Monitoring differential pressure (∆p) is critical in a wide variety of production processes. Often, two standard pressure gauges are used when a differential pressure gauge can indicate the difference in pressure with a single instrument. Measuring differential pressure is essential wherever small differences in pressure can have a significant effect, such as monitoring pressure drop across filters, strainers, separators, valves and pumps, displaying liquid levels in tanks, detecting system leaks, and measuring liquid flow. Applications include clean rooms, heat exchangers, refineries, petrochemical plants, and chemical plants,
A differential pressure gauge measures and visually indicates the difference between two pressure points within a process system. The differential pressure gauge has two inlet ports that are both connected to the pressure points being monitored. For example, if the pressure at one inlet port is 200 psi and the pressure at the other port is 50 psi, then the differential pressure is 150 psi (200 psi – 50 psi).
Differential pressure gauges can be used for many purposes, including:
One of the most popular applications for differential pressure gauges is filtration monitoring. As a filter cartridge starts to become obstructed with contaminants, the pressure between the inlet and outlet ports increases and efficiency drops. When the differential pressure reaches a certain limit, the operator knows it is time to change the filter, keeping the process running smoothly and efficiently and preventing damage to the system. Similarly, a differential pressure gauge can be used to detect scale buildup in pipes and pumps.
Large, heavy, expensive flowmeters are often used to measure the flow of fluids inside a pipe. Differential pressure gauges provide a less expensive and simpler option.
Bernoulli's principle states that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy.
An orifice plate, venturi, or flow nozzle is often used to reduce the diameter inside the pipe. A differential pressure gauge can then be installed, connecting the high-pressure side to the larger diameter and the low-pressure side to the smaller diameter, to measure the difference in pressure before and after the orifice and determine the flow rate. This provides a high accuracy solution at a low cost.
In open tanks, the high-pressure DP gauge port is positioned on the bottom of the tank, and the low-pressure side is vented to the atmosphere. The DP gauge then provides a reading that indicates the height of the liquid in the tank, often in inches or feet of water, or percent full.
In a sealed tank, the atmospheric pressure of the gas at the top of the tank above the contained liquid adds to the hydrostatic pressure of the liquid. A standard gauge mounted at the bottom of the tank would measure both the gas and liquid level pressure, rather than just the pressure of the water column.
A differential pressure gauge works much more effectively in this application. By connecting the high side to the bottom of the tank and the low side to the top, the gauge measures the difference in gas pressure from the total pressure to provide a much more accurate liquid level reading. This can be a cost-effective solution for tanks measuring liquid oxygen, nitrogen, argon, and similar mediums, as well as measuring the difference between the pressure of the reservoir and bottom-hole pressure in production and injection wells.
Detecting leaks in a system is one of the lesser known applications for differential pressure gauges.
Simply measuring the differential pressure between a potentially leaking vessel and a reference vessel can determine if a leak is present. This can save time and provide a more accurate measurement as opposed to waiting a long period of time for a drop in pressure on a standard pressure gauge.
Types of Differential Pressure Gauges
Piston type differential pressure gauges are designed primarily for liquid or gaseous media applications where low differential pressure ranges and static working pressures are needed. As the fluid crosses from the high to low pressure port, the differential pressure gauge senses a slight amount of bypass via the movement of a floating piston/magnet in a precision bore against a calibrated spring. Any change in pressure on either side of the piston/magnet causes the magnet to move proportionally to the difference in differential pressure. A rotary pointer magnet located close to the internal magnet, but outside the pressure housing, follows the movement of the piston magnet and indicates differential pressure on the dial. Piston type DP gauges indicate a slight amount of bypass as the fluid crosses from the high to the low-pressure port.
Diaphragm type differential pressure gauges isolate the high and low-pressure ports from each other, so there is no bypass. These gauges can be used in applications measuring air, gas or liquids. The differential pressure is sensed by the movement of an elastomer diaphragm against a precision calibrated range spring. The change in position of the diaphragm in response to the change in Differential Pressure moves an internal magnet. This magnet, in turn, causes a rotary magnet external to the gauge body to rotate. This rotary magnet has a pointer attached which indicates the differential pressure on the dial. The diaphragm provides total separation.
Many differential pressure gauges are available with built-in switches or transmitters. These switches or transmitters can be set to turn on or off, or trigger alarms at specified pressure points.