MEMORANDUM 

TO: Docket EPA-HQ-OAR-2012-0522 

FROM: Gerri G. Garwood, P.E., EPA/OAR/OAQPS/SPPD 

DATE: March 16, 2015 

SUBJECT: Use of Pressure Drop as an Operating Parameter 

A. INTRODUCTION 

During the proposal of the Phosphoric Acid Manufacturing and Phosphate Fertilizer 

Production Risk and Technology Review and Standards of Performance for Phosphate 

Processing, the EPA received comments about the removal of pressure drop as an operating 

parameter on certain scrubbers. This memo explains the rationale behind the agency’s decision. 

B. DISCUSSION 

 Pressure drop is a critical scrubber design element and an important indicator of 

performance for most wet scrubbers. Because pressure drop remains fairly constant during 

steady state operations, a significant change in pressure drop (either an increase or a decrease) 

indicates a problem with the scrubbing system. This is particularly true for high energy 

scrubbers. However, the EPA has previously noted that pressure drop across the scrubber is not 

a parameter of interest in spray towers and other low energy scrubbers because it is not directly 

related to the effectiveness of particle capture1. Additionally, due to the small magnitude of 

pressure drop in a low energy scrubber, it is more difficult to measure a significant change in 

pressure drop. 

A low energy scrubber is generally defined as a scrubber that utilizes a pressure drop 

through the system of less than 5 inches of water column2

. Low energy scrubbers, such as spray 

 

1 Wet Scrubber Inspection and Evaluation Manual. Prepared for U.S. Environmental Protection Agency, Office of 

Air Quality Planning and Standards. September 1983. EPA-340/1-83-022. 2

 The EPA notes that 5 inches of water column is the general cut-off for low energy scrubbers in several documents, 

including the instructor’s guide to APTI course 413 – Control of Particulate Emissions, March 1980 and the student 

manual for APTI Course 445 - Inspection of Particle Control Devices, Third Edition, February 2003. This is also 

noted in the Monitoring Knowledge Base (see: http://cfpub.epa.gov/oarweb/mkb/contechnique.cfm?ControlID=27). 
towers, are effective in applications requiring control of gaseous pollutants or large particles. 

High energy scrubbers, such as venturi scrubbers, are useful in applications requiring high 

removal efficiencies of small particulate. 

It is important to note that we are not saying that a low energy scrubber is not an effective 

control device, nor should a low energy scrubber be forced to operate at higher pressure drops. 

We are simply stating that monitoring pressure drop on a low energy scrubber is of limited value. 

There are different operating parameters that are more informative of a low energy scrubber’s 

performance. Although not inclusive, the following are examples of operating parameters that 

can be useful in determining the performance of a low energy scrubber. Generally more than 

one operating parameter is necessary to gain a complete picture of the scrubber’s performance. 

(1) The ratio of the liquid flow rate and gas flow rate (or L/G) in the scrubber is an 

indicator of whether enough water is present to provide adequate scrubbing for the 

amount of gas flowing through the system. 

(2) In acid gas scrubbers, pH measurements of the scrubber liquor is an indicator of 

whether enough caustic is available in the system to achieve neutralization of the 

acid. 

(3) Changes in scrubber liquid specific gravity can be an indication of changes in 

pollutant concentration in the scrubber liquid. An increase in pollutant concentration 

in the scrubber liquid can decrease the scrubber’s efficiency. 

(4) Spray nozzle pressure can provide an indication of proper atomization of the 

scrubbing liquid. Water droplet formation increases the surface area for liquid and 

gas contact. Loss of water droplet formation can result in loss of absorption.