Improving the reliability of the piston compressor

Illustration

SK Energy: Founded in 1962 as the first oil company in South Korea, SK Energy produces various petroleum products at the Ulsan Complex with a daily processing capacity of 840,000 barrels of crude oil, selling them to both domestic and overseas markets.

In response to increasing demand for increased production, three reciprocating compressors for gas pressure boosting in the Continuous Catalyst Regeneration (CCR) reforming process had been operating without backup and without backup equipment since their reconstruction in 2009. Subsequently, the number of unplanned shutdowns and related costs began to increase due to unexpected failures and subsequent maintenance work. In fact, these three CCR compressors consumed up to 90% of the total maintenance costs among the 100 reciprocating compressors at the facility.

This article describes how SK Energy overcame challenges in process control, mechanical design, and condition monitoring of reciprocating compressors to improve the reliability of these assets and reduce maintenance costs.

Equipment description

Three reciprocating compressors in the Continuous Catalyst Regeneration (CCR) reforming process were installed in 2005. The booster compressors in the CCR reforming process pump the rich hydrogen (H 2 ) from the reactor to the hydrogen production plant.

Illustration

Problem formulation

Since the equipment was upgraded in 2009 to increase production capacity, these three compressors have been operating without reserve.

After the upgrade project, the pressure did not change significantly, but the outlet temperature increased significantly compared to the original values. After the upgrade project, the outlet temperature was set at 145 °C.

    • №

    • Process revamp

    • Before

    • After

    • №

    • №

    • Process revamp

    • Process revamp

    • Before

    • Before

    • After

    • After

    • 1

    • Compressor at operating

    • 2 run out of 3

    • 3 run out of 3

    • №

    • 1

    • Process revamp

    • Compressor at operating

    • Before

    • 2 run out of 3

    • After

    • 3 run out of 3

    • 2

    • Pressure Ratio 1st stage

    • 80,983 Nm3/h

    • 102,648 Nm3/h

    • №

    • 2

    • Process revamp

    • Pressure Ratio 1st stage

    • Before

    • 80,983 Nm3/h

    • After

    • 102,648 Nm3/h

    • 3

    • Pressure Ratio 1st stage

    • 2.20

    • 2.24

    • №

    • 3

    • Process revamp

    • Pressure Ratio 1st stage

    • Before

    • 2.20

    • After

    • 2.24

    • 4

    • Pressure Ratio 2st stage

    • 2.39

    • 2.41

    • №

    • 4

    • Process revamp

    • Pressure Ratio 2st stage

    • Before

    • 2.39

    • After

    • 2.41

    • 5

    • Pressure Ratio 1st stage

    • 2.31

    • 2.32

    • №

    • 5

    • Process revamp

    • Pressure Ratio 1st stage

    • Before

    • 2.31

    • After

    • 2.32

    • 6

    • Discharge Temperature 1st stage

    • 105 °C

    • 112 °C

    • №

    • 6

    • Process revamp

    • Discharge Temperature 1st stage

    • Before

    • 105 °C

    • After

    • 112 °C

    • 7

    • Discharge Temperature 1st stage

    • 101 °C

    • 106 °C

    • №

    • 7

    • Process revamp

    • Discharge Temperature 1st stage

    • Before

    • 101 °C

    • After

    • 106 °C

    • 8

    • Discharge Temperature 3st stage

    • 106 °C

    • 113 °C

    • №

    • 8

    • Process revamp

    • Discharge Temperature 3st stage

    • Before

    • 106 °C

    • After

    • 113 °C

Illustration

2. Compressor maintenance events from 2009 to 2016.

Illustration

2. Compressor maintenance events from 2009 to 2016.

Illustration

2. Compressor maintenance events from 2009 to 2016.

Cause and effect analysis

SK Energy conducted a cause and effect analysis to identify the root cause of the valve and piston cracking problems, covering all aspects of the process and mechanical design and integrity of the compressor.

Process

The process review found that the increase in iron (III) oxide (Fe 2 O 3 ) and chloride (Cl 2 ) in the dust contributed to the polymerization of green oil residues due to the changed process conditions. (H 2 /HC) is one of the factors contributing to the increase in light olefins, another factor in the formation of green oil.

Illustration

Image 4. H 2 /HC ratio in the process gas composition.

Fe 2 O 3 was formed in the pipe under the influence of the atmosphere during the break, and Cl 2 dust was generated in the initial start-up period and when using a new type of catalyst. Fe 2 O 3 and Cl 2 dust are catalysts that lead to the formation of viscous green oil, which accumulates as condensate in the compressor cylinder.

Mechanical design and equipment integrity

Before the modernization project, a concentric ring type of compartment valve was used, but it was found that the valve did not have sufficient strength to withstand foreign substances.

Seizure occurs due to foreign substances between the valve plate (seat) and the ring.
Delayed opening and increased pressure between areas A and B (see Figure 7).
Increased impact and overturning between the ring and the fence. (Since the rigidity of the outer ring is the lowest, the outer ring is easily broken.)

Illustration

Image 5. Valve failure on the shut-off valve.

One of the unique design features of these compressors is the use of a free-floating piston (FFP). Due to the unique operating principle, the piston is made of a lightweight material. It was found that the strength of the material was not sufficient to withstand the high loads created by green oil.

Illustration

Image 6. Piston failure due to green oil.

Improvement

Based on the results of the study, the following measures were taken on both the process and mechanical integrity sides. In addition, the condition monitoring system was upgraded to more accurately monitor the condition of the machines.

Process

To prevent further formation of green oil, catalysts such as Fe 2 O 3 and Cl 2 were completely removed from the piping system and the separation drum on the pumping line during scheduled maintenance.

Illustration

7. Improving pipeline cleaning.

To minimize green oil formation, the reforming process reduced the amount of light olefins and adjusted the start-up operating conditions to reduce chloride (Cl) content.

Mechanical Design and Integrity

Replaced the outlet concentric ring shut-off valves with plunger-type valves, which have high reliability in unwanted liquid ingress and seizure scenarios.

Illustration

8. Improvement of valve type.

The geometric structure of the piston has been improved to increase its mechanical strength. Due to the change in material and production method, strength has increased by approximately 48% compared to the original state, while maintaining the same weight.

Illustration

9. Geometry improvement.

Additionally, to further increase strength, the piston manufacturing process was changed to drop forging, resulting in a 26% increase in strength compared to the original design.

Condition Monitoring System for Reciprocating Compressor

The original condition monitoring system could only monitor frame and crosshead vibration, as well as simple process parameters (inlet pressure, outlet pressure, outlet temperature). Due to the limited capabilities of the system, root cause diagnostics could not be performed.

    • Type of Measurement

    • Original

    • Recommended

    • Type of Measurement

    • Type of Measurement

    • Original

    • Original

    • Recommended

    • Recommended

    • Frame Vibration

    • YES

    • YES

    • Type of Measurement

    • Frame Vibration

    • Original

    • YES

    • Recommended

    • YES

    • Crosshead Vibration

    • YES

    • YES

    • Type of Measurement

    • Crosshead Vibration

    • Original

    • YES

    • Recommended

    • YES

    • Rod Position V

    • YES

    • YES

    • Type of Measurement

    • Rod Position V

    • Original

    • YES

    • Recommended

    • YES

    • Rod Position H

    • NO

    • YES

    • Type of Measurement

    • Rod Position H

    • Original

    • NO

    • Recommended

    • YES

    • Cylinder Pressure

    • NO

    • YES

    • Type of Measurement

    • Cylinder Pressure

    • Original

    • NO

    • Recommended

    • YES

    • Multi-Event KPH

    • NO

    • YES

    • Type of Measurement

    • Multi-Event KPH

    • Original

    • NO

    • Recommended

    • YES

Illustration

10. Equipment diagram in the Bently Nevada System 1.

A continuous monitoring and diagnostic system for the reciprocating compressor was recommended. It included measurements and logic for the installed cylinder pressure, two-plane rod position, and multi-valued (MEW) key phasor sensors. Appropriate diagnostic software, such as the Bently Nevada System 1, allows for accurate diagnostics through PV diagram analysis, as well as monitoring compressor events and vibrations for every degree of crankshaft rotation.

The following extended diagnostic variables were included in the System1 software installed on a fully instrumented reciprocating compressor with cylinder pressure sensors and MEW Keyphasor:

- Outlet Volumetric Flow Efficiency
- Inlet Volumetric Flow Efficiency
- Indicated Power
- Adiabatic Outlet Temperature
- Outlet Volumetric Flow
- Inlet Volumetric Flow
- Average Volumetric Flow
- Adiabatic Average Volumetric Flow
- Flow Balance
- Adiabatic Flow Balance
- Power to Average Flow
- Indicated Clearance Volume
- Outlet Power Loss
- Inlet Power Loss
The inspection determined that the maximum pressure inside the chamber tended to increase above normal values ​​when foreign matter entered the chamber and subsequently settled or formed deposits around the shut-off valve, preventing it from normal operation. Therefore, it was assumed that the effective compression ratio was higher than the normal outlet pressure to inlet pressure ratio due to the sticking effect (sticky valves), which in turn resulted in a greater thermal effect of compression before the stuck shut-off valves opened.

Illustration

Image 11. Determining valve maintenance time.

Based on the recommendations of Bently Nevada and the technical review, new rod proximity sensors and pressure sensors were installed in each chamber to verify piston rod behavior and valve condition. Previously, the improvement in the available quantitative data was not sufficient for accurate assessment. With the expansion of online condition monitoring options, it was demonstrated that chamber pressure measurements can provide rich information about compression as well as valve condition.

Result

Illustration

12. Results of improvements.

Shortly after the plant was upgraded in 2009, the maintenance costs of these three compressors increased more than 30 times due to the aforementioned process and mechanical issues. Thanks to improvements in the condition monitoring system in 2016, the maintenance rate has decreased to just four incidents per year and no cracked pistons. These figures are the lowest in the last 10 years, demonstrating successful and collaborative efforts across the various business areas.

For this reciprocating compressor in the CCR reform process, the following considerations were key to successfully improving reliability:

Process

• Optimization of the operating mode to minimize liquid carryover.

• Flushing the piping system to reduce the green oil catalyst after a break.

Mechanical design

• Based on the improvements, a piston with higher material strength was required in the event of foreign matter.

• The ring valve did not have sufficient strength to withstand external foreign matter.

Condition Monitoring

• Increased discussion with quantitative data coming from System1 and process data. Calculated pressure measurement variables provide a wealth of information for each position on the machine, allowing corrective action to be taken. • System1 pressure monitoring also provided accurate timing for valve maintenance using the PV diagram.

Illustration

13. Result of valve replacement.

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+38 066 1555747+48 660 871645

Created by: smelov

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Menu:

Address:

1 Generała Grochowskiego Street,Office 48, Warsaw, Poland.

Phone number:

+38 066 1555747+48 660 871645