One third of global electricity consumption is by industrial electric motors. I just heard this statistic on a recent Pumps & Systems podcast and went in search of verification for it. Holy moly. It turns out that’s true! It’s one of these points that an up until recently outsider like myself may well have never known. How are they and other process instrumentation regulated and why does it matter? It’s important to keep asking the relevant questions and looking for answers, so let’s do it.
Piping, pumps, motors and other equipment have a critical role in global safety, security, and standards of living. This equipment is designed in adherence to rules established by standards organizations, government agencies, and trade association standards. Engineers can also employ their own project-specific specifications. This matters because when a project calls for instrumentation, it’s mandatory to know what the applicable standards are that apply to an informed project design. And there are caveats to be mindful of in this process.
In the interest of saving time, engineers will sometimes recycle codes, standards and specifications from past projects onto a new project. Brian Silowash, author of Piping Systems Manual, has seen this firsthand. It can be problematic if any regulations specified are out of date. Apart from recycled codes, projects tend to have multiple revisions. The danger in printing projects on paper is that various parties may not have the same revisions in hand. A Building Information Modeling (BIM) program can solve that problem by storing project revisions to the internet cloud, allowing all parties associated with projects access to the same revisions.
Though some effort has been made through the years to unify codes and standards, there are still many to sort through by relevant issuing associations. The first photo below shows standards issuing trade associations. My reference text shows eighteen of these associations, though there may be more nationally and internationally.
This next photo shows one page of individual standards, their issuing organizations, ID numbers and titles pertaining to valves and fittings. We should be mindful that every project is subject to a number of codes, specifications, and standards.
Codes, standards, and specifications are typically identified like this:
ACRO is the organization that developed the code, standard or specification
SPEC is an alphanumeric identifier
YR is the year of the latest revision
I recently came out of a piping and instrumentation diagram seminar session where a wastewater department standard drawing references legends, symbols and abbreviations. The photo below depicts engineer specified project specific instrument letter identification, symbol configuration, and instrument or function symbols.
While instrumentation and projects are subject to codes, standards, and specifications from many sources and there are pitfalls to avoid, the good news is that these are categorized for searches. Take care to ensure that any recycled codes are current. And save time for all parties affiliated with the project by use of a BIM cloud storage application.
Best regards and thank you for reading.
Sources: Piping Systems Manual by Brian Silowash
On the Job Site: Construction course by Jim Rogers
Merriam Webster defines affinity as: “a likeness based on relationship or causal connection.” The Affinity Laws are a heuristic governing pump sizing for one pump based on known constants for another of that same pump. Knowing of these laws helps, but really understanding to apply them in problem solving goes a long way. I’ve been holding off on writing this because the topic is one I had been struggling with. It’s important to know, so it’s worth the struggle.
I once had someone tell me, “I’m not paying you to learn all of this theory!” That’s fair enough, but know that the trade-off is between a culture of striving for a good enough versus one that’s striving for excellence. Which standard would you rather hold? If we’re honest, we can’t have it both ways. For this, I challenge you. Be honest. Hold your torches high!
- Pump performance involves relationships between performance [ie, head, shaft speed, volumetric flow rate] and power. If speed or impeller diameter are known with one pump, performance based on speed or impeller diameter change for a another of that same pump can be determined. Also, if these are known for just one pump with a VFD (variable frequency drive), which is a commonplace scenario, new H-Q (head-capacity relationship) and BHP (brake horsepower) curves with a different speed than published on a pump performance curve can be plotted.
- There are two sets of Affinity Laws, and both are based on the premise of a pump’s specific speed not being changed once its been calculated. One law holds impeller diameter constant. The other holds speed constant.
- What’s nice is that this can all be seen on a pump performance curve. These relationships are what we’re typically seeing most manufacturers including in their pump H-Q curves. The curve is designed to provide this information/knowledge as given information. What I’m writing about here is less about that knowledge, itself, and more about understanding relationships based on knowledge.
- Though you could just see it all on the curve and not bother with knowing the why and how of it, don’t cheat. Learn how it all ties together foundationally. This is like the difference between seeing a movie in just two dimensions versus in all three (seeing versus “seeing”). Wouldn’t it be more far more entertaining to have it all come to life in vivid detail – not once, but every single time?
- Here are the two sets of Affinity Laws presented in basic and practical terms:
Affinity Laws Set 1.
Holding the impeller diameter, D, constant, let’s solve for speed:
Q1/Q2 = N1/N2
- Q is capacity (flow in GPM) and N is speed (motor speed). 1 is the first baseline pump; 2 is a second of that same pump with a capacity change having affinity in predicting second pump speed based on capacity change.
What that’s saying is this:
Q1(capacity of baseline pump 1)/Q2(capacity change for pump 2) = [N1(speed of baseline pump 1)/N2(speed change for pump 2)].
Visualizing this, I see two of the same exact pumps sitting side-by-side. Each has the same unchanged impeller size inside the respective volutes. But each pump calls for a different capacity (flow in GPM). How will the flow difference in that second pump change the required motor speed, since capacity and speed are related? We’re about to find out!
Let’s solve an iterative trial and error problem with these known terms, referencing this pump curve:
Using an example pump curve, what speed is required with a full diameter impeller to make this rating: 3000 gpm @ 225′? Here, I’m referencing an ITT Goulds Model 3196 centrifugal pump performance curve, 6 x 8 – 15 at 1780 RPM. (This model has a 6″ discharge, an 8″ suction, and a 15″ full impeller diameter). Let’s try out 2200 RPM to see if it gets to the desired rating:
Holding the impeller diameter, D, constant, let’s solve for speed:
Q1/Q2 = N1/N2
- Q1 = 3000 GPM; H1 = 225 ft; N1 = 1780 RPM
- Q2 = Q1 * (N2/N1); Q2 = 3000 * (1780/2200) = 2427 gpm
- H2 = H1 * (N2/N1) squared; H2 = 225 (1780/2200) squared = 147′
This first test doesn’t work out. We’re trying for a motor speed to accommodate 3000 gpm given an untrimmed full 15″ impeller diameter. 2427 gpm isn’t a high enough flow for the full impeller to make sense on the example 1780 RPM pump curve. It’s below the 15″ full diameter curve line. Ditto for the 225 feet of head requirement. 147′ isn’t a high enough head for the full 15″ impeller size.
We tried a 2200 RPM motor speed for 3000 gpm @ 225′ and it didn’t work out. I’ll try again, this time @ 2000 RPM:
Holding the impeller diameter, D, constant, let’s try again to solve for motor speed:
Q1/Q2 = N1/N2
- Q1 = 3000 GPM; H1 = 225 ft; N2 = 1780 RPM
- Q2 = Q1 * (N2/N1); Q2 = 3000 * (1780/2000) = 2670 gpm
- H2 = H1 * (N2/N1) squared; H2 = 225 * (1780/2000) squared = 178′
Referencing the performance curve published for this pump, 2670 gpm @ 178′ does fall on the curve line for the 15″ full size impeller. (It wasn’t slow enough of a speed to be above the line.) 2000 RPM speed is the test winner! Awesome.
Q1/Q2 = N1/N2
- For this first set of Affinity Laws (that being the set with the impeller diameter held constant), it’s also true that:
H1/H2 = (N1/N2) squared
- H1(ft/hd requirement for baseline pump 1)/H2(ft/hd change for pump 2) = [N1(speed of baseline pump 1)/N2(speed change for pump 2) squared].
BHP1/BHP2 = (D1/D2) cubed
- BHP1(brake horsepower for baseline pump 1)/BHP2(brake horsepower change for pump 2) = [N1(speed of baseline pump 1)/N2(speed change for pump 2) cubed].
In summary, this first set of laws holds impeller diameter as constant. The second set of affinity laws is different. It holds speed constant in order to solve for impeller diameter trim.
Affinity Laws Set 2:
The speed is held constant. Let’s solve for impeller trim.
- Flow in GPM is the same as shaft speed (1780 RPM, for example).
- Head is shaft speed squared. (1780 * 1780 RPM).
- Power (BHP) is the cube of shaft speed (1780 * 1780 * 1780 RPM).
Q1/Q2 = D1/D2
- Q is capacity (flow in GPM) and D is impeller diameter (imp dia). 1 is the first baseline pump; 2 is a second of that same pump with an imp dia change having affinity in predicting second pump imp dia based on capacity change.
What that’s saying is this:
Q1(capacity of baseline pump 1)/Q2(capacity change for pump 2) = [D1 (imp dia of baseline pump 1)/D2(imp dia change for pump 2)].
Visualizing this scenario, I see two of the same exact pumps sitting side-by-side. Each runs on the same motor speed (1780 RPM, for example). But each pump calls for a different capacity (flow in GPM). How will the flow difference in that second pump change the impeller trim, since capacity and impeller diameter are related? Let’s do this!
In the second set of Affinity Laws (the set holding speed as the constant), it’s also true that:
- H1/H2 = (D1/D2) squared
- BHP1/BHP2 = (D1/D2) cubed
To summarize, the first set of Affinity Laws holds the impeller diameter as an unchanged constant in order to solve for motor speed required to accommodate a pump rating. In set one of the Affinity Laws, the problems can be solved by either changing capacity, feet of head, or brake horsepower to solve for speed. The second Affinity Laws set holds the speed constant in order to solve for impeller diameter trim. In set two, the problems can be solved by either changing capacity, feet of head, or brake horsepower to solve for impeller diameter trim.
To conclude, the Affinity Laws are a good rule of thumb, but can have up to a 15%-20% margin of error when solving for impeller trim. Slower motors tend to allow for greater impeller trim while following the Laws than higher specific speed motors. I personally find it interesting that capacity is equal to, while ft/head is squared and brake horsepower is cubed to solve for these variables. It’s so neat and tidy to have these variables line up for solving that way. I hope this information brings value to you. Please feel free to hold onto this for your next learning “curve” adventure!
With warm regards,
PS: Here’s my reference source: Pump Characteristics and Applications, 3rd Edition by Mike Volk.
I’m a lifelong figure skating fan. If someone were to ask me why, the first thought that comes to mind is the power and elegance in it. There is a convergence of power, speed and grace, where these coincide to create the breathtakingly captivating experience we see on the ice. If you’ve ever watched a performance, you’ll quickly see that circular motions are what make the shows. What now? You’re not into figure skating, so you say? Try it, try it, and you may! In that case, let’s get on a race track and rev up your engines. If neither figure skating nor cars are gonna do it for you, so much more is governed by circular shapes that there aren’t enough hours in the day to cover it.
- For those of us involved with water stewardship, circles are everywhere. They are in all places water transport and water treatment. I work in a pump shop. We call it rotary equipment. It’s only in these last few months that circles being in every facet of water stewardship has become a stark observation. It really came together for me in this last week, while I was taking a close look at pump stuffing box drawing. The diameter for gland, shaft, lock nuts, and bearings among other things, are all specified with diameter symbols throughout the page.
- Some people have visions of sugar plums dancing in their heads. I was looking at the page, seeing nothing but diameter symbols everywhere because each of these components is circular. We wanted to know what it would take to possibly fit a cartridge type mechanical seal into the tight clearance, possibly boring through the box to accommodate a packing-to-seal conversion.
- In tandem, I was overhearing an inquiry for a sump basin diameter. I walked out into our shop and looked at the wall of gaskets, baskets of o-rings, and shelves of couplings, pipes, fittings, impellers, and yes, at FRP basins being prepped for pump station installations. It was this unreal recognition. There are just circles, circles, and nothing but more circles all over the place. You have to leave the shop to get away from them, even for one moment! When calculating new installations and retrofits, this has implications for how it’s done.
If we know a diameter, calculating a circumference is straightforward. Here are some basic definitions:
- Radius: A straight line extending from the center of a circle to one end of the circle (diameter / 2).
- Diameter: A straight line across the center of a circle, from end-to-end (2 * radius, since a radius is half of a diameter).
- Circumference: The linear distance around the circle. (2 * pi * radius), or (pi * diameter).
- pi: A little over 3x the diameter of a circle. 3.14….
Why are there so many circles in the structures of water stewardship? It turns out that circles are the most efficient shape for handling pressure because pressure force is evenly distributed around a circumference. With other shapes, pressure forces concentrate at the corners, requiring expensive non-standard inefficient reinforcement. So, let’s say we have flow through a square. Is the velocity the same or is it slowed down? Well, if pressure is not evenly distributed, it can’t accelerate the same.
- From the above example, we know that circumference is (2 * pi * radius). For circular motion, here’s how it’s determined:
- Average speed = distance/time = (2 * pi * radius)/time. In other words, circumference is the distance we’re talking about here divided by time, which gives us the average speed.
I’ve been talking about circles in terms of their mechanics. Bacteria and other buildup such as scaling also love to hide in corners and crevices that come from shapes other than circular. This, too, means circles are a winning shape.
All of this had me curious about the larger picture above and beyond water. Circles are elegant and the most powerful of any shape to be found in the universe and beyond. They might be the shape of choice in water stewardship, but they didn’t start there. That shape is found everywhere in nature, and its use in water stewardship is simply a mirror to it. Everything from atoms to cells to the earth, planets, sun, moon, and even black holes are all circular shapes. It’s only fitting that the shape of the basic building block of life is also the best conductor to what flows through it.
That’s fascinating. And beautiful.