Richard Smith of AESSEAL writes:
Engineers that specify an appropriate mechanical sealing system on their pumps can significantly improve the energy efficiency of a manufacturing plant as it is estimated that around 10 % of electrical power is used for pumping equipment [1]. Unsurprisingly, regulatory bodies and engineers are focussed on improving the energy efficiency of pumps and pumping systems. Choosing the right mechanical seal is one of the most effective ways of doing this.
The purpose of a mechanical seal is to ‘seal’ the process fluid - whether it is toxic or expensive, the objective is to keep it within the system and pipe work to avoid it seeping out and resulting in a cost for lost process fluid and clean up. And, of course, if the liquid or process fluid that is leaking is heated, it is losing costly energy from the system. So, be it with flanges, valve stems or pump seals, it is necessary to think of the seal not only as preventing process fluid contamination and leakage to external atmosphere but as an important part of conserving energy within the system.
Mechanical seals on pumps are probably the most delicate components and we use seal flush plans to change the environment that the seals operate in and that makes them flourish and provide reliable operation. Flush plans are formalised by the American Petroleum Institute in their standard API-682, the custodian of the flush plans, where they are detailed in standardised formats.
Energy efficiency of the most popular flush plans.
A survey [2] was carried out by the Fluid Sealing Association (FSA) for pumps incorporating a total of 28,000 seals where the process temperature exceeded 200°C (400°F). The chart below shows the most popular flush plans used and, by comparison, the energy absorption of each. It is interesting to note that flush plan 32, the second most commonly specified in the survey, is by far the highest energy user. Looking at studies on different flush plans undertaken by the FSA, we compare some single seal flush plans (21, 23, 32 and 62) with a hypothetical case study. The configuration is a hot oil pumping application with single stage, end suction centrifugal pump (API 610 compliant), based on:
Pumped fluid: hydrocarbon at 315°C (600F) Specific gravity: 0.8 Specific heat: 1.67 kJ-°C (0.4 BTU/lb-°C) System pressure: 345 kPag (50psig) in seal chamber Pump driver: 50 HP (typical) Sealing devices: Mechanical seals Assumptions: Heat lost at the pump must be replaced at the system boiler / heat exchanger
* If well maintained (if not 1.8kW)
To put these figures into context, if a pump is running 24 hours a day, 365 days a year the difference between flushing a seal using API Plan 32 and API Plan 23 can see a business save around 400,000 kW of energy a year. When this saving, which equates to a 96% reduction in energy costs, is spread across multiple pumps and multiple locations the potential savings are huge.
Conclusion
Selection of inappropriate sealing systems can have a very significant impact on the energy efficiency of a plant. Sealing systems found in many industrial applications, even when functioning as intended, are extremely wasteful of energy. However, the sealing industry has technologies that can overcome some of this energy wastage and there is increased awareness about these solutions.
Being able to choose between different sealing systems or flush plans requires a good understanding of the principals of their operation and why they’re used. The best way to access this knowledge is to work with a sealing partner that has the understanding of this specialist area.
