AUTOMATED COMPRESSOR-PACKING SYSTEM MEETS CALIFORNIA EMISSIONS REG

John Lemanski
Atlantic Richfield Co.
Los Angeles

In 1991, Southern California fell under the most stringent fugitive emission regulation in the nation: South Coast Air Quality Management District (Scaqmd) Rule 1173.

Under the new rule, 27 reciprocating compressors with a total of 53 cylinders presented a concern for ARCO Products Co.'s Los Angeles refinery. Even under the best conditions, the compressors' existing packing systems could not reliably provide a tight enough seal to meet the stringent requirements of Rule 1173. A new approach was required.

Rule 1173 took effect Feb. 1, 1991, reducing permissible minor gas leaks from machinery and valves to 1,000 ppm from a previous standard of 10,000 ppm (see box). Measurements of leaks were to be taken within 1 cm of the source.

ARCO's problem was approached by a team with key personnel from engineering, maintenance, and operations. The team wanted a long-term solution that could survive any "worst-case" scenario, while offering advantages to maintenance and operations personnel.

ARCO's success in accomplishing this in less than 6 months rests on many factors, including the hardware chosen and the teamwork between ARCO and the hardware vendors. Today, the refinery has a solution that not only keeps emissions well within the rule, but also provides a valuable predictive maintenance tool.

THE PROBLEM

Reciprocating compressors lose gas along the piston rod and past the packing (Fig. 1). Leaks can occur at several points:

Nose gasket leakage. Conventional nose gaskets may not seal effectively enough to prevent some gas from leaking.
Cup leakage. Packing is held in place around the piston rod by a series of cups that seat against one another. The metal-to-metal contact area between cups is another place where gas can leak.
Ring leakage. This leakage takes three forms:
1. Rod rings may slide back and forth as the compressor rod moves, allowing gas to leak past the sides of the rings.
2. At low pressure, rings may not fit the rod well, allowing leakage between the ring and the rod.
3. Leakage may occur through the overlapping joints in the rings.
Regardless of the source, leaking gas will eventually find its way to the distance piece and may even cause problems in the crankcase.
As one of its first decisions, the project team determined that ARCO's existing vapor recovery system (operating at a vacuum of 7-11 in. mercury) would be used to collect emissions.

All compressor distance piece and crosshead frames would be enclosed, with the distance-piece vent connected to vapor recovery via a three-way valve and vacuum regulator. Normally open to atmosphere, the three-way vent valve would provide a convenient port to monitor emissions from the packing box. If emissions increased beyond a trigger level (1,000 ppm), the valve could be switched to connect that cylinder frame to vapor recovery without having to shut down the compressor for unscheduled maintenance.

The biggest challenge, however, was to find a better way to seal the cylinder chambers. This was not simple, because the compressors were of many types and up to 40 years old. Because all of this equipment had to be upgraded, ARCO was determined to find packing that could be standardized.

PACKING SYSTEM

After numerous investigations and interviews with equipment manufacturers, ARCO chose C. Lee Cook's nitrogen-purged packing system (Fig. 2). In this system, nitrogen is introduced at 15 psi above vent pressure (vent pressure is governed by the vapor recovery pressure). This nitrogen-purged system offers several advantages:

Spiral-wound nose gasket. This composite metal/graphite gasket has greater conformability, providing a better seal than the typical nose gasket.
Cup-to-cup sealing. The system prevents cup leakage by providing O-rings between the cups. ARCO was concerned about the initial installation of the O-rings because nothing held them in place. The O-rings could become dislodged when a compressor was packed, causing problems and requiring extended maintenance. The solution, developed by C. Lee Cook, was a dovetailed groove to hold the rings in place even when cups are pulled apart (Fig. 3).
Flange configuration. Part of the packing system is a flange with a raised face that can be lapped to ensure a tight seal. Many of the previously used flanges had a recessed area for the final cup, making it difficult or impossible to create a good seal.
Teflon (TFE) packing rings. The packing case uses TFE packing rings, which provide both an exceptional seal and good lubricity. The sealing rings are supported with high-strength nonmetallic rings to prevent extrusion down the rod.
Axial-loaded AL ring. The AL ring uses nitrogen purge to maintain an effective barrier seal (Fig. 4). It has five symmetrical TFE rings designed so that nitrogen pressure forces the center wedge to side-load the rings in the cup. This design helps prevent ring leakage and offers two unique advantages: dowelled AL rings cannot be put in backward, and maintenance is simplified.
Purged wiper packing. To ensure protection of the crankcase under catastrophic circumstances, compressor upgrades also incorporated nitrogen-pressured AL wiper packing. This packing confines crankcase oil and prevents contamination of the crankcase with process gases. The crankcase is also swept with nitrogen to prevent any accumulation of explosive gases.

With the help of C. Lee Cook personnel, ARCO designed standard packing systems for all compressors. Although ARCO still stocks about the same number of parts in inventory, there are far fewer types of parts. Both operator training and maintenance have been simplified.

EFFECTIVE SYSTEM USE

The use of nitrogen to pressurize AL rings and ensure the packing seal may sound simple, but it is not.

Before ARCO started this project, the Los Angeles refinery already had installed a prototype purged-packing system. The problem with this system was that it had to be operated manually. It worked for steady-state conditions, but readjustments were required to respond to system upsets or changes. Requiring the operator to make frequent manual adjustments was not practical.

With the general trend toward automation, the logical solution was an automated purge-control system mounted on a modular local panel. At this point, the Autocator division of T.F. Hudgins Inc. joined the team effort.

AUTOMATED PURGE SYSTEM

The three companies developed a control system to automate the flow of nitrogen to the compressor packing and to pass packing vent gases to vapor recovery.

Modularized panels, designed for one or two cylinders plus crankcase, were combined to accommodate each compressor (Fig. 5). Each panel system also included a pressure regulator to break down the nitrogen header supply pressure from 200 psi to 60 psi.

The panel combination is designed to manage the three key areas of the fugitive emission control system (Fig. 6):

Wiper pressure and flow. The small amount of nitrogen that leaks into the doghouse from the wiper case is taken into vapor recovery, while nitrogen leakage into the crankcase is swept with the nitrogen purge to atmosphere.
Any unusual increase in nitrogen flow indicates ring wear.
Buffer pressure and flow. A buffer pressure gauge shows the pressure of nitrogen on the AL rings in the compressor packing. According to API specification 618, the differential between the purge pressure and the disposal pressure should be at least 15 psi.
The flow of nitrogen (measured by a rotameter) indicates the condition of the packing. When packing is new, the flow is almost zero, because little gas is leaking. An increase in nitrogen flow (shown by the rotameter) indicates packing wear.
Vent pressure and flow. The packing vent flow to vapor recovery is monitored by a rotameter on the panel. (The vent gases flow first through a liquid collection pot to separate entrained oil before passing through the panel rotameter.) Most of the time at ARCO, vent flow varies between 0.5 scfm and 2 scfm. The magnitude of the vent flow to vapor recovery indicates packing wear.

IMPORTANT BENEFITS

When the compressors are operating, the monitoring panel runs in automatic mode and maintains a pressure differential of 15 psi between the vent and the packing buffer. The constant pressure differential means that the compressor leakage reliability meets requirements without operator intervention.

One key part of the control panel is the manual/automatic selector.

When compressors come down for any reason (for example, process turnaround), the nitrogen purge is switched to manual operation. This maintains pressure on the AL rings in both the compressor buffer and the wiper, preventing dirt and debris from falling between rings and causing wear that could result in complete failure of the AL ring.

When compressors come back up, they are manually switched back to automatic operation and continue to operate with the same reliability as before the shutdown.

ARCO has not had a single compressor down for an emission problem since installing this system.

Finally, ARCO now has a valuable predictive maintenance tool, in the same class with vibration analysis. The flow indicators on the Autocator panel show how much packing has worn, so maintenance can be planned 6 months or more in advance. This feature is expected to save considerable maintenance hours because compressors will be overhauled only when necessary.

This automated system was not the least expensive alternative, but it will easily pay for itself in long-term compliance and ease of maintenance.

Note: Atlantic Richfield Co. does not endorse or recommend any particular seals made by any specific manufacturer.