泵NPSH计算教程

This tutorial will focus on pump NPSH calculation, given the details of inlet tank, pump elevation profile and suction piping.

NPSHr(required) is normally specified by the泵制造商。Hence, NPSH calculation usually refers to the calculation of available NPSH (NPSHa). Calculation of available NPSH or NPSHa is important for deciding the line sizes on suction side of a centrifugal pump. Selecting a large enough line size is crucial foravoiding cavitation in the pump。

NPSH calculation - problem statement

Calculate the Net Positive Suction Head for a pump handling 100,000 kg/hr flow of water coming from an atmospheric storage tank. The water temperature can be taken as 25⁰C The line size of pump suction line is 6" and the suction line is 20m long. The pump suction nozzle is 0.4 m above ground level. The tank is elevated on a 1 m high platform. The minimum liquid level in the tank is 300 mm.

Solution

Before solving the sample problem, the equation used to calculate NPSH must be understood.

where, h_Lis the head loss between 0 and 1, p₀is the pressure at the water surface, p_Vis the vapour pressure (saturation pressure) for the fluid at the temperature T1 at 1, Δz is the difference in height z1 − z0 (shown as H on the diagram) from the water surface to the location 1, and ρ is the fluid density, assumed constant, and g is gravitational acceleration.

以下4个基本步骤解决了此样本问题。

Step1

Important physical properties of water are first determined. Since the water is stored in an atmospheric tank at ambient conditions,

Water temperature = 25⁰C

Water pressure = atmospheric pressure = 1.031 bar = 1 bar (approximately)

使用Enggld乐动体育appCyclopediaLiquid Density Calculator, waterdensity在25岁⁰C =994.72 kg/m³

使用Enggld乐动体育appCyclopediaLiquid Viscosity Calculator, waterviscosity在25岁⁰C =0.90 cP

使用Enggld乐动体育appCyclopediaVapor Pressure Calculator，25处的水蒸气压力⁰C =0.032 bara

步骤2

Next step to solve this sample problem is to calculate the frictional pressure loss in the suction line.

This NPSH calculation can be performed usingld乐动体育appEnggcyclopedia的管道压降计算器。根据线的大小计算器，吸气线中的摩擦压力损失为1.3 bar/km。因此，对于20m的管道长度

suction line pressure loss = 1.3 X 0.02 = 0.026 bar

This pressure loss can be converted to frictional head loss,

head loss = pressure loss /ρg = 0.026X10⁵/(994.72X9.81) = 0.27 m

In this sample problem, fittings in the suction line have not been considered. In case there are some fittings present in the pump suction line such as valves, elbows, reducer etc.,ld乐动体育appEnggcyclopedia的K因子计算器can be used to determine the frictional pressure loss caused by these fittings.

步骤3

Next step to solve the sample problem is to determine elevation difference (Δz) between pump suction and the liquid level in the storage tank. Note that for NPSH calculation always the lowest liquid level in the suction vessel or tank is considered for conservative estimate.

In the present sample problem,

Δz = elevation of tank + minimum liquid level - pump suction nozzle level

= 1 + 0.3 - 0.4 = 0.9 m

Step4

In the final of NPSH calculation, the above formula derived from NPSH definition is used.

Here, p0 = 1 bar ,

pv = 0.032 bara (water vapor pressure)

ρ = 994.72 kg/m³(water density)

g = 9.81 m/s²(gravitational acceleration)

Δz = 0.9 m

h_L= 0.27 m

使用上面的NPSH方程，

NPSH = (1-0.032)/(994.72X9.81) + 0.9 - 0.27

= 10.56 m

注意ld乐动体育appEnggcyclopedia的泵尺寸计算器使用相同的equation and method for NPSH calculation. It can be used to quickly determine the NPSH, instead of manually calculating it in 4 steps.

Related tutorials & calculators for NPSH calculation

• Net positive suction head- NPSH definition
• Tutorial -NPSHa calculation(for available NPSH)
• Difference betweenNPSHr (required) and NPSHa (available)
• Pump sizing calculator- for quick NPSHa calculation