Of all the loading conditions that a piping system may experience in service, hydraulic transients are among the most damaging. The most common form of damage caused by hydraulic transient loads is the failure of pipe supports and supporting structures. However, occasional breaches of pressure integrity are also experienced, particularly where large-diameter thin-walled pipe is involved. Two common types of hydraulic transient loads are water hammer and relief valve discharge. These two load sets are discussed in detail below.
If the velocity of water or other liquid ﬂowing in a pipe is suddenly reduced, a pressure wave results which travels up and down the piping system at the speed of sound in the liquid. Depending upon the initial velocity and physical properties of the liquid and the mechanical properties of the piping system, the peak value of the pressure wave may exceed the steady-state pressure.
Water hammer frequently occurs in systems that are subject to rapid changes in ﬂuid ﬂow rate, including systems with rapidly actuated valves, fast-starting pumps, and check valves. It is most severe in systems which convey fully condensed liquids; however, it is possible to develop water hammer-type pressure transients in systems containing two-phase ﬂuids and gases, although the magnitude of the pressure rise for these systems will generally be lower.
Water hammer must be considered in the design of those systems where it is likely to occur. For systems designed to codes that provide higher allowable stress criteria for occasional loads, the water hammer-induced peak pressure should be evaluated under that loading category. For systems designed to codes which do not provide alternative design criteria for occasional loads, the design pressure may be set high enough to envelop the water hammer-induced peak pressure. The designer is cautioned that this approach can result in an extremely conservative design, which may be prohibitively expensive. Consequently consideration of alternatives may be required.
Relief Valve Discharge Loads.
Because of their rapid opening characteristics and generally high ﬂow rates, the actuation of relief valves frequently results in the application of signiﬁcant loads to the associated piping system. These loads are caused by the differential pressures across the valves, differential pressure between the valve discharge and the downstream discharge piping, and differential pressure between the discharge piping and the receiver or atmosphere. In addition, momentum effects caused by velocity changes at high ﬂow rates result in secondary loads,which act on the attached (or non-attached) piping. These secondary loads must be reacted to by the supporting structures. Actual pipe stress levels in inadequately designed relief valve installations can exceed code-allowable values, and failures of such installations are common. Depending upon the pressure and temperature conditions of the system ﬂuid, such failures can represent a personnel safety hazard as well as a costly economic issue.
To assist the piping designer in developing a safe and functional relief valve installation, Appendix II of ASME B31.1, Power Piping, was developed and issued. This non-mandatory appendix to the code provides an extensive treatment of the relevant factors which must be considered to produce a successful design. Relief valve discharge loadings typically occur during a very small percentage of the total system operating time; consequently, they can be treated as occasional loads. The design for load combinations that include these hydraulic transients can therefore be based upon the higher code-deﬁned stress limits.