Positive pressure explosion-proof refers to a method that maintains the pressure of the protective gas inside the casing higher than the external atmospheric pressure to prevent explosive gases from entering the casing. Motors using this method for explosion protection are called positive pressure explosion-proof motors. With the advancement of science and technology in China, positive pressure explosion-proof motors are gradually being produced by major motor manufacturers domestically. As a type of explosion-proof design, positive pressure explosion-proof motors have significant advantages in the structural applications of large explosion-proof motors and are a commonly used explosion-proof structure in the production of large explosion-proof motors.
The specific explosion-proof measures for positive pressure explosion-proof motors are as follows: clean air or inert gas is supplied to the motor and a certain pressure is maintained to prevent external explosive mixtures from entering the motor and coming into contact with sparks and heat-generating components inside the motor, thereby achieving the purpose of explosion protection. Based on the gas supply method, they can be divided into two types: explosion-proof ventilated type and explosion-proof gas-filled type.
To ensure explosion-proof performance, positive pressure explosion-proof motors must undergo a series of specialized tests. Ms. Can will analyze these specialized test items one by one and share positive pressure explosion-proof technology and experience with you.
●Shell strength test
The structural strength and sealing performance of the motor and its auxiliary pipelines are checked using air or inert gas. The casing strength must be able to withstand 1.5 to 2 times the maximum overpressure value specified during normal operation, but the minimum is 200 Pa. The test time is 1 minute. The casing, pipelines, and sealing rings are considered合格 (qualified) if there is no damage.
● Ventilation dead zone inspection test
The motor casing is filled with hydrogen, nitrogen, or carbon dioxide gas at a concentration of 60% ± 1%. Ventilation is performed with an airflow rate five times the total volume of the motor casing and associated piping, or according to the airflow rate and time specified in the product instruction manual. Then, a gas analyzer is used to measure the gas concentration at 3-5 points in the expected ventilation dead zones. A gas concentration ≤ 1% is considered合格 (qualified).
●Minimum wind pressure test when the motor is stationary and running
Connect the fan and ventilation duct according to the design requirements, and connect the valves for adjusting air pressure and air volume at the air inlet. Determine the air pressure measurement point based on the motor structure, airflow path, and changes in air duct dimensions.
Pressure testing points should be set in areas of low wind pressure and in gaps where explosive gases may enter the casing. They are generally located in the following positions:
(1) Air inlet and outlet;
(2) Determine the location where the pressure measuring instrument will be installed;
(3) Locations and gaps in the casing where leaks may occur, such as the shaft extension location;
(4) Places where the airflow changes drastically.
U-tube manometers, micromanometers, and Pitot tubes are generally used to measure air pressure. Adjust the inlet flow rate and pressure to meet the technical requirements, then measure the positive pressure at each measuring point to ensure it does not exceed 50 Pa. Tests should be conducted with the motor stationary and at its rated speed.
● Reliability test of the interlocking device between the motor and the fan.
Before operating a positive pressure motor, the motor and ventilation ducts must be ventilated and cleaned.
Before the ventilation system operates, the motor must not be powered on. The motor can only be powered on and put into operation after the air inside the motor and duct has been flushed with a measured amount of air (at least five times the volume of the motor and duct cavity). When the motor stops, ventilation can only cease when the surface temperature of the internal components is below the permissible overheating temperature for explosive gases.
This test verifies the accuracy and reliability of the system in ensuring positive pressure control. The motor is considered合格 (qualified) if it can operate reliably or ensure that the positive pressure explosion-proof conditions are met.
● Reliability test of wind pressure protection device
Positive pressure explosion-proof motors must maintain the air pressure specified by the product during normal operation. When the air pressure at the air outlet is artificially reduced to 100Pa or the minimum air pressure specified by the product, the air pressure protection device should activate.
This test was performed five times. If the pressure relay cannot be installed at the air outlet, the operating value of the pressure relay is often increased to check and monitor the air pressure at the outlet and ensure the air pressure at the motor outlet. This test should be performed not only when the motor leaves the factory but also during trial operation at the installation location.
● Tests to measure the temperature of the motor casing and the cooling time of internal heating elements after power failure.
The temperature of the casing and internal heating elements of a positive-pressure motor must be measured under rated load. The air pressure and volume during measurement should be at minimum flow and pressure. It is also necessary to measure the maximum temperature of the motor casing when ventilation is stopped. A stopwatch should be used to measure the time required for the surface temperature of the internal heating elements to cool to the limit temperature (i.e., the maximum permissible surface temperature of the equipment) after power is cut off. This time is the shortest time that ventilation must continue after power is cut off when the motor has finished operating.
● Spark prevention test
The air inlet and outlet of the motor ventilation duct should be located in a safe place without explosion hazards. If the exhaust outlet is located in an explosion hazard area, it should be equipped with a device to prevent sparks, electric arcs, and flammable materials from being blown out of the casing. If there is a risk of sparks being ejected from the shaft clearance and duct joints, sparks can be artificially created, such as by igniting some flammable material, and then normal ventilation can be performed to observe whether sparks can be blown out from the gaps.
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