HG/T 20570.9-1995 Calculation of pressure loss at equipment inlet and outlet
Some standard content:
Calculation of pressure loss at the inlet and outlet of equipment
HG/T20570.9-95
Compiled by: China Huanqiu Chemical Engineering Corporation Approved by: Ministry of Chemical Industry,
Implementation date: September 1, 1996 Compiled by:
China Huanqiu Chemical Engineering Corporation Yang Qinglan Wang Qingyu Reviewed by:
China Huanqiu Chemical Engineering Corporation Yang Yi
Ministry of Chemical Industry Process System Design Technology Center Gong Renwei 1 Scope of application and instructions
1.0.1 This regulation is applicable to the separate calculation of the pressure loss at the inlet and outlet of equipment. This part of the resistance is part of the system pressure drop and is called local resistance. It is formed when the liquid passes through the inlet or outlet of the equipment, and the flow velocity or flow direction of the fluid suddenly changes, resulting in vortices, increasing the relative motion of the fluid particles and the internal friction.
1.0.2 There are three ways to express the pressure loss at the inlet and outlet of the equipment: 1.0.2.1 Expressed by the velocity head of the fluid in the pipe and the local resistance coefficient. 1.0.2.2 Expressed by pressure drop.
1.0.2.3 Expressed by equivalent length, the local resistance generated by the fluid is converted into the resistance equivalent to the resistance generated when the fluid flows through a pipe of the same length and diameter.
1.0.3 The local resistance coefficient of the pipe inlet and outlet resistance is expressed together with the fluid velocity head, which is related to the connection form of the equipment (see Figure 7.0.1). The pressure loss at the inlet and outlet of the equipment can generally be conservatively estimated based on the sharp-edged inlet and outlet (velocity head loss).
1.0.4 Unless otherwise specified in this regulation, all pressures are absolute pressures. 335
2 Expression of pressure loss at the inlet and outlet of equipment 2.0.1
Expression expressed by velocity head and local resistance coefficient ht=Ku=/2g
h-Pressure loss at the inlet and outlet of equipment, m liquid column; (2.0.1)
K-Local resistance coefficient, depends on the shape and configuration of the inlet and outlet of the pipeline, refer to Figure 7.0.1; g
-Gravity acceleration, 9.81m/s;
-Flow rate of fluid in the inlet pipe or outlet pipe of equipment, m/s. 2.0.2 Expression expressed by pressure drop
APhipg
AP.Pressure loss at the inlet and outlet of equipment, Pa(2.0.2)
-Fluid density determined according to the temperature (T), pressure (P) and molecular weight (M) of the upstream fluid, kg/m°;
The meanings of other symbols are the same as before.
Formula (2.0.2) is usually used in hydraulic calculations expressed in terms of pressure drop. 2.0.3 Expressions expressed in terms of equivalent length
hr=in2g
hr—Pressure loss at the inlet and outlet of the equipment, m liquid column; D—Inner diameter of the inlet or outlet pipe of the equipment, m; L. —The pipe outlet of the equipment is equivalent to the equivalent length of the pipeline, see Table 7.0.2, m; in—Friction coefficient;
The meanings of the other symbols are the same as before.
3.0.1 Compressible fluid
3 Matters to be noted in calculations
When a compressible fluid flows in a pipe, the pressure is reduced due to overcoming the fluid resistance, and the volume (density) of the fluid will change. Therefore, when performing hydraulic calculations on compressible fluids, the change in resistance caused by the change in fluid density must be considered. In high-pressure systems, the density change caused by the inlet or outlet pressure loss (measured at the same velocity head) is very small and can usually be ignored; however, in low-pressure systems, the impact of the density change caused by this pressure loss cannot be ignored, especially in vacuum systems. 3.0.2 Fluids prone to flashing
When the pressure or liquid column head of the fluid in the container changes, the fluid in the inlet and outlet pipes of the equipment may flash when the flow rate is constant and the pressure is reduced. Therefore, it is necessary to provide sufficient liquid column head and/or sufficient equipment inlet and outlet pipe diameter in the container to reduce the fluid velocity to prevent flashing, thereby reducing the pressure loss of the fluid entering and exiting the equipment pipe and reducing the pressure change caused by acceleration in the equipment pipe. 337
4.0.1 Definition
Calculation of equipment inlet pressure loss
The equipment inlet pressure loss is the pressure loss of the fluid entering the equipment (such as a heat exchanger or container) from the pipeline. 4.0.2 Calculation formula
Take Figure 4.0.2 as an example to show the equipment inlet pressure loss calculation formula. Pi.u
Figure 4.0.2 Pressure loss of fluid from the equipment inlet The energy balance formula is listed inside and outside the container inlet uipzp
P2P+车
P.-—Pressure at the equipment inlet, Pa;
Pressure inside the container, Pa;
APr Friction pressure loss, Pa;
ui-Fluid flow rate in the inlet pipe, m/s;u2
-Fluid flow rate in the container, m/s;
-Fluid density at the equipment inlet pipe, kg/m(4.0.2)
Assume u=0, the inlet pipe is considered as a sharp edge, and the local resistance coefficient at the equipment inlet is K=1.0 (see Figure 7.0.1). The pressure loss when the fluid enters the equipment from the inlet pipe is AP;=k\P, which is exactly equal to the loss of the fluid's inlet kinetic energy\e,
, then Pi=P2.
5.0.1 Definition
5 Calculation of equipment outlet pressure loss
The equipment outlet pressure loss is the pressure loss of the fluid from the equipment into the pipeline. 5.0.2 Calculation formula
5.0.2.1 Gas
Figure 5.0.2-1 shows the calculation formula for the pressure loss of gas from the equipment into the outlet pipeline. Pu
Figure 5.0.2-1 The pressure loss of the fluid from the equipment outlet is listed inside and outside the container outlet. The energy balance formula P is listed, =P+ ie_e
(5.0.2-1)
Assuming u1=0, when the pipe mouth is a sharp edge, the friction pressure loss (see Figure 7.0.1, K=0.5) is obtained: AP/0.5X
Pi-Pz=1. 5×up
Pi—Pressure inside the container, Pa;
P. Pressure at the equipment outlet, Pa;
-Friction pressure loss, Pa;
ulFlow rate of the fluid in the container, m/s;
-Flow rate of the fluid in the equipment outlet pipe, m/s, u2
-Fluid density at the equipment outlet pipe, kg/m3. (5.0.2—2)
From the above, it can be seen that when the fluid flows from the equipment (such as a container or a heat exchanger) to the outlet pipe, the outlet 339
pressure loss at the outlet pipe mouth, when the pipe mouth is a sharp-edged pipe mouth, can be taken as 1.5 times the velocity head. 5.0.2.2 Liquid
(1) The saturated solution must have a high enough working liquid level to overcome the pipe mouth loss to prevent the liquid from flashing at the pipe mouth, as shown in Figure 5.0.2—2.
Figure 5.0.2-2 Pressure loss density of saturated solution from the equipment outlet p2=p3
Assume u2—0. In order to prevent flash evaporation, h>1.5X
PaP+hp2g
P3-P2-1.5X
In the above formulas,
-gravitational acceleration.9.81m/s2
h-liquid level, m;
Pi-liquid surface in the container Pressure, Pa; P, - pressure at the equipment outlet pipe, Pa; P. - pressure inside the equipment outlet pipe, Pa
(5.0.2-3)
(5.0.2-4)
(5.0.2-5)
Note ① The "" here can only ensure to overcome the pressure loss from the equipment outlet to the outlet pipe. If it is to be ensured that the entire downstream system does not vaporize, "" should be high enough to overcome all pipeline pressure losses in the downstream system. 340
- velocity of the fluid at the equipment outlet pipe, m/s; u3 - velocity of the fluid inside the equipment outlet pipe, m/s; P2 - density of the fluid at the equipment outlet pipe, kg/m; p3 - density of the fluid inside the equipment outlet pipe, kg/m. For horizontal equipment, the outlet pipe is mostly of the inserted type, and the local coefficient of the outlet pipe K=0.78 (see Figure 7.0.1). In order to prevent flash evaporation, it is necessary to make
P,=P+hp2g
P3=P2-1.78X
where the symbols have the same meaning as before.
（2）Unsaturated solution
(5.0.2—6)
(5.0.2—7)
(5.0.2—8)
Unsaturated solutions generally do not flash when flowing through the pipe mouth, but it is necessary to prevent the gas on the liquid surface from being brought into the pipe mouth, so a certain liquid level height must be maintained. Therefore, it can be treated as a saturated solution. (3) Gas-liquid mixture
The pressure loss of the equipment outlet of the gas-liquid mixture is calculated according to the average density method and the mixing speed, and the calculation formula is as follows: V
4P=K×VpH
In the above formulas,
A is the pipe mouth section, m;
K is the local resistance coefficient, as shown in Figure 7.0.1; △P is the pressure loss at the equipment outlet, Pa; (5.0.2-9)
(5.0.2-10)
(5.0.2-11)
(5.0.2-12)
(5.0.2-13)
Note ① The \\ here can only ensure that the pressure loss from the equipment outlet to the outlet pipe is overcome. If it is to be ensured that the entire downstream system does not vaporize, "" should be high enough to overcome the total pipeline pressure loss of the downstream system. 341
W.--liquid phase flow rate at the equipment outlet, kg/s; V.--gas-liquid phase mixing speed at the equipment outlet, m/s; Wv--gas phase weight flow rate at the equipment outlet, kg/s; VL--liquid phase volume flow rate at the equipment outlet, m\/s; Vv--gas phase volume flow rate at the equipment outlet, m\/s; PH
--gas-liquid phase uniform density at the equipment outlet, kg/m3Liquid phase density at the equipment outlet, kg/m; Gas phase density at the equipment outlet, kg/m. 6.0.1 Horizontal equipment with inserted outlet pipe
Calculation example
u=2.225m/s
620kg/ms
Figure 6.0.1 Calculation example diagram (I)
Solution: Since the outlet pipe is inserted, it can be seen from Figure 7.0.1 that K0.78, so the liquid level height required to overcome the pressure loss at the pipe mouth is:
Pressure loss at the pipe mouth
=0.449m (less than the liquid level in the equipment)wwW.bzxz.Net
X620-2732Pa||tt| |Outlet pressure loss when velocity and density change Vapor-liquid
U,=35.2m/s
p=4.24kg/m2
In the figure: A
u-23.5m/s
Pz=6.35kg/m
Figure 6.0.2 Calculation example diagram (II)
Reactor: B, C-
Heat exchanger: D
u,=17.0m/s
Ps=8.67kg/m
Separator.
Solution: The pressure loss of each section is:
4P=1.5×35.22
X4.24=3940Pa
4P=1.5×23.5
X6.35=2630Pa
4P=1.5×17.0*
X8.67-1879Pa
Here u3 and 0s are calculated according to two-phase flow. 344
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