Net Positive Suction Head (Npsh) Pump in Series and Parallel- Chemical Engineering

NET POSITIVE SUCTION HEAD (NPSH) PUMP IN SERIES AND PARALLEL NPSH

is a flow parameter that measure the difference between the pump’s inlet stagnation pressure head and the vapor pressure head. There are two types of NPSH which are NPSHr (required) and NPSHa (actual). NPSHa is use to describe absolute pressure of a fluid at inlet of the pump minus vapor pressure of liquid. NPSHr is use to describe energy loses that occur within many pumps as the fluid volume is allowed to expand within the pump body. NPSHr depends on impellor design & diameter, inlet type, flow rate, pump speed and other factors.

FIGURE 3: The curve shows when two (or more) pumps are arranged in serial, their resulting pump performance curve is obtained by adding heads at the same flow rate. FIGURE 3

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2       NPSH     g 2 g  g   pumpinlet

FIGURE 4: When two or more pumps are arranged in parallel, their resulting performance curve is obtained by adding their flow rates at the same head.

NPSHa = Fluid surface pressure + positive head – pipework friction loss – fluid vapor pressure FIGURE 1: If the NPSHa is greater than the NPSHr cavitations should not occur.

FIGURE 4

If the NPSHa is lower than the NPSHr then gas bubbles will form in the fluid and cavitations will occur. FIGURE 1

In this case we focus on centrifugal pump to describe the relationship between NPSH with the location of pump in series and parallel. Shutoff head Pump Performance Curve

BEP H

H H available

H, η pump, or bhp

η pump

Operating Point

H* bhp*

bhp

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V*

System curve H required

Free Delivery

0

0 

0

V

FIGURE 2(a)

0

BEP

FIGURE 2(b)

APPLICATION & BENEFITS OF PARALLEL PUMP Parallel pumping don't need to use bigger inline or base-mounted pumps as often because two smaller pumps in parallel will handle the job just as well. If you get stuck in a jam and your supply house doesn't have that one big pump you need to do the job, by applying parallel pumping you will be able to use two smaller stock pumps. Parallel pumping with variable speed can save energy. Two smaller pumps could be less costly than running one large pump. Parallel pumps provide good operating flexibility in static head-dominated systems.

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V

REFERENCES Fluid Mechanics Fundamental and Application; Yunus A. Cengel;John M. Cimbala ;1st Edition in SI units McGraw Hill 739-750. http://www.engineeringtoolbox.com/npsh-net-positivesuction-head-d_634.html

FIGURE 2(a) and FIGURE 2(b) shows the pump performance curve for a centrifugal pump with backward inclined blades. The operating point of a piping system established as the volume flow rate where the system curve and pump performance curve intersect. PREPARED BY: SITI THAIYIBAH SHAFIE (141257), HAFIZAH NAIHI (142023), NURULAIN ABU BAKAR (141180) ECH 3103 (DR. SITI ASLINA HUSSAIN)