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Pumps & Systems, May 2008

In principle, a successful sealing program is centered on tracking performance levels to ensure the reliability of shaft sealing components in process systems. This article focuses on the pumping asset only, less the other capital equipment also considered to make-up a "system." The ultimate goal of a sealing program is reducing the total cost of ownership of the asset for which the seal is installed. The focus should be a "pumping system" approach and include root cause failure awareness.

Understanding how failures occur will eliminate everyday ignorance and permit the seal user to implement programs or initiatives that will address the operational, mechanical and design aspects of any pumping/sealing system.

Operational Causes for Seal Failure

Equipment operational factors account for approximately 40 percent of seal failures. For the operations functions, a basic understanding of pump operation and its effects on sealing components can make a huge difference. Broadly speaking, operational factors can become a problem when the human element is introduced. To avoid many problems, knowledge of the following is required:

• Minimum flow requirements
  • Operating a recirculation line properly
• Sufficient NPSH and operating on BEP
  • Throttling a discharge valve when required
  • Ensuring the supply source is sufficient
• Variable head conditions
  • Using a variable speed drive when available
• Valve positioning
  • Avoiding dead heading against a closed discharge valve
  • Avoiding start-up with a closed suction valve
  • Understanding variable positioning of control valves upstream
• Start-up
  • Air bound prevention
  • Vapor locked prevention
  • Ensuring operational seal support systems
  • Using the heat tracing when needed

It has been proven that 50 to 70 percent of these operational seal failures can be avoided when this practical knowledge is applied by the people who operate and monitor these pumps. Good operating training, with a pump focus, will empower the operations department and further reduce potential failures resulting from improper operation of equipment.   

Mechanical Causes

Mechanical causes account for approximately 24 percent of seal failures. These failures can be contained with awareness, or with the implementation of standards that eliminate the opportunities for defects. Mechanical failure avoidance starts with the development of plant standards such as seal, bench, operational and field inspection. The umbrella of a "precision pump" repair standard can cover all of these and include the proper design specification, sizing and operation of the pump. Partnering with pump vendors places the onus on them to understand plant systems, specify the pump accordingly and use their knowledge to meet your goals. Mechanical considerations include:

• Applying the correct seal design based on the plant standard
• Considering L3/D4 ratio for the window of operation as it pertains to centrifugal pumps
• Validating the pump design and speed based on flow requirements
• Applying industry best practices to minimize turbulent flow
• Using the manufacturer's recommended tolerances for bench inspection
• Ensuring coupling alignment standards are met
• Standardizing a flexible coupling program
• Reducing pipe strain where possible
• Correcting poor foundation or baseplate conditions
• Balancing standards for impellers and rotors
• Using bearing isolators to eliminate contamination from dirt and moisture
• Understanding proper impeller/clearance based on pump design

The seal is only as good as its foundation. When considering all the above, one common failure mode is bearing failure. Addressing the root causes for bearing failure will inherently drive greater seal reliability.