Why does the valve electric device appear overload phenomenon? What are the protection methods?
Petrochemical, power station, metallurgy and other industries require continuous, smooth, long cycle operation. Therefore, require the use of the valve should have high reliability, safety factor is big, can’t because the valve failure caused serious production safety and personal casualty accident, meet the requirement of long period run of the equipment, continuous production is benefit for a long period of time, in addition, to reduce or avoid due to valve leakage, to create a clean and civilized factories, implementation of the HsE (health, safety, environment) management.
First, the valve selection principle
(1) Safety and reliability
Petrochemical, power station, metallurgy and other industries require continuous, smooth, long cycle operation. Therefore, require the use of the valve should have high reliability, safety factor is big, can’t because the valve failure caused serious production safety and personal casualty accident, meet the requirement of long period run of the equipment, continuous production is benefit for a long period of time, in addition, to reduce or avoid due to valve leakage, to create a clean and civilized factories, implementation of the HsE (health, safety, environment) management.
(2) Meet the process production requirements
Valve should meet the use of medium, working pressure, working temperature and use needs, which is the most basic requirements of valve selection. Overpressure protection for valve, discharge excess medium, should choose safety valve, relief valve, the need to prevent the reverse flow of medium in the process of operation, should use the check valve, automatic eliminate steam pipes and equipment in the need to continue to produce the condensed water, air and other non-condensable gases, to prevent the steam to escape again at the same time, should choose trap. In addition, when the medium is corrosive, the material with good corrosion resistance should be selected.
(3) Convenient operation, installation, inspection (maintenance) repair
After the valve is installed, the operator should be able to correctly identify the direction of the valve, the opening mark, indicating signal, easy to deal with various emergency faults in a timely and decisive manner. At the same time, the selected valve type structure should be single as far as possible, easy installation, inspection (maintenance) repair.
(4) Economy
Under the premise of meeting the normal use of process pipelines, valves with relatively low manufacturing cost and simple structure should be selected as far as possible to reduce the cost of equipment, avoid the waste of valve raw materials and reduce the cost of valve installation and maintenance in the later period.
Two, valve selection steps
1. Determine the working condition of the valve according to its use in the device or process pipeline. For example, working medium, working pressure and working temperature.
2. According to the working medium, working environment and user requirements to determine the sealing performance grade of the valve.
3. Determine the type and driving mode of the valve according to its use. Type such as truncation valve, regulating valve, safety valve, other special valve, etc. Drive such as worm gear, electric, pneumatic and so on.
4. Choose according to the nominal parameters of the valve. The nominal pressure and dimensions of the valve shall be determined in accordance with the installed process piping. The valve is installed in the process pipeline, so its use condition should be consistent with the design choice of the process pipeline, the standard system used by the pipeline and the nominal pressure of the pipeline is determined, the valve nominal pressure, nominal size, valve design and manufacturing standards can be determined. Some valves are based on the rated time flow through the valve or displacement of the nominal size of the valve.
5. Determine the connection form of valve end face and pipeline according to the actual operating conditions and the nominal size of the valve. Such as flange, welding, clamping or thread, etc.
6. According to the installation position of the valve, installation space, nominal size to determine the structural form of the valve type. Such as dark rod gate valve, Angle globe valve, fixed ball valve, etc.
7. According to the characteristics of the medium, working pressure and working temperature, to correctly and reasonably choose the material of the valve shell and inner parts.
Why does the valve electric device appear overload? What are the protection methods?
Valve electric device is to realize the valve control, automatic control and remote control indispensable equipment, its movement process can be controlled by stroke, torque or axial thrust size. Because the operating characteristics and utilization rate of the valve electric device depend on the type of valve, the device working specification and the position of the valve in the pipeline or equipment, therefore, the correct selection of the valve electric device is very important to prevent overload phenomenon (the working torque is higher than the control torque). However, it can be overloaded if:
1, the power supply is low, can not get the required torque, so that the motor stops rotating.
2, incorrectly set the torque limit mechanism, so that it is greater than the stop torque, resulting in continuous generation of too large torque, so that the motor stops rotating.
3, such as the point of intermittent use, the heat generated up, more than the allowable temperature of the motor.
4, for some reason the torque limiting mechanism circuit failure, so that the torque is too large.
5, the use of ambient temperature is too high, relatively make the motor heat capacity decline.
The above are some reasons for overload, for these reasons the phenomenon of motor overheating should be considered in advance, and take measures to prevent overheating. In the past, the way to protect the motor is to use fuses, overcurrent relays, thermal relays, thermostatic devices, etc., but these methods also have their own advantages and disadvantages. For electric equipment with variable load, there is no reliable protection method.
Therefore, a combination of methods must be adopted. However, due to the different load of each electric device, it is difficult to put forward a unified approach. But for the most part, common ground can be found.
The overload protection methods adopted can be summarized into two types:
1. Judge the increase or decrease of motor input current;
2. Judge the heat of the motor itself. The above two ways, no matter which should consider the motor heat capacity given time margin. It is difficult to make it consistent with the heat capacity characteristics of the motor in a single way.
Therefore, we should choose a combination of methods based on reliable action according to the cause of overload to achieve optimal overload protection. The motor of Rotock electric device, because it is embedded in the windings of the thermostat with the same insulation level of the motor, when the rated temperature is reached, the motor control loop will be cut off. The heat capacity of the thermostat itself is small, and its time-limited characteristics are determined by the heat capacity characteristics of the motor, so this is a reliable method.
The basic protection methods for overload are:
1, the motor continuous operation or point operation of overload protection using thermostat;
2. Thermal relay is used for the protection of motor blocking
3. Fuse or overcurrent relay is used for short circuit accident. The correct choice of valve electric device and the prevention of overload are related and should be paid attention to.
Post time: Jun-23-2022
