| Suppose it’s lunch time and you’re really starving. All day your boss has been dumping work on your desk and you’re really busy. You checked your wallet and you only have a couple of bucks. Your favorite deli sells inexpensive sandwiches that are really good, but it’s over four blocks away and the lines are long. The only thing that’s going to work for you is to find something to eat that’s both fast and cheap, so you head for the lunchroom and get one of those nasty looking sandwiches out of the vending machine. As expected, it’s not very good. It’s downright disgusting. You toss it in the trash and end up feeling angry and disappointed as you head back to the stack of work on your desk.
So what does a disgusting vending machine sandwich have to do with engineering projects? Well, engineers are often called upon to feed a very strong appetite for the design of consumer products, industrial products, and manufacturing systems. What few people outside of the engineering profession realize is that engineering design projects operate according to three constraints: cheap, fast, and good. These constraints have been around for a very long time but they have become more critical in recent times in large part due to globalization. They are shown in the engineering project triangle in Figure 1 below.
Figure 1 – The Engineering Project Triangle
Here’s how the triangle works. Pick any two of the constraints but exclude the remaining third. For example, in our lunchtime scenario above you chose fast and cheap, and you ended up with food that wasn’t good. It works the same way in engineering design projects.
A number of years ago I was working as a project manager at a small engineering firm. One of our customers wanted us to design a consumer electronic product chocked full of really cool features. He wanted the design completed on a fast track schedule. As I worked up a quotation, I determined that if we were to design all of the features into the product on the desired tight schedule, I would have to buy expensive design tools and put a large number of engineers to work on the project. Kind of like having the whole family pitch in to clean house versus you doing it alone. The bottom line was that although it was possible to give the customer what he wanted when he wanted it, it would cost a lot of money to pull off. This meant we could only fill two of the proscribed parameters for production, fast and good, but not cheap.
In today’s fast paced, highly competitive, and minimally staffed business environment, engineers are often under a lot of pressure to somehow beat the project engineering triangle. No one wants to give up cheap, because budgets are slim and it’s extremely difficult to get more funding. No one wants to give up fast, because the marketing folks are always looking for ways to get a jump on the competition. So where does that leave “good?” Well, unless someone is going to let go of either cheap or fast, good isn’t going to happen. The end result is most often that sales and marketing end up very disappointed, dissatisfied customers proliferate, sales go down the drain, service costs go through the roof, and potential liability issues start popping up.
Moral our story? Don’t fight the engineering project triangle, work with it. Start by carefully considering the project scope and all the requirements the design must satisfy. Involve engineers in the consideration process, since they’re going to be the ones who are responsible for producing the design. They know their own capabilities and what it takes to meet your expectations. Based on engineering input, set up your budget and/or schedule to ensure that you get a good result.
Turns out that with engineering design, as with most things in life, effort-in equals result- out.
Archive for October, 2010
| Over the last few weeks we looked at the dangers associated with pressurized containers, also known as “pressure vessels.” We also looked at overpressure devices that can keep the pressure from building to the point where the vessel ruptures. But what about keeping pressure vessels from rupturing under normal operating pressure? You know, pressures well below the point where an overpressure device would kick in. This can happen if there is some sort of weakness in the pressure vessel caused by things like poor design, defective materials, or bad welds.
In the 19th Century the machines of the Industrial Revolution were driven by steam. Those magnificent machines advanced our civilization and standard of living. Sounds like a win-win situation, right? Wrong! The downside was that there were no standards for the design of pressure vessels like air storage tanks and boilers. Every engineer had their own ideas as to how they wanted to approach pressure vessel design. I use the word “engineer” loosely because most “engineers” of that time were not college graduates. Some approaches were good, some were bad, and some were in between. The end result was often not good. There were many pressure vessel leaks and explosions that damaged property, caused injury, and took lives.
By the turn of the 20th Century industrialization spread far and wide, intensifying safety concerns about pressure vessels. One deadly incident was the straw that broke the camel’s back. On March 10, 1905, the boiler failed in a shoe factory in Brockton, Massachusetts. 58 people were killed and another 117 were injured. The factory was completely destroyed. This tragedy prompted Massachusetts to form a Board of Boiler Rules to write boiler laws. Ohio followed with their own boiler laws. This was a step in the right direction, but each state law was different and a boiler that was legal in one state was illegal in another. There was no standardization between states.
In 1911 the American Society of Mechanical Engineers (ASME) formed its Boiler and Pressure Vessel Committee to address the lack of standardization. The committee’s work resulted in publication of the Boiler and Pressure Vessel Code (BPVC). In a nutshell, the BPVC establishes standardized rules governing the design, fabrication, testing, inspection, and repair of boilers and other pressurized vessels and containers. The BPVC set the standards that can be adopted by all states to minimize risk to the public.
The ASME is not a government agency, so it cannot enforce compliance with the BPVC. As a matter of fact, compliance with the BPVC by manufacturers has been completely voluntary. However, most state laws now require that pressure vessels must be certified by their manufacturers to be in compliance with the BPVC before they can be sold and put into operation. A certified pressure vessel must be permanently and conspicuously marked with the manufacturer’s name, the date built, serial number, and information about its construction and the type of use it’s designed for.
That wraps it up for our series about pressurized containers. Next time, we’ll shift gears and take a look at the project triangle and how it influences the outcome of engineering designs.
| 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.
| Have you ever come home to a basement full of water? The sinking feeling in your stomach at the moment of discovery is soon followed by a cascade of other emotions: fear, anger, and you probably had a few choice sailor’s words to round off the experience.
What’s just happened? Well, it may very well have been a water heater explosion, and the water on the floor may be just the beginning of the damage. Perhaps you even have a hole blown into the side of your house! Watch this video for excellent graphic footage of just such an explosion:
You probably didn’t realize that the water heater in your home has the potential to become a pressure vessel, and with that present all of the potential dangers that a pressure vessel presents. Remember our discussion on the Boyle-Charles Law a few weeks ago? We learned that in the fixed volume environment of a pressurized container if the temperature keeps climbing, the pressure keeps building, and the outcome of this coupling is precisely what we’re observing in the video. The water in the water heater has turned to steam, causing pressure to build in the vessel until rupture occurs.
None of us wants to come home to a basement filled with water, much less a hole blown through our house by a rocketing water heater, so how can we prevent it from happening? One answer is to have your water heater regularly serviced by a qualified plumber. The plumber would make sure that the overpressure device on the water heater tank, namely the relief valve (a.k.a. T&P valve), is in proper working order.
Now you may have noticed a circle drawn around the water heater’s relief valve in the video. As their name implies, relief valves are used to relieve pressure buildup, generally of liquids. If the pressure within the water heater reaches a certain limit, set by the heater’s manufacturer, the relief valve will automatically open to vent off the pressure. A pipe on the outlet of the valve safely directs the water and steam that is let off to the floor where it can flow down to a drain. That’s why floor drains are usually located in close proximity to water heaters.
Besides relief valves, overpressure devices come in many configurations, including: safety valves, rupture discs, fusible plugs, and temperature limiting controls. They may be used singly or jointly in order to perform the same basic function, that is, to keep the pressure within a vessel from building to the point where it may fail. Next week we’ll explore the other overpressure devices mentioned and where they are generally employed.
“Danger! Danger, Will Robinson!” What science fiction fan isn’t familiar with this warning cry, or the whirli-gigging robot that shouted it? The Lost in Space robot was a real scene stealer, able to glide effortlessly between Robinson’s spaceship, the vacuum of outer space, and the atmosphere of yet another alien planet without any ill effects. He was a machine. Humans are much more fragile, so the Robinson had to wear pressurized space suits for protection. These suits were pressurized containers for their bodies. Even the smallest leak in a space suit would prove disastrous. In outer space, the big rip in a suit would result in a sudden loss of pressure and expose the wearer’s body to vacuum. The vacuum would rapidly suck all of the oxygen out of the lungs and bloodstream and death would come quickly. On an alien planet with a poisonous atmosphere, the poisonous gases could leak into the suit through the rip. Back here on Earth, leaks in pressurized containers can be just as deadly.
A few weeks ago we discussed pressurized containers, otherwise known as pressure vessels, and how they are no stronger than their weakest point. We were also familiarized with the Boyle-Charles Law and the way it can be used to predict how heat increases the pressure of gas within a sealed pressure vessel. In today’s article we’ll discuss some safety concerns, as when a pressure vessel fails and begins to leak.
Pressurized vessels can pose a danger for various reasons. Suppose for instance that the substance leaking from it is flammable or toxic. An example would be when propane gas leaking from a storage tank mixes with air surrounding the tank. This can create an explosive mixture, readily ignited by static electricity or a nearby ignition source, such as a spark from a worker’s tool. When a toxic substance is released by a leak into an occupied area, it can be inhaled or come into contact with skin eyes, nose, and mouth, where it can enter the body and injure or kill.
It might be obvious that toxic, flammable substances can prove threatening, but it is not quite as obvious that some nontoxic, nonflammable substances can be just as dangerous. For example, a substance can be heavier than air, and as it leaks out of the pressure vessel, it will roll along the floor, sinking into adjacent low spots such as basements and tunnels. This substance would then displace the air, creating a suffocation hazard for anyone who is unlucky enough to be there.
Perhaps the most obvious source of danger posed by a pressurized vessel is when its contents rapidly and violently discharge. This scenario presents the same hazards mentioned above, coupled with dangers associated with flying objects and shock waves. For example, if a pressure vessel is not securely held in place and it fails, the rapid release of its contents could literally turn it into a missile careening out of control.
I happened to be in close proximity to one of these “unintentional flying objects” one day. The valve broke off an unsecured pressurized gas cylinder of the type that welders use. Its gas escaped with sufficient force to cause the cylinder to fly across a concrete floor and crash through a cinderblock wall. The violent and rapid release of a pressure vessel’s contents also has the potential to create shock waves strong enough to move heavy objects and people, capable of catapulting them through the air.
So it’s obvious then that the release of substances from a failed pressure vessel can lead to serious problems, but are there ways to prevent these failures from happening? Yes, there are. We’ll discuss that subject in my next installment where we will discuss, among other things, “overpressure devices.”