For many process optimized control valve, it is important to quickly reach a designated location. For small signal change (1% or less) make a quick response is one of the most important factors to provide optimum process control aspects. Automatic adjustable control occasions, the large number of signals from the controller to accept change in order to obtain a small valve change. If an adjusting valve (control valve) components able to quickly respond to these small signal changes, the process of the degree of deviation will be improved.
The valve response time is measured through a process called the parameters of the T63. T63 is started from the input signal is changed play the output reaches 63% of the corresponding change in the measurement obtained time. It includes dynamic time delay time (a static time) of the valve assembly and valve assembly. This dynamic time is a measure of the time required for the execution mechanism from once begins to move to reach 63% of the points.
The dead zone, whether it is from the friction in the valve body and the implementing agencies, or from a locator, can greatly affect the delay time of the valve assembly. Important Delay time is such that as small as possible. Overall, delay time should be no more than one-third of the the valve overall response time. However, the relative relationship between the Delay time and process time constant are critical. If the valve assembly is placed in the fast loop time constant of a process close to the delay time, delay time can severely affect the performance of the circuit. In these fast loops, key To select the Delay time is as small as possible control equipment.
Delay time in the two-stroke movement of the valve direction from the loop tuning perspective remain relatively constant, is also very important. Some valve assembly structure in a stroke action direction than in another 3-5 times longer delay time. This characteristic is typically caused by the asymmetric features designed by the locator. It will severely limit the loop tuning to the best overall performance.
Once the Delay time has passed, and the valve response, the remaining portion of the valve response time from the valve assembly of dynamic time. This dynamic time is determined by the combination of dynamic characteristics locator and implementing agencies. These two components must be well matched in order to reduce the overall response time of the valve. Example, in a pneumatic valve assembly, the locator must have a high dynamic gain of the valve assembly in order to reduce dynamic time. This dynamic gain provided by the power amplifier in the locator. In other words, the locator amplifier or slide valve sooner to provide a lot of compressed air to the actuator, the valve response time will also be the faster. However, this high dynamic gain power amplifier Delay time there is a small effect, unless it is intentionally designed to reduce the static air consumption in one of the dead. Of course, the design of the implementing agencies have a great impact on the dynamic time. For example, the greater the need of filling the volume of the actuator air chamber, the slower the response time of the valve.
First, the solution may seem to be the volume of the implementing agencies to minimize, and locator dynamic power gain increased to the maximum, but it is not so simple. From the stability point of view, which may be dangerous combination of a plurality of factors. To know locator / actuator combination of its own feedback loop. Type of the implementing agencies are using the locator / implementing agencies loop gain is too high may guide the valve assembly into an unstable oscillation state. In addition, to reduce the volume of the implementing agencies have a negative impact on the thrust / friction ratio. This will increase the dead zone of the valve assembly, resulting increase in the delay time.
For a given applications, if no adequate overall thrust / friction ratio, a selection is by using a large-size actuators to increase the execution driving force mechanism, or to increase the pressure to executing agencies. The high thrust / friction ratio will reduce the dead zone. This helps to reduce the delay time of the valve assembly. However, these two options are meant to require a large amount of compressed air supply to the implementing agencies. As the exchange is to produce a devastating impact on the valve response time by increasing the dynamic time.
To reduce actuator air chamber volume method is to use a piston actuator instead of spring diaphragm actuators, but this is not a panacea. Piston actuator is usually greater thrust than the spring diaphragm actuators, but they also have higher friction, which may cause the problems related with the valve response time. Order to obtain the thrust of the required piston actuator, it is usually necessary to use a source of gas pressure is higher than the film actuator, because the typical piston has a smaller pressure receiving area. This implies the need to supply a greater amount of air, and the consequent is the negative effects of the dynamic time. In addition, more oriented surface of the piston actuator. Due to the inherent difficulties of the complex aspects, and the friction of the O-ring, they tend to have higher friction. These frictional force also tends to increase with time. Regardless Initially these O-rings is how good, due to abrasion or other environmental conditions, these resilient material will over time reduce the performance. Similarly, the guide surface will wear to increase the friction, lubrication is also reduced. These friction problem will generate greater piston actuator dead. This will increase the response time of the valve by increasing the Delay time.
Instrument supply pressure is also likely to have a huge impact on the dynamic performance of the valve assembly. For example, it can significantly affect the locator gain and total gas consumption.
The fixed gain locator is usually already in a particular supply pressure is optimized. However, this gain may be small changes in supply pressure within a twice or more changes. For example, a supply pressure of 20 Psig optimal positioning may be found when the supply pressure increased to 35 Psig reduce its gain half.
The supply pressure can also affect the supply to the implementing agencies air __. The amount of air is determined the movement speed. It is directly related with the air consumption. High gain spool valve positioner consumes 5 times the amount of air required by the amplifier in the power amplification stage more efficient high-performance secondary locator. Minimize the valve assembly Delay time needed to minimize the dead zone of the valve assembly, regardless of the dead zone is caused due to the friction of the sealing structure of the valve, or due to the frictional force of the filler material, the twist of the valve shaft, actuators or the positioning a structure is caused. As previously noted, the friction is the main reason of the dead zone control valve (control valve). For rotary valves, valve shaft torsion (see definition in Chapter 1) is also important cause of the dead zone. The type of the implementing agencies of the valve assembly friction. In general, within a very long period of time, the piston actuator to provide a greater frictional force than the spring diaphragm actuator to the regulating valve (control valve) components. As previously mentioned, this is due to the piston O-ring, with the result the problem of poor lubrication failure caused increasing frictional force.
The locator type with high static gain preamplifier can produce a big difference in reducing the dead zone. It will make a significant improvement on the resolution of the valve assembly (see definition in Chapter 1). The dead zone and the resolution of 1% or smaller of the valve assembly is not enough to meet the need for many reduction processes degree of deviation. Many processes require that the valve assembly has as low as 0.25% of the dead zone and resolution, particularly the valve assembly mounted on the occasion of a fast process loop.
In many studies of the response time of the control valve (control valve), there is an amazing thing. That is about spring diaphragm actuator piston actuator concept. Long been a misunderstanding in the process industry The piston actuator action up faster than spring diaphragm actuators. Studies have shown that for small signal changes, this is not correct.
This misconception from years of experience in the time of the test valve action. The operation time test typically is performed such that: a valve assembly to receive a 100% step change in the input signal, and then measuring the time needed to complete a full stroke movement the valve assembly in either direction.
Although the piston driven valve is usually faster than the most of the spring diaphragm driven valve operation time, but this test does not indicate the performance of the valve in the case where the control in the actual process. In the normal process control applications, valves rarely need the action of the whole trip. Typically, the valve only requires to respond in a 0.25% to 2% of the variation range of the valve position. A wide range of valve testing shows The spring diaphragm valve assembly performance of the small signal change is always more than the piston-driven pneumatic control valve, and the small signal change is more representative of the adjustable process control application conditions. The higher piston actuator friction is making them more slowly than spring diaphragm actuators for small signal response of a contributing factor.
Select the correct valves, actuators and positioners combination is not easy. This is not a simply find a matching combination in the physical things. Good engineering judgment must be incorporated into the calculation of the valve assembly and selection practices in order to obtain the best dynamic performance of the loop. And
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