In many industries, centrifugal pumps are often used to transport viscous fluid. For this reason, we often encounter the following problems: how much is the maximum viscosity that the centrifugal pump can handle; What is the minimum viscosity that needs to be corrected for the performance of the centrifugal pump. This involves the size of the pump (pumping flow), the specific speed (the lower the specific speed, the greater the disk friction loss), application (system pressure requirements), economy, maintainability, etc.
This article will introduce in detail the influence of viscosity on the performance of centrifugal pump, the determination of viscosity correction coefficient, and the matters needing attention in practical engineering application in combination with relevant standards and engineering practice experience, for reference only.
1. Maximum viscosity that centrifugal pump can handle
In some foreign references, the maximum viscosity limit that the centrifugal pump can handle is set as 3000~3300cSt (centisea, equivalent to mm ²/ s). On this issue, C.E. Petersen had an earlier technical paper (published at the meeting of the Pacific Energy Association in September 1982) and put forward an argument that the maximum viscosity that the centrifugal pump can handle can be calculated by the size of the pump outlet nozzle, as shown in Formula (1):
Where, Vm is the maximum allowable kinematic viscosity SSU (Saybolt universal viscosity) of the pump; D is the diameter of pump outlet nozzle (inch).
In practical engineering practice, this formula can be used as a rule of thumb for reference. Guan Xingfan’s Modern Pump Theory and Design holds that: in general, the vane pump is suitable for conveying with viscosity less than 150cSt, but for centrifugal pumps with NPSHR far less than NSHA, it can be used for viscosity of 500~600cSt; When the viscosity is greater than 650cSt, the performance of the centrifugal pump will decline greatly and it is not suitable for use. However, because the centrifugal pump is continuous and pulsatile compared with the volumetric pump, and does not need a safety valve and the flow regulation is simple, it is also common to use centrifugal pumps in chemical production where the viscosity reaches 1000cSt. The economic application viscosity of centrifugal pump is usually limited to about 500ct, which largely depends on the size and application of the pump.
2. The influence of viscosity on the performance of centrifugal pump
The pressure loss, impeller friction and internal leakage loss in the impeller and guide vane/volute flow passage of centrifugal pump largely depend on the viscosity of the pumped liquid. Therefore, when pumping liquid with high viscosity, the performance determined with water will lose its effectiveness The viscosity of medium has a great impact on the performance of centrifugal pump. Compared with water, the higher the viscosity of liquid, the greater the flow and head loss of a given pump at a given speed. Therefore, the optimal efficiency point of the pump will move towards a lower flow, the flow and head will decrease, the power consumption will increase, and the efficiency will decrease. The vast majority of domestic and foreign literature and standards as well as engineering practice experience show that viscosity has little effect on the head at the pump shut-off point.
3. Determination of viscosity correction coefficient
When the viscosity exceeds 20cSt, the effect of viscosity on the performance of the pump is obvious. Therefore, in practical engineering applications, when the viscosity reaches 20cSt, the performance of the centrifugal pump needs to be corrected. However, when the viscosity is in the range of 5~20 cSt, its performance and the motor matching power must be checked.
When pumping viscous medium, it is necessary to modify the characteristic curve when pumping water.
At present, the formulas, charts and correction steps adopted by domestic and foreign standards (such as GB/Z 32458 , ISO/TR 17766 , etc.) for viscous liquids are basically from the standards of the American Hydraulic Institute. When the performance of the pump conveying medium is known to be water, the American Hydraulic Institute standard ANSI/HI9.6.7-2015  gives detailed correction steps and relevant calculation formulas.
4. Engineering application experience
Since the development of centrifugal pumps, the predecessors of the pump industry have summarized a variety of methods to modify the performance of centrifugal pumps from water to viscous media, each with advantages and disadvantages:
4.1 A.J.Stepanoff Model
4.2 Paciga method
4.3 American Hydraulic Institute
4.4 Germany KSB method
The conversion chart and calculation formula are only applicable to homogeneous viscous liquid, which is commonly called Newtonian liquid (such as lubricating oil), but not to non Newtonian liquid (such as liquid with fiber, cream, pulp, coal water mixture liquid, etc.)
5.2 Applicable flow
Reading is not practical.
At present, the correction formulas and charts at home and abroad are the summary of empirical data, which will be restricted by test conditions. Therefore, in practical engineering applications, special attention should be paid to: different correction formulas or charts should be used for different flow ranges.
5.3 Applicable pump type
The modified formulas and charts are only applicable to centrifugal pumps with conventional hydraulic design, open or closed impellers, and operating near the optimal efficiency point (rather than at the far end of the pump curve). Pumps specially designed for viscous or heterogeneous liquids cannot use these formulas and charts.
5.4 Applicable cavitation safety margin
When pumping liquid with high viscosity, NPSHA and NPSH3 are required to have sufficient cavitation safety margin, which is higher than that specified in some standards and specifications (such as ANSI/HI 9.6.1-2012 ).
1) The influence of viscosity on the performance of centrifugal pump is difficult to be calculated by accurate formula or checked by chart, and can only be converted by the curve obtained from test. Therefore, in practical engineering applications, when selecting the driving equipment (with power), it should consider to reserve sufficient safety margin.
2) For liquids with high viscosity at room temperature, if the pump (such as the high-temperature slurry pump of the catalytic cracking unit in the refinery) is started at a temperature lower than the normal operating temperature, the mechanical design of the pump (such as the strength of the pump shaft) and the selection of the drive and coupling should take into account the influence of the torque generated by the increase in viscosity. At the same time, it must be noted that:
① In order to reduce leakage points (possible accidents), single-stage cantilever pump shall be used as far as possible;
② The pump shell shall be equipped with insulation jacket or heat tracing device to prevent medium solidification during short-term shutdown;
③ If the shutdown time is long, the medium in the shell shall be emptied and purged;
④ In order to prevent the pump from being difficult to disassemble due to the solidification of viscous medium at normal temperature, the fasteners on the pump housing should be slowly loosened before the medium temperature drops to normal temperature (pay attention to personnel protection to avoid scalding), so that the pump body and pump cover can be slowly separated.
3) Pump with higher specific speed shall be selected as far as possible to transport viscous liquid, so as to reduce the impact of viscous liquid on its performance and improve the efficiency of viscous pump.
The viscosity of medium has a great influence on the performance of centrifugal pump. The influence of viscosity on the performance of centrifugal pump is difficult to be calculated by accurate formula or checked out by chart, so appropriate methods should be selected to correct the performance of the pump.
Only when the actual viscosity of the pumped medium is known, can it be accurately selected to avoid many on-site problems caused by the large difference between the viscosity provided and the actual viscosity.
Post time: Dec-27-2022