There has been much discussion about the accuracy of the control valve test using the original equipment manufacturer (OEM) measurements compared to the use of independent valve analyzers.
In this article, we will identify these devices, as well as how they work and how they affect the measurements in the various diagnostic programs that use them.
Although many types of digital locators exist, only a few types of travel feedback sensors are available. This constitutes the basis for our discussion. Including the pros and cons of travel sensors and how to measure travel.
Thin film gauge Potentiometers: Resistive with an ohmic value between 35k – 50k ohms. They can be used for linear (slippage) or rotational feedback.
Magnetic Array (Hall effect): Magnetic field and calibration for a specific measuring range. Available from 0.5 “(12.7 mm) to 4” 101.4 mm). Although some manufacturers often produce them for longer shots. They can be used for both the linear axis (sliding body) and the rotating valve.
Extension cable detector (CET): Also known as wire pots, zipper sensors, string encoders or yo-yo sensors. They can operate as low voltage differential transformers (LVDTs), linear encoders and magnetic limiters. Although many types are provided, linear encoders are often used for independent valve performance. It can also be used on both linear and rotary control valve.
CET is a device used to detect and measure position and linear velocity using soft cable and spring discharge. Simply attach the probe’s body to a fixed surface and attach the stainless steel cable to the moving object. When moving the object, the probe generates a power signal proportional to the linear extension or velocity of the cable. This signal can then be sent to the PC via a suitable interface.
The CET consists of four main parts: 1) cable measurement; 2) pull pipe; 3) springs and 4) rotary sensors (potentiometer or encoder).
Inside the probe’s sheath, stainless steel cables are braided on a cylindrical lens of constant diameter and transformed into a measuring cable and unreels. To maintain the cable tension, a spring is connected to the cable. The coupler is connected to the shaft of a rotary sensor (an encoder or potentiometer). When the transducer cable extends along with the moving object, it will rotate the axis and rotate the sensor axis. The spindle generates an electrical signal proportional to the linear extension or velocity of the cable.
Moving and moving objects easily with CET, can be installed in minutes, mounted in unsafe or tight areas and does not require alignment. They also offer high flexibility, small size ratios for measurement and can be used in a wide variety of applications including industrial plant automation, high tech medical equipment, structural testing and Automobile, molding or injection molding, just about anything else you can think of.
The main controller of the valve usually has a flow characteristic, either the mounting part, the disc, the shade or the cage. So that the position of the element is important for controlling the flow through the valve and the shaft or control shaft. main element.
The parameters of the software program in the performance diagnosis relies on this location or travel measurement data to calculate the bench, spring speed, dynamic range and linearity.
Measurement data is also used to initially set up valve valve strokes when coupling the actuator to the valve body. Using this can significantly reduce the errors commonly associated with incorrect impulse stroke setting. Because you can set it precisely at a few thousandths of an inch.
With a 0.75 inch stroke (typical typical stroke for a slide valve), the CET can measure at a resolution of 0.001 inch. A few thousandths of an inch less may mean that it is impossible to obtain a full CV rating out of the valve and the ability of the stem to be damaged. Because the actuator is capable of overcoming the move to try to pull the plug through the gearbox assembly and bonnet, this can pull the body out. A few thousand percent too much and the stem or plug can be damaged if the drive’s torque is curved.
One of the major problems of the control valve is the incorrect setting of the valve valve. And this will show up in the diagnosis if you know what to look for. This will be discussed in more detail later.
Potentiometers, since they are resistors, must be converted from an ohmic value similar to something that can be translated to position or distance. A network of impedance in the electronic positioning device locates the resistor converter to count this. Typically, there are about 0-15,000 full hits and this varies between OEMs.
However, this is not a true tourist measurement. Instead, it is a reference to the value that technicians put into the device setup program.
If the valve valve stroke is 1,125 inches and we put 1,125 inches into the installation program, the calculation in the device will reflect that input. However, if we enter 1 inch into that program, the data will also reflect that. Consequently, errors can be put into the measurement because of inferences about what is actually the actual error.
A digital or analog signal is sent to the device, as this is the actual linear measurement of the CET encoder. They are extremely accurate and durable, and can be used indoors or outdoors. Actual actual measurements are obtained and can be used to accurately set valve valve strokes (travel). They are often used with independent valve performance analyzers.
As well as a better substitute for the potentiometer in that it can measure quite accurately since the arrays are only made for a specific band. In general, they should not be used with less than 60% of their full range or performance issues may result. Because there is no direct contact with the shaft or shaft, they are vibration-resistant, durable, easy to install and reliable, because measuring accuracy is usually very good.
Valve independent valve performance analyzes on control valve using standard pneumatic positioners can provide useful diagnostic information to technicians or trained engineers. While most of the information is drive performance data, some information about trim valve conditions can be obtained.
Because the diagnosis requires the valve to be fully open and closed. These tests are static and are usually not performed while the valve is in use. Some digital locators do, however, have diagnostics in place.
The next problem is, if the diagnosis can be made with a valve-independent performance analyzer, why do we need to position the digital valve instead of simply using the compressed air positioner standard?
The obvious answer is cost, $ 15,000 or more. They can not be used with asset management systems, so no online diagnosis can be made, and they are essential for time and labor time. It usually takes longer than running a diagnosis.
They are priced comparable to pneumatic locators, depending on the level of diagnosis needed or desirable. The higher the level, the higher the price.
Most number positions are HART or Fieldbus capable. This means that online diagnostics, execution times are available for asset management systems that operate with those protocols.
They are easy to install, maintain and edit. In addition, they have the advantage of having onboard diagnostic programs available either by handheld communicators or computer systems with diagnostic software installed.
Consider using independent valve performance analyzers to initially set up a control valve when:
The actuator must be installed on the valve as a result of a newly assembled valve package.
The valve package has been rebuilt or some maintenance to the required shaft / stem is disconnected from the actuator.
The valve is equipped with a pneumatic positioner.
If the valve and actuator do not have a locator, and are used for regulatory purposes, it is recommended that a device be installed.
All installation, diagnosis and repair on the control valve must be performed by a qualified valve engineer / engineer. The life of the control valve can be increased significantly when using PdM (Predictive Maintenance) using intelligent positioning and / or independent valve performance analyzers.
Remember, safety first, quality and desire to improve the management of your control valve assets will increase your ROI, since that is the bottom line.