THIS MONTH'S PUZZLER
|We have two parallel centrifugal compressors that deliver motive gas to an ejector system that has two parallel ejectors. The ejectors extract fines from a cyclone to prevent cyclone blockage. One compressor operates at all times, with the other in standby. We have been getting condensation in the standby compressor due to cooling some of the heavy material in the motive gas. The liquids cause the compressor to fail on start-up, defeating the entire point of having that unit. One idea we have is to allow some discharge gas from the on-line compressor recycle back through the standby compressor to keep it hot. This would lead to reverse flow in the standby machine. Is this okay? Or would some type of auto-drain, slow-roll operation, or aggressive heat tracing work better? Does anyone have experience with these ideas or with other solutions?|
Install a drip leg drain
Putting discharge gas from the operating compressor through the standby compressor is not recommended. Reverse gas flow is likely to cause the standby compressor to rotate in the reverse direction without control. Most centrifugal compressors are not designed for reverse rotation and operating in reverse rotation could damage the compressor.
If the compressors have down-oriented nozzles and have slow roll capability, a drip leg drain could be added to the discharge line together with level monitoring instrumentation. The compressor could be drained either manually or automatically depending on the instrumentation. If the compressors have up-oriented nozzles and casing drains, the drains can be manifolded to a drip leg using the method described for the down nozzles. If none of the methods mentioned are feasible, the addition of aggressive heat tracing can be considered.
Royce Brown, principal,
RNB Engineering, Houston
Pipe in a recycle line
Not knowing the details of your system, I will make some assumptions:
1. Common suction and discharge lines.
2. Automatic valves on suction and discharge that open/close when the standby compressor is needed.
Also, since you said you could slow roll, I assume you have a steam-turbine driver or VSD motor.
1. Do not run compressor train in reverse.
2. Simply pipe a small recycle line from the discharge of both compressors to the suction of the other compressors.
3. Locate the tie-ins for the recycle lines between the automatic isolation valves and the compressors.
4. Size the line to keep a minimum flow to prevent condensation. Use a
restricting orifice or a valve that could be adjusted while running to achieve desired results. If you want to get fancy, add an automatic valve with a temperature sensor to minimize recycle.
5. If not already done, insulate both compressors. Insulation blankets and thermal shrouds can be easily obtained.
6. If this results in the standby compressor slow rolling, all the better for start up when needed; this also should help prevent a bow in the shaft from occurring. It will be turning in the correct direction for operation.
7. Have isolation valves in the recycle line so that maintenance can be performed on one compressor while the other compressor runs.
8. With the arrangement described above, either compressor that is running will be able to recycle a small flow back through the suction of the other compressor on standby and back to the suction of the compressor running.
9. This system will use some energy because of the recycle, but should provide much greater reliability and lower maintenance cost.
10. Either compressor could be operated with the other compressor on standby with this system.
George Hogg, maintenance/reliability manager
El Dorado Chemicals, El Dorado, Ark.
Try a condensate trap
Could the fix be as simple as a condensate trap <em dash>— like one that would be used on a steam system?
Jason Englund, instrument tech II
Amgen, Bothell, Wash.
What about condensing heavies?
It would help to know what the “motive gas” is that the compressors boost to get the ejectors extracting. Guessing by your description, the motive gas might be a combination of solvent vapors with the “heavies” condensing at a cold spot in the system — the idle compressor. Are there any other factors such as environmental, product run material?
Online compressor recycle. We have used simple single-valve, manual kickbacks on spare, parallel liquid centrifugal pumps for many years as standard installations for freeze protection.
We have also considered an “on-line compressor recycle” as you noted in an effort to eliminate condensed ammonia gas at an idle, parallel reciprocating compressor. With our reciprocating compressor, a small-orifice solenoid valve would be installed on existing ports in the compressor head to allow discharge gases from the online compressor to back-flow across the idle compressor’s internal discharge reed valve and its internal suction reed valve. This hot-gas kickback solenoid valve would fail open when the compressor is off and timer-close a few seconds after the compressor starts (also serving as an unloading valve to lower startup costs). Our volume of kickback would be very small and within heat-traced and insulated piping.
There may be other considerations with your larger centrifugal compressor such as a much larger, inefficient kickback to achieve heating across all dead spots, possible seal damage due to condensation/seepage and safety issues relating to isolation and clearing. This approach would require a detailed “Management of Change” per PSM. Finally, any warranty might be voided on the compressor itself.