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Flow Simulation in Pump/Compressor Design

Pumps & Systems, February 2009


Computer simulations of pumps and compressors can now serve the same function as hardware testing. These simulations can be done in less time with less cost while providing engineering data of similar quality. Furthermore, computer modeling can be performed directly by the engineer doing the hardware design, thus providing a tight link between analysis and design optimization.

Computer Pump/Compressor Simulations

We will start by defining pump simulation. Modeling and simulation can take many forms, but in this article, pump simulation refers specifically to 3-D computational fluid dynamics (CFD), an example of which is shown in Figure 1.

Figure 1. CFD model of an external gear oil pump (surface pressures and x-y data plots)

In this context, Computational Fluid Dynamics applies to liquid or gas, compressors or pumps, and can be used to model fluid motors and a wide range of other fluid components. Such CFD codes usually start with a CAD model of the geometry and then create a 3-D numerical mesh representing the flow path through the device. This mesh is subsequently used to model the dynamics of the flow based on fundamental laws for conservations of mass and momentum. The output of these models includes plots and three-dimensional maps of flowrates, loads, head-rise, power, pressure ripples, velocities and torques, depending on whether it is for a pump, compressor or motor.
For liquid applications, the more advanced codes include aeration and cavitation. Aeration refers to the presence of non-condensable gases, such as air; and cavitation refers to the formation of vapor from the liquid. Both can have a significant effect on pump performance and life.
If temperature influences ...

PUMP AND PUMP SYSTEM GLOSSARY

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[TD]Absolute pressure: pressure is measured in psi (pounds per square inch) in the imperial system and kPa (kiloPascal or bar) in the metric system. Most pressure measurements are made relative to the local atmospheric pressure. In that case we add a "g" to the pressure measurement unit such as psig or kPag. The value of the local atmospheric pressure varies with elevation It is not the same if you are at sea level (14.7 psia) or at 4000 feet elevation (12.7 psia). In certain cases it is necessary to measure pressure values that are less then the local atmospheric pressure and in those cases we use the absolute unit of pressure, the psia or kPa a.


pa(psia) = pr(psig) + patm(psia), patm = 14.7 psia at sea level.

where pa is the absolute pressure, pr the relative pressure and patm the absolute pressure value of the local atmospheric pressure.

and in the metric system

pa(kPa a) = pr(kPag) + patm(kPa a), patm = 100 kPa a at sea level.

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[TD]Accumulator: used in domestic water applications to stabilize the pressure in the system and avoid the pump cycling on and off every time a tap is opened somewhere in the house. The flexible bladder is pressurized with air at the pressure[/table] ...

CENTRIFUGAL PUMP SYSTEM TUTORIAL 2

What is friction in a pump system (cont.) Another cause of friction is all the fittings (elbows, tees, y's, etc) required to get the fluid from point A to B. Each one has a particular effect on the fluid streamlines. For example in the case of the elbow, the fluid particles that are closest to the tight inner radius of the elbow lift off from the pipe surface forming small vortices that consume energy. This energy loss is small for one elbow but if you have several elbows and other fittings the total can become significant. Generally speaking they rarely represent more then 30% of the total friction due to the overall pipe length.

Figure 9
[hr][/hr] Energy and head in pump systems
Energy and head are two terms that are often used in pump systems. We use energy to describe the movement of liquids in pump systems because it is easier than any other method. There are four forms of energy in pump systems: pressure, elevation, friction and velocity.


Pressure is produced at the bottom of the reservoir because the liquid fills up the container completely and its weight produces a force that is distributed over a surface which is pressure. This type of pressure is called static pressure. Pressure energy is the energy that builds up when liquid or gas particles are moved slightly closer to each other and as a result they push outwards in their environment. A good example is a fire extinguisher, ...

CENTRIFUGAL PUMP SYSTEM TUTORIAL 1

What is total head
Total head and flow are the main criteria that are used to compare one pump with another or to select a centrifugal pump for an application. Total head is related to the discharge pressure of the pump. Why can't we just use discharge pressure? Pressure is a familiar concept, we are familiar with it in our daily lives. For example, fire extinguishers are pressurized at 60 psig (413 kPa), we put 35 psig (241 kPa) air pressure in our bicycle and car tires.For good reasons, pump manufacturers do not use discharge pressure as a criteria for pump selection. One of the reasons is that they do not know how you will use the pump. They do not know what flow rate you require and the flow rate of a centrifugal pump is not fixed. The discharge pressure depends on the pressure available on the suction side of the pump. If the source of water for the pump is below or above the pump suction, for the same flow rate you will get a different discharge pressure. Therefore to eliminate this problem, it is preferable to use the difference in pressure between the inlet and outlet of the pump.

The manufacturers have taken this a step further, the amount of pressure that a pump can produce will depend on the density of the fluid, for a salt water solution which is denser than pure water, the pressure will be higher for the same flow rate. Once again, the manufacturer doesn't know what type of fluid is in your system, so that a criteria that does not depend on density is very useful. There is such a criteria and it is called TOTAL HEAD, and it is defined as the difference in head between the inlet and outlet of the pump.

You can measure the discharge head by attaching a tube to the discharge side of the pump and measuring the height of the liquid ...

[QUOTE=Esam;176603]

CONNECTING and INTERPRETING LIMIT SWITCHES

© Walter ********, P. Eng., 2000 May 20. walter(at)********(dot)ca

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

This Adobe® file is available for download.

[IMG]http://www.********.ca/limitsw/LS-1.jpg[/IMG]INTRODUCTION. There is a great variety of possible combinations for installing and connecting limit switches on valves. The number of switches depends on the particular control objective and may be influenced by redundancy considerations. The way they are connected depends on the safety and reliability requirements.
In order to clarify this discussion, diagrams like Figure 1 will be used. All signals, switch positions, etc. are shown with the valve at the center of travel. No limit switches are actuated, all are shown in their shelf position as determined by their internal springs. Imagine the valve to be like a guillotine where the stem travels upward to open the valve and downwards to close it. The diagrams show the switches connected to indicating light bulbs but the logic is identical if a DCS or other form of MMI is used.
The limit switch that is actuated when the valve is fully open is labeled ZSO. The one at the extreme opposite end is labeled ZSC.
The terminals on the electrical switches are labeled Common (C), Normally Open (NO), and Normally Closed (NC). This unfortunate choice of terminology has nothing to do with the state of the valve nor even the "normal" position of the switch. It refers to the state of the switch when nothing is pushing on it.

[IMG]http://www.********.ca/limitsw/LS-2.jpg[/IMG]SINGLE SWITCH, DIRECT APPROACH. A single limit switch at the OPEN end of the valve (ZSO), as shown in Figure 2, will tell us when the valve is fully open. It cannot tell us if the valve is ...

Optimum Settings For Automatic Controllers
the American Society of Mechanical Engineers, 1942.

First published in "Transactions of the A. S. M. E.", November 1942.

An exact facsimile on Adobe ® is available for download.
"Optimum Settings For Automatic Controllers" – is the name of one of the most important publications in the history of automation, instrumentation, and control systems. Written by Ziegler and Nichols and published in the November 1942 issue of the Transactions of the American Society of Mechanical Engineers, it gave the "hit and miss" art of tuning controllers a practical basis. Ziegler and Nichols developed and published tuning rules for pneumatic PID controllers while working in Rochester, NY for Taylor Instruments, now a part of ABB. Although developed for pneumatic controllers, their rules are still widely used as a comparison for other methods. When Nichols died in April 1997, at age 82, and Ziegler not long after on December 9, at age 88, a chapter in industrial automatic control came to an end.
The ASME graciously granted permission for me to reproduce "Optimum Settings for Automatic Controllers", © 1942, on my website, walter(at)********(dot)ca. To generate this reproduction, a photocopy of the printed original has been scanned, run through an optical character recognition (OCR) program, formatted in MS Word, and edited to duplicate the original as closely as possible. The diagrams were scanned and embedded in the Word file in .jpg format. Finally, this MS Word "forgery" was converted into .html and .pdf formats.

I thank the ASME for allowing this important work to be made so freely available. The society can be reached at www.asme.org. I would also like to thank my daughter, Mika ********, for her patience with the OCR work.

Optimum Settings

...

Originally Posted by Esam

THE COMPRESSOR MONITORING SKETCH

© Walter ********, P. Eng., 2000 August 20. walter(at)********(dot)ca

First published in Intech , July 1990

This Adobe® file is available for download.


INTRODUCTION. Review meetings between mechanical engineers, control systems engineers and equipment manufacturing representatives do not always provide a final resolution of all outstanding technical issues relating to the precise scope of supply. Without some means of focusing the discussion a number of scenarios are possible.
Scenario number 1: Everybody thinks they understand.
A typical discussion may run like this:
Equipment Engineer, "Where will you provide bearing monitoring?"
Vendor Representative, "On all the bearings."
The control systems engineer writes this down in his note book and goes off to list and tag all the monitoring points. Some time later he finds out that there were more bearings than anyone thought, that axial probes appear on both sides of the thrust bearings and the key phasor is missing. At this point he realizes that his monitoring package is too small and that the proper monitor will not fit into the compressor control panel that has been ordered.
Scenario number 2: Everybody is totally mystified.
A typical discussion may run like this:
Equipment Engineer, "Where will you provide bearing monitors?"
Vendor Representative, "Radial X/Y probes will be provided on both the inboard and the outboard bearings but axial probes only on the active side of the outboard bearings."
Equipment Engineer, "Are those on the driven end or on the opposite from driven end?"
Controls Engineer, "Is that on the north end or the south end of the compressor?"
Vendor Representative, "It's on the high speed

...

[QUOTE=Esam;176596]CONTROLLING FIRED HEATERS © Walter ********, P. Eng., 2000 May 20. walter(at)********(dot)ca

First published in Hydrocarbon Processing , April 1997.

This Adobe® file is available for download.


[IMG]http://www.********.ca/ce5_fh/5-1.jpg[/IMG]INTRODUCTION. The purpose of a fired heater is very simple: To add heat to a process fluid. Its representation on a process flow diagram is also very simple. But, of course, fired heaters are among the most complex pieces of process control equipment. Each furnace is, after all, at least two pieces of equipment in one. Firstly, it is a special variant of the shell and tube heat exchanger since its purpose is to exchange heat. Secondly, it is a chemical reactor in which fuel and air undergo extremely exothermic reactions to produce the required heat.

In previous articles of this series^{1, 2, 3, 4}, the process aspects of controlling a piece of equipment were presented before dealing with protection and safety. This time the topics will be reversed: In the case of fired heaters, it must be safety first!
SAFETY. If fired heaters had not been invented and were being proposed for the first time, I would probably say, "You've got to be kidding. That thing will blow up in your face the first time you throw a match in it." However, at least a half a billion gas fired heaters are in service around the world (according to the American Gas Association). Most of them are operated by people with no technical experience whatsoever; few heaters blow up. Still, the average domestic water heater is not in the same league as a hydrogen reformer furnace. The fact that accidents and disasters are as few as they are, is due to the long experience the human race has in dealing with fire. A million years, I'm told. For the last century, this experience has been embodied in various codes and standards that ...

[QUOTE=Esam;176591]

CONTROLLING STEAM HEATERS

© Walter ********, P. Eng., 2000 May 20. walter(at)********(dot)ca

First published in Hydrocarbon Processing , November 1996.

This Adobe® file is available for download.


INTRODUCTION. Steam Heaters are simply heat exchangers in which one of the media is steam being condensed while the other is a process fluid being heated. In doing this, there is a phase change which puts special demands on the process control system. It is difficult to generalize about the various options for control. Special system requirements often put unexpected constraints on the process. Even the orientation of the exchanger can have peculiar and unexpected results.
[IMG]http://www.********.ca/ce4_sh/4-1.jpg[/IMG]A SIMPLE STEAM SPACE HEATER. Figure 4-1 shows a steam heater such as those used to heat a warehouse. This simple example demonstrates many of the characteristics of steam heaters of all sizes and applications. Steam enters the heater at the top. As the moving air draws away the heat, the steam condenses. The condensate flows down the tubes, through the steam trap, and into the condensate drain header.
The function of the steam trap is to prevent steam from blowing through into the condensate system. It is the one essential part of any steam heater and will receive further attention later. For now it is sufficient to say only that it passes condensate and blocks steam.
This system tends to be rather self-regulating. The moving air rises to some temperature approaching that of the steam and draws away as much heat as it can. Colder air will draw more heat, and warm air will draw less. The steam trap is essentially a level controller with a set point of zero.
This arrangement can be compared to a shell and tube exchanger where the room itself ...

CONTROLLING POSITIVE DISPLACEMENT PUMPS

© Walter ********, P. Eng., 2000 May 20. walter(at)********(dot)ca

First published in Hydrocarbon Processing , May 1996.


[IMG]http://www.********.ca/ce2_pdp/2-1.jpg[/IMG]
INTRODUCTION. The positive displacement pump is in some ways an even simpler device to control than the centrifugal pump discussed previously¹. It has the same function, namely to provide the pressure necessary to move a liquid at the desired rate from point A to point B of the process. Figure 2-1 shows a 'generic' process with a positive displacement pump (in this case a gear pump) connected to deliver liquid from A to B.
There is a great variety of positive displacement pumps. They are divided into two broad categories: Rotary and reciprocating. From the controls point of view, however, they are all similar. Their characteristic curve is so simple that it is rarely drawn. It is essentially a straight vertical line, as shown in Figure 2-2. (For some reason PD pump curves are usually shown with the pressure and flow axis exchanged. I will not follow that convention in this article.) All are constant flow machines whose pressure rises to whatever value is necessary to put out the flow appropriate to the pump speed. If the discharge is blocked, the pressure will rise until something yields -- preferably a relief valve. Close examination of the curve shows a slight counter clockwise rotation. This is due to internal leakage.
[IMG]http://www.********.ca/ce2_pdp/2-2.jpg[/IMG]For positive displacement pumps the major cause of leakage is the small amount of reverse flow that occurs before a check valve closes and possibly past the check valve after it is closed. Leakage past the piston is negligible. Diaphragm operated PD pumps have no cylinder to leak past. Rotating PD pumps, such as gear pumps or progressing cavity pumps have internal clearances which permit a small reverse flow, called ...