FEATURE

December 2011 Sealing Technology9

The oilfield industry has experienced sweeping changes during the past 25 years. Market con-ditions, coupled with advances in the quality and performance of pumps, drives, ‘‘steerables’’ and other equipment, have resulted in explora-tion and production practices that are far more aggressive and extreme.

Today’s contractors, for instance, are push-ing deeper into wells and seeking a much higher rate of penetration. Wash pipe swivels, once considered to have peaked at 120 rpm and 250 bar (3700 psi), now regularly run at 200 rpm, between 345 bar and 480 bar (5000–7000 psi). Well bore diameters have been boosted from 75 mm to 100 mm (3–4 inches), and mud volume has increased exponentially.

New frontierThis new frontier of higher temperatures, pressures and speeds is placing unprecedented demands on the design engineer whose job it is to protect expensive motors, sensitive electron-ics and other critical internal components.

It is also forcing a more careful evaluation and, in many instances, a need to seriously rethink traditional equipment sealing methodologies.

Deeper and hotter

In oilfield environments, seals commonly encounter drilling mud, sour gas, ambient pres-sures up to 1550 bar (22 500 psi) and tempera-tures in excess of 175ºC (350ºF).

For example, deep-water drilling ships are designed to drill to 9000 m (30 000 ft) – after penetrating the ocean bottom 3000 m (10 000 ft) below the ship.

In addition, modern oil exploration relies on complex in-situ instrumentation in this extreme environment. These instruments must be sealed

from conditions that could easily compromise performance or result in complete failure.

Expectations

The goal cannot be infinite service life and zero failure; such a target just is not realistic.

Instead, the focus must be on improving seal reliability and predictability. These key factors make one approach better than another. If they can achieve both, engineers ultimately will be able to offer products that give customers better value, more uptime and more productivity.

Uptime

“Uptime” is defined as the productive operation of a system. All systems have an operating life and defined intervals for the replacement of wear items.

In oil exploration, the untimely failure of a component, such as a seal, can be extremely costly.

For example, if a seal fails and causes an unsched-uled withdrawal and insertion of the drill stem, the operator can lose millions of dollars. Thus, the predictability of a given seal technology is some-times more important than its actual performance.

Environmental protection

An additional consideration, especially for seals involved in oil exploration and production, is environmental protection.

Many seals used in oil production operate unat-tended for long periods. A seal failure in these installations can go undetected, resulting in release of oil into the surrounding well-head environment.

Fines and mitigation are expensive outcomes, making predictable and reliable performance imperative.

Current technologyBefore discussing recent developments in seals, a survey of current technology is in order.

O-ring

Typically the first line of sealing defence, the O-ring seal works well in valves, logging while drilling (LWD) and measurement while drilling (MWD) tooling and rotary steerables.

Figure 1. The anatomy of a composite sealing system (LKS High PV seal).

Multi-component seals for aggressive oilfield applicationsMike Foster, Seal Application Engineer, Bal Seal Engineering Inc, Foothill Ranch, California, USA

Advanced oilfield equipment technology has opened the door to more aggres-sive exploration and production processes, characterised by much higher tem-peratures, speeds and pressures. These, in turn, have created a need for better sealing performance. By exploring the benefits and pitfalls of several popular sealing methodologies and materials this feature provides an introduction to the concept of multi-component sealing – an approach that can increase equipment productivity and reduce the potential for environmental issues caused by unex-pected failure.

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It is a relatively inexpensive, readily avail-able seal type that is easy to install. However, it offers only limited chemical compatibility and, depending on material type, can have narrow operating temperature and pressure ranges, mak-ing it susceptible to explosive decompression.

The O-ring can, in some cases, exhibit high “stiction” and high/inconsistent friction, causing it to bond to some hardware. It is typically made of an elastomer such as nitrile, fluoroelastomer, or ethylene propylene diene monomer (EPDM).

Compression packing

The sealing choice for many progressive cavity, mud, plunger and triplex pumps, wash pipe swiv-els and downhole tools, compression packing is an inexpensive, off-the-shelf product. Unfortunately, it also must be constantly maintained and tight-ened until, at some point, it begins to leak.

Typically composed of elastomer-impregnated fabric, chemical compatibility is also an issue with this seal, as the elastomer or braided material must be carefully paired with the sealed media.

Compression packing is widely considered as a fast, fairly effective fix, but it can pose prob-lems when used in a piece of equipment that is difficult to access and maintain.

Spring-energised U-cup seal (elastomeric)A seal type commonly found in rotary steera-bles, mud and piston pumps, and control valves, the spring-energised elastomeric U-Cup is moderately expensive and readily available.

It offers good chemical resistance, but has significant pressure and temperature limitations. The U-cup design is “pressure-assisted” in that the pressure of the media presses the annular

walls of the seal against the adjacent structure to ensure sealing.

Spring-energised engineered polytetrafluoroethyleneIdeal for use in valves, LWD and MWD tool-ing, and rotary steerables, the spring-energised polytetrafluoroethylene (PTFE) seal basically can be used anywhere an elastomeric ring or packing would go, but it is not as readily avail-able or easily installed.

It is chemically inert, and it offers higher temperature and pressure capabilities and low or controlled friction. This seal type is typically considered moderate in cost, as it is usually machined to order. However, some OEMs use its chemical compatibility to realise inventory savings by using a single part number for seal-ing more than one type of media (for instance, water, air and oil).

Mechanical face seal

This seal’s ability to perform at a high pressure and a high velocity makes it generally regarded as the standard for pumps, wash-pipe swivels and other high-pressure or high-velocity equip-ment applications.

It also offers good temperature resistance and chemical compatibility. However, performance can be costly; the mechanical face seal comes at a premium price. It also requires extremely tight tolerances and a means of continuous lubrication to work properly.

Common materials for the primary sealing faces are super-hard materials such as tungsten carbides and metal matrix composites.

Failure modesIn dynamic applications, where one sealing surface moves with respect to the seal, there are some common failure modes.

Thermal overload

Thermal overload occurs when the dynamics of the seal zone result in friction heating that cannot be carried away by means of some heat transfer mechanism (either convection or conduction).

This condition typically leads to structural failure. Seals made from PTFE lose their dimen-sional stability when their temperature climbs.

Structural failure

Structural failure occurs when the loads on the seal exceed the material limits of the seal. For example, in typical filled PTFE, the ultimate strength is about 27.5 MPa (4000 psi). This

Figure 2. Seal-life curves for the High PV seal.

Figure 3. Finite element analysis (FEA) pressure/force distribution for the LKS High PV seal.

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December 2011 Sealing Technology11

limit can be easily exceeded if the pressure is too high and the seal walls are too thin.

Extrusion

Perhaps the most common failure mode of an otherwise well-designed seal (one that avoids the prompt failure of thermal overload and struc-tural failure) is extrusion.

This occurs when the seal material is forced into the gap between the shaft and the hous-ing of a rotary seal. This extrusion is a form of creep, with increasing strain at constant stress.

The material that is extruded into the gap increases seal friction force and results in increased heat generation at the seal. This higher temperature accelerates the creep phenomena, dooming the seal to an unstable runaway failure. Soon, the material, which is migrating into the gap reduces the wall thick-ness of the seal to the point where structural failure of the seal will occur.

Emerging technologiesIn the context of the aforementioned strenuous requirements, the evolutionary development of designs and materials, and the failure modes which must be avoided, it would seem that equipment manufacturers face insurmountable odds in their pursuit of more uptime and better predictability. But there are new developments which hold promise for the next generation of oil exploration and product systems.

Composite design

“Composite design” refers to the engineer-ing practice of combining different, disparate materials in a single element to create improved properties or to accomplish a function not achievable with a single material.

This process of combining materials gener-ally is more costly than using a single material, but in demanding applications, the added expense is justified.

One example of just such an innovative amalgam can be found in a recently introduced multi-component seal (Figure 1, on page 9) that consists of a spring-energised, graphite-reinforced PTFE sealing ring coupled with a high-temper-ature engineered thermoplastic anti-extrusion element and a metal locking ring.

In this seal – Bal Seal Engineering Inc’s LKS High PV seal – each of the components has its own unique role to play. The metal locking ring prevents the outer diameter of the seal from dynamic movement, minimises friction and heat, and prolongs seal service life.

At moderate speeds, the graphite-reinforced sealing ring retains the ability to seal under

high-temperature, high-pressure service condi-tions while providing a high level of extrusion resistance because of its long-wearing properties and built-in lubricity.

The thermoplastic anti-extrusion element minimises seal jacket extrusion, provides a longer sealing life, forces wear to occur at the lip-contact area (and not at the hinge point), and extends seal performance range at low and high temperatures (-70ºC to 290ºC or -94ºF to 550ºF).

The Hastelloy Canted-coil spring exerts uniform sealing force against the shaft, reduc-ing heat build-up and extending seal life, while providing excellent corrosion resistance and chemical compatibility.

This seal and other similar multi-compo-nent seals represent a novel take on problem-solving, because they deal in capabilities rather than limitations. Their design considers, for example, how the high-temperature behaviour of polyether ether ketone (PEEK) thermoplas-tic – which becomes more flexible at 80ºC (180ºF) will complement the lubricity of PTFE, and how the force exerted by a spring can help the whole package provide longer service life. In other words, it is sealing from a holistic perspective.

PerformanceWhat are the performance benefits of using these new materials and design approaches?

Primarily, seal life (Figure 2) is the biggest benefit. Testing data have quantified an approxi-mate fourfold increase in seal life compared with conventional seals. In addition, more predictable seal friction and heat generation provide system-level benefits.

By carefully selecting and applying materials, and refining the design to take into account predicted deflections, improved seal perform-ance can be achieved.

Figure 3 shows the contribution of the stiffer PEEK material – preventing heel collapse onto the rotating shaft.

The properties of the PEEK material hold up well to about 150ºC (300ºF), but above 170ºC (340ºF) there is a sharp drop of the modulus. Therefore, thermal design is critical (Figure 4).

The seal is a heat source, and adds its own heat to the ambient environment. Here, the addition of the PEEK support ring keeps the seal lip cooler, allowing it to avoid direct and constant contact with the shaft.

Summary and conclusionThe ever more challenging demands of today’s oil exploration and production operations are pushing the limits of current sealing systems and products, requiring equipment designers to look at the use of unconventional technologies.

Recent breakthroughs in material science, when employed with the latest design and manufacturing tools, are helping equipment manufacturers achieve significant improve-ments. The service life of seals in some rotary applications can be doubled, or even quad-rupled.

In addition, by applying this design philosophy and using new materials, more capable seals will be made available to meet the challenges.

Contacts:

Bal Seal Engineering Europe BV,

1019 GW Amsterdam, The Netherlands.

Tel: +31 20 638 6523,

www.balseal.com

Figure 4. Thermal analysis of the LKS High PV seal performance.