Piping Pulsation and Vibration
Criteria recommended for early stage of piping layout design to minimize shaking forces: 1. Knock out drum/separator
mounted close to the compressor. 2. Minimize sources of
shaking forces generated in equipment and piping such as elbows, tees, size variation, etc. Extra supports must be provided near concentrated masses such as elbows, tees, valves,
etc. 3. Position normally closed valves on branch lines close to
the branch point. 4. Control maximum gas velocity (below 30
m/s, common speed range: 10 to 20 m/s, to avoid gas turbu-lence/vortex, high pressure drop, noise, etc.).
The maximum piping span between two consecutive supports can be calculated to control resonances. Piping must be
designed as close to the ground as possible. For fixed equipment such as filters, separators, etc., skirt support is strongly
recommended. Leg-type supports or other relatively flexible
supports must be avoided.
Pulsation Shaking Forces
Reduction of pressure pulsation (for example, using larger
pulsation vessels) can be accompanied by an increase in shaking forces (or unbalanced forces). The margin of separation
between the mechanical natural frequency (MNF) of a system
(including piping and bottles) and excitation frequency is 20%.
Also, MNF shall be greater than 2. 4 times maximum run
speed. If the design does not meet these limits, determining
the force response (including stress analysis) is required. The
cylinder gas forces (also called frame stretch or cylinder stretch
forces) can be a significant source of excitation (can cause
high-frequency vibration on the bottles and piping close to the
compressor) and can lead to excessive pulsation bottle and
piping vibration even if the pulsation shaking forces meet limits. The first step is to calculate these forces. The second step
is evaluating the mode shapes.
For pulsation vessels, symmetric inlet and outlet connections are necessary in order to minimize the shaking forces.
For example, cylinder connections can be routed to the
center of the pulsation bottle. Furthermore, the length of
piping between cylinder and bottle must be limited to the
minimum possible, because the longer it is, the more harmonic components may resonate.
Fabrication practice such as installation and mounting details to the skid (or foundation) are very important to provide
the required stiffness of components. As vessel design (length
vs. diameter) vs. mounting design can dramatically affect
MNFs, accurate finite element analysis (FEA) modeling techniques that closely match the real MNFs must be used.
Shortcuts in modeling (simplistic models such as those assuming a rigid base or generic estimate of stiffness) create high
risk. The model must include base details, mounting plate,
bolts, beams and local skid construction. More than 15% of errors is reported for simplistic models. High error means high
vibration and failure or excessive costs (conservative design).
Low Pressure Drop Strategically Located Orifice
Optimum pulsation reduction techniques tend to dissipate
less energy than reliance on special solutions such as orifices
to control pulsation levels. However, low pressure drop orifices may be necessary for some applications or specific situations. Insertion of orifices keeps pulsation vessel fabrication
schedules without need to wait for the accurate pulsation
study to be performed. For some cases, damping of the resonance is necessary to avoid excessive shaking forces inside the
vessel (to avoid damage).
These orifices must be located in strategic points for
most effective pulsation reduction and lowest pressure
drops (points with maximum velocity in the standing
wave field of resonance and optimum effects of dampening). The position can be obtained from trial and error
Orifice plates in the throat of flanged inlet nozzle connections are among the effective tools to reduce the amplitude of
acoustic resonance occurring between the cylinder and the
volume bottles. For effective operation, these orifices must be
located exactly at the outlet for suction (outlet flange of suction pulsation vessel connected to piping of cylinder suction
port) and at inlet for discharge of the volume bottles (inlet
point of discharge flow to discharge pulsation vessel). Because
the length of the section in which resonance is generated is
very short, pulsations can vary suddenly in these sections.
Preliminary and roughly predicted pressure drop values include: pulsation devices (total pressure drop for pulsation vessels, internals, orifices, etc.) — around 1% pressure; intercooler — around 10. 15 psi (0.70 bar). The use of orifice
plates, especially on high-speed, single-acting machines, can
contribute to significant pressure drops. These kinds of high
pressure drop orifices must be avoided. Based on experience,
correct design and proper location selection for low pressure
drop orifices will result in around 0.1 to 0.8% pressure drop
(commonly 0.3% pressure drop). Using an orifice requires an
“Orifice Justification Report” to be sure of orifice application
and proper design of orifice type and location.
Case Study
Analytical results are presented for heavy-duty special-purpose hydrogen reciprocating compressors. Table I shows
selected optimum pressure pulsation limit values at the vessel
outlet flange (plant side) as a percentage of “API 618 –
Approach 3” limit values for design stage (based on detailed
calculation and optimization processes).
Stage 1
Stage 2
Stage 3
Suction Pulsation
Limit
as of API618
Appr. 3 Limit
85%
88%
97%
Discharge Pulsation
Limit
as of API618
Appr. 3 Limit
91%
95%
90%
Table I. Selected optimum pressure pulsation limit values (plant side)
as percentage of API 618 – Approach 3 limit values.
Pulsation
at Cylinder
Flange (%)
3.3%
4.3%
6.1%
Pulsation
at Vessel
Flange (%)
0.8%
0.9%
0.6%
Full Flow
Half Flow
Alternative Gas
(Nitrogen)
Table II. Pressure pulsation in reciprocating compressor package in
various operating.
Table II shows pressure pulsation at the discharge of the
reciprocating compressor package. Half flow represents highest pressure pulsation values at pulsation vessel outlet flange.
The table shows high pulsation values at the cylinder flange
for operation with nitrogen during start-up. Figure 1 shows
pressure pulsation values at the pulsation vessel flange (
pressure pulsation-detected values in plant side) vs. harmonic
component for three different process operating conditions.
This plot presents pressure pulsation components (as percentage of total value) vs. harmonic components (1st to 20th
harmonic components). In case of full flow, the major pulsation component is in the 2nd harmonic ( 10. 9 Hz). Half-flow operation has peak pulsation in 1st harmonic ( 5. 45 Hz) because it
