Written by Bill Looman, Sales & Business Development for Webcast Experts℠, a division of Krishnan & Associates
There is a story about a young bride who is making her first holiday ham for her new in laws. As she prepares the ham she cuts the end of the ham off and puts the rest of the ham in the roasting pan. Her husband asks, “Why do you cut the end of the ham off?” To which she replies, “I’m not sure, that’s the way my mother did it.” So the bride calls her mother and asks “why did you cut the end of the ham off before roasting it?” Her mother replies, “That is the way my mother did it.” So the young girl, wanting to understand, calls her grandmother and asks the question, to which the old woman replied, “I had to cut the end of the ham off because the pan was too small to fit a full sized ham.”
Just because it’s always been done that way, doesn’t mean it is the best way to do it. For example, steam turbine tooling and hardware components have come a long way over the past 20 years, yet most plants are still embracing OEM methodologies from the seventies. Yes, they are still cutting the end off of the holiday ham. Torque and heat should be reconsidered and tensioning technologies should be evaluated. Tensioning is a proven technology that shortens project turnaround, improves safety and increases the longevity of expensive fastening hardware.
The first and very common method is affectionately referred to as heat and beat. Heating up the stud until expansion and using a slug wrench and hammer to tighten the stud bolt is both very time consuming and provides inaccurate loading. Safety should also be considered when using this very old method.
Torqueing, not to be confused with twerking, uses a torque wrench or possibly hydraulic tools. Problems arise when the friction between the nut and stud threads increase during torqueing resulting in thread surfaces ceasing to slide on one another causing the stud to begin to twist and windup. Torque can also result in friction between the nut face and the flange face as shown below on the right. You will also experience thread galling using this method.
Image Courtesy of Riverhawk Company
Hydraulic Tensioning is using hydraulics to stretch the stud causing axial preload allowing you to hand tighten the nut. (As shown above on the left) Hydraulic tensioning eliminates safety issues from stud heating with up to 75% reduction in installation time. It is repeatable, uniform, controlled loading that eliminates thread galling on the Inconel studs that are common with toque methods. Tensioning offers shorter turnarounds and offers the ability to daisy chain multiple tensioners to speed the job up.
Most steam plants are unaware of hydraulic tensioning but it is certainly something to consider. I have seen plants spend well over $150K on turbine stud and bolt assemblies only to cut them off in the next outage. Lost hardware and lost time. Converting to Hydraulic tensioned equipment has a very quick return on investment and you won’t be sorry you did it.