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It takes about 90 seconds for the Bypass Valve to normally close, but after only 25 seconds into that time, the unit starts vibrating and the crossheads start smoking. The healthy unit goes down on Pin Non-reversal with multiple damaged crossheads. What happened to cause this Shut Down?

The problem goes way back to the purchase of the unit. The end user, the packager, the OEM, and consultants all did a great job making sure that the unit was sized well to handle the end user’s operating requirements. This sizing is often accomplished by reviewing from around ten to over a hundred operating points across the unit’s expected operating map: Design Point, Max Flow Point, Min Ratio Point, Max Discharge Pressure, Summer Conditions, Winter Conditions, etc.

However, during the closing of the Bypass Valve, the initial effective “Discharge Pressure” is the same as the Suction Pressure, and then it is gradually increased from there until the Bypass Valve is fully closed at which time the effective Discharge Pressure becomes the station’s discharge line pressure. During this time that the unit is in Bypass Closing mode, the unit is compressing gas to non-standard (aka, never reviewed!) discharge pressures. Moreover, the unit is likely doing this at a Minimum Start Up Speed. There is usually not a lot of torque on the frame’s crankshaft (nor on the driver) during this Start Up process, but torque is not the issue – stress is. And while the frame may only be 30% loaded, the stress on any individual throw’s rod (or crosshead pin, or other components) may exceed allowed limits.

For pin non-reversal, there may be insufficient time and magnitude when switching between compression and tension forces to allow every crosshead pin to be properly lubricated. Different OEMs have different requirements for minimum allowed degrees of reversal and for opposing force magnitudes. But, based on inertia forces (determined by speed and reciprocating weights), and internal gas forces (determined by inlet and outlet pressures to the cylinder), the resulting combined forces may lead to pin non-reversal issues, which can then lead to catastrophic unit failure. In the case here, bushings started to smoke!

Thus, it becomes imperative that someone (the packager, the OEM, the consultant, etc.) needs to review Start Up conditions so that operators (and control panels) know when it is truly safe to start the unit. If/when issue(s) are identified, then some workarounds are: 1) set speed to a different (often higher) speed before closing the Bypass Valve, 2) change to a different load step prior to closing the Bypass Valve, or 3) re-pressurize (higher or lower) the unit’s gas pressure during Start Up to one that is determined to be safe.

The first option (changing speed) is ideal and can be readily reviewed to make sure it is a viable option. Figures 1 and 2 show Safe Start Up Maps at two different speeds. Figure 1 is at 1200 rpm and has issues (yellow cells) where load step needs to be changed prior to closing the Bypass Valve. Figure 2 is from the same unit, but now based on starting up at 1800 rpm. The conclusion here is that if the unit’s speed is ramped to 1800 rpm, then there is no need to change to a different load step prior to closing the Bypass Valve.

Figure 1: Start Up Map with unit starting at 1200 rpm.

Figure 2: Start Up Map with same unit starting at 1800 rpm.

Note: Operations can achieve the best of both by simply specifying that when the Unit’s Start Up Pressure is less than 250 psig, then unit’s Start Up speed is 1800 rpm, otherwise unit’s Start Up speed is the normal 1200 rpm.

But, the above conclusion is unique to this particular unit. Only a detailed review of a unit’s Start Up Map can help determine if changes to speed help, hurt, or offer no real benefit to Start Up Safety.

In cases where speed cannot be changed (fixed-speed motor, torsional concerns during Start Up, power requirements during Start Up, etc.), then a good option is to change the unit from its least-loaded load step, to a load step that does not have issues with rod loads, pin loads, pin non-reversals, etc. during that period of time when the Bypass Valve is closed. In this type of situation, a Safe Start Up Map (example in Figure 1) can be used to determine which load step to engage before closing the Bypass Valve. Load Step #5 is the unit’s Least-loaded load step, and it can safely be used throughout much of the operating map when starting the unit. However, at times, the operator or the Unit Control Panel needs to change to Load Step #1 before closing the Bypass Valve to keep the unit safe during Start Up.

One final thing to consider. Just because a unit did not break a rod or burn up bushings during Start Up does not mean that there are no issues with its Start Up. Starting units under stress can lead to component fatigue, and thus certain stressed components can later fail during normal operations. As such, it is best to make sure that your reciprocating compressor components are not being unduly stressed during Start Up. And your packager, your OEM, and your consultants can readily provide you with that information.

So, if you are seeing rod issues, pin issues, pin lubrication issues, and no one seems to be giving you a good reason to why those issues persist, then you might want to do a Safe Start Up Map review. Stress during Start Up could be the culprit.

Real-time monitoring systems, like MMS’s Sentinel-CPM®, can identify rod load, pin load, and pin non-reversal safety issues even during the Start Up process. They can be very useful even before the compressor is online and running.  When using the Sentinel-CPM for monitoring, ensure that:

·       “RPM Limit for Alarms” (Setup | Compressor page) is set at an appropriate value,

·       Limits for Rod Load Tension/Compression & Rod reversal are set appropriately,

·       The eRCM Pro model was configured properly before creating the ACI Viewer file (*.rvf):

o   Proper compressor OEM selected,

o   Proper compressor geometry and load step information entered, and

o   Appropriate “Road Load Methodology” selected.

If you need assistance creating the correct eRCM Viewer file, contact MMS or ACI Services.

If you would like more information about the MMS Sentinel-CPM product, please contact MMS.

Sentinel - CPM Key Features

• Real time HP for each cylinder end
• Real time measurements and alarming of rod loads and degrees of reversal
• Automated leak detection of suction & discharge valves, rings and packing
• Quickly alarm on Load Step or clearance pocket issues
• eRCM Kernel enhanced with MMS real time compressor performance calcs
• Changeable Usernames, Encrypted Passwords and Multi-Factor Authentication per TSA mandates
• Continuous monitoring of up to 6 double-acting compressor cylinders (up to 12 cylinders w/ an additional CPM module
• 12 pressure sensor inputs
• 2 Magnetic Pickup inputs (TDC & Deg)
• MODBUS (RS-485 & Ethernet) communication
• Class I, Division 2, Gas groups A, B, C & D
• Alarm Relay – Dry contacts

• Cylinder-End HPs
• Suction/Discharge Pressures
• Suction/Discharge VEs
• Rod Reversal/Loads
• Theoretical Discharge Temperatures
• Flows & Flow Balances
• Flow Profile Algorithm
• OEM Methods and Limits Used
• Theoretical PV Cards
• Calculated Clearances
• Interstage Pressure Predictions
• Gas Leakage Rate

The Compressor Diagnostics Program can reliably detect the most common reciprocating compressor faults in real time. We will continue to expand our fault detection to other serious but less common reciprocating compressor issues as we continue to refine this product. Currently detected faults include: suction and discharge valve leaks, rings leaks, packing leaks, unloader and clearance pocket issues overloading the compressor, exceeding rod load limits and lack of rod reversal.

Our motto, our customers come first is reflected in this new product. We are proud to share that our MMS Compressor Diagnostics program is installed on every MMS Sentinel – CPM board and connects directly to the Unit PLC to exchange data. This is a big win for our customers and best of all, there are no monthly subscription fees nor data that has to be uploaded to a company’s web application in the cloud.