A Packaged Metering Manhole (PMM) is a factory-integrated fiberglass manhole that mounts a flume, weir, mag meter, or other primary device to allow a user to measure or sample flows from full or non-full pipes.
Components of a Metering Manhole
At its most basic a Metering Manhole consists of:
- Manhole tube
- Top
- Primary device (flume, weir, mag meter)
- End connections (pipe stubs or flanges)
- Base
The manhole tube can also be supplied with a range of accessories, including: access ladders, vents, benching, cabling couplings, shelfs, and operator platforms.
Depending upon the primary device, stilling wells, flow meter mounts, gratings / covers, and other accessories can be mounted.
How to Install a Packaged Metering Manhole
Metering Manholes are extremely easy to install:
- Excavate the site, exposing the line the manhole it to be connected to.
- Pour a concrete pad
- Divert / stop flow through the line
- Cut the line
- Lower the manhole onto the pad
- Connect the manhole stubs / flanges to the line
- Secure the manhole to the pad
- Backfill the excavation
For a step-by-step walkthroughs on how to install a metering manhole, take a look at:
Installing a metering manhole is a simple as Dig It! Drop It! Done!
Metering Manhole Advantages
Depending upon whether you are a consulting engineer, installing contractor, or operator, Packaged Metering Manholes have a number of distinct advantages over other flow metering approaches.
In general, the advantages are:
- Single-Source Responsiblity
- The primary device is factory integrated into the manhole - no finger pointing between subcontractors.
- Accuracy
- Properly installed primary devices (and reduced inflow) mean that accurate measurements can be taken
- Longevity
- Installed lifespans are typically 25+ years
- Reduced inflow
- Measure and quantify only the flow going through pipe - not that spilling into the manhole
- Environmental
- Watertight, corrosion resistant construction makes it unlikely that a metering manhole will ever experience a spill or leak due to failure of the structure
- Economics
- Higher initial material cost is more than off-set by installation and life-cycle costs
- Usability
- Packaged Metering Manholes are highly customizable - allowing operators to configure them for maximum usability. They require little more than a periodic wash down to maintain them. They provide excellent operator access entering / exiting the manhole as well as working inside with the primary device.
Metering Manhole Drawbacks
The only true drawback to a Packaged Metering Manhole is the higher upfront manhole cost - usually 25% more than concrete construction. However, as the Packaged Metering Manholes are turn-key systems (and are not susceptible to Microbial Induced Concrete Corrosion (MICC)) single-source responsibility alone more than outweighs any difference in the initial costs.
