spring energized seal

When considering polymer jacketed seals — especially PTFE-based products — some form of energizer is typically required. These types of seals are usually specified to operate both in very high pressures, low pressures, or even in a vacuum.  

At certain pressures (typically above 100psi), the system pressure will energize the seal and prevent leakage. But at low pressures, additional energy is required to force the jacket material to mate with the hardware.  

The solution to this is to add a spring to the seal. The spring provides the needed sealing-energy to prevent leakage at low media pressures.

When considering a high pressure-application, there are start/stop conditions where the system is at low pressure. If the seal allows some amount of leakage at low pressure, it becomes possible for that leakage level to increase once as the pressure builds.  

This phenomenon is called “blow-by.” Once it occurs in a system, it’s difficult to get the seal to seat and seal correctly.

Canted Coil Spring Energizer

There are several types of energizers to consider when specifying a seal. These can be as simple as an O-Ring or some other elastomer. 

In a typical oil refinery or chemical processing plant, 60% of fugitive emissions are attributable to leaking valves. Of these, nearly 80% are released at valve stems. Also contributing to valve failures, are leaks at the bonnet, flanges and seats. As operating conditions in these and other industries continue to subject valves to ever more extreme temperatures and pressures, sealing them effectively poses a challenge that cannot be met with traditional materials and methods. Soft, compressible elastomers provide good sealing performance, but they are porous and cannot withstand temperatures in excess of 482°F (250°C). They also become hard and brittle in cryogenic service. Metal seals have much greater temperature capabilities, high mechanical properties, lack of porosity and long shelf life. Ductility and elasticity are typically the limiting factors.

Today more resilient, metal-to-metal seals are available and being used in valves operating under extreme conditions. Spring- and pressure-energized metal seals function much like a gasket between two flanges with little or no relative motion between them.

The sealing principle of spring-energized seals is based on plastic deformation of a jacket that is more ductile than the mating surface. This deformation occurs between the sealing face and an elastic core composed of a closely wound helical spring. The spring provides specific resistance to compression, during which the resulting pressure forces the jacket to yield and fill flange imperfections. Each coil of the helical spring acts independently, allowing the seal to conform to any surface irregularities. This combination of elasticity and plasticity provides an extremely effective seal even under the decompression cycles.

“How much pressure can this seal handle?”

The answer to this question depends on a number of parameters and conditions. But the principle limiting factor in the pressure handling of any seal system is the extrusion gap.

Commonly referred to as the “E-Gap,” the extrusion gap is one of the most critical design aspects in any high-pressure application. Seal design, type, and material are all influenced by the extrusion gap and the desired pressure handling capability.

What exactly is an extrusion gap, and why is it so important in the successful design of a sealing system? Let’s find out.

The Basics: What is an Extrusion Gap?

In terms of sealing systems, the extrusion gap is defined as the clearance between the hardware components.

In a piston configuration, this would be the clearance between the piston and bore. In a rod configuration, this is the clearance between the rod and housing it’s passing through.

The extrusion gap can be expressed in terms of radial or diametral clearance, which can lead to some confusion. Our partners at Eclipse define the E-Gap by stating it as the radial clearance. The radial clearance is equal to the diametral clearance divided by two.

It’s important to note that while hardware components might be machined to have a specified clearance, this gap might not be perfectly realized or maintained.

Spring-energized metal seals provide numerous advantages in oil and gas applications, including but not limited to MWD and LWD tools, couplings, subsea compressors, enclosures/vessels, christmas trees, electronic submersible pumps and flowmeters. Extreme operating pressures and temperatures, together with more difficult resource recovery, zero tolerance for failure and environmental concerns, are placing unprecedented demands on this equipment.

Traditionally this industry has used solid machined seals that provide high compression loads but lack resilience. They also tend to have relatively high rates of leakage over time as flanges deteriorate. Recent advances in metal seal technology provide controlled compression, high resilience and reduced leakage.

Background

Eclipse Engineering has in-house capabilities to manufacture seals up to 55 inches in diameter, and over 100 inches through production partners.

While seals with huge diameters certainly grant their own significant levels of intricacy, here we’ll look at the other end of the spectrum: the micro-sized seals.

We won’t just look at a simple seal ring, but an inherently more complicated and geometrically detailed spring energized seal. As we’ll see, very small diameters make multiple manufacturing aspects more involved and challenging.

The Client’s Issue

A sealing solution in a customer's epoxy dispensing equipment. They needed an effective seal for the reciprocating rod responsible for the flow-control and metering of the epoxy while being dispensed.

Operating Conditions:

• Reciprocating Rod Seal
• Epoxy Dispensing Head
• Rod Diameter: 1.2mm [0.047”]
• Stroke Length: 6mm [0.236”]
• Cycle Rate: 15 per min
• Media: Epoxy
• Operating Pressure: 1,500 PSI
• Temperature: 70° to 150°F

In general terms, most viscous media sealing solutions have three things in common:

1. A variant of UHMW for the seal jacket,
2. heavy spring loading, and
3. multiple point contacts with increased interference.

In most cases, multiple nested V-Springs are incorporated to provide optimal load and energize the compound contact points on the seal. With this formula, we’ve had great success sealing media like epoxy, urethane, silicones and acrylics.

The heavy loading is necessary to effectively wipe the reciprocating rod. This is balanced with the correct material and design geometry to provide long wear life of the seal, which has the potential to be compromised under such loading.

The challenge in this case was to incorporate these same proven principles in a micro-sized seal.

The Eclipse Solution

Innovative Subsea Processing Technologies

The Aasgard oil and gas field in the North Sea has been operating since 1999. Innovative subsea processing technologies were used to compress the oil and gas on the site’s seabed to improve recovery rates as the field ages and as equipment begins to draw from increasingly deeper subsea reservoirs. The operator required seals for the lid and body of the control power distribution unit in order to protect its vital electronics in the harsh, unforgiving subsea environment.

As a globally trusted source for engineered components, seals, assemblies, and sub-systems for demanding environments, Technetics was uniquely positioned to evaluate and specify a sealing solution for this application. The system designer and Technetics engineers subsequently underwent a two-year testing phase to examine the performance of a variety of sealing options.

Demanding Subsea Environment

Due to the extremely demanding environment in