Retrofitting

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Q.

Our high performance gear couplings are working fine. Why should we change them?
A. Some rotating equipment engineers are quite satisfied with the life of a gear tooth coupling. Typically, well-aligned and properly maintained gear couplings can last 10-15 years.

There are basically two types of flexible couplings, sliding and deforming types. A gear coupling is a sliding type. The intermeshed gears slide back in forth in every revolution and therefore the gear teeth wear. As they wear, the radial clearances become larger and consequently, their unbalance level increases. Gear couplings also require regular maintenance.
 
Dry couplings offer many advantages over gear tooth couplings as follows:

  • Dry couplings have no radial clearances resulting in a lower potential unbalance.

  • Dry couplings require no maintenance or inspection for a minimum of five years and are designed for infinite life. Coupling vendors recommend that gear couplings be inspected, cleaned and regreased every 12-18 months.

  • Dry couplings eliminate “lock-up” and spool cracking.

  • Dry couplings reduce hub to shaft fretting.

  • Dry couplings exert significantly lower forces on connected equipment. The sliding force exerted by a gear coupling is a function of torque. The deforming force of a disc or diaphragm coupling is spring force and independent of torque. The force dry couplings exert on equipment bearings is roughly 40% of the gear in bending and 10% axially.  Companies will sometimes include machine-bearing life when calculating the return on investment.
Q. Dry couplings generally cost more than gear couplings and lubricated couplings were originally approved by the O.E.M. Why should a replacement be considered?  
A. Actually, high performance gear couplings generally cost more today than disc or diaphragm couplings. Gear couplings are labor intensive to manufacture since they require many operations.  In addition, the cost of maintaining (period lubrication, checking, repacking grease, etc) gear couplings is significantly higher than dry couplings. Although the OEM approved lubricated gear couplings originally, a dry coupling has probably been designed and approved by the OEM to replace it. In today’s Turbomachinery, over 95% of new applications uses dry flexible element couplings.
 
Q. When retrofitting, who offers the greatest advantage: the OEM or coupling vendor?
A. The question often arises as to who should be involved in the retrofit process? There are two avenues available to the end user- the driven equipment O.E.M. or the coupling manufacturer. The O.E.M. generally offers the greatest advantages such as:
Turbomachinery technical expertise.

·        Availability of existing coupling data and envelope dimensions.

·        Historical data.

·        Rotor dynamics analyses capability.

·        Availability of taper plug/ring gages.

On the other hand, thousands of high performance gear couplings have been successfully retrofitted to dry directly through the coupling vendor. Coupling designers can generally select and “fine tune” a dry coupling to match the engineering data of the existing gear coupling so the machine train rotor dynamics do not change. It is important to have the necessary information to insure a successful retrofit.  

 
Q.  What information is necessary for retrofitting?
A. The application data must be known to insure a proper retrofit. These items include:

·        Power (HP or kW) and speed (min., max. and operating);

·        Driving and Driven equipment shaft end details;

·        B.S.E. (between shaft end) dimensions, also known as shaft separation;

·        Anticipated thermal growth, (amount and direction);

·        Any high angular misalignment requirements (High Performance couplings are generally designed for ¼ degree);

·        Service (pipeline compressor, ethylene train);

·        If coupling is to be per API-671 specification – required for turbo machinery equipment;

·        Environment (Salt water spray, chlorine, H2S).
 
To insure a successful retrofit, the coupling designer needs the weight and CG (Center of Gravity) location for each half of the coupling to match the overhung moment. By matching this data with the proposed dry coupling, the lateral critical speeds of the train should not be affected. There are guidelines based on application speed that have been established by some of the major turbo compressor O.E.M.s to insure a successful retrofit. They are as follows:

·        If the operating speed is less than 3600 rpm, the ½ coupling weight and CG location of the new coupling should be within 20% of the existing coupling.

·        Between 3600 and 6000 rpm, the ½ weight and CG should be within 15%.

·        Over 6000 rpm, the ½ weight and CG should be less than 10% away.
 
The torsional stiffness or Kt of the existing coupling should also be matched to prevent a change in the train’s torsional critical speeds. Generally, a two bodied train or driver direct coupled to driven equipment is not very sensitive to a change in the coupling’s Kt but if there is a gearbox or 2nd or 3rd rotor, a change can have a dramatic effect.
 
Q. What is most apt to fail on a flexible-element coupling?
A. Coupling designers generally try to make the flexible element the weak link. It is important to review the coupling drawing for service factors and minimum hub slippage values to insure that the coupling is designed to meet your requirements.
 
Q. Many steam turbines have a significant amount of thermal growth both thermally and axially. Can the flexible membrane coupling handle this growth?
A. Most dry coupling selections - disc or diaphragm - can handle the axial growth requirements of Turbomachinery. It is important to consider the axial thermal movement of the equipment when selecting a flexible membrane coupling because they are limited in capacity. If the axial thermal requirement exceeds 25% of the dry coupling’s capacity, the coupling designer will generally design the spacer piece to accommodate this movement.

For example, if a 20MW steam turbine grows toward the compressor 0.150 inches (3.8 mm), from static to “hot” operating condition and the coupling has a capacity of + .200 inches (+5.1 mm), the coupling spacer piece will be designed 0.150 inches (3.8 mm) short. By doing this, when the coupling is installed and all bolts are tightened, the coupling will be stretched by the amount of thermal growth. When the equipment train moves to its “hot” operating condition, the coupling should be close to its neutral or relaxed position.
 
Q. We would like to retrofit a reduced moment gear coupling? Are there any specific concerns retrofitting this type of coupling design?  
A. Yes. If the equipment requires a reduced moment gear coupling, it will need to be replaced by another reduced moment design. Reduced moment coupling hubs are fairly easy to install and remove if they are keyless hydraulic fit, but if the shaft is keyed it presents problems when attempting to remove it.

Reduced moment, keyed hubs can be mounted with heat, usually an oven or oil bath, but when unmounting, heat generally has to come from a torch. Even with the greatest care, the flexible membrane coupling can be damaged by localized heat. And the flexible element has a tendency to act like a radiator fin, dissipating the heat from the middle of the hub.

To solve this problem, Kop-Flex engineers have come up with a unique design for hydraulic removal of keyed hubs. Annular grooves are machined into the hub I.D. with connecting ports to the end of the hub’s barrel. By applying 500- 1500 psi of oil pressure to the ports, the hub “pops off” the shaft similar to a keyless hydraulic fit. This modification to keyed hubs has become very popular with refineries that would prefer not to use torches for hub removal.
 
Q. Dry couplings are generally larger in diameter than gear couplings. Will the dry coupling fit in the existing guard? Is windage a concern when retrofitting?
A. When establishing a retrofit criterion, it is important to include a dimensional check of the coupling guard. In addition, when the proposed dry coupling O.D. has a radial clearance of less than 1 inch (25 mm), windage calculations should be performed. The coupling vendors have developed computer programs to calculate the increase in guard temperature. It is also important when retrofitting to vent the guard giving the developed heat a place to exit.
 
Q. If dry couplings are so great, why isn’t everyone converting their critical equipment?
A. Most major users in North America introduced reliability programs in the 1980’s that included the coupling retrofit. This trend continues through the rest of the world. Retrofitting critical equipment trains in refineries and petrochemical plants has become a significant part of the coupling vendor’s business.