Introduction to Pressure Seal valves

Pressure seal construction is adopted for Valves for high pressure service, typically in excess of above 170 bar. The

unique feature about the pressure seal Bonnet is that the body-Bonnet joints seals improves as the internal pressure in the

Valve increases, compared to other constructions where the increase in internal pressure tends to create leaks in the body-

Bonnet joint.







The higher the internal pressure, the greater the sealing force. Easy dismantling is made possible by dropping the

Bonnet assembly into the body cavity and driving out the four-segmental thrust rings by means of a push pin.







Relying on fairly simple design principles, pressure seal

valves have proven their capability to handle increasingly demanding fossil and combined-cycle steam isolation

applications, as designers continue to push boiler, HRSG, and piping system pressure/temperature envelopes. Pressure seal

Valves are typically available in size ranges from 2 inches to 24 inches and ASME B16.34 pressure classes from #600 to #2500,

although some manufacturers can accommodate the need for larger diameters and higher ratings for special applications.


Pressure seal gate valves are

available in many material qualities such as A105 forged and Gr.WCB cast, alloy F22 forged and Gr.WC9 cast; F11 forged and

Gr.WC6 cast, austenitic stainless F316 forged and Gr.CF8M cast; for over 500°C, F316H forged and suitable austenitic cast

grades.


The pressure seal design concept can be traced back to the mid-1900s, when, faced with ever increasing pressures and

temperatures (primarily in power applications), Valve manufacturers began designing alternatives to the traditional bolted-

Bonnet approach to sealing the body/Bonnet joint. Along with providing a higher level of pressure boundary sealing integrity,

many of the pressure seal globe valves

design weighed significantly less than their bolted Bonnet Valve counterparts.






Bolted Bonnets vs. Pressure Seals


To better understand the pressure seal design concept, let's contrast the body-to-Bonnet sealing mechanism between

bolted Bonnets and pressure seals. FIG. 1 depicts the typical Bolted Bonnet valve. The body flange and Bonnet flange are

joined by studs and nuts, with a gasket of suitable design/material inserted between the flange faces to facilitate sealing.

Studs/nuts/bolts are tightened to prescribed torques in a pattern defined by the manufacturer to affect optimal sealing.

However, as system pressure increases, the potential for leakage through the body/Bonnet joint also increases.






Pressure Seal Gaskets


One of the primary components involved in sealing the pressure seal Valve is the gasket itself. Early pressure seal

gaskets were manufactured from iron or soft steel. These gaskets were subsequently silver-plated to take advantage of the

softer plating material's ability to provide a tighter seal. Due to the pressure applied during the Valve's

hydrotest, a set (or deformation of the gasket profile) between the Bonnet and gasket was taken. Because of the inherent

Bonnet take-up bolt and pressure seal joint elasticity, the potential for the Bonnet to move and break that set when

subjected to system pressure increases/ decreases existed, with body/Bonnet joint leakage the result.


This problem could be effectively negated by utilizing the practice of "hot torquing" the Bonnet take-up bolts

after system pressure and temperature equalization, but it required owner/user maintenance personnel to do so after plant

startup. If this practice was not adhered to, the potential for leakage through the body/Bonnet joint existed, which could

damage the pressure seal gasket, the Bonnet and/or the I.D. of the Valve body, as well as creating compounding problems and

inefficiencies that the steam leakage could have on plant operations. As a result, Valve designers took several steps to

address this problem.






Although many manufacturers still recommend hot torquing, the potential for leakage when this is not done is greatly

diminished. The seating surfaces in pressure seal Valves, as in many power plant Valves, are subjected to, comparatively

speaking, very high seating loads. Seat integrity is maintained as a function of tight machining tolerances on component

parts, means of providing the requisite torque to open/close as a function of gears or actuation, and selection/ application

of proper materials for seating surfaces.


Cobalt, nickel, and iron-based hardfacing alloys are utilized for optimal wear resistance of the wedge/disc and seat ring

seating surfaces. Most commonly used are the CoCr-A (e.g., Stellite) materials. These materials are applied with a variety of

processes, including shielded metal arc, gas metal arc, gas tungsten arc, and plasma (transferred) arc. Many pressure seal

Globe Valves are designed having integral hardfaced seats, while the

Gate Valves and

Check Valves typically have hardfaced seat rings that are welded into the Valve body.



Both butterfly valves and

ball valves offer unique benefits, so choosing between them in

certain processes isn't a hard choice. It can be problematic when a process requires valve characteristics that both

valves have. We need a deeper understanding of the butterfly valve vs ball valve in order to pick the correct valve.






Teflon is a polymer compound formed by the polymerization of tetrafluoroethylene with excellent chemical stability,

corrosion resistance, sealing, high lubrication and non-viscosity, electrical insulation and good aging resistance for media

such as strong acid, strong alkali, strong oxidant. Its operating temperature is -200 ~ 180℃, poor fluidity, large thermal

expansion. Teflon lined valves ensures extremely high

chemical resistance and longevity, can be widely used in corrosive applications in the chemical, electric machinery，

harmaceutical, petrochemical, fertilizer, pulp and paper, and metallurgical industries.

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