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渣漿泵汽蝕判別式分幾種
由式(1- 24)和式(1- 26)可以看出:
(1)NPSHa = NPSHr時，是泵發(fā)生汽蝕的臨界值，通過汽蝕實驗測定的就是這個臨界值。
(2)NPSH,>NPSH,時.泵內不發(fā)生汽蝕。但為了安全起見，在設計和使用過程中，一般在NPSH的基礎上加上一個安全余量.稱為允許汽蝕余量用NPSHJ表示[NPSH =NPSH. +0.3。在設計和使用泵的過程中，為保證不發(fā)生汽的NSH,要大FINPS:圖1-20給出了泵的有效汽蝕余量和必需汽蝕余量隨流量的變化曲線。由圖可則，隨著流量的增加，泵裝置提供的有效汽蝕余量在減少，而泵本身所必需的內油余質在均加。
吸上真空度和泵的安裝高度
1.吸上真空度
由于裝置的有效汽蝕余量所表示的泵入口的液體能量不易直接測量出來，而泵入口法蘭處的真空表讀數(以液柱高度表示)可直接測量出來，可用于監(jiān)測泵是否正常運行。吸上真空度就是泵入口處的真空表讀數，用Hs來表示，單位為m.其計算公式為:
Hs=Pa/pg- Ps/pg
式中 Pa----泵使用地點的當地大氣壓.Pa;
Ps----泵入口處的絕對壓力.Pa。
吸上真空度可用于表示有效汽蝕余量，將式(1-27)與式(1-23)聯立可得二者的關系式為:
NPSHa=Pa-Pv/pg+cs/2g+Hs
Hs越大，即泵入口處壓力Ps越小，則NPSHa就越小，泵越容易發(fā)生汽蝕。
[例1-1]如圖1- 19所示,用一臺離心泵輸送水，泵吸入口直徑為200mm,安裝高度Zg= 5m,流量為280m3 /h,此時泵的允許汽蝕余量[NPSH:]=4.4m。已知液面下降速度CA≈0,液面壓力為大氣壓pa=1.013X 105 Pa,該液體在罐內溫度條件下的飽和蒸氣壓p.=2. 35X10Pa,吸入管路阻力損失hf=0.5m。問該泵是否汽蝕?泵的吸上真空度Hs是多少?
解:根據已知條件可知，泵的有效汽蝕余量NPSH.為:
用汽蝕試驗可得到剛發(fā)生汽蝕時的吸上真空度，稱為最大吸上真空度,用Hx表示。運行時若真空表讀數H,小于Hmnx ,說明不發(fā)生汽蝕。但為安全起見,在設計和使用過程中，一般用允許吸上真空度[H.]作為泵的汽蝕特性參數,[H,]= Hsmax-0.3,該值需要由泵制造廠家提供。例如,IS65- 50- 125型單級單吸懸臂式離心泵的[H,]為7m,則泵工作時泵入口處的真空表讀數就應該小于7m,這樣泵才是在安全狀態(tài)下工作，不會發(fā)生汽蝕。
泵樣本上的[H;]值，是在標準大氣壓下抽送20°C 清水時測得的。如果泵的運行條件與上述條件不同，不能直接采用樣本上提供的[H,]值，應將其換算成泵使用現場條件下的[H]。換算關系如下:
泵的安裝高度
泵的安裝高度是由用戶自己確定的，安裝高度確定的正確與否，決定了泵工作時是否發(fā)生汽蝕。因此，用戶需要根據泵制造廠家給定的汽蝕參數[NPSH,]和[H]來確定允許安裝高度[zg]。實際安裝高度定要小于[zg]，這樣 才能保證泵工作時不發(fā)生汽蝕。
1)根據[NPSH,]確定[z]由式(1 - 24)得:
剛汽蝕時,NPSH. = NPSH,，則此時的安裝高度是最大安裝高度qgmx，有:
為安全起見，一般采用允許安裝高度，用x]表示,1ξsJ=zm -0.3.則:
根據Hs確定 的安裝高度
將式(1-23)與式(1 - 27)聯立，可得：
剛汽蝕時:
故得到允許安裝高度為:
若吸入罐液面上的壓力pA為當地大氣壓，即pa=p.上式簡化為:
采用式(1- 30)進行計算時需要注意，一般來說[H,]是隨著流量的增加而下降的(流量增
加，流動損失和速度水頭均增加，使得ps降低)，如圖1-21所示。因此，必須以泵在運行中可能出現的最大流量所對應的[Hs]為準，而泵銘牌上的[Hs]是指最高效率點的值。
[例1-2]用一臺水泵從湖中將水泵送到水池，流量Q為0.2m3/s，湖面高程(海拔)500m，水池水面高程530m,吸水管和排水管直徑d為500mm,長度分別為l吸=10m和l排= 100m，沿程摩阻系數λ=0.22,水泵允許吸上真空度[Hs]=4.5m,已知底閥的局部阻力系數ξ =2. 5,90°彎頭的局部阻力系數ξ b.=0.3，水泵入口前變徑的局部阻力系數ζ。=0. 1，設水溫為10C，試確定水泵允許安裝高度。
解:泵的允許安裝高度為:
由于該泵運行條件不是在標準大氣壓下抽送20C清水，因此需對水泵的允許吸上真空度進行修正。查表1-1, 海拔500m時,大氣壓為9.7m;查表1-2,水溫為10C時，飽和蒸氣壓
為0.12x 10-Pa(0.12m)，則:
吸入管內的流速為:渣漿泵廠家
所以泵的允許安裝高度為:
[例1一3]如圖1一19(d)所示，用一臺離心泵輸送罐內的液體,已知罐內液面下降速度CA≈0,罐內液面壓力pA= 10.1m,該液體在罐內溫度條件下的飽和蒸氣壓pv=6.3m,吸入管路阻力損失hf=2m,泵的允許必需汽蝕余量[NPSH,]=7m。若保證不汽蝕，則罐的最低液位是多少?
解:根據已知條件可知，泵的允許安裝高度為:
說明當罐最低液位為5.2m時，可以保證泵不發(fā)生汽蝕。
Several types of cavitation discriminant for slurry pump
It can be seen from formula (1-24) and formula (1-26):
(1) When npsha = NPSHr, it is the critical value of pump cavitation, which is determined by cavitation experiment.
(2) When NPSH > NPSH, no cavitation occurs in the pump. However, for the sake of safety, in the process of design and use, a safety margin is generally added on the basis of NPSH, which is called allowable NPSH expressed by npshj [NPSH = NPSH. + 0.3]. In the process of designing and using the pump, in order to ensure the NSH without steam, finps: figure 1-20 shows the change curve of effective NPSH and necessary NPSH with flow. It can be seen from the figure that with the increase of flow, the effective NPSH provided by the pump device is decreasing, while the necessary internal oil surplus of the pump itself is being added.
Suction vacuum and pump installation height
1. Vacuum degree
Because the liquid energy at the pump inlet indicated by the effective NPSH of the device is not easy to be measured directly, and the reading of the vacuum gauge at the pump inlet flange (indicated by the liquid column height) can be measured directly, which can be used to monitor whether the pump operates normally. Suction vacuum is the reading of the vacuum gauge at the pump inlet, expressed in HS, in M. its calculation formula is:
Hs=Pa/pg- Ps/pg
Where PA is the local atmospheric pressure at the place where the pump is used. Pa;
PS ---- absolute pressure at pump inlet. PA.
The suction vacuum degree can be used to express the effective NPSH, and the relationship between equation (1-27) and equation (1-23) can be obtained as follows:
NPSHa=Pa-Pv/pg+cs/2g+Hs
The larger HS is, the smaller PS is at the pump inlet, the smaller npsha is, and the more likely the pump is to cavitation.
[example 1-1] as shown in Figure 1-19, a centrifugal pump is used to transport water. The diameter of the pump suction port is 200mm, the installation height is ZG = 5m, and the flow is 280M3 / h. at this time, the allowable NPSH of the pump [NPSH:] = 4.4m. It is known that the falling speed of liquid level Ca ≈ 0, the pressure of liquid level PA = 1.013 x 105 PA, the saturated vapor pressure P. = 2.35x10pa and the resistance loss HF = 0.5m of suction pipeline under the temperature condition in the tank. Is the pump cavitating? What is the suction vacuum HS of the pump?
Solution: according to the known conditions, the effective NPSH of the pump is:
When cavitation happens, the suction vacuum degree can be obtained by cavitation test, which is called the maximum suction vacuum degree, expressed by HX. If the reading h of the vacuum gauge is less than hmnx during operation, it means no cavitation. However, for the sake of safety, in the design and use process, the allowable suction vacuum degree [h.] is generally used as the cavitation characteristic parameter of the pump, [h,] = hsmax-0.3, which needs to be provided by the pump manufacturer. For example, if the [h,] of is65-50-125 single-stage single suction cantilever centrifugal pump is 7m, the reading of vacuum gauge at the pump inlet should be less than 7m when the pump is working, so that the pump is working in a safe state without cavitation.
The value of [h;] on the pump sample is measured when 20 ° C clean water is pumped under the standard atmospheric pressure. If the operating conditions of the pump are different from the above conditions, the value of [h,] provided on the sample cannot be used directly, it shall be converted to the value of [H] under the field conditions of pump use. The conversion relationship is as follows:
Installation height of pump
The installation height of the pump is determined by the user. Whether the installation height is correct or not determines whether cavitation occurs during the operation of the pump. Therefore, the user needs to determine the allowable installation height [ZG] according to the cavitation parameters [NPSH,] and [H] given by the pump manufacturer. The actual installation height must be less than [ZG], so as to ensure that no cavitation occurs during the operation of the pump.
1) According to [NPSH,] it is determined that [Z] is obtained from equation (1-24):
When NPSH. = NPSH, the installation height is the maximum installation height qgmx, including:
For the sake of safety, the allowable installation height is generally adopted, expressed by x], 1 ξ SJ = ZM - 0.3
Installation height according to HS
If equation (1-23) and equation (1-27) are combined, we can get:
In case of rigid cavitation:
Therefore, the allowable installation height is:
If the pressure PA on the liquid level of the suction tank is the local atmospheric pressure, that is, PA = P. the above formula is simplified as:
When using formula (1-30) for calculation, it should be noted that [h,] generally decreases with the increase of flow (flow increases
In addition, both flow loss and velocity head increase, which makes PS decrease), as shown in Figure 1-21. Therefore, it must be based on the [HS] corresponding to the maximum flow that may occur during the operation of the pump, and the [HS] on the pump nameplate refers to the value of the highest efficiency point.
[example 1-2] a water pump is used to pump water from the lake to the pool, with the flow Q of 0.2m3/s, the lake surface elevation (altitude) of 500m, the pool surface elevation of 530m, the diameter D of the suction pipe and the drainage pipe of 500mm, the length of L suction = 10m and l discharge = 100m, the friction coefficient along the way λ = 0.22, the allowable suction vacuum degree of the pump [HS] = 4.5m, and the local resistance coefficient ξ = 2.5, the local resistance coefficient ξ B. = 0.3 of the 90 ° elbow , the local resistance coefficient ζ of the variable diameter in front of the water pump inlet. =0.1, set the water temperature to 10C, and try to determine the allowable installation height of the water pump.
Solution: the allowable installation height of the pump is:
Since the operation condition of the pump is not to pump 20c clean water under standard atmospheric pressure, it is necessary to correct the allowable suction vacuum of the pump. Look up Table 1-1, when the altitude is 500m, the atmospheric pressure is 9.7M; look up table 1-2, when the water temperature is 10C, the saturated vapor pressure
0.12 x 10-pa (0.12M), then:
The flow rate in the suction pipe is: slurry pump manufacturer
Therefore, the allowable installation height of the pump is:
[example 1-3] as shown in Figure 1-19 (d), a centrifugal pump is used to transport the liquid in the tank. It is known that the falling speed of the liquid level in the tank Ca ≈ 0, the pressure of the liquid level in the tank PA = 10.1m, the saturated vapor pressure of the liquid at the temperature in the tank PV = 6.3m, the resistance loss of the suction pipeline HF = 2m, and the allowable necessary NPSH of the pump [NPSH,] = 7m. If no cavitation is guaranteed, what is the minimum liquid level of the tank?
Solution: according to the known conditions, the allowable installation height of the pump is:
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