Centrifugal Compressors

1Q. We have dry gas seals in our compressors and they seem to be working OK. We are going to convert all our centrifugals. Do you have any cautions?

1A. If you're converting from oil bushing type seals, the damping may change and instabilities may arise. Make sure you have a rotor dynamic analysis done. Dry gas seals don't like impacts so have good surge control on your compressor. Gas seals don't like contamination from process, oil system or seal gas supply, either solids or liquids. Use 3 micron or smaller coalescing type filters and sometimes pre-filters are needed. High pressure services, over 3,000 psig present special concerns. Make sure you are aware of them if your thinking high pressure service. Compressor discharge seal gas supply can run into problems with start-up, shut down and hot idle, when pressure is too low. If you don't have gas available and have to install nitrogen compressors, you could be exchanging seal problems for nitrogen compressor problems. A good paper to read, available through Google, is by John S. Stahly, Dresser-Rand, “Design, Operation & Maintenance Considerations for Improving Dry Gas Seal Reliability in Centrifugal Compressors”.

2Q. What is the best way to make sure the new centrifugal compressor we are ordering, with dry gas seals will operate correctly after installed?

2A. With dry gas seals on over 80% of new process gas compressors it good to be concerned, especially on off shore injection gas higher pressure machines. These have to engineered correctly! Full pressure tests in the vendors shop are expensive but much less costly than continued on site failures. Have the compressor and seal manufacture field service personnel on site during start-up. Have a "Cold Eyes Review" by an expert in dry gas seals in the early design stage. Many failures are due to the auxiliary systems providing gas to the seals rather than failure of the seals themselves.

3Q. What is meant by a rotor dynamic study?

3A. Centrifugal compressors, steam turbines, gas turbines and other large similar machines can have vibration related problems not just by mechanical sources such as unbalance or thermal rotor bow. Instabilities due to oil whirl, and aerodynamic problems of all sorts can develop in these machines. A rotor dynamic analysis analyzes the system in the design stage and allows the designers to tune out some of these problems by the correct design of the bearings spring constants and other damping characteristics. By knowing where these events are located in the operating range, the system can be modified to remove them from the speed range or design the system to be able to tolerate the vibration level. Usually the amplitude of the lateral rotors vibration and frequencies of these vibrations are the outputs from such a study. They are also helpful in determining the cause of vibration concerns when compared to on line vibration signature analysis.

4Q. What are hydraulically fitted hubs and why are they used?

4A. Machines such as centrifugal compressors have to be connected to gear boxes, turbines or motors. On small equipment a shaft with a key was used in the past, or a straight or taper fit put on by expanding the hub with heat. They were difficult to remove in the field and had stress risers which caused shaft failures. Taper fitted hubs that are pulled up with hydraulic rams and released with oil pressure have replaced them in higher horsepower machines. They are removed by just pumping in some oil. They then "pop off". Extreme care is needed and safety procedures are required, as the hub comes off at 20 miles per hour or more and can travel quite a distance. Treat them like the barrel of a gun and stay out of the line of fire.

5Q. What is compressor surge and why is it a problem?

5A. Very simply, when a compressor discharge pressure gets too high, it is easier for the gas flow to flow through the compressor back to suction. As it does this the discharge pressure drops. The discharge pressure starts to rise again and the cycle starts all over again. So it's a flow reversal within the compressor. Low compressor flows, molecular weight changes, surge control problems and other factors can cause surge. Surge control methods usually keep machines out of surge by routing discharge flow back to suction. Machines in surge continuously, can damage thrust bearings and cause other problems.

6Q. My compressor at times sounds like someone has stepped on a cat's tail, what is the problem?

6A. Probably the compressor is in surge. The sound of surge is different for different systems and depends on the systems volume. It can sound like a cat whining or a dog barking and everything in between. It can be a real frightening sound!

7Q. What is a good book on compressors?

7A. I like "A Practical Guide to Compressor Technology, H.P.Bloch, ISBN 0070059373". It is quite practical and has a lot of information in one place. There are other excellent books out there too.

8Q. Why aren't bearings in centrifugal compressor just 360 degree shell type bearings like in automobile and diesel engines or ball or roller bearings? There seem to be so many different types for use in centrifugal compressors of multi-pad design.

8A. It is related to question 3. The bearings are of special design as they are used for tuning and alignment of the rotors. Their damping and stiffness constants can be varied. When designed and operated properly the rotor is supported by a hydrodynamic film and there is no metal to metal contact and thus little wear. They have a long life as long at the oil is clean and of the correct type.

9Q. We are bringing a major compressor down after many years of service, and plan on changing the rotor out. Any suggestions?

9A. Only to make sure you put together a checklist and expect the unexpected. Get the compressor manufacturer service engineer on site with drawings, make sure the spare rotor will fit in the case (it happens). Put your hands on the bearings and parts and don't think they are there just because someone says so. Be familiar with the shut down and start up procedures, all clearances, balance specs and alignment. You will probably want the spare rotor sent out to a shop, measured and balanced. Long term storage does strange things to rotor elements.