JPS629795B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for cooling down low-temperature liquefied gas flow piping.

[Conventional technology]

In order to prevent rapid cooling contraction of the piping when the flow of liquefied gas (LN ₂ , LNG, LO ₂ , LAr, etc.) starts, the flow piping for low-temperature liquefied gas (LN 2 , LNG, LO 2 , LAr, etc.) is It is cooled to a temperature at which the liquefied gas can be safely transported. This is called piping cooldown, and this cooling down is performed by flowing a low-temperature fluid through the piping.

When cooling down this piping, generally, first, the low-temperature liquid for cooling down (usually the low-temperature liquefied gas to be flowed is used) is gasified and flowed into the piping, and pre-cooling is performed by gas cooling. Next, a method is adopted in which full-scale cool-down is performed by liquid cooling by flowing a low-temperature liquid into the pipe.

By the way, when cooling down the piping in this way, the gas contains a large amount of liquid that has not been completely gasified in the form of mist when cooling the gas. The liquid drops in the pipe, becomes a liquid flow, and flows at the bottom of the pipe.Also, even when the liquid is cooled, the liquid does not flow all the way inside the pipe, so in any case, the bottom side of the pipe is the first to flow. It will be cooled down. Therefore, when cooling down the piping, the piping may be deformed into an arched shape (curved upward) due to the temperature difference between the upper and lower portions of the piping. This phenomenon is called the bowing phenomenon, and if the piping is not restrained, there is no problem even if bowing occurs, but in reality, the piping is restrained at appropriate intervals by supports, etc., so bowing does not occur. Stress is generated in the piping, and if this generated stress exceeds the allowable stress of the piping, the piping will be damaged.

For this reason, in general, the bowing of the piping is continuously monitored during cool-down, and when bowing occurs, the flow of low-temperature liquid in the pipe is temporarily stopped, or the flow rate is reduced to speed up the cooling of the piping. control is being carried out.

[Problem that the invention seeks to solve]

However, in the past, the cooling rate was controlled as soon as even the slightest bowing occurred in the piping, so even if the stress generated in the piping due to bowing was much smaller than the allowable stress of the piping, the cooling rate Therefore, there was a problem that the cooling down of the piping could not be completed efficiently and in a short time.

[Means for solving problems]

This invention prevents lifting of piping due to the bowing phenomenon during cool-down between a fixing device that supports and fixes the piping at appropriate intervals and an intermediate fixing device that supports and fixes the central part of the piping between the fixing devices. From the intermediate fixing device to the following formula l ₁ = 3-5α/2l α=σ _A /σ _O l: Distance between the fixing device and the intermediate fixing device σ _A : Stress σ that takes into account the safety level in addition to the allowable stress of the piping. _O : Monitor from a distance l ₁ determined by the stress generated in the pipe when α = 1.0, and when the pipe lifts up, the flow of low-temperature fluid into the pipe is stopped at once. By stopping the flow or reducing the flow rate to control the cooling rate of the piping, the piping can be efficiently cooled down.

[Effect]

In other words, instead of controlling the cooling rate of piping immediately when bowing occurs in the piping, as in the past, this invention elucidates the relationship between the situation in which bowing occurs during cool-down and the stress generated in the piping. Based on this relationship, by determining the position at which the piping should be monitored for lifting, the cooling rate of the piping is controlled when the piping lifts at this position. The lifting of the piping is monitored at the required position, that is, the position where the lifting occurs only when the stress generated in the piping due to the bowing phenomenon during cool-down exceeds the allowable stress of the piping plus the stress that takes safety into account. Since the cooling rate of the piping is controlled only when the piping lifts, it is possible to control the cooling rate in the most efficient manner while ensuring that the stress generated during cool-down does not exceed the allowable stress of the piping plus the stress that takes safety into account. Good cooling can be achieved.

〔Example〕

The cool-down method of the present invention will be explained below.

First, the position for monitoring piping lifting will be explained.

Figure 1 shows a typical piping system for low-temperature liquefied gas flow piping. 1 is restrained by fixing devices 2, 2 so as not to shift. Also, 3,3
is an intermediate fixing device provided between the fixing devices 2 and 2 to support and fix the central part of the piping between the fixing devices 2 and 2; 4 and 4 are provided between the fixing devices 2 and 3; It is a support member that supports the pipe 1 only from below, and the pipe 1 is fixed to the fixing devices 2, 2 via the expansion joints 5, 5 that absorb expansion and contraction in the axial direction. The part between the fixing devices 2, 2 can be viewed as one block.

Figure 2 shows a model of one block in the above piping system. In this model diagram, A and A are pivot points that support both ends of the piping 1, and B
is a central fixing point that fixes the central part of the pipe 1,
C and C indicate support points that receive the weight of the pipe 1, and P indicates a reaction force that occurs when the center of the pipe attempts to rise due to bowing.

By the way, suppose that bowing occurs in the pipe 1 during the cool-down of the pipe 1.
The magnitude of the virtual bending moment M _T generated in the pipe 1 due to bowing can be expressed by the following general formula.

M _T = Kα _T ΔT/D・EI……(1) K: Constant determined by the temperature distribution in the pipe cross section (the temperature gradient in the pipe cross section is almost a straight line, so K
= 1) α _T : Linear expansion coefficient of pipe (1.5×10 ^-5 /℃ for stainless steel pipe) E: Young's modulus (2.0×10 ⁶ Kg/cm ² for stainless steel pipe) ΔT: Coefficient of pipe cross section Temperature difference between top and bottom (°C) D: Pipe diameter (cm) I: Second moment of inertia of pipe cross section (cm ⁴ ) Also, based on the model in Figure 2 above, calculate the total span length when only both ends of the pipe are supported. Assuming a 2-liter piping model, and considering the deformation of piping 1 when bowing occurs in piping 1 as shown in Figure 3a in this model, assuming that there is no temperature difference in the pipe axis direction, the piping The magnitude of deformation y _M (x) of the pipe 1 due to bowing at a position x apart from the center 0 (which is the origin of the xy coordinates) can be expressed by the following equation.

y _M (x)=M _T /2EI(l ² −x ² )………………(
2) (-l≦x≦l) On the other hand, in the above piping model, the dead weight w (Kg/
cm), the pipe 1 is deflected as shown in Figure 3b, the amount of deflection y due to the weight of the pipe 1 at a position x apart from the center of the pipe
_W (x) can be expressed by the following equation.

y _W (x)=w/24EI(l ² −x ² )(x ² −5 l ² )…(
3) (-l≦x≦l) Then, since pipe 1 will be deflected by the sum of the deformation due to bowing and the downward deflection due to its own weight, the deflection of pipe 1 at a position x apart from the center of the pipe is The quantity y(x) is y(x)y _M (x)+y _W (x). Note that since the actual piping 1 is supported by support members 4, 4 as shown in FIG.
Deflection of the pipe 1 occurs only in the upward direction.

Here, for convenience in the following calculations, the magnitude of the virtual bending moment M _T will be expressed as follows.

M _T =5/12wl ² ·α (4) In the above equation (4), α is a dimensionless coefficient, and when α=0, it indicates that no bowing occurs at all.

Substituting equation (4) into equation (2), the amount of deflection y of piping 1 is calculated as follows:
Calculating (x)=y _M (x)+y _W (x), y(x)= w/24EI(l ² −x ² )(x ² +5 l ² α−5 l ² ) (−l≦x ≦l), and if α=1, y(0)=0.

Next, if we consider a pipe in which both ends are supported at pivot points A and A, and the center part is supported and fixed at central fixed point B, in the case of such three-point support,
A reaction force in the opposite direction to the direction of deflection of the pipe 1 acts at the center.

Therefore, assuming that only both ends of the pipe 1 are supported, there is no bending due to bowing or its own weight, and if a vertical pushing force P is applied to the center of the pipe, then the The amount of deflection y _P at the center of the pipe can be expressed by the following equation.

y _P = Pl ² /4EI(l-x) {1-(x-l) ² /
3 l ² }...(5) (The above equation holds true in the range of 0≦x≦l) When the above-mentioned pushing force P restrains the central part of the pipe 1, the pipe 1 is affected by bowing and its own weight. This acts as a reaction force when the pipe tries to bend, and if the center of the pipe 1 is restrained, the amount of deflection at the center of the pipe is 0 (y _M + y _W + y _P = 0).
Therefore, the reaction force P acting on the pipe 1 that is about to deflect at the central fixed point B is as follows: P=5/4(1-α)wl (6)

Therefore, when the pipe 1 is supported at three points, the amount of deflection y(x) of the pipe 1 at a position x apart from the center of the pipe can be expressed by the following equation.

y(x)=y _M (x)+y _W (x)+y _P (x)=w(l-x)/24EIx ² {x-3-5α/2l}......(7) (In addition, The above equation holds true in the range of 0≦x≦l) The deflection shape of the pipe 1 when the pipe 1 is supported at three points can be assumed from the above equation (7).

In other words, if 1/5 > α > 0 in equation (7), the amount of deflection (amount of upward deflection) y of the pipe 1 becomes y≦0 over the entire span length 2 l, which is shown in Figure 4 a. As shown, there is no bowing in the pipe 1 at all. In addition, Figure 4 a is y
<0 indicates that the pipe 1 is bent downward due to its own weight.

In addition, if 3/5 > α > 1/5 in equation (7), the amount of deflection y of the pipe 1 becomes y = 0 at a point a certain distance away from the central fixed point B, and from this point toward the center, y ≦0, and y≧0 on the end side. This is the fourth
As shown in FIG. b, a state in which bowing has occurred in the pipe 1 is shown, but this bowing does not occur near the central fixed point B and is relatively mild.

Here, if the distance from the central fixed point B to the point where y=0 is l ₁ when 3/5>α>1/5, then l ₁ =3−5α/2l ……………( 8).

Furthermore, if 1≧α≧3/5 in equation (7), the amount of deflection y of the pipe 1 becomes y≧0 over the entire span length 2 l, which means that the deflection amount y of the pipe 1 becomes y≧0 as shown in Figure 4c. This shows a state where excessive bowing is occurring.

Next, considering the bending moment occurring in the pipe 1 supported at three points, the bending moment M occurring in the pipe 1 is due to the own weight w of the pipe 1 and the upward reaction force P at the central fixed point B. (bowing itself does not act as a bending moment), the bending moment M can be expressed by the following equation.

M=w/2(l ² −x ² )−5/8(1−α)wl(l−x)
…… ………(9) (0≦x≦l) Also, the bending moment M becomes maximum at x=5/8(1-α)l, and this maximum bending moment M
(max) is M(max)=wl ² /2・(5α+3) ² /64...
...(10) becomes.

By the way, in the case of 1/5<α<1, (5α+3) ² /64≈α (11), so the above equation (10) can be expressed as the following equation.

M(max)=wl ² /2・α ……………(12) Therefore, the maximum bending moment M(max) in pipe 1
The stress σ ^generated in the pipe 1 when
) becomes.

Therefore, in order to keep the stress σ generated in the pipe 1 within a range smaller than the stress σ A, which takes into account the safety level σ _S in addition to the allowable stress _{σ a} of the pipe 1, The value of wl ² /2・D/2I in the above equation (14) corresponds to the stress σ _O generated in the piping when α = 1.0, so the maximum allowable value of α is can be obtained as α=σ _A /σ _O (15).

Also, by substituting this value of α into equation (8) above, α
The central fixed point B when is the value of equation (15)
The distance l ₁ from to the point where y=0 is becomes.

Considering this in reverse, in order to suppress the stress σ generated in the pipe within the range of σ≦σ _A , it _is necessary to The amount of deflection y of the pipe should always be y≦0, so in order to do this, the lifting of the pipe 1 should be monitored at the above points, and when the pipe 1 rises, the low-temperature fluid The cooling rate of the piping may be controlled by temporarily stopping the flow of water or by reducing the flow rate.

FIG. 5 shows an embodiment of this cool-down method. In the figure, 1 is a pipe, 2 is a fixing device, 3 is an intermediate fixing device, and 4 is a supporting member.
is supported by the fixing device 2 via the expansion joint 5, and among the supporting members 4, 4, the supporting member 4a closest to the central fixing device 3 is connected to the central fixing device 3 by the above equation (8). It is provided at a position at the determined distance _l1 , and its top surface is used as a reference surface for monitoring the lifting of the pipe 1.

Therefore, when cooling down the piping 1, the support member 4a closest to the central fixing device 3
Cool down is performed while monitoring the lifting of the piping 1 from above, and when the lifting of the piping 1 occurs at this position, for example, the flow of the low-temperature fluid is stopped until the piping 1 bottoms out on the support member 4a. In this case, by inserting a piece of paper or the like between the support member 4a and the pipe 1, it is possible to easily determine whether the pipe 1 is lifted up.

Note that the reference plane (or reference level) for monitoring the floating of the pipe 1 is not limited to the support member, and may be set by providing other members.
Monitoring of the floating may be performed using a dial gauge or the like.

Here, an example of calculation for determining the uplift monitoring point of the pipe 1 will be explained using specific numerical values.

Now, the allowable stress of the pipe σ _a = 1330Kg/cm ² (according to the notification specifying the details of technical standards for thermal power generation equipment) Weight of the pipe w = 2.52Kg/cm Pipe diameter D = 76.2cm Quadratic of the pipe cross section Moment I = 210000cm ⁴ Piping length l between fixing device 2 and intermediate fixing device 3
= 3000cm, the stress σ generated in the pipe 1 when α=1.0 is obtained from the above equation (13) as follows: σ=wl ² /2・D/2I・α=2.52×3000 ² /2×76.2 /2×210000×1.0=209
0 (Kg/cm ² ).

In addition, considering the degree of safety for the allowable stress σ _a = 1330 Kg/cm ² of the pipe, the maximum value of α when trying to suppress the maximum stress σ _A generated in the pipe 1 during cool-down to, for example, 1045 Kg/cm ² is calculated. When you ask,
From equation (15), α=σ _A /σ _O =1045/2090=0.
5, and by substituting this value of α into the above equation (8), l ₁ = 3-5α/2l = 3-5 x 0.5/2l = 1/4l = 3000/4 = 750 (cm) .

Therefore, in this case, the distance is 750 cm from the intermediate fixing device 3.
It is sufficient to monitor the floating of the pipe 1 at a distant position. In this case, if the pipe 1 does not float at the floating monitoring point, the stress σ occurring in the pipe 1
is less than 1045Kg/cm ² , but if pipe 1 rises, the stress σ occurring in pipe 1 exceeds 1045Kg/cm ² , so at this time, the cooling temperature (cooling strength) of pipe 1 All you have to do is drop it.

〔Effect of the invention〕

The cool-down method for low-temperature liquefied gas flow piping according to the present invention starts when the stress generated in the piping exceeds the allowable stress of the piping plus the stress that takes safety into account in the piping part that floats due to the bowing phenomenon during cool-down. According to this invention, the position where lifting occurs is determined by calculation, the lifting of the piping is monitored at this position, and the cooling rate of the piping is controlled when lifting occurs. Since the most efficient cooling can be performed while ensuring that the stress generated during operation does not exceed the allowable stress of the piping and the stress that takes safety into account, the cooling down of the piping can be completed efficiently and in a short time. .

[Brief explanation of the drawing]

Figures 1 and 2 are principle diagrams and model diagrams showing the piping system of general low-temperature liquefied gas flow piping,
Figures 3a and b are model diagrams when the piping is supported only at both ends; Figures 4a, b, and c are model diagrams when the piping is supported at three points; Figure 5 is an embodiment of the present invention. It is a principle diagram showing an example. 1... Piping, 2... Fixing device, 3... Intermediate fixing device, 4... Supporting member, 5... Expansion joint.

Claims (1)

[Scope of Claims] 1. In cool-down of low-temperature liquefied gas flow piping, lifting of the piping due to the bowing phenomenon during cool-down can be prevented by fixing devices that support and fix the piping at appropriate intervals and the center of the piping between the fixing devices. Monitoring is performed at a position separated from the intermediate fixing device by a distance l ₁ determined by the following formula between the intermediate fixing device that supports and fixes the pipe, and when floating occurs in the piping,
A method for cooling down low temperature liquefied gas flow piping, characterized by controlling the cooling rate of the pipe by temporarily stopping the flow of low temperature fluid into the pipe or reducing the flow amount. l ₁ = 3-5α/2l α=σ _A /σ _O l: Distance between the fixing device and intermediate fixing device σ _A : Stress that takes into account the safety level in addition to the allowable stress of the piping σ _O : α=1.0 Stress that occurs in piping at a certain time