Troubleshooting Low-Temperature Valve Issues

Common faults of 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.

Leakage at the gland of the handwheel or valve cover nuts can be tightened with a wrench when the storage tank is not pressurized; it is strictly forbidden to tighten under pressure.

Valve internal leakage is usually caused by solid particles in the valve core or wear from frequent opening and closing. Both inlet and outlet valves are typically dual-valve designs, 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 or 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, condensation, and water, and tighten the valve cover nut.

When a leakage is found at the valve-pipe connection, please have a qualified unit perform the repair. During the repair, drain the liquid completely, empty it, and replace the oxygen equipment to ensure an oxygen content less than 21%. Before welding, set the valve to a semi-open position 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 trembling.

1) Failure of the indicator can be categorized into two types: the pointer not returning to zero and the pointer remaining fixed at a certain value. If the former is the case, it is usually due to improper use, such as the combination valve of the differential pressure (level) gauge not being in a balanced state before filling, causing the pointer to not return to zero. Open the combination valve after filling from the lower inlet. If the deviation between the pointer and zero is minimal, you can loosen the screw securing the fixed indicator needle, move the pointer to zero, and then tighten the screw.

2) Pointer trembling is usually caused by a leak in the gauge tube, so a careful inspection of the gauge tube, combination valve of the level gauge, and pressure gauge connections is required. Additionally, if the liquid is overfilled, causing a differential pressure (level) gauge to intake liquid, the tube may also experience trembling.

(4) Summary of Fault Handling Methods

Potential faults during use, their causes, and solutions are listed in the following table.

Troubleshooting Methods

Fault Phenomenon    Cause    Solution

During use, internal pressure

Abnormal Increase Occurred a. The turbocharger's inlet valve is not sealed properly, causing internal leakage. The turbocharger's inlet needs grinding and inspection.

b. The pressure gauge indicates inaccurately. Inspect or replace the pressure gauge.

Pressure (Level) Gauge Indication Inaccurate    a. Pressure (Level) Gauge Failure.    Inspect the Pressure (Level) Gauge.

b. Leaks in the upper and lower pipes of the differential pressure (level) gauge. Repair and seal leaks.

c. Air leakage at the differential pressure (level) gauge connection. Repair and remove the air leakage.

Evaporation rate exceeds design specifications

Value, sweating and frosting at the top. a. The vapor vent valve is not tightly closed or has internal leakage. Valve grinding and maintenance required.

b. Vacuum level deterioration. Re-evacuate the vacuum.

c. Top pearlescent sand settled, sand not properly filled. Refill pearlescent sand, re-evacuate vacuum.

Leakage of liquid or gas from the valve cap or joint. a. The sealing packing is not properly tightened. Tighten the gland nut.

b. Seal ring damaged. Replace the seal ring.

c. Sealing surface damage. Repair or replace parts.

d. Weld Seam Leaking - Repair Welding (but should be carried out according to repair welding specifications).

Huzhou Boiler Factory Co., Ltd. _ Shandong Hekou Group

Regular inspections

1. The user unit should report for regular inspection to the inspection agency one month before the expiration of the regular inspection validity period. The first regular inspection of the storage tank is generally conducted within 3 years of its commissioning. The subsequent inspection cycles are determined by the inspection agency based on the safety status level.

2. The unit and its auxiliary units should conduct technical handling after shutdown and safety inspections before testing, ensuring the site conditions meet the requirements for inspection work and preparing the necessary arrangements.

3. During inspections, the unit's safety management personnel, operation, and maintenance staff should be present to assist with the inspection, promptly provide relevant documents, oversee safety, and establish reliable communication methods.

Safety Protection

Potential hazards in the operation of liquefied natural gas mainly arise from its physical properties:

Liquefied natural gas has an extremely low temperature at atmospheric pressure: -162°C.

The gas-liquid volume ratio is quite high; improper pressure reduction measures can lead to a rapid increase in pressure. The gas-liquid volume ratio of liquefied natural gas is approximately 620:1.

Natural gas is an asphyxiating gas.

Natural gas is a flammable gas (ignition point 538°C).

Explosions may occur in confined spaces (explosion limits in air: 5.3% to 15%).

2. When working on-site with natural gas or liquefied natural gas, the following safety knowledge is required:

Keep liquefied natural gas equipment away from flames or electric sparks.

No smoking or open flames are permitted in areas involved in LNG equipment maintenance, refueling, and storage.

When working in areas with liquefied natural gas, safety goggles, face shields, and insulated gloves must be worn. Pants should not be tucked into boots, sleeves should not be worn inside insulated gloves, and boots should not have nails.

During disassembly and component repair, it is necessary to empty and depressurize the liquefied natural gas storage tank.

Tools used for maintenance should not produce sparks.

Warning

Protect eyes and exposed skin.

Ensure good ventilation in the area where the equipment is located.

Ensure the system is depressurized before repairing or maintaining equipment.

Keep away from flammable materials or sparks.

The root valve must always remain open.