Flow Control Valves
Control valves with variable-flow plug (Fig. 7.65) are either operated manually or automatically and remotely with the aid of a pneumatic actuator. When the regulating handle is turned, the plug moves up or down, varying the passage and thereby the flow rate or the pressure. A scale on the valve indicates the setting. The plug-and-seat arrangement is similar in construction to an on/off linear plug and stem valve but has a tapered valve head. Its suitability for hygienic or aseptic applications is determined by the design of the valve stem seal (Bylund, 2015; CFCRA, 1997).
Nonreturn valves (also called check valves) are used to ensure that liquid flows in one direction only. When the flow is in the desired direction, the drag causes the valve head (disc), ball or shutter to move away from its seat. When the flow stops, the valve head or ball returns to the seat, thereby preventing flow in the reverse direction. Nonreturn valves must be installed in a position that allows full drainage (CFCRA, 1997; Schonrock, 2005; Moerman and Kastelein, 2014). Available types are:
• Nonreturn valves with a spring (Fig. 7.66) work on the principle that a light spring loading closes the valve head onto its seat once the flow stops. In this manner, flow in the reverse direction is prevented. As the
FIGURE 7.66 Stainless steel nonreturn valve with spring (tri-clamp type). Figure right: Courtesy of Tetra Pak Processing Systems AB; From Bylund, G., 2015. “Building-blocks of dairy processing”, Ch. 6, section 6.8 — Piping, valves and fittings, Ch. 21. In: Teknotext, A.B. (Ed.), Dairy processing handbook. Tetra Pak Processing Systems A/B, Lund, Sweden.
disc is forced against the flow by the spring, resistance to the inflowing fluid and pressure drop is high. Such valves are unsuitable for use with viscous liquids and may be a cleaning nightmare. There is also no indication as to whether the nonreturn valve is working. When spring- loaded nonreturn valves are used, the coil spring(s) having product contact surfaces shall have at least 2 mm openings between coils, including the ends when the spring is in a free position. Spring-loaded nonreturn valves must be fully disassembled for manual cleaning (CFCRA, 1997).
- • Nonreturn valves of the swing type (Fig. 7.67) use a hinged disc which swings open when the flow travels in the right direction. The disc closes toward the seat when the flow goes in the reverse and wrong direction. The spring-assisted closure tension holds the disc in-place. As the flappers, hinges and springs quickly become contaminated and could give rise to cleaning problems, nonreturn valves with hinged flapper should be avoided. In most cases, on a few exceptions, they are only applicable in horizontal pipe sections.
- • Springless floating ball nonreturn valves are more preferable. When the fluid enters the inlet of the Y-type ball check valve (Fig. 7.68), an elastomeric ball is pushed upward into the “Y” branch of the valve. When the flow stops, the pressure within the valve equalizes, and the ball will return from the “Y” branch of the valve, and rest itself against the smaller diameter of the valve near its inlet. The opposing pressure of the reverse flow will seat the ball firmly against the inlet of the valve. Springless inline floating ball check valves are mounted vertically, with simply the weight of the ball or poppet holding it against its seat. This ball-type check valve is hydraulically highly efficient, with flow passing straight
FIGURE 7.67 This nonreturn valve of the swing type consists of a disc (1) which opens in the right direction of flow by means of a hinge (2). When the flow goes in the reverse and wrong direction, the spring (3) forces the disc toward the seat (4). As an O-ring is contained in the groove of the body’s seat, a uniform zero leakage seal is obtained. This design contains plenty of dead areas, and the hinge and spring are sensitive to contamination. Also the valve’s drain- ability can be questioned.
FIGURE 7.68 Y-type ball check valve. Photo left, Dixon Valve & Coupling, © 2016.
through vertically. Its streamlined internal design reduces the potential for material to clog or hang up.
• A magnetic nonreturn valves with floating ball (Fig. 7.69A) or shutter (Fig. 7.69B) are also available. Magnets built in the valve body keep the floating ball or shutter in a closed position. The nonreturn valve opens when the inflow pressure exceeds that of the combined pressure of the outflow and the magnetic field. As the ball or shutter moves away from the magnet, it is less attracted to the seat and therefore starts to provide lower resistance to flow. The valve closes when the difference in pressure
FIGURE 7.69 Springless magnetic nonreturn valves with butt-welded end or clamp connections, suitable for liquids, steam, and food gases. The risk of contamination is minimal due to the springless design and the lack of flow obstructing components. The check-valve can be installed horizontally and vertically (up and down), although full drainage of the valve in horizontal position may be compromised. The minimum opening pressure is # 0.1 bar. (A) floating ball-type, (B) shutter type. Courtesy of Carollo SRL, division Ygros valves.
ceases or in cases of back pressure. The magnet will attract the ball or shutter back to its seat, to finally push it against the seat. Due to the design of the valve without flow-obstructing components, resistance to flow and pressure drop are low, as there are no springs, hinges, discs, or other components and there are no contamination or stagnation points. These check valves can be installed in horizontal as well as in vertical up-and-down positions.