Does Your Pressure Impulse System Measure Up?

One of the greatest growth areas for Wineman Technology are our lines of pressure impulse and pressure pulsation test systems. During the quoting process, our customers usually ask several questions to see how our engineered system’s capacities compare with several of our competitors “off the shelf” product lines. The main and perhaps the most important question that they ask is “Why are you asking me all of these questions about what we are testing and how we are testing it?” This blog post will hopefully highlight some of the differences in the WTI engineered system’s approach, why we ask so many questions, and if what you are buying really measures up.

The Base Pressure Impulse System Requirements

The overall approach to control and operational concepts of pressure impulse test systems varies as much as the number of suppliers currently in the market. As a point of discussion for this blog, let’s just look at the waveform’s pressure, volume, and test frequency requirements at the inlet of the unit under test (UUT).

Waveform Pressure Requirements:

The maximum test pressure requirement of an impulse test system is the peek pressure required for the utmost top of the pressure curve or waveform. Waveforms can take all shapes, from triangle type, square, modified square, and sinusoidal, to the adjustable, high-accuracy 20-point waveforms usually utilized in the aerospace industry. All are generally driven by industry-accepted specifications that identify the actual ramp times, pressures, and frequencies being utilized.


Figure 1 – A Common Waveform per J434-V004 / ISO6605 Standards


The actual peek test pressures are dependent on the UUT’s requirement, the specification being adhered to, and the on the type of test system being utilized. A few examples would be:

  • Pressure pulsation systems, or UUTs tested with continuous flow through the parts, are generally performed in the 50-500 psi range, although WTI has designed these systems upwards to 5,000 psi and 200 gpm. These types of systems are generally used for testing radiators, condensers, and for operational product testing.
  • Direct-drive impulse systems, or low-pressure component testing, can be performed utilizing either a direct-driven servo valve control system or a cylinder-driven control system. Pressures generally approach 3,000 psi in these types of systems and are used for smaller-volume or lower-frequency testing of general fluid control products.
  • Intensifier-(booster) driven systems are utilized for higher pressures, high frequencies, and higher response testing. System pressures are generally capped at 14,500 psi for standard systems, but can go as high a 60,000 psi in special applications. These types of systems are generally utilized for product life or fatigue testing of anything from a ball valve to an aircraft actuator.


Figure 2 – A typical, more complex, Waveform per AS603 Standards


The maximum, or peek, waveform’s pressure requirement is the number one driving force in determining what kind of impulse system is required and how much it will cost.

Waveform Volume Requirements:

This requirement is where the confusion starts creeping when specifying an impulse test system. Many of our customers will confuse the required fluid capacities required to perform the test. To break the total volume down into separate easily calculable portions, we use the following:

  • UUT capacity:  The amount of fluid, or volume, contained within the actual part being tested. An example would be how a one-quart filter may only require 20 cubic inches of fluid to fill it due to the element’s volumetric displacement.
  • Test capacity: The amount of fluid, or volume, contained within the circuit conductors, test manifolds, feedback transducer(s), isolation valving, etc. This volume can be substantially higher than the UUT volume in some cases and is dependent on the length of lines, the minimum flow requirements based upon the test frequency, and the number of UUTs being tested at one time.
  • Compressible capacity: The additional amount of fluid volume to be added that is required to raise the contained total volume, UUT capacity and test capacity from minimum to maximum test pressure.
It is important that the total system volume is considered when engineering a pressure impulse system. A common mistake is confusing an impulse test system’s compressible capacity with the UUT’s capacity.

Test Frequency Requirements:

Simply put, everyone wants higher test frequencies. The higher the frequency, the more throughput a system has and the higher return on investment a system can produce. Many of the standard waveforms/specifications used now list a minimum and maximum test frequency rate required to meet industry standardization requirements. Actual R&D testing requirements and available testing times may accelerate these requirements when and where possible.

The test frequency affects the overall system performance more than any other aspect of the test.

What to Look for When Specifying a System (Measuring Up)

Many system suppliers will outline their technical specifications as:

  •    Type of Waveform(s) Supported
  •    Maximum Test Pressure
  •    Minimum Test Pressure
  •    Displacement per Impulse
  •    Maximum Test Frequency
  •    Operational Temperatures

While all operational parameters outlined are correct, don’t simply assume that the system can provide all of the listed parameters simultaneously. We have been contacted by customers very frustrated that their “off the shelf” impulse test system seems to fall short when testing certain waveforms or operational parameters.

The operational parameters may vary based upon:

  • Test Volume: The overall test volumes, sometimes referred to as a displacement, may decrease based upon test frequencies.
  • Maximum Test Pressures: The higher your test pressure, the higher the test volume, or displacement per impulse that is required, becomes due to the fluid compressibility.
  • Minimum Test Pressures: Higher test frequencies may affect the minimum operational pressures. The control system may not have enough time to “recover” to minimum pressure at faster speeds. Many of the systems on market today are rated for a minimum operational pressure of 10 percent of the maximum pressures.
  • Test Fluids: The compressibility of a test fluid varies based upon the fluid being used and the test temperature. Changing fluids or temperatures may limit test pressure and test volumes. Are the parameters specified in a brochure based upon the test fluid you are actually using?
  • Test Frequency: The overall test volume may be limited by the test frequency and vice versa. Does the machine have the input drive to handle the rated volume at the maximum test frequency? Generally, the higher the frequency, the lower the available test volume (displacement per impulse).
  • Expansion: Do the specified operational parameters take the UUT’s and conductor’s expansion into account? Some hoses, for example, can increase their internal volume as much as 50 percent at higher pressures.
  • Displacement per Impulse: This is the limiting factor of any impulse test system. What operational parameters is a system’s displacement being based upon? The key factors to consider are pressure, temperature, frequency, total compressibility volume (UUT capacity plus test capacity), input power or control fluid capacities, and expansion. All of these factors are intertwined and need to be considered when selecting an impulse test system.

Considerations prior to a purchase:

  • Input Power: The machine’s input power is a good indication of maximum capacity. The higher the flows and pressures; the higher the input power must be. If the input power seems lower than other offerings, the overall performance is probably limited.
  • Waveforms: Are the waveforms adjustable and/or tunable to meet your changing UUT test requirements? All tests change over time.
  • Fluids: Are you limited to a single test fluid? The more flexible the test system, the fewer separate systems are required to test a range of UUTs.
  • Waveform Upgrades: Is the system’s control upgradable? Can additional waveforms be added in the future?
  • Cooling Capacities: Is the system’s cooling capacity designed for the water requirements within your facility? If not, additional closed-loop chillers may need to be added at additional cost(s).

A WTI System Requirement(s) Analysis

Wineman Technology’s engineering application team can provide a system requirement analysis to help you determine the best system to meet your testing requirements. This includes defining a system to meet your current and future lab testing life cycles. This may also include a dedicated impulse or pressure pulsation test stand, a combination test stand including burst pressure testing, dual or multiple fluid systems, multiple station test and/or automatic UUT selection/isolation fixtures, or any specialized design per application solution you may need.

As always, we welcome any comments on or suggestions regarding our blog articles to better support our customers’ requirements. For more information on hydraulic servo-based test systems, please contact Wineman Technology for detailed specifications or quotations to meet your testing requirements today.