Monday, May 2, 2016


In the United States there are two agencies responsible for workplace safety: the Occupational Safety and Health Administration (OSHA) and the Mine Safety and Health Administration (MSHA). These two agencies are part of the US Department of Labor and are under the administration of the Secretary of Labor.  Generally speaking, OSHA is responsible for safety regulations covering general industry and construction while MSHA is responsible for safety covering mining and other mineral based industries. The actual distinction between the jurisdictions of the two organizations are far more complex than my simple definition. If you search for the phrase "OSHA vs. MSHA jurisdiction" you will find a long list of articles about the sometimes not so clear jurisdictional boundaries.

While the two organizations are tasked with similar responsibilities in their respective jurisdictions, they have separate rules and regulations and enforcement procedures. At times these can be remarkably similar and at other times remarkably different. In the gray areas where the lines between the jurisdictions are blurred, operators can be in compliance with the rules of one organization and find itself out of compliance with the rules of the other organization. It is very important that the operator of any facility understand which set of rules govern their operation.

Please remember that I am not an attorney or expert of the law. I am just an engineer that has worked on a fair number of production facilities (both OSHA and MSHA jurisdiction) and interacted with Environmental, Health and Safety (EH&S) personnel whose job is to make sure that the workers are safe AND the applicable regulations are being followed. The things in this blog are not legal or even formal engineering recommendations. If you run into a situation about OSHA/MSHA compliance or procedure, go to a pro. Based on what I have read, you will need a professional who understands the process, the language, and the regulations. If you try to go it alone you risk scuttling your case due to procedural errors even if you have a rock solid, no way to lose case.

Monday, April 25, 2016

Willful Violations = Prison

Over the past few weeks two high profiles cases about business executives or business owners being sentenced to prison for fatal workplace accidents.

The first case comes under the jurisdiction of OSHA and is high profile mainly because it was referenced in a couple of publications/websites dedicated to industrial hygiene and safety. According to the owner of a roofing business was sentenced to 10 months in prison after pleading guilty to charges stemming from the death of an employee. The employee died when he fell approximately 45 ft from a scaffold while performing roof repairs on a church. There were several charges resulting from the incident which included four counts of making false statements, one count of obstruction of justice, and one count of willfully violating an Occupational Safety and Health Administration regulation causing death to an employee. (According to  "A willful violation is defined as a violation in which the employer either knowingly failed to comply with a legal requirement (purposeful disregard) or acted with plain indifference to employee safety."). In this case, the willful violation was due to the owner not supplying fall protection to his employees. The other charges stemmed from false statements made during the subsequent investigation.

The second case has had national and international news coverage. According to the U.S. Department of Justice ( Don Blankenship the former Massey Energy CEO was sentenced on April 6, 2016 " a year in federal prison and ordered to pay a $250,000 fine." He was sentenced after being found guilty of conspiracy to willfully violate mine health and safety standards. The charges were the results of a catastrophic mine explosion in April 2010 at the Upper Big Branch coal mine that killed 29 miners. The one year sentence is the maximum allowed for that charge.

Blankenship was acquitted of two charges of making false statements that would have carried the chance of much longer sentences. A Massey mine supervisor was sentenced to 21 months in prison for instructing an electrician to disable a methane monitor and another executive was sentenced to 42 months for pleading guilty to charges that he helped evade surprise mine inspections. Blankenship maintains that he did not commit a crime and will appeal the convictions.

Tuesday, April 19, 2016

GIGO Revisited

Last week I ordered a few small parts from a large, well known supplier. I had been thinking about ordering these for a while but had been putting it off. Then they sent me an email coupon for free shipping and it got me off of the fence and I ordered the parts. The total of the order was around $250. The online catalog indicated that the items "Usually Ship the Same Day". After a few days I hadn't received an email telling me that the order had shipped so I got online to check the status. When I opened my order summary, I was a bit surprised with what I found:

Wow! That is a bit excessive. For that price Scotty should have been beamed the parts to me instantaneously. After a quick phone call to Customer Service the error was fixed and things are back to the way they should be. 

In this case the error was harmless. The company didn't try to charge me $10,000,000 dollars and essentially it was just a typo on a website. But what if this data field were controlling something important?

Way back I wrote about GIGO and blindly trusting the output we get from computers. This glitch illustrates how the computer systems just do what they are told to do by their programmers. If we tell them to do the wrong thing, they will gladly do it, even if it results in disaster. In computer simulations if we put in the wrong boundary conditions, use poor meshing techniques, or even stop the iterations before the solution is converged, we risk basing our designs on flawed data. Any computer simulation must be compared to some benchmark or undergo some sort of "sanity" test. Without these checks, the risk of failures increase dramatically.

Monday, April 11, 2016

Engineering Blog Disclaimer

I need to make sure that every so often I write a disclaimer about the contents of this blog. I am a registered/licensed Professional Engineer in Colorado and several other states (Colorado is my primary state). According to the laws in Colorado (the other states are virtually identical): "Registrants shall at all times recognize that their primary obligation is to protect the safety, health, property, and welfare of the public."

In order to fulfill this obligation, I spend a lot of time performing analysis, simulations, design reviews, etc. for each particular project on which I am working. I am selective on which projects I will assume "Responsible Charge" and subsequently the engineering responsibility. Nothing in this blog should be construed as engineering advice in terms of specific projects or designs. The topics I write about will give general or background information only. Due to space limitations and the goal of a blog, the concepts are necessarily simplified, generalized, and not complete. Because of the general nature of the information, it cannot be blindly used as a design basis or a "how to" manual. For a complete view, consult one of the myriad of textbooks on the subjects I write about or take classes at your local university or college.

Unless it is specifically stated otherwise, I am not in "Responsible Charge" for any project or design that may use the information I have presented as a basis for the design. Anyone reading this blog should use the information I present as a beginning point for their own investigation into a subject matter. If you do want my professional help on a particular project and you want to hire me, please visit my webpage at and contact me.

Monday, April 4, 2016

Confined Spaces

You may be asking yourself "What is a confined space?" The official definition contained in the Code of Federal Regulations (29 CFR 1910.146(a)) is:

"Confined space" means a space that: (1) Is large enough and so configured that an employee can bodily enter and perform assigned work; and (2) Has limited or restricted means for entry or exit (for example, tanks, vessels, silos, storage bins, hoppers, vaults, and pits are spaces that may have limited means of entry.); and (3) Is not designed for continuous employee occupancy.

Confined spaces are divided into two types: Permit confined spaces and Non-Permit confined spaces. The definitions of these are also given in 29 CFD 1910.146(a):

"Non-permit confined space" means a confined space that does not contain or, with respect to atmospheric hazards, have the potential to contain any hazard capable of causing death or serious physical harm. 

"Permit-required confined space (permit space)" means a confined space that has one or more of the following characteristics: 

(1) Contains or has a potential to contain a hazardous atmosphere; 

(2) Contains a material that has the potential for engulfing an entrant; 

(3) Has an internal configuration such that an entrant could be trapped or asphyxiated by inwardly converging walls or by a floor which slopes downward and tapers to a smaller cross-section; or 

(4) Contains any other recognized serious safety or health hazard. 

Basically, if there are hazards or potential hazards within a room or area that meets the definition of "Confined Space" then it is a Permit Required Space. It is an employers duty to examine their workplace and determine if confined spaces exist and, if they do exist, evaluate each them and determine if they are "Permit" or "Non-Permit" spaces.

If permit spaces exist and the employer decides the employees WILL NOT enter the Permit space, then it is the employers responsibility to:
1. Warn employees of the danger with signs or "equally effective" means. (The employer must tell the employees that the dangers exist.)

2. Take effective measures to prevent employees from entering the permit spaces. (It is not enough to say "Don't go in there".)
3. Reevaluate any confined space if the use or configuration changes. (This is to make sure that a space isn't mistakenly classified as a "Non-Permit" space.)
4. Inform the contractor of the danger and ensure that any contractor employees that enter a space follow a permit program that is in accordance with 29 CFR 1910.

If permit spaces exist and the employer decides that employees will enter the permit spaces, then it is the employer's responsibility to develop AND implement a written Permit Space Plan in accordance to 29 CFR 1910. This plan must be made available for inspection by employees. Without going into any details about a written plan, it basically is a plan that describes;

1. How the hazards will be removed from the space.
2. How the workers will be protected from the hazards that exist within the space.
3, How injured or incapacitated workers will be removed from the space without exposing other workers to risk.
4. What emergency and/or medical equipment will be supplied.
5. Identifies the number of people that must be involved in any entry into a permit space and outlines the responsibilities of each team member.

The specifics of each plan are left to individual employer to develop since each work site is different and has different hazards.

There are a few places around the home that would qualify as confined spaces such as crawl spaces or attics. These two areas are large enough to enter and "do work", they are not designed for continuous occupancy and they typically have limited or restricted means of entry. Generally speaking, most crawl spaces and attics would qualify as "Non-permit confined spaces". I used to live in a house whose furnace was located in the crawl space that had so many Black Widow and Brown Recluse spiders that I think it met condition (4) above and actually should have been a Permit Space! In all seriousness, since the furnace was located in the crawl space, there was potential for a buildup of carbon monoxide (CO) within the space and so it would meet characteristic (1) listed above. If a furnace technician were to enter the crawl space to work on the furnace, it could qualify as a Permit Space and the technician would need to follow a procedure to ensure that the atmosphere was not hazardous and the other elements contained within a Permit Space Entry plan.

Monday, March 28, 2016

Flow Induced Vibration Part 2: Using CFD to find the forcing frequencies

Combining CFD with FEA can provide a powerful method to investigate flow induced vibration. In Part 1 of this post, the use of FEA to find the natural frequency of the spray lance was discussed. In this post, the use of CFD to find the frequencies found in the velocity fluctuations as the fluid flows across the spray lance.

The spray lance shown in the previous post was designed to be inserted into a vertical column with a turbulent airflow. An important question that had to be answered was whether the flow across the spray lance would induce vibrations within the spray lance that would lead to vibration or even to failure. Since the system was in the design phase, it was necessary to use computer simulations to investigate the expected performance of the system. A transient CFD analysis was performed to estimate the velocities that would be found within the system during normal operation.

The plot below shows the fluctuation in the average velocity along the length the spray lance as determined from the transient CFD analysis. This plot shows that significant fluctuations in the velocity can be expected and, as a result, fluctuations in the forces acting on the spray lance. 

A Fourier analysis of this velocity signal yielded a frequency spectrum for this velocity profile as shown in the figure below. This plot shows that the primary frequencies of the velocity are found below 10 Hz, with the two main frequencies at approximately 2-4 Hz.

This second look shows that there are no strong forcing frequencies found at the first or second natural frequencies of the spray lance. Like the results from the velocity analysis, these results also suggest that the configuration is unlikely to experience vibration due to the flow across the spray lance.

The method outlined in these two posts is a simple approach to looking at flow induced vibration. A complete analysis is far more complicated and takes more factors into account. This method however can be used as a screening tool to find potential problems. For example, if the velocity were at or near the critical velocity or if the frequencies in the velocity were found to be near the natural frequency of the spray lance, it would indicate that there was a strong possibility that excessive vibration would exist when the system was started. In this were indeed the case, more investigation or a redesign of the spray lance would be required.

Monday, March 21, 2016

Flow Induced Vibration Part 1: Using FEA to determine the natural frequency

In a recent project I had to design a spray lance to be inserted into a large vertical column. In an early design iteration, it was necessary to determine if the spray lance would suffer from flow induced vibration once the system was started. Since the system did not exist and there was no data about how it actually worked, a computer study of the system was performed to investigate the possibility of flow induced vibration. 

One of the useful tools in most FEA packages is the ability to determine the natural frequency of a part very quickly. Using FEA to determine the natural frequency rather than a using a hand calculation can help reduce the time and effort required to study the dynamics of a system. This is especially true if a part undergoes many design changes and the FEA package is linked to or part of the CAD package.

Knowing the natural frequency of a part is important when dealing with turbulent fluid flow or rotating machinery because if the natural frequency of a part happens to be close to the frequency of some forcing function that exists within a system the part is likely to fail at loads far below what a static analysis predicts. This occurs due to the fact that if the forcing function matches the natural frequency, the amplitude of the deflection due to the forcing function is magnified with each cycle, eventually leading to failure. This behavior is called resonance and in almost all engineering cases, is a very bad thing.

In the video example above, the first five calculated natural frequencies for the spray lance are 12.7 Hz, 14.7 Hz, 79.1 Hz, 90.4 Hz, and 218.4 Hz. 

As fluids flow across cylinders and other shapes, they can create a regular vortex pattern downstream of the shape. The frequency of these vortices can be expressed using the Strouhal Number which varies primarily as a function of shape and Reynolds number. In this case, the average fluid properties and velocity were used to calculate the Reynolds number across the spray lance to see if it falls within the zone (below Re<300,000) where vortex shedding occurs. In this case, the Reynolds number is approximately 3,600 which is in the vortex shedding zone. At this Reynolds number, the Strouhal Number is approximately 0.21. As a result, the critical velocity where the frequency of the vortex shedding equals the first two natural frequencies are 15.1 ft/s for the first natural frequency and 17.5 ft/s for the second natural frequency. The average velocity along the spray lance was found to be approximately 4.6 ft/s. At this velocity, the frequency of vortex shedding is approximately 3.9 Hz, which is well below the first natural frequency of 12.7 Hz. The fact that the average velocity is well below the first critical velocity suggests that the likelihood for vibration due to the vortex shedding is low.

Monday, March 14, 2016

Part 2: What is the basis for using ACH as a design parameter?

In a previous post I examined how the concentration of a pollutant decreased over time as a function of different ventilation rates. This examination was limited to the case where the pollutant was at some fixed level in a space and then ventilation was introduced into that space. An example of this situation would be a pollutant leaking from a pipe into a closed room and then a valve being closed which stops the flow of the pollutant and then a fan being turned on to provide ventilation to the space. While this scenario is possible, it is probably more useful to examine the case where a pollutant is being emitted at some rate and ventilation is being supplied to attempt control the level of that pollutant. For simplicity, it will be assumed that the initial concentration of the pollutant is zero. With this simplifying assumption, this case can be modeled using a relatively simple differential equation:

where   C(t) = Concentration at time t
G = Generation rate of pollutant
Q = Ventilation rate
V = Volume of the space
Dt = Change in time

This equation comes from ACGIH's book Industrial Ventilation A Manual of Recommended Practice. As with the previous case, the units for each of the parameters must be consistent. If G is given in CFM, then the time will be minutes and the volume will need to be given in cubic feet. So what impact does changing the ACH have upon the concentration of the pollutant in a space? For this example, it is assumed that the rate of pollutant generation is 1 CFM (0.5 L/s) in a room with a volume of 10,000 cubic feet (283 cubic meters), a space roughly 29’ wide x 29’ long x 12’ tall (8.8 m x 8.8 m x 3.7 m).

As in the previous case, the ACH has a dramatic impact upon the final concentration of the pollutant in the room. At small values for ACH the concentration of a pollutant increases for quite some time until a steady state concentration is reached. For example with an ACH = 0.5 the concentration continues to increase for about 10 hours until the final concentration of 12,000 ppm (1.2% by volume) is reached. Contrast this with an ACH = 4 where the final concentration of 1,500 ppm (0.15%) is reached after an hour. As ACH increases, the final steady state concentration decreases. This chart suggests that the ventilation rate can be used to control the final concentration of pollutants in a space.

It can be reasonably concluded that using the ACH as a design parameter for a ventilation system has merit. However, it is necessary to again mention that several simplifying assumptions were made in the previous analysis which can have a dramatic effect upon the performance graphs presented here. The limitations of this method will be examined in an upcoming post.

Tuesday, March 8, 2016

AIHce 2016: Understanding and Using ANSI/AIHA/ASSE Z9.2-2012

See the main conference and expo website here.

Keith D. Robinson, P.E. will be teaching a course entitled PDC 109:  Understanding and Using ANSI/AIHA/ASSE Z9.2-2012 at the upcoming AIHce conference. This course provides an in-depth look at the requirements for Local Exhaust Ventilation (LEV) systems that are set forth in this standard. It is intended for Environment Health & Safety (EH&S) personnel, facility managers, system operators, and engineers. 

Please visit my main website at for more information about Keith D. Robinson, P.E.

The System Curve, The Fan Curve, and the Operating Point Lunch and Learn

I am pleased to announce that I will be presenting a complimentary Lunch and Learn session on April 21, 2016 entitled "The System Curve, The Fan Curve, and the Operating Point". During this session, the theoretical background for how a fan and duct system interact will be presented. Following this brief theoretical introduction participants will take flow and pressure measurements on a small duct and fan system to develop the system curve, the fan curve, and the operating point for the system. The skills developed during this session can be used by the participants to determine the performance of the industrial ventilation systems at their individual facilities. A boxed lunch will be provided.

The Lunch and Learn will be held at 12303 Airport Way, Suite 200 in Broomfield, CO. Space is limited to 10 participants. Please contact Keith Robinson at or at 303-746-8904 to reserve your spot.

Please visit my main website at for more information about this Lunch and Learn or Keith D. Robinson, P.E.

Monday, February 29, 2016

What is the basis for using ACH as a design parameter?

So why is ACH used as a design parameter? The basis for this comes from the differential equations that describe concentration buildup and concentration decay. As a first step, let's examine the concentration decay due to purging. In its basic form, the concentration decay equation is of the following form:

where C(t) = Concentration at time t.
C0 = Concentration at time 0
Q  = Ventilation rate
V  = Volume of the space
Dt  = Change in time

In the equation, the units need to be in some consistent format. For example, if the time is given in minutes, the ventilation rate needs to be given in terms of volumes per minute. At the same time, the volumetric units used in the ventilation rate must match the units used to define the space volume such as cubic feet or cubic meters. For example, if the volume of the room is given in cubic meters (m3) and the time is given in hours, the ventilation rate must be given in terms of cubic meters per hour (m3/hr). When the value of Q is given in terms of cubic feet per hour or cubic meters per hour, the value of Q/V in the exponent is the ACH.

So how does changing the ACH effect the concentration of a pollutant in a space? Figure 1 shows how the concentration of a substance declines based on different ACH. The abscissa of this graph is change in time while the ordinate is the ratio of the concentration at a given time to the initial concentration. While there were several simplifying assumptions made in the development of this graph, it shows how increasing the ACH decreases the time required to reduce a concentration of a pollutant. 

Based on this simple analysis, it can be stated that using ACH as a design parameter does have a sound basis. However, it must be recognized that it is a very simplified approach. In an upcoming post I will examine how the ventilation rate affects the concentration of a pollutant that is being emitted into a space.

Thursday, February 11, 2016

Commonly Used Acronyms

It is customary to not use an acronym without first defining the acronym. To be proper the acronym should  be spelled out in each post. I am going to take a shortcut and define commonly used acronyms in this post. In subsequent posts I will use just the acronym to save space. My apologies to all the style manuals out there.

CFD = Computational Fluid Dynamics
FEA = Finite Element Analysis
STP = Standard Temperature and Pressure

AIHA = American Industrial Hygiene Association
ANSI = American National Standards Institute
ASHRAE = American Society of Heating Refrigeration and Air-Conditioning Engineers
ASME = American Society of Mechanical Engineers
ASSE = American Society of Safety Engineers
NFPA = National Fire Protection Association

acfm = Actual cubic feet per minute
ACH  = Air Changes per Hour
fpm = Feet per minute
gpm = Gallons per minute
HP = Horsepower
in w.g. = Inches of water gauge
psia = Pounds per square inch absolute
psig = Pounds per square inch gauge
rpm = revolutions per minute
scfm = Standard cubic feet per minute

More will be added as the need arises.

Monday, February 8, 2016

Understanding and Using ANSI/AIHA/ASSE Z9.2-2012

Keith D. Robinson, P.E. will be teaching a course entitled PDC 109:  Understanding and Using ANSI/AIHA/ASSE Z9.2-2012 at the upcoming AIHce conference. This course provides an in-depth look at the requirements for Local Exhaust Ventilation (LEV) systems that are set forth in this standard. It is intended for Environment Health & Safety (EH&S) personnel, facility managers, system operators, and engineers. 

Please visit my main website at for more information.

Is Air Changes per Hour (ACH) a valid ventilation design parameter?

One question discussed at the recent ASHRAE Winter Meeting in Orlando was whether or not Air Changes per Hour, commonly referred to as ACH, represents a valid design criterion for ventilation systems. There are several design standards that prescribe minimum ACH rates to keep contaminant levels at acceptable levels. In general, it is considered that the best way to decrease exposure to chemicals and pollutants in the indoor environment is to increase the ACH. However some research now suggests that increasing the ACH actually increases exposure rather than decreases exposure. Increasing ACH also increases the energy usage of the system due to fan energy and the need to heat or cool the incoming makeup air. So is this a valid method to use when designing ventilation systems?
The answer is an unqualified "Maybe." Each system and situation is different. Some situations, such as labs with fume hoods, need relatively still air to prevent recirculation of pollutants form entering the operator's breathing zone. Other situations, such as machinery rooms, need large amounts of air to evacuate potentially hazardous gasses if a leak occurs. Suffice it to say that it takes sound engineering judgment in cooperation with the Environmental Health and Safety (EH&S) department during the design phase to ensure that the correct design criteria are used for each specific system.

Please visit my website at to learn more about Keith D. Robinson, P.E.