When performing any sewage pumping bypass, the first few days are the most critical. According to MWI Pump’s Steven von Gontard, 90% of sewage bypass troubleshooting issues happen in the first few days.
“In these first few days, you are learning the system that you are bypassing,” says von Gontard, who has nearly two decades of experience selling, installing, and servicing bypass sewage systems. “These first few days require constant checking and tweaking of the bypass system, including adjusting float level height, engine RPMs, autodialer/GPS telemetry system monitoring floats, programming the control panel to providing delays in start up or shut down, as needed, and more.”
A bypass system, whether for sewage or stormwater, is a temporary system put in place to perform work expansion or maintenance on an existing structure. It typically consists of a primary pump (and an identical backup pump) which lifts the water from a manhole, line, or vault and sends it down to a predetermined discharge point. It is mostly used with underground storm or sewer lines that have aged and may have begun to leak. To keep the system flowing, a bypass pumping system is installed.
Through experience, von Gontard has learned that the best pump to use for a sewage bypass system is a MWI Primerite™ self-priming, centrifugal trash pump, especially if it is used in a residential or densely-populated area. If it is a larger space or a very deep line, an ideal solution may be a MWI Duraflo™ or Hydraflo™ hydraulic submersible pump.
The first step is to ask the contractor or an expert with the municipality a few critical questions:
- What is max flow (gpm)
- What is max break in pressure on force main (tdh or psi)
- When are the peak and low flow times?
- How much water infiltration do you have on system?
- How dirty is the system?
“If it’s a gravity line, you can base this information on the gravity line of the existing sewer line and look at the flow at 4 feet per second,” von Gontard says. “If it is a lift station rehabilitation and we can’t get the accurate information, the next thing we will look at is the size of the lines coming in and consider what size pumps already exist in the system. For example, if it’s a 10 horsepower, 6-inch pump you can estimate the type of flow and break-in pressure to expect. This helps to determine the correct size of equipment to use. In a gravity system when you are just picking up from one gravity hole to another gravity hole, you don’t have to worry about break-in pressure. However, if you are bypassing into a force main you may have increased break-in pressure. If you don’t have it sized correctly, you may have the ability to move the amount of water, but you don’t have the ability to open the force main and push the water in.”
With the exception of typical pump application and/or mechanical problems, here are the 8 most common issues that can occur during stormwater or sewer bypass pumping and von Gontard’s expert advice for solving them:
- Wrong information is given by the contractor on flow or break-in pressure of the project being bypassed resulting in the pumps being under sized (or oversized) for the application and not using proper fittings rated for the pressure of system.
To solve this problem, first run a total dynamic head calculation. This determines that you are moving X amount of water over X amount of feet through various fittings like 90-degree elbows, lines, check valves, gate valves, etc. All this calculates into one number which is the total dynamic head (TDH).
“On a pump curve, you look at the total dynamic head (let’s say it’s 50) and you look at the curve and know that you can pump whatever value, for example, 1000 gallons per minute (GPM),” von Gontard explains. “Then you know that if this system falls below that number, everything is good.”
Often, contractors will not have accurate information stemming from a lack of information from the municipality. If this happens, von Gontard recommends implementing common sense practices. “I do a pre-construction job site visit and look at the system, consider the size pipe and the amount of flow so that I can make an educated calculation. If the contractor gives me a number that far exceeds or is far below my calculation, then we have a conversation to figure it out. When that happens, it is generally a force main application where flow isn’t the issue as much as the break-in pressure. A pump can do 1000 gallons per minute at 50 feet of head, but it may not be able to do that same 1000 gallons per minute at 100 feet of head. If you have the additional break-in pressure that wasn’t accounted for, it causes problems.”
A valve downstream on a force main may have gone bad (for example, an 8-inch valve is 80% shut). This creates significant head pressure and doesn’t allow as much product to go through. The pumping system should be drawing down in a matter of minutes, but instead it’s slowly rising. The effort on the front end can prevent this from happening later, or provide you direction in resolving issues as they come up.
“There are environmental and health implications, so I also insist on 100% redundancy because it’s so important to have a high-quality, high-functioning backup pump,” von Gontard says. “Most major municipalities require 100% redundancy.”
- Suction/priming/vacuum issues due to improper connections, bad O-rings, debris caught in discharge check valves, venturi clogged by trash in the system, or a blocked venturi extension line.
The suction line must be completely airtight. During installation, some fittings can get loose, so it is important to check this at the beginning of the operation. Centrifugal pumps have a discharge check valve that must seal for vacuum purposes. If it does not seal properly because something small like a blade of grass or sand is present, this can be enough to keep it from priming. There are a lot of materials in a storm system and in a sewer system, so this is important to check upfront.
There are two types of priming systems in centrifugal trash pumps. The venturi system uses a compressor to blow air over a venturi, creating the vacuum. A vacuum-style diaphragm pump pulls the air out of the hose. In venturi systems there are small ports that discharge the air. Debris in the manhole can adversely affect the vacuum by clogging these ports. Vacuum-style pumps use a peeler valve which has a rod that goes up into a port and seals off the vacuum. Once you get the water in the pump it pushes a valve up and seals off the vacuum, so the water doesn’t contaminate the vacuum system. These peeler valves can get stuck in an open or closed position creating problems as well.
For more information, please contact MWI Pumps at 954-426-1500 or via the web contact forms.