Pressure gauges are a versatile way to monitor pump operation.

Prior to 1850, the only way to measure pressure was with a vertical tube (usually glass) with some liquid inside. The liquid was generally either water or mercury. The investigator would apply the unknown pressure to the bottom of the tube and observe how tall a column of liquid the pressure would support.

Using a liquid-filled instrument to measure pressure is simple and direct, but awkward for field measurements. For example, in 1647 Balise Pascal's brother-in-law Perier, measured atmospheric pressure at two different elevations: At the base and at the summit of the Puy de Dome. Can you imagine carrying a 30-inch tall mercury filled barometer 3,000 feet up a mountain? Pascal writes, “Thus between the heights of the quicksilver in these two experiments there was a difference of three inches and one and a half lines, which ravished us all with admiration and astonishment.” It did not take a lot to get people excited in those days. He actually invented the altimeter.

A liquid-filled tube can be a simple, robust device for pressure measurement and, through some methods, can be amazingly precise. High precision measurements with liquid-filled manometers take into account thermal expansion of the measuring scale, thermal effects on the density of the fill fluid, and shifts in the gravitational force due to altitude and longitude. However, these devices quickly become impractically tall as the pressure range increases, and liquid-filled glass tubing does not travel particularly well.

Figure 1. Fortin mercury-filled barometer

Coiled Tube Versus Liquid

In one of those historical moments of genius, in the 1840s Eugene Bourdon was witnessing a pressure test of a spiral wound heat exchanger. He noticed that as internal pressure in the helical wound tubing increased, the coil twisted and expanded. When the pressure was relieved, the coil constricted back to its original shape and diameter. He had the mental preparation and insight to realize that the rotational transit of the coil end was directly proportional to the pressure inside the tube. 

He carried the concept to its logical conclusion, designing a practical instrument with a coiled tube coupled to a gear set to amplify the tip deflection. A pointer and scale completed the system, comprising the modern mechanical pressure Bourdon tube pressure gauge.

This device provides a simple, inexpensive and robust way to measure pressure in countless practical applications. 

Figure 2. Mechanical elements of the Bourdon tube pressure gauge

Bourdon Variations

Countless variations on the design of the Bourdon tube gauge are available, but these can be roughly divided in to two categories:

  1. Open-front, drawn-case gauge. This type's principle characteristic is economy. These gauges generally range from a 1½-inch to 4-diameter dial, have a drawn steel or stainless steel case, and are ANSI Grade-B accuracy (±2 percent in the middle third). Since they are an “open-front” design, if they are over pressurized, the Bourdon tube can rupture violently, and the lens and internal components will be blown forward towards the observer.
  2. Solid-front / blow-out back turret-case gauge. Generally, these are more expensive than an open-front gauge. They are molded from phenolic plastic and feature a thick barrier between the Bourdon tube and the dial with just a small hole for the pointer axel through the wall. As a result, if they are over pressurized and the Bourdon tube ruptures, the fragments are blown out the back away from the observer. They are most frequently provided with a 4½-inch diameter dial, although 6-inch dials are available. Accuracy is generally ANSI Grade-2A (± ½ percent throughout the range).

Adaptations of this design have evolved through the years. For example, to operate at lower pressure levels, a bellows or accordion assembly can be substituted for the C-shaped Bourdon tube, but the geared travel amplification remains intact. These instruments generally operate at pressures well below 1 Atmosphere.

Alternate Applications

The beauty of the Bourdon tube or bellows pressure instruments is that they can be used to measure other process variables by inference.

A pressure gauge can be used as a barometer. As a general rule, the pressure being measured is applied to the inside of the Bourdon tube or bellows. The outside of the tube or bellows is subject to atmospheric pressure. However, this can be flipped around. If the entrance to the measuring element is plugged, a fixed volume of air is trapped inside. Atmospheric pressure exerts a force against the outside of the measuring capsule where slight variations in atmospheric pressure are amplified by the gear set and registered by the pointer. This provides an indication of the local barometric pressure.

A pressure gauge can also function as an altimeter. The same arrangement as was used for the mechanical barometer can, with a slight modification, function as an altimeter as well. Atmospheric pressure decreases with increases in altitude.

A pressure gauge can be used as a thermometer. By connecting a bulb with a specific volume of gas to the measuring element, a Bourdon tube gauge will function as a thermometer.

A pressure gauge can be a Flow Meter! The flow rate through any pump can be determined. For this to work, a pressure gauge must be placed on the suction and discharge of the pump. The end user will also need a copy of the pump performance curve.

Step 1 is to subtract the suction gauge reading from the discharge gauge reading. This provides the ∆P across the pump.

Step 2 is to look at the pump performance curve, trace across from the ∆P on the y-axis to the pump Q/H curve and drop down to the x-axis. Read the flow. Ignore efficiency and brake horsepower curves at this stage. 

In this example, if the difference between the discharge gauge and the suction gauge is 80 feet of head, then the flow through the pump is 4,500 gallons per minute.

Figure 3. Determining the flow through a pump with a pressure gauge

No Pressure Gauge?

Many pump installations do not have a pressure gauge on the discharge. This leaves the plant operators blind and makes pump problems difficult or impossible to diagnose. A frequent justification for leaving a pressure gauge off a pump installation is that the small passages into the Bourdon tube frequently clog, especially when handling fluids such as sewage, sludge, mining slurries, pigments or aggressive chemicals.

The solution for clogging is to provide an assembly consisting of an isolator ring and gauge. The isolator ring is installed between standard ANSI pipe flanges. It consists of a steel ring with a rubber inner tube inside. The internal diameter of the inner tube matches the ID of the adjacent pipe, providing a smooth transition in and out of the device. The rubber inner tube is filled with a benign fluid such as silicone instrument oil, which communicates the process pressure to the gauge. The isolator ring can be fitted with a gauge, pressure switch, or pressure transmitter, or any combination of the three. 

When the gauge is combined with a switch or transmitter, it provides a simple, robust sentinel that will monitor pump performance and provide an early warning of any impending performance issues.

 

Pumps & Systems, October 2011