Water hammer and other hydraulic phenomena

Dr. Otakar Jonas, PE, and Joyce M. Mancini, Jonas Inc.

Pages: 1 2 3 4 5

Water hammer—or hydraulic shock —is an apt description of the sudden impact of a mass of fast-moving water on a piping component such as a valve or an elbow. It usually occurs in longer steam lines and is caused by the opening of a shut-off valve or the mixing of steam from one pipe with condensate from another.

Several other thermohydraulic phenomena (Table 1) often are mischaracterized as water hammer. The observed damage includes mechanical damage, thermal shock, and high cycle fatigue. Water hammer happens rarely. But when it does, it can be destructive, expensive, or even fatal. Table 2 identifies the most common contributors to water hammer.

Table 1. Hydraulic and thermohydraulic shock events and where they occur. Source: Jonas Inc.

Table 2. Contributors to water hammer. Source: Jonas Inc.

Avoiding water hammer and similar events requires both a thorough understanding of their causes and contributing factors and following good design and O&M practices see box). As the four tragic case studies that conclude this article make clear, water hammer prevention should be given a higher priority at all power plants.

Considered as a whole, water hammer is a complex fluid mechanics process that may embody some or all of the following five phenomena.

Preventing water hammer and hydrolic shocks

A properly designed, operated, and maintained steam system should never, or rarely, suffer a water hammer event or hydraulic shock. Following are some steps that designers and O&M personnel can take to achieve that goal:

Pitch steam pipes away from the boiler toward a drip trap station.
Install a drip trap before each steam regulator valve to prevent condensate from accumulating behind the valve while it is closed.
Install drip trap stations ahead of any risers, at the end of the steam main, and at every 500 feet along the steam piping.
Inspect traps often and replace any found to be defective.
Repair damaged pipe insulation and replace any that has been removed.
Repair any sagging pipes or install steam traps at the low point.
Do not allow steam into any line filled with subcooled condensate or into any cold steam line suspected of containing condensate.
Do not "crack open" valves to avoid water hammer. Condensation-induced water hammer can occur at very low condensate flow rates.
If steam and condensate are in contact, stop the flow of steam before draining the condensate.
Wherever possible, use mechanical extensions or reach rods to operate manual valves.
Test deaerators during steam system commissioning and optimize their operation to follow load, temperature, and rate of pressure change with load.