Tuesday 28 May 2013

Control Valve Problems

Control Valve Problems

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Control valve problems can severely affect control loop performance and, unless eliminated, they can make controller tuning a challenging (sometimes impossible) task. Some problems are quite obvious to the trained eye and can easily be detected by loop performance assessment software. Others can be more difficult to detect without running specific tests. When doing on-site services, I always make sure to watch out and/or test for valve problems.
Four problems with control valves are found at a high frequency in poorly performing control loops. These are:
- Dead band
- Stiction
- Positioner overshoot
- Incorrect valve sizing
- Nonlinear flow characteristic

Let’s take a closer look at each of these problems.
Dead band
A valve with dead band acts like there is some backlash between the controller output and the actual valve position. Every time the controller output changes direction, the dead band has to be traversed before the valve physically starts moving. Although dead band may be caused by mechanical backlash (looseness or play in mechanical linkages), it can also be caused by excessive friction in the valve, an undersized actuator, or a defective positioner.

Many people use the term Hysteresis instead of dead band (I used to be one of them). But theISA and Wikipedia define hysteresis as something else. The ISA clearly calls the mechanical backlash phenomenon in control valves ”dead band”.

A control valve with dead band will cause oscillations in a level loop under PI or PID control if the controller directly drives the control valve (non-cascade). A control valve with dead band can also cause oscillations after a set point change in control loops on self-regulating processes – especially if the integral action of the controller is a little excessive.
Stiction
Another very common problem found in control loops is stiction. This is short for Static Friction, and means that the valve internals are sticky.

If a valve with stiction stops moving, it tends to stick in that position. Then additional force is required to overcome the stiction. The controller continues to change its output while the valve continues to stick in position. Additional pressure mounts in the actuator. If enough pressure builds up to overcome the static friction, the valve breaks free. The valve movement quickly absorbs the excess in pressure, and often the valve overshoots its target position. After this, the valve movement stops and the valve sticks in the new position.

Frequently, this overshoot in valve position causes the process to overshoot its set point. Then the valve sticks at the new position, the controller output reverses direction and the whole process repeats in the opposite direction. This causes an oscillation, called a stick-slip cycle. If loop oscillations are caused by stiction, the controller output’s cycle often resembles a saw-tooth wave, while the process variable may look like a square wave or an irregular sine wave.

Stiction might be caused by an over-tight valve stem seal, by sticky valve internals, by an undersized actuator, or a sticky positioner.
Positioner Overshoot
One control valve problem that is more common now than a decade ago, is that of positioner overshoot. Positioners are fast feedback controllers that measure the valve stem position and manipulate the valve actuator until the desired valve position is achieved. Most positioners can be tuned. Some are tuned too aggressively for the valve they are controlling. This causes the valve to overshoot its target position after a change in controller output. Sometimes the positioner is simply defective in a way that causes overshoot. If the process controller is also tuned aggressively, the combination with positioner overshoot can cause severe oscillations in the control loop.


Valve Sizing
The fourth common problem with control valves are oversized valves. Valves should be sized so that full flow is obtained at about 70%-90% of travel, depending on the valve characteristic curve and the service conditions. In most cases, however, control valves are sized too large for the flow rates they need to control. This leads to the valve operating at small openings even at full flow conditions. A small changes in valve position has a large effect on flow. This leads to poor control performance because any valve positioning errors, like stiction and dead band, are greatly amplified by the oversized valve.

Nonlinearity
A valve with a nonlinear flow characteristic can also lead to tuning problems. A control valve’s flow characteristic is the relationship between the valve position and the flow rate through the valve under normal service conditions. Ideally the flow characteristic should be linear. With a nonlinear characteristic, one can have optimal controller response only at one operating point. The loop could become quite unstable or sluggish as the valve position moves away from this operating point.



Conclusion
Before attempting to tune a control loop, check the valve for dead band, stiction and nonlinearity and have all problems attended to. This could save hours of effort tuning a loop in which the control valve is actually the item needing attention.


Also See-

Top Guided & Cage Guided Valves - By Defination

Friday 3 May 2013

Interview Q_Technical Quiz - Static (Mechanical - Heat Exchanger)

Interview Q_Technical Quiz - Static (Mechanical - Heat Exchanger)


Q         What are the limitations of TEMA Mechanical Standard?

Ans.    (As per RCB 1.11)
The TEMA Mechanical Std. are applicable to shell and tube heat exchangers, which do not exceed any on the following criteria :-
1)     Inside Diameter of 100”(2540mm) inches
2)     Product of nominal diameter(inch) and design pressure(psi) of 100000 psi.
3)     Design pressure of 3000 psi.

Q         What will be the corrosion allowance of part partition plate, when tube side corrosion allowance of a shell & tube heat exchanger is 3mm?
           
Ans.:  Nil, irrespective of Tube Side C.A. (as per RCB 1.518).

Q     As per TEMA, What is the allowable tolerance for Tube sheets & Girth Flange thicknesses?
Ans.: +1.6mm

Q         What is the Tube sheet ligament?
Ans.: It is the shortest distance between adjacent tube holes in the tube pattern.

Q     What is the max. distance between tube bend and nearest baffle/support Plate in a U -tube heat exchanger?
Ans. : (RCB 4.54) The sum of the bend diameter plus the straight lengths
measured along both the legs from supports to bend tangents does not
exceed the max. unsupported span determined as per Table RCB 4.52

Q    What is the minimum pass partition gasket width as per TEMA?
Ans.  As per RCB-6.4
For nominal shell diameter less than 23” (584mm)-6.4mm
For Nominal diameter above 23” (584mm)-9.4mm

Q      What is the max. ovality at the tube bends of U-Tubes?

Ans:     Max. ovality at the tube bends shall not exceed 5% of the nominal   dia.              

                                               
Q         In an expanded Tube to tube sheet joint, four grooves are required.Tube sheet material is C.S with SS cladding. As per TEMA where should be the location of grooves?
           
Ans.: As per RB 7.44 all the four grooves should be in base material i.e. CS part
of tube sheet.

Q         In a  horizontal Heat Exchangers tube ends are extended  3mm beyond tube sheet face. How much tube ends are extended  in vertical exchangers ?

Ans.: It is flushed with tube sheet faces to facilitate complete drainage.

Q         What is the formula for calculating min. part partition thickness as per TEMA?
Ans. : t = bsqrt(qB/1.5x S)   - RCB 9.132, 
            ‘,b= Plate Dimension –mm
              B=Value from table corresponding to  a & b
             ‘q= Pressure Drop across plate in –kPa
             S= Code Allowable material stress value- kPa

Q         What is TEMA recommended limit for channel cover deflection ?
Ans.:  As per TEMA RCB 9.21:
0.8 mm for nominal dia. Through 24”
            0.125% of Nominal dia. (Nominal dia./800) for larger size.

Q         How much chloride content in ppm in water for hydrotesting of SS & High Nickel alloy vessel is allowed as per our standard pecification?
Ans:    30 PPM
Q         What is the thinning percentage value of tube wall thickness after rolling?

Ans.    Thinning percentage value of tube wall thickness after rolling shall be in the range from 8% to 12% for CS & 5% to 8% for SS.

Q         What is the minimum tube pitch as per IBR?
 Ans.   As per clause No.580 ===
Tube Pitch (inch) =Tube OD+ Tube Hole dia. + 0.5’’ 
                                                                           8
Q         What is the minimum carbon steel baffle thickness as per our standard specification for shell & tube heat exchanger?
Ans.    As per clause No.8.4.1, Minimum thickness = 2 x CA

Monday 29 April 2013

Top Guided & Cage Guided Valves - By Defination

Top Guided & Cage Guided Valves

Top Guided Single Seated Control Valve, the trim is interchangeable in body. It's content to various technics liquid process by replacing the different trim assembled.


Top Guided Single Seated Control Valve is the furthest common control valve type, simple structure, are used for strict sealed close situation. single action valve can be controlled the maximum flow rate by changing the size of plug and seat ring. setting fluid characteristics by changing the curved surface. They are applicable in a wide scope of liquid services as well as steam/gas services.

As a result of high-pressure fluid is generally holded the load to add the whole area of bottom seat, made the single plug to be endured huge unbalanced force from fluid, relative to the control valve with balanced trim, the allowable pressure drops is much less for single seated valve on the Process Control.

Among different type valve body, single seated valve have trim structure with the easiest collocation, for example, angle valve.

Cage Defination - cage: A part of a valve trim that surrounds the closure member and may provide flow characterization and/ or a seating surface. It may also provide stability, guiding, balance, and alignment, and facilitate assembly of other parts of the valve trim



Cage Guided Control Valve, cage structure, straight stroke control valve. the trim is interchangeable in the body Its content to various technics liquid process by replacing the different trim assemebled.is designed focused on high pressure drops, effectively reduce the effects of cavitation and the noise in the high pressure drops services.favourable Interchangeability, be reached different flow characteristics by replacing different cage, to get much higher precision of control. Can be reduced noise, dispelled the effects of cavitation by replacing trim.

Cage Guided control valve size is in common use for 1-12, actuator for standard diaphragm type, Its content to most industrial process control. By metal seat,the suitable temperature range is -196 +538,and by soft seat,the suitable temperature range is -45 +200

The most excellent of cage guided structure control valve is balance characteristic, for plug balance ports function, fluid pressure acted on static unbalance force of plug, it reduced unbalance force, then valve can be configured actuator smaller than conventional single seat valve.

The most simple structure of cage guided control valve is twin-seat, twin-seat structure is applicable to situation that requirement of leakage is not high.

Cage guided single seat control valve have a sliding seal between plugs top with cage. by seals function, dispelling the leak that upriver fluid entered low-pressure downriver system. So it have the tight closed characteristic as conventional single seat valve, moreover have balance characteristic.