Bernoulli's Principle Revisited

One of the most popular posts I have written deals with the question of whether Bernoulli's Principle is behind the operation of hydromechanical grease interceptors, as some have argued.

The specific point I was making in that post (Is Bernoulli's Principle Behind Grease Interceptor Performance?) is that there is no evidence that deliberately adding baffles to the interior of an interceptor (to create regions of higher and lower pressure) enhances grease separation.

Gravity differential separation works best in a laminar low-velocity flow environment.

Attempting to enhance the natural separation of liquids with differing specific gravities by incorporating regions of higher and lower pressure by means of internal baffles, doesn't even make sense.

It would be like hooking your car up to a team of horses to add some "horsepower."

I guess technically the horses add horsepower, but do you really think the car is going to go faster!

So why am I writing about Bernoulli's principle again?

Because, there is a proper application of Bernoulli's Principle in grease interceptor design.

Lets begin with a couple of basic concepts in fluid hydraulics.

Liquids are generally non-compressible, which means you cannot change the volume of a liquid by adding pressure. 

The 'conservation of mass' principle requires every bit of mass (volume of liquid) to be accounted for during a process.

Think of a water hose with a spray nozzle attached.  As you close the nozzle the stream tightens up and goes farther.

The tighter the stream the higher the velocity (the speed at which the liquid is moving).

Since neither the mass nor the volume can change under steady-flow conditions, the velocity must change to allow the mass and volume of liquid to move from the larger area in the hose through the smaller area in the nozzle.

When I was a boy, my brothers and I discovered the awesome power of the tight stream from a hose nozzle which could clean dirt and mud from almost anything.

We often tried to clean each others faces off with this same technique - which invariably led to an opportunity to sit quietly for a while to contemplate our "actions".

I took contemplate to mean ponder all the ways to blame my brothers for the trouble we were in, while plotting appropriate avenues of revenge. 

Anyway, what does this have to do with grease interceptors?

The same is true in reverse!

Think of the nozzle as being the waste piping connecting to a grease interceptor.  The volume and mass of liquid is constant but the velocity changes as the liquid moves from the pipe through the interceptor.

Good interceptor designs will take the mass and volume of water and distribute it throughout the cross-sectional area of the interceptor to reduce the velocity - the speed at which the volume is moving through the interceptor - allowing for gravity differential separation (The real difference between Gravity and Hydromechanical grease interceptors).

Now it may be easier to understand why adding baffles inside the interceptor would not make sense.

The baffles add an obstacle inside of the interceptor that the liquid must flow around.  Velocity increases as the liquid moves around the edges of the baffles and while there is an associated pressure drop, the increase in speed and turbulence more than compensate for the pressure drop making it more difficult for gravity differential separation to occur.

Proof?

If you promise not to get mad, I'll demonstrate the point by comparing several PDI certified 20 gpm units to Schier's GB-20:

Yes, the Schier GB-20 holds more water than the other units.  That's why we are looking at lbs per gallon to compare each interceptor's efficiency in capturing and storing grease.  It simply illustrates that the baffles in these PDI certified units do not enhance the interceptors performance.

The Schier unit has no internal baffles - it's wide open. It does a better job of distributing the volume of liquid throughout the units cross-sectional area which reduces velocity and enhances separation.

So now you can rest easy - feel free to even snore a bit - knowing that Bernoulli's principle does figure into good grease interceptor design - just not the way you might have thought.

Is Bernoulli's Principle Behind Grease Interceptor Performance?

When it comes to how grease interceptor's work, there are many theories espoused that are supposed to be based on principles of fluid dynamics, however, when tested under controlled conditions many of these theories are debunked.

For example, I have heard the argument that Bernoulli's principle is behind hydromechanical grease interceptor performance. On its face the argument sounds plausible. I would, however, like to test the theory with historical research and current experience from thousands of tests conducted at our plant utilizing our own ASME A112.14.3 (PDI G-101) test apparatus.

In fluid dynamics, Bernoulli's principle states that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure.

Bernoulli's principle is commonly used as a rudimentary explanation of how airplanes fly and the way 'lift' works on a wing.

It also explains how curve balls, sliders, and sinkers work in baseball.

Some argue that baffles inside a grease interceptor, which increase velocity, serve to create regions of higher pressure underneath regions of lower pressure aiding in separation efficiency as grease in the higher pressure region is forced to rise towards the region of lower pressure based on Bernoulli's principle.

What the research says
In the early 1940s, research began at the Iowa Institute of Hydraulic Research to formalize a grease interceptor testing and rating system, initially for the construction branch of the US Army Engineers. The Plumbing and Drainage Manufacturer's Association took that research and in 1949 launched the first commercial standard for grease interceptors, PDI G-101.

In 1944 a symposium of four papers was presented at the Sixteen Annual Meeting of the New York State Sewage Works Association, one of which was titled, Symposium on Grease Removal, Design and Operation of Grease Interceptors, by Francis Murray Dawson and Anton Adam Kalinske.

Frank Dawson was Director of the Iowa Institute of Hydraulic Research (1936 - 1944), Dean of the College of Engineering (1936 - 1959) and Professor of Engineering (1936 - 1959) at the University of Iowa. He had worked very closely with the military on various earlier and concurrent projects and was considered an expert on fluid dynamics as well as the hydraulics and pneumatics of plumbing drainage systems.

As a result of the research conducted by the IIHR, any kind of interceptor, before it could be installed in an Army camp kitchen, had to have a rating certificate from the Institute (Iowa Institute of Hydraulic Research, Bulletin 30, 1946).

Dawson et al, established gravity differential separation as "the basic principle of grease interception." He defined this as, "the liquid greases and fats separate from the waste water in the interceptor, when the velocity of flow is reduced, owing to the difference in specific gravity."

"Since grease separation is due to gravity differential, a quantitative analysis of what occurs as waste water flows through an interceptor may lead to the establishment of some basic design data. For simplicity let us assume that pure grease and water enter near the bottom of a rectangular-shaped interceptor L feet long, B feet wide, and with a water depth of D feet. The interceptor will do a good job of separation if, as the flow goes through the interceptor, the mean velocity of the flow is such as to permit a grease globule to rise a vertical distance D in a length of L feet. If we neglect, for the moment, the presence of turbulence we see that the controlling item in the sizing of the interceptor for any particular rate of flow is the rate of rise of the grease globules," Dawson said.

Dawson then went on to explain the application of Stokes law, providing the formula and the basic calculations which led to the determination that 150 microns was the minimum size grease globule an interceptor could reasonably be sized to capture because, "the rate of rise of globules much less than this size is so small that gravitational separation is impracticable, and globules much larger than this size will be easily separated."

Bernoulli's principle was developed by Daniel Bernoulli and published in his book Hydrodynamica in 1738. It is a well known principle in fluid dynamics, yet Dawson et al. never made mention of it applying to grease interceptors. In fact, while Dawson et al. argued there was a benefit to some kind of baffle near the inlet with louvers to distribute the flow (throughout the cross-sectional area of the interceptor) and give it a gentle upward motion, he criticized the use of baffles in the interceptor body as, "undesirable since they induce turbulence."

Any positive effect that could be argued for baffles in the application of Bernoulli's principle is necessarily offset by the negative effect of the resultant turbulence created by the baffles.

ASPEs Plumbing Engineering Design Handbook 4, Plumbing Components and Equipment, Chapter 8, Grease Interceptors, states, "The ideal separation basin is one that has no turbulence, short-circuiting or eddies. The flow through the basin is laminar and distributed uniformly throughout the basin's cross-sectional area."

Distributing the incoming flow throughout the cross-sectional area of a grease interceptor is the key to reducing forward velocity. While flow-rate measures volume, velocity measures speed and reducing the speed of the flow is critical to good interceptor design as was indicated by Dawson et al. in their research and confirmed more recently by ASPE.

Practical experience
Schier has literally run thousands of tests on interceptors, because it’s one thing to sit at a desk calculating the effects of all of the various principles of fluid dynamics on a grease interceptor, but where the leather meets the road is when you take your engineered design to the test bench.

In all of our thousands of tests, we have never found a benefit to adding internal baffles to somehow take advantage of Bernoulli's principle. Instead we experienced far worse consequences in the resulting turbulence.

By far, our greatest successes in design effectively control and distribute the entering flow throughout the interceptors cross-sectional area, while simultaneously creating a laminar flow pattern in a wide open vessel (no baffles).

The argument that Bernoulli’s principle is somehow necessary to hydromechanical grease interceptor performance is a theory that cannot be proven and has no support in any research that has ever been done going back as far as the original testing and rating protocols developed in the early 1940s.

Nothing has ever been written by PDI, ASPE or any other research body that supports the idea that Bernoulli’s principle is behind hydromechanical grease interceptor performance.

Bernoulli's principle does explain how curve balls, sliders and sinkers work though, in case you're interested!