Value of Cylinder Pressure Measurement for Reciprocating Compressor

S-Oil produces lube base oil, petro-chemical products, and operates crude oil refining facilities of 669,000 barrels a day in the Onsan Industrial Complex of Ulsan, Korea, which is the world's largest paraxylene (PX) production facility, and bunker-c cracking center of worldwide standard.

The Aromatic process plant's Hydrogen rich gas compressors are four throw, three-stage, double-acting horizontal API618 reciprocating compressors with 40,122 Nm3/h output.

Table 1. Compressor Nameplate Information

In 2021, a Bently Nevada reciprocating compressor condition monitoring system, including on-line dynamic cylinder pressure, was implemented for this compressor to detect earlier indication of machine problems and provide better diagnostics capability. Figure 1 shows the transducer layout on one of these machines.

In addition to the monitoring parameters recommended by API and the machine manufacturer, S-Oil decided to plan for a machine protection and condition monitoring system to holistically observe key monitoring parameters for these new reciprocating compressors, at the machine design phase. Bently Nevada's Reciprocating Compressor best practice document was used as reference for the front-end engineering design (FEED) specification.

As part of S-Oil's ongoing reciprocating compressor management program, various machine parameters are continuously monitored and trended with the on-line system. Parameters monitored by the system are cylinder pressure (Pressure vs Volume, Rod load, Rod reversal, Flow balance...), valve temperature, suction and discharge temperature, frame velocity, crosshead acceleration and piston rod position. 3500 machine protection system provides alarm for key monitoring parameters such as low rod load reversal, high vibration and changes in gap voltage. All key parameters are collected and displayed on System 1® Asset Condition Monitoring System. The Multi Event Wheel (MEW) signal is an important parameter measured by 3500 system and System 1 to calculate extremely accurate crank angle of each piston to support advanced diagnostics and machinery protection.

On December 16th, 2021, the 3500 rack detected a hardware alarm on the frame vibration. As Figure 2 shows, the compressor shut down because of this high vibration, initiating a follow-up investigation.

The following trend plot, Figure 3, indicates that the frame vibration had a step change back on December 2nd. However, this plot is not sufficient to determine a root cause of the vibration trip. Deeper diagnostic analysis is required.

The header temperature and pressure trends, as measured by DCS, were reviewed (Figure 4), but root cause of the machine trip could not be determined with this data.  It was confirmed that both of CE (crank-end), HE (head-end) temperature and the pressure had both changed from around 2nd December as shown in the temperature trend plot below.

Generally speaking, compressor frame vibration can increase due to a number of reasons, as mentioned below, often requiring full inspection of the reciprocating compressor to identify the root cause of the problem. The potential failure modes may include the following:

• Rotation related vibration transmitted to the compressor frame
• Imbalance due to an unusual pressure differential or inertial imbalance due to:
  • Valve Failures
  • Piston Ring Failures
• Looseness in the foundation assembly, degradation of grout
• High moments caused by excessive rod load. (also related to pressure component failure)
• Rider Band Wear
• Pressure Packing Leaks
• Crosshead & Pin Wear
• Liquid ingestion into the cylinder
• Main Bearing wear or failure
• Broken or loose piston rod

Bently Nevada's System 1 ® is capable of measuring and monitoring the essential condition monitoring parameters for reciprocating compressors. In this case, System 1®'s P-θ (Pressure vs Crank Angle) plot for Throw 2 clearly indicated a more rapid pressure increase than the expected value curve, and a significant reduction in flow balance on the Head End chamber for the same Throw from around 2nd December. (Refer Figures 5, 6 and 7 below)

The P-θ plot is a valuable tool to diagnose valve problems by comparing the expected (theoretical adiabatic) vs actual current cylinder pressure curves as described below:

Flow Balance = n_s / n_d

If the cylinder is in perfect condition, with no leakage past the valves, packing or rings, Flow Balance is exactly 1. If process gas leaks through the suction valve, flow balance shall be higher than 1. When the flow balance is lower than 1, the discharge valve is leaking.

After reviewing the various data captured by System 1®, S-Oil concluded that the root cause of high frame vibration was, in fact, an issue related to the Head End discharge valve on Throw 2.  In particular, the P-θ plot with expected (theoretical adiabatic) and actual cylinder pressure curves provided clear evidence of valve condition, together with flow balance and rod load trends plots. An inspection was scheduled for the Throw 2 Head End discharge valve specifically, as opposed to the traditional approach of opening and inspecting all valves. Clear damage to the discharge valve ring plate was found as shown in pictures. (Refer to Figures 9 and 10 below)

If frame vibration monitoring alone was installed, without additional parameters such as cylinder pressure, the frame vibration would still trigger an alarm, but the extensive time and effort to identify and rectify the correct root cause would result in significant delays to machine startup for production.

Prior to commissioning the System 1 diagnostics platform, the normal practice was to replace all valves when a problem was suspected. Clearly, having limited access to quality diagnostic data to indicate exactly which valve is faulty incurs higher costs for parts and maintenance labor. This quick and simple conclusion on root cause has been demonstrated to minimize the cost for inspection and repair on this critical reciprocating compressor, reducing downtime from 2~3 days to just 1 day for root cause analysis, inspection and repair.

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