A
hydrocracker makeup compressor in a refinery in Saudi Arabia failed during
operation. Upon internal inspection of the compressor, it was found that the
cylinder liner was cracked and needed a replacement (FIG. 1).
Installed
in 1977, the compressor is a four-throw, three-stage double acting
reciprocating compressor operating at 300 revolutions per minute (rpm), driven
by a 5300 high-pressure steam turbine at 5,500 rpm through a double reduction
gearbox. The compressor takes hydrogen (H2) gas at 284 psia and
compresses it to 2,035 psia at the discharge with a normal flow of ̴34 MMft3d.
Problem statement.
Heavy-duty
cylinder liner replacement is a difficult procedure to perform locally, and
sending it to the original equipment manufacturer (OEM) to perform a
replacement may take several months. With the urgent need from operators to
return the equipment to service and avoid extensive production loss, an
in-house procedure and tools were developed to perform this task. To perform
the repair in-house, the following challenges needed to be tackled:
Upon
inspecting the compressor, the liner shape was found to be more tapered and
oval than acceptable, in addition to the aforementioned crack. This article details
how the challenges were overcome during the repair process, such as the
difficulties of fitting a new liner into a cylinder, which requires cooling the
liner to a specific temperature or heating the cylinder on the oven. Other
machining difficulties were tackled by fabricating a special jig to hold the
liner in place during machining and fabricating a special lifting tool to lift
and install the liner.
Solution. Upon checking the
integrity of the old installed liner and after planning the required action to
replace the liner with a new one, the old liner was removed, utilizing a computer
numerical control (CNC) milling machine. After removal, the cylinder inside
diameter (ID) sizes were measured as received in four transversal parallel
planes and three locations: the center, stuffing box end and outer end (FIG. 2).
The new liner
ID measurements were taken in three different areas to capture any ovality and
tapering. After that, the concentricity of the cylinder ID was checked using
the stuffing box bore and face as a reference to set up the compressor.
Utilizing the CNC machine and dial indicator, the compressor case was set up to
a 0.000-in. reading in the stuffing box bore and face. Subsequently, runout
readings were recorded in different areas of the liner, and each reading was taken
in four angles (0°,
90°,180° and 270°). It was found that the
cylinder case was not perfectly round; therefore, the cylinder ID was machined.
During the machining, the following were considered:
Meanwhile, a special jig was fabricated to hold
the liner in the lathe machine and the new liner’s outer diameter (OD) was
machined to be an interference fit with the ID of the cylinder liner (FIG. 3). Upon
correcting the ovality and tapering, a special lifting tool was fabricated to fit
the liner into the cylinder and both centerlines were aligned (FIG. 4).
The installation is critical because it has a shrink fit of around 0.006 in. and the liner should not get stuck in the middle during installation. Assembling the new shrinking liner in the cylinder is performed by cooling it in liquid nitrogen or dry ice. Alternatively, this can be accomplished by heating the cylinder in a furnace at approximately 220°C (428°F). In this case, the cooling procedure was chosen. To do that, the final temperature was calculated to acquire recommended shrink using Eq. 1:
∂D = D0α (T0 – T1) (1)
where,
∂D = Required reduction in the diameter
D0 = Original diameter
A =
Coefficient of thermal linear expansion/contraction
T1 = Final temperature
T0 = Initial temperature.
The center lines for the liner and cylinder were marked to ensure the suction and discharge holes were aligned, and the liner cooled by dry ice to –75°C. During the process, regular temperature and size checks verified the progress toward the targeted final size reduction. The final reduced liner size was reached and provided 0.012 in. of clearance between the liner and cylinder. The liner was installed successfully into the cylinder and machined to the required size. Consequently, the wet honing process was carried out by using honing fluid—a mixture of diesel and Society of Automotive Engineers’ oil 40 (International Standards Organization Viscosity Grade 150). Each honing stage was performed with higher stone grade to reach the desired surface roughness of 0.45 microns (µm) or close to it (FIG. 5).
Takeaway. After the compressor liner re-boring was completed,
the equipment was ready to be shipped back. This job was critical to the refinery
and performing the heavy-duty cylinder liner replacement is difficult to perform locally for such
compressors. Moreover, sending it to the OEM to perform the replacement will
take a minimum of a couple of months. Due to operational requirements and the
need to avoid production loss, the challenge of performing the procedure
locally for the first time was worth the effort. All resources were pooled together
to ensure a timely, efficient installation of the cylinder, maintaining the cylinder
centerline and liner, in addition to correcting any ovality/tapering that might
have existed before or after liner installation. HP
MOHAMMED ALQAHTANI is a rotating equipment specialist working for Saudi Aramco with more than 10 yr of experience in maintaining rotating machinery. His previous experience includes working in mechanical engineering, reliability, projects and technical support related to rotating equipment. Alqahtani is involved in developing and deploying 3D printing and reverse engineering at Saudi Aramco. He provides technical support and troubleshooting of failures during shop and field repairs/rebuilds of rotating equipment as per API and other international standards. Alqahtani earned an MS in mechanical engineering from the University of Minnesota.