VALVE TYPES
Although many different types of valves are used to control the flow of fluids, the basic valve types can be divided into two general groups: stop valves and check valves.
Besides the basic types of valves, many special valves, which cannot really be classified as either stop valves or check valves, are found in the engineering spaces. Many of these valves serve to control the pressure of fluids and are known as pressure-control valves. Other valves are identified by names that indicate their general function, such as thermostatic recirculating valves. The following sections deal first with the basic types of stop valves and check valves, then with some of the more complicated special valves.
Stop Valves
Stop valves are used to shut off or, in some cases, partially shut off the flow of fluid. Stop valves are controlled by the movement of the valve stem. Stop valves can be divided into four general categories: globe, gate, butterfly, and ball valves. Plug valves and needle valves may also be considered stop valves.
GLOBE VALVES.- Globe valves are probably the most common valves in existence. The globe valve derives its name from the globular shape of the valve body. However, positive identification of a globe valve must be made internally because other valve types may have globular appearing bodies. Globe valve inlet and outlet openings are arranged in several ways to suit varying requirements of flow. Figure 9-18 shows the common types of globe valve bodies: straightflow, angle-flow, and cross flow. Globe valves are used extensively throughout the engineering plant and other parts of the ship in a variety of systems.
GATE VALVES.- Gate valves are used when a straight-line flow of fluid and minimum restriction is desired. Gate valves are so named because the part that either stops or allows flow through the valve acts somewhat like the opening or closing of a gate and is called, appropriately, the gate. The gate is usually wedge shaped. When the valve is wide open, the gate is fully drawn up into the valve, leaving an opening for flow through the valve the same size as the pipe in which the valve is installed. Therefore, there is little pressure drop or flow restriction through the valve. Gate valves are not suitable for throttling purposes since the control of flow would be difficult due to valve design and since the flow of fluid slapping against a partially open gate can cause extensive damage to the valve. Except as specifically authorized, gate valves should not be used for throttling.
Gate valves are classified as either RISINGSTEM or NONRISING-STEM valves. On the nonrising-stem gate valve shown in figure 9-19 the stem is threaded on the lower end into the gate. As the handwheel on the stem is rotated, the gate travels up or down the stem on the threads, while the stem remains vertically stationary. This type of valve almost always has a pointer-type indicator threaded onto the upper end of the stem to indicate valve position.
The rising-stem gate valve, shown in figure has the stem attached to the gate; the gate and stem rise and lower together as the valve is operated.
Gate valves used in steam systems have flexible gates. The reason for using a flexible gate is to prevent binding of the gate within the valve when the valve is in the closed position. When steam lines are heated, they will expand, causing some distortion of valve bodies. If a solid gate fits snugly between the seat of a valve in a cold steam system, when the system is heated and pipes elongate, the seats will compress against the gate, wedging the gate between them and clamping the valve shut. This problem is overcome by use of a flexible gate (two circular plates attached to each other with a flexible hub in the middle). This design allows the gate to flex as the valve seat compresses it, thereby preventing clamping.
BUTTERFLY VALVES.- The butterfly valve, one type of which is shown in figure 9-21 may be used in a variety of systems aboard ship. These valves can be used effectively in freshwater, saltwater, JP-5, F-76 (naval distillate), lube oil, and chill water systems aboard ship. The butterfly valve is light in weight, relatively small, relatively quick-acting, provides positive shut-off, and can be used for throttling.
The butterfly valve has a body, a resilient seat, a butterfly disk, a stem, packing, a notched positioning plate, and a handle. The resilient seat is under compression when it is mounted in the valve body, thus making a seal around the periphery of the disk and both upper and lower points where the stem passes through the seat. Packing is provided to form a positive seal around the stem for added protection in case the seal formed by the seat should become damaged.
To close or open a butterfly valve, turn the handle only one quarter turn to rotate the disk 90°. Some larger butterfly valves may have a handwheel that operates through a gearing arrangement to operate the valve. This method is used especially where space limitation prevents use of a long handle.
Butterfly valves are relatively easy to maintain. The resilient seat is held in place by mechanical means, and neither bonding nor cementing is necessary, Because the seat is replaceable, the valve seat does not require lapping, grinding, or machine work.
BALL VALVES.- Ball valves, as the name implies, are stop valves that use a ball to stop or start the flow of fluid. The ball (fig 9-22) performs the same function as the disk in the globe valve. When the valve handle is operated to open the valve, the ball rotates to a point where the hole through the ball is in line with the valve body inlet and outlet. When the valve is shut, which requires only a 90-degree rotation of the handwheel for most valves, the ball is rotated so the hole is perpendicular to the flow openings of the valve body, and flow is stopped.
Most ball valves are of the quick-acting type (requiring only a 90-degree turn to operate the valve either completely open or closed), but many are planetary gear operated. This type of gearing allows the use of a relatively small handwheel and operating force to operate a fairly large valve. The gearing does, however, increase the operating time for the valve. Some ball valves contain a swing check located within the ball to give the valve a check valve feature. Ball valves are normally found in the following systems aboard ship: seawater, sanitary, trim and drain, air, hydraulic, and oil transfer.
Friday, 18 May 2007
Monday, 14 May 2007
Flexgate Slurry Knife Gate Valve
Flexgate Slurry Knifegates are Designed for Heavy-Duty Use Flexgate Slurry Knifegate Valve
- Ideal for difficult abrasive slurry service
- Replaceable elastomer cartridge seats
- Heavy-duty 150-psi stainless steel gate
- Bi-directional droptight shutoff
Materials of Construction
- Cast Iron Body to 24"
- Fabricated Steel Body over 24"
- Optional 304SS or 316SS body
- Gate ASTM A240 T-316
- Optional 17-4 Ph, Hastelloy C, or Teflon coated 316 SS
- ANSI Class 125/150#
- Slurry sleeves: Pure Gum Rubber, Buna-N, EPDM, and Viton.
Red Valve's Flexgate valve is designed with a cast iron body or fabricated steel body, stainless steel gate, and two heavy-duty elastomer cartridge slurry seats. The port is formed by two heavy duty cartridge elastomer slurry seat halves. The cartridge slurry seats are available in various elastomers for chemical compatibility and abrasion resistance. A flush area at the valve's base provides a clean-out port. The Flexgate valve provides bi-directional shutoff. A heavy-duty topworks is standard.
The relatively simple design eliminates expensive overhauls, unscheduled shutdowns, and costly replacement parts - the only replacement parts are the slurry sleeves and the packing. Flexgate valves of 3"-12" are provided with standard handwheel mechanisms featuring cast handwheels, machined 303 stainless steel stems with yoke sleeves, and thrust washers designed to reduce operating torque. It is recommended that Manual Flexgate Valves 12" and larger be specified with a 4:1 bevel gear actuator. The 4:1 bevel gear actuator is available to reduce rim pull and operate the valve. Pneumatic actuators are also available, and allow the Flexgate to provide long-term abrasion resistance under high cycle conditions.
The valve self-cleans each time it is cycled, making it ideal for automated plants. Red Valve manufactures its actuators using black Amalgon, a fiberglass reinforced epoxy resin. The actuator material is inert to most corrosive chemicals, hydraulic fluids, water, and oil. It has an operating temperature range from -90° to 225° F, and can withstand maximum working pressures of 250 psi. Inside actuator walls are polished and impregnated with a moly-disulfide for reduced friction and wear. The piston seal is a long-lasting O-ring. The piston rod is sealed with a lip seal, which is protected by a rod wiper ring. This wiper ring prevents dirt from being dragged though the seal.
Red Valve's Flexgate Valves are also available with hydraulic or electric actuators. Other options include limit switches, solenoids, air regulators, and chainwheels.
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