Plant operators rely on plant air pressures to maintain properly working equipment and systems. One of the most common problems affecting processing plants that use compressed air systems is the ability to maintain reliable air pressure throughout the distribution system.
Undersized distribution piping may create insufficient air pressures, but more often the cause is 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.
Pressure drop is the loss or reduction of pressure from the compressor discharge to the actual point of use. The system will appear as though there is a lack of air pressure, but the real problem is pressure drop, which will cause excessive energy consumption. This article will explain 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. Every one pound per square inch (PSI) of excess operating pressure increases air compressor power consumption by around 0.5 percent. As can be seen from the pressure drop formula, increasing the system pressure will actually increase the pressure drop (see Equation 1). Unregulated equipment on the system will increase demand and increase inefficiency of the system.
Users can minimize pressure drop by selecting equipment such as filters, dryers, flow meters, air treatment equipment and instrumentation with the lowest amount of pressure drop. Additional ways to decrease pressure drop would be to maintain air filtering and air treatment to reduce moisture that can create corrosion and increase friction on the piping system. Also, users should reduce the distance the air travels through the distribution system so as to create the shortest runs possible to reduce loss.
Select the correct pipe sizes and pipe material with an effort to avoid 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 under 20 to 30 feet per second. Increased velocities will increase pressure drop. The calculation for pressure drop is called the empirical formula (see Equation 1).
dp = 7.57 q1.85 L104/(d5 p)
dp = the pressure drop measured in kilograms per square centimeter (kg/cm2)
q = the air volume flow at atmospheric conditions measured in cubic meters per minute (m3/min)
L = the length of pipe measured in meters (m)
d = the inside diameter of pipe measured in millimeters (mm)
p = initial absolute pressure in the system measured in (kg/cm2). This is the rating of the compressor, which gives the pressure expected at its outlet valve.
Another major influence to pressure drop has to do with the quality of the air flow. Air that flows in a swirling and turbulent path inside the pipe is called turbulent flow, which is the most inefficient flow condition. Air molecules spend so much energy bouncing off of pipe walls in all directions that they have less energy for moving down the pipe.
The ideal flow condition is for the air molecules to all move in a direct path down the pipe together. This flow condition is called laminar flow. Turbulent and laminar flow can be calculated as well, using Equation 2 to determine the Reynolds number.
Re = ρvd/μ
Re = the Reynolds number
ρ = the air density
v = the mean velocity
d = the diameter of the pipe
μ = the dynamic viscosity
An efficient air flow system should have a Reynolds number of less than 2,300. If it is greater than 3,000, the air flow in the pipe will be turbulent, and corrections should be made.
Making smart measurements of the compressed air system will reduce maintenance and energy cost and ensure proper operation for all equipment requiring consistent air delivery. Measurement equipment is inexpensive.
Monitoring power use, air flow and air pressure is all that is required to ensure a properly working system. Other measurements like dew point and temperature can also help your system operate with better results.
By measuring the air flow, pressure and energy use, users can generate a baseline of the system operation. Typical flow data in a compressed air system is measured in cubic feet per minute (CFM). By knowing the volume of air use, system pressure measured in pounds per square inch (PSI) and energy use measured in kilo-watt per hour (KWH), you can create a baseline of efficiency.
Volumetric flow meters, also called variable area (VA) flow meters, are typically used to measure compressed air since they do not require any straight runs of pipe before or after the meter installation. They also are unaffected by turbulent flow conditions, which can create inaccurate or no flow readings in a velocity type of flow meter. Some volumetric flow meters give accurate flow readings even with differing system pressure by using pressure compensation into a flow monitor.
One of the greatest benefits of using a VA meter in a compressed air system is that the meter is unaffected by water or moisture in the compressed air. Unlike velocity meters or thermal mass flow meters, volumetric technology is only able to read flow ranges within a 10:1 flow ratio, also known as the effective flow range of the flow meter. Some meters are can read flow ranges up to 30:1.
Thermal mass flow meters are also an excellent choice for measuring flow in the compressed air line as long as the meter is installed after the air dryer and the air dryer is functioning properly. Thermal mass meters typically measure a flow range of 100:1, meaning they can read very low flow rates and detect air leaks in the system. Some thermal mass meters will insert into the pipe and included hot-tap ability for easy installation without shutting the system flow down.
Since there are no moving parts, the meters are easy to maintain, and the flow system, when inserted into the pipe, will provide accurate flow with almost no pressure drop across the flow meter. Insertion type thermal mass flow meters can also be used as a portable meter that can measure flows at different locations by insertion through isolation ports installed throughout the system. The meters will detect velocities well above 30 fps and as low as 0.05 fps.
Smart metering allows the system to optimize velocity and operating pressure to reduce pressure drop. Typically, flow meters measure plant demand, not the full load of the compressor output.
The baseline of the operating system can be generated with flow, pressure and electrical load measurements taken on a continual basis through a customized meter. This flexible process meter will calculate the baseline with time and date stamped data that is stored in the memory of the unit. The data can also interface with a PC-based monitoring software for use when analyzing data during production or when flow data analysis is convenient.
Some benefits to using a flexible process meter are that leaks can be easily detected. This happens when the flow reading indicates something besides zero flow when the plant is not in operation. The pressure trend will also indicate a loss of pressure at a greater rate than historically trended if the air leak has increased over time. Logged data can also be analyzed to evaluate system performance and provide predictive maintenance on the compressor head, vanes and lower unit. It is possible for the compressor to consume the same amount of energy and appear to be working properly but not produce any pressure or air flow.
Each of these operational and maintenance issues can be reduced by smart metering analytics and system alarming strategies.