Causes of water hammer in pipelines

Water hammer is a term used to describe fluid transients, when banging sounds are sometimes heard as check valves slam shut or piping strikes adjacent structures. The terms water hammer and fluid transient are frequently used interchangeably, but fluid transient is a generic technical term, since it describes any pressure or flow change in a system.

This article contains excerpts from the paper, "Water hammer and piping stresses" by  Robert A. Leishear at the 2018 Turbomachinery and Pump Symposium.

Pressure surges during transient operations occur when flow rates are changed in any piping system containing vapors, gasses, liquids, or combinations of these fluids. As valves are opened in pipelines containing pressurized gasses or vapors upstream of those valves, pressure transients occur in the downstream piping. These transients are further complicated by the fact that sonic velocities may occur at the valve or in the piping, and fluid densities and temperatures vary significantly during these transients.

During all fluid transients, pressure waves are transmitted through the fluids. In liquid filled systems, these transients typically occur when valves and pumps are operated.

Fluid transients due to valve operations cause pressure surges of varying magnitudes, which depend on flow rates, piping dimensions, piping materials, the types of fluid, and the opening or closing speeds of valves. An example of sudden pressure changes, suddenly closed valves may cause high system pressures. Note that the resultant pressure due to this applied pressure increase is multiplied times the DLF to determine the pressure effects on maximum piping stresses. Suddenly opening valves causes pressures lower than those caused by sudden closing. Sudden valve openings also cause increased pressures in the system. Note that a finite difference computer codes, referred to as the Method of Characteristics (MOC) are typically used for fluid transient calculations.

Slow valve closure speeds result in lower pressures than those that would be caused by sudden valve closures. In fact, pressure surges may be eliminated if the closure speed is low enough, and the pressure will slowly rise to the operating pressure when a pump is started.

Pump operations are complicated by their installation and pump performance. In a long pipeline, a pump can act like a suddenly opened or closed valve. Pump Startups and Shutdowns Fluid transients occur when pumps are started or stopped. In a closed loop system, the flow will coast to a stop, and reverse flow through a pump will not occur. In a system pumping uphill using a single pump, a check valve is required to prevent reverse flow through a pump. Pressure surges will occur when the check valve closes.

If pumping downhill, there will be flow separation in the piping. The piping will be completely drained if the lower end of the piping is open to atmosphere. Otherwise fluid transients will occur due to vapor collapse on restart of the pump or opening of the valve, when upstream liquid is present.

When one of two pumps shuts down, the flow from the operating pump will reverse the flow through the stopping pump and hammer the check valve to a closed position. Resulting high pressures will occur, which are affected by the closing characteristics of the check valve.

Reflected pressure waves occur at all tees, changes in pipe diameter, and changes in pipe wall material. The maximum reflected pressure waves occur at the closed ends of piping, where the magnitude of a pressure wave is doubled as the impinging wave reflects back into the piping.

In general, trapped gas, or air, collects at high points in piping systems. These noncondensable gas pockets act to reduce the pressures caused by fluid transients when valves are closed or systems are pressurized. When systems are depressurized, noncondensable gasses expand, and system surge tanks or cooling tower levels open to atmosphere will suddenly increase, which may cause overflows.

Vapor cavity collapse and increased pressures occur in many different circumstances. Only some of those conditions are presented here. Any time that a vapor pocket is present in a system, valve or pump operations will cause pressure increases, and additionally valve and pump operations may form vapor pockets that create pressure increases

During Valve and Pump Operations When valves are suddenly closed, or pumps are shut down, vapor collapse may occur at high points in the system, where this vapor collapse is dependent on the hydraulic grade line of the system. High pressures may occur near the pump or valve as well as at the high points of the piping system.

When the pressure in a pipe reduces to the vapor pressure of the liquid contained in the pipe, the liquid vaporizes. During void formation, this low pressure occurs when the liquid column in the pipe separates due to motion. When the column rejoins, the vapor collapses. Waves are induced throughout the liquid filled system. Again, computer simulations are recommended, but when computer simulations are unavailable systems should be designed to prevent occurrence of vapor collapse. Where elbows occur near a starting pump, high forces and stresses may occur, since the pump may be operating near runout and the resultant flow rates that induce piping stresses will cause off-normal design conditions.

Slug flow transients are another example of vapor cavity collapse, and occur when a volume of water moves through a pipe due to an applied pressure. The slug will cause high pressures when it strikes elbows or the closed end of a pipe. For example, when a condensate slug moves through a system, it will impact every elbow as it travels and induce bending stresses at each of those elbows. When the slug reaches a closed end of the piping, the vapor in the piping will condense and collapse with negligible resistance to the moving slug of condensate, where increased pressures and reflected pressure waves will then occur.

When relief valves or safety valves open to atmosphere through their c piping, significant forces occur on the piping. ASME B31.1 [6] provides simplified methods to calculate these forces. Flare Headers Flare headers connect system piping to safety valves. Depending on design, forces and stresses can be minimized.

Slug flow transients are another example of vapor cavity collapse, and occur when a volume of water moves through a pipe due to an applied pressure. The slug will cause high pressures when it strikes elbows or the closed end of a pipe. For example, when a condensate slug moves through a system, it will impact every elbow as it travels and induce bending stresses at each of those elbows. When the slug reaches a closed end of the piping, the vapor in the piping will condense and collapse with negligible resistance to the moving slug of condensate, where increased pressures and reflected pressure waves will then occur.