Plant operators rely on plant air pressures to maintain properly working equipment and systems. One common problem affecting processing plants that use compressed air systems is the ability to maintain reliable pressure throughout the distribution system.
Insufficient air pressures may be due to undersized distribution piping, but more commonly the causes are flow restrictions in the distribution system such as filters, check valves and fittings, as well as flow meters and gauges.
Leaks in the distribution system actually rank low as the common cause of pressure drop--the reduction of pressure from the compressor discharge to the actual point of use. The system will appear as if there is a lack of air pressure but the real problem is pressure drop, which will cause excessive energy consumption. This article explains the causes of pressure drop, a variety of solutions and ultimately how to eliminate costly downtime.
Unfortunately, the solution is not as easy as just cranking up the air pressure at the delivery point, as this will cause other issues such as leaks, excessive component wear and maintenance issues. Every pound per square inch (psi) of excess operating pressure increases air compressor power consumption by about 0.5%. As can be seen from the pressure drop formula (Figure 1), increasing the system pressure will actually increase the pressure drop. Unregulated equipment on the system will increase demand and increase inefficiency of the system.
Minimizing pressure drop can be obtained by selecting equipment such as air treatment equipment, filters, dryers, flow metering and instrumentation with the lowest amount of pressure drop. Additional ways to decrease pressure drop include maintaining air filtering and air treatment to reduce moisture that can create corrosion and increase friction on the piping system. Also, reduce the distance air travels through the distribution system to create the shortest runs possible to reduce loss.
Select the correct pipe sizes and pipe material with the effort to stay away from hoses or corrugated products that will reduce pressure. Sizing pipe is critical for efficiency with the goal of keeping air velocities in the distribution header less than 20-30 ft./sec. Increased velocities will increase pressure drop.
The calculation for pressure drop is called the empirical formula:
dp = 7.57 [q.sup.1.85] L[10.sup.4]/([d.sup.5] p)
* dp is the pressure drop measured in kg/ cm2.
* q is the air volume flow...