Press diaphragm problem causes work stoppage, a case study

"There it goes again!" shouts one of the press operators as the firecracker-like sound of a bursting load diaphragm rings out. All eyes glance up to the nip where the now lifeless top roll hangs on the mating bottom roll. With the nip lost, the operator shuts the press section down and everyone settles in for a work stoppage of at least three hours for repairs.

The problem

The maintenance crew and the operators are stumped. A total of three press load and unload diaphragms have ruptured this week alone, and each case looks the same. The diaphragms, during roll loading and unloading, either burst or tear out of their clamps. In one case, the diaphragm blew even before the press section was running. A blown diaphragm cripples the operation by eliminating or lowering nip pressure on one side of the press. Unfortunately, the end result can be catastrophic or at best case, a wetter sheet and unsalable paper.

Earlier in the week, maintenance experimented by rotating the diaphragms 180 degrees, hoping for a solution. Obviously, that wasn't it. How could it be that new diaphragms, under apparently normal operating conditions, could burst in a week's time?

The analysis

Upon arriving at the mill, the service technician from the machine's manufacturer pulled the original drawings of the press section. He was drawn to the pressure plates, which have finished surfaces that the diaphragms push against during loading and unloading. The technician suspected that if misaligned, the plates could be a factor in the diaphragm failures.

A trip up the ladder into the frames confirmed his suspicions - the pressure plates were badly misaligned. On top of that, they were corroded. It's likely the corrosion was putting undue pressure on the diaphragms in their clamps, forcing them to burst.

A check of the diaphragms showed that they, too, were misaligned. In fact, in their attempt to correct the problem, the maintenance crew had installed one of the diaphragms backwards. (Diaphragms are designed to stretch in only one direction.)

Making matters worse, the mill was using diaphragms that didn't meet the manufacturer's design specifications. The clamping surface of each was an eighth of an inch too thick, which made for a bad fit in the clamps.

Additional inspecting revealed that stop screws, used to prevent damage to the diaphragms, were either missing, bent or corroded. Missing or misaligned stop screws would be enough to allow excessive amounts of pressure onto the diaphragms. The maintenance department didn't notice the damaged screws because they never knew to look for them.

But the question still remained, what caused all this?

The culprit

The answer was found in the nip area. Measurements of the nipping rolls showed that their outer diameters were different. Over time, whether through regrinds or overuse, the OD of the rolls had changed and set off an unfortunate chain of events. The irregular OD forced the swing arm out of alignment, which, in turn, misaligned the stop screws, pressure plates and the diaphragms. In essence, the entire set-up was misaligned, which overextended the elasticity of the diaphragms and forced them to burst or pull out of the clamps.

The solution

Most or all of the diaphragm problems could have been avoided had the mill referenced the machine's original drawings and taken the necessary corrective steps. Fortunately, after the service technician recommended new diaphragms and re-alignment of the pressure plates and stop screws, the press section was up and running once again.

For the most complete and accurate information on press maintenance, contact your Valmet Representative.