Understanding Gauge and Absolute Pressures in Pump OperationsBy Welcome / October 15, 2019
Pumps and their associated systems are designed to produce and retain pressure. Professionals involved in building, maintaining, and repairing such systems, need a good understanding of how pressure works. Two methods of measuring pressure—gauge and absolute pressures—are foundational concepts for pump manufacturers and users.
Gauge Pressure vs. Absolute Pressure
Pressure is a force acting over a certain area. In the United States, pressure is often described as pounds per square inch, or PSI. But PSI is only the most common unit for measuring pressure. To get a firm grip on the pressures in a fluid system, one also needs to know about absolute pressure and gauge pressure.
Absolute Pressure (PSIA)
Absolute pressure (PSIA) is measured relative to a perfect vacuum (0 PSIA). An absolute pressure measurement of 5 PSIA simply means that the system is operating 5 PSI above the pressure of absolute vacuum. Because it is referenced against an absolute zero pressure, PSIA is always expressed as a positive value.
Gauge Pressure (PSIG)
Gauge pressure (PSIG) is measured with reference to the local ambient atmospheric pressure. A PSIG is typically higher than the local atmospheric pressure (for example, 100 PSIG indicates a system pressure that is 100 PSI greater than ambient). But it can also be lower (for example, -10 PSIG).
A system’s pressure can be measured using either gauge or absolute pressure, and one may easily convert one to another as needed. A PSIG value can be converted to PSIA by adding the local atmospheric pressure to the PSIG value. For example, at sea level (14.7 PSIA) a pressure may be read as 100 PSIG or as 114.7 PSIA.
Absolute and gauge are also used with metric units, such as the Bar (0.1N/mm2), which expresses them as BarA or BarG, respectively. For reference, 1 Bar = 14.504 PSI.
Below is a sketch showing examples of pressure values, where the dotted red line is the pressure that is measured. This illustration assumes that the pressure being measured is located at sea level.
How to Convert Pressure to Head
You can use pressure values to calculate a pump’s net positive suction head available (NPSHa) value.
A pump’s NPSHa value is crucially important to understanding if the pump is correct for the job at hand. NPSHa, or Net Positive Suction Head Available, is crucial to avoiding cavitation and other inefficiencies. It is determined by subtracting the liquid vapor pressure from the suction pressure:
NPSHa = Suction Pressure – Liquid Vapor Pressure
It sometimes happens that the component pressures of the NPSHa calculation are measured in different units. In such cases, the values need to be converted to a common unit. Here’s an example:
A maintenance team wishes to measure the NPSHa of a pump located in a facility that is 1,500 feet above sea level. At that elevation, the atmospheric pressure is 13.91 PSIA. The system’s instruments indicate a suction pressure of 74 PSIG and a liquid vapor pressure of 72 PSIA. The team elects to convert the suction pressure to PSIA:
Gauge Pressure + Atmospheric Pressure = Absolute Pressure
74 PSIG + 13.91 PSI = 87.91 PSIA
Now that the team has the pump’s suction pressure in PSIA, they can easily calculate NPSHa:
NPSHa = Suction Pressure – Vapor Pressure
NPSHa = 87.91 PSIA – 72 PSIA = 15.91 PSIA
NPSHa is always expressed in head units. A PSI value can be converted to head feet using the following formula:
Head = PSI x 2.31/SG
The team knows that the pumped fluids have a specific gravity (SG) of 0.8. Therefore:
Head = 15.91 PSI x 2.31/0.8 = 45.9 feet
PumpWorks Helps Customers Understand Pressure
At PumpWorks we are passionate about helping customers match their pump selections to the particular conditions of their systems. We can help your business with pressure analysis, maintenance, and training.