Hosing down your losses

Almost every compressed air system uses flex hose to make the final connection to production machinery. Proper selection and application of this air hose and the quick disconnects is critical to achieving optimum performance.

Compressed air system audits often uncover significant opportunity for savings at such locations. Typically, total system pressure is unnecessarily high to offset pressure drops in small-diameter hose and incorrect quick disconnects.

The most important sizing data for any process is the air flow and minimum pressure required at the tool entry. If you don’t know these data, it’s easy for system analysts to measure them on-site. In areas where the pressure or flow are critical to productivity or quality, economical mass flowmeters and pressure gauges can be rigged for continuous machine monitoring.

Working by hand

Air-driven tools can illustrate the effect of hose and connector selection on productivity and quality. Most air tools are designed for a hose feed pressure of 90 psig. The tool designer really sizes for full flow at about 80 psig for optimum performance. Depending on the tool, pressure significantly higher than 90 psig may not increase performance, but lower pressure certainly will reduce it. In many cases, out-of-range air pressure can damage tools and reduce the time between rebuilds.

Standard impact tools, screwdrivers, grinders, chippers and banders prefer a constant 80 psig to 90 psig inlet pressure. The phrase “at rest pressure” has no meaning. Table 1, abstracted from selected air tool technical data sheets, clearly shows the general magnitude of performance loss at low pressure. At 70 psig, most tools will still operate, but below rating. At 60 psig, performance is seriously degraded and probably will be unacceptable. Operating below 60 psig isn’t really a viable option. However, unless specifically stated, no tool is designed for inlet pressure greater than 100 psig. Table 2 shows the approximate performance losses at various inlet pressures in 1-hp to 3-hp vane motor grinders and sanders. The power drops may preclude effective job performance. Along with the loss in power, which is most important, there’s also a loss in speed. Both factors affect productivity.

Performance data for air-operated tools

Inlet air press (psig)		1/2 hp	3/4 hp	1 hp	1.5 hp	2 hp	3 hp
60	rpm at max load	8,500	5,809	3,810	5,550	3,730	3,900
	Max hp	0.35	0.47	0.765	0.927	1.74	2.32
	scfm at max hp	20	20.1	27.5	30	51	67
	Max torque ft/lb	0.36	0.88	1.67	1.67	3.7	5.0
70	rpm at max load	9,000	6,184	4,060	5,900	3,975	4,160
	Max hp	0.41	0.58	0.95	1.15	2.16	2.88
	scfm at max hp	21	53	32	35	60	78
	Max torque ft/lb	0.42	1.0	1.95	1.95	4.3	5.8
80	rpm at max load	9,500	6,429	4,250	6,190	4,160	4,350
	Max hp	0.5	0.69	1.13	1.38	2.58	3.44
	scfm at max hp	22	27	36	40	68	89
	Max torque ft/lb	0.5	1.2	2.2	2.2	4.9	6.7
90	rpm at max load	10,000	6,700	4,400	6,400	4,300	4,500
	Max hp	0.6	0.8	1.4	1.5	3.0	4.0
	scfm at max hp	24	30	39	42	76	100
	Max at torque ft/lb	0.55	1.3	2.5	2.5	5.5	7.5
100	rpm at max load	10,500	6,888	4,520	6,580	4,415	4,630
	Max hp	0.6	0.9	1.5	1.8	3.4	4.6
	scfm at max hp	26	33	45	50	85	111
	Max torque at ft/lb	0.6	1.4	2.8	2.8	6.1	8.3

Beware of 3/8-inch. hose

Never select air hose unless you know the air flow and hose length the tool requires. The most common hose sizes for plant use range from 3/8 inch to 3/4 inch and handle 300 psig. Hose choice is often left to the operator, who usually wants 3/8-inch hose, regardless of application, because:

• 3/8-inch hose appears to be the lightest and easiest to handle.
• A 50-foot length of 3/8-inch or 1/2-inch hose weighs about 13 pounds, depending on grade but a 50-foot length of 3/4-inch hose weighs 22 punds.
• The operator might not be trained regarding the hose size required to run the tool.

A 3/8-inch hose isn’t a viable supply hose for industrial tools. The smallest size you should use is 1/2-inch. Table 3 refers to premium black industrial air hose. The data leads us to specific conclusions:

• 1/2-inch hose in 50 foot lengths is suitable only for 1 hp or smaller tools (approximately 30 cfm/hp).
• 3/4-inch hose is acceptable for 2 hp to 3 hp (60 scfm to 90 scfm), depending on the length of run.
• For runs greater than 50 foot, use larger hose or pipe, supported on the walls or ground as required, to eliminate pressure drop.
• For more comfort and easier operation, adding an 8 foot to 10 foot whip hose to the larger 3/4 inch or 1 inch main line will have minimal effect on performance, but still gives the operator the feel of a lighter hose.
• Don’t use any more hose than necessary. Coiling the extra just adds pressure drop. Cut the hose to the proper length and install fittings.

Don’t forget about OSHA safety requirements. Going from a 3/8-inch to 1/2-inch hose still allows personnel to handle smaller hose without the mandatory automatic air shutoff valve or safety velocity fuse. These fuses are an excellent safety device when applied correctly. Refer to U.S. Department of Labor, Occupational Safety and Health Administration – Power Operated Tools 1926.302, page 2, paragraph 1926.302(b)(7), which mandates a safety velocity fuse on all hoses larger than 1/2 inch inside diameter.

A real-world example

More often than not, a process requires some minimum pressure. Trace these so-called requirements to their origin to determine if they are actual OEM specifications or simply an operator’s perception.
A recent client was running the plant headers at 100 psig to 110 psig because a critical hand tool grinding process was believed to require 98 psig to run correctly. Therefore, they reasoned, the system should run at 98 psig or more.

When you hear things like this, dig for more information. If the system header pressure falls below 98 psig, the grinders probably don’t work well. Production personnel probably don’t know the actual pressure at the tool or how much air the tool uses. The rest of the plant could have run at 80 psig, but it operated at 98 psig because the grinding area supposedly required it. Grinding accounted for only 20% of the demand, so 80% of the plant was supplied with air at a much higher pressure than needed. We didn’t calculate how much the higher pressure was costing, but intuition says it amounts to thousands of dollars a year.

Testing with a needle gauge at full operation revealed that the actual inlet pressure to the tool was 63 psig at load, but the header pressure stayed at 98 psig. In other words, there was a 35-psig pressure drop between the header and each grinder. Further testing revealed that the grinders only needed 75 psig for optimum performance.

The operators argued that they found the recommended 3/4-inch hose to be too heavy, so they used 3/8-inch hose instead. The smaller hose restricted the air flow, which produced a substantial pressure drop. Furthermore, the 3/8-inch hose used standard quick disconnects, which added their own 23-psi pressure drop.

We changed the standard 3/8-inch quick disconnects to industrial quick disconnects costing only $2.50 per pair — a whopping $5 per station — to reduce the pressure drop to 5 psig. Then, we replaced the 3/8-inch hose with 1-inch pipe routed to the base of the work stations at a cost of $30 each. Next, we installed a regulator that delivered full flow to the grinders at 75 psig with 80-psig feed pressure. Finally, we reduced the header pressure to 85 psig. About 18 months later, grinder repair costs had decreased and production throughput increased by 30% with the addition of more equipment. The cost of materials to implement these changes was $1,362 for nine grinders. Even with the production increase and new equipment, the average total air demand fell from 1,600 to 1,400 cfm.

The key to this success was monitoring the workstation inlet pressure while simultaneously monitoring header pressure. If the header pressure stays steady, and the process inlet pressure falls, then the restriction is in the feed line from the header to the process.

Break down the connection

This case study demonstrates that small hose represented only 12 psid while the quick disconnects represented 23 psid. Often, but not always, a quick disconnect is the best answer for overall productivity. But, size the quick disconnect for the maximum expected flow and the allowable pressure loss. Read the manufacturer’s performance data sheet.

• Never select by connection size — select by acceptable performance at specified flow and entry pressure.
• If you want to use the same quick disconnect everywhere for flexibility — do it. But, size them for the single largest flow demand at the lowest expected pressure.
• Remember that each feed has at least two quick disconnects.
• Use quick disconnects that shut off the flow when disconnected to eliminate potential hose whipping.
• Consider ISO 4414 exhaust-type quick disconnects that bleed off the air trapped inside the connection to eliminate blasting compressed air onto the operator at disconnect. It’s easier to uncouple a depressurized fitting.
• Quick disconnects should have proper safety latches, wires and keepers or be of a design that won’t open when dragged over the ground, floor or machinery.

Seek tested performance curves

Don’t assume that because couplers appear similar the performance is similar. Review the performance curves or, even better, measure the pressure loss at specific flows. On a recent audit to help select the proper disconnect for a major tool operation, we tested the pressure drop on two specific types of quick disconnect  — a lock-ring coupler with a ball-check nipple versus an exhaust-type coupler with a standard nipple. Both had 1 1/4-inch diameter coupler bodies and ports sizes of 3/8-inch, 1/2-inch and 3/4-inch The 3/8-inch nipple on the lock-ring type coupler didn’t have a ball check to shut off the air. The 1/2-inch and 3/4-inch units did. The exhaust-type couplers had the full shutoff and exhaust to allow disconnect at zero pressure.

Figures 1, 2 and 3 are the three sets of performance curves that reflect the measured pressured drop of each quick disconnect at various flows and inlet pressures. The results will probably vary by manufacturer. The key is to optimize performance by investigating.