Menghitung NPSHA dan NPSHR Pompa

Istilah di dalam sistem perhitungan pompa :

Pengertian istilah tsb ada disini

Untuk menghitung nilai NPSH_Akita menggunakan rumus :

NPSH_A = (static head + surface pressure head + pressure gauge) - (vapor pressure + friction losses in the piping, valves and fittings)

Istilah2 yg harus dipahami :

Static Head : tinggi yg diukur dari tengah suction pipe ke atas sampai permukaan atas cairan, dan kalau arah ke bawah berarti nilai static head negative (-)
Surface Pressure Head : tinggi yg didapatkan dari mengkonversi nilai tekanan yg ada pada permukaan cairan dan dihitung harus dalam pressure absolut (P_abs)
Vapor Pressure : disediakan grafik T vs P (Grafik 1) utk berbagai cairan, disini akan didapatkan nilai pressure (P) dan dikonversi ke head
Friction Loss : menggunakan data yg ada dalam tabel utk penghitungan (Grafik 2)
Pressure Gauge : selisih pressure antara pressure absolut dengan pressure lingkungan (1 atm)

Macam konversi pembacaan grafik dari "pembacaan tekanan" diubah ke "head" :

(Inches of mercury x 1.133) / s.g = feet of liquid

(Pounds per square inch x 2.31) / s.g = feet of liquid

Millimeters of mercury / (22.4 x s.g) = feet of liquid

Grafik 1. Tekanan Absolut Cairan berdasar Tekanan Uap

Grafik 2. Koefisien Tahanan (K) utk Valve dan Fitting

Contoh soal : (adapted from www.mcnallyinstitute.com)

Sumber : www.mcnallyinstitute.com

Atmospheric pressure = 14.7 psi
Gage pressure =The tank is at sea level and open to atmospheric pressure.
Liquid level above pump centerline = 5 feet
Piping = a total of 10 feet of 2 inch pipe plus one 90° long radius screwed elbow.
Pumping =100 gpm. 68°F. fresh water with a specific gravity of one (1).
Vapor pressure of 68°F. Water = 0.27 psia from the vapor chart.
Specific gravity = 1
NPSHR (net positive suction head required, from the pump curve) = 9 feet

PERHITUNGAN :

NPSH_A = (static head + surface pressure head + pressure gage) - (vapor pressure + friction losses in the piping, valves and fittings)

Static head = 5 feet
Atmospheric pressure = (pressure x 2,31) / s.g = 14,7 x 2,31 / 1 = 34 feet absolute
Gage pressure = 0 (karena tdk vakum)
Vapor pressure of 68°F. Water converted to head = (pressure x 2,31) / s g = 0,27 x 2,31 / 1 = 0,62 feet
Looking at the friction charts:

100 gpm flowing through 2 inch pipe shows a loss of 17,4 feet for each 100 feet of pipe or 17.4/10 = 1.74 feet of head loss in the piping
The K factor for one 2 inch elbow is 0,4 x 1,42 = 0,6 feet

Adding these numbers together, 1,74 + 0,6 = a total of 2,34 feet friction loss in the pipe and fitting.

NPSHA (net positive suction head available) = 34 + 5 + 0 – 0,62 – 2,34 = 36,04 feet

The pump required 9 feet of head at 100 gpm. And we have 36.04 feet so we have plenty to spare.

RUMUSAN SINGKATNYA :

Referensi : Diolah dari berbagai sumber

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