American Oil and Gas Reporter - October 2016 - 75

FIGURE 1B
Operating Envelope for 3606 Package
190
170

Pressure (psig)

150
130
110
90
70
50
10/3/2013

10/8/2013

10/13/2013

10/18/2013

10/23/2013

Field Pressure

Date

10/28/2013

11/2/2013

11/7/2013

11/12/2013

11/17/2013

Suction Control Setpoint

FIGURE 2
Load Step Curves for 3606 Package

1,800

Power, BHP (Driver: 1,630)

Step 3

Step 4

1,700

Step 2

Step 1

Step 0

1,600
1,500
1,400
1,300
1,200
1,100
13

12

Flow, MMcf/d (Driver: 1,630)

package at rated speed and a typical discharge pressure.
The boundaries of the operating envelope are defined as minimum (fullyclosed VVCP) and maximum clearance
(fully-open VVCP), and rated power
(maximum compressor power with siterated driver power/auxiliary power) and
minimum power (minimum compressor
power because of driver stability and reliability). In addition to driver site rating
and VVCP design volumes, the safe operating envelope may be reduced by rod
load, volumetric efficiency, discharge
temperature, and other limitations.
The operating envelope shows that
the 3606 package could achieve a maximum of 12.1 million cubic feet a day at
site-rated power, rated speed and maximum
safe suction pressure (minimum compression ratio). Also illustrated in Figure
1B is the typical operating point found
near the nominal throughput of 10 MMcf/d
for the 3606 package. Thus, with the
high field pressure seen at the time, an
increase of approximately 2 MMcf/d per
compressor could be achieved by adjusting
suction control set points and VVCP positions appropriately.
VVCPs can provide infinitely variable
clearance and often are installed on cylinders in different stages. If one is not
careful with adjusting VVCPs on different
stages, load and differential pressures
can become imbalanced, leading to equipment damage from excessive rod load,
low volumetric efficiency, or high discharge temperature.
Consequently, the load-step schedule
was developed during the modeling as a
discreet position of all VVCPs configured
to span the range from minimum to maximum clearance in steps of percent load
as equal as possible. The percent load increment was targeted at 5-7 percent to
give a sufficiently fine step, but not so
fine as to require excessive adjustment.
Figure 2 shows the resulting load-step
curves for the 3606 packages. The graph
is directly from the Ariel Performance
Program™, which was used extensively
since the majority of the Wattenberg fleet
utilized Ariel compressors.
Manually adjusting two-four VVCPs
on several like-kind compressors at a station is a physical workout and takes time.
Therefore, the load-steps had to be practical for field operators to perform. In
addition, the actual VVCP positions for

11
10
9
8
7
6
5

40

50

60

70

80

Ps (psig)

90

100

110

120

130

FIGURE 3
Load Step Schedule for 3606 Package
CYL 2

CYL 4
1st Stage

Load Step

CYL 3

CYL 1

2nd Stage

3rd Stage

VVCP Postition - A inches (see drawing)

1st Stage
Pkt Stroke
inch

0

85⁄8"

85⁄8"

4 ⁄8"
(closed)

n/a

4"

1

75⁄8"

75⁄8"

45⁄8"
(closed)

n/a

3"

2

65⁄8"

65⁄8"

45⁄8"
(closed)

n/a

2"

3

55⁄8"

55⁄8"

45⁄8"
(closed)

n/a

1"

4

45⁄8"

45⁄8"

45⁄8"
(closed)

n/a

1"

5

FULLY CLOSED A =
FULLY OPEN A =
WCP S.N.
REMOVE ALUMINUM THREAD PROTECTOR WHERE APPLICABLE

OCTOBER 2016 75
American Oil and Gas Reporter - October 2016

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