Pressure peaks in hydraulic systems: A risk to sensors and other equipment

Pressure peaks in hydraulic systems: A risk to sensors and other equipment

Pressure peaks occur in virtually all gas and liquid-filled pipelines. Those pressures arising in just a few milliseconds can exceed the overload pressure of the pressure transducers employed and also destroy them.

Pressure peaks, or very high pressures existing over a short timeframe, are usually noticed only when the damage has already been done. They are the result of pressure surges and also other physical phenomena (cavitation, micro-diesel effect) that occur wherever liquids or gases are transported through pipes. Pressure peaks, however, are less important among gases due to their high compressibility and thus only rarely represent a danger. In the context of water pipes, the term ‘water hammer’ is often used. With these terms, a dynamic pressure change of the liquid is ultimately implied. When, for example, a valve is quickly closed, water flow will stop instantaneously. This triggers a pressure wave, which flows through the medium against the direction of flow at the speed of sound and is then reflected back again. Within milliseconds, there is a sharp pressure increase which can cause damage to pressure sensors and other equipment (damage to pipe fittings and pipe clamps, as well as to pumps and their footings etc.). In the first line, however, it is the measuring devices that are affected, upon which we will be concentrating in the following. These damages can appear as a tiny “rupture” or a deformation (see Figures 1 and 2).

Figure 1: “Rupture” as a result of pressure spike

Figure 2: Deformations due to pressure peaks

If the pressure acting on the pressure transducer exceeds the overload pressure, then this will sustain permanent damage. There are two possible scenarios here: As paradoxical as it may sound, the complete destruction of the measuring instrument due to pressure peak is the mildest of consequences. Users, after all, do notice the damage immediately here. If the sensor is merely deformed as the result of a pressure peak, however, it will continue to operate, but deliver only inaccurate measurements. The financial consequences here are disproportionally higher than with a totally destroyed sensor.

How to prevent damage caused by pressure peaks

The golden path to preventing damage caused by pressure peaks lies in the integration of pulsation dampers or pressure chokes. Other means, such as the use of valves, would not lead to satisfactory results, because they are too slow to react to pressure peaks which actually arise in mere milliseconds.

The purpose of a choke is to dampen pressure peaks so that they no longer exceed the overload pressure of pressure transducers and then damage them. For this purpose, the choke is placed in the pressure channel in front of the sensor cell. As a result, pressure peaks will no longer reach the membrane directly and unchecked, since they must first pass through the choke itself:

Figure 3: Pressure channel with Pressure choke

Because of their very good protection from pressure peaks, the use of pressure chokes remains the best option. This variant, however, does have its pitfalls. It can lead to a blockage of the pressure channel due to calcification and deposits, especially in media with solid and suspended particles. This results in a slowing down of the measurement signal. If chokes are used in relevant applications, then regular maintenance should be carried out here.

A supplementary protection from pressure peaks can be achieved with a higher overpressure resistance, as opposed to the standard one. Whether this is advisable depends upon the particular application: If high accuracy readings are required, these can no longer be achieved in certain circumstances of very high overpressure resistance relative to the measurement range.

Preventing Corrosion Caused by Aggressive Liquids in the Food Industry

Preventing Corrosion Caused by Aggressive Liquids in the Food Industry

When testing proportional pressure regulators as part of the development of complex hydraulic systems, high impulse capability and precision are required from the pressure measurement sensors employed.

Carbonic acid and alcohol can put a strain on measuring equipment. A manufacturer of automatic in-line and laboratory liquid analyzers has approached STS to find a durable and accurate pressure transmitter.

When exposed to aggressive fluids such as alcohol or carbonic acid, standard materials suffer from corrosion. For example, carbonic acid causes an increase in the hydron (H +) concentration and therefore leads to hydrogen corrosion. Once the corrosion eats through the membrane of the pressure sensor, it becomes unusable. That is why regular stainless steel will not suffice for applications with high levels of carbonic acid.

Other than being highly corrosion-resistant, the pressure sensor for this particular application in a bottling plant has to be able to deal with extremely low pressures close to a vacuum. As this application is part of the food industry, hygiene standards are very high. The near-vacuum conditions that the equipment is regularly exposed to is part of the sterilization process (similar, although not as extreme, as what happens in an autoclave). Low pressures below 0 bar can present a danger to the integrity of pressure sensors. The vacuum may cause the membrane to be sucked off from it position in the sensor. False measuring results or a completely broken sensor are the consequence.

Due to these requirements, we had to assemble a customized solution for this manufacturer of automatic in-line and laboratory liquid analyzers based on the pressure transmitter ATM.ECO. As material, we chose an extremely corrosion-resistant Hastelloy steel. To ensure membrane stability during low pressure conditions, we applied a special glue to fixate the membrane in place.

Since the pressure transmitter operates under room temperature conditions in this application, no special temperature compensation was necessary. The accuracy of 0,25 percent of the total scale is also more than enough for this particular application. The full scale ranges from 1 to 15,000 psi and is hence perfectly suitable for low pressure.

Mud Pulse Telemetry: MWD Data Transmission with Pressure Sensors

Mud Pulse Telemetry: MWD Data Transmission with Pressure Sensors

Hydraulic data transmission requires sensitive pressure sensors capable of enduring high pressures. This is particularly true when used in measurement while drilling (MWD) applications.

MWD has become a standard application, especially for offshore directional drilling. Real-time data collection is essential for measuring the trajectory of the hole as it is drilled. For this purpose, various sensors are mounted on the drill head to provide information about the drilling environment in real time. Inclination, temperature, ultrasound and also radiation sensors are used. These various sensors are physically or digitally connected to a logic unit that converts the information into binary digits. The downhole data are transmitted to the surface via mud pulse telemetry. In addition to monitoring and controlling the drilling process, the data are used for further aspects, including:

  • Information about the condition of the drill bit
  • Records of the geological formations penetrated by the borehole
  • Creation of performance statistics to identify possible improvements
  • Risk analysis for future drilling

Mud pulse telemetry is a binary coding transmission system used with liquids. This is achieved by a valve that varies the pressure of the drilling mud within the drill string and thus converts the recordings of the sensors mounted on the drill head into pressure pulses. The pulsations reach the surface via the drilling mud. The pressure pulses are measured on the surface by a pressure transmitter and converted into an electrical signal. This signal is transmitted to a computer and digitized.

STS provides offshore directional drilling companies with analog pressure transmitters optimized for mud pulse telemetry. The sensors have to meet high demands: They must be extremely sensitive in order to reliably register even the smallest pressure differences. At the same time, the sensors must withstand pressures of up to 1,000 bar. Very high pressures are required to power the drill head in very deep drill holes. The pressure transmitters used for mud pulse telemetry on the surface are also exposed to these forces.

In addition to the high sensitivity, very fast response times are required to ensure good data communication in real time. In order to exclude falsified measurement results, the measuring instrument should be low-noise. The mud pumps in particular can cause the most signal noise in drilling applications. The drive of the drill is another source of interference. For this reason, analogue sensors with a 4 – 20 mA output signal are the best solution for mud pulse telemetry.

The Right Leak Testing Equipment

The Right Leak Testing Equipment

Many applications have components that must be absolutely leakproof to ensure proper operation. Leak tests are commonly carried out with pressure transducers that have to meet high requirements.

Applications and components that have to be leakproof are, amongst others:

  • Engines, brake systems, air conditioning systems, cylinder heads, valves, filters, fuel and injection systems
  • Packaging in the food industry or medical technology
  • Electric appliances
  • Refrigeration systems
  • Hydraulic systems

Components that have to be tight are usually sealed before installation. The equipment used for leak testing must therefore work very reliably during production.

Usually, leak testing is carried out by means of a pressure measurement. Pressure is applied to the component. The pressure is measured again after a rest period. If a pressure drop has occurred between both measurements, the component can be considered leaking.

The stable and precise function of the pressure sensor used for the test is crucial for the detection of leaks. In particular, the requirements regarding stability and the adverse effects of atmospheric noise are very high. Even minimal pressure losses must be detected reliably.

For example, accuracy values should not exceed 10 … 20 Pa or 0.001% … 0.002% of the full scale for a 10 bar sensor.

STS has been manufacturing leak detection sensors for years, including the ATM series analog pressure transmitters with a 4 … 20 mA output signal. The high-precision measuring element detects even low pressure losses in the mbar range and thus meets the high requirements of leak testing applications.

The mechanical design (process connection and electrical connection) does not affect the behavior of the sensor and can be configured thanks to the modular design principle employed by STS.

The pressure sensors of the ATM series are available with different output signals. In this application, however, it is important to use 4 … 20 mA as this robust output signal is largely unaffected by atmospheric noise.

Read more about leak testing here.

Accurate pressure measurement is critical to safe, cost-effective, motor vehicle development

Accurate pressure measurement is critical to safe, cost-effective, motor vehicle development

The principle of hydraulic power to carry out work has been around since ancient Egyptian times, but as systems have evolved, so too have the tools required to design and develop these sophisticated, often critical circuits.

From the earliest manometer invented by Evangelista Torricelli in the 1600’s to the mechanical Bourdon gauge and finally today, the piezoresistive pressure transducer, developers have always sought the best equipment to measure pressures and optimize the design. And in recent times automotive engineers, in particular, have come to rely on these high-quality, accurate pressure sensors when carrying out vehicle testing and development.

These current pressure transducers are typically capable of recording full-scale deflections from about 350 mbar to 700 bar under sustained temperatures ranging from -40OC to 150°C; and best of all, quality sensors such as those produced by STS, are capable of a hysteresis and repeatability of typically around 0.001%!

Image 1: High precision pressure transmitter ATM.1ST with accuracy of up to 0.05% FS

High-quality pressure sensors are used in the development of key automotive systems.

This level of repeatability is critical in the design and development of cooling and fuel delivery systems, amongst others. During development, designers rely on stable pressure measuring equipment to accurately record information so that the effect of even the smallest of design changes can be documented without concerns that the sensor is incapable of repeatable results.

In a recent redesign of an engine cooling system to take advantage of the reduced parasitic losses made possible through electrification, the engineering team at a luxury OEM was initially faced with a pressure drop across the pump of around 250kPa. Before a redesign of the new electric pump was possible, accurate pressure measurements had to be recorded allowing engineers the opportunity to identify the problem. After studying the results logged by the array of pressure sensors the design was modified, reducing the drop to less than 100kPa and cutting the parasitic losses by 500W.

And although electrification and electronic controls are playing increasingly significant roles in vehicle systems, hydraulic pressure is still relied upon to guarantee smooth operation of many critical circuits.

By way of example, during the development of an automatic transmission, port line pressures have to be measured in real time and then compared to design norms to confirm that design parameters are being met. At the same time, shift times and quality are measured and subjectively evaluated to ensure drivability and performance meet customer requirements.

Notwithstanding the value of high-quality pressure sensors in recording valuable data during testing and development, in industrializing future technologies these tools can also significantly reduce design costs.

Pressure sensors make sure future technologies measure up to expectations.

In an attempt to improve the performance of severely downsized engines, manufacturers are taking advantage of the additional power 48V electrification offers, by replacing the turbocharger with an electrical supercharger.

Being a maturing technology, not much research and testing data are available to engineers wishing to optimize eCharge superchargers. Although fluid dynamics and electrical engineering provide a sound platform from which to build, it’s still vital that theories are validated under real-world test conditions.

To achieve this, manifold pressures must be mapped to optimize engine performance while maximizing the energy recovered from the exhaust gas. For this, extremely accurate pressure sensors that provide precise readings over a wide range of manifold boost pressures and temperatures are required. These sensors must also be resistant to vibration and chemical degradation.

And while manufacturers around the world continue to carry out research into electric vehicles, several groups are considering ways to harness hydrogen to generate electricity instead of relying on storage batteries.

Hydrogen fuel cells employing proton exchange membranes, also known as polymer electrolyte membrane (PEM) fuel cells (PEMFC), have already seen limited series production in vehicles such as Toyota’s Mirai.

Although small PEM fuel cells commonly operate at normal air pressure, higher powered fuel cells, of 10kW or more, usually run at elevated pressures. As with conventional Internal Combustion Engines, the purpose of increasing the pressure in a fuel stack is to increase the specific power by extracting more power out of the same size cell.

Typically the PEM fuel cell operates at pressures ranging from near atmospheric to about 3Bar, and at temperatures between 50 and 90°C. While higher power densities made possible by increasing the operating pressure, the net system efficiency may be lower due to the power needed to compress the air; hence the importance of balancing the pressure to the requirements of the particular fuel cell.

As with ICE boost pressures, this can only be done by taking accurate pressure measurements using high-quality pressure sensors. These measurements are then compared to the fuel stack outputs to minimize the parasitic losses while optimizing the gains in electrical output.

So, irrespective of the course the automotive industry chooses for future technologies, accurate pressure sensors will remain key to the development of safe and efficient vehicles.

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