Many times I have come across so called “rules of thumb” in delivering my training courses. And I am surprised at how often many people “blindly” accept them – almost with the same reverence as accepting the words of a legally binding standard. I do try to drive home the point that a rule of thumb is a “starting point” for doing a calculation or an assessment, but it may not be the final answer.
There are many rules of thumb, and it seems that each discipline has them. And I have developed some of my own rules of thumb for when I travel. Here are some of the rules of thumb I have come across – while I have no problems if you correct me, I would like to hear from you WHY you are correcting me.
+ For a conventional fixed steel leg offshore platform, 1 kg of equipment list weight required 6 kg of structural steel to hold it up out of the water
+ Lowering Compressor Inlet Air Temperature 5° C will result in a 2% energy savings
+ If you are thirsty, you have waited too long to drink water
+ Mains water velocities are often between 0.75 and 1 m/s to manage sediments
+ If it takes 8 minutes to fly from east to west, it will take 7 minutes to fly from west to east (due to prevailing winds at aviation altitudes).
Recently I have been delivering a lot of courses where we have been discussing velocities in in-plant piping systems, where the distance is too short for pressure drop to be significant. These pipes are often sized with the allowable velocity method, and then checked (if critical) for pressure drop.
But where did the rules of thumb originate? And what were the underlying assumptions behind them? Projects in Australia are an interesting mix of logistics. Some are within metropolitan areas, some are remote outback locations (with not access to any infrastructure except an unsealed road), and some are offshore. A rule of thumb that is based on the cost of electricity surely must not be a universal rule of thumb if electricity is low priced (metropolitan area) or high priced (remote outback location).
I have started to encourage people in my training courses to challenge the basic concepts of the rule of thumb, and not blindly use the middle value as the starting point. Once they start to do this, they have more confidence in WHY the rules of thumb can be used, and also where they should not be used. As an example, there is a rule of thumb for an acceptable velocity for a fuel gas pipe. Most fuel gas applications are based on buying gas from the gas company, but if fuel comes from landfill gas, the supply pressure is quite different, so the rule of thumb may not be valid at all.
I have received good feedback from the delegates at the end of the training sessions, but I have not received long term feedback, so I don’t know if this idea is effective. I look forward to hearing your thoughts.