Chemical Processing - October 2007 - (Page 48)

>> Response time 100 10 Responses, seconds 1 0.1 0.1 1 Step size, % Figure 2. Various positioners on small actuators all exhibit much longer times for smaller step changes. Source: Ref. 3. 10 calibration and have no position feedback or diagnostics, there’s considerable justification in terms of performance and maintenance for replacing such positioners. In the days of analog control, a guideline advised using boosters instead of positioners on fast loops. With digital process loops and smart positioners, this no longer is an issue. It’s essential that every control valve have a smart digital positioner. A booster, if needed on a large actuator to reduce pre-stroke dead time and stroking time, should be installed on the outlet of the positioner with the booster bypass adjusted to prevent cycling by allowing the positioner to see a small portion of the actuator volume [5]. Achieving fine results Resolution and deadband play a crucial role in valve response and highly depend upon the total valve package. Resolution is the smallest change in signal in the same direction that will result in a change in position. Pneumatic positioners can adversely affect this but an even bigger potential problem originates from friction in the packing and seating through a behavior known as stick-slip where the valve closure member doesn’t move (sticks) and then breaks free and jumps to a new position (slips). Older designs of high-temperature and environmental packing as well as manual tightening of the packing beyond specifications can cause the resolution to deteriorate to 10% or worse. A more insidious source of resolution problems is the high seat or seal friction particularly associated with valves designed for isolation. On/off (block) valves aren’t 48 • October 2007 control valves and vice versa. If an application must prevent leakage, install an on/off valve whose action is coordinated with the opening and closing of a control valve. If a valve must operate near the seat due to rangeability requirements, it’s essential to ask the vendor for resolution measurements near the seat. Don’t rely on stated resolution because this normally is for the valve at mid-position where the seating or sealing friction is lower. High friction and the discontinuity in process gains near the seat cause loops to oscillate around the split range point where there is a transition from one valve to another [6]. The realistic rangeability of a control valve (the largest flow divided by the smallest controllable flow) is set by the installed characteristic and resolution near the seat. Furthermore, repeatability isn’t as important as simply the ability of the valve to respond, as the process loop will correct for changes in the magnitude of the response. Deadband is the smallest change in signal in the opposite direction that will result in a change in position. The deadband also is known as backlash and lost motion because it primarily originates from shaft connections and linkages. It’s particularly noticeable in rotary valves when there’s a translation of linear motion of an actuator to rotary motion of the ball or disk. Don’t count on rotary actuators as a solution; these actuators typically have been designed for on/off service and consequently don’t generally provide adequate resolution and deadband specifications. The effect is aggravated by high sealing friction and may result in shaft windup where the actuator shaft twists but the closure member is stuck and then considerably overshoots the desired position when it breaks free. >> Resolution 4-in. segmented ball valves with metal seals, diaphragm actuators and standard positioners 70 65 60 (%) 55 50 45 40 65 60 (%) 55 50 45 40 35 0 50 Input signal Actuator position Flow rate (ﬁltered) 100 150 200 250 300 350 400 450 0.5% steps 1% steps 2% steps 5% steps 10% steps Valve A (good resolution) Valve B (poor resolution) Time (seconds) Figure 3. Even at mid-throttle the flow through Valve B only responds to larger-thannormal changes in the input signals. www.chemicalprocessing.com http://www.chemicalprocessing.com

Table of Contents Feed for the Digital Edition of Chemical Processing - October 2007

Chemical Processing - October 2007 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Biofeedstocks See Real Growth Become a Cyber-Security Pacesetter Go Beyond Condition Monitoring Disposable Equipment Earns Lasting Role Improve Control Loop Performance Ethanol Plant Boosts Output and Saves Energy Process Puzzler Plant InSites Equipment & Services Ad Lits Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - October 2007

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