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介質(zhì)黏性對渣漿泵性能的影響
在油氣儲運工程中，有許多油品的黏度上與清水不同，用離心泵輸送這些油品時,因為黏度的增加，使得泵內(nèi)部的能量損失加大，揚程和流量都要減小，并導(dǎo)致效率下降，而軸功率卻增加，泵的特性曲線發(fā)生變化，如圖1-34所示。當輸送液體運動粘度v>20mm2/s時，泵的特性曲線就需要換算，為此應(yīng)該號慮不同黏度下泵性能的換算方法。
2. 特性曲線換算方法
目前，還沒有成熟的純理論推導(dǎo)方法能精確地確定出抽送黏性液體的泵性能曲線。常用的換算方法有前蘇聯(lián)國家石油機械研究設(shè)計院的標準曲線法、美國水力協(xié)會的圖線換算法和德國KSB公司的換算方法。其共同特征都是在大量實驗的基礎(chǔ)上,給出各性能曲線的修正系數(shù)圖并以此進行換算。
目前，國內(nèi)多采用美國水力協(xié)會方法進行黏度換算。當已知某離心泵輸送20C清水的特性曲線時,利用下列關(guān)系式換算成輸送黏性液體的特性曲線:
使用本方法時，不需知道離心泵葉輪尺寸，只需知道被輸送液體的運動黏度和輸送清水時ηmsx工祝點的Q0和Ho (多級泵應(yīng)取單級葉輪的揚程)，然后利用圖1-35或圖1-36查出各換算系數(shù)，并將換算系數(shù)代入式(1-44) 即可。
使用圖1-35或圖1-36時，在橫坐標上取Q=Q0點作垂線，與H=H0的斜線相交，自交點作水平線與所輸送液體的運動黏度為v的斜線相交，從該交點再作垂線與圖上方的各換算曲線相交，得到各換算系數(shù)。
應(yīng)用此法換算范圍較寬: (0.6~1.2) Q.，換算誤差在v<865mm2/s時不超過士5%，特別當v<400mm2/s時更準確。
應(yīng)當注意，該二圖只適用于一般結(jié)構(gòu)的離心泵，且在不發(fā)生汽蝕的工況下進行換算。它不適用于混流泵、軸流泵，也不適用于含有雜質(zhì)的非均相液體。此外，圖中各曲線不能用外推法延伸使用。
美國水力協(xié)會對世界范圍內(nèi)大量的實驗數(shù)據(jù)進行了分析和研究，給出了近似的換算公式，形成了技術(shù)報告，并由國際標準化組織在2005年發(fā)布了此報告(ISO/TR 17766-2005)。
根據(jù)ISO/TR 17766 的公式計算法對性能曲線進行換算時，公式計算法的適用范圍為:介質(zhì)必須是牛頓型流體;泵只能是離心泵，不適用于混流泵、軸流泵和旋渦泵;運動黏度v= 1~4000mm2/s;比轉(zhuǎn)速ns ≤60;泵有效汽蝕余量NPSH.足夠大。
首先引入黏度換算過程參數(shù)B,其定義為:
B的范圍為B< 40(B≤1時，則設(shè)定CH=1,Cq=1)。
流換算系數(shù)Co的計算公式為：
揚程換算系數(shù)Cu的計算公式為:
效率換算系數(shù)C,的計算公式為:
[例1-4]某泵在轉(zhuǎn)速n= 1475r/min輸送清水時在4個工況點0. 6Qur w、0. 8QeF-W、1.0QEP-w.1.2QBEP-w的流量、揚程和效率，見表1-4。現(xiàn)用該泵輸送原油，密度ρ=956kg/m3，運動黏度vi = 200mm2/s。采用美國水力協(xié)會方法對性能參數(shù)進行換算(分別采用查圖法和公式計算法)。
從圖1-35橫坐標的流量Qw處垂直向上，直到與給定的揚程Hw相交于一點,從此相交點水平向右或向左,再與原油介質(zhì)的運動黏度v.i相交，并由此垂直向上，與各換算系數(shù)CH、Cq、C,相交，即可得出各換算系數(shù)，即Cq=0.93,C,=0.63,CH在4個工況點0.6QsEP-w、0.8QkE w.1. 0QwEP- w.1.2Q w的值分別為0.95 .0. 93.0.91.0.87。泵軸功率P的計算公式為:
P=HQs/367η
根據(jù)式(1- 4)式(1- 50)可計算出輸送原油時離心泵的流量.揚程、效率和軸功率，見表
根據(jù)ISO/TR 7766的公式計算法對性能曲線進行換算。把Vu = 20mm2/s，Q =139m3/h.N= 1475/min.Hur w= 48m帶入式(1- 46)得:
流量換算系數(shù):
根據(jù)公式(1 - 48)計算可求得揚程換算系數(shù)CH在4個工況點0. 6QBEP-w、0.8QuEP- w、1. 0QeEP- w、1. 2QEP- w的值分別為0. 930、0. 913、0. 897、0.882。
根據(jù)公式(1- 49)計算可求效率換算系數(shù)C,為0. 641。渣漿泵
把流量、揚程、效率的換算系數(shù)帶入式(1-44)、式(1- 50)可計算出輸送原油時離心泵的流量、揚程、效率和軸功率，見表1- 6。
1-5。
Effect of medium viscosity on the performance of slurry pump
In the oil and gas storage and transportation engineering, the viscosity of many oil products is different from that of clean water. When the centrifugal pump is used to transport these oil products, the increase of viscosity will increase the internal energy loss of the pump, reduce the head and flow, and lead to the decrease of efficiency. However, the shaft power increases, and the characteristic curve of the pump changes, as shown in Figure 1-34. When the kinematic viscosity of the conveying liquid V > 20mm2 / s, the characteristic curve of the pump needs to be converted. Therefore, the conversion method of pump performance under different viscosity should be considered.
2. Conversion method of characteristic curve
At present, there is no mature theoretical derivation method to accurately determine the performance curve of the pump pumping viscous liquid. The commonly used conversion methods are the standard curve method of the National Petroleum Machinery Research and Design Institute of the former Soviet Union, the chart line conversion algorithm of the American Hydraulic Association and the conversion method of KSB company of Germany. The common feature is that on the basis of a large number of experiments, the correction coefficient diagrams of each performance curve are given and converted.
At present, the method of American Hydraulic association is widely used in viscosity conversion in China. When the characteristic curve of 20c clear water delivered by a centrifugal pump is known, it can be converted into the characteristic curve of viscous liquid delivered by the following formula:
When using this method, it is not necessary to know the impeller size of the centrifugal pump, only the kinematic viscosity of the delivered liquid and Q0 and ho of η MSX working point when delivering clean water (the lift of single-stage impeller should be taken for multi-stage pump), then use figure 1-35 or figure 1-36 to find out each conversion coefficient, and substitute the conversion coefficient into equation (1-44).
When using figure 1-35 or figure 1-36, take point q = Q0 on the abscissa as the vertical line, intersect with the oblique line of H = H0, and self intersection point as the horizontal line intersects with the oblique line of the kinematic viscosity of the liquid delivered as v. from this intersection point, make the vertical line intersect with each conversion curve above the figure to obtain each conversion coefficient.
The conversion range is wide: (0.6 ~ 1.2) Q. the conversion error is less than ± 5% when V < 865mm2 / s, especially when V < 400mm2 / s.
It should be noted that the second drawing is only applicable to the centrifugal pump of general structure, and the conversion is carried out under the condition of no cavitation. It is not suitable for mixed flow pump, axial flow pump, and heterogeneous liquid containing impurities. In addition, the curves in the figure cannot be extended by extrapolation.
The American Hydraulic association has analyzed and studied a large number of experimental data in the world, given approximate conversion formulas, and formed a technical report, which was issued by the international organization for Standardization in 2005 (ISO / TR 17766-2005).
When the performance curve is converted according to the formula calculation method of ISO / TR 17766, the application scope of the formula calculation method is: the medium must be Newtonian fluid; the pump can only be centrifugal pump, which is not suitable for mixed flow pump, axial flow pump and vortex pump; the kinematic viscosity v = 1 ~ 4000mm2 / S; the specific speed ns ≤ 60; the effective NPSH of the pump is large enough.
Firstly, parameter B of viscosity conversion process is introduced, which is defined as:
The range of B is B < 40 (when B ≤ 1, set ch = 1, CQ = 1).
The calculation formula of flow conversion coefficient CO is:
The calculation formula of head conversion coefficient Cu is as follows:
The calculation formula of efficiency conversion coefficient C is:
[example 1-4] see table 1-4 for the flow, lift and efficiency of 0.6qur W, 0.8qef-w and 1.0qep-w.1.2qbep-w at four operating points when the speed of a pump is n = 1475r / min to deliver clean water. The pump is now used to transport crude oil with density ρ = 956kg / m3 and kinematic viscosity VI = 200mm2 / s. The performance parameters are converted by the method of American Hydraulic Association (using the method of looking up the chart and the method of formula calculation respectively).
From the flow QW in the abscissa of Fig. 1-35, it is vertically upward until it intersects with the given lift HW at a point, from which it is horizontally right or left, and then intersects with the kinematic viscosity V.I of the crude oil medium, and then vertically upward, intersects with each conversion coefficient ch, CQ, C, to obtain each conversion coefficient, that is, CQ = 0.93, C, = 0.63, The values of CH at four operating points are 0.6qsep-w, 0.8qke w.1.0qwp-w.1.2qw, respectively, 0.95.0.93.0.91.0.87. The calculation formula of pump shaft power P is:
P=HQs/367 ETA
According to formula (1-4) (1-50), the flow, head, efficiency and shaft power of the centrifugal pump can be calculated when delivering crude oil, as shown in table
According to the formula calculation method of ISO / TR 7766, the performance curve is converted. Take Vu = 20mm2 / s, q = 139m3 / h, n = 1475 / min, HuR w = 48m into the formula (1-46)
Flow conversion factor:
According to the formula (1-48), the values of head conversion coefficient ch at four working points, 0.6qbep-w, 0.8qep-w, 1.0qeep-w and 1.2qep-w, are 0.930, 0.913, 0.897 and 0.882 respectively.
According to the formula (1-49), the efficiency conversion coefficient C is 0.641. Slurry pump
Take the conversion coefficient of flow, head and efficiency into formula (1-44) and formula (1-50) to calculate the flow, head, efficiency and shaft power of centrifugal pump when delivering crude oil, as shown in table 1-6.
1-5.
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