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Reduce Energy usage through efficient mechanical seal flush plans

The following article looks at how it’s possible to reduce energy usage by up to 96% through selecting more energy efficient mechanical seal flush plans.

Mechanical Seals, Systems & Energy Efficiency

In Europe and the United Kingdom it is estimated that around 10% of electrical power is used for pumping equipment (https://www.youtube.com/watch?v=FvPRTowCK-E). Mechanical seals and the appropriate seal support systems can have a significant impact on the efficiency of the pumping equipment in a plant.

Energy Efficiency of Mechanical seals

If you look at the mechanical seal in isolation the energy savings are relatively small, a 3 - 4 inch mechanical seal will consume over 3HP (2.24Kw) as a maximum power absorption. In the overall system this is relatively low, however if the focus is on the thermal inefficiency of mechanical seal API piping plans the energy that can be lost with inefficient systems can be significant. 

A survey was carried out by the Fluid Sealing Association (FSA) (www.fluidsealing.com) on 28,000 mechanical seals where the process temperature exceeded 200°C (400°F). Figure 2 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.

Why use API Piping Plans?

To function correctly a mechanical seal needs to be flushed with a cool, clean liquid to remove heat from the seal faces. This is accomplished through the use of sealing systems and the various configurations of seal flush plans (API Plans) that can achieve this.

If a pump is pumping a hot process and the cool flush liquid is then added into the process this cools down the process temperature, and there is then a requirement for downstream separation/evaporation, re-heating and/or effluent treatment. However there are flush plans available that are more energy efficient closed loop systems that only remove heat from the seal faces and don’t inject the cool flush liquid directly into the process.   

The following API piping plans will be used along with a hypothetical case study to highlight just how much energy can be saved through selecting more energy efficient seal flush plans.

API Plan 21

 

API plan 21 takes a simple side stream from the discharge, through an orifice plate which controls the flow, then through a heat exchanger before this cooler liquid is injected over the seal faces and then is fed back into the process. An issue with API Plan 21 is that the seal flush liquid enters the process and with the process fluid being of a higher temperature this additional cool flush liquid dilutes the process.

API Plan 23

 

API Plan 23 is probably the most efficient way to remove heat from a mechanical seal. Plan 23 is significantly more efficient than Plan 21 because only heat from the seal faces and any heat that comes through the pump casing are removed, and no heat is being removed from the process.

API Plan 32

 

Plan 32 involves injecting a clean, cold liquid from an external source, through various controls, through the seal faces and into the process. The flush liquid is generally significantly cooler than the process fluid & effectively this liquid is diluting the process having a significant effect on the thermal efficiency of the process.

A typical case study

It is possible to apply the above API Plans to a hypothetical hot oil pumping application, which illustrates the relative energy efficiency of each. This configuration would be a 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 (37kW) (typical)

Sealing devices Mechanical seals

Assumptions Heat lost at the pump must be replaced at the system boiler / heat exchanger


API Plan

Description

Observation

Sealing system power consumption

API Plan 21

Cooled by-pass flush

A hot process continually cooled by-pass flush means poor energy efficiency

37.8kW/hr

API Plan 23

Cooled closed-loop flush

More efficient closed-loop circuit that dissipates heat generated at seal faces only

1.7kW/hr

API Plan 32

Cool external flush

Constant cold clean external flush directly into process fluid is very energy inefficient. Heat required to compensate for injection of cool flush.

47.4kW/hr


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 can be significant.

Conclusion

The selection of inappropriate sealing systems can have a very significant impact on the thermal efficiency of a plant and plant utilities. It’s clear from the above hypothetical case study that the energy savings that can be achieved by switching to closed loop API piping plans can be significant by removing the need for downstream separation/evaporation, re-heating and/or effluent treatment.  

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