GSWW Provides Innovative Solution to a Difficult Flow Monitoring Problem

GSWW, Inc. encountered a collection system flow monitoring application that required the development of alternative equipment installation methods in a 90-inch wastewater outfall. Monitoring this 90-inch outfall line was determined to be a critical component to the overall flow monitoring study. The site selected on the 90-inch outfall was one of many locations originally investigated. Access points (manholes) were investigated both upstream and downstream of the proposed site, only to find that the atmospheric and site hydraulic conditions were less favorable. In order to avoid monitoring on the outfall line, other system wide monitoring plans were investigated. However, at the conclusion of the planning and site investigation phase, it was determined that a site on the outfall line was required and the access point with the most favorable atmospheric conditions was selected. To ensure all safety guidelines were observed by GSWW personnel, a methodology was developed to install and calibrate flow-monitoring equipment from the ground surface without entering the confined space.

Monitoring atmospheric conditions is a critical component in any confined space related work. By OSHA requirements, the atmosphere of any confined space must be monitored prior to and during any entry. Atmospheric monitors calibrated to register alarms at pre-determined limits monitor Hydrogen Sulfide (H2S), Carbon Monoxide (CO), Oxygen (O2) and Lower Explosive Limit (LEL) levels. While confined spaces with certain levels of H2S, CO and low O2 can be entered if the entrant wears fresh air apparatus and/or a Self Contained Breathing Apparatus (SCBA), when an LEL alarm is registered at a site, entry into the confined space is prohibited.

Traditional flow monitoring of wastewater flows requires manned entry into confined spaces to install the flow monitor sensor and verify the sensed flows with physical measurements. As confined space entry was not permitted due to atmospheric conditions, the installation of any equipment would be required to be performed from above ground. In order to install a flow monitor that did not require a traditional sensor installation, GSWW utilized Marsh McBirney Flo-Dar equipment for this site.

The Marsh McBirney Flo-Dar, which utilizes radar velocity sensing technology and ultrasonic level measurement. The Flo-Dar sensor is positioned inside the manhole, above the flow, typically mounted just below the pipe crown. While this device does not require a sensor to be mounted in the flow, to ensure an accurate measurement the sensor must be level and the distance to the water surface and the pipe diameter must be known. To meet this requirement a mounting bracket is typically required to properly position the sensor.

GSWW found that an innovative method to install the sensor from above must be developed. A pole mounting system that would allow the sensor to be lowered into the manhole was developed to overcome the no-entry requirement. The system incorporates the Flo-Dar mounting bracket to hold the sensor in a level position. The sensor is attached to the pole mounting system and then lowered into the manhole in sections. Since the overall device was approximately 19 feet long and constructed of stainless steel (to resist the elements in the manhole) a concern was developed regarding the weight of the device. To combat this issue and eliminate sensor sway, control arms were added to the pole at strategic locations.

The design of the 'installation armä was completed and a steel fabricator was hired to construct. During fabrication, minor changes were made to the design to facilitate the construction and installation process. The device was built in four separate pieces for ease of delivery to the site and installation. Each constructed piece was delivered to the monitoring site prior to installation. A man rated winch and rescue tripod was positioned over the manhole and the first section of the device, including the mounting bracket and sensor, was lowered into the manhole.

As one piece of the installation arm is installed, the next pipe segment was attached via a threaded connection. Finally the manhole support arm is threaded to the third piece, to sit across the manhole ring. The support arm held the weight of the device and sensor and was shimmed to provide a level support for the Flo-Dar sensor.

Once the device was secured, three sets of mounting arms were secured to the wall to hold the pipe at the center of the manhole and to hold the sensor at a level position.

The position of the sensor was confirmed using a pole mounted video camera, which was lowered into the manhole to view the sensor and the level bubble on the sensor. The monitor site set-up was performed once the device was secure. A distance from the bottom of the sensor to the surface of the water was measured using a graduated pole, lowered from above.

The sensor was installed for a period of 4 months, with site visit and profiles performed every week to confirm flow meter operation and the verify that the mounting device was operating properly. Each site visit included velocity and level verification from above ground. Flows were monitored for a period of four months and all associated equipment was removed at the conclusion of the monitoring period.

GSWW, Inc. is a full service civil and environmental engineering firm founded in 1975. The firm offers a complete range of civil engineering services including information technology, water and wastewater systems, planning and development and transportation system services. With an experienced staff of engineers and technical professionals, GSWW provides clients with the highest quality service, utilizing the latest technology and resources. GSWW has offices in Dallas, Austin, Fort Worth and Midland, Texas.

This article was written by:
David Koberlein, P.E., Associate - GSWW
Leigh Cerda, P.E., Senior Project Engineer - GSWW