The centrifugal pump system is one of the basic units that make up a chemical plant, and its design is the basis of engineering design and an important part. The head of the wear-resistant and corrosion-resistant centrifugal pump is an important basis for choosing a pump. Too large or small head will reduce pump efficiency and increase operating costs.
1. Steps to calculate the head of a wear-resistant and corrosion-resistant centrifugal pump
1.1 Calculate the equivalent length and static head. Draw the direction of the centrifugal pump pipeline and mark the length, pipe diameter, elevation, valves and pipe fittings, etc., and calculate the equivalent length and static head of the pipeline.
1.2 Calculation of pressure difference of wear-resistant and corrosion-resistant centrifugal pump adopts "Calculation of Pipeline Pressure Drop" to calculate pipeline pressure drop.
1.3 The head of the wear-resistant and corrosion-resistant centrifugal pump is determined according to the formula H=Δp/ρg (H—pressure head under the design flow of the pump).
2. A case of calculation of the head of a wear-resistant and corrosion-resistant centrifugal pump
The wear-resistant and corrosion-resistant pump of a chemical project has a design flow of 1.8m3/h, the inlet pipe diameter of the pump is DN20, and the outlet pipe diameter of the pump is DN20. The temperature of the conveying fluid is 25℃, the density is about 1,000kg/m3, the viscosity is 0.893 7m Pas, the suction side pressure is 101k Pa, and the discharge pressure is 101k Pa (25℃). There is a precision filter on the pump discharge pipeline, and the pressure drop is 50k Pa under normal flow. The flow diagram is shown in Figure 1.
Case diagram of head calculation of wear-resistant and corrosion-resistant centrifugal pump
2.1 The pressure drop of the outlet pipe of the pump is calculated for the flow velocity in the pipe: According to the pipe material regulation: the inner diameter of seamless steel pipe of DN20 is φ21. Pump suction and discharge pipeline u1=u2=Q/A=1.8/(3 600×0.785×0.0212)=1.44m/s
2.2 The resistance coefficient of the pipeline is known from the seamless steel pipe material, and the absolute roughness ε=0.2mm. Pump inlet and discharge pipe: Reynolds coefficient Re1=Re2=ρud/μ=1 000×1.44×0.021/(0.893 7×10-3)=33 836
ε/d=0.2/21=0.009 5 Re and ε/d look up the graph to get λ1=λ2=0.039.
2.3 Resistance drop
(1) The resistance of the pump inlet pipe is reduced
①The pressure drop of the straight pipeline at the inlet of the pump
Δp1′=λL1u2ρ/2d1=0.039×2.3×1.442×1 000/(2×0.021)=4.428k Pa
②From the container nozzle to the pump inlet, there are 3 90° elbows, 1 tee, 1 filter, and 2 stop valves. The look-up table shows that the sum of the equivalent length when the fluid flows through the elbow valve is ∑Le=0.63×3+1.26+7.14×2=17.43, then the resistance of the pipe fittings on the inlet pipeline is reduced to:
Δp1″=λ∑Leu2ρ/2d1=0.039×17.43×1.442×1 000/(2×0.021)=33.56k Pa
③The total resistance of the inlet is reduced to, Δp1″=Δp1′+Δp1″=37.988 k Pa
(2) Pressure drop of pump outlet pipeline
①The pressure drop of the straight pipeline at the outlet of the pump Δp2′=λL1u2ρ/2d2=0.039×20×1.442×1 000/(2×0.021)=38.509k Pa
②There are 9 elbows, 5 stop valves, 1 three-way and 1 check valve from the pump outlet to the container inlet. The look-up table shows that the sum of the equivalent length when the fluid flows through the elbow valve is ∑Le=0.63×9+7.14×5+1.26+12.6=55.23, then the resistance of the pipe fittings on the pump outlet pipeline drops to:
Δp2″=λ∑Leu2ρ/2d2=0.039×55.23×1.442×1 000/(2×0.021)=106.344k Pa
(3) The total resistance of the pump outlet drops to Δp2=Δp1″+Δp2″+p filter+ρgh=38.509+106.344+50+9.81×9.81×2/1000=214.473 k Pa
2.4 The pressure difference between the inlet and outlet of the wear-resistant and corrosion-resistant centrifugal pump ΔP pump suction port pressure:
pA=1 0 1×1 03+1 0 0 0×9.8×11.6-3 7 9 8 8=1 7 6 6 9 2 P a=176.692k Pa
Pump discharge pressure: pB=p2+Δp2=101+214.473=315.473k
PaΔp=pB-pA=315.473-176.692=138.781k Pa
3. Calculation of head piece of wear-resistant and corrosion-resistant centrifugal pump
H△p/g=138781×1000/(1000×9.81)=1415m
Because the pipe resistance calculation often has errors, and the pipe fouling and carbon deposit during operation also make the pipe resistance greater than the calculated value, the head should also be 105-1.1 times the calculated value. Then the head of the pump is taken as 1415×1.1=15.6m, which is 20m.
