Chemical Engineering December 2010 - 42

Engineering Practice
1.5
oil pumps, both sized for a capacity
that is 20% greater than the maximum
oil flow required for the compressor.
At a minimum, two pumps should be
used. Either a run-down tank (this is
a stainless-steel tank that allows the
supply oil to safely coast down the
machine in the event that both pumps
have failed), or a crank-shaft-driven
main oil pump is required. Dual (two)
removable bundle shell-and-tube oil
coolers (TEMA C), double oil filters
with a removable element and stainless-steel
piping are also necessary.
Liquids should not be allowed to accumulate
inside the compressor cylinder.
For any application, a suction
drum (sized appropriately with regard
to application-specific retention time,
flow velocity, and, if required, a mistcollection
system to capture contaminants)
with a drain provision should
be provided. It may be part of pulsation
control. To control pulsation, a
vertical vessel is sometimes used as
both suction drum and suction pulsation
vessel, but this not recommended
since vertical pulsation vessels cause
relatively long piping to the compressor,
which may lead to dynamic problems.
Similarly, these are sometimes
conflicting requirements for pulsation
damping and suction separation, so
this combined approach is not preferred.
It may be used only for small
compressors, let's assume below 250
kW, with relatively light gases, such
as those lighter than nitrogen.
Cooling-water systems are generally
used as a heat sink for reciprocating
compressors, to avoid hot spots
and improve machine stability and
reliability. To design a cooling system,
first, the generated heat should
be calculated. Then, the anticipated
temperature rise should be identified.
The cooling-water inlet temperature
should be selected between 6°C and
16ºC above the inlet gas temperature.
When selecting the pump to deliver
cooling water to the compressor cylinder
and packings, consider this:
* With regard to the slope of the operating
curve (discussed in greater
detail below), the selected operating
point should not be in the
flat or near-flat part of curve;
rather, enough slope is needed for
proper operation
Full capacity
50% Capacity
1
0.5
50
100
150
200
250
300
One rotation of the crankshaft angle, degrees
FIGURE 3. This plot of reciprocating compressor torque versus crank angle shows
the variation in compressor torque represented by the ratio of compressor torque/
motor torque, in one revolution of compressor crankshaft (0 to 360 deg.). The
curves show compressor torque in two operation conditions - 50% capacity and
100% (full) capacity
* There should be a continuous rise
from a selected operating point to
shut off
* There should be proper shut-off
pressure compared to a selected operating
point (preferably 10% but a
minimum of 6%)
Note: usually a larger size pump
shall be selected to meet these recommendations.
Any
reciprocating compressor system
should be designed with a margin
of excess flow capacity for the cooling
system, to enable it to respond to situations
that deviate from normal operation,
where the need may arise later
for additional cooling flow to remove
excess generated heat (for example,
unloaded operation when the compressor
is idle, overload conditions, or
future expansion, if applicable). The
recommended cooling pump capacity
margin is 10-25% (that is, pump
rated capacity is 10-25% more than
required normal flow).
Users should consider suitably sized
pulsation vessels and correct any potential
pulsation resonance in piping
rather than using damping devices,
such as orifices, choke tube and so on
to dampen pulsation. Acoustic reviews
should be performed during compressor
system design to guarantee all anticipated
combinations of pressures,
speeds and load steps (including the
use of flow-reduction steps that rely
on unloaders, which can vary the compressor
flow).
Pulsation limits are recommended
42 ChemiCal engineering www.Che.Com DeCember 2010
around 85-95% of API 618 (Approach
3) limits to provide some margin
(5-15% of API 618 limit values) to
mitigate risk during construction and
installation periods, and to cope with
unanticipated deviations and problems.
Similarly, pulsation vessels are
generally fabricated before finalization
of the piping-design-and-pulsation
study, and enough margin should
be provided to meet potential risks.
For nearly all applications, horizontal
suction and horizontal discharge
vessels are preferred. Long distances
between vertical pulsation vessels and
compressors increase the likelihood of
pulsation problems.
Improving performance
The maximum predicted discharge
temperature for any API 618 reciprocating
compressor for CPI applications
must not exceed 150ºC, and must
not exceed 135ºC for hydrogen-rich
service. In general, gas discharge temperatures
below 118ºC tend to lead to
longer life for the wearing parts.
When it comes to optimum pressuredrop
values for pulsation dampeners
and suppression devices, the pressure
drop maximum is 1% of absolute
pressure. For the intercooler, pressure
drop around 0.70 bar or 2% of absolute
pressure is recommended.
Readers should note that the use
of orifice plates to dampen pulsation,
especially on high-speed, single-act
compressors (that is, those that compress
gas on only one head of cylin350
Ratio
of compressor torque, kNm /
electric motor torque, kNm
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Chemical Engineering December 2010

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