As natural forces go, pressure is pretty straightforward. It involves less mysterious physics than electromagnetism, it's easier to observe than thermodynamics, and its calculations are simpler than often turbulent flows. However, there are still some important aspects of pressure that must be remembered to apply its technologies properly, and this is especially important when calibrating pressure-related devices.
To reacquaint users with pressure's crucial details, two 90-minute webinars were delivered recently by ISA and Beamex. The three presenters were Hunter Vegas, project engineering manager at Wunderlich-Malec, Ned Espy, technical director at Beamex and Roy Tomalino, professional services engineer at Beamex. They decided to do the webinar on pressure because a recent Beamex survey found that about 60% of applications in process plants use pressure.
The trio reported that calibration begins with the International System of Units (SI-Units), and that international, national, reference and working standards are essential for maintaining the agreed-upon building blocks of precise and accurate calibration, process measurements and efficient performance. "When we're talking about good measurement, we're really talking about good metrology practice and data with demonstrable pedigree that can show traceability back to international standards," says Espy. "And the reason we do calibration is to bring transmitters that were installed and have drifted back to their good-as-new condition."
Basic Units and Scales
While pressure is defined as equaling force divided by unit area, Vegas reminded viewers that this simple equation can occur in some unexpected ways. For instance, if a large force is spread over a relatively large area, then the net local force is small, while a small force over a small area can have a high net local force. "Both sides of this equation need to be taken into account," says Vegas. "With tanks, a common myth is that the shape of a tank can affect the pressure at the bottom, but this is not true because 1 in. x 1 in. x 23 ft of water always weighs 10 lbs regardless of its shape, so the shape of a tank has no impact on the 10 psi pressure at its bottom. All that matters is the height of the liquid."
If this tank were filled with mercury, then this initial 10 psi would be multiplied by mercury's specific gravity (SG) of 13.6 to produce a pressure of 136 psi, and if a 1-psi blanket of nitrogen is added at the top to suppress fumes, then it would bring the bottom pressure up to 137 psi. "When calibrating a differential pressure (dP) transmitter that's reading pressure at the bottom, three things matter—height of the liquid, SG of the liquid and any pressure on top." Likewise, a 23-ft storage tank at 100% will read 276 inches of water column (in.wc), and a 10-in.wc nitrogen blanket will bring it up to 286 in.wc, until a compensation leg, bubbler or other device is added (Figure 1).
Espy adds that, while absolute pressure begins with zero in a vacuum and gauge pressure begins with zero at ambient barometric pressure (14.7 psi at sea level), dP happens in a closed system that looks at the difference between two pressure signals coming from a high leg and a low leg, and zero differential happens when those two legs are connected.
The primary pressure units are atmospheres, pounds per square inch (psi), Newtons per square meter (kPa), bars that are 0.01 kPa, in.wc, millimeters of mercury (mmHg, Torr) and inches of mercury (in.Hg). "People tend to get confused because there are so many units, and then ambient pressure is also affected by altitude, temperature, humidity and even latitude," adds Vegas. "Depending how your scale is set, at sea level you may see any of these: 0 psig (gauge), 14.7 psia (absolute), 1 atmosphere, 30 in.Hg or 760 mmHg. Inches of water column are based on the weight of a 1-in. cube of water, and 27.7 in.wc equals 1 psi."
Vegas added it's also important to remember that, when using a standard orifice place in an air line, dP is multiplied by four when the flow is doubled, and dP is multiplied by nine when the flow is tripled. "Flow and dP have a squared relationship, so the dP's square root is needed to convert or relate to a given flow," adds Vegas. "This is usually done in the DCS, so if it's done in the field, you need to make sure the DCS doesn't do it again."