Pressure Problems? Check Your Pipe Size First
By: Michael Camber
When it comes to compressed air piping, there are several options. Most manufacturers or process industries use black iron, galvanized, copper, aluminum or stainless. These may be threaded, brazed, welded or connected with various proprietary fittings. Generally, we recommend copper or aluminum as cost effective upgrades to black iron and galvanized. Lighter weight makes them easier to install, and their smooth inner walls reduce pressure drop. In some cases, stainless steel piping may be required to withstand water, cleaning agents or other corrosive substances in immediate proximity.
But regardless of pipe material, they all come in a variety of sizes, and a key point to understand is the relationship of pipe diameter to how much flow it can handle. It’s quite common for users to experience low pressure somewhere in their system. In some cases, this may mean there simply isn’t enough flow to meet demand and it’s time to increase system capacity with another compressor. But in many cases, new compressors are installed and the problem persists.
Pushing too much air though a pipe causes pressure drop. If your plant has grown over time and added production capacity, chances are it has acquired more compressors too. But if the piping hasn’t been upgraded, you may be suffering pressure drop caused by internal friction. You may mitigate the problem by adding storage downstream closer to points of use with large demands, but sometimes up-sizing the air lines is the only effective solution to get reliable pressure.
If you are putting in a new air system, you have the opportunity to plan for growth. It’s common for operators to allow space for future compressors as they grow, but adding compressors is far less disruptive and expensive than trying to re-pipe later when you find you cannot stuff all the air you need into the original piping. Better to bump up the pipe a size or two to allow for additional air flow that may be added later when you expand operations.
Below are two scenarios that illustrate the impact of pipe on pressure loss and how planning ahead can help.
Small system example:
You build a shop with two 10 hp units (total flow is approx. 80 cfm at 125 psig). Using 1″ diameter pipe, you will lose about 7 psi through 500 ft of piping at full flow. If you bumped it up to 1 ¼ ” that pressure drop would be less than 2 psi.
Two years later, due to business growth, you add another 10 hp. Now you have a max flow of 120 cfm. If you built the shop with the 1″ line, you would suffer 20 psi pressure drop. In the 1 ¼” line, the loss would be only about 4 psi. If you doubled the flow from 80 to 160 cfm, the pressure drop would be 30 psi in the 1″ line and only 7 psi in the 1 ¼”.
Larger system example:
You open a plant with two 100 hp compressors (total flow is approx. 900 cfm at 125 psig). Using 3″ diameter pipe, you will lose about 6 psi through 1000 ft of piping at full flow. If you bumped it up to 4″ that pressure drop would be less than 2 psi.
Two years later, due to business growth, you add another 100 hp. Now you have a max flow of 1350 cfm. If you built the shop with the 3″ line, you would suffer 15 psi pressure drop. In the 4″ line, the loss would be only about 2 psi. If you doubled the flow from 900 to 1800 cfm, the pressure drop would be 30 psi in the 3″ line and only 7 psi in the 4″.
The calculations above were based on Table 8.15 Loss of Air Pressure Due to Friction in the CAGI Handbook, Sixth Edition. If you are suffering pressure loss, check your pressure/flow combination in this chart before thinking of adding another compressor. If it seems like you have adequate piping, the next step is to find and fix leaks. Try to get leakage under control (less than 10% of demand) before adding compressors. Pressure drop through dryers and filters may compound the problem. There will always be some drop through air treatment components, but keeping up with maintenance will minimize pressure losses.
Another home remedy to overcome pressure drop is adjusting the compressor discharge pressure higher. If this is effective and necessary, by all means do it, but keep in mind that power consumption increases by 1% for every 2 psi increase. The larger the system, the larger the additional power costs from cranking up the pressure. Adding 5-10% to the 300 hp air system above could easily add $10,000-15,000 to your power bill. And in many cases it just won’t help. The pipe may simply be too small for the volume of air going through it.
Harder to calculate —but often more costly— are the scrap, lost production and downtime resulting from equipment getting low or inconsistent pressure. While those numbers vary from business to business, we’ve read the average cost of downtime for manufacturers is $23,000 per hour. Before you dismiss the need to upgrade your air piping, you owe it to yourself to do that math .
P.S. Our website has a handy pressure drop calculator!