What is a Pressure Seal and How Do They Work

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The history, advantages and applications of pressure seals.

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A pressure seal is a valve design that offers distinct advantages over other sealing mechanisms. Using valve stem pressure, it keeps thousands of pounds of pressure contained within.

Metal seals do not respond well to cycling but there is little or no effect when using graphite.

A pressure seal is a valve design concept that offers distinct advantages over the conventional bolted body-to-bonnet sealing mechanism. It uses the valve system pressure to provide sufficient force against the valve body ID and the bonnet surfaces. As system pressure increases, so does the force on the pressure seal gasket.

Although the only working part of the gasket is the apex or “toe”, the pressure seal conforms to the inside diameter of the valve. When the system pressure is activated, the toe forms a seal that can keep thousands of pounds of pressure and system media contained in the valve.    

Most often, it’s used for high pressure in power generation, pulp and paper, refineries and even chemical plants. Due to the reliance on system pressure to maintain a seal, these valves are best applied in systems where the minimum operating pressure is over 500 psi.

Is the pressure seal something new?
The pressure seal valve design can be traced back to as early as the 1900’s. Use was increased significantly in the late 1940’s, early 1950’s when war technology began to be applied for a consumer application. The pressure seal was further developed to remove weight from large ships. All ships, at the time, operated using a steam engine for propulsion and the removal of weight made the vessel run and maneuver better.

With all the valves necessary for steam generation, reducing the weight of these valves results in a significant overall weight reduction. There are shut-off valves, control valves and a host of boilers with blow-down and shock-valves that were paralleled into the high pressure system and then there was all the plumbing that held it all together. Let’s say that you can reduce the weight in each valve by 100 pounds. Now multiply that by a thousand valves and it’s a huge amount of weight.

Prior to the development of the pressure seal valve, all valves used bolted bonnets to keep the pressurized media in the valves. The bonnet was extremely heavy and removing them was seen as a method for losing a lot of weight. The new design incorporated a metallic pressure seal and eliminated the need for bolted bonnets. It worked extremely well, saved a ton of weight and changed valve design forever. One of the leaders instrumental in pressure seal technology and development was Edwards Valve.

How does the pressure seal work?
The pressure seal gasket has to conform to the inside diameter of the valve body and the clearance between the seal and the body has to be small. The pressure seal gasket is typically made of a malleable iron and then treated with silver to aid in conformance. The structure of the seal consists of a wider top, an angle consisting of 45 degrees or 30 degrees a wire-thin toe at the bottom.

The pressure seal gasket is placed just below the keeper ring. With enough force, metal will move into its correct position and force the toe between the body ID and the bonnet surface. When the system pressure is turned on, the seal becomes a permanent part of the valve until it has to be replaced.

The pressure on the gasket in pounds per square inch is enormous. If you take the system pressure times the area of the bonnet it equals the amount of force generated by the system. For example, if you take 2000 pounds per square inch times a 10 inch pipe that equals 157,000 pounds of load. Considering that the Space Shuttle weighs 292,000 pounds, it wouldn’t take a whole lot more, in terms of load, to launch it.

Temperature cycling can have an impact on metal pressure seals.
Thermo-cycling is when a temperature oscillates with a metal gasket during a temperature increase or cools down with a temperature decrease. This results in expansion or contraction of the gasket. Pressure cycling happens when operators are modulating the system or turning the system on or off. When the system goes off, the metal gasket starts to contract. When you begin to pressurize once again, it wants to expand. If you have a combination of both types of cycling happening at the same time, the metal gasket will literally weld itself into position. And, as any service guy who works in the power industry can tell you, bonnet removal is the pits.

Are there materials other than malleable iron that can function as a pressure seal gasket?
Carbon steel, brass, stainless and aluminum have been candidates but most of them were deemed unsatisfactory. They either cracked or couldn’t tolerate the enormous temperature, pressure and chemistry parameters.

Arguably, the best alternative to metal is a flexible graphite or graphitic pressure seal and it can be used as a replacement seal for the metal aftermarket and, in many cases, as an OEM pressure seal.

From a valve manufacturers perspective, thermo and/or pressure cycling is a big problem. Metal seals don’t really respond well to cycling because of their high coefficient of expansion (CTE). But there’s little or no effect on a graphitic seal. Graphite works with every start up and it rebounds after every cycle regardless of the number of times it happens daily.

The difference in replacing metal vs. a graphitic pressure seal.
A problem that’s shared by both the aftermarket and the OEM is replacement of a metal pressure seal gasket. Metal can become galled or welded to the valve body which takes a jackhammer to remove it. Then, once it’s removed, the valve body has to be re-machined to restore its ultra-smooth finish so that another metal seal can take its place.

Not so with a graphite pressure seal. First of all, it takes no more effort to remove the graphite seal as it does to install it and most damaged areas, related to a metallic gasket operation and removal, are accommodated by the graphitic gasket. Seldom is there a need to re-machine valve surfaces when using a flexible gasket when servicing a pressure seal style valve.

Corporate Profile
EGC is a recognized world leader in the engineering and manufacturing of graphite and graphite composites for high temperature applications in fluid sealing or thermal systems management.

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Jeff Bernthisel
since: 11/2012
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