Dynamic Field Torque Test

By Peter Khramov, Project Engineer

There are many reasons to perform a dynamic field torque test. The test can detect potential problems before the equipment is put into service. It is much easier to fix the problem sooner than later. The torque test is also used to prove that the drive is capable of producing the manufacturer’s claimed torque. The test demonstrates the structural integrity of the drive. It shows that the drive unit can be safely loaded to its rated torque. In addition, it demonstrates that the rake arms are capable of handling the required load. The accuracy of the DBS torque gauge is verified against the load cell readings. If the gauge is found to be off, it can easily be calibrated.

At the heart of the dynamic torque test kit are two long stroke hydraulic cylinders. The cylinders are lightweight and compact, allowing them to be lowered into the tank manually without the need to rent expensive cranes. Set up is a breeze, just attach the cylinders to the tank floor with half inch concrete anchors and attach the other end of the cylinder to the rake arms with supplied nylon straps. Using nylon straps instead of chains ensures that the rake arm coating is not damaged. It takes just two people to perform the test. One operator controls the load on the rake arms while simultaneously observing the reading of the drive unit torque gauge. The operator controls the hydraulic backpressure, and therefore load, on the cylinders by adjust a needle valve. Torque load on the drive unit is calculated by multiplying the load on the cylinder by the distance from the center of the tank to the point on the rake arm where the cylinder is attached. The other operator starts and stops the drive unit, making sure the cylinders do not bottom out. The entire test, from set up to break down, can be performed in a matter of hours. The test rig is routinely weight test calibrated to ensure maximum accuracy.

Many clarifier and drive manufacturers offer a static torque test. The test involves restraining the tips of the arms with straps and running the drive. While this procedure checks the structural integrity of the drive and rake arms, it says little about the running performance of the drive. The load is not controlled and the drive goes to maximum torque very quickly. During the DBS dynamic field test the load is applied very slowly and the test can be stopped if the rake mechanism starts to show any signs of damage.

Some clarifier manufacturers offer a dynamic field torque tests that involves dragging a weighted sled with the rake arms. This procedure has several distinct disadvantages. Depending on the size of the tank and drive unit, a considerable amount of weight must be placed on the sled, which may require a crane to lower the weights inside the tank. Because the rake arms must be equally loaded, both of the sleds must weight exactly the same, which is difficult to do in practice. The hydraulic cylinders in the DBS test insure that both arms are equally loaded. Dragging the sled on the concrete floor can also damage the bottom of the tank.

Case Study: Hagerstown WWTP

In October 2009, DBS replaced two Envirex model C40 units with D30-BE’s on Ø80 ft primary clarifiers at Hagerstwon, MD Wastewater Treatment Plant. The consulting engineer specified a dynamic field torque consisting of pulling a weighted sled with the rake arms. DBS proposed an alternative test which was gladly accepted. DBS engineer successfully performed the tests and demonstrated that the torque gauges were accurate within 1% of the gauge’s full scale reading.