In the situation of flow control under streets there are two common practices, and one new method to maintain standard regulations; on-site metering, off-site metering, and near-site metering.
On-site metering locates the flow meter in the same manhole as the pipe(s) to be measured. If the manhole installation is classified as a confined space, calibration and maintenance may pose a problem. In this situation multiple operators, a hoist, and gas detection equipment may be mandatory and pose increased costs for maintenance. Some of the difficulties may be avoided by installing remote reading equipment, but to calibrate the flow meter the operator must enter the manhole.
To avoid traffic congestion due to maintenance off-site metering utilizes a manhole placed outside of traffic flow for reading, measuring, and calibrating the flow meter. Concerns with the off-site metering methodology is lag-time between visual observation of the flow rate and calibration of the flow meter. As distance between the measurement location and the remote calibration location increases the possibility and degree of error increases as well.
A compromise of both methodologies above is the near-site metering practice. In this situation a short manhole is located directly next to the metering manhole, yet outside of traffic. In this shorter manhole the flow meter and remote reading equipment is placed, preferably on a telescoping stand. By sealing both manholes from each other, even if the operator must enter the new manhole it is not calssified as a confined space therefore avoiding additional operators and equipment.
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