EFFECT OF SUCTION - LIFT:

For a centrifugal pump to develop its normal head - capacity characteristics, the suction lift against which it operates must not exceed a certain value. If this value exceeded, the head developed by the pump and its effiency begin to decrease to below normal for a given capacity. Normally, the pump should operate under a combination of capacity and suction conditions that permit it to develop normal head and capacity characteristics. Under these conditions the pump also operates at a best efficiency for a given capacity.

EFFECT OF VISCOSITY OF LIQUID:

The disk - friction losses of a pump increases with the increase in viscosity, because the thicker a liquid, more power is needed to rotate the impeller through it. Disk friction varies as the fifth power of the diameter of the impeller and varies as the cube of the speed.

As two of the losses in a centrifugal pump, disk horse power and friction heac increases with increased vicosity, the head delivered would be less for the some capacity or for the same head capacity discharged would beless. The efficiency of the pump handling viscous fluid will be lower than one handling water. How much it will decrease will depend on the type of pump, its hydraulic design and the relation of its operation to the conditions at best efficiency as well as to the viscosity of the liquid.

Example: when handling water at 26 lit/min, the pump develops 13 mtr. head (point A) but when handling a liquid of 1000 SSU viscosity at 26 lit/min, it develops a head of 8.8 m (point B).

EFFECT OF SPEED:

The discharge capacity of a pump varies directly as the speed, its head as the square of the speed and its power input as the speed cubed. Example: A pump that delivers 220 lit/min against 30 m total head at 1000 rpm (point A) and requires. 30 kw to drive it (point B), will deliver 200 lit/min against 25 m total head when run at 900 rpm (point C) and will require 22 kw to drive it under those conditions (point D).

However if the speed is increased sufficiently and the limit of operation at the prevailing suction conditions are exceeded, its head and capacity range will be reduced. Likewise if the speed is reduced excessively, the efficiency of the pump will be somewhat reduced so that its power input will no longer follow the speed - cubed relationship.

EFFECT OF IMPELLER - DIAMETER:

Since the peripheral speed of the impeller changes directly at its diameter, the pump capacity varies directly as the diameter, where as the head varies as the diameter squared and the power input varies as the diameter cubed.

for example: With a impeller diameter of 37 cm, operating at 1800 rpm, the pump has a capacity of 500 lit/min against a head of 66 mtr (Point A) and requires 112.6 kw to drive it (point B). If the impeller diameter is reduced to 35 cm, its capacity at 1800 rpm will be 474.4 lit/min against a head of 59.4 mtr (point C) and will require 89.5 kw.

EFFECT OF IMPELLER - WIDTH:

Variation of capacity at constant speed and diameter, can be facilitated by fitting impellers of different widths. Higher the width, higher would be the capacity. These curves also have points where performance is at its highest. Obviously, by altering impeller diameters and widths, a pump can be tailored to requirements.

EFFECT OF THROTTLING PUMP DISCHARGE:

Throttling the discharge of the pump increases the friction head in the system and discharge capacity is reduced. However head developed by the pump increases, and at complete shut-off it will create excessive head but will not cause excessive pressure. Due to churning and turbulance, pump will be overheated.

MATERIALS:

a)                   Pumps for engine cooling water, fresh or potable water: high grade cast iron casings with bronze internals - shaft of bronze or stainless steel (EN 57, 18 Cr/2Ni), the stainless steel gives better wear life.

b)                    Sea water pumps also for harbour, river and canal water : either gun metal casing with aluminium bronze impeller (BS 1400 A B 29, 5 Al - 5 Fe - 5.5 Ni) - shaft either stainless steel (EN57) for soft pack stuffing boxes, or EN 58 (18 Cr, 10 Ni, 3 Mo) under mechanical seals or bearings.

c)                    Boiler feed pumps: Because of high pressure and temperatures casings are of cast steel - shaft and impeller are of stainless steel.

d)         Cargo pumps : stainless steel casings, impeller and shaft suitable for most chemicals - nickel steel for low temperature liquefied gas.

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