When a vacuum sewer system is planned in an area, there is usually an options study done showing whole of life costs. In the study there is an expectation that, over the life of the system (100 years), there are items that will wear out and will need replacing. Just as there are other components that will need parts replaced through normal maintenance (asset life).
For the rotary vane vacuum pumps, that means oil and filter changes. According to the pump manuals, it is expected to do a fairly extensive motor overhaul at year 5 with a possible replacement at year 15. Through good care, we have seen many vacuum pumps have their lives extended considerably.
Many of the systems built prior to 1996 used liquid ring pumps. Since then, the use of rotary vane pumps has become widespread. Dry running claw pumps are also used for small systems. Also, there have been a few outlier pumps like oil sealed screw pumps and even some rotary lobe pumps.
The vacuum pumps in any system are sized according to an air to liquid calculation based on the length of vacuum mains and the volume of the mains. Operationally, we always look to the amount of air it takes to move the liquid through the system. A well run fully developed system should run at about an 8 to 1, or 10 to 1 air to liquid ratio. We know that if that creeps up to 30 to 1, there is too much air in the system and it should be tuned.
A high rate like that would suggest that the vacuum pumps are running far more than they need to. If the ratio though was lower at about 4 to 1 or 6 to 1, we would know there is too much water relative to air. This might mean the discharge pumps are running longer than they should, relative to the vacuum pumps. This might be due to I&I (infiltration and inflow) or higher flows than expected in the catchment.
With a good air to liquid ratio, vacuum pumps are expected to run at about 6 to 8 hours per day, in total, in a fully developed system. If the pumps are running longer than that, the air to liquid ratio should be checked to see what is impacting the system. Two of the biggest mistakes that we commonly see when utilities are looking to upgrade their pumps are:
Increasing the size of the pumps might be a great idea if done for the right reason. If it is possible to increase capacity at the station, that might allow more flows from subdivisions, new areas, or higher density (more revenue). If though the size is only increased because the pumps are running a long time, then the problem needs to be fixed first. The questions that should be asked in that scenario are: Why are the pumps running so long? Is the system tuned correctly? Are there any leaks in the valves or pipework?
Oversized pumps can increase the problems in a network by creating more leaks. For this reason, once the system is tight and you have an accurate idea of your air to liquid ratio, then more informed and accurate decisions can be made.
If the change is because you are using too much oil and too many filters, that would suggest that the pump run hours are too long. In small systems, Mink pumps and their equivalent have improved substantially, but in large systems, there are a number of downsides that utilities are now discovering.
In the larger sizes the Mink pumps run a lot hotter and potentially noisier. The heat has an impact on pipe materials in the station, venting of the station, and will badly affect odour control measures. On larger systems there is also a limit to the depth of vacuum that these type of pumps will achieve and often the result is an increase in energy use and pump run hours.
Some utilities have also tried the addition of variable speed drives in their pumps as a way of increasing vacuum in the lines and decreasing line flooding. The speed is often varied due to the amount of vacuum pressure in the lines or at the end. It is rare to see this used in wet/sewer type systems and is more common in dry/food processing applications. As vacuum sewers are based on a ratio of air to liquid, the use of a VSD disguises problems in the system and can ultimately increase problems in the networks.
Upgrading of the vacuum pumps is inevitable eventually. At Flovac, we can help you with finding the right solution for your system. You can contact us to organise the following as a first step:
System Audit. To correctly size the vacuum pumps an analysis of the air to liquid ratio is needed. The system needs to be air tight to do this. If there are leaking vacuum valves, due to old age, air leaks in the pump station or elsewhere, these need to be fixed prior to any upgrade as any larger or new vacuum pumps could make these harder to find and just lead to higher energy costs and high oil use. An audit will check the current state of your system and give you an understanding of what will be required to accomplish the outcomes you want.
Capacity Audit. What was the system designed to handle and what can it potentially handle for the next 15 years.