In today's economic climate, it has become increasingly important to reduce processing costs. Premature pump failures can easily lead to thousands of dollars in maintenance and operations losses. Site surveys conducted in numerous process plants across North America have concluded that 70 percent of all pump failures are attributed to some form of misalignment. Detailed failure evaluations performed at specific plant sites have revealed that the cause of more than 80 percent of chronic failures involved pipe strain.
Piping should not impose excessive forces and moments on the pump to which it is connected. Pipe flanges must be brought squarely together before the bolts are installed and tightened. Suction and discharge piping, along with all associated valves, strainers, etc., should be adequately supported and anchored near to and independent of the pump to prevent strain from being transmitted to the pump casing.
It is widely accepted in the pump industry that pipe strain leads to unreliable equipment operation. With such emphasis placed on eliminating bending moments in high-speed centrifugal pumps, it is important to define pipe strain, consider the effects it has on pumping systems and identify methods of eliminating it.
Defining Pipe Strain
Pipe strain is caused by misalignment between the pump suction and discharge flanges and the corresponding pipe flange connections. Unacceptable pipe strain can be defined as any forces from unanchored piping that will cause equipment deformation of more than .002-in.
Free-bolting is one method that can be used to ensure that no bending moments will be transmitted to the pump. This practice involves confirming that all bolts will slide freely through the holes in both the pump flanges and the pipe flanges without exerting force. A common engineering practice is to specify the bolt holes in flanges to be drilled 1/8-in larger than the diameter of the connecting bolts. This practice is consistent with the alignment specification in ANSI/ASME B31.3, which states:
"Flanged joints shall be aligned to the design plane within 1/16-in/ft measured across any diameter. Flange bolt holes shall be aligned within 1/8-in maximum offset."
Effects of Pipe Strain on Pumping Systems
Parallel and angular misalignment of piping flanges at the pump nozzle results in excessive nozzle loads. Excessive nozzle loads create stresses in pump hold-down bolts as well as distortion in pump supports and baseplates. Other than serious unbalance of pump components, there is no single detractor of equipment reliability more significant than poor alignment. Incorrect alignment between pump and driver couplings can cause extreme heat in couplings that leads to hub, keyway and grid failure. Reverse bending fatigue creates excessive loads that can bend, crack or break a pump shaft and excessive radial and thrust loads lead to premature radial and thrust bearing failure.
Forcing piping in place for attachment to the pump suction and discharge flanges can easily create excessive loads in pump nozzles that stress materials and produce bending moments. These distort internal moving parts and affect critical radial clearances. Rubbing caused by radial clearance losses between rotating and stationary elements rapidly damages component parts and requires more power to rotate the pump shaft.
Piping misalignment can also be costly from a leakage standpoint. Impending forces are placed on piping components that will relieve compression on casing gaskets as well as gaskets that seal between the suction and discharge flanges and the corresponding pipe flanges. Damage from piping misalignment can also cause failures of supports, expansion joints and fittings. Eccentricity of the pump shaft to the bore of the stuffing box or seal chamber caused by pipe strain will also impact performance of packing and mechanical seals. This is of utmost importance given the tremendous interest placed on reducing dilution and waste associated with flushing fluid and eliminating costly product leakage to the atmosphere.
The choice of using packing versus mechanical seals to control leakage is largely based on the end user's process conditions and requirements. If slight leakage can be tolerated, or it is undesirable to pull the pump off-line, then packing may be the preferred choice for sealing the stuffing box. Sealing water-based slurries and water at elevated process temperatures are examples. Packing relies on radially expanding soft flexible materials to control leakage by filling the annulus between the rotating shaft/sleeve and the bore of the stuffing box.
To achieve cost-effective leakage control with packing, it becomes increasingly important to ensure leakage is consistently maintained at allowable rates for the application conditions and the type of packing used. If the pump shaft does not remain concentric with the bore of the stuffing box, the radial gap is compromised, preventing equal compression on the packing. This condition then leads to product dilution, leakage to the atmosphere and dramatically increased process costs.