NB: This article first appeared as part of the Talking the Torque series in Water Magazine
Martin Hardwidge explains how a critical ‘single point of failure’ asset, the actuated inlet penstock, can be kept running more smoothly and more accurately, for longer, and with a reduced need for operator intervention. It’s not even witchcraft.
The inlet penstock on a WWTW is a critical control asset, managing flows into the works, to storm, and protecting from surges. The inlet penstock can, however, also be that thing that we’re taught to avoid – a single point of failure (SPOF). They are, unfortunately, also very often overlooked, many are in a poor state of repair and not well maintained.
The right choice of actuation for the penstock can really help here, because one of the main issues impacting the operation of an inlet penstock is the mechanical interface between the actuator and the penstock spindle – the humble drivenut. These softer, phosphor bronze drives wear over time, protecting the much more expensive stainless steel penstock spindle. The drivenut will wear out eventually, but there are ways in which we can significantly extend its lifespan.
Accurate machining of the drivenut is important, so it mates with the spindle properly. Incidentally, cover tubes are not just there so you can see the spindle. They play an important part in keeping the top of the drive dry and avoiding washing grease out so it’s important to make sure they are in good condition, especially the end caps.
Protecting the drivenut boils down to the need to reduce friction, to reduce wear. A penstock can be heavy, with a lot of inertia, so the starting torque to get it moving is often high. In the same way, if the end position isn’t accurately set or there is overrun on the travel, there is a lot of pressure as the penstock impacts its closed position.
There are a few ways that AUMA can help the operator understand what’s happening, and to mitigate the wear of the drive to keep the penstock running efficiently:
The built-in asset management functions in the actuator help the operator to be aware of issues. As the drivenut wears, the torque requirement increases and that can be tracked in the actuator. At a pre-set torque level an alert can be triggered to let the operators know that its time to either solve a problem or to trigger a maintenance process.
AUMA’s variable speed actuators are increasingly being used in penstock operation, where their soft start and soft stop, along with exceptional positioning accuracy with no overrun and no ‘hunting’ reduces stresses on the drivenut. The ability to set up a velocity profile also means that the penstock can be slowed as it approaches the closed position, creating a choke effect that helps to clear the penstock channels.
One of the simplest and most obvious ways to reduce friction and wear is, of course, to get the lubrication right. The drivenut and the spindle both need to be lubricated, as do the bearings in the thrust base assembly that holds the drivenut in place. This is a necessary and important maintenance task, but it’s often difficult and awkward to do.
At AUMA, we have a very straightforward solution – because AUMA thrust bases operate using a sequence of roller bearings we’re able to cross-drill the base so a single application of grease can lubricate the bearings, the drivenut and the spindle all in one. Marry that with a simple grease ‘dribbler’ to deliver lubrication over an extended period and you have an easy, powerful way to both improve operation of a critical component and reduce the need for operator intervention, at very low cost (though keeping a spare is still a great idea!). If you add to that an extended, extra-long drive nut to spread the load you can substantially extend the life of this small-but-important component and in the process remove a high proportion of the asset failure risk from critical penstock operations.