Maintenance Pumps

Mechanical-Seal LCCs Hold Savings

Michelle Segrest | November 15, 2016

Mechanical seals may represent the largest cost of operation in some facilities and their reliability is a direct proxy for overall pump reliability.

The result of good reliability engineering is elimination of field failures, yet making the connection between reliability and prevention of these failures is not always obvious, according to Lloyd Dewey Lee, Jr., CMRP, MBA, CRL, Reliability & Asset Management SME, FacileX, Knoxville, TN.

During his presentation at the 24th Annual SMRP Conference in Jacksonville, FL, in Oct. 2016, Lee said communication issues that many reliability personnel have with management can limit exposure to justification of their programs.

He posed the question, “How does one show and promote the ongoing contribution to ROI to justify the reliability program?” A quick answer is that one of the biggest, most overlooked, contributions to maintenance costs for pumps (which are also one of the most prevalent equipment types found in manufacturing plants) is the mechanical seal.


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Mechanical seals (depending on their piping plans) may represent the largest cost of operation in some facilities, and their reliability has a direct impact on overall pump reliability. Because of these factors, reliability personnel should be aware of the life-cycle costs (LCCs) of these seals.

Total cost of ownership (TCO) involves, at a minimum, five factors:

Design — This includes considerations such as expected design life and service criticality. Studies show that as much as 80% of machinery reliability is determined in the design phase.

Acquisition — The acquisition cost of an individual mechanical seal is dependent upon many variables, including metallurgy, elastomers, shaft size, cartridge or single-spring type, face materials, whether it has single or double faces, and any ancillary equipment needed for a flush plan. Acquisition costs aren’t significant, compared with operational costs. Implementing an alliance program with a seal vendor can improve acquisition costs.

Operation — Far and away the costliest component of mechanical-seal usage is in the operation. This is where the real savings to LCC can be achieved. Numerous factors affect pump reliability from an operations point of view. Once operational life is underway, the optimum life of the pump and system will only be realized if the pump is operated near its best efficiency point (BEP).

Maintenance — The opportunity for repair should be viewed as a maintenance upgrade event. For example, an analysis of pump curves may reveal that a change in the impeller size could move the pump closer to its BEP. With regard to mechanical seals, it is a normal practice to remove the entire seal, document the failure mode on a travel ticket, and send the seal either to the seal manufacturer for an analysis and/or execute a core return if the plant is under an alliance contract.

Disposal — Failed mechanical seals are among the most frequent reasons for removing pumps from service for repairs. That’s because leaks are obvious visual evidence of a failure. Impending seal failure may also be indicated if pressure, temperature, or level-gauge alarms on ancillary equipment are active.

Single-face seals leak along one of five paths (dual-face designs have similar static and dynamic leak paths:

Seal face leakage is visible at the shaft exit of the gland or at the drain connections.

Dynamic secondary seal leakage is also visually noticeable where the shaft exits the gland or at the drain connections.

Static secondary seal leakage is visible at the point where the shaft exits the gland or at the drain connections.

Gland gasket leakage is visible at the gland-seal chamber interface.

Hook-sleeve gasket leakage or cartridge-sleeve secondary seal leakage is visible at the point where the sleeve ends outside of the seal chamber.

Number-crunching is essential to capturing equipment ROI. Many companies have not performed a thorough cost-benefit analysis on the preventive-maintenance function. Therefore, it is difficult to analyze, with financial credibility, the cost of preventive-maintenance tasks and the contribution of the reliability program to reducing costs.

Thus, reliability and maintenance personnel should understand and be able to apply key financial concepts regarding return on investment (ROI). Common methods for analyzing payback include:

Net Present Value (NPV) — The total present value (PV) of a time-series of cash flows.

Investment Yield — The internal rate of return (IRR) for an investment is the discount rate that makes the net present value of the investment’s income stream total to zero.

Payback Period — The time it takes the cash inflow from a capital investment project to equal the cash outflow is typically expressed in years. The payback period is a simple and well-understood metric by most personnel because it simply calculates the length of time for the cash flow or savings generated by the project to pay back the project’s cost.

Cost of Capital — This is an important financial metric to understand when discussing the payback on an investment. It is not unusual for an organization to use its weighted average cost of capital (WACC) as a discount, or “hurdle,” rate in the payback evaluation of capital expenditures.

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ABOUT THE AUTHOR

Michelle Segrest

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