Operation methods of leakage detection, blowdown and vacuum pumping in refrigeration system

Update:19 Aug 2020
Summary:

The refrigeration system is essential for the machine r […]

The refrigeration system is essential for the machine room. In the refrigeration system, leak detection, blowdown and vacuum pumping play a protective role in the refrigeration system, so as to prevent the dirt in the system from entering the compressor and throttle valve, causing dirty blockage, cylinder scuffing, valve plate sealing not tight, affecting the normal operation of the compressor.

The following gives you the refrigeration system leak detection, blowdown, vacuum operation method.
The purpose of blowdown of the refrigeration system in the machine room is to remove the dirt in the refrigeration system. Although every equipment and pipeline has been derusted and blown before installation, during the whole system installation process, some welding slag, iron filings and dust will fall into the pipe from the welding joint.
Through the blowdown of the whole system, the dirt can be further removed, and the vacuum operation of the refrigeration system should be carried out after the system cleaning, blowdown, pressure test and leakage detection. Vacuumizing can not only further check the air tightness of the system, but also eliminate the air, moisture and other non condensable gases in the system, so as to prepare for the system to be filled with refrigerant.
Vacuum pump shall be used for vacuum pumping. For small refrigeration systems using medium and small open and semi enclosed compressors, the compressor of the system itself can also be used for air extraction without vacuum pump, except for those prohibited in the operation manual provided by the equipment manufacturer.

Vacuum operation method:
The operation method of vacuumizing with vacuum pump is to close the valves (such as filling valve, air vent valve, etc.) connected with the outside of the system, open all connected valves in the system, screw off the plug of the bypass hole of the exhaust valve, open the bypass port, and connect the vacuum pump to start the vacuum pump for air extraction. The air in the system is difficult to be pumped out. In order to achieve a certain degree of vacuum, vacuum is necessary The residual pressure of the ammonia system should be less than 7.999 kPa (60mmhg), and the residual pressure of the Freon system should be less than 1.333kpa (10mmhg). After the vacuum degree reaches the standard, the bypass port should be closed first, then the vacuum pump should be stopped, and the suction pipe should be removed, and the plug of the bypass hole of the exhaust valve should be screwed on If the pressure in the system does not exceed 666.61pa (5mmhg), it is qualified. If the pressure rises rapidly, the cause should be found out and eliminated in time.
The operation method of the compressor vacuumizing is to close the suction valve and exhaust valve, screw off the screw plug of the bypass hole of the exhaust valve, install the exhaust pipe, open the bypass hole, so as to discharge air, close the valves (such as filling valve, air vent valve, etc.) that are connected with the atmosphere in the system, and open all other valves in the system. If the condenser of the system is a water-cooled condenser, the cooling water in the condenser shall be drained Otherwise, the water in the system is not easy to evaporate due to the low temperature of cooling water and it is difficult to be drained out. Force the contact points of the oil pressure controller and the low pressure controller to be normally open, start the compressor, and slowly open the suction valve after the oil pressure is normal, and put the energy regulating device in the minimum gear.

Due to the small diameter of the discharge valve of the refrigeration compressor, the suction valve can not be opened very large at the beginning, and the energy regulating device can not set the high gear. As the pressure in the system decreases, the suction valve can be opened gradually and loaded gradually to increase the suction volume. In the process of air extraction, the minimum oil pressure of the refrigeration compressor shall not be less than 50kPa, and the intermittent evacuation method shall be adopted for vacuum pumping. When the compressor continuously exhausts until no sound of air flow can be heard, the exhaust pipe shall be immersed into the refrigerator oil cup to observe the bubbling at the pipe orifice.
If there is no bubble in 5min, it can be considered that the gas in the system has been basically exhausted. If the exhaust pipe orifice has bubbles for a long time, it indicates that the compressor itself or the system has leakage, which should be checked and eliminated.
When checking, first close the suction valve of the compressor and check whether the compressor itself leaks. If the compressor does not leak, there will be no bubbles in the oil container. At the same time, it also indicates that there is something wrong with the system. If the compressor has leakage, bubbles will be generated continuously, which is often caused by the shaft seal not sealing. If the bubble is larger at the beginning and then becomes smaller, the interval between bubbles is longer and longer, which indicates that the shaft seal is not closed to gradually.

If it is found that the refrigeration oil is repeatedly sucked in and out of the pipe end (when the insertion surface is not deep), the phenomenon can not be seen when the pipe end is inserted into the oil depth. This situation is generally caused by the valve plate not tightly closed, which will be improved after heavy load use. After the spoon is vacuumized, the emptying hole should be closed first, and then the machine should be shut down to prevent the valve plate from being closed after shutdown The air is backflow.

Precautions for vacuum pumping:
(1) After vacuumizing, the compressor should be disassembled and cleaned and replaced with new lubricating oil.
(2) When the vacuum degree reaches 8.659kpa, the oil pressure of the compressor is very low, and it can not continue to vacuum.
(3) In the process of vacuumizing with its own compressor, if the compressor is equipped with a lubricating oil pump, the working conditions of the lubricating oil pump will deteriorate with the increase of the vacuum degree in the system, resulting in the damage of the moving parts of the machine. Therefore, when the oil pressure (pressure difference) is less than 26.7kpa, the machine should be shut down immediately.

Leakage detection and blowdown operation method:
First, connect the pressure reducing valve to the nitrogen cylinder, connect the pressure reducing valve of the nitrogen cylinder with the ammonia feeding valve of the system with the nitrogen pipe, open all the valves except the valves connecting the atmosphere, open the ammonia feeding valve and nitrogen valve, fill the system with nitrogen to low pressure and some experimental pressure, close the ammonia feeding valve and nitrogen valve, and dip the brush with soapy water or detergent to all the welding joints, pipe joints, etc Flange, valve and other joint and sealing parts shall be altered for leakage detection.
(1) Remember to find out the leakage points and mark them. After finding out a number of leakage points, the operator should be located in the safe position of the blowdown operation, and screw off the screw plug at the lower part of the low-pressure circulating liquid reservoir (or gas-liquid separator), blow down and discharge nitrogen.
(2) Then make up the leakage, screw on the plug of the low-pressure circulating liquid reservoir (or gas-liquid separator) and fill it with nitrogen from the beginning. The operator is located in the safe position of blowdown operation. Screw off the plug at the lower part of the central cooler, blow down and discharge nitrogen, and repeat the above two steps until there is no leakage point.
(3) Close the inlet valve and drain valve of the low-pressure circulating liquid reservoir (or gas-liquid separator), close the liquid outlet valve and oil drain valve of the drain drum, the suction valve and oil drain valve of the compressor, and fill the system with nitrogen to some high-pressure experimental pressure.
(4) Some high-pressure leak detection, find out the leakage point, mark, find out a number of leakage points, the operator is located in the safe position of sewage operation, screw off the screw plug at the bottom of the drain drum, blow down and discharge nitrogen.
(5) To make up the leakage, screw on the plug of the drain tank, and fill in nitrogen from the beginning. The operator is located in the safe position of the blowdown operation. Screw off the screw plug at the lower part of the high-pressure liquid reservoir, blow down and discharge the nitrogen. Repeat the above two steps until there is no leakage point.

Precautions for leak detection and blowdown process:
(1) Blowdown is usually not less than 3 times until the gas is discharged without water vapor, oil and rust.
(2) In practice, the drain outlet can also be blocked with a wooden plug. When the system has a certain pressure, the plug is pulled out to make the air quickly discharged. This method is very good. However, there are certain risks, so we must be careful and pay attention to safety during operation.
(3) Before the operation of the refrigeration system, the equipment and pipelines must be drained to remove the welding slag, iron filings, sand and other dirt left in the system during the process of the device. Prevent dirt from damaging the parts of the refrigerator and valves in the system, and prevent the system pipeline from blocking.
(4) When ammonia refrigeration system blowdown, compressed air can be supplied by air compressor or ammonia refrigerator, and the pressure of compressed air is usually less than 0.6MPa. The sewage outlet should be set at the lowest part of the pipeline. The sewage discharge operation can be divided into groups and carried out by sections and layers.
(5) In order to effectively use the explosive force and high-speed air flow of compressed gas, a valve can be installed on the drain outlet. When the pressure in the system increases, the valve can be opened quickly, so that the gas can be quickly discharged and the dirt can be brought out.
(6) In the process of blowdown, if it is found that the pipeline flange valve has significant leakage, it should be made up in time. After the system blowdown is completed, the valve cores and filters of all valves in the system shall be disassembled and cleaned.

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