There has been discussion for some years about the accuracy of measuring control valve travel with the use of the original equipment manufacturer (OEM) measuring element versus the use of independent valve analyzers.
In this article, we will define these devices, as well as relate how they work and how they influence measurements in the various diagnostics programs that use them.
Although many types of digital positioners exist, only a few types of travel feedback sensors are available. This forms the basis of our discussion, which includes the pros and cons of travel sensors and how they measure the travel.
Here are the types of travel sensors used for travel measurements:
Thin Film Potentiometers. Resistive potentiometers with an ohmic value between 35k – 50k ohms. They can be used for linear (sliding stem) or rotary position feedback.
Magnetic Array (Hall effect). Magnetic and factory calibrated for a specific measurement range. Available from 0.5” (12.7 mm) to 4” 101.4 mm), although some manufacturers custom produce them for longer strokes. These can be used for both linear (sliding stem) and rotary valves.
Cable Extension Transducers (CET). Also known as string pots, draw-wire sensors, string encoders or yo-yo sensors. They can operate as low-voltage differential transformers (LVDT), linear encoders and magneto-restrictive devices. While many types are offered, the linear encoder is most often used for independent valve performance. It can also be used on both linear and rotary control valves.
A CET is a device used to detect and measure linear position and velocity using a flexible cable and spring-loaded spool. Simply mount the transducer's body to a fixed surface and attach the stainless-steel cable to the movable object. As the object moves, the transducer produces an electrical signal proportional to the cable's linear extension or velocity. This signal can then be sent to a PC via a suitable interface.
CETs are composed of four main parts: 1) measuring cable; 2) spool; 3) spring and 4) rotational sensor (potentiometer or encoder).
Inside the transducer's housing, a stainless-steel cable is wound on a precisely machined constant-diameter cylindrical spool that turns as the measuring cable reels and unreels. To maintain cable tension, a spring is coupled to the spool. The spool is coupled to the shaft of a rotational sensor (an encoder or potentiometer). As the transducer's cable extends along with the movable object, it causes the spool and sensor shafts to rotate. The rotating shaft creates an electrical signal proportional to the cable's linear extension or velocity.
Measuring the movement and displacement of objects is easy with CETs, which can be installed in minutes, fitted into precarious or tight areas and do not require perfectly parallel alignment. They also offer great flexibility, a small size-to- measurement ratio and can be used in a variety of applications including industrial factory automation, high-tech medical devices, structural and automotive testing, die-casting or injection molding, hydraulic cylinder control or just about anything else you can think of.
Accuracy is import for several different reasons:
- The valve’s primary control element is usually flow characterized, either the plug, disc, ball or cage assembly, so the position of that element is crucial to controlling flow through the valve, and the stem or rotary shaft is controlling the primary element.
- Software program parameters in the performance diagnostics rely on this travel or position measurement data to calculate bench set, spring rate, dynamic error band and linearity.
- The measurement data is also used to initially set up the valve stroke when coupling up the actuator to the valve stem. Using this can significantly reduce the errors that are often associated with improper stroke set-up as you can precisely set it to with in a few thousandths of an inch.
Example: For a 0.75-inch stroke (a typical stroke for a sliding stem valve), the CET can measure with a resolution of 0.001 inches. A few thousandths of an inch less can mean not being able to get the full CV rating out of the valve and the possibility of the stem being damaged. Because the actuator has enough over-travel to try to pull the plug through the packing & bonnet assembly, this can physically pull the stem apart. A few thousandths too much and the stem or plug could be damaged if the closing force of the actuator bends the stem.
One of the major problems with control valves is improper setup of the valve stroke, and this will show up in the diagnostics if you know what to look for. This will be discussed in more detail later.
Differences between cable extension transducers, potentiometers and magnetic arrays
Potentiometers, since they are resistive, must be converted from an analog ohmic value to something that can be translated into position or distance. A resistance divider network in the positioner electronics that converts the resistance to counts accomplishes this. Typically, 0-15,000 counts range for full travel and this varies somewhat between OEM vendors.
However, this is not a true travel measurement; rather it is a reference measurement to the value of what the technician inputs into the device-setup program. An example: If the valve stroke is 1.125 inches and we put 1.125 inches into the setup program, the counts in the device will reflect that input. However, if we input 1 inch into that program, the counts will reflect that as well. Thus, errors can be introduced into the measurement because of the inference to what the actual stroke really is.
Cable Extension Transducers: A digital or analog signal is sent to the device, as this is an actual linear measurement made by the CET encoder. They are extremely accurate and durable, and can be used indoors or out. Actual true measurement is obtained and can be used to accurately set up the valve stroke (travel). These are typically used with the independent valve performance analyzers.
The Magnetic Array. Also a better alternative to the potentiometer in that it can measure the stroke fairly accurately as the arrays are made only for a specific range. Generally, they should not be used with less than 60% of their full range or performance problems can result. As there is no direct contact with the stem or shaft, they are vibration-resistant, durable, comparably easy to install and reliable, as measurement accuracy is generally good.
Independent valve performance analyzers on control valves that use standard pneumatic positioners can give useful diagnostic information to the trained technician or engineer. While most of the information is actuator-performance data, some information about the valve trim condition can be obtained. Such information includes:
- Valve travel (stroke)
- HDL (hysteresis, dead band and lag), also known as dynamic error band
- Dynamic linearity
- Actuator bench set (sliding stem) or initial set (rotary)
- Actuator spring rate
- Friction, maximum, normal and minimum, expected total friction
- Input signal zero range, input signal full range
- Seat load as tested, service seat load and required seat load (sliding stem)
- Torque: maximum, normal, minimum (rotary)
Because the diagnostics require the valve be fully opened and closed, these tests are static and are generally not performed while the valve is in service. Some digital positioners do, however, have in-service diagnostics.
The next issue is, if diagnostics can be done with an independent valve- performance analyzer, why would we need digital valve positioners instead of simply using standard pneumatic positioners?
The obvious answer is the expense, as much as $15,000 and up. They can’t be used with asset management systems, so no on-line diagnostics are possible, and they are very labor- and time-intensive to set up. It often takes more time than to run the diagnostics.
The reasons to use a digital positioner are:
- They are somewhat price comparable with pneumatic positioners, depending on what level of diagnostics are needed or desired. Higher levels of diagnostics increase the price substantially.
- Most digital positioners are HART or Fieldbus capable. This means on-line, real-time diagnostics are available to asset management systems that operate with those protocols.
- They are easy to install, maintain and calibrate. Also, they have the advantage of on-board diagnostics available either by hand-held 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 the result of a new valve package being assembled.
- The valve package has been rebuilt or some maintenance to the valve required the stem/shaft be disconnected from the actuator.
- The valve is only equipped with a pneumatic positioner.
If the valve and actuator package has no positioner, and is used for throttling purposes, the recommendation is that one be installed.
All installation, diagnostics and repair on control valves must be done by a qualified control valve technician/engineer. The life cycle of the control valve package can be significantly increased with the use of PdM (predictive maintenance) using smart positioners, and/or independent valve performance analyzers.
Remember, safety first, quality and the desire to improve management of your control valve assets will increase your ROI, as that is the bottom line.
Louis Reeves formerly worked as an electrical and instrumentation technician for control valve asset management at International Paper.