Pumps & Valves
Valves are essential components of a piping system in any machinery package, packaged equipment or facilities; valves allow the working fluid to be controlled and directed on its journey through the package. They are expensive engineered items, and it is important that the correct valve is specified for the function and that it is constructed of the correct material for the working fluid. There are two methods of operating a valve: manually, with a hand-wheel, lever, wrench, or actuator; or through automatically controlled valves. This article focuses on valves; it discusses the practical note and important guidelines for selection, operation and reliability of valves in packages.
Components of valves
Components of a valve are usually categorised by the requirements of the valve’s task; the pressure containing envelope is that volume exposed to the full operating conditions of the fluid temperature and pressure. Wetted describes a component directly exposed to the process fluid, either fully or partially; pressure-containing components and components with areas in contact with the handled fluid( such as the body and bonnet) need careful material selection. They are usually fabricated from metallic materials. All components in this group should have both the mechanical strength to cope with the design conditions and the correct material chemical composition to handle the corrosion characteristics of the handled fluid. If the component falls into the non-pressure-containing group, then pressure containment is not an issue, but the material chosen should have the mechanical strength for its chosen function. For example, a stem material should be able to support the torque applied to open and close the valve without failure. Also, as a wetted component( in contact with the fluid ), the stem should have corrosion resistance characteristics for the fluid. There have been many bolts used in different valves; bolts should be of sufficient strength to seat the gasket when bolt loads are applied and create an effective seal.
Hand-wheels should be constructed of a robust material to ensure that they do not crack and fail when being operated. Environmental conditions should also be considered, and some valve components may require an additional coating, as is the case of valves in corrosive or harsh environment of some plant’s locations, which may require a special coating and painting. Examples of non-metallic materials in valves:
• Primary seals – pressure containing and wetted.
• Secondary seals – pressure retaining and partially wetted.
• Soft seats – pressure containing and wetted.
• Gaskets – pressure containing and partially wetted. All non-metallic components form some sort of seal, either a primary seal (the first seal, and directly in contact with the process fluid and exposed to full design conditions, pressure, and temperature) or a secondary seal (any seal after the primary seal and not in direct contact with the process fluid and full design conditions, pressure, and temperature).
There are different valve design standards, such as ASME, BS, API, and in each references the numerous components included in the various types of valves. It is essential that all the valve components are suitable for the process fluid and the design conditions. A chain is as strong as its weakest link, so it is pointless to select suitable material for all but one component, because this inferior part may lead to the total failure of the valve and costly maintenance.
Valve technical details
The ASME/ANSI standard B16.34 is a valve standard used in conjunction with the ASME codes on boilers and power piping in ASME design installations and, separately, as a valve standard in itself. The standard is relevant to flanged, weld-neck, and threaded end-types for all applications. Theoretically it covers all valves down to the very smallest sizes but, in practice, its main application is for those with a nominal bore of above approximately 60 mm. The parts of ANSI B16.34 relevant to design, manufacture, and inspection are spread throughout various sections of the document. The key ones are:
• Different valve classes.
• Materials and dimensions.
• NDT scope.
• NDT techniques and acceptance criteria.
• Defect repair.
Valve classes divides valves into two main ‘classes’: “”standard classes” and “special class” (ASME/ANSI B16.34). There is a third one called ‘limited’ class, but it is not used very often. There are a series of pressure–temperature ratings for each type, designated as 150, 300, 400, 600, 900, 1500, 2500, and 4500#; these are related predominantly to the valve inside diameter and its minimum wall thickness. The higher the number, the larger the wall thickness and the maximum design pressure.
Ball valve designs
Ball valves have been specified in different designs for example, in forms of split-body ball valves and floating-ball ball valves. Regardless of the materials for construction of a split body, floating ball valve, all such valves have (more or less) the same principal components. The body in a split body design can be made in two pieces or three pieces. Both designs allow the ball valve to be removed from the line and repaired locally or, ideally, in a workshop. The three-piece version is more expensive but easier to maintain, because you can work on both side soft he ball. The floating ball design means that the ball is suspended from the stem and rests on the soft seats. It is used for smaller sizes and lower- and medium-pressure classes. As the line size increases, the mass of the ball increases and reaches a weight at which it should be supported from below with a trunnion. The valve is usually available with a reduced port (usually one size down from the line size, e.g., 6 × 4 in.) or a full port (the port and line size are the same, e.g., 6 × 6 in.) An antistatic device is also included to prevent a static charge as the metal ball travels over the soft seats, which could be made of PTFE. Depending
on the process conditions, some of the materials could change; others remain the same. This particular valve is often designed to a combination of API-6D and BS-5351 specifications. The flanged ends are designed and drilled to the specifications of ASME B16.5. The antistatic device can be according to BS-5361. The face-to-face dimensions are from API-6D and ASME B16.10. It is fire safe to an undefined code.
Another important type of ball valve is split-body, trunnion mounted type; trunnion-mounted valves are specified when the mass of the ball is such that it requires additional support at its base or for service at higher pressure ratings, when it is essential that the construction of the valve be more robust and the ball maintained in a fixed position when the valve is fully closed and not forced up hard against the soft seats, which risks squeezing them out of their retaining seat ring. There are many other types of ball valves which are not mentioned in details here, they are usually used in specific applications.
Control valves are valves used to control conditions such as flow, pressure, temperature, and liquid level by fully or partially opening or closing in response to signals received from controllers that compare a “set-point” to a “process variable” whose value is provided by sensors that monitor changes in such conditions. The opening or closing of control valves is usually done automatically by electrical, hydraulic or pneumatic actuators. Positioners are used to control the opening or closing of the actuator based on electric, or pneumatic signals. A control valve consists of three main parts in which each part exist in several types and designs. Because of its design, the globe valve pattern is the most suitable valve to control fluids for a wide range of pressures and temperatures and the most commonly specified. Although globe valves are available in sizes (say above 14 inch), for commercial reasons, at the larger sizes a butterfly valve is often specified, for the saving on space and weight.
An example of a control valve application is the pump control valve; this type of valve is used on pumped systems to control or eliminate surges caused by pump start and stop. It often operates by using a springloaded closure member that opens or closes slowly to restrict the initial flow of water when a pump starts and stops.
Safety relief valves
All conventional pressure relief valves operate on the principle of system pressure overcoming a spring load, allowing the valve to relieve at a defined set-point. When the relief valve is closed during normal operation, pressure acting against the seating surfaces is resisted by the spring force. Ideally with the system pressure below set pressure by more than one or two percent, the relief valve should be completely leak- free. As system pressure is applied to the inlet of the valve, force is exerted on the base of the disc assembly. The force produced by the compression of the spring counters this upward force. When the operating system is below set pressure, the spring housing (or body) of the valve and the outlet are at atmospheric pressure; as operating pressure begins to approach set pressure of the relief valve, the disc will begin to lift. This will ideally occur within one to two percent of set point value and an audible sound will be produced, termed the ‘simmer’ of the valve. As the disc lifts the gas is transferred from the seat area to the additional area, hence substantially increasing the area being acted on. The result is that the amount of force being applied against the spring compression is dramatically increased. This causes the disc assembly to rapidly accelerate to the lifted position or ‘open’ condition, resulting in a ‘popping’ sound.
The disc will not stay in the full open position and will begin to drop until an additional pressure build-up occurs. This over-pressure condition will maintain the relief valve in the full open position and allow it to discharge at maximum rated capacity. As the system pressure begins to drop and the spring force overcomes the force created by the disc, the valve will begin to close. The system pressure should drop below the set pressure before the relief valve will close. This process is termed the ‘blowdown’ of the valve.
Considerations regarding nozzle opening are important for relief valve operations. Nozzle openings in equipment or machineries above a certain diameter (specified in individual vessel codes for vessels) need the addition of a reinforcing pad to restore shell strength. When strength compensation is needed, nozzles may be ‘set-in’ or ‘set-through’ the vessel shell. An alternative type of nozzle used for some applications (typically on thick-walled headers) is the ‘weldolet’. These are heavily chamfered where they fit into the vessel or header shell and require a large multi-layered weld. In all nozzle fittings, the specification of the weld leg length and throat thickness is critical to the joint strength; in some cases it plays a part in the compensation for loss of strength from the nozzle opening.
Check valves automatically check or prevent the reversal off low. Basic types are the swing check, lift check, ball check, and wafer check designs. Another designation used for some applications (such as some waste systems) is a backwater valve. The swing check has a hinged disk( sometimes called a flap per) that swing son a hinge pin. When flow reverses, the pressure pushes the disk against a seat. The flap per may have a composition disk, rubber or Teflon, rather than metal when tight closure is required. Swing checks offer little resistance to flow. The lift check has a guided disk that is raised from the seat by upward flow pressure. Reversal of flow pushes the disks down against the seat, stopping back flow. Lift checks have considerable resistance to flow, similar to that of a globe valve. They are well suited for high-pressure service.
Another common check is a wafer design which fits between flanges in the same fashion as a butterfly valve. Wafer checks come in two types: a dual flap per that is hinged on a centre post and a single flapper that is similar to the standard swing check; they are generally used in larger size piping (4 in and larger) because they are much lighter and less expensive than traditional flanged end swing check valves.
A demand check value is of two-piece construction, with one piece having a springloaded closure similar to the air values found on automobile tires. The second piece, when inserted into the first, opens the valve, allowing free passage of air. The demand check valve is used for connecting gauges, allowing removal without permitting air to escape from the pipe.