Hydrocarbon Processing - August 2021 - GP-34

EQUIPMENT
long stroke) with average speed and pressure
loading at the wear band is equal to
a high-rotational-speed compressor (API
11P, short stroke) and may be expected to
have similar sliding wear rates. However,
the former does not normally have a cylinder
liner, and the risk of other types of
wear are still high.
Valves are not the subject of this paper;
however, it is worth mentioning that increasing
the rotational speed has a significant
impact on the valve service life as one
of the most critical compressor parts. Each
suction or discharge valve in a compression
cycle opens and closes one time. The
number of openings and closings of a valve
in a single stage of a single compressor with
two different speeds is shown in TABLE 4.
In an RGC with a constant capacity, if
the stroke is decreased and the rotational
speed is increased, then reciprocating
forces acting on the piston, cross-head,
connecting rod, piston pin, etc., will increase
and a higher load will be imposed
on the crank shaft. In parallel, higher rotational
speed causes increased acceleration
of gas coming to or exiting from the cylinder
via valves, which leads to greater wear
of the valve components.
Under constant
rotational speed, a
compressor with higher stroke causes
more wear on cylinder and piston rings.
As an example, suppose a packaged, separable
RGC (API 11P) with a doubleacting,
gas engine drive with speed of
1,000 rpm experiences a 1-in. increase
in stroke, which causes: 1 in. × 2 × 1,000
rpm × 24 hr × 60 min × 365 d × (1/12) ft
= 87,600,000 ft/yr = 26,700,480 m/yr =
26,700.5 km/yr additional movement of
the reciprocating parts and the equivalent
Category
amount of additional wear. This is about
three times the east-to-west distance of
Russia-i.e., 10,000 km!
Increasing the cylinder diameter does
not impact the velocity or acceleration of
the piston. However, the volume of gas displaced
by the piston increases significantly.
The larger cylinder requires a larger piston,
which causes higher piston weight and
higher pressure in the wear band ring and
cylinder. The cylinder diameter, stroke
and piston velocity must be decided carefully
to mitigate the risk of wear and vibration
and also to minimize clearance losses.
High-speed compressor without cylinder
liner. Consider a single-stage, singleacting
reciprocating compressor. The theoretical
capacity or the displaced volume
by piston is calculated as shown in Eq. 4:
Qt = ∏ × (D2
where:
Qt
/4) × S × N × n
(4)
= Compressor theoretical capacity
or the displaced volume by piston,
m3
/min
D = Cylinder internal diameter, m
S = Stroke, m
N = Compressor rotational speed, rpm
n = Number of cylinders
∏ = Pi.
With reference to Eq. 3, if the piston
speed is assumed to be constant, then the
stroke and compressor rotational speed
have a reverse relation. For a constant capacity,
to decrease the piston stroke, it is
necessary to increase the compressor rotational
speed. Decreasing the compressor
stroke means decreasing the dimension of
the compressor, which results in less material,
less space and lower cost. In general,
TABLE 4. Number of openings and closings of valves at diff erent speeds
Speed, rpm
High speed
Low speed
1,000
500
API Working hour
24
24
11P
618
Open
Close
720,000
Total
1,440,000 1,440,000 2,880,000
720,000
1,440,000
high-speed compressors are cheaper than
low-speed API 618 compressors due to
their lower stroke and lower dimension.
Adding a liner to the cylinder of a highspeed
compressor has a negative impact
on compressor performance and volumetric
efficiency because the fixed clearance
in the valve area is increased (FIG. 6). The
valve area cannot be increased to compensate
for the clearance loss due to the liner
because a larger cylinder is required. A
larger cylinder needs a larger piston, and
a larger piston means heavier load in the
wearing band, cross-head, connecting rod,
bearings and crank shaft. Also, a larger piston
will increase the clearance loss due to
greater gas leakage from the rings.
Adding a liner in the cylinder of a highspeed
compressor can reduce the volumetric
efficiency and flexibility of the variable
volume clearance pocket. This is largely
why manufacturers of high-speed compressors
do not use liners for cylinders.
The volumetric efficiency of a reciprocating
compressor is the ratio of the
discharged gas from the compressor to
the displaced volume of the first stage, as
shown in Eq. 5:
ηv = 1 - {[Z1 ÷ Z2] × [(P2
- 1} C-L
where:
ηv
(P2
÷ P1 ]1/γ)i
(5)
= Volumetric efficiency
Z1, Z2 = Gas compressibility factor at
suction and discharge condition
)i
÷ P1 = Compression ratio at ith
stage, largest compression ratio
P2, P1= Absolute pressure at discharge
and suction, respectively
γ = Polytrophic coefficient = 1.3-1.35
C = Clearance loss (%), to be
determined by vendor
(e.g., 10% < C < 15%)
L = Ring clearance for lubricated
cylinder, 0.05-0.06
L = Ring clearance for non-lubricated
cylinder, 0.09-0.1.
It is obvious that if C = 0, then ηv = 1;
however, practically, ηv < 1 (e.g., 0.97).
Therefore, Eq. 5 can be rewritten as Eq. 6:
ηv
= 0.97 - {[Z1 ÷ Z2] [(P2
- 1} C-L
Eq. 7 can be used:
ηv
FIG. 6. Adding a cylinder liner results in a higher clearance pocket in the valve area.
34 JULY/AUGUST 2021 | GasProcessingNews.com
= 0.97 - {[(P2
- 1} C-L
÷ P1 ]1/γ)i
÷ P1 ]1/γ)i
(6)
For ideal and real gas at low pressure,
(7)
For Eqs. 4 and 5, the actual capacity
of a single-stage, single-acting reciprocat
http://www.GasProcessingNews.com
Hydrocarbon Processing - August 2021

Table of Contents for the Digital Edition of Hydrocarbon Processing - August 2021
