JP3279892B2 - Submersible pump device for liquefied gas tank and control method therefor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a immersion pump device for a liquefied gas tank and a control method thereof, and more particularly to a immersion pump device provided with a tank for a liquefied gas tank for storing a liquefied gas such as liquefied natural gas. For liquefied gas tanks suitable for supporting the unbalanced shaft thrust in the pump generated during the excessive discharge operation until the inside of the liquid filling pipe is filled, greatly reducing the load on the bearing and extending the life of the bearing The present invention relates to a submerged pump device and a control method thereof.

[0002]

2. Description of the Related Art A conventional immersion pump device for a liquefied gas tank using an active type thrust magnetic bearing is disclosed in Japanese Patent Application No. 5-268852 proposed by the present applicant. FIG. 2 is a sectional view of a submerged pump device for a liquefaction tank using a general active thrust magnetic bearing described in Japanese Patent Application No. 5-268852. In FIG. 2, the submerged pump device includes a liquid pumping pipe 2 suspended in a liquefied gas tank 1,
A suction valve 3 attached to the lower end of the pumping pipe 2 and a submerged pump 5 installed on a seat 21 on the valve, a wire 4 for lifting the submerged pump, and a pump lifted to the top of the pumping pipe 2 Head plate 2a having a mechanism, hoist 3
0 and others.

[0003] A submerged pump 5 shown in FIG. 2 is arranged directly from a ceiling plate 1a of a liquefied gas tank 1 (in a direction perpendicular to a horizontal plane).
Is suspended from the head plate 2a to a depth of, for example, 50 m by the suspension wire 4 from the head plate 2a, and is seated on the submerged pump seating surface 21 below the liquid supply tube 2. . The discharge pipe 10 of the pump is provided with a discharge pressure detecting means, for example, a pressure switch 41, and its output signal is supplied to a power supply section of an active thrust magnetic bearing mounted on the submerged pump 5, a control cable. It is connected to the controller 60 together with the cable from the take-out terminal block 42.

When power is supplied to the submerged pump 5 by the power supply cable 17 and the operation is started, the liquefied gas is sucked from the suction valve 3 and pressurized, and the pressurized liquefied gas is provided in the center of the pump 5. The liquid is discharged from the discharge port 12 into the liquid pumping pipe 2, rises the liquid pumping pipe 2, and is sent out to the discharge pipe 10. In such a submerged pump device, when the pump is started, power is supplied to the active thrust magnetic bearing 61 (see FIG. 1 described later) to lift the pump shaft system,
When the pump is started and the liquid pipe 2 is filled with the liquefied gas and a predetermined discharge pressure is determined, a power supply to the active thrust magnetic bearing 61 is provided by an output signal of a pressure switch 41 which is a discharge pressure detecting means. Cut off.

[0005] On the other hand, when the pressure switch 41 detects a pressure drop at the time of stop, it is supported by the active type thrust magnetic bearing 61. After a lapse of a predetermined time, the power supply of the active thrust magnetic bearing 61 is cut off. During the operation of the active thrust magnetic bearing, the gap between the rotating part (pump shaft system) and the stationary part, for example, the axial gap between the balance disk 59 (see FIG. 1 described later) and the stationary part, is formed. The size is detected by means for detecting the distance of the narrow portion, and the current to the active type thrust magnetic bearing 61 is controlled so that the distance is within a predetermined range. By these technical means, the unbalanced axial thrust force in the pump generated when the submerged pump is started and stopped does not act on the thrust bearing (see ball bearings 57 and 58 in FIG. 1 described later), so that the life of the thrust bearing is extended. Can be longer.

[0006]

In the above-mentioned prior art,
Protection of the rotating system when the magnetic attraction force becomes uncontrollable for some reason while the submerged pump 5 having the axial thrust balance device having the balance disk 59 is magnetically levitated by the active type thrust magnetic bearing 61. As a device, when a pump rotating shaft (hereinafter simply referred to as a pump shaft 50) moves vertically below a prescribed value, a downward supporting thrust bearing (a ball bearing 58 in FIG. 1 described later) attached to the pump shaft 50. ) Receives the pump shaft 50, prevents further downward descent, and prevents contact between the pump rotating part and the stationary part.

Similarly, when the magnetic attraction force becomes uncontrollable for some reason and the pump shaft 50 moves vertically above the specified value, that is, the balance disk 59
When the distance between the narrow portion and the stationary portion becomes equal to or less than the minimum allowable value, the upward supporting thrust bearing (a ball bearing 57 in FIG. 1 described later) attached to the pump shaft 50 is used.
0, the structure prevents further upward movement and prevents contact between the pump rotating part and the stationary part. In the case of a submerged pump 5 having an axial thrust equilibrium device having a balance disk 59 suspended in a liquefaction tank, in particular, a thrust bearing for supporting the upward direction has a gap between the balance disk 59 and the stationary part, and the shaft formed by the balance disk 59 It must be installed so that it will be supported for the first time when the gap is equal to or less than the gap in the thrust balanced state.

However, the gap in the axial thrust equilibrium state due to the balance disk 59 is determined by the pump discharge pressure and is constantly fluctuating. Further, in some cases, the gap is as small as about 0.1 mm. It is difficult to determine the installation position of the thrust bearing. Further, depending on the installation position of the upward supporting thrust bearing, the balance disk 59 may come into contact with the
9 has a problem that the axial thrust equilibrium state cannot be realized.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art, and an object of the present invention is to provide a submerged pump for a liquefied gas tank having an active thrust magnetic bearing. An object of the present invention is to provide a submerged pump device for a liquefied gas tank that ensures protection of the pump rotation system even when the magnetic attraction force becomes uncontrollable, and a control method therefor.

[0010]

In order to achieve the above object, a liquefied gas tank submerged pump apparatus according to the present invention has a configuration in which a liquid-gas tank is seated on a bottom seating surface of a liquid-lifting pipe suspended in the liquefied gas tank. A pump body having a motor, an impeller connected to the motor via a rotating shaft, and pressurizing the sucked liquefied gas, and a plurality of discharge holes for discharging the pressurized liquefied gas into a liquid pumping pipe; A thrust bearing and a radial bearing for supporting the rotating shaft, a shaft thrust equilibrium device having a balance disk, a detecting means for detecting a distance of a narrow portion between the balance disk and a stationary portion, and a thrust bearing mounted on the rotating shaft. Active type thrust magnetic bearing, detecting means for detecting the discharge pressure of the pump, and turning on / off the power of the active type thrust magnetic bearing according to the discharge pressure value detected by the detecting means. A submerged pump device for a liquefied gas tank provided with a control device, wherein a thrust bearing for supporting the rotary shaft is mounted at a position for preventing excessive vertical downward movement of the rotary shaft, and When the displacement value detected by the detecting means that detects the distance of the slit becomes less than or equal to the minimum allowable value, or the minimum allowable value
When that has elapsed since it is determined that more than a predetermined time, the protective device, and after forced cutoff having a control function to force off the power of the active thrust magnetic bearing so as to avoid excessive movement of the vertically upward of the rotational axis Is supported by the thrust bearing.
To carry .

In order to achieve the above object, a control method for a liquefied gas tank submersion pump device according to the present invention uses the above-described liquefied gas tank submersion pump device.
According to the displacement value detected by the detecting means for detecting the distance between the narrow portion of the balance disk and the stationary portion, the active thrust magnetic bearing of the active type thrust magnetic bearing so as to avoid excessive vertical upward movement of the rotating shaft. The power supply is forcibly shut off.

More specifically, a state in which the displacement value detected by the detecting means for detecting the distance between the balance disk and the stationary portion is smaller than the minimum allowable value of the narrow portion has continued for a predetermined time. Sometimes, the power source of the active thrust magnetic bearing is forcibly shut off. Further, when the detected displacement value indicates that the distance of the narrow portion has become zero, the power supply of the active thrust magnetic bearing is forcibly shut off. Further, when the detected displacement value is equal to or less than the minimum allowable value of the narrow portion and the rate of change of the detected value with respect to time is equal to or greater than a predetermined rate, the power supply of the thrust magnetic bearing is forcibly shut off. It is something to do.

After the power of the thrust magnetic bearing is forcibly shut off by the protection device, the rotary system including the rotary shaft is supported by the thrust bearing supporting the rotary shaft to continue the operation.

The function of the above technical means is as follows. Usually, when starting a submerged pump equipped with an active thrust magnetic bearing having an axial thrust balancing device having a balance disk, the current to the active thrust magnetic bearing is set so that the gap between the rotating part and the stationary part is within a predetermined range. Control. Thereafter, the power supply to the active thrust magnetic bearing is shut off by the output signal of the discharge pressure detecting means. In the state of magnetic levitation, when the magnetic attraction force becomes uncontrollable for some reason and the pump shaft moves vertically downward beyond the limit value, the downward supporting thrust bearing attached to the pump shaft rotates the pump. It receives the shaft, prevents further downward descent, and prevents contact between the pump rotating part and the stationary part.

Similarly, when the magnetic attraction force becomes uncontrollable for some reason and the pump shaft moves vertically upward beyond the limit value, the value detected by the displacement detecting means becomes the default value. At this point, the power of the thrust magnetic bearing is forcibly shut off, ignoring the control by the output signal of the discharge pressure detecting means, in order to avoid excessive movement of the pump shaft vertically upward.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG.
This will be described with reference to FIG. FIG. 1 is a longitudinal sectional view of a submerged pump according to an embodiment of the present invention. FIG. 3 is a flowchart of gap detection value processing of a narrow portion of a balance disk in an active thrust magnetic bearing of the apparatus of FIG. FIGS. 4 to 6 are diagrams showing the time change of the gap of the narrow portion of the balance disk in the apparatus of FIG. In addition, FIG.
FIG. 3 is an enlarged detailed view of a submerged pump portion of the submerged pump device for a liquefied gas tank shown in FIG. 2, and the same parts as those in FIG.

In FIG. 1, 50 is a pump shaft, and 51 is a pump shaft.
The pump shaft 50 includes a rotor 51a and a stator 51b.
Sub-merged motor for rotating the pump shaft 52
Inducer attached to 0 to improve suction performance, 5
3 is a multi-stage impeller constituting a pump, 54, 55,
56 is a static pressure bearing, 57 and 58 are auxiliary ball bearings serving as thrust bearings, 59 is a balance disk fixed to a rotating system to constitute an axial thrust balancer, and 61 is composed of an electromagnet 62 and a ferromagnetic rotor 63. The active thrust magnetic bearing 64 is a gap detection sensor related to a detecting means for detecting a distance between a narrow portion between the balance disk 59 and the stationary portion.

As shown in FIG. 1, the structure of the submerged pump 5 is such that an inducer 52, a plurality of stages of impellers 53, and a rotor 51a of a sub-merged motor are fixed to a pump shaft 50 and integrally rotated. I do. This rotating system is supported in the radial direction by static pressure bearings 54, 55, 56 which are self-liquid lubricated.
This rotating system is supported by auxiliary ball bearings 57 and 58 for supporting the axial thrust force. When the pump is running
The axial thrust is self-balanced by an axial thrust balance device including a balance disk 59 fixed to the rotating system, the rotating system floats in the liquid, and the auxiliary ball bearings 57, 58 for supporting the axial thrust force have thrust forces. Has no effect. When the pump is stopped, the weight of the rotating system is supported by an auxiliary ball bearing 58 for supporting an axial thrust force.

An electromagnet 62,
Active thrust magnetic bearing 6 comprising ferromagnetic rotor 63
1 is equipped. When the self-weight of the pump shaft rotating system and the residual fluid force acting on the impeller 53 and the balance disk 59 always act vertically downward, the electromagnet 62 of the magnetic bearing is set only on the upper side as shown in FIG. -T6
3 is controlled so as to be pulled upward by an attractive electromagnetic force corresponding to the thrust force.

Generally, the attraction electromagnetic force of the electromagnet is
2. It is affected by the inter-plane dimension δ1 between the ferromagnetic rotors 63. Therefore, the gap detection sensor 64 for detecting the gap δ0 of the axially narrow portion of the balance disk 59 is provided, and the current is controlled so as to control the gap δ0 within a predetermined range. The gap δ0 is maximum when the pump is stopped and the rotating system is supported by the auxiliary ball bearing 58, and the state where the axial thrust balance device including the balance disk 59 and the like functions is minimum. It is controlled to an intermediate value between the two, so that the thrust force acts on the auxiliary ball bearing 58, and the control is performed so that the rotating system does not come into contact with the stationary system due to the narrow space of the balance disk 59 becoming zero. Supported.

The starting sequence is as follows. That is, when the start signal of the submerged pump 5 is issued, first, power is supplied to the active type thrust magnetic bearing 61 composed of the electromagnet 62 and the ferromagnetic rotor 63, and the rotating system floats and is supported. . Next, when power is supplied to the sub-merged motor 51, the pump rotates to suck liquefied gas from the suction valve 3 and is pressurized by a plurality of stages of impellers 53. It is discharged into.

In this state, in addition to the own weight of the rotating system, the fluid force of each impeller 53 acting downward and the fluid resultant force acting on the balance disk 59 acting upward are applied to the thrust force. To increase. Accordingly, in response to this, the active thrust magnetic bearing 61 is controlled so as to increase the current value to increase the attraction electromagnetic force. When the liquid level in the pumping pipe 2 rises and reaches the discharge pipe 10, the discharge pressure rises. Therefore, the discharge pressure is detected by the pressure switch 41, this signal is taken into the control device 60, and the active type thrust The power supply of the magnetic bearing 61 is shut off.

On the other hand, when the pump is stopped, when a pressure drop is first detected by the pressure switch 41, the power of the active thrust magnetic bearing 61 is turned on, and the own weight of the rotating system is supported by the active thrust magnetic bearing 61. After a lapse of a predetermined time, the power supply of the active thrust magnetic bearing 61 is cut off. These series of controls are performed by incorporating a control circuit into the control device 60.

A basic control method in the submerged pump apparatus according to this embodiment will be described with reference to FIG. 1 and FIG. The control device 60 compares the displacement value of the rotation system detected by the detection sensor 64 that detects the gap δ0 of the narrow portion between the balance disk 59 and the stationary portion with a predetermined value set in advance, and determines whether the rotation system has an excessive value. A gap determination function 71 for determining movement, and a gap control function 7 for controlling the displacement value of the rotation system detected by the detection sensor 64 to a predetermined range.
0.

As shown in FIG.
Is detected by the gap detection sensor 64. On the other hand, the detected value is used as an input signal of a gap control function 70 for controlling the gap δ0 of the narrow portion of the balance disk 59 to be an intermediate value between the allowable maximum value and the allowable minimum value,
At the same time, the other is used as a signal of the gap determination function 71 for determining whether the gap δ0 of the narrow portion of the balance disk 59 exceeds or does not exceed the maximum value or the minimum value.

Therefore, the active type thrust magnetic bearing 61 is controlled by the gap control function 70 so that the gap δ0 of the narrow portion of the balance disk 59 falls within a predetermined range, and at the same time, the value of the gap The determination function 71 monitors whether the allowable maximum value or the minimum allowable value is exceeded. Further, normally, the discharge pressure is monitored by the pressure switch 41, and when the discharge pressure reaches a predetermined pressure, the power supply of the active type thrust magnetic bearing 61 is cut off.

With the active type thrust magnetic bearing 61,
During the magnetic levitation of the rotating system, the control by the gap control function 70 becomes impossible for some reason, the magnetic attraction force is insufficient, and the gap δ of the narrow portion of the balance disk 59 is reduced.
When 0 exceeds the maximum value, that is, when the rotating system including the shaft moves vertically below the range that can be controlled by the magnetic attraction force, the rotating system is supported by the auxiliary ball bearing 58.

When the magnetic attraction force becomes excessive for some reason during the magnetic levitation of the rotating system and the gap δ0 of the narrow portion of the balance disk 59 becomes smaller than the minimum value, that is, the rotating system including the shaft Is moved vertically upward, regardless of the value of the pressure switch 41, the power of the active type thrust magnetic bearing 61 is cut off by the gap determination function 71 and the magnetic attraction force is cut off. It falls downward due to physical strength and is supported by the auxiliary ball bearing 58. Thereby, contact between the rotating system and the stationary part can be avoided.

The gap determining function 71 shown in FIG. 3 for determining whether or not the gap δ0 of the narrow portion of the balance disk 59 exceeds or does not exceed a specified value allows the gap δ0 of the narrow portion of the balance disk 59 to be reduced. If the maximum or minimum value is exceeded, the user will be alerted with some kind of warning.

Next, the control method will be described more specifically with reference to FIGS. As shown in FIG. 4, a predetermined time Δt has elapsed since the gap δ0 of the narrow portion of the balance disk 59 was determined by the gap determination function 71 shown in FIG. After that, the power supply of the active type thrust magnetic bearing 61 is cut off to avoid contact between the rotating system and the stationary part. Thus, when the gap δ0 of the narrow portion of the balance disk 59 only exceeds the minimum value instantaneously, the power supply of the active thrust magnetic bearing 61 is not shut off, and stable control can be performed.

Next, as shown in FIG. 5, when the gap δ0 of the narrow portion of the balance disk 59 becomes zero, the gap determination function 71 shown in FIG.
The power supply of the active thrust magnetic bearing is shut off to avoid contact between the rotating system and the stationary part.

Next, as shown in FIG. 6, when the gap δ0 of the narrow portion of the balance disk 59 becomes equal to the allowable minimum value, the change amount Δδ of the gap with respect to the time before the predetermined time Δt is obtained. When the ratio, that is, the change rate of the gap is larger than a predetermined value, the power supply of the active type thrust magnetic bearing 61 is cut off.

In the submerged pump for a liquefied gas tank having the active thrust magnetic bearing 61, the magnetic bearing cannot be controlled during the magnetic levitation by the active thrust magnetic bearing 61, and the active thrust is prevented by the protection device. After the power supply to the magnetic bearing 61 is cut off, the operation is continued by the auxiliary ball bearing 58 in a sequence.

FIG. 7 is a longitudinal sectional view of a submersible pump according to another embodiment of the present invention. In the figure, components having the same reference numerals as those in FIG. 1 are the same as those in the previous embodiment, and the description thereof will be omitted. The embodiment shown in FIG. 7 is different from the apparatus shown in FIG. 1 in that a displacement detection sensor 6 for detecting a gap δ0 of a narrow portion of the balance disk 59 is provided.
5 is installed at the uppermost end of the pump shaft 50, and the other functions have the functions shown in FIGS. According to the embodiment shown in FIG. 7, the same effects as those of the previous embodiment shown in FIG. 1 can be obtained.

[0035]

As described above in detail, according to the present invention, in a submerged pump for a liquefied gas tank having an active thrust magnetic bearing, when the magnetic attraction force of the active thrust magnetic bearing cannot be controlled. However, it is possible to provide a submerged pump device for a liquefied gas tank and a control method thereof, which ensure protection of the pump rotation system.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a submersible pump according to one embodiment of the present invention.

FIG. 2 is a sectional view of a submerged pump device for a liquefaction tank using a general active type thrust magnetic bearing.

FIG. 3 is a flowchart of a gap detection value process of a narrow portion of a balance disk in the active thrust magnetic bearing of the apparatus of FIG. 1;

FIG. 4 is a diagram showing a time change of a gap of a narrow portion of a balance disk in the apparatus of FIG. 1;

FIG. 5 is a diagram showing a time change of a gap of a narrow portion of a balance disk in the apparatus of FIG. 1;

FIG. 6 is a diagram showing a time change of a gap of a narrow portion of a balance disk in the apparatus of FIG. 1;

FIG. 7 is a longitudinal sectional view of a submersible pump according to another embodiment of the present invention.

[Explanation of symbols]

1. Liquefied gas tank, 2. Pumping pipe, 3. Suction valve, 5.
Submersion pump, 10: discharge pipe, 12: pump discharge port, 21
... Submerged pump seating surface, 41 ... Pressure switch, 50 ... Pump shaft, 51 ... Submerged motor, 51a ...
52, an inducer, 53, an impeller, 54, 55,
56 ... static pressure bearing, 57, 58 ... auxiliary ball bearing, 59 ... balance disk, 60 ... thrust magnetic bearing control device, 61
... Active thrust magnetic bearing, 62 ... electromagnet, 63 ... ferromagnetic rotor, 64 ... gap detection sensor, 65 ... displacement amount detection sensor.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Mitsuo Agatsuma 209-15 Fukasaku, Omiya City, Saitama Prefecture (72) Inventor Koichi Morito 15-10 Miyamatsucho, Hiratsuka City, Kanagawa Prefecture Tokyo Gas Hiratsuka Apartment 402 (56) References Special JP-A-7-119680 (JP, A) JP-A-2-22197 (JP, A) JP-A-57-116931 (JP, A) JP-A-2-85012 (JP, U) (58) Fields surveyed ( Int.Cl. ⁷ , DB name) F04D 7/02 F04D 13/08 F04D 15/00 F04D 29/04 F16C 32/04

Claims (9)

(57) [Claims] 1. A motor and an impeller which is connected to the motor by a rotating shaft and pressurizes the sucked liquefied gas while being seated on a bottom seating surface of a pumping pipe suspended in a liquefied gas tank. A pump body having a plurality of discharge holes for discharging pressurized liquefied gas into a liquid lifting pipe, a thrust bearing and a radial bearing for supporting the rotary shaft, a shaft thrust balance device having a balance disk, and the balance disk Detecting means for detecting the distance of the narrow portion between the shaft and the stationary part; an active thrust magnetic bearing mounted on the rotating shaft; detecting means for detecting the discharge pressure of the pump; and the discharge pressure detected by the detecting means. A submersible pump device for a liquefied gas tank, comprising: a control device for turning on and off the power of the active thrust magnetic bearing according to a value of the thrust bearing. Attached to a position that prevents an excessive movement in the vertical below the rotary shaft, the displacement value detected by the detecting means for detecting a distance slit portion of said balance disc and the stationary portion is less than the allowable minimum value
, Or is judged to be below the minimum allowable value
When passed Luo predetermined time, so as to avoid excessive movement in the vertical above the rotary shaft, the protective device having a control function to force off the power of the active thrust magnetic bearing, Oyo
A submerged pump for a liquefied gas tank, wherein the submerged pump is supported by the thrust bearing after the forced shut-off . 2. A protection device, comprising: comparing a displacement value of a rotating system detected by a detecting means for detecting a distance of a narrow portion between a balance disk and a stationary portion with a predetermined value set in advance; 2. A submerged pump apparatus for a liquefied gas tank according to claim 1, further comprising a function of determining an excessive movement and a function of controlling a displacement value of a rotating system detected by said detecting means within a predetermined range. . 3. The submersible pump device for a liquefied gas tank according to claim 1, further comprising detection means for detecting a displacement of the rotary shaft at a shaft end of the rotary shaft. 4. A motor and an impeller which is connected to the motor by a rotating shaft and pressurizes the sucked liquefied gas is seated on a bottom seating surface of a liquid lifting pipe suspended in the liquefied gas tank. A pump body having a plurality of discharge holes for discharging pressurized liquefied gas into a liquid lifting pipe, a thrust bearing and a radial bearing for supporting the rotary shaft, a shaft thrust balance device having a balance disk, and the balance disk Detecting means for detecting a distance between a narrow portion of the rotary shaft and the stationary portion; an active thrust magnetic bearing mounted on the rotating shaft; and detecting means for detecting a discharge pressure of a pump. In a control method of a submerged pump device for a liquefied gas tank, the power source of the active type thrust magnetic bearing is turned on and off in accordance with a discharge pressure value. Depending on the detected displacement value by detecting means for detecting a distance slit portion, to force off the power of the active thrust magnetic bearing so as to avoid excessive movement of the vertically upward of the rotational axis of the, Forcibly shut off
A method for controlling a submerged pump device for a liquefied gas tank, comprising: 5. A warning is issued when a displacement value detected by a detecting means for detecting a distance between a narrow portion between a balance disk and a stationary portion becomes equal to or less than an allowable minimum value of the narrow portion. 5. The control method for a submerged pump device for a liquefied gas tank according to claim 4, wherein a power supply of the active type thrust magnetic bearing is forcibly shut off. 6. A state in which a displacement value detected by a detecting means for detecting a distance of a narrow portion between a balance disk and a stationary portion is equal to or less than an allowable minimum value of the narrow portion for a predetermined time. 5. The control method for a submerged pump device for a liquefied gas tank according to claim 4, wherein the power supply of the active thrust magnetic bearing is forcibly shut off. 7. When the displacement value detected by the detecting means for detecting the distance of the narrow portion between the balance disk and the stationary portion indicates that the distance of the narrow portion has become zero, the active type 5. The control method for a submerged pump for a liquefied gas tank according to claim 4 , wherein the power supply of the thrust magnetic bearing is forcibly shut off. 8. A displacement value detected by a detecting means for detecting a distance of a narrow portion between a balance disk and a stationary portion is equal to or less than an allowable minimum value of the narrow portion, and a rate of change of the detected value with respect to time. Is greater than or equal to a predetermined rate of change,
5. The control method for a submerged pump for a liquefied gas tank according to claim 4 , wherein the power supply of the thrust magnetic bearing is forcibly shut off. 9. After the power supply of the thrust magnetic bearing is forcibly shut off by the protection device, the rotation system including the rotation shaft is supported by the thrust bearing supporting the rotation shaft to continue the operation. The control method for a submerged pump device for a liquefied gas tank according to any one of claims 4 to 8.