Turbo Tips: How to Manage Bolt Connections & Bolt Joints in Turbomachines

Turbomachines feature several types of joint connections. Bolt connections, flange connections, and bolt joints are the most widely used; however, flange and bolt connections are major points for leakage and operational problems.

Many turbomachinery components, such as casings and piping, need to be dismantled or disconnected. Bolt connections, which make disconnection possible, may seem simple, but they can be complex in their mechanical behavior, performance, and operation. Typically, the behavior of a bolt connection is highly nonlinear and complicated, so bolt connections should only be used to disassemble or adjust for operation or maintenance. In addition, bolt connections are expensive compared to welding and other types of connections, and they are relatively heavy and need special care and provisions, such as inspection, maintenance, etc.

CRITICALITY OF DIFFERENT BOLT CONNECTIONS

Bolt connections only account for a small amount of the overall budget of a turbomachine. However, they are responsible for many leaks, operational problems, shutdowns, and failures. Bolt-connection requirements often determine the size and design of attached items, modules, and subsections. Specific nonstandard bolt connections may require extra thought in connection sizing, configuration, and installation.

NONLINEARITIES, YEILDING & COMPLICATIONS

Many components are involved in a bolt connection. Bolt details (size, patterns, etc.), the application/details of nuts/ washers, connecting plates/components, and the main connected assemblies all have a significant effect on the performance and reliability of the bolt connection. Any one of the involved parts can fail or cause failure in the bolt connection—bolt failure, cracks, and/or damages at bolt holes, failures due to bending stresses, buckling of parts, such as the compression flange, and others.

Despite widespread applications, bolting connections are surprisingly unfamiliar to engineers, operators, and experts. Some sources of difficulties are:

• Local yielding in different parts. This involves localized loads and highly localized stresses.
• High nonlinearities. For example, material nonlinearity due to stress-strain relationships and the involvement of complex contact and contact stresses.

As components yield and plastic zones extend, the behavior of the bolt connection gets more complicated and becomes highly nonlinear. After some yielding, there are bolt load (plastic) redistributions that usually lead to an increased moment capacity of the bolt connection and reduced stiffness of the connection and connected components. However, the redistribution of plastic/inelastic bolt loads/forces is a complex phenomenon. In general, many theories and methods, even modern ones, cannot predict the behavior and performance of bolt connections accurately. Some underestimate the forces and stresses, and others overestimate the forces/ stresses.

BOLT TENSION

Bolts should be tensioned using proper methods before a turbomachine assumes loading. This is a necessary step for any bolting joint to ensure that the load-bearing and load-transmitting plies are brought into effective contact. The final tensioning of the bolt connection should proceed from the stiffest part of the connection toward the free edges. Previously tensioned bolts could have been loosened by adjusting adjacent bolts, so re-tightening and -tensioning may be required.

OPTIMIZATION & KEY AREAS OF CONCERN

Bolt connections should be sufficiently strong, rigid, and stiff to prevent any leakage or operational mishaps under adverse scenarios. On the other hand, connections in general, and bolt connections in particular, should be lightweight, compact, and cost-effective. Given that bolt-connection components are expensive when compared to the connecting module or section, selecting the appropriate size and bolt optimizes installation and cost.

If bolts, relative to the members or parts they connect, are not strong enough, they will break. If the reverse is true, the member or part might weaken due to fatigue. If connecting plates or sections are not strong enough, they may fail in the bolt hole. Although the entire bolt connection should be strong and ideally designed for infinite life, it is easier to periodically replace a few bolts rather than break the entire bolted connection, which can be troublesome and expensive.

Key areas of concern for bolt connections are:

1. losses in bolt tension after installation (changes in bolt tension over time);
2. slip resistance of the joint;
3. the ability to tighten the bolts using standard installation techniques; and
4. washer requirements for normal bolt holes and oversized (slotted) bolt holes.

RELAXATION & LOSS IN BOLT PRELOADS

For typical high-strength bolts in normal circumstances, the total loss of the preload due to relaxation should be below 25% of the initial preload, and, in many cases, it should be below 20%—so on average, 10-18%. Preload is usually unaffected by small temperature changes. For bolts tightened to their yield point, bolt relaxations between 6 - 12% can occur after 5 - 10 years of operation. A large portion of the loss in bolt load occurs within days or weeks after tightening.

Washers and their details play a vital role in this regard. The loss of preload is about 6 - 8% for a typical bolt connection with two washers and about 10 - 14% for a bolt connection with one washer (10 - 15 years of operation).

BOLT JOINTS UNDER VIBRATION

In general, bolt connections on turbomachines and rotating equipment under vibration or dynamic/shaking forces need focused care. Nuts that are subject to vibration should also be secured.

Some experts suggest not using slotted (bolt) holes under fatigue-loading conditions. However, others support the use of slotted holes in certain conditions. The risk of movement under dynamic/ cyclic load should be considered and properly addressed.

BOLT CONNECTION DETAILS

Bolt connections and their counterparts are often the weakest link in a turbomachine or connected piping. As such, the load capacities of each component and connection should be evaluated, including connectors such as bolts, nuts, and washers, and connection components such as plates or flanges.

There should be sufficient distance between bolt holes and between each bolt hole and the edge of the connecting plate/ part to avoid failure. Similarly, to tighten bolts, ensure there is access to each one and sufficient distances between bolts and the surrounding obstacles.

To prevent bolts from tearing, center-to-center spacing should be at least 2.5 times the bolt-hole diameter and edge distance, ranging from 1.5 to 1.75 times the diameter, depending on how the edge is cut. To be safe, it should be 1.75 times, or even two times, the diameter.

ABOUT THE AUTHOR

Amin Almasi is a Chartered Professional Engineer in Australia and the U.K. (M.Sc. and B.Sc. in mechanical engineering). He is a senior consultant specializing in rotating equipment, condition monitoring and reliability.