Resource Documents
Western Process Controls can provide access to a number of key resource documents. If you wish to request one of these documents please feel free to contact us for more information.
Dead Band Versus Dynamic Error Band

DYNAMIC ERROR BAND IS NOT THE SAME THING AS DEAD BAND!

It is important to recognize the difference between these two important parameters when it comes to meeting customer Dead Band specifications. This is because the values of Dynamic Error Band can be as much as 10-20 times larger than the value for Dead Band on the same piece of hardware!

The first step in investigating the difference between these two uniquely different phenomenon is to recognize that Dead Band is a purely static measurement whereas Dynamic Error Band involves the phase lag relationship of what is typically a second-order lag of the valve assembly. Some individuals may tend to discount the significance of this phase lag, but as we shall see it is not insignificant.

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Fundamentals of Gas Pressure Regulation

Gas pressure regulators have become very familiar items over the years, and nearly everyone has grown accustomed to seeing them in factories, public buildings, by the roadside, and even in their own homes.

As is frequently the case with many such familiar items, we all have a tendency to take them for granted. Even the gas man who handles regulators every day as part of his job frequently tends to view the regulator simply as a piece of hardware which fits in the line and regulates pressure. The fact that it will do precisely that, for months on end without human intervention, makes it easy to maintain such a view. It is only when a problem develops or when we are selecting a regulator for a new application, that we need to look more deeply into the fundamentals of the regulator’s operation.

Fundamental Essential Elements
The primary function of any gas regulator is to match the flow of gas through the regulator to the demand for gas placed upon the system. At the same time, the regulator must maintain the system pressure within certain acceptable limits.

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Guidelines for the application of Electro-Pneumatic Positioners and Controllers

Selection of the right electro-pneumatic instrument for a particular process control application has always been a controversy among those who select, operate, maintain and troubleshoot process control equipment.

Probably the biggest reason for the ongoing debate is that most application guidelines are either not completely reliable, or they fail to cover all the various applications that may arise. A superior application guideline, therefore, should be easy to apply, very reliable, and consistent with the realities of the process control system, mathematically and practically. One should be able to select equipment with confidence and know that it will result in the best possible over-all control system performance.

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Fundamentals of Flow Characterisation

Achieving optimal control system performance keys on selecting or specifying the proper flow characteristic.

Therefore, people who design, operate, or maintain process controls components and systems need to understand the fundamentals of flow characterization. Characterization is the establishment of a relationship between the output and input of any device. In particular, the relationship between valve flow and valve travel is called the valve flow characteristic. Understanding the link between good control performance and flow characteristics requires some knowledge of closed loop control.

Closed Loop Control
The purpose of process control is to maintain certain process parameters such as pressure, flow, temperature, liquid level, etc. at their desired values at all times despite changes in the process load. Load change usually, but not always, means a change in process throughput. Any change in the system, which requires a change in control valve percentage of opening, should be considered a load change.

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Fundamentals of Valve Sizing for Liquids

Valves are selected and sized to perform a specific function within a process system.

Failure to perform that given function, whether it is controlling a process variable or simple on/off service, results in higher process costs. The sizing function thus becomes a critical step to successful process operation. This paper focuses on correctly sizing valves for liquid service.

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This monograph describes the technology and performance of valve stem packing systems developed by Fisher Controls for use:

  • Where valve stem leakage must be minimized to meet environmental, health and/or safety standards; and/or 
  • Where the down-time and maintenance costs associated with valve packing must be reduced.
These packing systems use technology and packing elements that defy many of the valve packing methods that have been practiced by valve and packing manufacturers for decades. They employ some proprietary and/or patented packing materials, live loading, and unique packing arrangements that were developed by Fisher over a four-year program of research and development (R&D).

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UNDERSTANDING THE STATISTICS OF VARIATION

Within a manufacturing environment the expectation is that machines will operate as required and materials will be within their own tolerance limits. However, reality shows that there are unique causes of variation, such as equipment malfunctions, fouling or operator errors. Even raw materials may have minor variations in characteristics and performance.

As each machine, process and raw material varies randomly within its own tolerance band, the cumulative effect on the overall process is a band of performance values (PV) that form a distribution about some average. The distribution of data typically follows a “normal” or “Gaussian” frequency distribution, which is the familiar bell-shaped curve.

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One of the very powerful tools used in system analysis is the block diagram.

It provides an exceedingly simple method of pinning down the exact responsibility of each element involved in a given system. For our purposes any combination of elements, however they may be grouped, that fall within any prescribed set of boundaries is defined as a “system”. To illustrate the construction of a block diagram, assume that the behavior of any element in a system can be described by the ratio of its output to its input.

For example, a diaphragm actuator has an input of diaphragm pressure, (P), and an output of stem movement, (Y). The actuator would then be described by the ratio (Y/P). For another illustration, the input to a valve is stem motion, (CY), and its output is flow, (W). The valve can now be described by the ratio (W/Y). With the help of one more ratio, a simple system can be described. Consider a level “process”. The input is flow, (W), and the output is level or head, (h). The describing ratio for the process is (h/W). Combining the elements, a system such as shown in Figure 1-1 might result.

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