Best practice no. 25

PRESSURE (ENERGY) AT SURFACE OF LIQUID

Example 4 .- Open system with gauge at pump suction.

Problem: For the system shown in the drawing, determine neet positive suction head available (NPSHA) when liquid is 68 °F water and reservoir is at sea level. A gauge is attached to the suction side of the pump at: Y = 1 ft above the pump centerline and reads: P g = 2.4 psi . liquid velocity in suction line is: V = 10 fps. Solution : The general equation for this calculation is: Given data are: P g = 2.4 psi and V = 10 fps , and Y = 1 ft. From table 1 , at sea level, P a = 14.7 psi , and from table 2 at temperature of 68° F, P vpa = 0.339 psi , then, NPSHA = 2.31 (2.4 + 14.7 – 0.339) – 100/64.34 +1 = 38.72 + 1.55 + 1 = 41.27 ft Exempla 5 .- System analysis with suction lift . Problem: A pump application requires 300 gpm at 80 ft total discharge head. The pump will operate at 1750 rpm. A 10 hp pump is selected with a Flow capacity of 300 gpm and 80 ft total head capacity (see graph). Determine the maximum allowable suction lift this pump can safely handle when: A. Liquid is at 85°F and pump operates at elevation of 1000 ft above sea level. B. Liquid is at 85°F and pump operates at elevation of 1000 ft above sea level. Safety margin is 10 percent. Solution A : The general equation for these calculations is: Given data are: NPSHR = 16 ft (taken from NPSH curve in graph), h f = 5 ft and , h vpa = 1.38 ft. (from standard reference tables as vapor pressure of 85°F water). From Table 1, at 1000 ft, h a = 32.8 ft. Then, because NPSH available (NPSHA) must equal or exceed NPSH required (NPSHR), substitute the value of NPSHR from the pump performance curve (above) in the equation and solve for h s or maximum suction lift. NPSHA = h s – h f + h a - h vap

h vpa = 0.78 FT

h f = 12.0 FT

h a = 33.9 FT

NPSHA

ATMOSPHERIC PRESSURE AT SEA LEVEL

LIQUID

h s = 20.0 FT

PUMP

CL

REQUIRED NPSH (FROM PUMP CURVE)

18 FT

VAPOR PRESSURE OF 85°F H 2 O (GIVEN) FRICTION LOSSES IN SUCTION PIPING ( GIVEN)

1.38 FT

5 FT

PRESSURE)

h s OR MAX. ALLOWABLE

10.42 FT AVAILABLE NET POSITIVE SUCTION HEAD 32.8 FT OF LIQUID (ATMOSPHERIC

SUCTION LIFT FOR PUMPO AT STATED CONDITIONS

NPSHA = h s – h f + h a - h vap

Hence:

NPSHR = h s – h f + h a - h vap

16 = h s – 5 + 32.8 – 1.38

h s = - 10.42 ft