A Packaged Metering Manhole (PMM) is a fiberglass manhole into which a primary device has been integrated.
Packaged Metering Manhole Benefits
The benefits of Packaged Metering Manholes are:
- Lightweight – typically 1/10th the weight of a concrete structure
- Corrosion resistant – long life
- Factory integrated – no need to assemble pieces – proper primary device installation
- Watertight – eliminating inflow and reducing
- Allow integration of a wide range of primary devices
- Infinitely customizable
In addition to the above, Packaged Metering Manholes allow the integration of primary devices larger than would otherwise be able to be easily accommodated in concrete construction.
What Comes in a Packaged Metering Manhole?
Each Packaged Metering Manhole consists of – starting from the top down:
Manhole Cover
Openchannelchannel flow offers (5) different manhole cover options:
- Fiberglass cover: for grassy areas where the greatest degree of operator access is desired.
- Traffic reducer (or cone): for use with standard grade rings, frame, and cover sets.
- Aluminum hatch: for pedestrian foot traffic areas or non-traffic areas where a low-profile installation is desired
- Grating cover: full diameter fiberglass grating suitable for occasional foot traffic
- Open: for applications where operator access is desired, but where the operator can cordon off the opening so that it isn’t a fall hazard
Manhole Barrel
A fiberglass manhole barrel 48, 60, 72, or 96-inches in diameter. Ours are H-20 traffic rated regardless of application. The inside of the barrel conforms to ASTM D3753 and is smooth, unpigmented, and corrosion resistant. The outside of the manhole barrel is gel coated for purely aesthetic reasons.
Primary Flow Device
Packaged Metering Manholes integrate one of three different primary devices:
- Flume (normally a Parshall flume, but Palmer-Bowlus, H type, and Cutthroat flumes can also be integrated)
- Weir (smaller v-notch weir boxes)
- Magnetic flow meter
End Connection
Typically pipe stubs, but occasionally flanges (mag meters) and caulking collars are provided to get flow into / out of the primary device.
Base
Openchannelflow Packaged Metering Manholes come with an integral base which also extends outside the manhole to double as a manhole mounting flange.
Ladder
While decreasing in popularity for confined space reasons, Packaged Metering Manholes can be provided with stainless steel reinforced fiberglass ladders. The ladders provide for easy operator access and are normally provided when there is at least 4-feet of distance from the top of the primary device and the underside of the manhole cover
How Do you Install a Packaged Metering Manhole?
In existing construction, the line to be metered is exposed – underneath a concrete pad is poured to set the Packaged Metering Manhole on. The slab must support all of the Packaged Metering Manhole and be thick enough to counteract the buoyancy forces of the surrounding soil / ground water.
Once the pad has cured, flow in the line is stopped / bypassed. The line is cut and the manhole lowered into place. If the manhole has pipe stubs, flexible couplings are used to make a watertight connection between the manhole pipe stubs and the upstream / downstream line.
An alternative method is to pour a slab offsite, mount the Packaged Metering Manhole to it, and then moving the whole thing in place once the flow has been stopped.
Any area of the flume extending outside the manhole barrel is filled beneath. Then the area round the manhole / flume is backfilled in lifts.
Packaged Metering Manhole Drawbacks
While we are naturally advocates of the Packaged Metering Manhole approach, we do understand there are some concerns.
Packaged Metering Manholes do require concrete slabs onto which they can be mounted, however sizing a slab isn’t particularly difficult and competent civil engineers can do the necessary calculations quickly.
With short Packaged Metering Manholes or those integrating small primary devices, the capital costs of the Packaged Metering Manhole can be higher than those for concrete. However, Packaged Metering Manholes require less manpower, lighter weight machinery, and line downtime is considerably less.
