Positive Displacement Pump

POSITIVE DISPLACEMENT PUMP
A pump that first traps liquids and then delivers it is called positive displacement pump.
I t can be classified as:
1. Rotary pump.
2. Reciprocating pump.
ROTARY PUMP
A positive displacement pump having rotary motion is called a rotary pump. The names
of rotary pump parts are similar to that of a centrifugal pump. Introduction about few
types of rotary pumps is given below.
GEAR PUMP
The fluid in a gear type pump travels around the outside of the gear teeth. The meshing
of the gear teeth prevents the fluid from returning to the suction side. Gear pump can
be sub divided as spur gear pump, helical gear pump & herring bone gear pump. All
these pumps are the external gear pumps as the fluid is discharged around the outside
of the gear teeth. Only one shaft extends from the casing of the pump. It is called the
driving shaft. The other shaft with its gear or idler is called the driven shaft.
Helical and herringbone gears are used in larger pumps, which

handle larger capacities and operate under higher speeds and
pressures than spur gear pumps. A slight pulsation occurs in
discharge of spur gear pump.
TECHNICAL TRAINING CENTER 1

Another type of gear pump is the internal gear
pump. This pump is entirely different in construction
than the external gear pump. It consists of two
gears in mesh with one another. The outer or larger
gear of the set is the driving gear and the other
smaller gear is the driven or idler gear of the pump.
The crescent keeps the gears separate and
increases pump efficiency. Like external gear
pumps it may be operated in either direction.
SCREW PUMP
This pump with its single or double idler design is able to pump large amounts of oil
with relative ease. In operation the center rotor
drives the screw pump. It drives two other idler
rotors, which mesh it. Because the threads of
the power rotor mate with the idler rotors, the
fluid is forced along the opening in the threaded
area towards the discharge of the pump. This
type of pump is suitable to handle oil or heavy
viscosity fluids. All the three rotors or screws are
usually made of the same metal (high carbon
steel, steel alloy or stainless steel).
VANE TYPE PUMP

It is another type of rotary pump generally used for
transferring hydraulic oils or viscous materials such
as paints. The vanes of the pump are made of
softer material than the pump casing. They wear
less and are replaceable. It is very simple in
operation. As the impeller rotates its offset position
above the centerline allows the vanes or blades to
extend and draw in fluid one side. The vanes are
placed or mounted within the impeller with a small amount of clearance for free
movement. The vanes are spring loaded to assure
complete contact with the pump housing at all times.
LOBE PUMP
Lobe pump works very much like a gear pump. The
lobes are mounted on separate shafts and rotate in
opposite direction. The liquid enters the pump through

the suction port and trapped between the wall of the casing and the rotating lobes.
Lobe pumps are used for relatively low pressures.
RECIPROCATING PUMPS
A pump that operates using a back and forth straight-line
motion is called a reciprocating pump. There are many
types of reciprocating pumps.
Reciprocating pumps have two sections, the liquid

section (hydraulic) and the power section. The liquid
section does the pumping and the power section
provides driving force to operate the liquid section.
Reciprocating pumps may be plunger type or piston type. The main parts of this pump
are:
HYDRAULIC END
1. Plunger or piston with piston rod.
2. Suction & discharge valves.
3. Cylinder.
4. Stuffing box.
POWER END
1. Crank shaft.
2. Cross head.
3. Connecting rod.
4. Crack shaft.
OPERATION OF RECIPROCATING
PUMP:
Reciprocating pumps are simplex, duplex, triplex or quintrex etc. Simplex pump is with
only one cylinder, while duplex with two, triplex with three and quintrex with five
cylinders and pistons. These pumps may be single acting or double acting. If the fluid is
delivered only with the forth going stroke it will be the single acting. When a pump
discharges liquid on both forward and backstrokes, it is called double acting pump.

WORKING PRINCIPLE:
AS the plunger or piston moves forward, the
liquid is forced out from the cylinder through the
discharge valve and on the back coming stroke,
discharge valve does not allow the delivered
liquid to return back in the cylinder by closing
itself. In the meantime, the suction valve opens
and space is filled with liquid. The suction valve
stops liquid from cylinder by closing itself with
the pressure built in the cylinder.
METERING OR DOZING PUMP

Metering pumps frequently resemble the reciprocating pumps but they are very special
in design and construction,. Because of their precision construction, they are very
sensitive to temperature change in fluids. In addition they can deliver a measured and
adjustable amount of flow. An adjusting device is installed on it. As the pump can deliver
small capacities, it is very much suitable for adding specific amount of chemicals to
water or any other system. It can be classified as plunger, piston or diaphragm
reciprocating pumps.
DIAPHRAGM PUMPS
Diaphragm pump is used for handling corrosive fluids.

PULSATION DAMPENER
The flow of liquid in reciprocating pump is pulsating. The flow is more even from a
double acting pump than single acting.
A pulsation dampener is connected to the discharge line. It is a sealed chamber

containing air or Nitrogen.
Nitrogen is preferred when hydrocarbons are being pumped. The dampener helps to
maintain a more even flow of liquid through the line.
Reciprocating Pump with Pulsation Dampener
Liquids under pressure absorb gas. Some times a bladder is used in the chamber to
seal off the gas from the liquid.

When there is no bladder the liquid gradually
absorbs air or gas in chamber. The chamber may
be refilled by isolating the chamber and opening
the drain valve at the bottom if air at atmospheric
pressure is used.
The chamber may be refilled by forcing

compressed air or gas into the top of the
chamber. In low-pressure pumps a snifter is used
to recharge the air chamber automatically.
SUCTION STABILIZER
Suction stabilizers are used on high-
pressure pumps. This prevents the
suction liquid from hammering at the
cylinder value and piping. Stabilizer is
connected to the pump’s suction line. It
works like a pulsation dampener in
reverse.
While the pump is drawing liquid, liquid

flows out of stabilizer. When the
suction valve closes liquid flows from
the suction line into the stabilizer.
Prepared By
USAMA BIN RIAZ
(DAE MECHANICAL)

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POSITIVE DISPLACEMENT PUMP

Helical and herringbone gears are used in larger pumps, which

TECHNICAL TRAINING CENTER 1

VANE TYPE PUMP

TECHNICAL TRAINING CENTER 2

Reciprocating pumps have two sections, the liquid

TECHNICAL TRAINING CENTER 3

METERING OR DOZING PUMP

TECHNICAL TRAINING CENTER 4

A pulsation dampener is connected to the discharge line. It is a sealed chamber

Reciprocating Pump with Pulsation Dampener

TECHNICAL TRAINING CENTER 5

The chamber may be refilled by forcing

While the pump is drawing liquid, liquid

USAMA BIN RIAZ

TECHNICAL TRAINING CENTER 6

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