JPH06257586A - Method for controlling oil level in gas-liquid separation tank - Google Patents

Abstract

PURPOSE:To reduce the frequency of oil level control so as to enhance reliability by using a step function as a control function for establishing the opening of a delivery valve in a cargo pump relative to the level of oil in a gas-liquid separation tank, the step function varying depending on whether the level of the oil is raised or lowered and having a dead zone in between. CONSTITUTION:Oil in a cargo tank installed in a tanker or the like is sucked by a cargo pump 1 via a gas-liquid separation tank 5, and is fed to a land tank through a main check valve 4 and a delivery valve 3 after its pressure is built up. Should gas be collected in the upper portion of the gas-liquid separation tank 5 and the level of the oil be lowered, a control unit 10 operates and controls an air extracting valve 6 and a vacuum pump unit 7 according to signals from a level transmitter 8 so that gas is discharged outside. In this case, a signal of the level transmitter 8 is inputted also to a function controller 11. A hydraulic solenoid valve 12 for drivingly opening and closing the delivery valve 3 is opened or closed following a step function which varies according to whether the level of the oil is raised or lowered and which has a dead zone between the increase and decrease in oil level.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid level control method for a gas-liquid separation tank of a cargo pump stripping device installed in, for example, a tanker or a product carrier. It can also be applied to the separation and liquid level control of storage tanks used.

[0002]

2. Description of the Related Art For example, a stripping device for a cargo pump in an oil tanker prevents the discharge function of the cargo pump from being lost due to air or gas mixed in the pumping liquid when overcoming cargo oil near the bottom of a ship. It is installed to separate those gases from the pumped liquid in a gas-liquid separation tank provided at the pump suction port.

FIG. 2 is a block diagram showing an example of such a conventional stripping device and its control device. In the figure, (1) is a cargo pump, (2) is a drive turbine,
(3) is a discharge valve, (4) is a main check valve, (5) is a gas-liquid separation tank, (6) is an air extraction valve, (7) is a vacuum pump unit, (7a) is a vacuum pump, (8) ) Is a level transmitter,
(9) shows a positioner, and (10) shows a vacuum pump / air extraction valve control device, respectively.

Cargo oil in a cargo tank (not shown) is sucked in by the cargo pump (1) via the gas-liquid separation tank (5), and the pressure is increased to the main check valve (4) and the discharge valve (3).
And is sent to a land tank (not shown). When the liquid level in the cargo tank becomes close to the bottom of the ship and air or cargo oil gas becomes mixed in the pumping liquid, air or the like is separated from the pumping liquid in the gas-liquid separation tank (5) and the air is sent to the cargo pump (1). Will not be sucked in and its function will not deteriorate.

When the separated gas is accumulated in the upper part of the gas-liquid separation tank (5) and the liquid level is remarkably lowered, the vacuum pump / air extraction valve control device (10) which receives the signal from the level transmitter (8). ) Sends an open command and a start command to the air extraction valve (6) and the vacuum pump unit (7), respectively, and discharges gas from the gas-liquid separation tank (5) to the outside. When the liquid level is sufficiently recovered in this way, the vacuum pump / air extraction valve control device (10) is connected to the vacuum pump unit (7).
To the air extraction valve (6).

Thus, the vacuum pump unit (7) is
It is used to generate the negative pressure necessary for extracting the cargo oil gas and air accumulated in the gas-liquid separation tank (5) to the outside. The air bleeding valve (6) quickly shuts off between the vacuum pump (7a) and the gas-liquid separation tank (5) when the liquid level in the gas-liquid separation tank (5) is restored, and the bleeding pipe or vacuum It is installed to prevent the liquid in the gas-liquid separation tank (5) from entering the pump (7a). Vacuum pump unit (7)
And the air extraction valve (6) are simultaneously controlled, and when the liquid level falls below the reference level of the gas-liquid separation tank (5), start and open commands are issued, and when the liquid level exceeds the reference level, stop and close commands are issued.

When the amount of gas sucked from the cargo tank increases as the liquid level of the cargo tank decreases and cannot be exhausted by the vacuum pump unit (7), the liquid level of the gas-liquid separation tank (5) significantly decreases. In that case, the opening degree of the discharge valve (3) of the cargo pump (1) is adjusted based on the signal of the level transmitter (8).

Further details will be described. If the amount of the liquid phase portion of the pumping oil flowing into the gas-liquid separation tank (5) fluctuates due to the inclusion of air or the like, and there is an excess or deficiency with respect to the amount flowing out from the gas-liquid separation tank (5) toward the pump, The liquid level in the liquid separation tank (5) changes. Therefore, by controlling the opening of the discharge valve (3) provided at the discharge port of the cargo pump (1) and changing the discharge amount of the cargo pump (1), the liquid level in the gas-liquid separation tank (5) can be controlled. The fluctuation range should be within a safe range where air etc. does not flow out to the cargo pump (1) side. Specifically, the liquid level signal of the level transmitter (8) provided in the gas-liquid separation tank (5) is directly connected to the discharge valve (3) which is equipped with a positioner and whose operation is controlled by air pressure, and the liquid level is changed. When rising, the discharge valve (3) was opened to increase the amount of liquid flowing out of the cargo pump (1), and conversely, when the liquid level decreased, the discharging valve (3) was closed to decrease the amount of discharge. That is, as shown in FIG. 3A, the discharge valve (3) is uniquely controlled by the liquid level signal of the level transmitter (8).

[0009]

In the conventional discharge valve control based on the liquid level of the gas-liquid separation tank, the liquid level itself and the opening of the discharge valve are unambiguously irrelevant to the conditions of rising and falling of the liquid level. Since it is related to each other, it becomes high frequency due to local and short cycle fluctuation of the tank level, and due to instability or delay of control, air flows into the pump suction port and the discharge function of the pump is lost, This caused the tank liquid level to rise excessively, causing the high-viscosity pumping liquid to enter the bleeder pipe and the vacuum pump.

Further, just before the end of the stripping work,
The discharge valve is often operated for a long time with a small opening. Then, cavitation occurs in the valve seat of the discharge valve and the downstream piping, and excessive vibration, noise, and damage due to erosion occur. Therefore, it is not possible to adopt a completely sealed discharge valve using rubber or resin when fully closed, and it is necessary to additionally install a completely sealed discharge valve in addition to the control discharge valve.

In addition, since the control discharge valve is controlled so that the opening is uniquely determined by the liquid level signal and fast response is required, an air operated valve with a positioner is usually used. It This is different from a general hydraulically actuated valve normally used onboard a ship in that the driving force source is different, so that a separate pressure air source must be provided and the cost is high.

[0012]

In order to solve the above-mentioned conventional problems, the present invention relates to a gas-liquid separation tank which is installed on the suction side of a cargo pump and is used during stripping. With respect to the above, the control function for giving the opening degree of the discharge valve of the pump is a different step function when the liquid level rises and when the liquid level falls, and a dead zone is provided between them. The liquid level control method is proposed.

[0013]

In the present invention, a different control function is used when the liquid level in the gas-liquid separation tank rises and when the liquid level falls, and a dead zone is provided between them, so that local / short-cycle liquid level fluctuations are caused. If there is, it is possible to accurately detect the upper and lower peaks of the liquid level fluctuation, which is the criterion for determining the necessity of control, and to accurately determine the necessity of controlling the discharge valve opening / closing direction. Therefore, it is possible to reduce the control frequency and reliably avoid instability and conflict of control.

Further, since the control function is a step function that increases / decreases stepwise, a hydraulically actuated general valve normally used in a ship can be used as a pump discharge valve. Therefore, it is not necessary to provide an air pressure source as in the case of using a conventional pneumatically operated valve with a positioner, the price of the valve itself is low, maintenance is easy, and reliability is improved.

[0015]

1 is a block diagram showing an embodiment of the present invention, FIG.
(B) is a figure which similarly shows the control characteristic of a discharge valve. In FIG. 1, the same parts as those of the conventional one described with reference to FIG. 2 are given the same reference numerals to avoid redundancy, and detailed description thereof is omitted.

In this embodiment, the liquid level signal of the level transmitter (8) provided in the gas-liquid separation tank (5) is input to the function controller (11). In the function controller (11), as illustrated in FIG. 3 (b), the step function of the discharge valve (3) is changed by a step function having a dead zone between the rising and falling of the liquid level of the gas-liquid separation tank. When the hydraulic solenoid valve (12) that controls the opening and closing is controlled to open and close, and the upper limit value for each discharge valve opening specified in a stepwise manner is exceeded or falls below the lower limit value, the discharge valve opening degree is gradually increased and decreased. Then, the change in the liquid level of the gas-liquid separation tank is controlled in a direction to settle it so that it enters the dead zone.

That is, if the actual liquid level deviates from the allowable liquid level when rising or falling corresponding to the opening of the discharge valve at that time, the discharge valve (3) is opened or closed stepwise. The open / close control of the hydraulic solenoid valve (12) is performed. The operating oil pressure controlled to be opened / closed by the hydraulic solenoid valve (12) enters the actuator of the discharge valve (3) and controls the opening / closing of the discharge valve. Then, by opening and closing the discharge valve, the discharge flow rate of the cargo pump (1), that is, the suction flow rate increases and decreases,
When the liquid level in the gas-liquid separation tank (5) enters the dead zone of the control liquid level in the ascending and descending directions determined by the opening degree of the discharge valve at that time, the control ends and the liquid level exits the dead zone. Unless there is a local change in the liquid level, no control is performed and the system enters the waiting state unless the liquid level changes.

As described above, in the present embodiment, the dead zone is provided between the rising and falling control liquid levels, so that even if the liquid level fluctuates locally or for a short period, it becomes a criterion for determining the necessity of control. It is possible to accurately detect the upper and lower peaks of the liquid level fluctuation and to determine the necessity of controlling the discharge valve opening / closing direction, reduce the control frequency, and reliably avoid the control instability and delay. By introducing a dead zone into the control, it is also possible to set the discharge valve control function stepwise with respect to the liquid level in the gas-liquid separation tank, which is normally used onboard in place of the conventional pneumatic valve with positioner. Since hydraulically operated general valves can be used, equipment costs are reduced, maintenance is easy, and reliability is improved.

In the present embodiment, the valve opening immediately before the occurrence of harmful cavitation that causes erosion of the valve seat is set as the minimum valve opening point, and the level of the gas-liquid separation tank is further lowered. In this case, the discharge valve control function is set so that the discharge valve is fully closed at once without stopping halfway. If a valve is used for a long time with a small valve opening between the minimum valve opening point and the fully closed point, the valve seat and the pipe downstream of the valve may be damaged by high flow velocity and cavitation. Is set as described above, the time during which the discharge valve is used at such a minute valve opening can be minimized.
Therefore, even if a completely sealed discharge valve is used, damage to the valve seat can be avoided, so that it is not necessary to install a dedicated control valve, the cost is reduced, and the operation is simplified.

In the discharge valve control function, the maximum valve opening point is set corresponding to the maximum value of the control liquid level as in the above.
When the liquid level of the gas-liquid separation tank (5) exceeds the maximum value, the discharge valve (3) is opened all at once without stopping halfway from the maximum valve opening point to the fully opened state. This prevents the gas-liquid separation tank (5) from overflowing.

Further, in the present embodiment, the pumping liquid specific gravity data is input to the function controller (4), and the control function of the discharge valve is corrected by the specific gravity ratio of the reference pumping liquid and the actual pumping liquid of the differential pressure type level transmitter.
As a result, the fluctuation of the control level due to the difference in the specific gravity of the actual pumping liquid is eliminated, and the optimum control condition is always maintained regardless of the type of pumping liquid.

[0022]

According to the present invention, it is possible to reduce the control frequency of the liquid level control of the gas-liquid separation tank used when stripping is performed by the cargo pump. Therefore, the life of the discharge valve can be extended and the reliability is improved. Further, since the delay of the liquid level control of the gas-liquid separation tank can be reduced, it is possible to prevent the loss of the function of the cargo pump due to the suction of air and the damage due to the intrusion of the pumped liquid into the vacuum pump unit, and the reliability in this respect is also improved.

Further, as the discharge valve, a hydraulically actuated general valve can be used instead of the conventional pneumatically actuated valve with a positioner, so that the facility cost is reduced and maintenance is facilitated.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention.

FIG. 2 is a block diagram showing an example of a conventional stripping device and its control device.

3A and 3B are diagrams showing control characteristics of a discharge valve, FIG. 3A shows an example of conventional characteristics, and FIG. 3B shows an embodiment of the present invention.

[Explanation of symbols]

 (1) Cargo pump (2) Drive turbine (3) Discharge valve (4) Main check valve (5) Gas-liquid separation tank (6) Air extraction valve (7) Vacuum pump unit (7a) Vacuum pump (8) Level transmitter (9) Positioner (10) Vacuum pump / air extraction valve controller (11) Function controller (12) Hydraulic solenoid valve

Claims (1)

[Claims] 1. In a gas-liquid separation tank which is installed on the suction side of a cargo pump and is used during stripping, a control function that gives the opening of the discharge valve of the pump to the liquid level in the tank is A liquid level control method for a gas-liquid separation tank, characterized in that a different step function is used when the position is raised and when the position is lowered, and a dead zone is provided between them.