Use of differential pressure flowmeters expands with the petroleum industry

Pipeline & Gas Journal, July, 2007 by Jesse Yoder

Rising prices and expanding production in the petroleum industry has increased the need for flow measurement. Differential pressure (DP) flowmeters are among the most widely used in the industry. In fact, the use of DP flowmeters is on the rise in the industry and one reason is that they have been displacing turbine meters. In some cases, DP flowmeters are the only type that is suited to handle the pressures associated with deep sea operations. The surge in exploration and production has brought a corresponding surge in the sales of DP flowmeters, with a resulting increase in the sale of primary elements.

DP flowmeters rely on a constriction placed in the flow line that creates reduced pressure in the line after the constriction. A DP flowmeter requires a means to detect the difference in upstream versus downstream pressure in the flow line. While this can be done with a manometer, today's DP flowmeters use differential pressure transmitters that sense the difference in pressure, and then use this value to compute flowrate.

How DP Flowmeters Are Different

When most flowmeters are sold, the transmitter and sensor are sold together. This is true for ultrasonic, vortex, Coriolis, turbine and other types of flowmeters. All these flowmeters operate based on a correlation between flowrate, or mass flow, and some physical phenomenon. For ultrasonic flowmeters, it is the difference in transit time of sound waves sent across the pipe. For turbine flowmeters, it is the speed of the rotor. Differential pressure flowmeters also correlate flow with a physical phenomenon; the difference in pressure upstream and downstream from a constriction in the flowstream.

Where DP flowmeters differ from other flowmeter types is that the transmitter is often sold separately from the primary element that creates the constriction in the flowstream. In many cases, the primary element is bought from a company that specializes in primary elements, rather than from the DP transmitter supplier. A DP transmitter by itself is not a flowmeter; it does not become a flowmeter until it is connected to a primary element.

A DP flowmeter is a DP flow transmitter that is connected to a primary element for the purpose of making a flow measurement.

Some companies have brought out integrated products that include the primary element mounted together with a DP flow transmitter. Examples include Emerson Rosemount's ProBar and ProPlate products. These integrated products save customers from having to buy a primary element separately. They also have the advantage that they can be calibrated ahead of time with the primary element already connected to the DP flow transmitter.

Types Of Primary Elements

There are a number of different types of primary elements. Which type is best for a given application depends on multiple factors, including fluid type, flow velocity, viscosity and other considerations. The main types are described here, along with some of their advantages and disadvantages.

Orifice Plates

Orifice plates are the most widely used type of primary element. Orifice plates consist of a flat, usually round piece of metal, often steel, containing an opening or "orifice" in the interior part of the plate. The orifice plate needs to be held in place within the pipe by an orifice flange, orifice assembly or holding element. The plate cannot function as a primary element unless it is held in place at the proper location.

Orifice plates have the advantage of simplicity, and of being well studied and well understood. They have been used in industrial flow measurement for about 100 years. They can be used to measure the flow of liquid, steam and gas. One disadvantage of orifice plates is that they may cause significant pressure loss, and they are subject to wear over time.

Venturi Tubes

The Venturi tube is named after Italian physicist Giovanni Battista Venturi who invented it in 1887. It consists of a flow tube with a tapered inlet and a diverging outlet. Clemens Herschel developed the first commercial Venturi-type flowmeter in 1887, building on Venturi's work. Due to their relatively wide opening, the amount of permanent pressure loss they cause is relatively low. They can be used on slurries and dirty fluids. Venturi tubes have the disadvantage of being relatively expensive and somewhat difficult to install.

Pitot Tubes

The Pitot tube was invented by Henri Pitot in 1732. The first patent for the use of a Pitot tube was granted in 1889. Pitot tubes are of two types: single port and multiport averaging. Single port Pitot tubes consist of an L-shaped tube with an opening that measures impact pressure. A second opening measures static pressure. The difference between these pressure values is used to compute flow.

Instead of having just one port, multiport averaging Pitot tubes measure impact pressure and static pressure at multiple points, using multiple ports. The DP transmitter uses the average of the different pressure readings to compute flowrate. Pitot tubes are designed for use with clean liquids, gas, or steam, but work less well with viscous liquids and low velocity gases. They may be subject to clogging when used with dirty liquids or gases.