Pressure

Introduction-Transient Pressure

A transient pressure in a pipeline is a generic term for a wave phenomenon that accompanies a rapid change of the velocity of the fluid in the pipeline. Authors variously use the term “surge” pressure to denote a transient pressure that has no detrimental effect, whereas the term “waterhammer” may be used to denote a sudden transient pressure that will have serious consequences if not properly addressed and mitigated. Pressure transients can be positive or negative. The magnitude of these surges is independent of the operating pressure, and can be many times normal operating pressure. The duration of a transient pressure will vary from several minutes in relatively long pipe segments, to several hundredths of a second in the case of waterhammer events.

Pressure Transient shown in red

Pressure Transient shown in red

Pipelines are designed to withstand transient pressures among the various loads that will be placed on them during operation: external loads such as soil, traffic, and the weight of the pipe itself; and internal pressure loads. The hydraulic design of the pipeline will identify the normal operating pressures, and will estimate the transient pressures or surge pressures that will accompany any change in the rate of flow in the pipeline such as the start of a pump or the closing of a valve. The operator will strive to make sure the loads imposed on a pipe do not exceed the design loads, but a problem arises when unforeseen operational events cause transient pressures exceeding the designer’s estimate. A frequent refrain from pipeline operators who have experienced failures or ruptures is that transient pressures may have caused the problem, but there is no way of knowing because transient pressures, sometimes referred to as waterhammer, are impossible to detect. We have all heard loud, banging waterhammer in household water or steam pipes; in buried pipelines there is normally no one to there to hear the event which may have a duration of only a few thousandths of a second.

The TP-1, Transient Pressure Monitoring System is specifically designed to give pipeline operators detailed information about transient pressures within a pipeline. Like conventional data loggers the TP-1 can continuously collect pressure measurements at any pre-programmed interval. What sets the TP-1 apart is its ability to instantly begin recording pressure data at up to 100 times per second should an anomaly occur. This patented second level of high-speed data collection allows operators to detect any spikes that would have otherwise gone undetected or insufficiently recorded given the standard rate of data collection. Once the pressure stabilizes the TP-1 returns to its normal monitoring mode.

The TP-1 is portable and easy to install on any pipeline transporting any fluid. The system has a wireless interface that makes it easy to collect the data from even the remotest of locations and comes with easy to use analysis software that offers operators instant information.

Events that change the velocity of fluid in a pipeline will cause a pressure change. Typical causes of transient pressures include:

  • Opening and closing of control valves
  • Starting and stopping of pumps
  • Sudden electrical power failures
  • Increase and decrease of customer delivery rates
  • Operation of air release valves
  • Operation of pressure release valves
  • Operation of backflow prevention valves
  • Filter backwash operations
  • Change of refined petroleum product being delivered
  • Pulsations from cylinder-type positive displacement pumps
  • Oscillations from unstable pressure reducing valves
  • Leaks and ruptures

But there is a need to monitor these transients, as they may cause damage if

  • The highest pressures exceed the structural design of the pipe, pipe seals nd gaskets, and thrust restraint systems
  • Negative pressures are observed, which may lead to cavitation, external liquid intrusion, and waterhammer
  • Continuous or frequent pressure oscillation occurs which may lead to material fatigue and structural failure even though the design pressures are not exceeded.

Transients often go unnoticed due to insufficient monitoring. An event or a series of events can lead to transients exceeding the design pressure of a pipe. Left unchecked, such events can affect delivery efficiency and weaken a pipeline, making it vulnerable to leakage or rupture

The TP-1 can assist operators in finding:

  • Faulty or insufficiently placed air valves
  • Faulty valve actuators
  • Isolated surge chambers
  • Faulty or inadequate pumps
  • Poor pipeline management procedures
  • Many types of anomalous transients are present immediately following construction, while others develop over time. Some events can be controlled, while others such as an electrical power failure, are uncontrolled and occur without warning. The TP-1 offers a pipeline operator the ability to identify and quantify many types of transient in every type of condition