Goulds pumps, Phv - packaged hydrovar series, N1 q2 n2 h1 – ITT BPHV User Manual

Page 18: N1 h2 n2, N1 p2 n2, Q1 q2 q h1 h2 h, N2 n1 a b

Goulds Pumps

PHV - Packaged Hydrovar Series

PERFORMANCE WITH VARYING SPEED FOR CENTRIFUGAL PUMPS

Fitting the electric pump with a variable speed drive makes it possible to vary the pump rotation speed, normally accor-
ding to the system pressure parameter.

Variations in electric pump speed result in modified performances

according to the equivalence relations, called affinity laws.

n1 = initial speed;

n2= speed required.

Q1 = initial flow rate;

Q2= flow rate required.

H1 = initial head;

H2= head required.

P1 = initial power;

P2= power required

Frequency ratios can be used instead of speed in practical applications, keeping 30 Hz as the bottom limit.

Example : 2-pole 50 Hz electric pump n1 =2900 (point A)

Flow rate (A) = 100 l/min; Head (A) = 50m

By reducing the frequency to 30 Hz the speed is reduced to approx. n2 = 1740 rpm (point B)

Flow rate (B) = 60 l/min; Head (B) = 18 m

The power of the new work point B is cut to about 22% of the initial power.

SIZING THE DIAPHRAGM TANK IN SYSTEMS WITH SPEED VARIATION

Variable speed booster sets need smaller tanks compared to traditional systems. Generally speaking, a tank with a

capacity of just 20% of the nominal capacity of a single pump, expressed in gallons per minute, is needed. The

gradual

starting of the pumps controlled by the drive reduces the need to limit the number of hourly starts; the main purpose

of the tank is to compensate for small system losses, stabilize the pressure and make up for pressure variations caused by
sudden demand (fast acting valves).

Make the following calculation:
Set made up of three electric pumps, each with a maximum flow rate of 100 GPM, for a total capacity of 300 GPM. The

volume required for the tank is 20 gallons. This is total capacity, not drawdown. Mount downstream of the check valves

in discharge manifold.

Flow Rate

Head

Power

Flow

Head