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Abrasive media comes in many forms—from mining slurries to wood pulp and even substances as seemingly mild as liquid chocolate. This diversity rules out a one-size-fits-all solution for abrasive pumping applications. However, today’s broad range of materials, from carbon fiber packing to graphite-filled polytetrafluoroethylene (PTFE) bushings, includes products capable of meeting an equally broad range of abrasive wear requirements.

Carbon Fiber Packing

Few materials offer the abrasive resistance and heat dissipation of carbon fiber yarns. Braided compression packing made from this material excels under extreme conditions, including exposure to a variety of chemicals, temperatures approaching 850 F (454 C) in oxygen-rich atmospheres (up to 1,200 F/649 C in steam) and shaft speeds in excess of 4,000 feet per minute (fpm).

The strength of carbon fiber yarns combined with their ability to draw heat from pump shafts make them the material of choice for resisting abrasive wear. Unfortunately, they do not seal as well as graphite foil packing. Upon closer examination, however, the speed and temperature capabilities of these two materials are similar.

Consider this hypothetical scenario: An abrasive pumping application is under control using carbon fiber, except for excessive leakage across the packing set. Depending on the nature of the leaking media, it may not be desirable to have it pooling on the plant floor. Even if it is just water, excessive leaking media is lost profit. If the leakage is clean, the carbon fiber packing is effectively excluding abrasives.

Carbon Fiber Rings and Foil Packing

When dealing with clean media, graphite foil packing becomes an option. At a subsequent repacking, it may be advisable to use just two carbon fiber rings in the bottom of the stuffing box and use graphite foil packing for the remaining rings.

The complete assembly includes a fluorocarbon resin PTFE body, electroless nickle-plated ductile iron flanges, polyethylene-covered restriction zinc plated bolts, and stainless steel corrosion-resistant reinforcing rings.

Case Study

Galvanic corrosion is an electrochemical process that occurs between two dissimilar metals, or between a metal and a conductive non-metallic material, when both are exposed to an electrically conductive media. In the case of a packing gland, it occurs between a metal component and the carbon or graphite packing. Under these conditions, the material that is closest to the anodic end of the galvanic scale will be corroded in preference to the one that is closest to the cathodic end of the scale. (See Table 1.) As the distance between materials on the galvanic scale increases, a corresponding rise occurs in the rate and the extent of the corrosion.

In a valve or a pump using packing made of either graphite or carbon, a galvanic reaction may be initiated as soon as any electrically conductive fluid, such as water, is introduced. Since graphite is more cathodic than the metals that make up valves and pumps, it is the metal that may be subject to corrosive attack.

Liquid Phase Needed

Even though a valve or pump may be packed with a graphite or carbon packing, many cases exist in which the metal parts will not be subjected to galvanic corrosion. For example, an electrically conductive fluid in a liquid state must be present for the galvanic reaction to take place. The temperature of a superheated steam valve prevents the accumulation of any significant amount of water, thereby nullifying the possibility of galvanic corrosion.

Stainless Steels

Another example of when galvanic corrosion protection may not be necessary is when the equipment is constructed of austenitic stainless steels (e.g., 300 series, 630, etc.). These stainless steels are much more resistant to galvanic attack.

On the other hand, the martensitic stainless steels (e.g., 400 series) are highly susceptible to galvanic attack. If a valve or pump is constructed of martensitic stainless steel and if it will be exposed to an electrically conductive fluid for any period of time, then consideration should be given to incorporating a galvanic corrosion inhibitor system into the carbon or graphite packing sets used to seal it.

When Are Corrosion Inhibitors Needed?

Under pressure? Absolutely. The increasingly high temperatures and harsh conditions to which gaskets are exposed makes selecting the right gasket all the more important.

In industries such as chemical processing, hydrocarbon refining, and power generation, leakage from extreme temperature process  streams can result in loss of efficiency and production as well as adverse environmental impacts and compromised employee safety. One of the most commonly used sealing products in systems subject to high pressures and temperatures is a spiral-wound gasket. These gaskets typically consist of filler and winding materials selected on the basis of application requirements and end-user preference. Proper selection of these materials is critical to achieving the desired performance in all applications.

Material Selection

Sealing at temperatures above 850 ºF (454 ºC) is particularly challenging because of the limited number of filler materials that can resist thermal degradation at extreme temperatures – these temperatures affect both the sealing material and metal components. For instance, the yield strength of fasteners decreases as the temperature is increased. In addition certain chemicals can become more volatile and aggressive in high-temperature reaction processes.

The two most common filler materials in spiral-wound gaskets are graphite (can withstand temperatures up to 850 ºF) and polytetrafluoroethylene (PTFE; tolerance up to 500 ºF). Other filler materials are used mainly for their thermal insulating properties, not for sealability; these include mica, exfoliated mica, and ceramics. While graphite and PTFE perform satisfactorily in terms of temperature and chemical resistance, they have limitations. Graphite is not compatible with heavily oxidizing media at any temperature, nor can it withstand continuous operating temperatures above 850 ºF. Beyond 850 ºF, volume loss through oxidation becomes excessive and sealing effectiveness is compromised.

Many high-temperature systems, such as exhaust manifolds and flanged piping connections in exhaust systems, are oxidizing. Other services are oxidizing because of the operating temperature and media involved.

The Garlock Family of Companies has launched a new fully-coated isolation gasket known as EVOLUTION.

EVOLUTION® Isolation Gaskets

The next generation of isolation gaskets, EVOLUTION®, features easier installation, tight sealing, high-temperature operation, no permeation, hydrotesting isolation, fire-safety and chemical-resistance.

Featuring a thinner, 1/8-inch design, EVOLUTION minimizes the difficulties encountered when attempting to install thicker isolating gaskets. The full-coating encapsulation allows the gasket to be hydrotested and left in the pipeline with the same isolation properties as before it was tested.

EVOLUTION's coating is highly resistant to abrasion and impact while providing chemical resistance to hydrogen sulphide (H2S), steam, carbon monoxide, carbon dioxide and other chemicals often found in oil and gas pipelines. This fully encapsulated coating also prevents the need for expensive exotic cores, as it eliminates contact to exposed metal.

How the raised surface profile of PTFE sheet gasket helped a midstream oil and gas processor address leaky pipes

Bolted flange-gasket connections in process piping systems are common and given little thought – unless they start to leak.

Chronic leakage proved to be an issue for one of Garlock's clients, a midstream oil and gas processor and services provider. The site processes, stores, and transports natural gas, liquefied natural gas and petroleum products. Garlock was brought in to provide a solution to the problem.

Successful connections are dependent on a variety of things, including the state of the flange surfaces, alignment, bolt and nut grade and strength, bolt and nut thread condition, lubrication, bolt tightening process, service conditions, and choice of gasket.

When a flange-gasket joint is assembled, the gasket must first be compressed to fill the gaps between the flange surfaces, creating a seal when system pressure is applied. Secondly, it must maintain that seal as the system is brought on-line and temperature and pressure escalate.

As the temperature increases, a gasket made of non-metallic materials such as rubber, fibre, PTFE and inorganic fillers is prone to lose thickness, that is, creep. And the thicker the gasket is, the
more it is prone to creep (1/8-inch thick gaskets creep more than 1/16-inch).

The two most important performance qualities of a gasket are its ability to seal and its ability maintain that seal. These can be indicated by industry standard tests for sealability and creep.

On the surface, this particular case study would seem to be an application of little complexity. However, the details of the joint gave rise to several issues that caused the user chronic leakage problems. Here are the service conditions and background of this particular case:

• Temperature: 100°F to 120°F (38°C to 49°C)
• Flanges and use: 18-inch Class 150 raised face flanges used in the pipe systems of cooling tower water pumps. Multiple gaskets are on either side of spool piece
• Media: Water
• Pressure: 100 psig to 150 psig (7 bar-g to 10.3 bar-g)