Influence of pipeline resistance on lift and calculation of pipeline loss
As we all know, the pipeline system is a kind of solid matter, and water is easy to flow. If the water in the pipeline is flowing, there must be a part of energy converted into heat energy and “eliminated”, that is, a part of water pressure (or head) is lost. This is the reflection of objective things and the inevitable law of water flow movement. Usually, we call this phenomenon of energy conversion energy loss (or hydraulic loss, head loss). It is measured in meters.
What is the effect of pipe resistance on lift?
Some users have measured that although the vertical distance from the reservoir or water tower to the water surface of the water source is slightly less than the head of the water pump, the water volume is still small or unable to be lifted. The reason is often that the pipe is too long, there are many bends in the pipe, and the resistance loss of water flow in the pipe is too large.
In general, the resistance of 90 degree elbow is greater than that of 120 degree elbow. The head loss of each 90 degree elbow is about 0.5-1 meter, and the resistance of every 20 meter pipe can make the head loss about 1 meter. In addition, some users also arbitrarily change the diameter of the water pump inlet and outlet pipes, which also has a certain impact on the lift. Then, how much influence does the pipeline resistance have on the lift? Now, let’s look at the table below.
Pipeline pressure loss table
Do you know the cause of hydraulic loss caused by pipe flow?
- It is the blocking effect of rough wall.
- It is the relative movement between the layers of water flow.
- It is the vortex formed by the local rapid change of water flow in the pipe. The hydraulic loss of pipeline (Network) is composed of two parts: along the way and local. In engineering, we must calculate and know how many of them are, in order to correctly select the pump and determine the required pump head.
The loss along the pipeline is the frictional resistance that occurs in the whole flow process. It is related to the roughness of the pipe wall, the length of the pipe, the diameter of the pipe and the velocity of the flow.
The friction coefficient is proportional to the loss along the pipe wall roughness. Different pipes have different roughness. The friction coefficient of cast iron pipe is larger when it is rough; The plastic pipe is smooth, and the friction coefficient along the way is smaller. It is proportional to the length of the pipe; It is inversely proportional to the pipe diameter, that is to say, when the flow rate is constant, the loss along the way is large when the pipe diameter is small and the flow rate is fast; It is also proportional to the square of velocity. Of course, the calculation is cumbersome, simple method can be estimated.
The local loss of pipeline is the change of flow pattern caused by local devices in the process of water flowing through the bottom valve, valve, elbow, reducer and other accessories; The direction and size of velocity change, and vortex appears in the flow, which makes the flow collide and impact each other. The hydraulic loss caused by local resistance is called local loss.
The magnitude of local loss is directly proportional to the square value of water velocity passing through the pipe fittings, and is also related to the shape and quantity of fittings. If the shape of the cross section of the fittings changes greatly and the quantity is large, the greater the local loss will be.
When the pipeline layout scheme is determined, the loss head of the pipeline is generally obtained by calculation method, and then the design head of the pump station is determined, then the pump selection can be carried out. However, the calculation program is more complicated. For the sake of simplicity, the calculation data can be compiled into a table so that it can be obtained by looking up the table. In addition, it can also be roughly estimated that the loss head is equivalent to the actual terrain lifting height (measured) of 30% ~ 50%, the smaller the pipe diameter and the shorter the pipeline, the larger the value; If the pipe diameter is large and the pipe length is long, take the small value.
The existing software can be used to calculate the pipeline loss along the way and the pipeline local loss.
Pressure loss of liquid flowing in straight pipe
The pressure loss of liquid flowing in a straight pipe is caused by the friction of liquid flowing, which is called pressure loss along the pipe. It mainly depends on the length, inner diameter, velocity and viscosity of the pipe. The pressure loss along the flow path is different with different liquid flow patterns. The laminar flow of liquid in the pipe is the most common in the hydraulic transmission. Therefore, when designing the hydraulic system, it is often expected that the liquid flow in the pipe will keep the laminar flow state.
Pressure loss of fluid flowing in pipeline
Pressure loss in laminar flow
In hydraulic transmission, the flow state of liquid is mostly laminar flow. In this state, the pressure loss of liquid flowing through straight pipe can be obtained by theoretical calculation.
Laminar flow in a circular tube
(1) The velocity distribution of liquid in the flow section.
As shown in the figure above, the liquid moves laminar in a circular pipe with diameter D, and the pipe is placed horizontally. Take a small cylinder in the pipe that coincides with the pipe axis, and set its radius as R and length as L. The forces acting on the small cylinder along the pipe axis are: pressure P1 at the left end, pressure P2 at the right end, and the friction force on the cylinder surface is FF. Then the force balance equation is as follows:
It can be seen from formula (2-6)
In the formula: μ It is dynamic viscosity.
Because the sign of velocity increment Du is opposite to that of radius increment Dr, a negative sign is added to the formula.
In addition, Δ P = P1-P2 Δ p. If equation (2-45) is substituted by equation (2-44), then:
The result is as follows:
When r = R, u = 0, substituting (2-47) formula, we get:
It can be seen from the formula that the velocity u in the pipe is distributed along the radius according to the parabola law, and the maximum velocity is on the axis
(2) The flow in the line.
The volume of the projectile shown in figure (b) is the volume of the liquid flowing through the flow section in unit time, that is, the flow rate. In order to calculate its volume, a layer of tiny ring area with the thickness of D and R can be taken at the radius R. the flow through the ring area is as follows:
The flow Q can be obtained by integrating with the equation
(3) Average velocity.
Set the average velocity in the pipe as υ
The relationship between average velocity and maximum velocity can be obtained by comparison
(4) Pressure loss along the way.
In laminar flow, the pressure loss of liquid flowing through the straight pipe can be obtained from the equation
It can be seen from the formula that in laminar flow state, the pressure loss of liquid flowing through the straight pipe is directly proportional to the dynamic viscosity, pipe length and flow velocity, and inversely proportional to the square of pipe diameter.
In the actual calculation of pressure loss, in order to simplify the calculation, the μ=υ d ρ/ Re, and μ=υ d ρ/ Re, and the numerator denominator is also multiplied by 2G:
In the formula: λ Is the drag coefficient. Its theoretical value is λ＝ In fact, due to the influence of various factors, it is difficult to make smooth metal tube λ＝ 75 / re for rubber pipe λ＝ 80/Re。
Pressure loss in turbulent laminar flow the particles move regularly along the axial direction. There is no lateral movement.
One of the important characteristics of turbulence is that the particles of liquid no longer move in regular axial direction, but permeate and pulsate with each other in the process of motion. This kind of irregular motion causes the collision between particles and the formation of vortices, which makes the energy loss of turbulence much greater than that of laminar flow.
Because of the complexity of turbulent flow phenomenon, the theoretical method has not been used to study it up to now, and satisfactory results have not been obtained. Therefore, the experimental method is still used to study it, supplemented by theoretical explanation. Therefore, the pressure loss of liquid flow in turbulent state is still calculated by formula λ The value is not only related to the Reynolds number Re, but also to the surface roughness of the tube wall.
Local pressure loss
Local pressure loss is the pressure loss caused by the liquid flowing through the valve port, elbow and the change of flow section. When the liquid flows through these places, due to the change of the direction and velocity of the liquid flow, a vortex is formed, which makes the liquid particles collide with each other, resulting in greater energy loss.
Local loss at sudden expansion
The formula of local pressure loss can be expressed as follows:
In the formula: it is the local resistance coefficient, which can be obtained by theoretical derivation only when the liquid flows through the suddenly enlarged section, and other cases must be determined by experiment; It is the average velocity of liquid, which generally refers to the velocity downstream of local resistance.
The total pressure loss of pipeline system is equal to the sum of all pressure losses along the pipeline and all local pressure losses:
Source: China Pressure Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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