During the tire vulcanization process, it is extremely important that the stable supply and circulation of the overpressure water under the internal pressure prevails. In its complete closed loop system, the hot water circulation pump is as important as the heart of the human body and must not malfunction. However, the actual situation is inevitable accidental. Only cavitation, not only cause damage to the pump, in particular, can lead to large pressure fluctuations in the circulatory system, and even pressure loss at the moment, the tire during the initial vulcanization caused a fatal injury. Thus, it is necessary to recognize the cause of cavitation and take effective measures to prevent or properly solve the problem.
1 Cavitation Cause Analysis
1.1 Qualitative analysis
The water at the suction inlet of the pump is vaporized into bubbles which are crushed by the high pressure before the discharge of the pump (the mass of water moves continuously in the flow path of the impeller, increasing the energy, and the position where the bubble is crushed is also Only), due to the sudden disappearance of the bubble occupying, causing a strong impact of the water quality point, resulting in cavitation damage to the pump impeller, while the pump water pressure fluctuations, resulting in serious loss of pressure.
Water vaporization at the suction inlet of the pump is such that its pressure suddenly drops below the saturated vapor pressure corresponding to the water temperature there. A stable heating system, pressure, water temperature, flow and stability, in the event of one of the following conditions, it will reduce the water pressure at the pump inlet.
(1) for oxygen supply
(2) the temperature of the steam supplied to the deaerator suddenly decreases;
(3) a lot of deaerator to add a lower temperature of cold water;
(4) vulcanization plant water suddenly increased;
(5) Outside the pump outlet, the resistance of the pipeline suddenly and drastically decreases until the deaerator is recycled;
(6) Outside the pump outlet, there is a sudden large amount of leakage in the pipe network to be recycled back to the deaerator.
Cavitation can occur if the pressure at the pump inlet falls below the saturated vapor pressure due to the above conditions.
1.2 quantitative analysis
The figure is a schematic diagram of the oxygen removal heating system. Take deaerator liquid level as the reference height, defined as "1-1" interface. Pump entrance for the "2-2" interface.
(1) installation height calculation
Hg = P0 / Ïg-P saturated / Ïg -Δh-Σhf (1-2) (1)
Where Hg - calculate the installation height, m;
P0 - vapor pressure in the deaerator, Pa;
P saturated - hot water pump inlet, that is, "2-2" interface at the water vapor pressure, Pa;
Ï - liquid density, kg / m3;
g - acceleration of gravity, m / S2;
Δh - Pump NPSH, m;
Σhf (1-2) - resistance loss of hot water from deaerator to pump inlet, m.
When the hot water flows from the deaerator to the inlet of the water pump, the change of water temperature can be neglected. That is to say, P full = P0 and the pump cavitation allowance Δh, which is 3.9m water column high with pump data.
Input pipeline resistance loss Σhf (1-2) is estimated as 1.1m water column height.
Thus, from (1) type of calculation:
Hg '= - 3.9-1.1 = -5 m water column height
This is calculated according to 20 ℃ water, folded into 170 ℃ water:
Hg = Ï20gHg '/ Ï170g = 998.2 × (-5) /897.3=-5.5m Water column height
In other words, the installation height of the hot water pump is at least 5.5 m below the minimum operating level of the deaerator.
The actual example is low 10m, the installation elevation still 4.5m margin (calculated according to 170 ℃ water).
(2) deaerator pressure changes can occur cavitation
The oxygen scavenging system, which is already in stable operation, has a relatively stable vapor pressure, water temperature, pressure at the pump inlet, and water temperature. Suppose P0 suddenly decreased at this time, the system balance will be destroyed. However, P0 decreases at the same time, the pump inlet water temperature will never immediately decline, the existing pressure of 10m170 ℃ water is:
h '= 10 × 897.3 / 998.2 ≈ 9m high water column
Use (1) formula to calculate the amount of P0 decline:
Let [(P0-ΔP) -P sat] / Ïg-Δh-Σhf (1-2) + h '= 0
(P0 - ΔP) - P = [- h '+ h + Σhf (1-2)] Ïg = [-9 + 3.9 + 1.1] × 998.2 × 9.8 = -39129.44 Pa
∵P0 = ΔP -P full = P full -ΔP -P full = -ΔP
∴ΔP = 39129Pa
That is, if the water temperature is at 170 ° C, that is, when the saturated vapor pressure (gauge pressure) is steady at 0.678 MPa, the cavitation may occur when the vapor pressure suddenly drops below 0.639 MPa.
(3) Replenishment to achieve how much cavitation can occur
In the event of a large leak in the pipeline network, the system will be destroyed and the deaerator water level will drop rapidly. Therefore, a large quantity of water should be added to the relatively low temperature soft water quickly.
Set the steady state deaerator water storage capacity is:
25m3 (volume) × 0.7 (duty ratio) = 17.5m3
In a relatively short period of time, due to the sudden drop in water level, the amount of water stored has been reduced by Vm3, so that Vm3 is added to the low temperature water.
When the low temperature water is added, P0 will also be reduced, the steam flow will increase, the entrainment heat rate will be greater than the original steady-state operation. To simplify the derivation, only the heat exchange of hot and cold water is considered here.
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