A method for determining intervals for feeding powder to a conveying pipe by means of compressed air in a plug conveying system that uses an apparatus for feeding powder to a conveying pipe, the apparatus including:
- a hopper for storing the powder to be conveyed, wherein the hopper has an opening for discharging the powder at its lower portion,
- a tank body used as a structure of a pressure vessel having a receiving port at its upper portion and a discharging port at its lower portion, wherein the receiving port faces the opening for discharging the powder of the hopper,
- a valve box for the second check valve disposed at the lower portion of the tank body, wherein one end of the valve box is connected and sealed to the outer and lower surface of the tank body so that the valve covers the discharging port of the tank body, and wherein the other end of the valve box is connected to and communicates with a root of the conveying pipe,
- a first check valve disposed inside the tank body and just under the receiving port so that it can freely move up and down, and so that it can close the receiving port by its upward movement that is caused by means of compressed air,
- a second check valve disposed inside the valve box and just under the discharging port so that it can freely move up and down, and so that it can close the discharging port by its upward movement that is caused by means of compressed air,
- a first electromagnetic valve for closing and opening a first conduit pipe connecting the lower portion of the first check valve to a source of the compressed air,
- a second electromagnetic valve for closing and opening a second conduit pipe connecting the lower portion of the second check valve to the source of the compressed air,
- a third electromagnetic valve that can exhaust the compressed air in the first conduit pipe,
- a detecting means for sensing an existence or nonexistence of the powder in the tank body, and
- a controlling means for controlling the opening and closing of the first, the second, and the third electromagnetic valve based on the results of the detecting means, by interlocking a plurality of timers,
- the method comprising:
- measuring the time that is required for feeding the powder in the tank body to the conveying pipe,
- adjusting the times to be set by the plurality of timers to control the electromagnetic valves based on the measurements so that the pressure loss of the compressed air in the conveying pipe corresponds to the pressure of the compressed air supplied to the conveying pipe,
- exhausting the compressed air in the tank body by opening the third electromagnetic valve after closing the first electromagnetic valve when the detecting means senses the nonexistence of the powder in the tank body,
- conveying the powder from the hopper into the tank body by opening the receiving port by descending the first check valve,
- supplying the compressed air to the second check valve and the valve box for the second check valve by opening the second electromagnetic valve,
- closing the discharging port of the tank body by ascending the second check valve,
- opening the first electromagnetic valve after closing the third electromagnetic valve when a predetermined time has passed from the time when the detecting means senses the existence of the powder in the tank body,
- closing the receiving port and pressurizing an inside of the tank body by ascending the first check valve by supplying the compressed air to the first check valve and the tank body,
- feeding the powder in the tank body to the conveying pipe by opening the discharge port of the tank body by descending the second check valve,
- calculating the time that is required for feeding the powder from the tank body to the conveying pipe based on two data points regarding time measured by the controlling means, after the detecting means senses the nonexistence of the powder in the tank body, wherein one data point regarding time is the time when the first electromagnetic valve is opened, and the other data point regarding time is the time when the detecting means senses the nonexistence of the powder,
- calculating the time when the powder in the tank body should be supplied to the conveying pipe at the next conveyance, by multiplying the time that is required for feeding the powder from the tank body to the conveying pipe by a constant value that is determined based on the ratio of an inner volume of the conveying pipe to an inner volume of the tank body,
- determining the time to be set by the timer to open the first electromagnetic valve and to close the third electromagnetic valve after the detecting means senses the existence of the powder in the tank body, based on the time when the powder in the tank body should be supplied to the conveying pipe at a next conveyance, and
- adjusting the times to be set by the plurality of timers so that the pressure loss of the compressed air in the conveying pipe corresponds to the pressure of the compressed air supplied to the conveying pipe.