Available NPSH calculation (NPSHa) is important for deciding the line sizes on suction side of a centrifugal pump. Selecting a large enough line size is crucial for avoiding空化in the pump.
This post will walk you through steps of calculating the available NPSH at the pump suction. It is important to note theNPSHA（可用）和NPSHR（必需）之间的差异。This post will focus on NPSHa calculation. NPSHr is normally provided by the泵制造商。
Problem Statement: available NPSH calculation
Estimate theNPSHA (Net Positive Suction Head Available)for a pumping system which is designed to pump 200,000 kg/hr of water. The water stream is available from astorage tankwhich operates at atmospheric pressure and 250C.
最低液位storage tank上面的泵吸气喷嘴保存为3m。吸气线的尺寸为6英寸，长10m。
The discharge from pump is to be sent to another vessel with a top connection for water inlet. The maximum height for the 6″ discharge line above the pump discharge nozzle is 12m. The discharge vessel operates at a pressure of 3 barg. There is no control valve in the discharge line. Discharge line to be assumed 100m long considering all the fittings and valves.
Solution
This sample problem can be solved by following these steps.
Step1
First physical properties (density, viscosity, vapor pressure etc.) of the process fluid (water in this case) must be determined at given operating pressure and temperature conditions.
使用Enggld乐动体育appCyclopediaLiquid Density Calculator， 水density在25岁0C =994.72 kg/m3
使用Enggld乐动体育appCyclopediaLiquid Viscosity Calculator， 水viscosity在25岁0C =0.90 cP
使用Enggld乐动体育appCyclopedia蒸气压计算器，25处的水蒸气压力0C =0.032 bara
Other adequate reference sources may be used for determination of the physical properties depending on design basis of the project.
步骤2
The second step of NPSHA calculation is to determine the frictional pressure loss in the suction line to the pump.
EnggCyclopedia's单相流量的管道压降计算器可用于此压力损失计算。有关使用此计算器的帮助，请参阅solved example for line sizing。在当前情况下，6英寸吸气和排放线的压降约为5 bar/km。对于10m吸力线，压降变为0.05 bar。
If details such as length of the suction line or line size have not be finalized, a suitable and conservative suction line pressure drop should be assumed. Other proprietary software or manual calculations may also be used to determine the suction line frictional pressure drop.
For suction line strainer, pressure drop can be calculated usingld乐动体育appEnggcyclopedia的过滤器压降计算器。For this example, the strainer pressure drop is around 0.09 bar. In case of an existing strainer the pressure drop may actually be measured using pressure differential indicators on the pump suction strainer. In case of new strainers being purchased, the strainer manufacturer can give a conservative value for maximum possible pressure drop across the strainer. This value can be used for pump sizing calculation and NPSH calculation.
步骤3
ByNPSHA的定义,
hLis the head loss between 0 and 1, p0is the pressure at the water surface, pVis the vapour pressure (saturation pressure) for the fluid at thetemperatureT1 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 fluiddensity, assumed constant, and g is gravitationalacceleration。
po = 1.013 bara (atmospheric pressure - Problem Statement)
PV = 0.032 BARA（25时的水蒸气压力0C-步骤1）
ρ = 994.72 kg/m3（25的水密度0C-步骤1）
ΔZ = 3 m (minimum liquid level in the suction tank above the pump suction nozzle - Problem Statement)
hL= head loss in the suction line
= pressure loss / ρg
= 0.05×105/ (994.72×9.81)
= 0.5124 m
Substituting all these values in the NPSHA equation,
NPSHA = (1.013 - 0.032)×105/（994.72x9.81） + 3.0-0.5124 = 12.54 m
Thus net positive head available at the pump suction (NPSHA) is 12.54 m.
References and calculators
- 净正吸头- NPSH definition
- Difference betweenNPSHR（必需）和NPSHA（可用）
- 泵尺寸计算器- for quick NPSHa calculation
- Here is anotherNPSH calculation tutorialthat makes extensive use of the EnggCyclopedia calculators for quick calculations.
