At zero flow (static or stagnant condition), a piping system experiences hydraulic forces due to the weight and static pressure of the liquid to be conveyed. At steady-state, these forces are typically balanced such that forces on most elbows are balanced by forces at another elbow or by a restraint, such as a thrust block. Codes such as ASME B31.3 refer to this balanced hydraulic steady-state as the "Operating" pressure and temperature. Pipe stress software can be used to ensure that supports, guides and restraints are sufficiently strong to hold the pipes in position without excessive displacement or vibration.

Hydraulic transients occur whenever a change in flow and/or pressure is rapid with respect to the characteristic time of the system. The rapid changes in pressure and momentum that occur during a transient cause liquids [and gases] to exert transient forces on piping and appurtenances. This is highly significant for in-plant, buried and freely-supported piping because:

• If pressures and flows change during the transient event, the force vectors will likewise change in magnitude and direction. This has fundamental implications for the design of thrust blocks and restraints.
• Due to weight, transient forces are always three-dimensional even for horizontal pipelines. For buried piping, these forces are also resisted in three dimensions at discrete points (thrust blocks), transversely due to contact with the earth, and longitudinally due to pipe friction with the soil.
• Transient forces are not linearly proportional to transient pressures. A small increase in transient pressure can develop proportionally larger transient forces. This is because the forces are not a linear function of the pressures.
• Thrust blocks or restraints designed for the steady-state or "operating case" times a (constant) safety factor can often be inadequate to resist transient forces, especially for systems with high operating pressures, temperatures or mass.

Codes such as ASME B31.3 refer to a fluid transient as a "Dynamic" operating case, which may also include sudden thrust due to relief valves that pop open or rapid piping accelerations due to an earthquake. It is advisable to investigate fluid-structure interactions (FSI) that can develop for dynamic cases but the decision to undertake such analysis is largely up to the designer; except for boilers or nuclear installations.

Prior to the advent of inexpensive computing, transient and pipe stress calculations were onerous and virtually impossible to perform for large piping systems or plants. The increased analysis and design involved can be justified in terms of achieving a greater understanding of the system to ensure safe operations with minimum down-time. Designers are well-advised to follow the following steps:

• Steady-state analysis using HAMMER: layout piping and equipment to convey the steady-state flow efficiently. This remains the essential design step and governs the economics of most systems by determining the number, material/thickness and length of pipe required.
• Transient analysis using HAMMER: revisit pipe class and/or add protective equipment to keep transient pressures as close to steady as possible. Check steady and transient forces to guide the design of thrust blocks. This may be the last step in the design of buried pipelines, or specialized pipe/soil models can be used to check for sufficient support and resistance to overburden and groundwater.
• Pipe stress analysis using Bentley AutoPIPE: verify supports, guides and restraints against steady-state (operating case) and transient (dynamic) plus thermal pipe stresses, if any. This may be the last step in the design of process plant piping, or additional time or frequency-domain analysis may be performed to check for flow-induced vibration or earthquakes.

HAMMER needs X, Y and Z (elevation) coordinates to calculate transient forces. Simulations for which transient forces are enabled have longer completion times but there are no additional steps. The results are available as tables or graphics in a similar way as transient pressures: transient force graphs show the X, Y and Z components as well as the resultant magnitude. Transient forces are also available from FlexTables: these can be used as input to pipe stress software such as Bentley AutoPIPE.