CONTROLLING EQUIPMENT

AUTHOR. Walter Driedger is Senior Process Control Engineer for Colt Engineering of Calgary, Alberta, Canada. His professional experience includes instrumentation and control systems engineering in oil, gas, refinery, petrochemical and heavy oil plant design primarily in Western Canada and world-wide. He graduated in 1973 with a B.Sc. in Electrical Engineering from the University of Calgary and is a Professional Engineer in the Province of Alberta, Canada. He is also a Senior Member of ISA, the Instrumentation, Systems, and Automation Society. He welcomes opportunities to discuss the contents of these articles and may be contacted at walter (at)driedger(dot)ca.

DISCLAIMER. The opinions and observations presented in these articles are based on many years of experience in a great variety of situations. They are necessarily generalizations. While it is hoped that they are instructive, they are under no circumstances to be taken as specifications, guidelines, or professional advice. It is expected that only skilled professionals apply any of the suggestion in these articles and only after a thorough understanding of all the special circumstances of their particular case. Indeed, a major motivation in writing this material was to provoke discussion for the sake of the author's own continuing education. To this end all feedback is welcome. A primary theme linking all the articles is that there are many options from which to choose and that no answer can be considered best for all situations; every application must be examined entirely on its own.

The material in these articles was written and published on the author’s own initiative. The fact that he is presently an employee of Colt Engineering does not imply the endorsement of Colt Engineering nor of any past or future employer or client.

COPYRIGHT. All content included in these articles including text, graphics, formatting, and any web site programming is the property of Walter Driedger and is protected by Canadian and international copyright laws. The content of this material may be used as a source of general information. Copies may be made for personal use as long as the usual courtesies are observed, i.e. no tampering with the contents, especially with this cover sheet. Copies may not be made for distribution without the written consent of the author.

SITE MAP. The links below are available for immediate viewing as web pages. Each article, including this introduction, has a target for downloading in Adobe® format. The author appreciates feedback.

CONTROLLING EQUIPMENT

0 INTRODUCTION

1 CONTROLLING CENTRIFUGAL PUMPS

First published in Hydrocarbon Processing, July 1995

2 CONTROLLING POSITIVE DISPLACEMENT PUMPS

First published in Hydrocarbon Processing, May 1996

3 CONTROLLING SHELL and TUBE HEAT EXCHANGERS

First published in Hydrocarbon Processing, March 1998

4 CONTROLLING STEAM HEATERS

First published in Hydrocarbon Processing, November 1996

5 CONTROLLING FIRED HEATERS

First published in Hydrocarbon Processing, April 1997

6 CONTROLLING VESSELS and TANKS

First published in Hydrocarbon Processing, March 2000

MISCELLANEOUS

C Classics (Part 1), (Part 2), (Part 3)

An anthology of really old and mostly unattributed engineering lore.

D Dirty Pictures (Part 1), (Part 2)

A collection of photographs of ‘incidents’ that have been sent to me over the years.

These include ‘Big Rock vs. Big Truck”, “Big Truck vs. Little Truck”,

“Plastic Bag vs. Storage Tank”, “Atmosphere vs. Rail Car”, “Local Hires in Alaska”, and others.

F The Care and Feeding of Black Holes

First published in Astronomy, May 1995

L Connecting and Interpreting Limit Switches

First published in Intech, January 1993 as “Limit Switches Key to Valve Reliability”

M Michezo ya Mbao – Mankala in East Africa

First published in Mila, July 1972 as “The Game of Bao or Mankala in East Africa”

V The Compressor Monitoring Sketch

First published in Intech, July 1990 as “Getting the Picture with Compressor Monitoring”

Z Optimum Settings for Automatic Controllers

By J.G. Ziegler and N. B. Nichols, Rochester, N. Y.

First published “Transactions of the A. S. M. E.”, November 1942,

Formatting and programming for this website by mika(at)driedger(dot)ca.

Hit counter by eXTReMe Tracking

CONTROLLING EQUIPMENT, INTRODUCTION

This Adobe® file is available for download at http://www.driedger.ca/.

n industrial process consists of a number of unit operations interconnected to produce the desired result: conversion of feedstock into product. Equipment is chosen to carry out the required unit operations. It is the performance of these pieces of equipment that is controlled by the control system. For example, a pump is required to move a liquid from one point to another. It must be selected to be adequate for a range of operating conditions. The only way to accomplish this to is to select a pump large enough to handle the most demanding condition and then to trim it back to the specific requirements at every instant in time. It is the responsibility of the process control engineer to adapt the pump to produce the appropriate result. In other words, what is usually called ‘process control’ is for the most part actually equipment control.

EQUIPMENT CONTROL. Every type of equipment has a variety of options available for controlling it. A question I am frequently asked is, ‘Which option is best?” The question may be more specific, “Why is this heat exchanger controlled on the steam inlet and the other controlled at the condensate outlet? Does it matter?”

It is the purpose of this series of articles to attempt to answer such questions. The simple answer is, “It all depends.” Of all the possible control options some will simply not work and some will have undesirable side effects. Those that do work will have different secondary effects. Some examples:

A control mode that does not work at all: Discharge throttling on a positive displacement pump.

A control mode with bad side effects: Suction throttling on a centrifugal pump.

A control mode with special characteristics: Bypass temperature control on a heat exchanger.

Once the ’bad’ options are eliminated, the ‘best’ of the remaining options depends very much on the detailed characteristics of the control mode itself and on the special requirements of the process. In other words, there is no universal ‘best’. It is the job of the process controls engineer to determine what is best for each individual case. The purpose of these articles is to outline the pros and cons of a variety of possible control modes for each type of equipment. However, the reader shall not, under any circumstances, expect to find the answer to the question, “Which is best?” What I am hoping to accomplish here is to introduce readers to a way of looking at things that will help them find the best solution to each unique case.

MEASUREMENT. Every type of equipment has its special measurement requirements. The guiding principle is to determine which is the actual value that is to be controlled and to install instrumentation that will measure that precise value. This is very much dependent on the specific purpose of the equipment. When this is not possible, adequate alternatives must be found. Such issues are addressed in these articles. They are not always self-evident.

EQUIPMENT PROTECTION. Every type of equipment has its particular weaknesses. These must be addressed by appropriate instrumentation. In some cases the strategy is to avoid trespassing into damaging regimes. In others it is possible to prevent damage from occurring.

SAFETY. Every type of equipment has its own dangers and safety concerns. These must be addressed by adequate controls. At the very least, instrumentation must be provided to detect and warn of unsafe conditions.

MINOR INSTRUMENTS. A good rule to follow when adding minor instruments to a Piping and Instrumentation Diagram (P&ID) is, “Everything that does something needs an instrument to indicate if it is actually happening.” In other words, a heat exchanger should have thermometers at its various nozzles, a pump should have pressure gauges, a tank should have level indicators, etc. Each of the equipment control articles has a section indicating key instruments.

ADVANCED CONTROL. There are frequently simple optimization techniques available that will help to make the most of a given type of equipment. Descriptions of these are provided. Occasionally there are digressions into specific control techniques that have applications beyond those mentioned in the article.

REFERENCES. It is not the purpose of the reference section of these articles to provide a comprehensive bibliography of available material. Such a bibliography would be very large. A particular effort has been made to list significant codes and standards that have a bearing on each type of equipment. Some articles, such as the one on fired heaters, consist largely of a summary and explanation of the code requirements that apply.