Low Temperature Valve Fault Handling

Common issues with low-temperature valves: leakage at the handwheel stuffing box; leakage at the valve cover nut; internal leakage of the valve; leakage at the valve-pipe connection.

When there is a leak at the gland of the handwheel or at the valve cover nut, tighten the bolts with a wrench when the storage tank is not pressurized. Absolutely no tightening under pressure is allowed.

Valve internal leakage is usually caused by solid particles in the valve core or wear from frequent opening and closing. Both inlet valves and outlet valves are typically dual valves, with one in reserve. When internal leakage is detected, close the reserve valve and repair the leaking valve. First, loosen the valve cover nut with a wrench, remove the valve stem, and dry the valve disc and cover stem with a hairdryer; then clean the valve body sealing surface and the valve disc sealing gasket, and wipe away any condensation and water. If the valve disc gasket shows unevenness, cracks, or grooves, replace it immediately with a spare gasket. If wear marks are found on the valve body sealing surface, smooth them with metallographic sandpaper (use fine wood sandpaper on the back if unavailable). Before reassembling, carefully remove dust and condensation and tighten the valve cover nut.

When leakage is detected at the valve-pipe connection, please have a qualified unit perform the repair. During the repair, drain the liquid completely and empty it, and replace the oxygen equipment to ensure oxygen content is less than 21%. Before welding, position the valve halfway open and loosen the valve cover nuts. After welding, retighten the nuts and conduct an air tightness test on the pipeline.

Differential Pressure (Level) Gauge Fault Handling

Common issues include malfunctioning indicators and pointer tremors.

1) Failure in the indicator can be categorized into two types: the pointer not returning to zero and the pointer remaining fixed at a certain value. If it's the former, it's usually due to improper use, specifically the combination valve of the differential pressure (level) gauge not being in a balanced state before filling, preventing the pointer from returning to zero. Wait until liquid enters from the lower inlet port before opening the combination valve. If the deviation between the pointer and zero is minimal, you can loosen the screw securing the fixed gauge needle, move the pointer to zero, and then retighten the screw.

2) Pointer jitter is usually caused by a leak in the gauge tube, so carefully inspect the gauge tube, combination valve of the level gauge, and related connections of the pressure gauge. Additionally, if the liquid is overfilled, causing a differential pressure (level) gauge to intake liquid, the tube may also exhibit a jittering phenomenon.

(4) Summary of Fault Handling Methods

Possible Faults During Use, Causes, and Solutions Are Listed in the Following Table

Fault Resolution Methods

Fault Phenomenon    Cause    Solution Method

During use, internal pressure

Anomaly of Abnormal Increase: a. The turbocharger's inlet valve is not sealed properly, causing internal leakage. The turbocharger's inlet is to be grinded and inspected.

b. The pressure gauge reading is inaccurate. Inspect or replace the pressure gauge.

Pressure (Level) Gauge Indication is False    a. Pressure (Level) Gauge Malfunction.    Inspect the Pressure (Level) Gauge.

b. Leaks detected in the upper and lower pipes of the differential pressure (level) gauge. Repair and sealing work required.

c. Leaks detected at the differential pressure (level) gauge connection. Inspect and rectify the leaks.

Evaporation rate exceeds design specifications

Value, Sweat and Frost on Top    a. Inadequate closure of the vapor vent valve and internal leakage.    Inspect and grind the valve for repair.

b. Degradation of vacuum level. Re-evacuate the vacuum.

c. Top pearlescent sand settled, sand not fully packed. Re-pack pearlescent sand, re-evacuate vacuum.

Leakage of liquid or gas from the valve cap or joint: a. Sealing gland not tightly compressed. Tighten gland cover.

b. Sealing ring damaged. Replace the sealing ring.

c. Seal face damage. Repair or replace parts.

d. Weld Seam Leaks - Retouch welding (but should be carried out according to the retouch welding requirements).

LNG stands for Liquefied Natural Gas, a premium natural gas product primarily composed of methane. Known for its high calorific value, low pollution, and ease of storage and transportation, it has become one of the quality energy options in modern society. LNG is the result of purifying and treating natural gas from gas fields, removing all impurities, and then cooling it to approximately -162°C at atmospheric pressure, condensing it into a safe, colorless, odorless, and non-toxic low-temperature liquid. It boasts high purity, safety, storage efficiency, and flexible transportation. Its volume is about 1/600th that of gaseous natural gas, and its weight is approximately 45% of the same volume of water.

　　Safety Advantages of LNG

Due to excellent physical and chemical properties, it is safe: with a higher ignition point and an auto-ignition temperature of 590°C, the combustion range is narrow at 5-15%, lighter than air and easy to disperse. LNG will quickly heat up and vaporize upon leakage, and the gas density of the vaporized natural gas is approximately half that of air, making it highly volatile but non-flammable and non-toxic. Methane has relatively stable chemical properties, and it can burn quietly in the air without supporting combustion.

Due to excellent storage conditions, the tank materials exhibit mechanical strength, such as tensile and compressive strength, low-temperature impact toughness, and thermal expansion coefficient, which are suitable for operation under low temperatures. The tank foundation can withstand the low temperatures associated with direct contact with LNG. The insulation material is non-flammable and has sufficient rigidity to withstand the impact of firefighting water, and the safety protection system is reliable. Mobile LNG storage is contained in insulated containers at pressures ranging from 0.05 to 0.5 MPa, which is lower than the storage pressure of CNG (20 MPa) and offers higher safety. The low-temperature mobile cylinders have undergone strict inspections including high-temperature resistance tests, impact tests, and mobile simulation tests by the National Quality Supervision and Inspection总局. Even in the event of an accident, LNG leakage is unlikely to occur.

　　Economic Advantages of LNG

High calorific value makes it economical. Compared to coal, natural gas, and liquefied petroleum gas, LNG has a higher calorific value and a relatively stable price that is less affected by international fluctuations. The calorific value of LNG is 12,000 kcal, while good coal only has about 8,000 kcal (per kilogram). LNG vehicles of the same displacement can save about 20% in fuel costs compared to gasoline and diesel vehicles, and their maintenance costs can also be reduced by about 10%. Although there is a price difference of about 80,000 yuan between LNG heavy trucks and diesel trucks, the operating advantage is significant due to lower fuel expenses. The driving range equivalent to 1 cubic meter of LNG (after gasification) is equivalent to 0.95 liters of diesel (Number 0). Calculations show that a tractor can save about 80,000-140,000 yuan in fuel costs annually. Dump trucks can save about 100,000 yuan in fuel costs annually.

　　Advantages of LNG transportation

Low-temperature, high gas-liquid expansion ratio, high energy efficiency, easy to transport and store. Natural gas is piped, providing a continuous supply, with simple and easy-to-operate equipment. It is straightforward and convenient for long-distance transportation, and has low costs. With the rapid development of the global natural gas industry, liquefied natural gas (LNG) has become an important part of international natural gas trade. Compared to ten years ago, the world's LNG trade volume has doubled, showing a strong growth momentum.

　 Convenience advantages of using LNG

Convenient due to LNG refueling time. LNG refueling takes even less time than gasoline. Long-distance LNG buses equipped with two cylinders can safely travel over 1,000 kilometers, with a single refill taking just a few minutes.

Convenient due to LNG's power performance. LNG's power performance is not inferior to diesel vehicles, with a longer driving range. A LNG truck equipped with two 450-liter gas cylinders can travel 800-1000 kilometers.

Due to the well-developed LNG industry chain. China has established a complete technical and industrial chain for LNG vehicles. The core technology of natural gas engines and cryogenic vehicle cylinders is mature, with vehicle assembly technology, spare parts production, and maintenance technologies all meeting international standards.

The clean and environmental benefits of LNG

LNG is considered to be a clean fossil fuel on Earth, with good environmental benefits.

Clean due to the chemical properties of the main component of natural gas. Compared to coal as a fuel, it can significantly reduce emissions of carbon dioxide, carbon monoxide, sulfur monoxide, sulfur dioxide, dust, and other hydrocarbon compounds and nitrogen oxides, as well as coal slag and other pollutants, leading to a substantial improvement in environmental quality and tremendous social benefits.

Cleaned due to the liquefied natural gas (LNG) production process. During the process of cooling natural gas to a liquid state, it first passes through a gas-liquid filter to remove mechanical impurities and free liquid droplets from the gas. Then, a raw gas heater uses MDEA to absorb carbon dioxide and hydrogen sulfide from the natural gas. Subsequent processes include decarbonization, dehydration, dehydrocarbonation, and mercury removal. A dust filter is also used to remove dust from the natural gas, among other purification steps, resulting in LNG at -162 degrees Celsius. Data shows that using engines fueled by LNG reduces overall emissions by approximately 85% compared to gasoline or diesel, with a reduction of 72% in hydroxides in vehicle exhaust, 97% in carbon monoxide, 39% in nitrogen oxides, 24% in carbon dioxide, 90% in sulfur dioxide, 40% in noise, and contains no lead or benzene, which are carcinogenic substances. Sulfur compounds are virtually absent.