- What is Net
Positive Suction Head (NPSH)?
- What is
Cavitation?
- What is Static
Head?
- What is Reverse
Osmosis?
- What is Salinity?
- What
is Secondary Treatment?
What is Net Positive
Suction Head (NPSH)?
Simply stated, NPSH is an
analysis of the energy conditions on the suction side of the pump to determine
if the liquid will vapourise at the lowest pressure point of the pump.
The pressure that a liquid
exerts on its surrounding is dependent on its temperature. this pressure is
called its vapour pressure. It is a unique characteristic of every fluid and it
increases with temperature. When the vapour pressure of a fluid equals the
pressure of its surroundings, the fluid begins to vapourise, or boil.
If we wish to pump a fluid
effectively we must keep it in liquid form. NPSH is simply a measure of the
amount of suction head present to prevent this vapourisation at the lowest
pressure point in the pump.
NPSH required is a function
of the pump design and varies with speed and capacity.
NPSH available is a
function of the system the pump is operating in. It is the excess pressure of
the liquid, in metres absolute, over its vapour pressure as it arrives at the
pump suction.
NPSH available must always
be great than NPSH required at the maximum required flow rate
Cavitation is a term used to describe the phenomenon,
which occurs in a pump when there is insufficient NPSH Available. When the
pressure of the liquid is reduced to a value equal to or below its vapour
pressure the liquid begins to boil and small vapour bubbles or pockets begin to
form. As these vapour bubbles move along the impeller vanes to a higher pressure
area above the vapour pressure, they rapidly collapse.
The collapse, or "implosion" is so rapid that it may be
heard as a rumbling noise, as if you were pumping gravel. In high suction energy
pumps, the collapses are generally high enough to cause minute pockets of
fatigue failure on the impeller vane surfaces. This action may be progressive,
and under severe (very high suction energy) conditions can cause serious pitting
damage to the impeller.
The accompanying noise is the easiest way to recognize
cavitation. Besides possible impeller damage, excessive cavitation results in
reduced capacity due to the vapour present in the pump. Also, the head may be
reduced and/or be unstable and the power consumption may be erratic. Vibration
and mechanical damage such as bearing failure can also occur as a result of
operating in excessive cavitation, with high and very high suction energy pumps.
The way to prevent the undesirable effects of cavitation
in standard low suction energy pumps is to insure that the NPSH Available in the
system is greater than the NPSH Required by the pump. High suction energy pumps
require an additional NPSH margin, above the NPSH Required. Hydraulic Institute
Standard (ANSI/HI 9.6.1) suggests NPSH margin ratios of from 1.2 to 2.5 times
the NPSH Required, for high and very high suction energy pumps, when operating
in the allowable operating range.
Source:
gouldspumps.com
When water is not moving, the vertical
distance (in feet) from a specific point to the water surface is the static
head. (The static pressure in psi is the static head in feet times 0.433
psi/ft.).
Reverse osmosis refers to a process of water
purification that has been used primarily for the desalination of seawater. To
understand reverse osmosis, it is first necessary to understand osmosis. Osmosis
is the term for the phenomenon whereby if a semi-permeable membrane separates
two salt solutions of different concentration, water will migrate from the
weaker solution through the membrane to the stronger solution, until the
solutions are of the same salt concentration. Reverse osmosis subverts this
process. It involves applying pressure to reverse the natural flow of water,
forcing the water to move from the more concentrated solution to the weaker. The
semi-permeable membrane is porous, allowing water to pass through, but blocking
the passage of the bulkier salt molecules (Binnie, Kimber, & Smethurst, 2002).
The end result is water sans salt on one side of the membrane.
The semi-permeable membranes for reverse osmosis treatment are generally
constructed from polyamide-based materials. These materials are resistant to
biological degradations, but are subject to chemical attacks from chlorine.
Reverse osmosis has been used as a method of purification for ground and surface
fresh water, in addition to its role as a desalinating agent. Working with such
water sources creates some problems for the reverse osmosis system. Because of
the very small pore sizes involved in the membrane, it is vital that ground and
surface water is adequately pre-treated prior to the reverse osmosis process.
Depending upon the hardness of the water involved, scaling of the membrane is
likely to occur. If the concentration of the calcium or magnesium in the water
(the chemicals that determine water’s hardness) is at a high enough level where
the chemicals are insoluble, it will create a hard mineral on the inside of the
membrane, rendering it impotent (Vigneswaran & Visvanathan, 1995).
Source:
historyofwaterfilters.com
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1. The relative
concentration of dissolved salts, usually sodium chloride, in a given
water sample.
2. A measure
of the concentration of dissolved mineral substances in
a given water sample.
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A wastewater treatment process used to
convert dissolved or suspended materials into a form more readily
separated from the water being treated. Usually the process follows
primary treatment by sedimentation. The process commonly is a type of
biological treatment process followed by secondary clarifiers that allow
the solids to settle out from the water being treated. |
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