Fiberglass Manholes Integrating Parshall Flumes

Parshall Flume Metering Manholes are an excellent solution to the problem of how to monitor below-grade piped flows. Integrating the popular Parshall flume into a fiberglass manhole, Parshall Flume Metering Manholes are complete, factory-integrated structures capable of direct burial with little line downtime.

Fiberglass construction means that Parshall Flume Metering Manholes are extremely rugged and highly corrosion resistant. Monolithic in nature, these manholes are watertight and are suitable for installations up to 35-feet [10.67 m] deep.

domed top Parshall Flume Metering Manhole

At the core of each manhole is a fiberglass barrel built to the exacting requirements of ANSI/ASTM D 3753 “Standard Specifications for Glass-Fiber-Reinforced Polyester Manholes and Wetwells”.

At the top of the barrel, a cover (or highway loading reducer), specific to the application. is mounted. At the base, a 1/2-inch 1.27 cm] thick fiberglass base is laminated the manhole barrel to make the unit watertight. Extending this base 4-inches [10.16 cm] past the barrel wall results in an external mounting flange that is used to secure the manhole to the concrete slab.

Inside, a fiberglass ladder is attached to the manhole barrel. The ladder complies with OHSA 29 CFR 1910.27 for fixed ladders. Each ladder rung is reinforced with a stainless steel safety rod threaded through solid cored fiberglass. A high-visibility abrasive antislip tape is then applied to the top surface for maximum safety.

fiberglass ladder in Parshall Flume Flow Metering Manhole

Finally, at the top ladder rung, fiberglass grating is mounted to aid in enter / exiting the manhole.

ladder grating in Parshall Flume manhole

Tops available for Parshall Flume Metering Manholes include:

Domed Fiberglass
- Fully opening with stainless steel hinge, lockable hasp, and strut
- Intended for non-traffic / non-pedestrian areas
- Provides unobstructed access to the manhole interior
- Intended to sit 1-foot [30.48 cm] above the surface grade
Aluminum Top
- Flush mounted aluminum hatch with non-slip surround
- Intended to site at or just above the surface grade
- Optionally available in water-tight and gas-tight construction
- Intended for foot traffic may be expected (sidewalks) or where aesthetic require a lower profile
Traffic Reducer Top
- AASHO H-20 applications
- Available with 31-3/4-inch [80.65 cm] and 38-inch [96.52 cm] openings
- Manhole reducer shoulder is 1-foot [30.48 cm] below grade to allow for grade rings, frame, and cover
- Requires grade rings, frame, and cover to the complete the installation

Flume Sizes

Depending upon the manhole barrel diameter, Parshall flumes from 1-inch to 36-inches in size can be integrated. All Parshall flumes are provided with end adapters to transition flow into / out of the flume in a controlled manner to maximize measurement accuracy.

Nested flumes configurations are also available with Parshall Flume Metering Manholes. Here a smaller, inner flume is installed in a larger, outer flume. In new construction, the idea is that the smaller, inner flume is size for the intial flow expected. In the long term, as flows increase, the inner flume is removed, leaving the larger, outer flume to handles the flow. Nested flumes maximize measurement accuracy over the life of a installation and maximize the manhole's value to the operator.

Piping Connections

Pipe stubs, caulking collars, or flanges are molded into each end adapter to connect to the upstream / downstream piping. The connection type or size does not need to be the same for both the inlet and the outlet.

pipe stub on Parshall flume outlet end adapter with flexible coupling mounted

Manhole Sizes

Parshall Flow Metering Manholes are available in four standard diameters:

48-inches [1.22 m]
60-inches [1.52 m]
72-inches [1.83 m]
98-inches [2.43 m]

Depending upon the application, larger or intermediate manhole diameters may be available.

Parshall Flume Metering Manholes are available in lengths from 2-feet 6-inches to 35-feet 0.762 to 10.67 m] high. Most manholes, though range between 6 and 8-feet [182.9 and 243.8 cm] deep; incorporating a Parshall flume from 3-inches to 9-inches in size.

Manhole depth is dependent upon a number of factors including: flume size, top type, and manhole diameter.

Construction Details

For Parshall Flume Metering Manholes where one (or both) of the end adapters extended well beyond the manhole barrel (6-inch sizes and up), stainless steel legs are chemically bonded to the end adapter to assist in anchoring the flume to the concrete pad. Each leg is pre-drilled with a 5/8-inch [1.5875 cm] hole to aid in the installatiion of anchor bolts.

A 2-inch [5.08 cm] diamter threaded fiberglass coupling is provided with each manhole. The coupling is a convenient way to bring sensor cables, suction lines, and power / signal lines into / out of the manhole. Unllke screw-on couplings that can cross-thread or not seal properly to the manhole, the threaded coupling is laminted to the manhole barrel. Laminating the coupling on maximizes the coupling strenght and ensures the manhole is water-tight.