Centrifugal Pumps Controlling

The centrifugal pump is one of the simplest pieces of equipment from the controls and instrumentation point of view, says Walter Driedger.

Introduction
The centrifugal pump is one of the simplest pieces of equipment from the controls and instrumentation point of view. It is a two port device with a well defined characteristic.
Its purpose is to provide the necessary pressure to move liquid at the desired rate from point A to point B of the process. Fig. 1 shows a ‘generic’ process with a centrifugal pump connected to deliver liquid from A to B.
Fig. 2 shows the characteristic curve of an actual pump (a single stage vertical turbine pump) together with the characteristic curve of the process, known as the system curve.

CONTROLLING EQUIPMENT

The Author Walter Driedger is a world renowned Instrumentation and Control professional and is with Colt Engineering of Calgary, Alberta, Canada. This article marks the beginning of a series of articles. He is willing to discuss any questions arising out of these articles in the news group sci.engr.control.

Introduction An 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 it is 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. A control mode that : Discharge throttling on a does not work at all 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 is being hoped, 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.

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 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, would typically consist largely of a summary and explanation of the code requirements that apply.

The intersection of the two curves defines the operating point of both pump and process. It would be fortunate indeed if this operating point is the one actually specified for the process. It is impossible for one operating point to meet all desired operating conditions since the operating point is, by definition, exactly one of infinity of possible operating points. In fact, the entire point of controlling the pump is to modify its characteristic so that its actual operating point is the one that is required at every instance in time.

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