Selection of structural parameters of centrifuge settlement zone and analysis of influencing factors

Abstract: There are many parameters that affect the separation performance of the centrifuge. These parameters are not independent but affect each other and restrict each other, and must be determined through comprehensive analysis. The article analyzes the influence of the structural parameters of the settlement area on the separation performance of the centrifuge, and gives the selection range of the structural parameters, which provides a reference for the further design of the drilling fluid high-speed centrifuge.

Keywords: drilling fluid; centrifuge; settlement; structural parameters; analysis

As a drilling fluid solid control equipment, the centrifuge is designed and used reasonably, which can effectively control the density of the drilling fluid, remove the harmful solid phase in the drilling fluid, improve the mechanical drilling speed, increase the service life of the subsequent equipment, and save mud The dosage of drugs and the protection of oil and gas layers are of great significance. China has only begun to use medium-speed centrifuges since the mid-1990s, and has recognized the huge role of centrifuges in the solid phase control of drilling fluids, especially in some special process wells. With the increasing difficulty of the drilling process, the requirements for solid control continue to increase, and only the medium-speed centrifuge cannot solve the solid-phase control problems of drilling fluids today. To adapt to the development of new drilling technology and improve the solids control level of drilling fluids, the performance of the centrifuge must be improved. In particular, high-speed centrifuges with large displacement and high reliability must be developed. There are many factors that affect the performance of the centrifuge, and the selection of structural parameters is a very important aspect. It must be selected according to the requirements of the centrifugal separation process and the principle of economic balance. This article will focus on the analysis of the drilling fluid high-speed centrifuge, the selection of the structural parameters of the settlement area and the influencing factors, to lay the foundation for the further design of high-speed centrifuges [1-2].

Analysis of the Drum Half Cone Angle α

The half cone angle α of the drum is the most important parameter in the structural design of the centrifuge, and its sensitivity is the largest. This is because when the centrifuge separates solid-liquid materials with poor density (solid phase and liquid phase in drilling fluid), the separated sediment is pushed out of the liquid pool by the screw conveyor along the cone surface of the drum, and then passes through the drying section of the cone surface ( After the dehydration zone), it is discharged from the slag outlet. Due to the high speed of the centrifuge for solid-liquid separation, the sediment will be subjected to a reverse flow force proportional to the centrifugal force when moving along the cone surface. This is similar to a river, where the current is narrow where the river bed is narrow, while the current is slow where the river bed is wide. If the cone angle of the cone section of the drum is large, there will be a phenomenon of sludge backflow, and the screw feeder cannot convey the slag.

It can be seen from the slag discharge process of the centrifuge that when the slag settled on the inside of the centrifuge drum is pushed along the cone end of the drum to the discharge port, due to the centrifugal force, it is affected by the downward return force. The larger the half-cone angle, the greater the slag squeezed by the centrifugal force, the greater the torque of the screw thrust, the greater the blade wear, and even the phenomenon of backflow of the sediment, which causes the screw pusher to be unable to discharge the slag. If the half-cone angle is small, the effective settlement area is reduced, which reduces the performance of the centrifuge.

As shown in Figure 1, when the depth h of the liquid pool and the radius r3 of the slag discharge port are fixed, the half-cone angle α increases, and the length of the drying zone LG = (r1-r3) / tgα (that is, between the slag leaving the liquid surface and the slag discharge port Distance) is shorter, the dehydration time becomes shorter, and the moisture content of the sediment increases; on the contrary, if the cone angle is small, the length of the drying zone is longer, and the dehydration time is longer, the moisture content of the sediment is lower. In addition, the size of the half-cone angle also affects the effective length and length-to-diameter ratio of the centrifuge drum.Under certain conditions of the inner radius of the drum and the radius of the slag discharge port, the smaller the half-cone angle, the longer the drum, the longer the aspect ratio The larger, the more difficult to manufacture. Therefore, when designing a centrifuge, a larger half-cone angle should be selected while ensuring separation conditions and performance.



In the formula, β is the angle of elevation of the spiral blade; δ1 is the angle between the sliding direction of the sediment along the drum wall and the radial plane perpendicular to the drum axis; α is the half-cone angle of the screw conveyor; λ1 and λ2 are the sediment and Friction angle of the drum wall and the spiral blade surface; f1, f2 are the friction factors of the sediment and the drum wall and the spiral blade surface; A∑ is the equivalent settlement area; r2, r1 are the inner radius of the drum and the surface radius of the liquid pool, respectively ; rs is the outer radius of the spiral inner cylinder; g is the acceleration of gravity; ω represents the rotating angular velocity of the drum; the rest are the structural parameters L1, L2 as shown in Figure 1.





According to formulas (1) to (3) and taking the structural parameters of LW500 × 1200-N drilling fluid high-speed centrifuge as an example, the relationship between the slag discharge efficiency EP and the settlement area AΣ and the half cone angle α can be calculated as shown in Figures 2 to 4 As shown. With the gradual increase of the half cone angle α of the drum, the angle δ1 of the sludge along the drum wall sliding direction gradually decreases, the slag discharge efficiency and the drum settlement area also decrease, and the amount of sludge discharged per unit time will also decrease Therefore, the half cone angle of the drum is not easy to be too large. With the gradual decrease of the half-taper angle α of the drum, the angle δ1 of the sludge along the sliding direction of the drum wall gradually increases, the slag discharge efficiency and the settlement area of ​​the drum also increase, and the amount of sludge discharged per unit time will also increase, but The length of the drying zone is extended, the dehydration time is long, the water content of the sediment is low, and it is also easy to cause slag discharge and accumulation and clogging, so the half cone angle of the drum is not easy to be too small.

Since the slag transfer is a necessary condition for the normal operation of the centrifuge, the best design must first meet the slag transfer conditions. In order to avoid the problem that the screw pusher cannot transport slag, for difficult-to-separate materials (that is, drilling fluids with small density differences or high viscosity), the half cone angle of the drum should be reduced to less than 8 ° in order to reduce the return of sediment in the cone section speed. For ordinary materials, the half cone angle should be within 10 °, so that the sediment can be transported. In the design of drilling fluid centrifuges, the general half cone angle of the drum is selected between 8 ° and 10 °, and the choice of Ep value is not suitable. Below 85%, the δ1 value must be at least greater than 45 °, and it is better to achieve Ep> 90% and δ1> 50 °.