Spherical Disc Valves

The powders and/or solvents to be processed will dictate the product contact material. Cast iron, carbon steel, and aluminum can be used for many industrial applications. Corrosive processes will call for the use of stainless steel, and in some cases, material laden with solvents or other corrosives will justify the extra expense of using Hastelloy or other high-nickel alloys to prolong valve life.

For stainless steel valves of welded construction, 316L grade is preferred. It is more corrosion-resistant than 316 stainless steel, and the lower carbon content prevents carbon precipitation from the welded joints.

Very abrasive materials will tend to dictate the use of metal seats rather than the more commonly used reinforced Teflon seats.
The metal seal will give longer life and can be used at higher temperatures than Teflon, but the shut-off sealing is limited to ANSI Class IV or Class V. Reinforced Teflon can be used up to 450 degrees F (230 C) and provides Class VI shut-off.

The sanitary requirements of the process will govern the surface finishes and other sanitary options. We define our finishes as follows:

No surface finish (Ra) specified. Sometimes referred to as “mill finish” or “as cast”. Fabricated valves only have the internal welds ground smooth and flush. Common in general industrial applications where cross-contamination or cleanability are not a concern.

#2 finish – Ra 33 to 65 microinch – 150 to 180 grits. Also referred to as “buffed, blended, or uniform appearance finish”. Often specified for the outside of equipment that will be washed down.

#4 polish – Ra 16 to 32 microinch – 180 to 240 grit. All fabrication and handling marks removed. Surface is buffed to a uniform satin finish. The #4 polish is typical for the interior of equipment that needs cleaning between batches to prevent cross-contamination.

#7 polish – Ra 10 to 15 microinch – 240 to 320 grit. All surface imperfections are repaired. Often referred to as “pit-free” and “mirror finish”. Used for high-purity applications such as processing potent pharmaceutical actives. Also used for products that tend to adhere to surfaces.

For automatic cleaning, spray balls or jets should be considered. Another option is a valve that can be dismantled by hand for inspection and cleaning. For safety reasons, the size of such valves is typically limited to an 8″ port diameter due to the weight of individual components.

The weight of a valve is especially important for mobile or rotating equipment. High-performance and heavy-duty models can weigh four times as much as their regular-duty counterparts. Dust-tight, full-vacuum, and 1 bar service are considered regular-duty service. 90 PSI (6 bar) to 150 PSI (10 bar) is high performance. An 8-inch manually operated regular duty valve can weigh 55 lbs. (25Kg), versus nearly 200 lbs. (90Kg) for an 8″ high performance model.

The typical ANSI 150 # and DIN drilling are used as industry standards for heavy-duty and high-performance models. Quick clamps can also be used for applications up to 30 PSI (2 bar) and port sizes up to 8″.
Some valves will include blind-tapped holes, which may be a problem if the valve mates with existing blind-tapped holes. Another option is to choose oversized flanges, which allow for through holes. For regular-duty service valves, ANSI and DIN bolting can be overkill; alternative bolting patterns are available.

As with other quarter-turn valves, levers, gear drives, or chain operators are available. Pneumatic and hydraulic operators are available in double-acting or fail-safe modes. Pneumatic operation should be the first choice for price, reliability, and speed. When handling solids, a higher factor of safety is used to calculate seat torque requirements. The factor is typically 1.5, rather than the 1.25 used for liquid and gas valve calculations.

For solids that “set up” or harden, oversized actuators and specially designed discs are used to break through the hardened cake. Actuators are typically sized for 80-PSI (5.3 bar) pressure. If the available air pressure supply is consistently higher (100 to 120 PSI) or lower (40 to 60 PSI), this will factor into actuator sizing.

For fail-safe operations, spring return actuators are the norm. When a spring-return type actuator is used, it is oversized to compensate for the spring as well as the unseating, run, and seating torques required for valve operation. This can lead to weight and space issues as well as additional costs. An alternate fail-safe option is to use a double-acting actuator with a pneumatic accumulator sufficiently sized to close the valve. If pneumatic pressure drops, the pressure switch activates the accumulator and opens the valve.

Fast-acting (1 to 5 seconds) quarter-turn valves are ideal for solids flow control. A pneumatic (3 to 15 psi) or electro-pneumatic (4 to 20 mA) positioner can receive a signal from a manually adjusted pressure regulator or a computer controller. The pneumatic positioner is often used in manually operated filling stations, while the electro-pneumatic positioner is typical for automatic loss-in-weight systems.