Posts Tagged ‘temperature control’

Manipulating Water Temperature to Control Cavitation

Monday, April 9th, 2018

    As we learned previously, cavitation bubbles form at a centrifugal pump’s inlet when the thermodynamic properties of water, namely temperature and pressure, are right.   Today we’ll see how just manipulating water temperature can control cavitation.

 Manipulating Water Temperature to Control Cavitation

Manipulating Water Temperature to Control Cavitation


    Some centrifugal pumps draw water from an external heat source such as a heat exchanger in order to provide heat to buildings, generate power, and perform manufacturing processes.   On some exchangers heat is applied at a fixed rate and can’t be varied.   On others heat can be varied by using a heat exchanger fitted with a temperature control.   This makes it easy to reduce or lower water temperature introduced at the pump’s inlet.   If the temperature is kept low enough relative to the pressure at the inlet, cavitation bubbles won’t form.

    Let’s say water enters the pump’s inlet from a heat exchanger at 59ºF and internal pump pressure is 0.25 pounds per square inch (PSI).   With these parameters in place water boils and cavitation bubbles will form in the pump inlet.   But if the heat exchanger is adjusted so that temperature is lowered by a mere two degrees to 57ºF, cavitation ceases.   This is in accordance with the boiling points of water, listed for various pressures and temperatures, as published in engineering thermodynamic texts.

    If it’s not possible to lower water temperature at the pump inlet, an alternate method to control cavitation is to raise water pressure, which can be accomplished in different ways.   We’ll review those options next time.

Copyright 2018 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog




Food Manufacturing Challenges – HACCP Design Principle No. 4

Sunday, November 6th, 2011
     Imagine going on a diet and not having a scale to check your progress, or going to the doctor and not having your temperature taken.  Feedback is important in our daily lives, and industry benefits by it, too.

      Generally speaking, feedback, or monitoring, is a tool that provides relevant information on a timely basis as to whether things are working as they were intended to.   It’s an indispensable tool within the food manufacturing industry.  Without it, entire plants could be erected exposing workers to injury and consumers to bacteria-laden products.  It’s just plain common sense to monitor activities all along the way, starting with the design process.  Now let’s see how monitoring is applied in HACCP Design Principle No. 4.

     Principle 4:  Establish critical control point monitoring requirements. – Monitoring activities are necessary to ensure that the critical limits established at each critical control point (CCP) established under Principle 3 discussed last week are working as intended.  In other words, if the engineer identifies significant risks in the design of a piece of food processing equipment and establishes critical limits at CCPs to eliminate the risk, then the CCPs must be monitored to see if the risk has actually been eliminated.

     Monitoring can and should be performed in food manufacturing plants by a variety of personnel, including design engineers, the manager of the engineering department, production line workers, maintenance workers, and quality control inspectors.  For example, engineering department procedures in a food manufacturing plant should require the engineering manager to monitor CCPs established by the staff during the design of food processing equipment and production lines.  Monitoring would include reviewing the design engineer’s plans, checking things like assumptions made concerning processes, calculations, material selections, and proposed physical dimensions.

     In short, monitoring should be a part of nearly every process, starting with the review of design documents, mechanical and electrical drawings, validation test data for machine prototypes, and technical specifications for mechanical and electrical components.  This monitoring would be conducted by the engineering manager during all phases of the design process and before the finished equipment is turned over to the production department to start production. 

     To illustrate, suppose the engineering manager is reviewing the logic in a programmable controller for a cooker on a production line.  She discovers a problem with the lower critical limits established by her engineer at a CCP in the design of a cooker temperature control loop.  You see, the time and temperature in the logic is sufficient to thoroughly cook smaller cuts of meat in most of the products that will be made on the line, however the larger cuts will be undercooked.  The time and temperature settings within the logic are insufficient to account for the difference.  

     This situation illustrates the fact that monitoring does no good unless feedback is provided with immediacy.  In our example, the design engineer who first established the CCP and the critical limits was not informed in a timely manner of the difference in cooking times that different size meats would require, resulting in the writing of erroneous software logic.  Fortunately, continued monitoring by the engineering manager caught the error, leading her to provide feedback about it to the design engineer, who can then make the necessary corrections to the software.

     Next week we’ll see what design engineers do with the feedback they’ve received, as seen through the eyes of HACCP Principle 5, covering the establishment of corrective actions. 

Food Manufacturing Challenges – HACCP Design Principle No. 3

Sunday, October 30th, 2011
     How do parents make life safer and healthier for their kids?  One of the ways is to  impose limits on things like roaming distance within the neighborhood, curfews, and insisting that you eat your vegetables.  Just common sense, right?  Let’s take a look at some more of it.

     Limits are also necessary within the food manufacturing industry.  Let’s take a look at Hazard Analysis and Critical Control Point (HACCP) Principle No. 3 to see how they’re established and why.

     Principle 3: Establish critical limits for each critical control point. – You can think of a critical limit as a boundary of safety for each critical control point (CCP).  So how do you determine that boundary of safety?  It’s difficult to generalize, but if you’ve ever watched the TV show Hoarders, you have an excellent example of one that has not only been breeched, but torn asunder.   

     In order to prevent things in the commercial food industry from getting anywhere near Hoarders bad, maximum and minimum values are set in place, representing safeguards to physical, biological, and chemical parameters at play within the industry.  Critical limits can be obtained from regulatory standards and guidelines, scientific literature, experimental studies, as well as information provided by consultants.  These critical limits come into play with issues as varied as machine design, raw material temperatures, and overall safe processing times.

     How could the hoarders let things get so bad?  If you listen carefully, you’ll hear bits of information that provide a clue.  They’ll say it started with a few things falling to the floor which they didn’t feel like picking up and it escalated from there. 

     Now all of us live within environments which differ as to their cleanliness, but by and large we live within space where we feel comfortable and consider to be reasonably clean.  We don’t all habitually move stoves and refrigerators to clean, for example.  But if we were so inclined, refrigerators do come with front access panels that are easily removed.  Trouble is the space they provide access to often isn’t large enough to accommodate hands and a vacuum cleaner nozzle comfortably.  You can imagine how frustrating and potentially dangerous it would be to public health to have commercial machinery that provided such limited access for cleaning.    

     To cope with this problem design engineers institute minimum and maximum parameters, such as in the critical limit dimensions of a removable cover.  Their guideline would ensure that enough space is provided so that personnel can fully access all aspects of machinery with tools for cleaning.  That same cover can also have established maximum critical limits, so that dimensions aren’t too large and heavy to be manipulated by hand.  Human nature being what it is, something that is too difficult to remove may be “forgotten” and parts of the machine may never get cleaned.

     Raw meats and many produce can contain hazards like salmonella, E. coli, and other nasty critters that are dangerous to human health.  One of the ways the commercial food industry works to ensure that these contaminants aren’t unleashed on the public is to install programmable control systems into processing machinery that essentially cooks the meat at an established minimum temperature for a minimum amount of time.  Utilizing this type of temperature control in conjunction with an established maximum cooking parameter for temperature and time will virtually eliminate the possibility of overcooked or burnt food products.  When you buy that frozen dinner in most cases it’s completely cooked, but it’s a rarity to find it’s been burned.  

     Another situation in which critical limits are utilized is in the maintenance of machinery, such as when they limit the number of hours a machine can be operated before it is shut down for servicing.

     Next week we’ll move on to Principle No. 4 and see how it establishes monitoring requirements for each CCP.  ____________________________________________

Industrial Ventilation

Sunday, March 27th, 2011
     On a hot, sticky day, what price would you pay for a cool breeze?  Imagine for a moment that it’s 90 degrees in the shade and humidity is 85%.  There are few human beings that wouldn’t consider this uncomfortable weather, although I have some die hard neighbors who rarely close their windows during the summer to engage the air conditioning, a rather recent modern convenience.  My mom told me stories of how it was common in the 1940s for entire Chicago neighborhoods to head to Lake Michigan, spread a blanket, and sleep on the beach to keep cool on the hottest nights.  As for myself, I remember being really happy when Dad broke down and finally purchased a window air unit.  It was as big as a small refrigerator and took two men to lift.  It was loud and drew so much power it frequently “blew the fuse.”  It was so much nicer when central air conditioning came along a few years later and we could finally retire that old clunker.

      Ultimately, it’s ventilation that makes air conditioning work, the principle here being a continuous circulation of air, exchanging hot for cooled.  If you’ll remember, hot air gives up its heat to coils containing coolant, and the newly cooled air is released back into the room. 

     In addition to cooling, another major function of ventilation is to remove odors and refresh the air.  Everyone likes a fresh smelling home, but even more importantly, proper ventilation reduces the concentration of contaminants in the air, things which tend to make us sick, like mold.  That’s why many states’ building codes require whole house air ventilation systems to be installed in new homes. 

     In industrial settings ventilation performs the same functions, but it’s necessary for other reasons as well.  Industrial facilities often house processes that create airborne toxins and other contaminants.  These byproducts of manufacturing can be dangerous if allowed to collect unchecked within the confines of a building.  Air containing certain concentrations of contaminants, such as vapors emitted by paints and solvents, can ignite, resulting in fire or explosion.  For safety of both workers and equipment, fresh air must displace air contaminated with fumes and dust.

     There are three types of ventilation that can be found in industrial facilities.  These include indoor air quality ventilation, dilution ventilation, and local exhaust ventilation.  Indoor air quality ventilation provides freshly heated or cooled air to buildings as part of the normal heating, ventilating and air conditioning system, much like we have in our own homes.  Dilution ventilation gets its name from the fact that it dilutes contaminated air by displacement, the blowing in of clean air and exhausting of dirty.  The last type of ventilation, local exhaust ventilation, captures contaminated emissions at or near the source and exhausts them directly outside.  Depending on the type of industrial application one, two, or all three of these ventilation types may be employed to keep air quality safe.

     Next week we’ll discuss dilution ventilation in detail, followed by local exhaust ventilation, and we’ll gain a better understanding of how they are used to protect worker health and safeguard property.


Pressurized Containers – Industrial Overpressure Devices

Sunday, October 17th, 2010

     Perhaps you went out on a drive to enjoy a nice summer day.  As you ventured into uncharted territory, you might have ended up in an industrial area.  There, you noticed factories, chemical plants, and oil refinery complexes, each surrounded by a huge system of pipes and tanks.  You might have considered it to be an eyesore, but if you’re an artist and engineer like I am, you might look at it as a form of art, composed of interesting shapes, colors, and patterns.  No matter how you look at it, you can bet that there are at least a few pressurized containers in there.

     Last time we saw how something as seemingly harmless as a home water heater could become a dangerous missile if the pressure inside builds to the point where the tank ruptures.  You can imagine what kind of explosive forces, steam, and chemicals would be unleashed into the surroundings if an industrial sized pressurized container failed due to overpressure.  Let’s explore some other types of overpressure devices that are commonly used in industrial settings.

     One type of overpressure device is a safety valve.  They are similar to a water heater relief valve, but they are generally used to relieve overpressure of gases and steam.  How do they work?  Basically, a safety valve is attached to the top of a pressurized container as shown in the cut away view in Figure 1 below.

Figure 1 – A Basic Safety Valve In The Closed Position

A powerful spring in the valve body is designed to force down on the valve and keep it closed if there is normal pressure inside the container.  Once the pressure begins to rise to an unsafe level, it pushes up against the valve and overcomes the force of the spring.  The valve opens, as shown in Figure 2 below, and the contents of the pressurized container are safely vented out to an area that is normally unoccupied by people.  In case you’re wondering, safety valves are commonly used on pressurized storage tanks and boilers.

Figure 2 – A Basic Safety Valve In The Open Position

     Another way to address the overpressure scenario is to employ a rupture disc.  This is in fact a purposely constructed weak spot.  It is intentionally built into a pressurized container and is designed so that it will fail when pressure starts to rise.  In fact, this disc is designed to fail at a pressure point just below the pressure at which the container itself would fail.  The disc is usually located within a vent pipe, which is in turn connected to the container.  Should the disc rupture in an overpressure situation, the contents of the pressurized container will safely flow out of the vent pipe to a place normally unoccupied by people.  The advantage of using a rupture disc is that they are made to safely release huge quantities of pressurized substances very quickly.  The disadvantage in their usage is that they’re a one-time fix.  That is, unlike relief or safety valves which may perform their function a multitude of times, a rupture disc is destroyed once it does its job.  They are generally used in industrial settings where potential hazards are greater than at home, so once the rupture disc blows, the complete system generally undergoes a shut down so that the disc an be replaced before the pressurized container can be used again.

     Another option to pressure containment is the use of a fusible plug, usually constructed of a metal that will melt if the temperature within a pressurized container rises above a certain level. The metal plug melts, and excess pressure is vented through the aperture formed into a safe location. These are often used on locomotive boilers and compressed gas cylinders. Like rupture discs, fusible plugs are a one-time fix and must be replaced once they have done their job.

     Yet another option to pressure containment is to use a temperature limiting control.  This category includes devices that monitor temperature and pressure within a pressurized container.  If a dangerous situation should develop, the control system reacts, effectively reducing the pressure to prevent failure of the vessel.  Automatic combustion control systems for boilers in electric utility power plants use temperature and pressure sensors to keep pressures within safe limits by regulating fuel and air input to the boiler.

     Next time we’ll cover the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC), which establishes rules governing the design, fabrication, testing, inspection, and repair of boilers and other pressurized containers.