Friction Smiction….. Water pressure and friction loss, a big waste of money…..
Over the last year or so we have been focusing on both specific products and inputs to systems. Our hope is to bring out some of the lesser known details on how things work. An important detail that is often ignored is pipe sizing. In many cases, the end user will choose to use the same size pipe as the inlet or outlet of the pump or blower that they are installing. The thought process is often along the lines of “well if it needed a bigger pipe, wouldn’t the manufacturer make the inlet or outlet bigger?” The answer is generally no.
The manufacturer chooses the inlet and outlet sizing based off their most common application for the equipment. Suffice it to say, aquaculture is not the first thing on the list of common applications for most manufacturers. There are exceptions, such as where the equipment is made specifically for our industry.
Why does pipe size matter? If the pipe is undersized, the pressure to push the air or water through the pipe is increased. Higher pressure requires more energy, more energy requires more work; which equals more power consumption. If the piping is oversized, there can be unnecessary material cost as well as waste accumulation. When designing an aquaculture system, there are several points along the way where decisions must be made in order to avoid causing long term expenses.
We’ll break this down by air and water to make it easier. We’ll further breakdown the water component by inlet and outlet, pressure and gravity. They all have factors that need to be understood.
The two most common air supply equipment pieces are compressors and regenerative blowers. Compressors supply low volumes of air at high pressures while regenerative blowers supply high volumes of air at low pressures. All air equipment should have at least a particulate filter on the inlet side to avoid contamination of your system. If the intake is located remotely, as is sometimes done for algae culture and other temperature sensitive applications, the vacuum calculations will need to be used to avoid starving the equipment.
On compressor systems, it is important to avoid oversizing delivery lines as this can overwork the compressor which can lead to premature failure.
Compressor lines, if possible, should not be made of PVC pipe since it can explode under too much pressure and many compressors can exceed the ratings on PVC. It’s best to use copper, galvanized pipe or even heavy walled vinyl tubing. If PVC must be used, it can be used only if the maximum pressure of the compressor doesn’t exceed the rating on the pipe. For instance, rotary vane compressors that we see in the industry max out at 15-20 psi which is well below the pipe rating. Compressors need some back pressure to operate correctly.
These are the most common air delivery systems in aquaculture. Blowers are often built with an undersized outlet since in most industrial applications, the blower is attached directly to the equipment where it is required. Unless the overall pipe run is really short, the pipe will need to be upsized. One of my favorite and easy demonstrations that Bob Heideman used back in the day; was to compare blowing through a coffee stirrer and a drinking straw. This demonstration showed us how much easier it is to blow through the drinking straw due to having less friction. Regenerative Blowers need to operate with the least amount of backpressure as possible, ½ psi-2.5psi. High-pressure Regenerative Blowers can operate as high as 5-6 psi.
Another thing to keep in mind is that undersized delivery pipe can overheat the PVC and cause warping and fittings to separate as well as bearing failure in the motor.
When sizing water piping systems, it is necessary to take into consideration the velocity of the water. This is true on both the intake side of the pump and the outlet. An oversized inlet pipe can cause the pump to cavitate during priming and is easier to lose prime. An undersized inlet pipe can starve the pump and cause the pump to create a vacuum and cavitate which can lead to premature failure.
On the outlet side of the pump, an undersized piping system can lead to excessive pressure and velocities. Remember that adds to the operating budget without any payback. Velocities over 8 feet per second over long periods of time can cause wear on equipment internals, plumbing parts, gaskets, and valves. All of this puts undue stress on the system which is hard to see until there is a catastrophic failure.
The only real major danger in oversizing the pressure side of the plumbing is wasting capital investment on the actual supplies needed to complete the project. This occurrence is very rare since most companies are working with tight budgets and try to use the smallest pipe diameter possible.
We’ve covered some of the dangers of incorrect pipe sizing, and now it is time to review pipe friction loss calculations. There are two common equations used to calculate friction loss. They are the Darcy-Weisbach Pressure and Major Head Loss Equation or the Hazen-Williams Equation. Both require coefficients and some relatively intricate calculations. Another option is to use charts and graphs to estimate the friction loss. Of course, the most accurate option is to hire an engineer to do the calculations which are not always possible.
In order to calculate friction loss for either water or air, a general schematic needs to be sketched, this will help determine the possible layout of the piping and allow the total number of fittings and linear pipe runs. This can be as simple as a napkin sketch or as detailed as construction level engineered drawings. It’s important to note that when building large systems, it is always worth the money to pay a good, experienced “in our application,” engineer. Eye-balling 1000 gpm pipe runs is not recommended at all.
Once the list of the total number of fittings and total linear pipe runs is compiled, the calculations begin. To start, the total number of fittings needs to be converted into the linear pipe. The chart in Table 1 shows the equivalent lengths for some of the typical threaded fittings up to 4” PVC. Keep in mind there are several versions of this type of chart available for all kinds of pipe and fittings. Most manufacturers also have them available as well.
Table 1: Equivalent Lengths for PVC Fittings
Now that we have the total equivalent length of pipe and the total linear feet of straight pipe we can use the sum of those two to determine the friction loss in different size pipes using Table 2.
Table 2: Water Flow – Pressure Loss Chart
As you can see these calculations are a little cumbersome but well worth your time to do them. Saving on electricity, operating expenses, capital expenses and keeping your pump and equipment safe is always worth a little homework.
One last thing to mention is the pipe sizing for gravity drains, basically the piping between the tank and any collection basins or sumps. In this application, the pipe size must be determined using two factors. One is minimizing friction loss which adds to the overall freeboard that is required in the culture tank and the settling velocity of the waste stream. This is different for each species and must be accounted for or there will be waste build up in the piping system which can cause all sorts of trouble. The last thing we want to have is waste breaking down in the system and adding the potential for pathogen production.
Table 3: Pressure and Flow in Various Pipe Sizes
Table 4 is a good way to see how much pressure is needed to overcome undersized piping system. Excessive pressures are an energy sink that only reduces the profitability of the system output.
At the end of the day, balancing economical solutions and efficiency is the crux of most facility’s problems. High-pressure plastic pumps are inexpensive and common, but they tend to be shoehorned into the application to save capital. Meanwhile, operating costs will continue to rob the system of profit. Finding the balance is key.