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Recent Entries

Flow Simulation in Pump/Compressor Design

by Esam on 08-08-2011 at 08:11 AM
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

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Categories
Mechanical

Pump and pump system glossary

by Esam on 08-08-2011 at 08:00 AM
PUMP AND PUMP SYSTEM GLOSSARY

[table]
[TR]
[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.

[/TD]
[TD][/TD]
[/TR]
[/table]


[hr][/hr] [table]
[TR]
[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]

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Categories
Mechanical

Centrifugal pump system tutorial 2

by Esam on 08-08-2011 at 07:35 AM
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,

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Categories
Mechanical

Centrifugal pump system tutorial 1

by Esam on 08-08-2011 at 07:31 AM
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

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Categories
Mechanical

CONNECTING and INTERPRETING LIMIT SWITCHES

by Esam on 08-08-2011 at 07:19 AM
[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

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Instrumentations