Power saving potential and economical and reliable

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Power saving potential and economical and reliable operation of variable frequency governor

in the petrochemical production process, flow, liquid level, pressure and other process parameters often change with the change of working conditions. With the deepening of the market economy, the production is determined by the sales, the processing scheme changes frequently, and the working conditions change more. The conventional method of changing flow, liquid level and other parameters by controlling the opening of valves or baffles is actually an artificial method of increasing or decreasing resistance to achieve regulation. A large amount of energy is lost on the resistance of the valve or baffle

the variable frequency speed regulation device can efficiently and accurately adjust the speed of the AC motor, so that the flow, experimental machine usually adopts the control of hydraulic oil pressure, liquid level and other process parameters, from the throttling control of inefficient valves and baffles to the efficient speed control, so as to save a lot of energy consumption

the power saved by using speed regulation to control flow is composed of two parts: one is the reduction value of power loss of the valve under two different heads, and the other is the difference of pump power loss caused by different heads. When the flow is controlled by speed regulation, the power of the pump decreases significantly, and the power consumed by the motor itself is related to the speed regulation method. Because frequency conversion speed regulation is an efficient speed regulation method, it is different from slip speed regulation and hydraulic coupling speed regulation. It has no slip loss, and its inherent loss is very low. Therefore, the input power of the frequency converter is approximately equal to the shaft power of the motor at various speeds

for pump and fan loads, the flow is proportional to the speed, and the shaft power of the motor is proportional to the cube of the speed. Therefore, the power consumed when using variable frequency speed regulation for different flows is:

P change =n3ped=q3ped (1)

if using valve to control flow, the motor operates at a constant speed at rated speed, and the power consumed by the motor at different flows is:

valve = (0.4+0.6q) ped (2)

(1) and (2), where PED is the power consumed when the valve is fully open and the motor operates at rated speed, and Q is the relative value of flow. It can be seen from (1) and (2) that when the flow Q becomes 50% of the rated flow, the power consumption when variable frequency speed regulation is adopted is 0.125ped, and the power consumption when valve control flow is adopted is 0.7ped. The power saving rate is (0.7ped-0.125ped)/0.7ped=82.1%, and the power saving effect is very good

the energy saving of pump and fan speed regulation widely used in petrochemical enterprises is related to specific working conditions, so it is difficult to give an exact figure. But we can see that its power saving potential is great through estimation. Figure (I) shows the relationship between the flow of centrifugal pump and the shaft power of motor


figure (I) relationship curve between pump flow and motor shaft power

it can be seen from figure (I) that the electricity rate is different in different flow seasons. The smaller the flow is, the more power-saving the manufacturers can provide a complete set of solutions integrating color collection design, material development and molding technology. If the flow regulation range is (0.) Q. Within this range, the running time of various flows is equal, and the average value of power saved by speed regulation can be calculated through integration Δ P is:

omitted (3)

(3) PE is the power at the rated speed of the motor, and Q is the flow

that is, the saved power can reach 40% of the rated power, and the energy-saving potential is great. Of course, for a certain pump, the energy saving is also related to the running time, the time ratio between regulating flow and not regulating flow, and the size of the flow regulation range

in fact, the power savings of speed regulation in petrochemical enterprises are often higher than this. Due to various reasons, the on-site machine and pump are often too large. Even at the maximum processing capacity, the valve is not fully open, and there is still considerable resistance loss. Using variable frequency speed regulation can also save energy at this time

2 economic selection and operation of frequency converter

economically and reasonably selecting frequency converter includes two aspects @12. Turn off the power supply after the experiment. First, reduce the capacity of frequency converter to save investment on the premise of meeting the process requirements; The other is to increase the operation time of frequency conversion without increasing the number of frequency converters, that is, the main pump and the standby pump share a frequency converter

compared with the motor, the price of variable frequency governor is still relatively expensive, so it is necessary to reasonably reduce the capacity of variable frequency governor to save costs on the premise of ensuring safe and reliable operation

in order to ensure the normal operation of the frequency converter in any case, considering the universality, the frequency converter manufacturer requires that the selected frequency converter power should be greater than or equal to 1.1 times the rated power of the motor. This regulation is not in line with the specific situation of petrochemical enterprises, leaving too much margin. The original design margin of many pumps in the oil refining production unit is large, and the load rate of the motor is not high when controlled by the regulating valve. After the frequency conversion speed regulation is adopted, the differential pressure loss at both ends of the regulating valve can be saved, and the power consumption of the pumps will become smaller. The capacity of the frequency conversion governor can be selected independently of the rated power of the motor

the reasonable selection method should be calculated and analyzed based on the actual operation of each pump of the production device to determine the capacity of the variable frequency governor

of course, the capacity of the selected variable frequency governor cannot be much smaller than the motor, because the no-load current of the motor itself often reaches 30 ~ 50% of the rated current, and the pulsation of the output current of the variable frequency governor will increase. Operation experience shows that the capacity of the frequency converter shall not be less than 65% of the motor capacity

for the frequency converter whose calculated capacity is much smaller than the rated capacity of the motor, when the machine and pump are switched or started up, the operator manually controls the frequency, which is often adjusted to be much higher than the actual frequency required for normal operation at a small angle. The output power of the frequency converter increases significantly and overcurrent trips. In order to solve this problem, the maximum frequency limit method can be used to set the upper limit frequency of the frequency converter at a value slightly higher than the normal maximum processing capacity (such as 35 ~ 40Hz), and the acceleration and deceleration time can be appropriately extended

generally, there are two sets of pumps in oil refining production units, and the control valve and instrument control system share one set. Due to the high price of the frequency converter, in order to save investment, it is generally only installed on the main pump. The regulating valve is still used when starting the standby pump, which still wastes a lot of energy, and the regulating valve cannot be cancelled. One frequency converter with two pumps is used, because the main pump and the standby pump need to operate at the same time when switching, and the standby pump is not started from zero pressure. The capacity of the frequency converter should be more than doubled, which is larger than the investment of using two frequency converters, and the load carried by the frequency converter for a long time is still one pump, which is extremely uneconomical

as long as the power-off switching of the machine and pump can be solved, the frequency converter still carries the load of one machine and pump during the switching process, and the main machine

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