Industrial valves are complex mechanical devices manufactured by assembling multiple parts: the body (i.e the outer shell, a critical pressure containing cast of forged steel component), the bonnet (gate, globe, and check), the trim (a collective name to designate the disc, the seat, and the stem), the packing and, the actuator (manual, gear, or automated).
EXAMPLE OF VALVE ASSEMBLY
The body is a key valve part: it contains the other critical components and it has to withstand pressure loads from the connected piping system. Valve are joined to other piping components by different connections as, for instance, butt weld or socket weld, threaded or flanged types. The body can be manufactured out of cast or forged steel in a variety of shapes, designs and material grades. The most common grades are high-temperature carbon steel, low-temperature carbon steel and austenitic stainless steel (SS304, SS316, SS321, etc).
For specific applications, special material grades with stronger corrosion resistance are used, such as super austenitic stainless steels (SMO 254), duplex and super duplex steels, and nickel alloys (Inconel, Incoloy, Hastelloy).
For marine applications, non-ferrous materials or alloys are used (Monel, cupronickel, aluminum bronze). Iron bodies are used in less critical applications as water distribution systems.
Bonnets are another important valve parts. When dismantled from the body, it allows the access to the internal components of the valve to execute maintenance activities or replacement of parts (such as the disc, seat, stem, etc).
Valve bonnets are available in many designs and models (the most typical are: bolted, round bolted, welded, pressure seal) and are manufactured in cast steel or forged steel (generally using the same material grade of the body).
The bonnet is connected with the body by a threaded, bolted, or welded joint connections – and gaskets are used in between the body and the bonnet. Manufacturers strive to optimize the design of this part (to reduce the overall dimension of the device and to ensure that the connection is tight and leakage-proof).
The valve trim is a collective name for all the internal parts of the valve that can be removed and replaced and that have a direct contact with the fluid flowing through the pipeline.
Typically, the trim includes components as the disc, the seat, the stem, the glands, the bushings and the sleeves needed to guide the stem (but the specific elements that constitute the trim of a valve depends on the valve type).
The trim is a fundamental component of the performance and the operation of the valve and the trim materials combination shall be chosen with care. The most used trim combinations for gate, globe and check valves are defined by the API trim chart.
The disc is the part of the valve that opens, closes or modulate the flow of the fluid within the pipeline, depending on its relative position to the seat. In the case of a gate valve, the gate (wedge) is the disc of the valve (whereas the disc is called “ball” for ball valves).
After the body and the bonnet, the disc is the third most important element that determines the performance of the valve and its tight seal. Discs are generally manufactured in forged steel and are often reinforced (hard faced) to enhance the mechanical properties of the base material.
Seats accommodate the movements of the disc, and valves may have one or multiple seats. Globe or a swing-check valve are fitted with one seat that, in connection with the disc, creates the seal that interrupts the flow of the fluid.
Gate valves have instead two seats, one on the upper side and the other on the lower side of the valve. Seats are often hard-faced. A proper finish of the seats is required to ensure a good seal when the valve is in the closed position.
The stem of a valve is used to maneuver the valve (open/close) because it moves the disc inside the valve.
The stem is linked to the valve actuator or to the manual handwheel (or lever) at one end and is connected to the valve disc on the other end. In the case of gate and globe valves, the stem exercises a linear motion on the disc, whereas for a ball, butterfly and plug valves the disc rotates to open or close the valve (therefore such valves are called “quarter turn valves”).
Stems are made of forged steel and are connected to the disc by threading or other means. To prevent the leakages of the valve, a proper finish of the stem surface is necessary.
There are five types of valve stems:
- Rotary Stem: This is a standard type of the ball, plug, and butterfly valves. A quarter-turn motion of the stem opens or closes the valve.
- Sliding Stem: In this case, the stem does not execute any rotation. The stem slides in and out the valve to open or close it. This design is common in hand-operated lever rapid opening valves. It is also used in control valves are operated by hydraulic or pneumatic cylinders.
- Rising type with outside screw and yoke (“OS&Y”): the external side of the stem is threaded while the part of the stem which is inside the valve is plain. The threads of the stem are isolated from the medium by the packing. Two alternative designs are available. The “OS&Y” design is common for valves above 2″.
- Rising type with an inside screw (“IS&Y”): The threaded part of the stem is positioned inside the valve body, whereas the stem packing lays outside. With this design, the stem threads are in touch with the medium flowing through the pipeline. Once rotated, the stem and the hand wheel rise together and open the valve.
- Non-rising stem type with inside screw: The threaded part of the stem is inside the valve and does not rise. The valve disc floats on the stem, like a nut if the stem is rotated. Stem threads are in contact with the media of the pipeline, and as such, may be exposed to its corrosive impact. This is the reason why such design is used when the available space to position the valve is too narrow to permit linear movement, and the media does not cause erosion, corrosion or abrasion of the stem material.
The gasket that seals the stem with the bonnet is called packing, and comprises the following components:
- Gland follower, which is a sleeve that compresses the packing, by a gland into the stuffing box.
- Gland, a type of bushing, which compresses the packing into the stuffing box.
- Stuffing box, a chamber in which the packing gets compressed.
- Packing, available in different materials, like PTFE, elastomers, fibrous material, etc.
- A backseat is a seating inside the bonnet. The back seat provides a seal between the stem and bonnet and prevents system pressure from building against the valve packing once the valve is fully open. Back seats are often used in gate and globe valves.
- The valve packaging shall be properly designed and manufactured to minimize the possible damages to the stem and minimize the risk of leakages of fluids. On the other hand, it is necessary to observe that a too tight packing may affect the stem.
A hand operated or manual valve is generally equipped with a hand wheel that can be rotated clockwise or counter-clockwise to open and/or close the valve (typical for gate and globe valves). Ball, plug or butterfly are actuated using a lever (manual quarter turn valves).
In the following cases, it is not either possible nor advisable to use manual valves:
- Large dimension valves that operate at high pressures
- Valves that need to be controlled from a remote location
- Valves that require, for the nature of the process, a very fast open or close operation
In all these cases, a valve actuator is needed. The actuator produces linear and rotary motion able to open or close a valve (the actual movement depends of course on the type of the valve, linear or quarter turn). The main types of valves actuators are:
- Gear Actuators
- Electric actuators
- Pneumatic actuators (pneumatic control valves use this type frequently)
- Hydraulic actuators
- Gas over oil actuators
- Solenoid Actuators
Rotork actuators and Auma actuators have the largest market shares within the petrochemical industry. Actuators are costly parts of valves.