| HEAD
Centrifugal pump curves show 'pressure' as head, which
is the equivalent height of water with S.G. = 1 (ie:
metre or feet). This makes allowance for specific gravity
variations in the pressure to head conversion to cater
for higher power requirements. Positive Displacement
pumps use pressure (ie; psi or kPa) and then multiply
power requirements by the S.G.
STATIC
HEAD
The vertical height difference from surface of water
source to centreline of impeller is termed as static
suction head or suction lift ('suction lift' can also
mean total suction head).
The vertical height difference from centreline of impeller
to discharge point is termed as discharge static head.
The vertical height difference from surface of water
source to discharge point is termed as total static
head.
TOTAL HEAD / TDH
Total height difference (total static head) plus friction
losses & 'demand' pressure from nozzles etc. (plus other
small head losses ie: velocity head) ie: Total Suction
Head plus Total Discharge Head = Total Dynamic Head
(TDH).
S.G.
Specific gravity. weight of liquid in comparison to
water at approx 20 deg C (SG = 1). |
|
SPECIFIC
SPEED
A number which is the function of pump flow, head, efficiency
etc. Not used in day to day pump selection, but very
useful to determine the characteristics of a pump relating
to efficiency, NPSH, solids handling, if you can start
against open / closed valve, etc. See
Fig.1
VAPOUR
PRESSURE
If the vapour pressure of a liquid is greater than the
surrounding air pressure, the liquid will boil.
VISCOSITY
A measure of a liquid's resistance to flow. ie: how
thick it is. The viscosity determines the type of pump
used, the speed it can run at, and with gear pumps,
the internal clearances required.
FRICTION
LOSS
The amount of pressure / head required to 'force' liquid
through pipe and fittings.
NPSH
Nett positive suction head - related to how much suction
lift a pump can achieve by creating a partial vacuum.
Atmospheric pressure then pushes liquid into pump. A
method of calculating if the pump will work or not. |
