Heat tracing and insulation. If the original installation was heat-traced and insulated, then it seems logical to improve coverage to eliminate cold spots.
A different approach — condensing heavies. If the motive gas and ejectors are part of an overall closed loop (as opposed to makeup air going in and finally exhausted to atmosphere), then perhaps the heavies need to be condensed and “purged” before the inlet of both of the compressors. If the motive gas composition has changed over time and is at dew point in an idle compressor, maybe this aspect needs to be evaluated.
Hopefully the condensed liquids can be recycled to recover costs. If the heavies have to be eliminated at some point anyway, then perhaps the compressor inlet is the place? The inlet cooling and liquids separation should also improve overall compressor efficiency and maintenance.
Tobin D. Kueper, acting plant manager
CYANCO, Winnemucca, Nev.
Add a drip tank
A simple drip tank with an integral demister chamber and an automatic drain valve installed between the discharge of the standby compressor and the tie point of the discharge from the primary compressor should completely eliminate any condensation from entering the standby compressor. It should be sized so that pressure drop is not a concern.
Sean Taylor, mechanical installation manager
Mueller Field Operations Inc., Springfield, Mo.
Consider gas re-routing issues
Has the compressor failure on start-up been proven to be due to condensed gases, that is, to liquids in the casing? If not, some more work should be done to confirm why the compressor is failing. Is it possible the failure mode may be related to the fact that the compressor on standby is cold and then gets hit by hot gases on start up — particularly, the rotor thermally growing faster than the casing and losing clearance? This latter point may be a contributing factor of the compressor failing.
If the failure mode has been narrowed down to “condensation,” then simply have the field operators drain the compressor casing on their rounds a couple times per shift (or as needed). Of course, this assumes the compressor can get this type of attention on a daily basis.
If the failure mode is due to additional factors like cold start up and thermal issues, then the solution may be the re-routing of some of the discharge gases off the main compressor and back through the standby compressor; the benefit of the re-routed gas will be to keep the standby compressor hot and (hopefully) liquid free.
If hot gas is to be rerouted back through the standby compressor, the following issues need to be addressed:
1. Can fines from the reverse gas “lay down” in the standby compressor, creating a new problem? The re-routed gas should come off after the ejectors to minimize fines lay down.
2. Reverse rotation will (likely) need to be avoided. The equipment manufacturer should be contacted to confirm this, as should the seal manufacturer (many seals only rotate in one direction). Reverse rotation of the compressor can be prevented with either a locking device that permits rotation in the “allowable” direction or the reverse flow can be restricted so as not to overcome the machine stationary momentum, but sufficient to keep things hot.
3. I assume there is a check valve in the piping system of the standby compressor (if only to avoid reverse rotation); this will need to be bypassed (obviously) depending on location in order to get reverse flow.
4. Bearing lubrication may be an issue. Are the bearings force-fed? Do they receive oil only when the compressor is running? If so, they may need to be fed while on standby to prevent heat damage to the bearings.
5. Are there other ancillary systems on the compressor that don’t run when the standby compressor is not running? For instance, is buffer gas/oil to the seals off when the compressor is on standby? Do these systems now need to be running when the compressor is not?
6. A complete review of how the compressor is intended to operate versus how it will now operate should be completed, including a review of the start-up procedure for the standby compressor, which should include draining the casing and checking to ensure it is free to rotate; obviously the startup procedures are irrelevant if the standby compressor auto-starts, but can be made into checks (standard operating procedures) as part of the daily field operators’ rounds.
Conrad J. Horvath, P.E., rotating machinery engineer
Syncrude Canada Ltd., Fort McMurray, Alberta
Install a sump
Your best bet is to install a sump at the lowest point of the pipe with a drain valve located at the bottom of the sump actuated by a level switch. Interlock the valve to the compressor motor so that if the level in the sump is high or the drain valve is open, the compressor cannot start. Include a position switch on the valve so that the operator in the control room can see the on/off position of the valve.
A tee can create the sump: The compressor discharge line begins with a tee. The top of the tee goes to the destination. The bottom is extended to about 5-ft long. This section serves as the sump with a level switch and a drain valve. The switch opens the drain valve on high level and closes the valve on low level.
Dilip K. Das, P. E., principal engineer (pressure safety specialist)
Bayer CropScience, Kansas City, Mo.
A fuel-gas pipeline in a pipe trench went under a road bridge inside a plant. A flanged joint fitted with a compressed asbestos fiber (CAF) gasket was located only 2.5 m (8.2 ft) from the roadway (Figure 1). According to the area classification code used by the company, there was a Division 2 area for a radius of 3 m (9.8 ft) around a CAF joint and road vehicles should not be allowed unrestricted access to Division 2 areas. However, the plant wanted to open the bridge to unrestricted traffic. Two courses of action were suggested: