Developments in Control Valves Technology

By admin May 26, 2012 7:31 am IST

Kiran Diwanji, President at Qu Con Inc. talks about the latest developments in the control valves technology
Now as technology advancement is on rise, new concepts and newer process technologies make plants very advanced. Such plants use and need state-of-the-art technologies and products in all areas including valves. With massive globalisation, now there is no physical limitation for buy and sell. In such competitive market, every industry has to be one up in everything – process technologies, waste control, productivity and zero maintenance. Therefore, dynamic progressive industrial units want long lasting fit and forget valves requiring zero or very little maintenance with guaranteed performance. There is more and more stress now on size and weight of valve provided they do not sacrifice performance in doing so. Industry needs small, compact, low weight reliable valve with ‘fit & forget’ feature. Here is one such valve for control functions.
Small is beautiful!Small, compact, light-weight, smart, and precise control valves. For handling low temperature media, air and water, there are such valves available locally. These valves with Belimo actuation are very reliable, high performance, low weight, compact ball valves with built in electronics for precise control. Normal ball valves have severely distorted flow characteristics on account of very high CV value as compared to globe valve used for control function. Belimo ball type control valves have special contoured ball so, actual flow of media flowing in valve opening is controlled by not the hole in ball but buy contour of the ball. This design offers many advantages like, equal percentage flow characteristics, higher rangability, high close off pressure, tight sealing and no sudden pressure jump while opening or closing. In a 3 way ball valve, operation from 0 to 15° of rotation of ball result in tight shut off performance. Further rotation from 15 to 90° gives regulating performance.
For critical service applications, it is desired that valve should not have leakage through seat when it is closed. Valves with Belimo actuation too offers such zero leak ball type valves with excellent seat leakage values which are less than 1/50,000 of maximum CV value for given valve.
These ball valves coming in size from ½ inch to 8 inches – are now used for control functions. These valves are offered with round or profiled opening for very precise flow control. For precise flow control of media, valves are fitted with fast response motorized actuators with minimal loose linkage, complete with feedback potentiometer and smart control electronics for very precise flow control. The smart control electronics of Belimo need just sensor having 4-20mA output signals, and an auto tuned PID controller with fuzzy logics.
Whenever valve is actuated, the Belimo actuated valves go in to self diagnostics and span monitoring. It is automatically travelling to fully open to fully close position for self re calibration. Secondly, the valves have no end limit switches at all. Smart electronics sense and fix end limits themselves. Thirdly, actuator is protected for over torque stress failures. When ever there is valve jamming or obstructions etc., the torque starts increasing. Each Belimo valve is selected and fitted with dedicated actuator with designed torque rated for said valve. When this torque increase beyond safe limit provided, valve stop automatically. Thus the valve stops functioning before external parameter damage them. These features make Belimo valves unique.
In most valves, due to hard stem movement, static friction exceeds dynamic friction and so initially, static friction prevents stem from moving for small input changes. Then stem suddenly moves when force is large enough to free it. Since force is excessive than needed to go to desired stem position, Movement is jumpy and control is shoddy. Underpowered actuator pronounces this effect. It can de-stabilise loop if action is fast. Secondly some valves do not behave same way during opening as during closing. Further since the torque that moves valve varies according to valve opening and process parameters, valve dynamic behaviour is not constant over entire range and that varies dead time, and positioner overshoot. As a result, control valve fail to control temperature precisely. Most control action for precise temperature control need very small incremental change, even 1 per cent or so, to which standard control valve may not even respond at all. These common problems associated with conventional control valve make valve to operate bumpy and jumpy.Belimo actuated flow control system is basically a decentralized intelligent-control loop that operates on its own. It has control put in to valve which receives temperature signal directly from sensor and responds immediately and also in proper algorithm moved into valve electronics giving feedback signal and command to PID controllers so as to position valve opening in such a way that its movement is just incremental in proportion to signal change received. Belimo actuated valves have so simple calibration and commissioning procedure that the whole procedure takes hardly few minutes. It is documented that the reason of failure to reduce process variability to acceptable level in as much as 80 per cent control loops audited was on account of poor dynamic performance of control valve due to not so good interfacing of communication module with valve actuator. A good valve positioner must control valve accurately in terms of dead band, Hystersis, Repeatability and Sensitivity. Smart valve have to have process gain less than 3, linearity less than 3 and hysteresis and dead band also less than 3. This is what Belimo does.
Auto-tuned PID controller with fuzzy logics evaluates close loop response from its existing PID setting and compare with set value. Deviations are converted in to corrections to PID settings, and then it actually and automatically close loop around PID controller. As long as this tuning loop is closed, it can readjust PID settings to correct any variations in process parameters as soon as they occur by observing resulting variations in damping, period etc.
PID controller acts in such a way that valve opens or closes just enough in response to intelligent signal required to position valve opening to regulate flow and hence desired parameter accurately.

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