The difference between Good and Poor equipment performance is all about attention to detail

API (American Petroleum Institute) plans are standardised guidelines that specify best practice arrangements for process systems to achieve the best operational uptime performance. Understanding the benefits and drawbacks of each API plan is essential to maximising rotating equipment and mechanical seal longevity. This paper looks at API Plan 23.

API Plan 23 is often used with rotating equipment such as centrifugal pumps which process fluid at elevated temperatures. Typical applications include boiler feed and circulation pumps, and hot oil pumps.

API Plan 23 is designed to provide fluid to the mechanical seal to cool and lubricate the seal faces, which in turn helps to improve mechanical seal and pump longevity.

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WHAT REALLY IS API PLAN 23?

Figure 1– Plan 23

Figure 1 shows a typical API Plan 23 configuration.

Mechanical seal longevity is achieved with Plan 23 because of its incredible cooling performance.

Plan 23 is essentially a ‘closed cooling circuit’ system that creates an idealised environment at the mechanical seal faces by separating the temperatures and conditions of the process pump to the conditions at the seal faces.

There are clearly adaptions, enhancements and add-ons such as temperature gauges and finned tubing to such a system, however the basic Plan 23 system essentially employs a fluid circulation device, which is often part of the mechanical seal, and an external heat exchanger.

In the Plan 23 system, ‘isolated’ process fluid is circulated in a substantially closed loop circuit, from the seal to the heat exchanger and back to the seal.

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LET’S NOW LOOK AT SOME BASIC ENGINEERING PRINCIPLES.

Question: “If you want to effectively cool fluid with a heat exchanger in a closed-circuit system, is it best to try and cool a large volume of fluid or cool a small volume of fluid?”

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Answer: With all other things being equal, the smaller the volume of fluid passing through a heat exchanger in a closed-circuit system, the greater effectiveness of the heat exchanger at being able to dissipate the heat from the fluid.

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This is because the small reservoir of fluid will pass through the heat exchanger many more times compared to a large reservoir of fluid, in any given time period. This is shown in Figure 2.

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Figure 2– Good Plan 23 v Bad Plan 23

Remember: Cool Seal Faces = Happy Seal Faces.

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So, when you look to install a Plan 23 system, the first thing to consider is the volume of fluid that is present in the closed-circuit Plan 23 system.

Now let’s look at some examples of actual seal/pump Plan 23 designs.

API682 is considered as the premium mechanical seal specification used across many industry sectors but specifically used in oil and gas and petrochemical processing.

API682 defines large seal chamber areas in the process pump, to essentially maximise the volume of fluid around the seal faces in order to facilitate seal face cooling and lubrication.

However, this large volume of fluid works against you in a Plan 23 application especially if the equipment designer places the restriction bush at the bottom of the seal chamber, as shown in Figure 3.

Figure 3- Large Volume of Fluid in the Plan 23 Closed circuit.

As shown in Figure 3, the entire ‘large swimming pool’ volume of process fluid in the seal chamber is part of the closed-circuit system! Not only is this a large volume of fluid, the fluid is also being constantly super-heated by thermal convection through the pump casing from the hot part of the pump. This adds unnecessary heat into the Plan 23 closed-circuit. As far as the seal faces are concerned, this arrangement is a DOUBLE NEGATIVE!

As a result of this poor equipment design, the seal faces will either overheat or the system supplier  will have to specify an over-sized heat exchanger to handle the huge ‘lava-pool’ of fluid. This adds unnecessary cost to the system and performance risk to the mechanical seal faces.

Figure 4 shows a much better Plan 23 design.

From Figure 4 you will see that the cartridge mechanical seal incorporates its own integral restriction bush to isolate the seal chamber process fluid from the process fluid at the seal faces.

Firstly, the reader will see that the volume of fluid in the mechanical seal ‘closed-circuit’ is significantly less with the solution offered in Figure 4 compared to Figure 3.

Secondarily, the reader will see from Figure 4 that the process fluid circulated around the mechanical seal closed circuit is isolated from the thermal convection heat acting on the seal chamber fluid from the process pump.

Not only is this arrangement beneficial for the mechanical seal faces, heat exchanger performance and size/cost, it means that heat is not being constantly sucked and cooled from the hot process fluid, as shown with arrangement in Figure 3.

By simply attending to the detail, the mechanical seal design of Figure 4 saves significant energy and cost of the holistic pumping system which can be a huge saving over the years in which the pump operates.

Figure 4- Small Volume of Fluid in the Plan 23 Closed circuit.

FURTHER ENVIRONMENTAL AND COST BENEFITS OF ELEGANT PLAN 23 DESIGN?

Now that the required size/capacity of the heat exchanger is minimised with the Plan 23 seal design of Figure 4, alternative heat exchanging equipment can be used.

The experienced reader will relate to the fact that most conventional Plan 23 heat exchangers are ‘shell and tube’ design. These heat exchangers are cooled by running water through the heat exchanger casing as shown in Figure 4.  Sometimes this water is fed from a closed-circuit ring main system and sometimes the used water from heat exchanger is simply sent to drain.

In some areas of the world, the mains supply water is ‘hard’. This means it has high levels of calcium and magnesium which can deposit on internal surfaces of pipework and ultimately restricts water flow and heat transfer efficiency of the system. Therefore before using hard-water, some plants treat the water to remove the calcium and magnesium. This adds to the plant costs and makes the system physically ugly in all aspects.

So, whatever the water supply approach, the reader will note that there is an element of cost involved supplying the water to a conventional Plan 23 heat exchanger.

Now lets get back to our elegant mechanical seal design of Figure 4.

Now the size/capacity of the heat exchanger is minimised, alternative, perhaps less efficient but more cost effective, heat removal solutions can be explored.

Take a look at the solution shown in Figure 5.

Figure 5- Environmentally friendly Plan 23 Closed circuit.

From Figure 5, the reader will see that there is no longer a mains water supply to the heat exchanger. The system shown is an air-cooled design with a combination of a heat exchanging seal support system and finned tubing.

As this system as no external water supply, it saves on

- water supply costs,

- initial supply equipment CapEx costs,

- ongoing maintenance costs and

- system complexity.

As the mechanical seal has been designed elegantly, it has a huge effect on the requirements of the supporting system….. winner winner chicken dinner!

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SUMMARY:

API Plan 23 is a brilliant system for hot process applications but it’s performance can be significantly impaired by poor attention to detail and poor equipment design.

Common sense shows that it is a far better approach to minimise the volume of fluid that is circulated in a closed-circuit system. This ensures the heat exchanger size and cost is minimized.

Common sense shows that it is a far better approach to isolate the fluid at the mechanical seal faces from the fluid in the seal chamber which is being super-heated by convectional heat transfer from process pump.

By attending to these two common sense details, many direct and indirect benefits result.

Not only are you creating an idealised cooled environment for mechanical seal longevity, you are also saving huge amounts of money on capital expenditure costs of purchasing over-sized heat exchanging equipment.

Furthermore, you are saving on water supply costs either by totally eliminating water from the system, or reducing the flow of water to the heat exchanger.

Lastly, and probably the most significant saving, elegant seal design creates thermal/heat cost savings by making the process pump more efficient when operating at high temperatures.

So, attention to detail when selecting, designing and running Plan 23 equipment is sooooooooooooo important.

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FURTHER INFORMATION

Contact the technical sales team at Reliability Seals for further information or to enlist on our 'Online Reliability Uptime training program'.

"Learning is a constant process of discovery without end" - Bruce Lee (Martial artist and actor)