What is the structure and working principle of an axial flow pump?

　　Axial pump is a kind of vane pump, its vane unit for a series of wing type, around the hub constitutes a cylindrical vane grille, due to flow through the impeller fluid microgroup traces theoretically located in the cylindrical plane concentric with the rotating axis, after the guide vane deconcentration is considered to be along the axial flow, so this pump is known as an axial flow pump, also known as the Kaplan (Kaplan) pump, the axial flow pump flow rate is larger, up to 60 m3 / s; single-stage The flow rate of axial flow pump is larger, up to 60m3/s; the head is lower, usually 1~25m, therefore, the specific speed of axial flow pump is higher, and its common range is between 500~1600.

　　Axial pumps can be divided into fixed, semi-regulated and fully regulated according to the vane fixation and adjustment methods; according to the pump shaft position is divided into vertical axial pumps and horizontal axial pumps, today, Schaederbauer will briefly introduce you to common axial pumps structure and working principle.

　　First, the structure and working principle of axial flow pumps

　　1、The structure of axial flow pump

　　The main components of axial pump are impeller, pump shaft, guide vane, suction pipe, etc., which are described as follows.

　　(1)Impeller

　　The role of the impeller and centrifugal pumps, the mechanical energy of the prime mover into the fluid pressure and kinetic energy, which consists of blades, hubs and dynamic leaf regulator, etc., blades are mostly wing-type, generally 4 to 6 pieces, hubs are used to install the blades and blade regulator, there are three kinds of conical, cylindrical and spherical. Small axial pump (impeller diameter of 300mm or less) of the blade and hub cast into one piece, the angle of the blade is not fixed, also known as fixed blade axial pump; medium-sized axial pump (impeller diameter of 300mm or more) is generally used in the semi-adjustable impeller structure, that is, the vane by the nut and locating pins are fixed to the hub, the vane angle can not be changed arbitrarily, only according to the pin holes corresponding to the vane angle to change, so it is called semi-adjustable axial pump. Therefore, it is called semi-adjustable axial pump; large axial pump (impeller diameter of 1600mm or more), generally using a spherical hub, the dynamic leaf adjustment mechanism installed in the hub, relying on the hydraulic drive system to adjust the vane angle, it is called dynamic leaf adjustable axial pump.

　　(2)Pump shaft

　　For large-capacity and vane adjustable axial pump, the shaft are made of high-quality carbon steel hollow, chrome-plated surface, both to reduce the quality of the shaft, but also to facilitate the installation of regulating agencies.

　　(3)Guide vane

　　The guide vane of the axial pump is generally installed in the outlet side of the impeller, and the function of the guide vane is to change the rotary motion of the water flowing out of the impeller into axial motion, and at the same time, part of the kinetic energy is changed into pressure energy.

　　(4)Suction pipe

　　Suction pipe and centrifugal pump suction chamber of the same role, small and medium-sized axial pumps with trumpet-shaped suction pipe, large axial pumps more elbow-shaped suction runner.

　　2、Working principle of axial flow pump

　　The working principle of the axial pump is different from the centrifugal pump, it is relying on the impeller rotation of the water flow generated by the lift and work, this pump because the water flow into the impeller and out of the guide vane are along the axial direction, so it is called axial flow pump.

　　The axial flow pump is based on the theory of lift of the wing in aerodynamics. When the fluid goes around the airfoil, it separates into two streams at the first end point of the airfoil, which pass through the upper and lower surfaces of the airfoil, and then converge at the tail end of the airfoil at the same time. As the distance along the lower surface of the airfoil is longer than that on the upper surface, the flow rate of the fluid along the lower surface of the airfoil is larger than that along the upper surface of the airfoil, accordingly, the pressure on the lower surface of the airfoil will be smaller than that on the upper surface, and there is a downward force of the fluid on the airfoil. Similarly, the airfoil for the fluid will also produce a reaction force, and this force is equal in size and opposite in direction, acting on the fluid. Under the action of this force, the water is pressed up to a certain height.

　　3、The performance characteristics of axial flow pump

　　Compared with centrifugal pump, it has the following characteristics:

　　(1)The head increases dramatically with the reduction of flow, and the Q-H curve increases suddenly, mainly because: when the flow is small, the return flow is generated at the inlet and outlet of the impeller vanes, and the water flow is repeated many times to get the energy, similar to the state of multi-stage pressurization, so the head increases dramatically, which results in the phenomenon of the increase of the axial power.

　　(2) Q-N curve is a steeply falling curve. The shutdown head (when flow Q=0) is 1.5~2 times of the rated value. Therefore, the axial flow pump should be started with the gate valve fully open to start the motor.

　　(3) Q-η curve is hump-shaped, that is, the range of high-efficiency work is very small, the flow rate in the deviation from the design condition point not far away from the efficiency of the rapid decline. According to this characteristic of the axial pump, take the outlet valve mediation flow is unfavorable, generally can only be taken to change the vane device angle or change the speed of the method to regulate the flow.

　　(4) axial pump impeller is generally submerged in the liquid, so do not need to consider the vapor corrosion, start-up is not required to fill the pump.

　　4、The advantages of axial flow pump are:

　　① simple structure, in the given operating parameters, cross-sectional area (perpendicular to the plane of the rotary axis) and weight than other types of vane pumps small; ② in the shutdown state, whether in the liquid, the impeller is generally submerged in the liquid, so there is no need to take into account the start of filling the pump.

　　② Whether in the shutdown state or running state, can easily change the flow rate by changing the vane placement angle; ③ Axial pumps are usually the same as the other types of pumps, but they are not the same as the other types of pumps.

　　③ axial flow pumps are usually vertical structure, so its footprint is small, in addition can be installed in the open air.

　　5、The disadvantages of axial flow pump are:

　　① limited self-priming capacity; ② low single-stage head.

　　② low single-stage head; ③ steep efficiency curve, high efficiency.

　　③ steep efficiency curve, high efficiency zone is narrow, if there is no impeller blade angle adjustment device, when deviating from the design operating conditions when the economy is poor.

　　Axial pumps are widely used in irrigation, water supply and drainage, dredging of inland waterways, power engineering and other areas where large amounts of liquid need to be conveyed but the head requirement is not high, and they are also used in nuclear power engineering and ship propulsion.