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PUMPS, MOTORS & DRIVES

Pulsation of flow and

pressure in piping of
reciprocating pumps and
compressors
Practical guidelines are presented to help avoid the
problem of pulsating flow in reciprocating machinery.
Amin Almasi
Sept. 1, 2020
An example of a reciprocating compressor LATEST IN
package with pulsation vessels (horizontal PUMPS, MOTORS
pulsation bottles at top and bottom of
& DRIVES
cylinders), auxiliaries and piping.

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Reciprocating compressors
and reciprocating pumps are
mainly used to produce a high
discharge pressure that is either
difficult or uneconomical for the
centrifugal machineries or other
types of machineries to achieve.
They have been used in many
services and applications. In
reciprocating machineries
(reciprocating compressors and
pumps), the working piston(s) or
plunger(s) are moving in a back-
and-forth fashion. They can
theoretically deliver the same
adjusted volume regardless of the
discharge pressure. In other
words, ideally, they can discharge
the same adjusted volume of fluid
— independent of behavior of the
downstream.

A major problem in reciprocating

compressors and pumps is the
pulsation of the flow due to the
intermittent action of the piston
and cylinder valves. The pulsating
flow causes vibration in the piping
and its supporting structure. The Bearings
& Seals
problems, issues and risks
Instrumenta
associated with this pulsation is & Measurem
quite complicated and
Piping,
widespread. There have been Tubing
&
many complaints about pulsation Hosing
of reciprocating machineries and
Process
particularly on the pulsation of Control &
Automation
flow in the associated piping in
many plants and facilities. This is
a widely reported area of concern Pumps,
Motors
and a hot topic in the flow &
movement industry. Drives

Valves &
This article discusses pulsation of Actuators

flow in piping of reciprocating

View All
machineries. The focus is on Companies >
practical guidelines and useful
knowledge to avoid this major risk
in fluid movement systems using
reciprocating pumps and
compressors.

Pulsation

The discharge from a

reciprocating machinery is not
continuous; rather, it is
intermittent. Similarly, the
suction flow to the machinery is
also intermittent. The piston is
pushed back and forth by the
connecting rod connected to the
crank, which is usually revolving
at a constant speed. With a
constant angular crank speed, the
linear speed of the piston varies
closely to a sinusoidal shape. If
the connecting rod is infinitely
long, then the piston speed follows
a pure sinusoidal form. With a
finite-length connecting rod, the
piston speed tends to be slower
than given by the sine curve at the
side closer to the crankshaft and
quicker toward the other side.

Because the pulsation problem

involves the reciprocating
machinery, pulsation vessels,
piping systems and accessories in
a multiple level of interaction, its
adverse phenomena is difficult to
determine. On one hand, some
reciprocating machinery
installations operate quite
satisfactorily without a detailed
pulsation study. On the other
hand, many ordinary preventive
pulsation studies do not
necessarily always eliminate the
pulsation problem.

Many plants and facilities have

adopted a wait-and-see policy. If
the system is not operating
smoothly, then all types of
questions are asked and used to
solve the problem. The fact is,
there are many reported problems
due to pulsation. Once the piping
starts shaking due to pulsation,
the entire facility is also likely to
shake. This could be a serious
situation. In some cases, an
immediate shutdown of the plant
is required.

Multiple cylinders,
interactions and variable
speed

Many reciprocating machineries

have more than one set of piston
and cylinder. The cylinder can
also be single-acting or double-
acting. Double-acting means the
piston is working both ways,
having inlet and outlet ports at
both ends of the cylinder. As flow
is discharged from both sides of
the double-acting cylinder or
different cylinders to a common
header, a combined volumetric
pulsation is created. The
combined pulsation flow
theoretically averages out by the
multiple outlets spaced at
strategically straddled crank
phase angles. More cylinders
result in a higher pulsation
frequency, but lower pulsation
amplitude, in theory. However,
there could be interactions.
Therefore, the pulsation matter is
more complicated where there are
different sources of pulsation, as
these can interact with each other.

Similarly, when a part-load

method is used, it can usually
complicate the pulsation patterns.
Another source of complication is
the speed variation. There have
been variable speed reciprocating
machineries, and the issues and
problems with their pulsation are
more complicated.

Cyclic shape and harmonics

The dominant (or fundamental)

pulsation frequency is the
crankshaft speed, multiplied by
the number of actions of piston(s)
or plunger(s) in each revolution.
For instance, in a single-cylinder
double-acting machinery, the
dominant pulsation is at two
cycles of pulsation per one
crankshaft revolution. This
fundamental (or dominant)
pulsation frequency is important,
as highest excitations are usually
experienced in this frequency.
In practice, the shape of the
pulsation has deviated from
sinusoidal. The shape of the
reciprocating compressor
pulsations is irregularly shaped.
Because of its imperfect but
nevertheless cyclic shape, the
pulsation is regarded as the
combination of many sinusoidal
pulsations at crankshaft rotation
frequency, fundamental frequency
and their higher harmonic
frequencies.

Compressors versus pumps

In a compressor, the gas should be

first compressed from the inlet
pressure to the outlet pressure
before it is discharged to the
outlet system. Because of its
compressible nature, it requires
the piston to move to a specific
point to compress the gas to reach
outlet pressure. The gas is then
discharged volumetrically
according to the speed of the
piston. This assumes that the
outlet pressure maintains a
constant pressure without being
influenced by the discharge of the
compressor.

The discharge shape of the pump

is quite different due to a
practically incompressible nature
of the pumped liquid. In a pump,
the liquid starts to discharge
almost instantaneously as the
piston starts to move. Therefore,
the volumetric shape is similar to
the sinusoidal shape without the
initial silent period. In high
pressures, liquid would become
slightly compressible. In very high
pressures, gases become dense,
and their behavior would be
similar to dense fluids.

Acoustic and structural

resonances
Acoustic and structural resonance
should be avoided. The pulsation
flow has two potential resonance
mechanisms that need to be
avoided. First, the pulsation
pressure wave can generate
acoustic resonance if the length of
any discontinuous section in the
piping has an acoustic natural
frequency that coincides with the
pulsation frequency.

Another potential problem is the

structural resonance of the piping
natural flexural vibration
frequency with the pulsation
pressure frequency. Acoustic
resonance can be largely avoided
by running a study of the piping
configuration and equipment
performance. Regarding piping
structural resonance, one of the
tactical solutions is to support the
piping in such a way that changes
the natural frequency of the
piping to be higher than the
pulsation frequency and not near
its harmonics. However, this is
usually challenging, as the
pulsation has many harmonic
modes. Too often, it is impossible
to provide the piping so that all
piping natural frequencies are
located away from all pulsation
frequencies (including
harmonics).

A commonly used policy is to

make the piping so stiff that its
fundamental frequency is at least
50% higher than the fundamental
frequency of the pulsation.
However, even this approach is
difficult to achieve in some cases.
A more practical rule-of-thumb
approach is to support the piping
with support spacing reduced to
one-half of the standard spacing.

Pulsation vessels (pulsation

bottles)
At a constant speed of machinery
and a constant setting of the
capacity control system, the
average mass flow passes the
entire system can be assumed
unchanged. However, the
volumetric pulsation entering the
pulsation vessel (pulsation bottle)
is partly absorbed by the volume
(capacitance) of the vessel
(bottle), leaving the rest to
discharge through the piping. The
pulsation flow to the piping
creates the pulsation pressure,
which produces the pulsation
force to shake the piping system.

A peak-to-peak pulsation
pressure is needed to push the
peak-to-peak volumetric pulsation
flowing through the piping. This
same pressure pulsation also
compresses the fluid volume
inside the vessel (bottle) to make
room for some of the incoming
pulsation flow. The bigger the
vessel volume and the higher the
fluid compressibility, the more
incoming pulsation is absorbed by
the bottle, leaving less pulsation
being transmitted through the
piping.

For incompressible fluids such as

the ones handled by the pump, the
portion of pulsation absorbed by
the pulsation vessel bottle is
negligible. Therefore, for pumps,
the residual pulsation flow is very
close to the original pulsation
flow.

For compressor installations, the

residual pulsation is roughly
determined by an attenuation
factor which is estimated as
1/(1+n), where “n” is the ratio of
the pulsation bottle volume to the
piston displacement volume. For
instance, if the volume of the
pulsation bottle is 7 times the total
cylinder displacement, pulsation
is roughly attenuated to around
0.125 (1/8) times the original
value. Thus, the bottle achieves
roughly an eightfold reduction of
the potential pulsation pressure.

The pulsation bottles used by the

reciprocating pump is more
complicated than the simple
conventional bottle. This is mainly
due to the incompressible nature
of the liquid. For liquid flow, a gas
volume is generally needed to
absorb the pulsation. The gas-
filled surge chamber is one such
example. With the compressible
chamber, a large portion of the
pulsation flow is absorbed by the
compression or expansion of the
gas volume. Thus, the residual
pulsation flow through the piping
is substantially reduced. Such a
pulsation dampener can be in the
form of a bladder-type dampener,
gas-filled surge chamber or
suction standpipe.

Piping connected to
reciprocating machineries

When dealing with a reciprocating

compressor or pump, the piping
can be excited by pulsation or
vibration. Therefore, special
provisions are needed for these
piping systems. The first
consideration is an independent
support system. The piping
vibration can propagate to the
entire facility or plant when the
piping is supported from a
common structure. Therefore, it is
important that proper spaces have
been allocated so the piping can
be independently supported
(independent from reciprocating
machinery and independent from
all other facilities).
Furthermore, the supports offered
should have sufficient stiffness to
effectively control the dynamic
motion of the piping and elevate
the natural frequencies to high
levels. Stiff supporting members,
such as concrete sleepers located
at grade level, should be used if
practical.

Another consideration is secured

clamping. The connection
between the piping and the
support structure is critical to the
effectiveness of the support. A
good connection starts with good
clamps, which is the first link
between the piping and support.
Without a good connection, a
purposely built heavy support
structure is not worth the cost.
The clamp should be stiffened and
provided with belting material to
offer some damping effect in
addition to securing a good
connection. Some successful
support types have been provided
with two squeezing wedges to
ensure a snug fit of the pipe and
clamp. This is often used in large
piping.

Different types of special supports

have been used for piping of
reciprocating machineries. In
general, simple and strong
systems are preferred. It is
important to investigate how
support is reacted in each
direction. Attention is needed for
details. A key point is, while the
support scheme should be stiff,
thermal movements should be
properly considered. Thermal
movements of the piping should
be accommodated without causing
overstress. However, most
vibration piping operates at a
rather moderate temperature. If
the rated temperature is overly
conservative, the piping might be
too flexible to prevent vibration.
Therefore, it is important to set a
realistic rated temperature.

If vibration or pulsation is
suspected, priority should be
given to these issues, and only
realistic thermal movements and
stresses with absolutely minimum
margins should be considered. For
many continuously operated
processing plants, thermal load
cycles only once or twice a year,
yet the vibration stress occurs
hundreds of cycles per minute. 

Amin Almasi is a
lead mechanical
engineer in
Australia. He is
a chartered
professional
engineer of Engineers Australia
(MIEAust CPEng – Mechanical)
and IMechE (CEng MIMechE) in
addition to a M.Sc. and B.Sc. in
mechanical engineering and
RPEQ (Registered Professional
Engineer in Queensland). He
specializes in mechanical
equipment and machineries
including centrifugal, screw and
reciprocating compressors, gas
turbines, steam turbines, engines,
pumps, condition monitoring,
reliability, as well as fire
protection, power generation,
water treatment, material
handling and others. Almasi is an
active member of Engineers
Australia, IMechE, ASME and
SPE. He has authored more than
150 papers and articles dealing
with rotating equipment,
condition monitoring, fire
protection, power generation,
water treatment, material
handling and reliability. He can
be reached at
amin.almasi@ymail.com.

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