What is a jet pump, and how does it differ from a centrifugal pump? A jet pump actually is two pumps in one – a centrifugal pump and a jet assembly, commonly called an injector. Figure 1 shows a typical shallow-well jet pump. Parts 1, 2 and 3 – the adapter, nozzle and venturi on the left side of the pump – are the injector or jet components. We refer to the entire package, the centrifugal pump and injector, as the jet pump. The centrifugal pump part of a jet pump package is specifically designed to operate in conjunction with an injector, and the injector enhances a centrifugal pumps pressure capability by about 50 percent.
Shallow-well jets, which have the injector attached to the pump, are limited by
atmospheric pressure as discussed last month, to about 25 feet of lift, just
like straight centrifugal pumps. Their only advantage over a straight
centrifugal is in their pressure-boosting capability.
Deep-well jets, on the other hand, have their jet injector down in the well
below the water level, so they push the water to the surface (see Figure 2).
Deep-well jet pumps are not limited by atmospheric pressure to 25 feet of lift.
A good deep-well jet can pump water from as deep as 200 feet. Remember, when we
say we are pumping from 200 feet, we are referring to the distance from the
surface of the water in the well to the discharge point at or above ground
level, not from the injector to the discharge point.
A third type of jet pump, called a convertible jet pump, is designed such that
the injector can be either attached directly to the pump or located down in the
well. Convertible jet pumps, therefore, can be operated as either a
shallow-well jet or a deep-well jet. With that background, we now will look at
the jet assembly to see how it functions.
The jet assembly or injector consists of three major parts – the jet body, the
nozzle and the venturi. Here is how it works: With the centrifugal pump primed
and pumping, a portion of the water leaving the impeller is diverted to the
injector. The amount of water diverted is determined by the pump design in a
shallow-well jet, and is not adjustable. In deep-well jets and convertible
jets, it is adjustable by means of a control valve, which is the subject of
next month’s article. Figure 3 shows a deep-well injector.
The diverted portion of water that powers the injector, called drive water, is
directed through the nozzle where it accelerates, just like water passing
through the nozzle at the end of a garden hose. The drive water stream is
directed through a gap toward the venturi, creating a partial vacuum at the
gap. Here, atmospheric pressure forces product water (well water) to enter the
injector from the suction pipe and mix with drive water as it enters the
venturi. The outward flair of the venturi reduces the velocity of the stream as
it passes through, converting it back into pressure, and directing it into the
eye of the impeller where it is further pressurized. Upon leaving the impeller,
a portion exits the pump to become service water, and the rest is returned to
the injector as drive water.
In a shallow-well application, a single pipe is connected to the inlet of the
injector and extended down into the well. In a deep-well application, the
injector is down in the well, requiring two water passages from the pump into
the well – one for drive water, and the other for product water. As you will see
in the next paragraph, one of the passages can be the well casing. For now, we
will refer to both passages as pipes. The drive water pipe often is called the
pressure pipe, and product water pipe often is referred to as the suction pipe,
which is a bit of a misnomer since it too is under pressure. I prefer to call
the product water pipe the delivery pipe or discharge pipe, and it should be
one size larger than the drive pipe because it carries more
Deep-well systems are broken down into two sub-types – double-pipe and
single-pipe. When the well casing is 4 inches or larger in diameter, a
double-pipe system normally is preferred. Where the well casing is smaller than
4 inches in diameter, a single-pipe deep-well injector can be used. It differs
from a standard two-pipe deep-well injector in that it is smaller in diameter,
is hung from a single suction pipe, and includes packers that seal against the
casing. A well casing adapter seals the top of the casing, and provides a means
of introducing drive water into the casing. With both ends sealed, the well
casing acts as the second pipe for the drive water.
To keep jet pumps primed, it is important to install a foot valve at the bottom
of the suction pipe to prevent the water in the system from draining back into
the well when the pump is off. This applies to all types of jet pumps.
A question that often comes up: “Will a jet pump work without the jet?” The
answer is yes, but .... Yes, because it will pump water, and no, because it may
destroy itself by pumping too much water. One of the advantages of a jet pump
is that it cannot be overloaded on the horsepower curve because we create
artificial head with the pressure regulator and nozzle. Without the jet
assembly, a jet pump can pump beyond its curve, draw too much power, overheat
the motor, and possibly burn it up. The bottom-line: Don’t use a jet pump
without the injector. We will continue this discussion of jet pumps next month
with a close look at jet pump control valves. ’Til then ….
Tech Topics: Jet Pumps - Part 1
August 1, 2010