Question: How can I improve the reliability of my pumping equipment fitted with dual seals?
Expert: Richard Smith, AESSEAL plc
Dual seals are increasing in popularity for pumping applications in all aspects of the oil and gas industry. Plant hazard safety requirement, reduced fugitive emissions and reliability are the main drivers.
Mechanical seal designers face opposing challenges when developing dual seals. Historically the requirement to provide optimised seal face cooling has conflicted with the need to provide a seal that is tolerant of upset conditions and contaminated process fluids. The American Petroleum Institute (API) mechanical seal standard API682 offers users a choice of three different configurations. Each has its own advantages and disadvantages. Elegant designs are now available that can satisfy contradictory requirements.
API682 describes pressurised dual seals (see figure 1) as arrangement 3 and having two seals per cartridge assembly that utilise an externally supplied pressurised barrier fluid. The three different configurations are illustrated with the principal difference of the face to back configuration being that the process fluid is on the outer side of the seal faces. The two other traditional configurations both have the process liquid on the inside diameter. These ‘back to back’ or ‘face to face’ designs are less suited to contaminated services. Entrained abrasive solids will be centrifuged into the seal faces with the potential for damage. The ‘dead zone’ (figure 2) formed by the small volume of process fluid underneath the inner seal creates susceptibility for fluids to congeal, wax or solids to accumulate causing seal ‘hang up’.
The face to back configuration seals overcome these limitations as eloquently described in the API standard. ‘The advantages of this series configuration is that abrasive contamination is centrifuged and has less effect on the inner seal. Liquid barrier seal designs arranged such that the process fluid is on the OD of the seal faces will help to minimise solids accumulation on the faces and minimise hang-up’.
Upset conditions such as loss in barrier pressure or increases in process pressure can compromise traditional back to back configurations. Resultant thrust forces in a pressure reversal may cause a non-retained mating ring to be dislodged from its mount or hydraulic force to open the inner seal faces.
Face to back seals have pressure reversal capability providing for greater safety and tolerance of upset conditions. As the API standard explains ‘In the event of a loss of barrier fluid pressure, the seal will behave like a tandem or arrangement 2’
However the cooling of inner seal in a face to back configuration has always been a challenge for the seal designer. In 2002 the API standard commented ‘Restricted seal chamber dimensions and the resulting cartridge hardware construction can affect the ability of the barrier fluid flush to adequately cool the inner seal.
Elegant modern designs solve the inner seal cooling issues. The inclusion of flow baffles deflect the barrier fluid flow. Separation between inlet and outlet of barrier flow, results in deposition of cool barrier fluid at the inner seal faces, the most important part of the mechanical seal. This technology is now common practice and is made possible by the utilisation of modern CNC machine tools. This has allowed integrating of seal components into the sleeve and gland plate, providing a compact design and opening up the inner cavity for the flow deflector baffle.
Circulation devices
Circulation devices performance for API plan 52 & 53 systems: Dual wet seals barrier fluid circulation is normally achieved by internal circulating devices incorporated in the seal cartridge. Effective cooling of the seal depends upon the efficiency of these devices. Optimisation of cooling improves reliability and reduces cost of ownership
Traditional internal circulating devices fall into two groups,(figure 4a&b) parallel slot (castellation) & helical vane. Parallel slot devices induce a radial flow and need to be used with tangential seal ports both devices are uni-directional. Reduction in internal radial clearance can improve performance of traditional designs.
Manufacturers frequently ignore API682 requirement for 1.5mm clearance between rotating and stationary components. However, this practice can compromise safety as clearance is required to prevent contact under fault conditions.
The use of multi axis CNC machine tools has provided their designers with far more freedom in sculpturing efficient arrangements. Figure 4c illustrates a modern bi-directional large clearance ‘taper vane’ that provides improved circulation. Bi-directional devices can be used on either end of a between bearings pump, reducing risk of errors in seal rebuilds and reducing spares inventory costs
The original taper vane bi-directional device has now been in commercial service since 1999. Recent research into vane profile and angles has provided a further increase of up to 40% in flow. The vanes are contoured into a ‘swan neck’ shape (figure 5) which prevents reverse flow on the back edge of the vane. The performance of this device is virtually equal to that of a similarly configured uni-directional taper vane.
The swan-neck design retains 1.5mm (figure 6) radial between the rotor and stator, thus fully compliant to the API-682 standard. The results of a (typical medium size pump) 50mm seal ‘swan-neck’ tapered design are shown in figure 6 (right).
Case Study: gas condensate
In 2006 AESSEAL were invited to improve the reliability of traditional back to back seals on a gas condensate pumping service. The pumps were installed on a FPSO in the North Sea. The duty on paper was relatively simple; temperature 20 to 100°C, Pressure 3.79 Bar (g) shaft speed 1775 rpm. However, the reliability of the seals was poor due to wax formation under the inner seal .The resultant face ‘hang up’ caused catastrophic seal failures and contamination of the barrier fluid system.
The seals were replaced with AESSEAL CAPI seals which are of a face to back format incorporating flow deflector baffle and bi-directional taper vane circulating device. With the process fluid now on the outside of the seal faces, are operating in the larger volume of the seal chamber, the waxing problem has been eliminated. The operator now enjoys reliable operation avoiding expensive down time.
Mechanical seal technology continues to develop. Robust face to back designs are now available with enhanced cooling features, which will increase the application base, improve equipment reliability safety and reduce operating costs.
