FC = 1 | |
- PBACKPRESSURE = 8.829(z3- z1) + 0.226(L + 2.3E + 3.0T + 0.6BV) + 25FC
- PBACKPRESSURE = 8.829(3 - 1) + 0.226(10 + 2.3(6) + 3.0(1) + 0.6(1)) + 25(1)
- PBACKPRESSURE = 8.829(2) + 0.226(27.4) + 25
- PBACKPRESSURE = 48.9 kPa
48.9 < 60 kPa = Back pressure is OK
Note: Refer to Engine News, SEBD9580, "Pressure Requirements for the Fuel Supply of the C175 Engine" for more information on requirements for the fuel pressure from the customer supply for C175 Engines.