In the tire vulcanization process, the stable supply and circulation of the internal pressure superheated water is extremely important. In its complete closed-circuit circulation system, the hot water circulation pump is as important as the heart of the human body and cannot be faulty. However, the actual situation is inevitable. In the case of cavitation alone, not only the damage of the water pump, but also the large pressure fluctuations in the circulation system, and even the sudden loss of pressure, caused fatal injuries to the tire during the initial vulcanization. It can be seen that it is necessary to recognize the cause of cavitation and take effective measures or properly solve the measures.
1 Analysis of causes of cavitation 1.1 Qualitative analysis The water at the suction port of the pump is vaporized into bubbles, which are crushed by high pressure before the pump discharge port. (When the particle of water moves on the impeller flow path, the energy is continuously increased. The position where the bubble is crushed is also unique. Because the bubble's space suddenly “disappearsâ€, it causes a strong impact on the water quality point, causing cavitation damage to the pump impeller, and at the same time causing the pump water pressure to fluctuate. Loss of pressure.
The vaporization condition of the water at the suction port of the water pump is that the pressure is suddenly lower than the saturated steam pressure corresponding to the water temperature at the water. A heating system that is operating steadily, with stable pressure, water temperature, and flow rate, reduces the water pressure at the pump inlet when the following conditions (one of them) are encountered.
(1) The vapor pressure supplied to the deaerator suddenly decreases;
(2) The temperature of the steam supplied to the deaerator suddenly drops;
(3) adding a large amount of cold water to the deaerator at a lower temperature;
(4) The water consumption in the vulcanization workshop suddenly increased;
(5) The pipe resistance in the pipe network except the pump outlet is reduced to a large extent;
(6) There is a sudden large amount of leakage in the pipe network except the pump outlet until the circulation of the deaerator.
Once the pressure at the pump inlet drops below the saturated vapor pressure due to the above conditions, cavitation will occur.
1.2 Quantitative Analysis The drawing is a simplified diagram of the oxygen removal heating system. Take the liquid level in the deaerator as the reference height and define it as the “1-1†interface. The “2-2†interface is at the inlet of the pump.
(1) Installation height calculation Hg=P0/Ïg-P full/Ïg-Δh-Σhf(1-2)(1)
Hg in the formula - calculate the installation height, m;
P0——Vapor pressure in the deaerator, Pa;
P full - the inlet of the hot water pump, that is, the saturated vapor pressure of the water at the "2-2" interface, Pa;
Ρ——liquid density, kg/m3;
G——gravity acceleration, m/S2;
Δh——the NPSH of the pump, m;
Σhf(1-2)——The resistance loss of hot water flowing from the deaerator to the inlet of the pump, m.
When the hot water flows from the deaerator to the inlet of the pump, the change of the water temperature can be neglected, that is, P full = P0, and the NPSH of the pump is given as 3.9 m water column height with the pump data.
The input side pipe resistance loss Σhf(1-2) is estimated to be 1.1 m water column height.
Thus, calculated by (1):
Hg'=-3.9-1.1=-5m water column height This is calculated as water at 20 ° C, when converted to 170 ° C water:
Hg=Ï20gHg'/Ï170g=998.2×(-5)/897.3=-5.5m The height of the water column means that the installation height of the hot water pump is at least 5.5m lower than the operating level of the deaerator.
The actual example is a low 10m, and the installation height difference has a margin of 4.5m (calculated as 170 ° C water).
(2) The oxygen depletion power system in which the cavitation pressure is already in a stable operation state, the steam pressure in the deaerator, the water temperature, the pressure at the inlet of the pump and the water temperature are relatively stable. Assuming that P0 suddenly drops at this time, the system balance is destroyed. However, at the same time that P0 is lowered, the water temperature at the inlet of the pump will never fall immediately. The pressure formed by the existing 10m170°C water is:
h'=10×897.3/998.2≈9m water column height Calculate the drop of P0 by formula (1):
Let [(P0-ΔP)-P satiety]/Ïg_Δh-Σhf(1-2)+h'=0
(P0-ΔP)-P satiety=[-h'+h+Σhf(1-2)]Ïg=[-9+3.9+1.1]×998.2×9.8=-39129.44Pa
∵P0=ΔP-P satiety=P satiety-ΔP-P satiety=-ΔP
∴ΔP=39129Pa
That is, if the water temperature is 170 ° C, that is, the saturated vapor pressure (gauge pressure) is 0.678 MPa, the steady state operation may cause cavitation when the vapor pressure suddenly drops below the gauge pressure of 0.639 MPa.
(3) The amount of water replenishment can cause cavitation. If a large leak occurs in the pipe network, the system balance is destroyed, and the water level of the deaerator will drop rapidly. Therefore, it is necessary to quickly and relatively fill the relatively low temperature soft water.
Set the deaerator steady-state operation water storage capacity:
25m3 (volume) × 0.7 (duty ratio) = 17.5m3
In a short period of time, due to the sudden drop in water level, the water storage capacity was reduced by Vm3, so the low temperature water Vm3 was added.
When cryogenic water is added, P0 will also decrease, the flow rate of steam will increase, and the rate of heat carried will be greater than when the original steady state operation is performed. To simplify the derivation, only the effect of heat exchange of hot and cold water on P0 is considered here, ignoring the heat exchange effect of the increased steam flow.
The supplementary water temperature is 60 °C; the water temperature during steady-state operation is 170 °C; the (17.5 m3-Vm3) water at 170 °C is mixed with the Vm3 water phase at 60 °C (ignoring the difference between the total volume after mixing and 17.5 m3):
ΔQ1=m1 (TCP12-60CP11)
ΔQ2=m2(170CP21-TCP22)
M1=VÏ60=983.2V
M2=Ï170(17.5-V)=897.3(17.5-V)
When the absolute pressure of saturated steam is 0.7377MPa, see the previous calculation, and T is 168.13 °C.
CP11=0.988; CP12=CP22=1.0445; CP21=1.046
Let ΔQ1=ΔQ2 be substituted for each parameter value:
983.2V(1.0445×168.13-60×0.998)=897.3(17.5-V)×(170×1.046-1.0445×168.13)
Solve V=0.31m3
When cold water is added, P0 is lowered and the steam flow rate is increased. It is not simply a mixture of water at two temperatures. It is possible to relax the estimation, and when the hydration of 60 ° C is added to 1 m 3 in a short time, cavitation may be caused.
(4) When the pump outlet flow increases, it can cause cavitation. When the production load suddenly increases, the pipe line on the pipe suddenly decreases or there is a large amount of leakage on the pipe network, which will increase the pump outlet flow.
When these conditions occur, the deaerator liquid level in the steady state operation is suddenly lowered, and cold water is added. The influence of cold water replenishment has been discussed in the past. This factor is not considered here, and only the static pressure at the pump inlet caused by the increase in flow rate is reduced.
The flow rate suddenly increases, the flow rate in the pump inlet pipe increases, the water leakage degree increases, the dynamic pressure head and the resistance loss increase, and the enlarged portion is converted by the static pressure head.
At the flow rate of 150m3 / h, the original input side pipeline loss:
Σhf(1-2)=1.1m water column height, according to Σhf=ξu2/2
U=Q/S=150÷3600/π÷4×0.082≈8.29m/S
ξ=2Σhf/U2≈0.032
It is known in the past that the existing 10m installation height difference is equivalent to 9m water column height. The 9m water column height is deducted from the NPSH and the original resistance loss is 5m water column height, and the remaining 4m water column height.
Let ΔU2/2+ξΔU2/2=4
ΔU≈2.784m/s
Also ΔQ=ΔUS=2.784×π/4×0.082=0.014m3/S=50.38m3/h
That is, if the sudden increase in flow rate is greater than or equal to 50.38 m3/h, there is a possibility of cavitation.
You can use a sentence to summarize the three quantitative analysis conclusions: half a vapor pressure, square water, and fifty flow can be a ghost.
2 Countermeasures for Preventing and Eliminating Cavitation According to the above analysis, the cause of cavitation is a sudden decrease in vapor pressure in the deaerator, a sudden decrease in water temperature, or a sudden increase in pump flow. Therefore, the following countermeasures are proposed:
(1) If the steam source pressure and supply capacity are rich, the deaeration pressure automatic control device should be installed to ensure the stability of P0.
(2) If the steam source pressure and supply are not rich, the pressure automatic control device should be equipped after the pressure increase, to ensure the stability of P0.
(3) Reduce the number of vulcanizers and tanks entering the line at the same time, that is, reduce the flow rate growth rate.
(4) Reduce to prevent pipeline leakage.
(5) Improve the water temperature of the hydration water source.
(6) Under the premise of ensuring the effective deaeration heat transfer effect of Zui, the deaerator liquid level control point should be set as high as possible.
(7) The water supply capacity of the pump should be greater than the production load to consider the local leakage problem.
(8) Set the exhaust valve at the pump outlet, and when the cavitation occurs, open the valve to discharge the generated vapor. Or it can increase the steam supply pressure of the deaerator at the same time.
(9) Set the differential pressure measurement display instrument between the steam pressure in the deaerator and the water pressure at the inlet of the water pump to monitor the change. If the differential pressure is greater than a certain value, the occurrence of cavitation is detected (this differential pressure is not a fixed value, the higher the water temperature, the larger the flow rate, the smaller the difference).
(10) When a large amount of running water occurs, increase the number of water supply pumps, so that the flow rate of each pump will be smaller, and the static pressure loss at the pump inlet will be smaller.
2.CE, FDA ,NSF and ISO approval
1 Analysis of causes of cavitation 1.1 Qualitative analysis The water at the suction port of the pump is vaporized into bubbles, which are crushed by high pressure before the pump discharge port. (When the particle of water moves on the impeller flow path, the energy is continuously increased. The position where the bubble is crushed is also unique. Because the bubble's space suddenly “disappearsâ€, it causes a strong impact on the water quality point, causing cavitation damage to the pump impeller, and at the same time causing the pump water pressure to fluctuate. Loss of pressure.
The vaporization condition of the water at the suction port of the water pump is that the pressure is suddenly lower than the saturated steam pressure corresponding to the water temperature at the water. A heating system that is operating steadily, with stable pressure, water temperature, and flow rate, reduces the water pressure at the pump inlet when the following conditions (one of them) are encountered.
(1) The vapor pressure supplied to the deaerator suddenly decreases;
(2) The temperature of the steam supplied to the deaerator suddenly drops;
(3) adding a large amount of cold water to the deaerator at a lower temperature;
(4) The water consumption in the vulcanization workshop suddenly increased;
(5) The pipe resistance in the pipe network except the pump outlet is reduced to a large extent;
(6) There is a sudden large amount of leakage in the pipe network except the pump outlet until the circulation of the deaerator.
Once the pressure at the pump inlet drops below the saturated vapor pressure due to the above conditions, cavitation will occur.
1.2 Quantitative Analysis The drawing is a simplified diagram of the oxygen removal heating system. Take the liquid level in the deaerator as the reference height and define it as the “1-1†interface. The “2-2†interface is at the inlet of the pump.
(1) Installation height calculation Hg=P0/Ïg-P full/Ïg-Δh-Σhf(1-2)(1)
Hg in the formula - calculate the installation height, m;
P0——Vapor pressure in the deaerator, Pa;
P full - the inlet of the hot water pump, that is, the saturated vapor pressure of the water at the "2-2" interface, Pa;
Ρ——liquid density, kg/m3;
G——gravity acceleration, m/S2;
Δh——the NPSH of the pump, m;
Σhf(1-2)——The resistance loss of hot water flowing from the deaerator to the inlet of the pump, m.
When the hot water flows from the deaerator to the inlet of the pump, the change of the water temperature can be neglected, that is, P full = P0, and the NPSH of the pump is given as 3.9 m water column height with the pump data.
The input side pipe resistance loss Σhf(1-2) is estimated to be 1.1 m water column height.
Thus, calculated by (1):
Hg'=-3.9-1.1=-5m water column height This is calculated as water at 20 ° C, when converted to 170 ° C water:
Hg=Ï20gHg'/Ï170g=998.2×(-5)/897.3=-5.5m The height of the water column means that the installation height of the hot water pump is at least 5.5m lower than the operating level of the deaerator.
The actual example is a low 10m, and the installation height difference has a margin of 4.5m (calculated as 170 ° C water).
(2) The oxygen depletion power system in which the cavitation pressure is already in a stable operation state, the steam pressure in the deaerator, the water temperature, the pressure at the inlet of the pump and the water temperature are relatively stable. Assuming that P0 suddenly drops at this time, the system balance is destroyed. However, at the same time that P0 is lowered, the water temperature at the inlet of the pump will never fall immediately. The pressure formed by the existing 10m170°C water is:
h'=10×897.3/998.2≈9m water column height Calculate the drop of P0 by formula (1):
Let [(P0-ΔP)-P satiety]/Ïg_Δh-Σhf(1-2)+h'=0
(P0-ΔP)-P satiety=[-h'+h+Σhf(1-2)]Ïg=[-9+3.9+1.1]×998.2×9.8=-39129.44Pa
∵P0=ΔP-P satiety=P satiety-ΔP-P satiety=-ΔP
∴ΔP=39129Pa
That is, if the water temperature is 170 ° C, that is, the saturated vapor pressure (gauge pressure) is 0.678 MPa, the steady state operation may cause cavitation when the vapor pressure suddenly drops below the gauge pressure of 0.639 MPa.
(3) The amount of water replenishment can cause cavitation. If a large leak occurs in the pipe network, the system balance is destroyed, and the water level of the deaerator will drop rapidly. Therefore, it is necessary to quickly and relatively fill the relatively low temperature soft water.
Set the deaerator steady-state operation water storage capacity:
25m3 (volume) × 0.7 (duty ratio) = 17.5m3
In a short period of time, due to the sudden drop in water level, the water storage capacity was reduced by Vm3, so the low temperature water Vm3 was added.
When cryogenic water is added, P0 will also decrease, the flow rate of steam will increase, and the rate of heat carried will be greater than when the original steady state operation is performed. To simplify the derivation, only the effect of heat exchange of hot and cold water on P0 is considered here, ignoring the heat exchange effect of the increased steam flow.
The supplementary water temperature is 60 °C; the water temperature during steady-state operation is 170 °C; the (17.5 m3-Vm3) water at 170 °C is mixed with the Vm3 water phase at 60 °C (ignoring the difference between the total volume after mixing and 17.5 m3):
ΔQ1=m1 (TCP12-60CP11)
ΔQ2=m2(170CP21-TCP22)
M1=VÏ60=983.2V
M2=Ï170(17.5-V)=897.3(17.5-V)
When the absolute pressure of saturated steam is 0.7377MPa, see the previous calculation, and T is 168.13 °C.
CP11=0.988; CP12=CP22=1.0445; CP21=1.046
Let ΔQ1=ΔQ2 be substituted for each parameter value:
983.2V(1.0445×168.13-60×0.998)=897.3(17.5-V)×(170×1.046-1.0445×168.13)
Solve V=0.31m3
When cold water is added, P0 is lowered and the steam flow rate is increased. It is not simply a mixture of water at two temperatures. It is possible to relax the estimation, and when the hydration of 60 ° C is added to 1 m 3 in a short time, cavitation may be caused.
(4) When the pump outlet flow increases, it can cause cavitation. When the production load suddenly increases, the pipe line on the pipe suddenly decreases or there is a large amount of leakage on the pipe network, which will increase the pump outlet flow.
When these conditions occur, the deaerator liquid level in the steady state operation is suddenly lowered, and cold water is added. The influence of cold water replenishment has been discussed in the past. This factor is not considered here, and only the static pressure at the pump inlet caused by the increase in flow rate is reduced.
The flow rate suddenly increases, the flow rate in the pump inlet pipe increases, the water leakage degree increases, the dynamic pressure head and the resistance loss increase, and the enlarged portion is converted by the static pressure head.
At the flow rate of 150m3 / h, the original input side pipeline loss:
Σhf(1-2)=1.1m water column height, according to Σhf=ξu2/2
U=Q/S=150÷3600/π÷4×0.082≈8.29m/S
ξ=2Σhf/U2≈0.032
It is known in the past that the existing 10m installation height difference is equivalent to 9m water column height. The 9m water column height is deducted from the NPSH and the original resistance loss is 5m water column height, and the remaining 4m water column height.
Let ΔU2/2+ξΔU2/2=4
ΔU≈2.784m/s
Also ΔQ=ΔUS=2.784×π/4×0.082=0.014m3/S=50.38m3/h
That is, if the sudden increase in flow rate is greater than or equal to 50.38 m3/h, there is a possibility of cavitation.
You can use a sentence to summarize the three quantitative analysis conclusions: half a vapor pressure, square water, and fifty flow can be a ghost.
2 Countermeasures for Preventing and Eliminating Cavitation According to the above analysis, the cause of cavitation is a sudden decrease in vapor pressure in the deaerator, a sudden decrease in water temperature, or a sudden increase in pump flow. Therefore, the following countermeasures are proposed:
(1) If the steam source pressure and supply capacity are rich, the deaeration pressure automatic control device should be installed to ensure the stability of P0.
(2) If the steam source pressure and supply are not rich, the pressure automatic control device should be equipped after the pressure increase, to ensure the stability of P0.
(3) Reduce the number of vulcanizers and tanks entering the line at the same time, that is, reduce the flow rate growth rate.
(4) Reduce to prevent pipeline leakage.
(5) Improve the water temperature of the hydration water source.
(6) Under the premise of ensuring the effective deaeration heat transfer effect of Zui, the deaerator liquid level control point should be set as high as possible.
(7) The water supply capacity of the pump should be greater than the production load to consider the local leakage problem.
(8) Set the exhaust valve at the pump outlet, and when the cavitation occurs, open the valve to discharge the generated vapor. Or it can increase the steam supply pressure of the deaerator at the same time.
(9) Set the differential pressure measurement display instrument between the steam pressure in the deaerator and the water pressure at the inlet of the water pump to monitor the change. If the differential pressure is greater than a certain value, the occurrence of cavitation is detected (this differential pressure is not a fixed value, the higher the water temperature, the larger the flow rate, the smaller the difference).
(10) When a large amount of running water occurs, increase the number of water supply pumps, so that the flow rate of each pump will be smaller, and the static pressure loss at the pump inlet will be smaller.
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