Valves regulate fluid flow regulating a pressure drop in a flow line. Valves are a common final control element for a process control system to maintain pressure, temperature, composition, or other process quantities. There are many types of valves with strengths and weaknesses for particular applications. For example, some valves are better suited for applications where the valve is mostly for starting and stopping flow in a pipeline. In this case, a type of valve design that has low pressure drop is desired and a ball valve or gate valve may be selected. In other cases, it is desirable to precisely maintain a flowrate into a process and a globe valve may be the preferred solution. When backflow is a concern, a swing-check valve can be installed to ensure that fluid only flows in one direction. Each valve has unique performance characteristics.
A final control element is a device used to control the output of a process in order to maintain a desired operating condition. Final control elements are typically used to regulate temperature, pressure, flow, level, or other process variables. Common types of final control elements include valves, dampers, switches, and actuators. These devices can be manually operated or controlled by an automated system, such as a programmable logic controller (PLC) or a distributed control system (DCS). Final control elements are used to ensure the safe and efficient operation of a process and are the last step in the control loop of a process control system.
The valve design equation relates the pressure drop `\Delta P_v` across the valve to the volumetric flow rate `q`.
$$q = C_v f(l) \sqrt{\frac{\Delta P_v}{g_s}}$$
The `C_v` is a measure of the size of a valve and valve suppliers have different valve body sizes, each with a different `C_v` value. The valve position or lift `l` is adjusted to regulate flow through the valve. The lift function `f(l)` depends on the type of trim installed. The trim is relatively easier to modify but the size of the valve `C_v` cannot be adjusted without replacing the valve body. Rearranging to solve for `P_v` gives the pressure drop across the valve as a function of flow `q`.
$$\Delta P_v = g_s \left(\frac{q}{C_v f(l)}\right)^2$$
In designing a valve for a particular process, it is important to maintain a linear relationship between lift `l` and flow `q`. A nonlinear relationship makes flow control tuning more complicated that requires gain scheduling, piecewise linearization in the control software, or other measures to compensate for the nonlinearity. Equal percentage valves are popular valve trims to compensate for typical process nonlinearity and create a combined valve + system response that is linear.
There are several types of valve trim that influence the `f(l)` function. A linear value trim is `f(l)=l` but this is rarely used in practice. An equal percentage valve trim is `f(l)=R^{l-1}` with `R=20` to `R=50`. An equal percentage valve trim opens more at higher flow rates and compensate for typical processes that have higher pressure losses at higher flow rates. The valve exercises show why this is needed.
Valve positioners measure the valve travel and automatically adjust the actuator to precisely lock in on a commanded valve opening. Valve positioners improve the precision with which a valve can follow commands. It is an inner feedback loop with the valve to match a valve opening set point.
A valve design involves selecting the type of valve, the body size `C_v`, and trim that gives a particular lift function `f(l)`. A few key concepts are important in valve design:
$$\Delta P_{pump} = \Delta P_v + \Delta P_{system}$$
$$\Delta P_{system} \propto q^2$$
Assignment
See the Valve Design Exercise for further details on valve design calculations.
While flow control valves are used to allow or prevent bulk material discharge from different locations, diverter valves are used to reroute powders, pellets and granular bulk materials from one source to multiple destinations (or reverse) as part of a dilute or dense phase pneumatic conveying system.
Butterfly Valves
Butterfly valves consist of a disc which rotates 90 degrees in circular hollow body. When fully open, it turns and becomes parallel to the flow direction, allowing minimum flow resistance. When fully closed, it is perpendicular to the flow direction and sealed against a seat.
Slide Gate Valves
In slide gate valves, a rectangular blade made of thick sheet metal moves horizontally in the valve body. When the blade is in fully open position, bulk material in powdered or granular form falls freely through the valve. It can also be kept open at intended level to control product flow speed.
Flap Type and Flexible Tube Diverter Valves
Flap type gravity discharge diverter valves are designed to divert the gravity flow of bulk solids from one feeding point to two or more receiving points. A wear-resistant swivel flap is used for diverting the flow inside the valve. Depending on the application it consists of flexible polyurethane rubber inner surfaces for better sealing.
The small angular deviation between the inlet and outlet ports and minimum change in cross-section area through the flap type pneumatic conveying diverter valves provide smooth material flow and reduce pressure drop in the conveying line.
Flexible tube diverter valves are suitable for pressure and vacuum conveying lines. Since there is no change in the cross-sectional area through the valve, the pressure drop is kept minimum.
Multiport Diverter Valves
A distributor pipe rotates around an axis to connect a single port to multiple ports with maximum sealing. The pressure drop is kept minimum.
Flow Control Gate Valves
Flow control gate valves provide controlled discharging of bulk solids like cement and fly ash from silos and bins. They have upper and lower chambers divided by a fabric that maintains a constant airflow to the upper side. The upper chamber has a rotating drum with an aperture around its perimeter. Rotating the drum in one direction provides dust-tight shut-off. Rotating the drum in the opposite direction provides an adjustable cross-sectional passage for material flow. By adjusting the exact position of the drum, bulk material flow speed can be adjusted with precision.
Custom Designed Valves
In addition to the standard product range, custom-designed valves are available for the most demanding and special applications. Different bore sizes, unique flange connections, non-standard body designs, and special accessories are accomplished through our engineered products group.
For abrasive materials and food applications, the valve parts are constructed from 304 or 316 stainless steel which ensures maximum durability and safety. Pipe connections can be supplied as either plain pipe ends or flanged as required. Custom engineered production incorporates problem definition, conceptualization, design, validation, production, and installation.
Atex Certified Diverter Valves
Potentially explosive atmospheres occur where fine dust of combustible bulk materials like coal, sugar, wood, and starch are handled. These risks may occur when the combustible dust suspends on-air, accumulated on hot surfaces, slides through chutes or conveyed within pipes, etc.
Any kind of ignition source like a spark, overheating, excessive friction might lead to hazardous dust explosions in the zone. To decrease dust explosion risk to acceptable levels, industrial processes should be designed and operated carefully. Polimak produces ATEX certified diverter and flow control valves for such processes.