Pulsation Analysis in Positive Displacement Pump Systems Using Waterhammer, Modal and Animation Software
AFT Impulse™ Technical Paper
Authors: James M. Blanding, Ph.D., DuPont Company; Trey Walters, P.E., Applied Flow Technology
Presented at the 45th Turbomachinery & 32nd Pump Symposia Conference, September 13, 2016
ABSTRACT
High-pressure and flow metering systems often include a positive displacement (PD) pump and a network of piping and process equipment. Steady state pressure and flow dynamics (pulsation) is a common problem in PD pump systems, which can cause high vibration, fatigue failures, frequent maintenance outages, and flow uniformity or product quality problems.
A number of engineering consulting firms have developed computational capabilities in pulsation, which they offer in the form of client services, but these are expensive. So too is the limited amount of commercial pulsation-specific software.
This paper describes a methodology to computer model pulsation using a combination of general-purpose and readily available, widely used software, such as AFT Impulse. Waterhammer Software – Normally used to simulate transients, if periodic flow forcing is modeled, the transients die out, and leave only the steady-state pulsation Modal Software – Frequency analysis of high-speed digital time history data can be done with many software platforms, including those for Experimental Modal Analysis (EMA), Data Acquisition, or general-purpose Statistical/Mathematical modeling Animation Software – Typically included with EMA software, animation is not necessary, but is useful to visually show pressure and flow response across the system at acoustic natural frequencies and for PD pump flow forcing An example problem shows how to efficiently model complex systems to determine acoustic natural frequencies, mode shapes, and pulsation response to PD pump forcing. Fundamentals of waterhammer and important modeling techniques are discussed. An introduction to EMA and digital signal processing is also provided.
CONCLUSION
Historically, waterhammer simulation has been viewed as an area only to be evaluated by specialists in the field of waterhammer. This was due in part to the unavailability of robust commercial software solutions. As the commercial engineering software market has
evolved and matured, robust and viable solutions for waterhammer simulation have become available. While these software solutions can never replace the expertise of waterhammer specialists, they have lowered the barrier to entry for non-specialists to be able to do productive work as a waterhammer analyst, especially when using supported commercial software. Waterhammer software often uses a graphically-driven user interface with a “worksheet” and drag-and-drop modules.
View the paper for the proposed 5-STEP methodology for pulsation analysis.
Below is an excerpt. Use the links above to view the full paper.
Introduction
High-pressure and flow metering systems often include a positive displacement (PD) pump and a network of piping and process equipment components. An example is shown in Fig 1 of such a suction system. Steady state pressure and flow dynamics, henceforth referred to simply as “pulsation”, is a common problem in PD pump systems, which can cause high vibration, fatigue failures, frequent maintenance outages, and flow uniformity or product quality problems.
This paper describes a methodology to computer model pulsation using a combination of general-purpose and readily available, widely used software. Waterhammer Software – “Classical Waterhammer” refers to the transient pressure spike and subsequent acoustic wave reflections that occur when a valve is suddenly closed in a long pipeline with established flow. Modern waterhammer software deals with all manner of transient pressure and flow dynamics in liquid piping networks. Any of several robust and benchmarked programs is suitable; see Ghidaoui, et al (2005).
No matter the program of choice, henceforth it is simply referred to generically as “the waterhammer software”. As will be seen in this paper, its use in pulsation analysis is atypical. For example when periodic once-perrev flow forcing of a PD pump is input, the simulation is run until all transients die out, leaving only the steady-state pulsation. Modal Software – Signal processing of high-speed digital time history data in the frequency domain can be done with many software platforms, including those for Experimental Modal Analysis (EMA), Data Acquisition, or general-purpose Statistical/Mathematical modeling. No matter the program of choice, it is henceforth simply referred to generically as “the modal software”. Animation Software – While animation is not necessary to do pulsation analysis, it is useful to visually show pressure and flow response throughout the piping network at acoustic natural frequencies and for PD pump flow forcing.
The pulsation modal and forced response shapes in the piping network can be shown in 2D animation. This capability is typically included in EMA software. General Steps in Pulsation Analysis and Typical Results – To computer model pulsation the following general steps are proposed.
- STEP 1. Model the piping and process equipment system
- STEP 2. Determine the natural frequencies and mode shapes of the system
- STEP 3. Identify “worst-case” pump speeds that excite natural frequencies
- STEP 4. Simulate operating pulsation response to PD pump flow forcing STEP 5.
Modify the system and reanalyze if pulsation is too high To illustrate what pulsation results look like, Fig 2 shows the waterhammer model and Fig 3 the modal and forced response animation depiction of the Fig 1 physical system. Note that the Fig 3 animation model displays all piping, including branches, to be aligned along the X-axis (left-to-right). This is so that animation can be shown in the Y-axis direction to achieve a 2D representation of pulsation response throughout the piping network.
Figure 4 shows the forced response and animation of simulated pressure pulsation for the West pump running at 250 RPM. Maximum pulsation is about 6 PSI-PP or 10% of the mean suction pressure. Figure 5 shows pulsation for a lower speed, 215 RPM. Figure 6 shows pressure pulsation animation for these speeds using alternative animation software. Note that pulsation is much worse at the lower speed – 29 PSI-PP (47%)! Among the goals of the remainder of the paper are to answer questions such as, “Why is pulsation so much higher at a lower pump speed?”, and “How does one determine this and gain a thorough understanding of the pulsation behavior in the system?”