Centrifugal Compressor Hot Gas Bypass

[RKJustice]

Hi there,

I am looking at the attached centrifugal compression system and I am considering the unmitigated consequences (ignoring all trips/interlocks - there are limit switches on the HGB valves and high temperature trips that trip the compressor but these are assumed to not act) of the hot gas bypass (HGB) failing open on either the 1st or 2nd stage compressor.

If the HGB spuriously opens then hot gas will be recycled resulting in increasing suction/discharge gas temperatures until the the system design temperature is exceeded and loss of containment, assuming there is a significant pressure drop around the HGB route. Is this correct? Could the gas temperature theoretically rise indefinitely if the system had an infinitely high design temperature?

 

[rotw]

You need to do a dynamic simulation to define the dynamics of the system upon failure of one of the HGBV's as it seems to me that both HGBV's failing at the same time seems quite unlikely.

Upon failure of one HGBV during normal operation, depends on the cv / sizing of the valve, one compressor section will see rapid increase of flow combined with increase of temperature. That will shift your operating map and operating of compressor will be affected. How it will be affected is the question.

So before looking at temperature behavior, the whole system behavior in terms of how far/ how long compressor operation can be kept stable without the available performance envelope - just by the unmanned intervention of the compressor control logic - needs to be verified with help of a dynamic simulation. Otherwise telling what is going on is just guessing.

By the way, it impacts if your two sections are mechanically coupled or not, means both run at same speed or are independantly controlled. When sections are coupled, I guess will make even more narrow the operability range of the whole system. In case this is a back to back compressor, also think that you have a common set of design pressure/temperature for both sections.

Ultimately if the compressor will continue operating, say without strange behaviour - which again seems hypothetical, then under failure of one of the HGBVs the temperature will in theory continue to increase until the discharge temperature reaches the design limit of the compressor. The design temperature of the piping, intercooler etc are not to be overlooked.

In certain pipeline applications where there is a hot anti surge /recycle (no cooler), where the pipeliner compressor is driven for instance by gas turbine - it often takes time for the driver to start and reach the sufficient speed for the compressor line to go on export mode. During that timeframe the temperature increase due to hot recycle is sometimes sufficiently substancial that adopting certain measures in the configuratiin are necessary to avoid exceeding limits problems. In this example the timeframe is about couple of minutes, just to give an order of idea.

"If you want to acquire a knowledge or skill, read a book and practice the skill".

 

[RKJustice]

Thanks for having a look.

Perhaps my wording was not the best but I am considering one off the HGB valves spuriously opening for either individually not both at the same time.

In any case as you say a dynamic analysis is required to see how the system actually reacts, which would be a very large undertaking (it is actually 2 trains of compression, each train mechanically linked with a complex speed/load/performance/anti-surge controller) to the opening of one HGB valves but in the absence of this I will assume that the temperatures will eventually exceed the design temperature of the system. Simplistically, there is nowhere for the heat of compression to be removed and will be continuously recycled until the design temperature exceeded.

 

[LittleInch]

Whilst I can understand your desire to simply see what would happen, ignoring multiple safety devices (double/triple jeopardy) is normally regarded as looking at events which are simply not credible.

I'm probably going outside my knowledge envelope here, because I had to go look up what HGBs were. From my small snapshot, they appear to be required for rundown of large volume compressors during a trip / ESD when otherwise the ASV would either be oversized for normal use or not big enough and hence at certain speeds during rundown you could enter the surge zone. Hence they appear normally to be large bore, low diff pressure on/off valves. The requirement for which is only apparent during a dynamic analysis and many compressors are in service with only ASVs.

The impact of opening one in service depends on the overall flow. It would seem feasible that providing through flow continues through the train, the impact is really one of reduced performance, though the machine in question could easily stone wall or over speed depending on its configuration and type of driver.

Only if there is no / very small through flow would the temperature start to seriously rise, but you cannot look at multiple failures of a control / ESD system and stay sane.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[RKJustice]

Thanks for having a look littleinch.

I am not sure if you are familiar with LOPA but for such reviews you need to consider the consequences of a single cause (in this case spurious opening of a HGB valve) without any layers of protection (combination causes would also be considered but the probability of this accounted for and thus not normally an issue). This is where I am coming from.

For the 1st stage HGB valve - I think there would be significant flow via the HGB route and very little through flow resulting in the compressor speeding up. The compressor would then continue at the maximum speed the controller allows recycling a large proportion of hot gas back to its suction.

A similar scenario with I think similar results is loss of cooling medium to one of the coolers while the compressor is running in full recycle (e.g. after plant upset or during start-up).

 

[LittleInch]

I get that - my point is that at max flow but with low increase in pressure across the compressor due to this valve being spuriously oened, the temperature rise is not so high. It all depends on your system and where the gas is going. Essentially you would be loosing probably 80-90% of that compressor normal increase in pressure hence the second unit would only be getting a fraction of its normal suction pressure and hence it will probably impact on its outlet pressure. If you're pumping into a fixed pressure system then this might result in no flow at all, but if your d/s system will still take gas, but at a lower flow due to the lower output pressure of your compressor train. Hence there might be not much impact on temperature.

Dynamic system analysis is the only way you'll find out.

Compressor running in full recycle with normally a decent pressure gradient but loss of coolant would definitely have an impact on temperature and that would gradually spiral upwards as there is no through flow, but having your HGB open during normal operation is not so easy to second guess.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[RKJustice]

Thanks littleinch.

 

[MortenA]

@RKJustice: Aswering your initial question: No the temperature could not rise indefenitely because as the temperature increase so will the heat losses through the pipes etc and at some stage the heat input would correspond to the heat loss - thats the theory.

Best regards, Morten

 

[RKJustice]

Thanks Morten.

 

[georgeverghese]

Hot gas bypass feature on centrifugals are some times necessary to keep the compressor away from surge on sudden shutdown.
Though I'm not party to this particular design, the valve capacity on a HGB ( which is usually a quick open high capacity ball valve) may well be that the compressor goes well down on it Q-h curve when this valve opens, for which compressor discharge temp would be much lower.