Pressure drop can be defined as the quantity of line pressure, which is lost forever when gas flows through a device in a gas line. This loss of pressure is due to the frictional resistance of the parts exposed to the gas. Each instrument and fitting in a line causes a certain amount of drop in pressure. For instance, even the walls of the pipe produce some friction, which leads to some pressure drop.
How to Calculate Pressure Drop
To calculate pressure drop, the difference between the pressure of the gas when it enters the instrument and the pressure of the gas when it leaves the instrument is noted (Figure 1). Measuring this difference is completed easily by pluming the outlet and inlet of the device to a differential pressure gauge, similar to one of Alicat’s P-PSID series gauges.
Figure 1. Pressure drop is determined by calculating the difference between the pressure of the gas when it enters the instrument and the pressure of the gas when it leaves the instrument.
The measurements can be made to considering certain relationships. They are listed below:
- Pressure drop is comparative to volumetric flow rate under laminar flow conditions. When the flow rate is double, there is double the pressure drop
- Pressure drop decreases when common mode pressure increases
- Pressure drop increases as the square of the volumetric flow rate under turbulent flow conditions. When the flow rate is double, there is four times the pressure drop
- Pressure drop increases as gas viscosity increases. As rising temperature of the gas increase its viscosity, similarly pressure drop also increase as gas temperature rises.
In order to regulate Alicat’s pressure drop specifications, Alicat’s instrument data sheets denote the highest pressure drop at the full-scale flow of the instrument, while venting to the atmosphere at standard conditions. In other words, Alicat’s pressure drop specification identifies the lowest amount of inlet pressure needed to operate the instrument at full-scale flows, while venting to atmosphere.
Figure 2. Measuring pressure drop.
Why Pressure Drop Matters
The available system pressure has to be greater than the total pressure drop of the parts in the system at the anticipated operating flow rates and temperatures, for any gas process to function. If too little pressure is provided at the inlet of the whole system, there would not be adequate gas pressure to pass via all of the parts of the process at full-scale flows. If several gases are used for the same process, pressure drop of the most viscous gas will be the highest.
Alicat’s several valve customizations are based on this principle. The broader the valve orifice, the lower the valve’s pressure drop when fully open. Maximum control accuracy is realized when the largest portion of the valve’s operating range is used. Alicats aim when constructing a flow or pressure controller is to choose the smallest valve to facilitate full-scale flows of all gases being used in the system.
If customers have little inlet pressure available, or an excess amount of back pressure, Alicat’s standard line of mass flow instruments can sometimes exhibit a pressure drop too high to accomplish full-scale flows. Alicat’s Whisper series of mass flow meters and controllers is the ideal solution for such situations.
Alicat is able to achieve very low-pressure drops, usually by a factor of 10, as it has modified the flow body design to be more open, and changed the spacing and configuration of Alicat’s laminar flow elements. The Whisper series also uses a highly sensitive pressure sensor package to take total advantage of the lower pressure readings.
Alicat’s Pressure Drop Demonstration
The method to measure pressure drop under a range of flow conditions is shown in the following video. The video also shows how a Whisper flow meter can make a difference under low differential pressure conditions.
Ask Alicat: What Is Pressure Drop?
Download the brochure for more information about Alicat Whisper Series of Mass Flow Meters and Controllers
This information has been sourced, reviewed and adapted from materials provided by Alicat Scientific.
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