Intermittent vs. continuous service 3

[rotw]

Hi,

Can anyone help me to figure out the impact on compressor design when specifying the machine in intermittent service (instead of continuous) ?
As I see it the driver (say a motor), gearbox, auxiliaries etc. shall be designed to withstand extended idle/stand still periods, frequent start/stop cycles etc. also intermittent service shall be accounted for mechanical fatigue of components (impact on service factors, oversizing ?)

- Is there a definition of an intermittent service as per codes (API) ?

I also guess that a machine in intermittent service is still designed for 20 years lifetime if it follows API ?

Any help is really appreciated.

Thanks

 

[JJPellin]

Intermittent service can be a challenge. The majority of our compressors run constantly. But, for the ones in intermittent service, we have seen a number of problems that have required us to make changes.

We have a spare coker wet gas compressor in one of our cokers. They used to leave that machine in a ready-to-run condition, blocked in from the process and under nitrogen purge. But, that compressor has cone seals that cannot function correctly when they are not spinning. Even with the N2 purge, they would get slight leakage through the block valves which accumulated liquid in the compressor case. The liquid corroded and fouled the rotor resulting in high vibration on start-up. Now, we keep that compressor fully blinded from the process. If we need to run it, it takes 4 to 8 hours to get the blinds pulled before it is ready to start-up.

We have a spare induced draft fan on one of our hydrogen plants. That big blower sits idle most of the time. There are no block valves, only inlet louvers. The louvers leak by and water condenses out of the hot flue gas. This corroded the wheel over time. As the stiffener ribs on the back of the wheel got weaker, the natural frequency of the rotor dropped down into run speed range. We experienced extremely high vibration or start-up. We had to change our operating practices and now we run both fans in parallel all the time to prevent this. We are considering an upgrade to a wheel material that won’t corrode as readily.

We have a large barrel compressor with a steam turbine driver that sits idle during winter. The compressor has not had problems associated with this. But the turbine has had issues. The turbine and associated support systems were not designed to sit idle in extreme cold. They had freeze up problems on the surface condenser, gland condenser and seal steam systems. The operators responded by introducing steam into the condenser. But this is a bottom exhaust turbine. Heat came up from the condenser and we found that the turbine was sitting there idle and quite hot. We were very lucky that they didn’t severely bow the turbine rotor. We offered them two options to address this. They could fully winterize the systems with thermostatically controlled electric tracing and insulation or they could run the turbine year-round. They opted to run the turbine all the time. They idle the turbine down to minimum speed and recirculate the gas on the compressor to keep the entire system hot.

Synchronous motors can suffer hard starts depending on the driven machine. If I was buying a machine for intermittent service, I would probably specify an induction motor rather than a synchronous motor. There would be an efficiency penalty, but the motor could be designed to take the additional starts. We have a coker jet pump that is driven by a 3500 HP induction motor. Because of the nature of decoking, they motor sees as many as 1000 starts per year. The motor is an enclosed motor with air-to-air exchanger. It has a heavy duty rotor with copper bars. The rotor was designed as a stiff rotor design so that it does not have to pass through critical speed on each start-up and shut-down. The motor has been very reliable.

For intermittent service, I would increase the service factor requirement for the coupling from 1.5 up to 2.0 or more. I would insist on a lube oil system with two pumps (no shaft driven oil pump). I would size the lube oil heater to be sufficient to maintain full normal oil temperature (100 °F minimum) when the machine was not running. I would keep the oil in circulation when the compressor was down. This would help reduce the chance of water condensing in the oil system when the compressor was not running. Depending on size, I would insist on a continuous monitoring system armed to trip on radial vibration or thrust position. I would verify that the gearbox design has a substantial overdesign. I would insist on a full torsional and rotor dynamics analysis of each major component and the full train. I would consider increasing the minimum separation margin from any critical speeds beyond the minimum API requirements.

I could be more specific if you provided additional details about the compressor, driver, service, expected number of starts per year, etc.


Johnny Pellin

 

[rotw]

JJPelin,

Thanks for the valuable input.

The subject compressor is intended to be an integrally geared compressor, motor driven rated about 2.2 MW power.
API 617 will be applied for machinery design.
The compressed fluid is fuel gas (almost 100% methane) with possible traces of water.

This machine will be operated only during a process upset thus is expected to be operate on unfrequent basis.
Probably compressor will operate for a brief period and then will stand idle for a quite long time (several months / possibly years).

So in my view it will have a very limited number of starts, possibly special considerations should be given to design of seals, valves.

Do you think the Service factors should be increased (gearbox, couplings) and focus should be given on torsional and lateral dynamic aspects ?
I have some worries about bearings and sealing.

in case of long stand still period, I was thinking to consider starting the compressor on recirculation mode once in a while?

What are your thoughts?

Thanks

 

[JJPellin]

We have two integrally geared, high speed compressors in H2 recycle service in our Penex unit. Because of the risk of liquid hitting the compressor, they are not allowed to switch compressors on a running unit. So, the spare unit is never started (often for years). Some of the items I listed are not relevant for this type of compressor. I would not request an over-design of the gearbox. The coupling rating is also probably not critical since you will have relatively few starts. Based on the history of our machines, I have additional concerns that I did not think of before. These units are prone to rust forming on the gears. I would suggest a well designed external lube oil system with heater, cooler and auxiliary oil pump. I would keep the oil system in circulation at all times. Our stream is not wet. Since your is wet, my comments above about blinding and/or purging are relevant. I would make sure to have a good set of double block valves. I would put a purge on the compressor. Bone dry nitrogen would be ideal. If that is not available, dry instrument air might work. I would still ask for a torsional and lateral rotor dynamics analysis. But, I would do that whether it was an intermittent service or a continuous service.

I would suggest running the compressor on some periodic basis in order to collect vibration data and prove out performance. If it can be started up on pure recirculation, that is fine as long as the recirculation loop has adequate cooling. I would run it every 2 or three months. I am assuming one particular configuration (like our compressors). If the compressor and gearbox have any rolling element bearings, you should probably run it more often. If all bearings are hydro-dynamic, then this is less important. The seal may dictate additional requirements. A dry gas seal should be fine in this condition as long as the seal gas is always in service. If it is an oil pressurized seal, then I would also keep the oil to the seal in normal operation at all times. I don't know what other type of seals might be used.

I am getting ready to start-up a two stage FCC wet gas compressor, so that is all I have time to offer.

Johnny Pellin

 

[rotw]

Very good points JPPellin. Thanks.