1. System composition and working principle of the pump?
1. System composition?
Ground—
Underground screw pump
Intermediate-intermediate pipe or intermediate cable
Workflow; The surface power drives the sucker rod string-the pump rotor rotates-the well fluid is sucked from the lower part of the pump-discharged from the upper part-and flows out from the tubing to the wellhead-the surface pipeline to the metering room.
2. The structure and working principle of screw pump?
(1) Structure of screw pump?
It consists of stator and rotor. The rotor is a high-strength screw, finished and chrome plated; Stator is a pump barrel, made of strong, oil-resistant and corrosion-resistant synthetic rubber, and then permanently bonded in a steel shell. Screw pump besides single screw pump, there are many screw pumps (twin screw pump, three screw pump and five screw pump, etc. ), mainly used to transport petroleum products.
(2) The working principle of screw pump?
Screw pump discharges oil through cavity, that is, the closed cavity formed between rotor and stator is not connected with each other. When the rotor rotates, the closed cavity moves axially from the suction end to the discharge end. When the closed cavity disappears at the discharge end, the crude oil in the cavity will be evenly squeezed from the suction end to the discharge end. At the same time, a new low-pressure cavity is formed at the suction end to suck crude oil. In this way, closed cavities are constantly formed, moved and disappeared, and crude oil is continuously filled, squeezed and discharged, so that crude oil in the well is continuously sucked and lifted to the wellhead through tubing.
3. Shortcomings
Second, the determination of the basic parameters of screw pump?
1. Theoretical displacement of the pump?
Any cross section of the screw is a circle with a radius of R. The shape of the whole screw can be regarded as consisting of many extremely thin disks with a radius of R. The centers of these disks move forward at a certain pitch t and rotate around the axis O2-Z of the screw itself, with an eccentricity of e ... that is, the locus of the center of the disk O 1 is a helix with a pitch of t and an eccentricity of e?
The bushing is made of rubber, and its cross section is a rectangle consisting of two semicircles with a radius of r (equal to the cross-sectional radius of the screw) and two linear segments with a length of 4e, as shown in figure 1 1-2 1. The double-line internal helicoid of the bushing is formed by the above-mentioned section rotating around the bushing axis OZ and moving forward according to a certain lead T=2t. ? The rubber bushing is easy to wear, the lower radial thrust bearing is damaged, the eccentric coupling is not reliable enough and the cycle is short. ?
When the position of the screw in the bushing is different, the contact points between them are different. When the cross section of the screw is at both ends of the oblong cross section of the casing, the contact between the screw and the casing is semi-circular; At other positions of the bushing, there are only two contact points between the screw and the bushing. Because the screw and the bushing are continuously engaged, these contact points form a sealing line, and a sealing chamber is formed in the lead T of the bushing. In this way, along the whole length of the single screw pump, the inner helicoid of the bushing and the helicoid of the screw form a closed chamber. It can be seen that the length of the bushing thread pair is at least one lead of the bushing to form a complete sealed cavity.
Where Qt—— is the theoretical displacement of the pump, m3/d. ; ?
E- eccentricity of screw, m? ; ?
N- screw speed,? r/min？ ; ?
DP- the diameter of the screw section, DP? =2r，m； ?
T—— lead of casing, t = 2t, m; ?
T- pitch, m?
2. The volumetric efficiency and system efficiency of the pump?
1) The ratio of the actual displacement q of the pump to the theoretical displacement Qt is called the volumetric efficiency of the pump, which is expressed by a formula. It is essentially a displacement coefficient, which is related to the pump head, the interference between the rotor and the stator, the rotor speed and the viscosity of the lifting liquid. It is a multivariable function.
2. The system efficiency η of the pump is defined as the ratio of the active power (hydraulic power) Ph of the pump to the input power Pin of the pump, namely
These include:
3. Torque of the pump?
Due to the pressure difference between the suction end and the discharge end of the screw pump, the liquid in the screw bushing pair will exert a force on the screw. At the same time, there is interference between stator and rotor, which will produce friction resistance moment between stator and rotor. (1) Rotor active torque The screw-bushing pair converts mechanical energy into hydraulic pressure energy, which can be obtained from the energy conversion relation without considering the loss.
(2) Friction torque between stator and rotor?
Because of the interference between the stator and rotor of screw pump, when the rotor rotates in the stator, friction will occur between the stator and rotor. Influence of stator on friction torque applied to rotor.
(3) Starting torque?
Starting torque is related to the length of screw pump sealing line, the interference between stator and rotor, the hardness of rubber and working pressure, as well as the length of static time and the roughness of friction surface. The more stages, the greater the roughness, the higher the rubber hardness, the greater the interference between the stator and the rotor, the higher the working pressure of the pump and the greater the starting torque of the pump. ?
3. Working characteristic curve of screw pump and its influencing factors?
1. Working characteristic curve of screw pump?
Relationship among pump volumetric efficiency, system efficiency, torque and lifting height. The curve reflecting this relationship is called the working characteristic curve of screw pump.
2. What are the factors that affect the working characteristics of screw pump?
What is the effect of (1) interference?
The contact line between stator and rotor surface is completely sealed, and the degree of sealing depends on the interference between rotor and stator.
The interference is large and the pump efficiency is high, but the sucker rod torque increases, which is easy to cause sucker rod fracture and stator rubber wear.
Small interference, low pump efficiency, without the above problems.
Therefore, the size of the interference directly affects the efficiency of the pump.
(2) The influence of rotor speed? The higher the rotor speed, the greater the displacement. However, the higher the rotating speed, the greater the centrifugal force of the sucker rod, the more serious the bending vibration of the sucker rod, and the greater the friction between the sucker rod coupling and the inner wall of the tubing. At the same time, due to the increase of loss along the way and the acceleration of stator rubber wear, the lifting height will also decrease. Therefore, the rotor speed is not easy to be too high, and should generally be less than 500 r/min.
(3) The increase of other viscosities reduces the leakage, which is beneficial to improve the volumetric efficiency and system efficiency of the pump; On the other hand, the increase of viscosity will increase the flow resistance, reduce the fullness and head of the pump, and the volumetric efficiency and system efficiency of the pump will also decrease. At the same time, the increase of pump friction will increase the resistance moment.
Fourth, the choice of screw pump?
Selection steps of screw pump:
1) The output of the oil well should be determined according to the productivity of the oil well, and the displacement of the screw pump used should be determined;
2) Determine the lifting height of the pump according to the working characteristic curve of the pump, and calculate the required pump stage according to the oil well conditions. At the same time, the reasonable interference is determined according to the needs and the actual situation of the oil well.
3) Select the materials and specifications of sucker rod, motor and other accessories according to the load.
1. How to determine the rotor speed?
The determination of rotation speed of ground-driven single screw pump is influenced by many factors. The first thing to consider is the viscosity and wear of the medium and the fatigue strength of the stator rubber. ?
The viscosity of the medium will affect the filling coefficient of the pump. When the pump rotates, the volume of the cavity at the pump suction inlet gradually increases. At this time, as long as there is a certain pressure difference, the liquid can quickly fill the cavity. When the viscosity of liquid is high, its fluidity becomes worse.
Therefore, the filling coefficient decreases and the volumetric efficiency of the pump decreases, and its influence degree increases with the increase of liquid viscosity.
In oil wells with high sand content, the life of pump depends on the fatigue strength of stator rubber. Because there is a certain interference between the stator and the rotor, when the rotor rotates in the stator, the stator rubber will be periodically compressed, which will cause the temperature rise and fatigue of the friction surface. The temperature rise of the friction surface can often reach tens of degrees higher than the temperature of the medium, which accelerates the recombination of rubber molecular chains and reduces the elastic modulus, thus reducing its fatigue characteristics and the bonding strength between metal and rubber. This kind of temperature rise and compression fatigue increase with the increase of rotating speed, so the rotating speed should be selected reasonably in practical application to ensure the life of the pump.
2. How to determine the pump stage and the length of stator and rotor?
Single-stage screw pump can not meet the actual lifting height (head) needs, just like submersible pump needs multi-stage pump. The number Z of the pump can be determined according to the actual pump lift H and single-stage lift Hj of the oil well, namely
After determining the pump stage, the stator and rotor can be determined.
Length. The length of stator and rotor is determined by the number of pump stages and the lead of bushing. The stator length Ls is
Rotor length Lr is generally taken as stator length Ls plus 250~350 mm? , to ensure that the rotor can be installed in place.
3. How to determine reasonable interference?
The interference fit between the stator and rotor of screw pump is shown in figure 1 1-25, and the interference amount is δ=(b-a)/2. In order to make the screw pump have the characteristics of positive displacement pump, the cavity between stator and rotor must be kept in a good sealed state.
That is to say, there must be a certain interference value. The reason is:?
1) Due to the limitation of machining technology, it is impossible to ensure that the stator and rotor have ideal geometric shapes; ?
2) Stator rubber is an elastomer, which will be elastically deformed and leaked under a certain pressure difference; ?
3) Because the rotor will generate inertia force and hydraulic radial force during operation, the resultant force of these two forces will make the rotor compress the stator rubber in the resultant force direction, resulting in displacement, thus creating a gap between the stator and the rotor. ? When the screw pump works underground, its total interference δ consists of three parts: initial interference δ0, interference δ 1' caused by thermal expansion and interference δ2' caused by oil-immersed expansion. The total interference δ can be estimated according to the pump and oil well conditions, and δ 1' and δ2' can be determined by experiments. In this way, the initial interference δ0 can be determined, which provides a basis for design and manufacture.
At present, the single-stage working pressure difference of screw pump is mainly realized by the interference between stator and rotor. The greater the interference, the greater the working pressure difference between stages and the greater the rotor torque; If the interference is too small, the single-stage working pressure difference will be smaller, which can not meet the needs of oil well lifting. Therefore, there is a reasonable interference between stator and rotor. For the determination of interference, it is necessary to master the physical characteristics of stator rubber, especially the thermal expansion and swelling characteristics of rubber, so as to realize the rationality of the determination of interference.