A valve is a device that controls the flow of a fluid or gas. A valve is a product rarely noticed by the average person, yet it plays an important role. Each time you turn on a faucet, use your dishwasher, turn on a gas stove, or step on the accelerator of your car, you operate a valve. Without modern valve systems, there would be no fresh pure water or automatic heat in your home. One of the mostly widely observed but least recognized type of valve is the fire hydrant. They are specialized underground valves that can be opened and closed from ground level when needed in emergency situations. Today’s valves can control not only the flow, but the rate, the volume, the pressure or the direction of liquids, gases, slurries or dry materials through a pipeline or similar passageway. They can turn on and turn off, regulate, modulate or isolate. They can range in size from a fraction of an inch to as large as 30 feet in diameter and can vary in complexity from a simple brass valve to a precision-designed control valve made of an exotic metal alloy. Valves can control flow of all types, from the thinnest gas to highly corrosive chemicals, steam, abrasive slurries, toxic gases and radioactive materials. They can handle temperatures from cryogenic to molten metal, and pressures from high vacuum to thousands of pounds per square inch.
Mechanical polishing is accomplished using aluminum oxide abrasives on rotary equipment. Mill finishes, welds and surfaces that have been in service have differing surface characteristics when viewed under magnification. Mechanical polishing reduces all surface ridges, pits and discrepancies to a uniform roughness. Mechanical polishing can be achieved by hand held tools for large surface areas, such as reactors and vessels in place, or by automatic reciprocating machines for pipe or tubular components. A series of grit polishes is applied in a successively finer sequence until the desired finish or surface roughness is achieved. Electropolishing is the electrochemical removal of microscopic irregularities from metal surfaces. It results in a general leveling or smoothing of the surface, that when viewed under magnification, appears virtually featureless. Stainless steel has a natural resistance to corrosion due to its high chromium content. Electropolishing enhances this natural resistance because the process dissolves more iron than chromium. This leaves higher levels of chromium on the stainless steel surface.
Hydraulic shock is the sudden elevation in line pressure caused by the change in speed of a liquid. A shock wave is created in manufacturing lines when moving liquids suddenly stop. The liquid bounces back and forth until it eventually loses momentum and stops. Hydraulic shock can damage or destroy the manufacturing line or the valve itself, and is often overlooked when valve selections are made. A banging sound is an indication that a shock wave may be traveling through the piping. Strong shock waves will actually cause the pipes to move. Ball valves are susceptible to quick opening and closing. Their relatively small size and low torque make them easy to quickly open or close. Damage can be avoided during the design stage if the length of the piping run and the potential for hydraulic shock are taken into consideration. The time it takes to close a valve must be greater than twice the length of the upstream pipe. The more rigid the pipe, the slower the valve must close. The best methods of prevention of hydraulic shock are to limit the velocity of non-compressible Liquids and to choose the right valve for specific applications.
PBM’s Adjust-O-Seal® design feature allows in-line adjustment to compensate for normal wear on seats, reducing downtime, maintenance and repair costs, thus increasing the time between seat replacements. The adjustment can usually be done several times before the seats have to be replaced. The adjustment is accomplished by slightly tightening the body bolts, which compresses the seats against the ball and restores the valve to a leak tight condition. The valve seats are always compressed against the ball which keeps process media out of the body chamber surrounding the ball. This seal also creates a double chamber or “Dual ChamberTM”. One chamber is inboard of the seats, and the other chamber is outboard of the seats. In the closed position, the Dual Chamber allows CIP/SIP flow to enter into purge ports through the body while process media is completely shut off at the upstream seat.
Cv is defined as the number of US gallons per minute, of ambient temperature water that will flow through a valve 1 psi pressure drop. Cv basically gives an idea of flow when the valve is open. Cv is important to know when sizing the proper valve to a valve system. Cv is also important to pressure drops. PBM measures the Cv of its valves in a flow loop, which gives a very accurate Cv factor. PBM also verifies the Cv values through calculations. By controlling the surface finish to reduce cracks and crevices, the Cv of a valve can be improved.
Cast valves are produced utilizing the “lost wax” method. A wax impression is created for the shape required. The wax impression is then covered with a ceramic material and fired in a kiln. The wax evaporates during the firing process leaving behind a hard ceramic mold into which molten material is poured. Machining is minimal. Ferrite content may vary depending on wall thickness and metallurgy of the material, but is generally higher in cast valves. Forged valves are produced from round stock which has been processed from an ingot. The round stock is compressed between two halves of a forging tool at very high temperatures. The result is a shape which is then machined to create the valve shape. Forged valves require more machining than cast valves. However, forged valves have a lower ferrite content and better surface finish than cast valves.
It depends on the application. Many valve applications in pharmaceutical and biotechnology manufacturing require forged valves because of the low ferrite content and better surface finish. Cast valves can sometimes have surface imperfections where bacteria can grow and cause contamination. Forged valves are also denser than cast. If rouging (a reddish deposit associated with corrosion of metal) is a possible problem, then forged valves should be considered.
TFM™, a trademarked name of Dyneon, a 3M company, is a modified polytetrafluoroethylene (PTFE) fluoroplastic material that maintains the exceptional chemical and heat resistance properties of conventional PTFE, but has a significantly lower melt viscosity. Ball valve seats are just one recommended application of TFM because of required cold flow without the use of fillers. This material is generally used for compression molding, isostatic molding, automatic molding, and ram extrusion. TFM is also used in semiconductor and chemical processing system components, as well as in the manufacture of thermoformed products. In addition to TFM’s reduced melt viscosity and reduced cold flow, it’s features also include lower porosity and permeability, lower void content, excellent non-stick properties, and a higher elastic modulus. The advantages of TFM include smoother surfaces, reduced deformation under load, reduced extractables, and improved design flexibility. Dyneon TFM™ also has as an unlimited shelf life as long as it is stored in a clean, dry place.