Question: Which seal management systems both reduce initial capital expenditure cost and increase equipment reliability?
Expert: Stephen Shaw, AESSEAL Group Technical Director
Traditionally off/onshore production platforms installed on new fields processed clean, relatively solid-free, hydrocarbon liquids.
Therefore many of the older platforms were furnished with traditional centrifugal pumps, which run typically at minimum speeds of 3000rpm. This kept the pumps small and compact which was important given the space and weight constraints offshore.
The mechanical seals furnished with equipment such as the Main Oils Line Pump (MOL), Sea Water Injection (SWI), Closed Drains, Test Separators and Flare Drums were nearly always single seals with single helical coil springs or multiple spring designs.
Single mechanical seals were often favoured by operators as the supporting system, pipework, instrumentation and operation of them is far less complicated than that of dual seals.
However, single seals rely on the process fluid to lubricate the seal faces. Increasingly sand contaminants are being extracted with the oil. Sand is unfortunately not a good lubricant and seal and equipment longevity is reduced.
SEPARATOR ISSUES
In an attempt to improve the sealing conditions at the single seal faces of such applications, cyclone separators have traditionally been recommended.
Cyclone separators, used in Plan 31 arrangements, shown in Figure 1, are designed to separate solids from the process fluid. The separated clean fluid is then used as the flush medium over the mechanical seal faces.
However, cyclone separators have drawbacks, specifically their inability to operate successfully if the particles being filtered are lighter than the mother liquor or where a constant pressure drop is not maintained across the unit.
This means there will always be a degree of carry-over of abrasive particles, which are injected into the sealing chamber causing further damage and impairing equipment longevity.
A major concern for plant engineers using cyclone separators in Plan 31 arrangements is internal wear on the separator body given that the failure mode of the unit is catastrophic once the wall thickness of the separator is no longer sufficient to withstand the internal operating pressure of the system.
Since cyclone separators are treated as part of the flush recirculation pipework, there has been no way for an operator to detect how much wear has taken place. This compromises platform safety and reliability-focused owner-users try to avoid them.
What options do I have to replace single seals?
Dual seals work differently to single seals as the inboard seal faces mostly operate in a pressurized clean barrier fluid environment (Plan 53) and are therefore lubricated and cooled by the barrier fluid rather than the abrasive process fluid.
This dramatically improves the reliability of the equipment as the seal face conditions are now much more favourable. In addition to the clear reliability advantages, many modern thinking exploration operations install dual mechanical seals in their operations because of the emission reduction advantages. However, while the cost difference between a dual and single seal may not be that great, the cost of the supporting dual seal system can be. Dual seals essentially need an independent pressure system connected to a pressure source.
This not only increases initial capital equipment costs, it dictates the need for increased physical space and space is a premium in confined applications as found in offshore platforms.Plant operators are therefore faced with a commercial and physical dilemma when looking to upgrade single seals to dual seals and systems on offshore platforms.
I want a Dual seal but what is the best solution?
There are many API Plans that have been designed for various applications. Clearly the Plan selected needs to consider a whole host of issues, from reliability to maintenance, but most importantly the safety of the operators and plant. Unfortunately there is no one magic system or plan solution for all applications. However, whenever a pressurized barrier fluid system is selected, a modern system favoured in many off/onshore platforms is a hybrid Plan 53/54 system. This system, marketed by AESSEAL as the Compact 107 and shown in Figure 2, offers all the benefits of Plan 53 and Plan 54 systems, yet saves significant space and capital equipment costs. It also increases the reliability of the operations, as highlighted above.
Case Study
In 2004, four AESSEAL CAPI Type A dual cartridge seals were installed in CPC vertical inline API 610 process pumps (Figure 3), sealing natural gas liquid on an offshore platform in Alaska, USA.
Before the AESSEAL installation, the equipment had a Mean Time Between Failure of 6 weeks.
The four CAPI dual seals were supplied by a single innovative stand mounted support system – the AESSEAL Compact 107.
The Compact 107 is a hybrid Plan 53/54 system with a small footprint, (0.7m x 1.0m), therefore ideal for restricted offshore space, with an installed cost of $40,000.
By 2007 the seals had successfully operated for over three years without failure. The customer was clearly delighted with the seal and system performance and subsequently ordered an additional Compact 107 system for a similar application.
To date, the systems are still performing excellently and the savings for this one application are over $350,000 based on repair costs, prior to the AESSEAL installation. This represents a payback period of less than 15 months based on cost savings alone and an Internal Rate of Return (IRR) of 38.7%.
This benefit excludes the significant gains from production improvements while using the AESSEAL solution.
If this solution was offered with a traditional Plan 53B system and dual seal arrangement, the initial capital cost would be more than 30% greater than with the Compact 107 Solution.
