Table of Contents
- Dead Band & Back lash
- Actuator-positioner design
- Response time
- Even though
- Dead Time
- Intrinsic feature
- Shaft Wind-Up
- Valve capacity (Cv) and (Kv)
- Relative current capacity
- Fail-Safe Action
- Leakage class Volvo
Dead bandage and backlash
Dead band is a general phenomenon in which a range or band of output controller (CO) values cannot change the measured process variable (PV) when the input signal reverses the direction. When a load fault occurs, the process variable (PV) deviates from the set point, this deviation initiates a corrective action through the controller and returns during the process. However, an initial change in the controller output can not cause any corresponding correction in the process variable. Only when the output of the controller has changed enough to advance in the dead band does the corresponding change in the process variable occur.
According to Fisher-Resmont (1999), the dead band is “the range in which the input signal can be changed by changing the direction without making a visible change in the output signal.” Dead band is the name given to a general phenomenon that can be applied to any device. One of the causes of dead valve band is the feedback reaction, which is defined as: “The general letter given in the form of a dead band that results from a temporary discontinuity between the input and output of a device when the input direction of the device changes. The looseness or looseness of a mechanical connection is a common example.
Actuator and positioner design
The design of the actuator and the positioner must be considered together. The combination of these two pieces of equipment strongly affects the static performance (dead band) as well as the dynamic response of the control valve assembly and the overall air consumption of the valve instrumentation. Positions are used with most of the control valve applications identified today. Positioners, when used with a conventional digital control system, enable positioning accuracy and faster response to processing disturbances. With increasing emphasis on the economic performance of process control, positions must be considered for each milk application where process optimization is important.
In automatic and tuning controls, the major changes are the signal received from the controller for small changes in position. If a control valve assembly can respond quickly to these small changes, process variability will improve. Valve response time is measured by a parameter called T63. T63 is the measured time from the start of the input signal change until the output reaches 63% of the corresponding change.
In the control valve body, the special components that perform the throttle function (or complete shut-off) of the fluid flow are collectively referred to as the valve trim. For each major type of control valve, there are usually many variations in the trim design.
The time interval (Td) at which no response from the system following a small input (usually 0.25% -5%) is detected. From the time the phase input starts until the first detectable response of the test system is measured. Dead Time can be applied to the valve assembly or the whole process.
Interest in the most general sense is the ratio of the amount of output change of a given system or device to the magnitude of the input change that caused the output change.
Profit has two components:
Static gain and dynamic gain. Static gain is the gain relationship between input and output and indicates the ease of input, which can change the output when the system or device is in a stable state. Sensitivity is sometimes used to mean a steady increase. Dynamic gain is the gain relationship between input and output when the system is in motion or flux. Dynamic gain is a function of the frequency or rate of change of input.
Hysteria, in the case of control valves, is the principle that a control valve depends on the position of the previous valve. Depending on the opening or closing of the valve to reach that position, a typical opening of the valve may be associated with different discharges. This indicates that changing the valve may require consideration in a control system to obtain the desired flow rate. If the waste becomes too large, it can force the system control architecture to oscillate around a desired point.
The relationship between the flow coefficient and the movement of the closed member (disk) when it is transferred from the closed position to the nominal path with a constant pressure drop across the valve. These characteristics are usually plotted on a curve in which the horizontal axis is labeled as the percentage of travel and the vertical axis as the percentage of current (or Cv).
Inflate the shaft
The phenomenon in which one end of the valve shaft rotates and the other end does not rotate is called inflating the shaft. This usually occurs in rotary valves where the actuator is connected to the valve closing member by a relatively long shaft. While the sealing friction in the valve holds one end of the shaft in place, the rotation of the shaft at the end of the actuator is absorbed by rotating the shaft until the actuator input transmits sufficient force to overcome the friction.
Valve capacity ( Cv ) and ( Kv )
Because a control valve acts like an orifice, the position of the plug determines the opening area of the opening. In the control valve, it is called the “valve throat”. Since the flow rate of an orifice can be expressed in terms of the differential pressure in it and the open passage. So for the right size of a control valve, we are interested to know how much we can pass through the valve for each valve opening and for any given pressure difference. The relationship between pressure drop and flow velocity through a valve is easily expressed by the flow coefficient (Cv) and (or) Kv.
Here, to define the flow capacity of a control valve, Cv is used by US-based control valves, and Kv is expressed by the rest of the world. Cv is the flow coefficient in imperial units. & Kv is the current coefficient in metric units.
The flow coefficient of a control valve (Cv) is a numerical value that expresses the number of gallons per minute of water flow at 600F when the valve passes through the valve at a constant pressure drop of 1 PSI. This rating is usually given for the valve when it is open.
While Kv is defined as the numerical value of cubic meters per hour (Nm3 / hr) of water at 160 ° C, which passes through a valve with a pressure drop of 1 bar.
Obviously, each control valve must be large enough (ie, have a maximum sufficient Cv capacity) to deliver the maximum expected flow rate in each specific process installation. A valve that is too small to use can not pass enough process fluid if needed.
The relationship between Cv and Kv is given below
Relative current capacity
The flow capacity of a valve (Cv) is a quantitative rating of its ability to pass fluid flow for a set of fluid pressure and density conditions. Therefore, not all types of control valves show the same Cv coefficients for the same tube size. For example, we have a butter valve and a butterfly valve with the same diameter has different Cv. A globe valve is simply more efficient at creating fluid turbulence and thus dissipating the kinetic energy of the fluid than a butterfly valve of the same tube size, because the design of the spherical valve forces the fluid to change direction further and in different ways. Therefore, to overcome this problem, quantifying the ability to design a specific valve to reduce fluid flow indicates this ability as the ratio of the flow coefficient (Cv) to the cross section of the pipe. This is called relative current capacity.
Since the cross section of the pipe:
We can simplify this ratio by removing all constants such as π and simply relating the Cv factor to the square of the tube diameter (d2). This ratio is called the relative current capacity, or Cd.
Safe action with failure
It is important to note that each control valve installed in a process system must have a specific failure position. This is a basic process requirement for safe shutdown of a process for human safety and also does not affect the process cycle or minimize product losses.
Characteristic of a valve and its actuator, which causes a closed valve member to fully open, fully open, or remain in the last position in any position necessary to protect the process in the event of a loss of active energy. Safe failure performance may include the use of auxiliary controls connected to the actuator.
- Do not close (open air)
- Do not open (air to close)
- The type was locked
Leakage class Volvo
In some process applications, it is important that the control valve be able to completely stop the flow of fluid when it is in the “closed” position. The ability to completely stop a liquid from passing through it under tight shut-off conditions is defined by the Leakage class. This class of leakage is tested using a procedure called “tightly closing bubbles” under a certain pressure. To classify the control valve based on its ability to turn off very tightly, seat leak tolerances are provided using Roman numerals. ANSI identifies six different leak classes, the “leak” being defined in terms of full open valve capacity, as follows: it has been shown.
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