Gauge pressure = absolute pressure – atmospheric pressure

Vacuum degree = atmospheric pressure - absolute pressure

Absolute pressure = gauge pressure atmospheric pressure

Solid: P=F/S (stress area)

Liquid: P=ρgh

ρ(water)=1000kg/m³

ρ(mercury)=13600kg/m³

ρ(air)=1.29kg/m³

ρ(alcohol)=789kg/m³=0.789g/cubic centimeter

One standard atmospheric pressure = 1atm = 101325pa (Pascal) = 101.325KPa = 0.101325MPa = 10.33m water column = 760mm mercury column

Atmospheric pressure of a project =1at =98066.5Pa =1000kgf/㎡=10m water column

U-shaped differential pressure gauge

spring pressure gauge

The larger the β value of the compressible fluid, the easier it is to compress. The continuity equation of stable flow u1A1/ρ1=u2A2/ρ2

Incompressible fluid dp/dp=0 (the density of the fluid does not change with pressure changes)

The essential difference between turbulent flow and laminar flow (turbulent flow has radial flow, while laminar flow has no radial flow)

Regardless of turbulent laminar flow, the flow velocity is maximum at the center of the tube, the closer to the tube wall, the smaller the flow velocity, and the flow velocity at the tube wall is zero.

120mm×20mm. Outer diameter × wall thickness. Then radius = (120-20*2)/2 equivalent diameter = 4×cross-sectional area/perimeter (usually rectangle)

What is equivalent length?

In round tube: u1/u2=(d2/d1)^2

Parallel connection (the resistance loss of any branch is equal)

series connection

Pitto tube

As the flow rate increases, the pressure difference increases

When the flow rate increases and the pressure difference remains unchanged, first use water as a benchmark to measure the range; when the alcohol flow rate is actually measured, the actual flow value of alcohol is larger than the scale value v2/v1 = root (ρ1 (ρf-ρ2)/ρ2 (ρf-ρ1))

① Keep the pipeline as short as possible and in a straight line. ② Try not to install unnecessary pipe fittings and valves ③ The pipe diameter should be appropriately large

When fluid flows, the internal friction force generated between adjacent fluid layers on the unit contact surface is 1.

Liquid: As the temperature increases, the viscosity decreases; Solid: As the temperature increases, the viscosity increases

Reciprocating pump, gear pump, screw pump

Reciprocating pump: small flow rate, high pressure head liquid transportation, not suitable for transporting corrosive liquids and suspensions with solid particles. Reciprocating pumps have self-priming capabilities. Centrifugal pumps do not have self-priming capabilities. Both need to be filled with liquid before starting (the reciprocating pump does not need to be filled with water to start)

① Bypass adjustment; ② Change the piston stroke; ③ Change the number of piston reciprocations. The flow rate of the reciprocating pump is only related to the geometric size of the pump itself and the number of reciprocations of the pump; it has nothing to do with the pressure head and management characteristics of the pump.

①Adjust the outlet valve ②Change the pump speed ③Turn the outer diameter of the impeller

Reason: Before the centrifugal pump is started, if the pump is not filled with liquid, the centrifugal pump has no self-priming ability: since the density of air is less than the density of liquid, the centrifugal force generated is very small, and the vacuum formed by the suction port cannot suck the liquid in. Solution: Before starting the pump, prime the pump and install a one-way bottom valve with a suction filter on the suction pipe. (The filter is to prevent solid matter from entering the pump, thereby damaging the blades of the impeller or hindering the normal operation of the pump) A: There is air binding. ① Check whether there is leakage in the inlet and outlet pipelines; ② Stop the pump and pump into the pump. Filling A: After starting, how does the liquid increase the pressure in the pump? After starting, the pump shaft drives the impeller to rotate, and the liquid between the blades rotates with the impeller. Under the action of centrifugal force, the liquid is thrown from the central inlet position of the impeller to the periphery of the impeller along the channel between the blades. After the fluid flows into the scroll channel at a very high flow rate, most of the kinetic energy is converted into static pressure energy due to the gradual expansion of the cross-section. .

The main reasons why the centrifugal pump does not absorb liquid after starting are: ① Due to insufficient filling of the pump or leakage of liquid from the bottom valve that is not tight, causing air in the pump; ② due to blockage of the bottom valve or suction pipeline. ③The installation height is too high; ④The motor wiring is incorrect, causing the impeller to rotate reversely.

The pressure at the inlet of the centrifugal pump impeller is equal to or less than the saturated vapor pressure of the liquid being transported at the operating temperature. The liquid will partially vaporize or the gas dissolved in the liquid will precipitate and form bubbles. After the liquid containing bubbles enters the high-pressure area of ​​the impeller, the bubbles will Under the action of the bubble, it shrinks and bursts sharply. The disappearance of the bubble leads to a partial vacuum. The surrounding liquid rushes towards the space occupied by the original bubble at a very high flow rate, causing shock and vibration. The metal surface is exposed to high-pressure and high-frequency impacts, causing electrochemical corrosion of the metal surface by a small amount of oxygen and other active gases entrained in the bubbles, causing sponge-like and fish-scale damage to the impeller surface, causing the pump to fail to operate normally. This phenomenon is called cavitation. Cavitation phenomena and hazards: ① The pump body produces vibration and noise; ② The pump performance decreases; ③ The pump casing and impeller are eroded, and the equipment is damaged. Preventive measures: Limit the installation height of the pump. If there is cavitation, check the inlet pipeline for leaks.

What does the allowable installation height of a centrifugal pump depend on?

Relying on the centrifugal force generated by the high-speed rotation of the impeller, the liquid is sucked and discharged. When starting the machine, the outlet valve must be closed in order to minimize the starting power of the centrifugal pump to avoid burning the motor. Before shutting down the machine, first close the outlet valve and then cut off the power.

The disadvantage is increased flow resistance loss; the advantage is increased turbulence.

Comparative relationship between inertial force and viscous force during fluid flow

The effective mechanical energy provided by the pump to unit mass of liquid

① Mechanical energy obtained by unit volume of gas; ② The sum of static wind pressure and dynamic wind pressure

Fan characteristic curve

The flow rate at the highest efficiency

As the flow rate increases, the pressure at the pump outlet decreases.

Performance curve when transporting clean water at 20°C at a certain rotation speed.

The intersection point of the pump characteristic curve and the pipeline characteristic curve. The flow adjustment of a centrifugal pump actually changes the working point of the pump. The design point refers to the highest efficiency point.

Composed of pump casing, pump shaft and impeller. Impellers include closed, semi-closed and open types