This is the second of a two-part article on control valves. Part one appeared in the September 1999issue of Control Engineering.

There is a saying, 'When momma ain't happy, nobody's happy.' A parallel could be said about control valves, 'When control valves aren't working, the whole loop's not working.'

An analysis of 31 control valves in a pulp and paper mill several years ago revealed 70% of the valves had significant operating problems (see Positioners cause most control valve problems diagram). Similar analysis in other industries has produced similar results. Even though control valves may be the single biggest contributor to control loop cycling, even though numerous articles have been written about the importance of sizing, selecting, and maintaining control valves, as a group control valves don't receive the respect they deserve.

While providing process improvement services to the pulp and paper industry Bill Bialkowski, president of EnTech Controls (Toronto, Ontario, Canada), recognized control valve dynamics depend on many factors including: valve design and size, actuator design and size, positioner performance, process conditions, and test methods. When all factors are considered control valves turn out to be complex systems that require careful engineering.

Mr. Bialkowski explains the problem with many control valves, 'Controllers expect the control valve to move and actually change the fluid flow right away for output changes as small as 0.1%. Unfortunately control valves have difficulty reversing direction and/or responding to small input changes as they tend to stick in their last position. Even under ideal conditions it takes time to convert a change in the milliamp signal to a pneumatic signal, build pressure on (or release pressure from) the actuator diaphragm, and obtain control valve movement. It turns out, control valves often respond in a fraction of a second to large input changes but, because of friction and mechanical clearances, the same valve takes 30 or 40 seconds to respond to a single 0.2% change. In the real world controllers are not 'patient enough' to wait that long. When the controller's initial small change was not satisfied additional control algorithm calculations produced additional milliamp output changes. By now the valve is moving but it's going to go too far. The controller views this as overshoot and begins new calculations to reverse the valve's direction. The result is a control valve induced limit cycle.' (See Avoid control valve induced limit cycle diagram).

Mr. Bialkowski adds, 'The problem becomes more complex when control valve sizing is considered. Many installed control valves are oversized to ensure more than adequate capacity. This results in a high process gain and thus small input changes to the control valve produce big flow changes.'

In 1992 EnTech released its Control Valve Dynamic Specification and almost immediately valve manufacturers began accessing their products' ability to comply with this new performance standard. Today, many control valves meet or exceed the performance standards defined in the original EnTech control valve spec. ISA (Research Triangle Park, N.C.) recognized the significance of the EnTech specification and convened the SP75.25 Control Valve Dynamic Testing committee to write a standard that promotes uniform specifying, testing, and reporting of control valve dynamic performance (see Online Extra box).

ISA's SP75.25 committee is being careful to define the standard on how uniform testing is to be conducted and reported. ISA is avoiding establishing performance criteria, that is being left for each manufacture to determine and report. What users will eventually see in manufacturer's literature is a consistent set of installed control valve performance characteristics.

Why users should be interested in the installed characteristics and control range is explained by Dennis Beckman, chemical industry performance consultant of Fisher Controls (Marshalltown, Ia.), 'The style and size of the valve has a significant affect on 'loop process gain' and thus control loop performance. Prior to EnTech's specification, valve selection typically considered maximum flow rates, rangeability (ratio of maximum to minimum controllable flow rates), and inherent valve characteristics. These methods fail to recognize the installed characteristics of the process and the control loop components (see CE, Feb. '99, p. 77). Since the valve provides a variable gain it is important to size and select a control valve that is sufficiently linear to stay within specified gain limits over the operating range of the system. If too much gain variation occurs in the control valve, the controller has less flexibility to control the process.'

An installed loop process gain (percent transmitter span divided by percent controller output) of 1.0 is the most desirable but good dynamic performance can be achieved if the installed loop gain is between 0.5 and 2.0 (see Installed flow characteristics influence rangeability diagram).

In terms of valve selection, butterfly valves have the smallest control range and globe valves have the largest. Eccentric plug and segmented ball valves fit in between.

Selection of the valve style is only part of assembling a well performing control valve 'system.' Attention to detail must be applied to each system component, especially positioners (see Positioners cause most control valve problems diagram).

When properly applied new positioners added to existing control valves can help reduce process variability.

Positioner rules-of-thumb guidelines evolved over the years, but recent improvements in positioner designs are changing many of these rules (see Online Extra box).

Modern positioners use a variety of internal control and feedback mechanisms to provide adjustable gain and make a positioner 'ready to jump' on small input changes.

Positioner manufacturers recognize the contribution of positioners and have on-going efforts to further improve the 'variable time to respond' problem described earlier. A promising technique is to use a positioner's digital intelligence to manage an adaptive gain algorithm and provide a consistent gain response regardless of the size of change to the input signal.

One form of insanity believes different results will be achieved by continuing to do things the same way.

Efforts by ISA's SP75.25 committee and the control valve manufacturers are useless if users don't take an active interest and insist that new control valves comply with the standard. Users should also take steps to make existing control valves come as close to compliance as possible.

Users serious about augmenting control valve performance to improve product quality need to do four things:

• Acquire knowledge on how to benchmark existing control valve performance;
• Ensure persons responsible for purchasing, engineering, and maintaining control valves understand the importance of understanding control valves as systems;
• Ensure control loops are properly tuned; and
• Regularly revisit the above three steps and ensure all five control loop elements are behaving as expected (see CE, Feb. '99, p. 77).

Users who give existing control valves some attention and respect, can expect to see their efforts pay big rewards.