Calculating pickup velocity in a dilute-phase pneumatic conveying system
Q: We're experiencing a significant pressure drop in our dilute-phase pneumatic conveying system. Can you explain how to calculate the pickup velocity properly to improve our system's performance?
Calculating pickup velocity in a dilute-phase pneumatic conveying system is the key to operating your system at peak performance. The pickup velocity is the minimum conveying velocity in a lean- or dilute-phase pneumatic conveying system at the pickup point or solids loading point. Typically, the pickup velocity is 15 to 20 percent higher than the saltation velocity
. Saltation velocity is the minimum air velocity in a pneumatic conveying system at which particles begin to fall from a suspended state and begin depositing at the bottom of the pipeline. In short, saltation velocity is the minimum air velocity to keep solids and particles in suspension. Calculating the pickup velocity in a conveying system isn't straightforward because doing so depends on the pressure drop at the material pickup point.
In principle, a pneumatic conveying system works based on the system's pressure differential between its inlet and outlet. This difference in pressure is what moves material and gas or air through the system. When gas is compressed and released to expand in the pipeline, the gas velocity increases as the pressure drop decreases during gas expansion toward the end of the pipeline. Gas velocity is the lowest at the blower and highest at the endpoint near the receiver. Whereas pressure drop is highest at the blower's outlet and lowest at the endpoint near the receiver. The energy needed to create this pressure differential depends on the energy loss related to the gas and solids friction in the pipeline, elbows, solids loading ratio, pipeline diameter, pipeline length, and air velocity.
Predicting the system's required pressure differential — also known as pressure drop — using models will work if all the parameters are correct but it's not unusual that some parameters may be unknown. Even if all the parameters are known, the information may be deemed a company's proprietary information and may not be accessible to you. Because there could be uncertainty in the parameters, most equipment manufacturers depend on their own test results rather than modeling the pressure differential. The variation and inconsistency in the raw material's particle size distribution and impurities in its composition lead to these uncertainties. The reliable method to find the pressure drop in a pneumatic conveying system is by conducting pneumatic conveying testing in a test lab to mimic the actual pipeline system. Almost all equipment manufacturers have their own testing facilities to identify the pressure drop. Equipment manufacturers use higher pickup velocities, which are 30 to 40 percent higher than saltation velocity, to account for these uncertainties. Keep in mind that these higher pickup velocities increase the energy consumption for pneumatic conveying system considerably.
An example of the pressure drop that occurs in a conveying pipeline with a test material is shown in Figure 1. At the air mover's outlet (positive displacement (PD) blower), the pressure drop is the highest at 7.5 psig. At the system's pickup point, the pressure drop is 6.0 psig. The example shows that there's a remaining 1.5 psig pressure loss (energy loss) in the pipeline from the blower to the pickup point (as well as the cooling system).
The acfm needed for the required pickup velocity is a straightforward calculation. Airflow (acfm) is equal to velocity (fpm) multiplied by the pipe's inner cross-sectional area. For example, in order to have 4,000 fpm pickup velocity on a 3-inch Schedule 10 pipe, you first would calculate the pipe diameter: 3.5 - 2*0.12 (pipe thickness) = 3.26-inch inner diameter. Then you can insert the pipe's inner diameter into the equation below to calculate acfm:
To have 232 acfm (4,000 fpm) at 6 psi pressure at the pickup point, how much inlet cubic feet per minute (icfm) is needed at the blower? This is calculated using the compression ratio (Pa+Pg)/Pa term to be multiplied to the acfm. Pa is the barometiric pressure in psi at the location, and Pg is the pressure drop (gauge pressure) in psi. In this case, let's assume the pneumatic conveying system's location is Houston, TX, which has a barometric pressure of 30.12 inches of Hg (14.79 psi). Now the icfm calculation becomes 232 acfm*(14.75 + 7.5)/14.79 = 350 icfm. Using the blower curve, the required icfm is achieved by selecting appropriate blower speed. Blower curve is the blower performance curve or chart given by the manufacturer that gives information about, at a particular pressure and blower speed, how much icfm it can give. With some uncertainty, the pressure drop increased to 9 psi under the same blower speed, but what will the pickup velocity be? Based on the blower curve, your icfm is reduced. Assume your icfm is reduced to 320 cfm at 9 psi pressure, the acfm is now reduced to 320*14.79/(14.79+9.0) = 199 acfm. The pickup velocity for this acfm is 199/0.0579 ft2 = 3,437 fpm. Similarly, if the pressure drop is reduced at the same blower speed, the pickup velocity increases.