Coupling guard heating

Vishal Jariwala, Daryll Turner and James Hardin of Elliott Group presented a paper titled, ‘Computational Investigation of Coupling Guard Heating and Mitigation’ at the 43rd Turbomachinery & 30th Pump Users Symposia (Pump & Turbo 2014) that took place from September 23-25, 2014 at Houston, TX. Below are some of the excerpts:

Up to the middle 1970s, lubricated gear type couplings (either grease packed or continuous oil lubrication type) were the norm in the refining and petrochemical industry. Totally enclosed guards for these couplings were simple structures with two major concerns: proper drainage of oil and prevention of oil leakage. Later, dry couplings began gaining acceptance. By eliminating gears, they eliminated tooth wear and the potential vibration caused by them, as well as the need for an oil lubrication system. They also reduced thrust bearing loading.

Dry couplings also need guards for safety reasons. Equipment and coupling manufacturers, however, quickly learned that coupling guard design was an important issue with dry couplings. Dry couplings generally had larger flange diameters than gear couplings of comparable power capacity. The larger coupling flanges with higher number of bolt heads caused significant amounts of windage, resulting in frictional heating of the air in the coupling guard. Coupling guard volume, net cooling air flow, and clearance between the flanges and the guard internal wall became important factors. An improperly designed coupling guard results in unacceptably high coupling guard surface and internal coupling temperatures. Further, close radial clearances effectively create separated “hot” volumes inside the guard.

Coupling manufactures do provide coupling guard temperature prediction software, but these programs do not address details of coupling guard design such as location and number of inlet breathers, interior baffle design, and location and number of outlets. Such details are important to meet the API 671 requirement that guard temperature should not exceed 160°F (70°C) (API 2007).

The results presented below are far from a thorough parametric study to derive an exact methodology for coupling guard design. The following can be concluded.

• A CFD model of a rotating coupling inside a stationary coupling guard must include bolt and nut geometry to effectively predict coupling guard heating, temperature, and flow. A model without the bolts, using only smooth surfaces, predicts very little heating.

• A single horizontal tangential discharge at the bottom axial center of the coupling guard has little pumping action. Horizontal tangential discharges at the Copyright© 2014 by Turbomachinery Laboratory, Texas A&M Engineering Experiment Station bottom in approximate alignment with the rotating coupling flanges improve pumping action and increase mass flow through the coupling.

• A coupling guard with one axially centered horizontal tangential discharge nozzle operating in parallel with a second horizontal tangential discharge nozzle in line with a rotating coupling flange can create flow recirculation from the discharge nozzle at the coupling flange back into the coupling guard through the discharge nozzle at the axial centerline.

• Coupling guard volumes that are isolated by close clearances between the rotating coupling and the guard inside diameter and that do not have an air inlet will have the highest heating and temperature because of limited cooling flow.

• When total flow area is equal, a larger number of smaller diameter inlet air breathers spaced along the top of the coupling guard provides a more effective cooling air inlet than does a reduced number of breathers of larger diameters.

• Air flow through the coupling guard can be increased significantly by reducing discharge vent pressure by only a few inches of water. In the present work, a 35% mass flow increase was seen by reducing exit pressure by only 1.3" (38.1 mm) water pressure. Clearly, the mass flow rate is very sensitive to exit static pressure.

• Adding a high efficiency coalescing filter at the coupling guard discharge vent effectively eliminates oil misting but increases discharge static pressure due to the coalescing filter pressure loss.

Coupling Guard Design Recommendations:

• Locate fluid exits tangential to coupling flanges having bolt heads.

• Equally distribute, along the coupling guard axis, a reasonably greater number of small area inlet breathers rather than a small number of large area inlet breathers.

• Make the clearance between the coupling guard inside diameter and the coupling flanges as large as possible, while considering installation constraints.

• Avoid creating isolated high temperature internal volumes by using large internal radial clearances and increasing the axial distance between the coupling flanges and the ends of the coupling guard.

• Consider applying forced ventilation by creating a vacuum at the coupling guard fluid exits.

• Take “system” approach to designing a guard. Consider other elements in the piping system, and how they affect the air flow through the guard.

(Vishal Jariwala is an Aerodynamics Engineer in Research and Development at Elliott Group, Jeannette, PA, who designs centrifugal compressor stages using analytical methods including computational fluid dynamics (CFD).

James Hardin is a Senior Engineer in the Research and Development at Elliott Group, in Jeannette, Pennsylvania, where he performs computational fluid dynamics (CFD) and other aerodynamic analyses for turbines and compressors.

Daryll Turner began working at Elliott Company in Jeannette, PA in 1975. He joined CITGO Refining in 1987 as rotating equipment engineer. In 1991 he helped form an international power recovery expander Users group.)