Transient pressure monitoring is quickly becoming a standard across multiple industries and applications. While it was once used almost exclusively to record pressure from occasional water hammer events, industry experts now recognize the benefits of continuous transient pressure monitoring to ensure optimal fluid system operations.
By Amy Marroquin
Transient Pressure Monitoring
What distinguishes transient pressure monitoring from standard pressure monitoring is the scan and record rate of measured data. Typical DCS (distributed control system) and SCADA (supervisory control and data acquisition) monitoring systems measure and record pressure at frequencies ranging from 0.0167Hz (1x/min) to 1.0Hz (1x/sec). Transient pressure monitoring systems measure and record pressure at 100Hz to 500Hz and higher.
Findings from continuous transient pressure monitoring are revealing truer pressure recordings in all stages of operation including steady state. Pressure recordings using low scan rates generally result in smaller and fewer pressure fluctuations detected than those using high scan rates. To illustrate, Figure 1 depicts pressure recorded in a basic residential water system using two different scan rates. Significantly larger and more frequent pressure changes were revealed at the higher scan rate, 100Hz (Channel 1), compared to the concurrent recordings at the lower scan rate 1.0Hz (Channel 2).
Transient pressure monitoring is a relatively new capability and was, in the early stages of its release, considered only for specialized or problematic systems where it was necessary to capture rare, otherwise unexplained events associated with ‘water hammer’. This was due, in part, to how systems were commonly assessed. Systems with occasional pressure fluctuations and no water hammer issues or, relatively stable systems with rare water hammer events, were categorized as either ‘normal’ or ‘mostly normal’ with no need for transient pressure monitoring.
Only when system operations were unreliable and required continuous troubleshooting and maintenance were they categorized as ‘problematic’ and thus, transient pressure monitoring was deemed necessary. The challenge with this method of categorizing is the assumption that water hammer either occurs in a system or it does not.
What is ‘Water Hammer’?
Because pipelines do not operate at a constant velocity, water hammer occurs to some extent, quite often, in all piping systems. It is often described as a ‘phenomenon’ and has many aliases, including, surge, hydraulic transient, and acoustic pressure wave. All of these terms are misleading, however, as they allude to a singular, unanticipated ‘event’, when in fact, water hammer is quite simply any change in pressure resulting from a change in velocity, even if only a slight change.
One might notice transient pressure recordings resemble sound waves. This is for good reason as the pressure waves reverberate through the fluid system at the speed of sound. The particular speed at which they travel is primarily a function of the elasticity of both the pipe and the fluid. The speed of the pressure wave will be much higher for rigid piping systems (steel, ductile iron, etc.) with fluids that have high bulk moduli than for more elastic systems (PVC, plastic, etc.) with fluids that have lower bulk moduli. The wavespeed also has a direct impact on the degree of pressure fluctuation. Higher wavespeeds will result in more significant swings in pressure, both high and low. A system’s wavespeed is the primary reason why transient pressure waves, of all magnitudes, go undetected. They are simply moving too fast.
Transient Pressure in Pumps
For pumps, transient pressure monitoring is of particular interest. Pressure waves are emitted into the system whenever a pump starts up, shuts down, or changes speed. A system’s total rotating inertia and torque characteristics determine how fast a pump ramps up or down. Generally, a lower total rotating inertia means faster pump ramp up/ramp down. This may be the preferred scenario from a torque and power perspective but, from a transient pressure perspective, the resulting abrupt changes in flow lead to larger transient pressure waves.
A fast startup, for example, can induce a low pressure transient wave backwards into the suction piping, potentially risking vapor formation and subsequent cavitation. On the discharge side of the pump, quick and powerful startups create significant pressure spikes forward into the discharge piping. It is also worth noting that large imbalances of unbridled driver torque versus pump torque can lead to more significant transient pressure waves.
Opportunities to mitigate water hammer come at various stages of any project and involve a wide variety of tools and solutions. As transient pressure monitoring is becoming the new standard, so is hydraulic transient computer modeling. Transient modeling simulates a system’s design and operating scenarios, which allows engineers to predict the magnitude and dynamics of transient pressure waves. Accurate models are highly dependent on accurate data, and the comparison of field transient pressure monitoring data has become an invaluable method for both calibrating and verifying computer simulations.
Another opportunity, not yet fully realized, involves transient pressure monitoring in the research, development and functional testing of pump designs. Testing is somewhat limited since a manufacturer’s pump test can only feasibly capture a small fraction of potential applications. However limited, transient modelers can glean valuable data to improve the accuracy of models and simulation software. Understanding and matching the transient response of a pump’s published functional test to a computer simulation by means of transient pressure monitoring would not only improve the accuracy of these models, it might also offer insights into improved pump designs.
In summary, transient pressure monitoring is proving to be a necessary tool for providing more complete and accurate pressure data in piping systems. As the use of transient pressure monitoring continues to expand, the newfound data has the potential to spark shifts in current engineering methods and approaches so as to strengthen and stabilize our fluid piping systems. Time will tell.