A screw compressor is composed of many parts, but compared with the piston compressor in the past, it has many fewer parts. Generally speaking, the main
components of a screw compressor include the compressor head, air inlet valve, oil and gas separator, oil filter and air filter. One of the components is often
over looked by people, it is the minimum pressure valve, also known as the pressure maintenance valve.
Working principle and structure of minimum pressure valve
The working principle of the minimum pressure valve of the air compressor: When the pressure of the air compressor system is lower than the set minimum
working pressure, the minimum pressure valve opens, allowing air to flow through the valve into the system and increase the pressure of the system; when
the pressure of the system reaches or exceeds At the set minimum operating pressure, the minimum pressure valve will close, stopping air from flowing into
the system. In this way, the system pressure is kept within the set range.
Minimum pressure valve construction usually includes the following components:
Valve body:The shape and size of the valve body depends on the design and purpose of the minimum pressure valve. Usually made of pressure-resistant
materials such as cast steel or stainless steel.
Valve seat: The valve seat is the part of the valve body that is usually connected to the inlet. It usually has a circular or tapered opening that controls gas flow.
Spring: The spring is used to provide pressure, which is usually compressed and applied to the valve seat to keep the minimum pressure valve closed.
Diaphragm: The diaphragm is the component used to isolate the valve seat and outlet. It is usually made of elastic material and can sense changes in outlet
pressure.
Adjustment nut: The adjusting nut can be used to adjust the preload of the spring, thereby adjusting the opening pressure of the minimum pressure valve.
Exit: The outlet is the port connected to the system piping through which gas can flow out.
Seals: Seals are used to ensure the sealing performance of the valve and prevent gas leakage.
The above is the basic structure of a minimum pressure valve. The specific design and structure may vary depending on the application and system requirements.
The minimum pressure valve can basically be found in compressors of more than 20 cubic meters. Its size is so small that it can be held with a palm. However,
such a small and inconspicuous component can also cause various failures, affecting the normal operation of the entire compressor unit.
Main functions of minimum pressure valve
1. Pressure requirements to open the pressure valve
The opening pressure of the minimum pressure valve is 4bar (based on the compressor setting parameters of the air compressor station where the author works),
which means that the minimum pressure valve only opens at this pressure value. The purpose of opening is to prevent the air from flowing too fast. A flow rate that
is too fast or too slow will hinder the normal operation of the compressor. Reducing the air flow rate can avoid unnecessary failures.
An important function of the minimum pressure valve is to reduce the pressure of the cylinder so that the energy consumption of the cylinder will not increase too
quickly due to the increase in pressure. Excessive pressure will also increase the resistance, leading to electrical failure and misjudgment, increasing the current
and causing shutdown. This phenomenon occurs because the minimum pressure valve valve is not opened, causing suffocation.
2. Regular overload protection
The minimum pressure valve is equivalent to a safety valve inside the compressor, which can unload a certain pressure. When the pressure rises to the set value,
it will act as a safety valve to unload the pressure.
3. Prevent compressed air from flowing back
When the gas is transported out, if the compressed air flows back, the minimum pressure valve will act as a one-way valve to prevent the air from flowing back.
When the compressor is unloaded, if the minimum pressure valve does not work, air backflow will occur. Therefore, gas backflow is found on the monitor, and
the problem often occurs on the minimum pressure valve.
Minimum pressure valve failure phenomenon
Several malfunctions that can occur with minimum pressure valves include:
1. Jet
Jet failure is mainly caused by the pressure in the system being too high. For example, there is a blockage in the pipeline, which hinders the smooth flow of air;
or the pressure gauge fails, resulting in inaccurate readings of the pressure gauge. As the air pressure continues to increase, the compressor will continue to
increase the compression ratio, resulting in high internal pressure in the air compressor and the occurrence of jets.
2. Insufficient lubricating oil pressure
Once the minimum pressure valve fails, the lubricating oil pressure is not enough and the circulation pressure necessary for lubrication cannot be quickly established,
resulting in wear of some friction parts, causing the air compressor to make abnormal noise due to poor lubrication or causing the rotor clearance to be too large,
resulting in a decrease in gas production. And other issues.
3. Minimum pressure valve failure during unloading
If the minimum pressure valve is not closed tightly during unloading, the pressure of the oil and gas separator will increase, resulting in the complete separation of oil
and gas, affecting the quality of the compressed air.
4. Overload shutdown
When the minimum pressure valve cannot be opened or is difficult to open, the pressure inside the oil barrel will be too high and the motor load will increase, causing
the equipment to be overloaded and shut down. In addition, if the minimum pressure valve is blocked, the exhaust pressure will increase, causing the operating current
to be overloaded and causing the screw air compressor to shut down. Moreover, when shutting down, the compressed air in the pipeline will flow back to the oil separator
barrel, causing the unit to spray oil backwards.
There are many air compressor shutdown faults, but shutdowns caused by the minimum pressure valve rarely occur. Precisely because the frequency is low, it is difficult
to judge when it occurs.
Failure cause analysis
The minimum pressure valve plays an important role in the screw air compressor, but sometimes it is not closed tightly, damaged or fails due to various reasons. The
following are common reasons:
Poor air quality or foreign impurities entering the unit: During the operation of the screw air compressor, if the air quality is poor or external impurities enter the unit,
these impurities may be driven by the high-pressure air flow and impact the components of the minimum pressure valve. Cause it to be damaged or contain dirt, which
will lead to its failure.
Too much oil in the air compressor: If the air compressor is filled with too much oil, too much lubricating oil will increase the oil viscosity, making it difficult for the valve
plate to close or delay opening, causing problems with the minimum pressure valve.
Large fluctuations in working conditions: the minimum pressure valve is set according to specific working conditions. If the operating conditions fluctuate greatly, the
minimum pressure valve may fail quickly.
Long shutdown and then restart: When the screw air compressor is shut down for a long time and then restarted, the moisture contained in the lubricating oil and air
will enter the inside of the equipment and accumulate and corrode various parts of the minimum pressure valve, causing the oil to become viscous. , causing the
minimum pressure valve to fail.
Responses
1. Preventive measures
(1) Clean the air filter regularly to keep the inside of the unit clean;
(2) Control the oil injection amount of the air compressor and maintain appropriate lubricating oil levels;
(3) Stabilize working conditions and avoid large fluctuations;
(4) Try to avoid restarting after a long shutdown. If shutdown is necessary, measures should be taken to prevent moisture and lubricating oil from entering the equipment.
2. Maintenance measures
If a screw air compressor failure has occurred and it is highly suspected that it is a problem with the minimum pressure valve, the following steps can be taken to solve it:
First, turn off the air compressor, disconnect the power supply, and ensure that the equipment is in a safe shutdown state.
Second, find the minimum pressure valve. It is usually located on top of the oil and gas separator and should be checked for faults. If the valve core or spring of the minimum
pressure valve fails, new parts need to be replaced. If there is no problem with the minimum pressure valve, the equipment can be inspected, including cleaning and cleaning,
to ensure that the valve opens and closes freely, promptly and in place.
Finally, restart the machine and observe whether the equipment is running normally. If the fault still exists, it is recommended to replace the valve with a new one unless there
are special circumstances.
It should be noted that before carrying out repairs, be sure to ensure that the equipment is in a safe state, disconnect the power supply and release the internal pressure to avoid
unexpected situations. At the same time, it is recommended that people who are not familiar with maintenance operations do not perform maintenance on their own to avoid
causing greater losses or safety issues.