Optimal plant operation and profitability

Emerson Automation Solutions
By Gavin Light, Business Unit Director, Emerson Process Mgt, Valves Europe
Sunday, 13 July, 2003


Increase throughput, improve yields, enhance quality, conserve energy, eliminate waste, reduce downtime - these are the rallying points in today's processing world as pressure grows to improve overall profitability.

The key to this search for profit improvement is the final control element, the control valve. In fact, only within the last several years has the importance of the control valve in managing process variability, and thereby increasing profits, been recognised.

Consider this case in point - within a refinery, performance problems with the main feed valve to a cracking tower resulted in pressure fluctuations greater than ±10psi (±0.7 bar) which repeatedly caused lifting of pressure relief valves.

After attempts to run at reduced set points, the system operators eventually resorted to placing the loop in manual to avoid the pressure relief lifts. Subsequent tests revealed excessive dead band in the original main feed valve, therefore it was replaced with an alternative control valve assembly. The replacement globe valve reduced the pressure fluctuations to ±psi (±0.07 bar) and unit production increased by 2000 bpd, equivalent to Euros 1.9 million/year.

By minimising and controlling process variability, opportunities to improve a plant's financial performance abound. It reduces throughput, avoids cost of producing off-spec products, increased raw material cost, increased operating costs and thereby lowers profits. Being successful in achieving the required performance levels ultimately yields a first-grade product that is available for immediate profitable sale.

The control valve: a source of variability

There are many causes of process variability, ranging from the improper design of a control system to instruments being out of tune, including utilisation of a flawed control strategy to run a manufacturing process.

Reduced maintenance on the process device, without visibility to the health of a process device, can also contribute to increased variability.

Process control analysis experts have found, after audits of thousands of flow control loops, that up to 40 per cent of all process variability is caused by poorly performing control valves.

Impact on performance

Supervisory control and advanced control strategies have been considered important steps in achieving process optimisation. Yet if optimisation looks only at control room equipment and software and does not consider the performance of field hardware, the true benefits of these investments will not be realised.

To better understand this rationale, consider the little that can be gained by developing a sophisticated control room architecture that is capable of performing to 0.5 per cent accuracy, only to implement a control strategy on the plant floor with a final control element that may only be capable of 5 per cent accuracy.

The selection of a valve capable of performing to required levels is therefore imperative, in fact, critical. Once a selected valve performs at the appropriate level, the key to sustenance of performance is the visibility of valve performance from the control room.

The modern advantage

Modern control valves take advantage of today's digital valve controls and transmitters. These offer several new operating and maintenance capabilities.

The digital valve controller, for example, provides two-way digital communication that makes real-time valve information available both at the plant floor and in the control room. This data proves invaluable in reducing process variability and enhancing overall plant operation by:

  • Helping operators keep processes tightly controlled within specifications.
  • Optimising maintenance programs by providing advanced valve diagnostics, allowing remote visibility to valve health (including packing performance).
  • Allowing remote and automatic instrument calibration to keep workers out of hazardous areas.
  • Minimising process start-up time through remote tag identification.
  • Allowing improving conformance to environmental standards and documentation requirements.
  • Protecting the operating integrity of the process with device alarms and alerts.
  • Providing an audit log database for ISO quality standard support.

Time savings, personnel safety and higher and better performance will result. The impact of digital valve controllers is significant, increased use is seen across the processing industries.

Digital communication allows access to expertise by phone. Services offered range from diagnostic interpretation to online product monitoring. Thanks to an ever-increasing number of mounting kits being offered, the advantages of digital valve control can be applied widely within the plant, regardless of the age or brand of the control valve.

Design advantages

Valve stem packing is another important area involving changes to long-standing designs. The impetus to change was prompted by environmental concerns and legislation.

Valves and pumps are primary sources of fugitive emissions. Stem packing and flange gaskets are the main areas of leakages. Considering that a typical plant has thousands of valves, it's clear that a significant amount of emission can prove a threat to workers and the environment. In many countries, legislations limiting fugitive emissions are employed to avoid or minimise such situations.

Many standard plastic or elastomeric packing sets used today meet the low leakage rates required but only for a short while as such materials are easily damaged by extrusion loss. Graphite packing is susceptible to friction damage. Valves that stroke frequently or are subject to high or varying temperatures are susceptible to high leakages and low packing life. All these sets thus require frequent maintenance and replacement to keep leakage low.

A good packing system will yield extended service life, reduce overall maintenance costs and provide long-term solutions to control valve sealing problems. Generally, the four basic principles in the selection of a good packing system for emission control performance are:

  • An effective mechanism should be in place to prevent extrusion.
  • An effective mechanism that maintains and withstands packing stress.
  • The valve stem should have a smooth finish to prevent erosion, along with a guide bushing that aligns the stem.
  • The stem or shaft must be well guided, especially in rotary valves.

Interchangeable cage style trim

Another design feature in a control valve that contributes to high performance control when used with digital based controllers or positioner is an interchangeable cage style trim.

Within the same valve design platform, one basic valve body casting accepts different cage and valve plug configurations. Many other parts such as gaskets, valve stems, bonnets, packing sets, etc are interchangeable between globe and angle body styles. This interchangeability lets the user match more closely each valve to its intended service conditions. By the same reasoning of changeability, 'quick-change-trim' assumes/ takes on significance when this capability allows the valve body to be left in line while trim parts are removed or replaced during inspection or maintenance.

All these benefits signal definitive savings on the number of total parts needed to maintain a spares inventory. Other savings also arise when converting from one trim style to another. Often this need for minimal parts inventory is backed by an express parts service from the valve manufacturer that makes parts available within hours of ordering.

Noise abatement

The design flexibility in the cage style trim platform is put to test in unique tough application situations. An example would be excessive noise in turbulent aerodynamic flow. Special flow cage designs, which carefully orchestrate the flow path, have proved to be effective in keeping noise levels within acceptable limits.

Whisperflo - an innovation from Fisher in noise abatement - is one that offers promise. It combines several principles to reduce noise, vibration and pipe fatigue. First, the new trim divides pressure drop, and therefore stream power, over two stages. Noise generated by the first stage is attenuated within the cage. The pressure ratio of the second stage is minimised, resulting in the stream power exiting the second stage being significantly less than that of the single stage device.

The passage shape within the cage reduces flow turbulence entering the first stage and minimises shock-associated noise. The design also directs turbulence shear layers away from solid boundaries as flow exits the second stage, which further avoids dipole sound. Exit jets from the second stage are essentially parallel, thereby avoiding shock cell interaction between them. The overall noise reduction can be in the neighbourhood of 40 dBA, surpassing earlier developed noise abatement trims by 5-10 dBA.

The key to appreciating noise reduction of the valve is to have an independent means of calculating noise, irrespective of supplier. IEC 534-8 is the means for achieving this.

The conclusion is that control valve design is key to the overall performance and profitability of a plant. Generally, the changes in control valve designs, the role of the control valve remains at its core and most essential - it's all about overall plant operation and profitability. And the selection or choice of the right control valve impacts this bottom line.

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