Working with High Pressure in Fluid-Based Applications
by John W. Batts IV, Product Sales Manager, Cole-Parmer
When companies push to accelerate their discovery process and sample throughput, it can cause a decrease in sample size and fluidic pathway volumes, and an increase in flow rates. When these changes happen, it produces higher system pressures that can cause problems with lab equipment and the performance of the fluidic system. By taking a proactive approach, you can limit the impact of the elevated pressure.
The basics of system pressure
System pressure is the amount of resistance to fluid flow in any given fluidic system. This resistance must be overcome by a delivery device, such as a pump, to achieve a given flow rate along a fluid pathway. The most common units of measurement for system pressure reflect a force per unit area, such as psi (pounds per square inch) or bar (1 bar = 100,000 pascal or 100 kPa; 1 pascal = 1 newton per square meter).
The key to understanding system pressure is to realize the pump doesn’t create the system pressure; rather, it is the resistance to flow created by the fluid pathway extending away from the pump that leads to measurable pressure in a system. System pressure is the highest at the pump and gradually decreases until the fluid completely exits the pathway.
The standard contributing factors to system pressure that cannot be avoided but must be mitigated are:
- Viscosity of the fluid
- Flow rate
- Fluid pathway geometry – i.e., the length and inner diameter of the fluid pathway, but also considering any other non-linear barriers that are part of the pathway such as filters, splitters, etc.
Adjusting these three factors can provide the greatest impact on the amount of fluidic system pressure. In fact, in most formulae used to calculate system pressure, the following mathematical relationships exist between the above-listed factors and system pressure:
- Viscosity, flow rate, and fluid pathway length are directly related to system pressure. Change in these variables will result in a direct and proportional variation in system pressure. For example, if the viscosity is decreased by 50%, the system pressure will also typically decrease by 50%.
- Fluid pathway inner diameter, however, is inversely (and exponentially) related to system pressure. Change to the inner diameter of a fluid pathway will result in an opposite and scaled change to system pressure. For example, increasing the inner diameter of a fluid pathway by 20% can result in a lowering of system pressure by more than 50%*. (*NOTE: This assumes turbulent flow and no obstruction in the fluid pathway, such as with a filter.)
The effects of higher system pressures
The amount of pressure in a fluidic system should always be considered, since the higher pressures can have the following negative effects on the hardware and ultimate performance of a fluidic system:
Stress on fluidic connections.Any time there is increased fluidic pressure, the connections will feel the strain causing a greater chance for connection failure—either by the compression mechanism letting loose of the tubing, the soft-walled tubing slipping off the connection, or physical failure of the connection resulting in cracking or breakage.
Stress on fluid pathway tubing.System pressure is exerted against the inner walls of the fluid pathway tubing. With softer-walled tubing, elevated system pressures cause the tubing to expand, and the constant expansion due to pressure can lead to a reduced lifetime for most tubing materials.
Stress on the pump. If there are increased system pressures present, it forces the pump to work harder to generate the force needed to overcome the resistance along the fluid pathway. This can lead to wear and tear on the pump and more frequent maintenance and down-time.
- Reduction in flow rate.
The flow rate generated by a peristaltic pump is impacted by a number of factors. However, one of the biggest contributing factors is how effectively and quickly the soft-walled tubing can be completely collapsed by the rollers. Because the tubing is effectively being energized from the inside, higher pressures will cause soft-walled tubing to inherently resist being fully collapsed by a peristaltic pump’s roller mechanism; thus, with increased pressures often comes lower maximum flow rates.
Because of the many ways increased pressures can adversely affect a fluidic system, it is important to mitigate increased pressures in the following ways:
- Shorten the fluid pathway as much as possible.
- Increase the inner diameter of the fluid pathway and slow down the pump’s speed accordingly.
- Adjust the viscosity of the fluid, if possible, through dilution or temperature increases (as can be supported by system hardware), or simply by using a different reagent altogether.
- Change methodology to allow for a lower all-around flow rate.
- Remove the need to do inline filtration or other flow conditioning, opting to take those steps offline instead.
- Select hardware built with elevated pressures in mind (e.g., the Masterflex® High-Performance Pump Heads)
These steps can be useful in compensating for anticipated elevated system pressures, which will save both time and money.
Working with increased system pressures
The current trend towards developing fluidic systems that feature elevated pressures will likely continue. Even so, there are steps that can be taken to proactively limit the impact of the elevated pressure. This will create greater flexibility for components that system designers use during the design and implementation phases of new product development. Cole-Parmer offers a large selection of hardware and the design and engineering expertise to help successfully navigate the process of OEM product development—in high-pressure applications or any other fluid-based applications as well. If you are interested in learning more about the services we offer, please visit the Masterflex Custom Engineered OEM Solutions page.