Recycle comp. driven source change HP steam to electric

[1906]

Recently we are thinking changing hydrocracker unit recycle gas compressor driven source from HP steam(Pressure 40 Kg/cm2, 400'c) to eletric power considering refinery steam balance. Re this matter, if there are good infomation please comment on below .

a) Merits and demerits of steam driven comp.
b) Merits and demerits of electric driven comp.
c) Any experience of eletric driven recycle gas comp.
in high pressure such as hydrocracker.
d) Reliability of each driven comp.
e) Steam driven comp seems to be designed generally
,inform me the reason , if any.

 

[JJPellin]

I have never heard of anyone making the conversion that you are proposing. All of our recycle compressors (I belive we have eight of them) are steam turbine driven. The only large process compressors that we have that are motor driven are in wet gas service in cokers and FCC gas plants. The main problem with a motor driven machine would be control. All of ours are speed controlled. If you use a motor driver, you have to have a more complicated spill-back system for capacity control. You also have to make changes to your anti-surge system. In this case, it may actually become simpler rather than more complex. The existance of a hydrocracker generally implies the existance of a hydrogen plant. In our installations, the hydrogen plants tend to be big steam producers and it is most ecconomic to use that steam to drive turbines.

The main disadvantage of a turbine is that it will be more maintenance intensive and less reliable than a motor. The main advantage of a turbine is the ability to drop speed for unit start-up and shutdown and the use of speed control to control capacity. Depending on the size of the motor, the electrical infrastructure required can be quite expensive. As a maintenance engineer, I prefer motors. They are less trouble.

 

[svanels]

What I have heard from experienced process engineers is that the main disadvantage of electric motors is the dangers when you have a power failure in this application. It is critical to keep the gas flowing and preventing meltdown temperatures in the reactor. When you have a 15 minute window to restart an electric motor, a steam system has in most case enough system pressure to initiate a controlled shutdown.

 

[tgmcg]

We too are considering such a conversion, using a high-speed variable speed motor. This conversion was successfully implemented at a sister refinery. Our gas MW is declining by a factor of 2 and the existing steam turbine cannot meet the required re-rated compressor speed. Nor is there space to install a gearbox. We may possibly decide to install a new steam turbine.

However, Svanels comment regarding the consequences of a power outage absolutely warrants in-depth consideration.

 

[UmeshMathur]

I begin by apologizing to process engineers who already know these details for revisiting them for the benefit of engineers from other disciplines:

To me, the main issue with a recycle gas compressor in this application is that the recycle flow itself can vary hugely, depending on unit throughput, operating severity, and of course start-up, shut-down, and other abnormal conditions. The performance curves for the compressor dictate the required speed for a given suction flow and desired head. For most compressors, the head drops off as flow increases at constant RPM. Thus, the compressor controls require that speed be increased continually to maintain constant head (or pressure ratio if you prefer to look at it that way) when flow increases.

The compressor driver must therefore be capable of responding smoothly to varying speed demands at highly variable power loadings. Steam turbines have proven their capability in this respect over very long periods of time, although I agree absolutely with the others who have responded to this post that turbines require more maintenance than electric motors. However, there are a lot of electrical paraphernalia and complex control issues to consider for large horsepower variable speed electric drives. Also, although I am not an electrical engineer, I believe that the range of operability (speed variation v/s loading) for steam turbines is greater than for electric drives.

Therefore, the three main questions that occur to me from a process and economic standpoint are:
(a) How much load variation do you currently see in your unit (especially during abnormal events)?
(b) Do you have the assurance that an electric driver will provide the needed flexibility without requiring a complete revamp of the anti-surge control system? (Recall you are handling mostly hydrogen, a very low molecular weight gas).
(c) Is the cost of conversion reasonable (assuming that you have included the economic cost of down-time or scheduled the work to coincide with a planned maintenance outage)?

Finally, the safety issue for hydrocrackers stands out in my mind as pre-eminent: the loss of recycle hydrogen to cool a hydrocracker reactor just when it is close to the edge of being in a thermal run-away condition can lead to a catastrophe (large hydrogen filled reactor vessels at extremely high pressure).

 

[Ashereng]

By all means, revisit. This is one of the features that I like about this forum - it reminds me of all the stuff I should have remembered, but, forgo.