Identifying and Troubleshooting Pressure Changes in Fluid Systems
Pressures are always rising and falling in fluid systems. Sometimes the changes are extreme and lead to damage. What causes problematic pressure changes, and how can you trace them to their source?
Fluid systems in automation consist of any system that uses a fluid to create a force within the system. Fluids can consist of two different media, gas (usually air, or pneumatic) or liquid (usually a special hydraulic fluid). In most simple, low-cost applications, the fluid used in pneumatic systems is air, while the fluid used in more complex hydraulic systems usually consists of a viscous oil. Air is compressible but hydraulic oil is not, lending unique benefits to each kind of system.
Pressure in Fluid Systems
In what might be an oversimplified definition of fluid flow, we can say that high-pressure fluid will move toward any lower-pressure area in the system whenever a valve is opened. Pressure changes occur naturally in every fluid system whenever that fluid moves or performs work (energy conversion).
These everyday pressure changes occur within the normal rated system pressure limits and have no damaging effect on the system.
Figure 1. Pressure gauges on a compressor tank show both the tank (internal) and outlet (system) pressure. Image used courtesy of Adobe Stock
There are instances, however, when pressure changes within the system can have negative effects on the system. If the pressure is too low, it may simply fail to accomplish the desired task. But sometimes, the change may be damaging, as is the case with overpressure. Pressure and flow rate sensors are often used to detect such changes and can be used to alert the control system that something is amiss. Then it’s up to the engineer to track down where and why the failure has occurred.
There are a number of reasons for incorrect fluid pressure, and finding the cause can help to prevent premature machine failure, save on wasted energy costs, and prevent damage from catastrophic failure.
Troubleshooting Pressure Changes in Pneumatic Systems
Pneumatics are used for a myriad of different tasks in automation and generally encompass medium-duty work, including vacuum grippers, pneumatic cylinders, and motors. Pneumatic systems rarely recycle the used air, instead using a compressor that feeds a tank, creating a reservoir of air to be used to perform work when it is needed.
Most facilities use a single large compressor and tank that feeds most of the equipment within the building, meaning there is a large infrastructure of pipes and hoses to carry the air where it needs to go. Identifying the source of a pressure change greatly depends on when the pressure change is seen.
Leaks are the number one cause of pressure drops in a pneumatic system. Since air leaks leave no physical mess they can often go unnoticed, especially small leaks that can be inaudible over the sound of the noisy manufacturing environment. Pressure drops from leaks are seen continuously when the culprit line is pressurized, so a flow sensor can help locate the source.
Figure 2. Air lines feed both sides of every double-acting cylinder for quick, low-force motion. Image used courtesy of Adobe Stock
Valves and actuators contain air-tight seals. If these deteriorate from improper lubrication or moisture in the lines, air will escape when the travel of the spool or piston is finished. This kind of leak will register only a slightly lowered pressure, but will become visible when the piston or valve starts to retract slowly alongside the pressure drop.
Lines, filters, and valves can become blocked due to contaminants, corrosion, and physical deforming. If a line is partially blocked, the pressure drop will be abnormally high when the air is flowing, meaning only while a cylinder is in motion. As soon as the motion ends, the pressure jumps back to normal. When acting against a load, the cylinder is likely to move more slowly than expected since the force (pressure) supplying the piston will have been reduced.
High Pneumatic Pressure
High pressure is not something that happens frequently in a pneumatic system, and for the most part, does not happen naturally. The most likely cause of an abnormally higher pressure in a pneumatic system is due to a manual adjustment of the pressure regulator in the system at the compressor. A second issue might be a faulty pressure regulator or pressure switch. If the spring fails in the regulator, there may be an instant loss or rise in pressure, but if the spring in the switch fails, the compressor may continue supplying the tank well beyond safe limits.
Figure 3. A hydraulic pump and reservoir system with pressure-limiting components. Image used courtesy of Adobe Stock
Troubleshooting Pressure Changes in Hydraulic Systems
Hydraulic systems are mainly used in automation for heavy-duty applications. They are used to create large forces necessary for some applications. They are more expensive than pneumatic systems in most cases but have their own set of pros and cons: tremendous pressure at the expense of flow rate (speed of actuator motion). Correct pressure is fundamental to the efficient operation of a hydraulic system and should be checked periodically to ensure proper machine operation.
Fluid Levels and Restrictions
Low pressure is another cause for concern in a hydraulic system. Low pressure is mostly caused by pump starvation due to low oil levels or clogged filters and screens. Motor wear can also create faulty seals within the pump itself, eventually leading to a low-pressure situation. If the oil level becomes too low, bubbles can form which can damage the pump, so try never to let a fluid system run low.
Leaks can also lower hydraulic system pressure as described previously, but they are often much more obvious than for pneumatic systems as they create large oil spills and are more likely to be noticed and repaired quickly.
Figure 4. Inside a hydraulic or pneumatic cylinder, the seals (visible in yellow and green) ensure that fluid is not leaking out of the system. Image used courtesy of Adobe Stock
Temperature and Viscosity
Temperature changes can create changes in the fluid viscosity, and therefore introduce issues in hydraulic systems as well. Low temperatures can lead to an increase in viscosity making the pump lose its ability to properly create hydraulic pressure. It also causes fluid to flow more slowly in the lines, exaggerating the normal pressure drops through lines, valves, and fittings. The opposite can be true in the case of high oil temperatures which can cause lubrication issues and potential failures.
High Pressure in Hydraulic Systems
Since hydraulic systems operate at extremely high pressures, the pumps that create these pressures need to function under extreme conditions. The main problem resulting from overpressurization in hydraulics is premature wear and failure at the pump as a result of loss of lubrication. Overpressurization also results in mechanical damage in the form of failed components and broken hoses and fittings.
Overpressurization comes from several different sources, often connected to the pressure devices built into the system, most notably the pressure regulators and any variable speed pumps. Thermal expansion can also cause overpressurization in the case of load-holding scenarios.
Pressure Changes: Good or Bad?
Pressure changes in fluid systems, although inevitable, can have negative impacts on plant automation if they extend beyond safe limits.
Leaks in pneumatic systems can waste energy since air is getting compressed only to be exhausted right back into the atmosphere without completing any useful work. Hydraulic systems must operate within a pressure range that allows useful work to be completed without overpressurization that might damage components. Plugged filters and undersized fittings and hoses can all be a source of improper pressures within either type of system.
As if it’s not stated throughout industry enough already, proper maintenance and regular pressure checks can be absolutely critical to saving money on downtime and preventing premature failure within any fluid system.