Part of a good compressed air assessment is the measurement of the plant pressure gradient. This can be accomplished by placing data loggers at various points in the plant along the distribution piping. If this is done, a time-based plot of the data can show where the biggest pressure loss occurs.
Often times, loggers might be placed only at the compressor room and the end of the piping. This will also capture the total pressure gradient but not identify where the loss develops. Frequently, the loss location can be accurately estimated by an experienced assessor — but sometimes this fails due to unexpected circumstances and faulty assumptions.
One recent assessment found a pressure loss of 15 to 20 psi across a long 500 foot section of 4-in. pipe carrying about 1,000 cfm. Without looking at piping loss tables, it might be easy to assume that with this significant flow, and internal corrosion, there might be a high pressure loss. But checking the tables, it was determined that this flow should have produced only 2.2 psi differential.
Placing pressure loggers at regular intervals along the pipe showed something strange: the high pressure drop was occurring along a short length of pipe located near the compressor room. Inspection of this run turned up an old orifice plate flow meter installation that had thought to have been removed. This device was installed when the air flow was about one-quarter of the current flow and was sized for lower flow. Measurements across the device confirmed the worst — there was a high restriction.
A maintenance work order has been raised to remove this offending device. Compressed air users, who are accustomed to complaining about the low pressure in this plant, are in for a treat.
The results of this exercise show the danger of assuming. Actual measurement of compressed air systems can cut straight to the facts.
