lng troubleshooting

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Common Troubles and Troubleshooting for LNG System The LNG fuel system consists of the cylinder (fluid reservoir), vaporizer, line regulator, buffer tank, pressure building pipeline and other systems. The LNG cylinder (fluid reservoir) is a cryo-insulation pressure vessel which can be used to multiply fill a gas. With the double-layer vacuum insulation structure, the reservoir consists of the inside liner, housing, insulating layer, valves and etc. The natural gas is stored in the low-temperature low-pressure vacuum insulation reservoir with the form of liquid-state, the inside temperature of the fluid reservoir is generally between -138℃ and -162℃, and the normal operating pressure of the cylinder shall be no more than 1.89MPa. Vaporizer: The LNG gas shall be vaporized by the vaporizer to the CNG gas after its temperature rises, and then the CNG gas is supplied to the engine. Much more heat will be absorbed during vaporization. The vaporizer uses the engine circulating coolant to heat and vaporize the liquefied natural gas (LNG), and thus make the natural gas meet the engine operating temperature and flowrate requirements. The main function of the line regulator is to decrease the pressure of natural gas after vaporized by the vaporizer and lower the pressure down to the set pressure of the pressure stabilizer so as to ensure that the pressure of gas entering into the engine meets the operating requirement. The function of the buffer tank is to store a certain amount of gas in the tank, and provide the additional gas consumption if the higher gas consumption (e.g., high power required) is required while the vaporized gas is insufficient, which avoids the shortage of gas supply. For the LNG supply system, the gas supply and pressure will directly affect the engine performance and its operating reliability. The schematic diagram of a LNG system:

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In the LNG system, the symbol, name and function description for its functional parts are as follows:

Symbol Chinese name model Function description

C1 The filling connection DJK-25B The device used while filling the LNG fluid

C2 The vent connection DHK-10A The special connection for discharging the gas

Vv Vent valve (globe) DJ-10P6-S2 It is used to exhaust and reduce the pressure inside the cylinder while filling the LNG fluid. Bleed the pressure inside the cylinder in case of emergency.

Vu Use valve (globe) DJ-10P-S1 It is used to supply the LNG or cut off the supply

Va Vaporizer Convert the LNG to the CNG

FCv The filling check valve DH-15C It is used to prevent the outflow of LNG filled into the cylinder

DCv The delivery check valve DH-06 It is a small check valve which is used to prevent the backflow of the high-pressure gas

Er Economizer regulator DYJ-06A It is used to preferentially use the gas and thus ensure the pressure inside the cylinder. The set value 0.9MPa

Ef Excess flow valve DL-06-Φ10 It is used to prevent the excessive outflow of the LNG gas from the cylinder when the external pipeline is loss of pressure.

P1 Cylinder Pressure Gauge Display the inside pressure of the cylinder

P2 Piping Pressure Gauge Display the pressure of the buffer tank

PBr Pressure Building Regulator DYS-06A1 It is used to control the pressure inside the cylinder during pressure building, with the set value of 0.80MPa

Pr Pressure Building Coil By vaporizing the LNG to the CNG, it is used to increase the pressure inside the cylinder

Pv Pressure Building Valve DJ-10P5 It is used to control whether the pressure building is required.

LG Liquid Fuel Level Gauge It is used to display the liquid capacity inside the cylinder

Svp Primary Safety Valve CRVP-253B-K-275

It is used to control the safety pressure inside the cylinder. The set value 1.89MPa

Svs Secondary Safety Valve CRVP-253B-K-413

In case of the primary safety valve failure, it is used to control the pressure inside the cylinder and ensure the safety of the cylinder. The set value 2.85MPa

LSv Line Safety Valve CRVP-253B-K-348

It is used to release the excessive pressure and thus protect the safety of the line system. The set value 2.4MPa

LRe Line Regulator DYS-20 or DYS-15C

It is also called the pressure stabilizer, which is used to regulate the pressure to the desired pressure. The set value 0.86MPa

Cb Convert Box It is used to convert the analog signal to the digital signal output.

SV Solenoid valve It is used to control opening and closing of a valve through the power supply.

Here, the operating principle of the LNG system isn't described in details. Now, the common trouble diagnosis, possible cause analysis and troubleshooting for the LNG system are summarized as follows:

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Common Troubles and Troubleshooting for LNG System:

Trouble LNG system appearance phenomena description Inspection Method Possible Cause Troubleshooting

Underpowered

The cylinder pressure normal; The vaporizer surface condensed, frosted; The buffer tank pressure low; (shortage of gas supply)

1. Check whether the gas supply pipeline is leaked, broken; and whether it is frosted, condensed, or frozen

The pipeline leakage, results in the low engine gas-supply pressure

Tighten the pipeline leakage connector; Since the materials of connection and valve are different and the coefficient of thermal expansion is varied, the leakage easily occurs at the beginning of driving.

2. Check whether the water inlet and outlet pipes of vaporizer are crushed, flattened or leaked

Shortage of vaporizer heat supply, results in the insufficient vaporization capacity

Check whether the water inlet and outlet pipes are leaked; whether there is the crushed flat, deformation, and the reduction of water flowrate; and whether there is any deterioration;

3. Check whether the engine coolant temperature and water pump function are normal, and check the flowrate

Shortage of vaporizer heat supply, results in the insufficient vaporization capacity Replace the water pump

4. Check whether there is any leakage inside the vaporizer. Whether the gas flows out from the expansion tank.

The internal leakage inside the vaporizer results in the shortage of gas supply Replace the vaporizer

5. Low ambient temperature, the engine coolant temperature doesn't reach 60℃

When the ambient temperature is below 0℃, the engine coolant temperature reaches 60℃ or higher.

Preheat the engine

Underpowered

The cylinder pressure continuously decreases; And the pressure building coil isn't frosted; The buffer tank pressure can't reach the specified value; (Not pressure building)

1. Check whether the pressure building line is properly opened

The pressure building line isn't opened, and the pressure building is unavailable. Result in the shortage of gas output

Confirm that the pressure building line is opened

2. Regulate the pressure of the pressure building regulator cylinder, find that the pressure doesn't increase yet, it indicates that the regulator fails.

The pressure building regulator failure, which can't build the pressure. Result in the shortage of gas output

Regulate, replace the pressure building regulator See the below in this document: Regulation of the pressure building regulator

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3. Replace the pressure building regulator, and find that the cylinder doesn't build the pressure yet

The pressure building regulator is blocked. LNG liquid can't flow in, and the pressure can't be built. Result in the shortage of gas output

Remove the pressure building line, and purge and rinse it with the dry nitrogen

Underpowered

The cylinder pressure continuously decreases; The pressure building coil is frosted;The buffer tank pressure is low; (Poor pressure building)

1. Increase the set value of the pressure building regulator, and check the variation of the cylinder pressure

The set value of the pressure building regulator is low, and although the pressure can be built but the pressure building effect is bad

Adjust the set value of the pressure building regulator See the below in this document: Regulation of the pressure building regulator

2. Increase the set value of the economizer regulator. Check the variation of the cylinder pressure

The set pressure of the economizer regulator is low and thus results in the reduction of the cylinder pressure. Although the pressure can be built, the pressure building effect is bad

Adjust the set value of the economizer regulator See the below in this document: Regulation of the pressure building regulator, Regulation of the economizer regulator

Underpowered

The cylinder pressure is normal and steady; The pressure building coil is frosted;The pressure of the buffer tank is normal;

Check the set value of the engine stabilizer; Increase the set value of the regulator;

The set value of the line regulator is lower than the pressure of the engine stabilizer

Adjust the set value of the line regulator See the below in this document: Regulation of the economizer regulator

Power unstable

The cylinder pressure gauge is stable; The pressure gauge of the buffer tank fluctuates; The pressure building coil is frosted;

Adjust the line regulator, and check the variation of the pressure gauge of the buffer tank

The line regulator failure results in the unstable gas supply pressure

Adjust or replace the line regulator See the below in this document: Regulation of the economizer regulator

The cylinder pressure gauge fluctuates; The pressure gauge of the buffer tank fluctuates; The pressure building coil is frosted;

Adjust the pressure building regulator, and check the variation of the cylinder pressure and the pressure gauge of the buffer tank

The unstable pressure building results in the fluctuation of the cylinder pressure and the unstable delivery

Regulate or replace the pressure building regulator See the below in this document: Regulation of the pressure building regulator

Engine fails to start up or suddenly halt

The cylinder pressure gauge is normal; The pressure building coil is frosted;The pressure gauge of the buffer tank fluctuates;

Check whether the pipeline and connection are frosted and frozen

The pipeline leakage, results in the low engine gas-supply pressure Replace or tighten

The cylinder pressure gauge is normal; The pressure building coil is frosted;The pressure gauge of the buffer tank reduces in reading;

Check the pressure gauge of the buffer tank, and find that its value can't almost increase after the pressure decreases

The delivery check valve is quickly opened, which results in the take-off of the excess flow valve

Warm the excess flow valve with lukewarm water; or wait for a period of time, the excess flow valve is automatically opened

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The cylinder pressure gauge is normal; The pressure building coil is frosted;The pressure gauge of the buffer tank is low in reading;

Check whether there is any leakage point in the pipeline

The pipeline suddenly fails, which results in the take-off of the excess flow valve, and prevents the leakage of the LNG

Tighten the leakage point, and replace the damaged part

The cylinder pressure is low; The pressure building coil is frosted;The pressure gauge of the buffer tank is low in reading;

Check the pressure gauge of the buffer tank, and find that its pressure reducesand then slowly rises

The set value of the economizer regulator is low and thus results in the reduction of the cylinder pressure.

Adjust the set pressure of the economizer regulator See the below in this document: Regulation of the economizer regulator

The cylinder pressure is normal; The pressure building coil is frosted;The pressure of the buffer tank is normal;

Check the set values of the regulator and then engine stabilizer

The pressure of the rear-end of the regulator is lower than the engine stabilizer pressure requirement

Adjust the set pressure values of the regulator and then engine stabilizer

The cylinder pressure quickly or continuously rises

The cylinder surface is normal, its pressure exceeds the set value of the safety valve, without exhaust

Check the safety valve by using the vibration method; The exhaust failure exists still, and checks the pressure gauge of the cylinder.

The safety valve is seized or fails Calibrate or replace the safety valve; Calibrate or replace the pressure gauge;

The cylinder surface is normal, its pressure quickly rises, without exhaust

Check the frosting of the economizer regulator circuit; the frosting is abnormal, and the set value of the economizer regulator is wrong

The economizer regulator is overhigh in set value or fails, the cylinder pressure can't be released according to the specified value

Adjust the set pressure value of the economizer regulator or replace it

The cylinder surface is normal, its pressure quickly rises, without exhaust

Check and find that the economizer regulator circuit is normal, and the delivery check valve fails

The delivery check valve fails and can't be closed. The gas fills back and thus result in the increase of pressure

Replace the delivery check valve

The safety valve exhausts for a long time

The cylinder surface is normal, and the pressure is normal

1. Check whether the pressure building coil is frosted

The pressure building coil is opened for a long time, and the pressure quickly rises Close the pressure building regulator

2. Check it by using the vibration method,and find that it still exhausts The safety valve isn't closed after opening Calibrate or replace the safety valve

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3. The set value of the safety valve is low or fails The safety valve isn't closed after opening Calibrate or replace the safety valve

The safety valve slowly exhausts for a long time

The cylinder surface is normal, and the pressure is normal

Check it with a screwdriver with a wooden handle by using the vibration method

The safety valve is frozen Normal

The safety valve exhausts for a long time

The cylinder pressure continuously rises, its surface is condensed or frosted, and the safety valve continuously exhausts until drains out

Check the condensation and frosting of the cylinder surface, and check the variation of the pressure gauge of the cylinder

The vacuum of the cylinder reduces or fails, and the thermal insulation property decreases

The cylinder is subjected again to the vacuuming treatment or replaced

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Note three pressure parameters of the LNG system: The set pressure value of the pressure building regulator: 0.9MPa; The set pressure value of the economizer regulator: 0.8MPa; The set value of the line regulator, i.e., pressure stabilizer: 0.86MPa. Since the failures of the pressure gauge, liquid fuel level display and transmitter are simply, visually diagnosed, they aren’t described in details.

lng troubleshooting

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