Using flowmeters for process cooling water can help increase equipment reliability. But which type to use?

Magnetic flowmeters are widely used and offer the advantage of being obstruction-less, so they are not subject to plugging.
Source: ABB


Paramount in industry is the maintenance of the on-stream reliability of equipment - a task that seems straightforward in principle but is not necessarily simple in practice. Ensuring such reliability often entails cooling certain portions of operating equipment so that equipment components are not damaged or degraded excessively. Stated differently, on certain components, higher equipment operating temperatures tend to accelerate equipment degradation and reduce equipment reliability. While some equipment may be cooled with air, many processes use cooling water to keep the equipment sufficiently cool so as to provide reliable long-term service.

In some applications, measuring the equipment temperature in strategic locations is a viable approach to detect when the equipment is being stressed. When the equipment requires more protection, both the equipment temperature and the flow of cooling water may be monitored. Detection of low cooling water flow can alert the operator to a problem sooner than the temperature measurement, so the operator can take appropriate action before the equipment temperature has time to rise. In many applications, normal cooling water flow may be much higher than the minimum cooling water flow requirement. Setting the flow alarm trip well above the minimum flow - but below the normal flow - can give the operator even more time to react to a potential problem.

There are a number of flowmeter technologies that commonly are applied to process cooling water service. They include differential pressure, magnetic, vane, variable area and vortex shedding flowmeters. These flowmeters typically offer sufficient accuracy to ensure that sufficient cooling water is maintained to protect equipment. Here is a closer look at each type.

Differential pressure flowmeters offer simplicity, but they can be subject to plugging when solids are present.
Source: ABB

Differential Pressure Flowmeters. With this device, mechanical restriction in the flow stream creates a pressure drop across the restriction. Differential pressure flowmeters have a known geometry (restriction), so the pressure difference between the upstream and downstream taps (differential pressure) is proportional to the square of the flow rate. Therefore, by measuring the differential pressure across the restriction, the process cooling water flow rate can be calculated. Differential pressure flowmeters offer simplicity, but they can be subject to plugging when solids are present.

Magnetic Flowmeters. Magnetic flowmeters use Faraday’s Law to measure the voltage generated when a conductor moves through a magnetic field. In magnetic flowmeters, a nonconductive liner isolates the conductor (cooling water) that flows through the magnetic field produced by the flowmeter. The voltage generated at the electrodes by cooling water flowing through the magnetic field is proportional to the cooling water flow rate. Magnetic flowmeters are widely used and offer the advantage of being obstruction-less, so they are not subject to plugging.

Vane flowmeters are commonly considered to be mechanical flowmeters because they have parts that move.

Vane Flowmeters. Vane flowmeters have a vane in the flowmeter that rotates more as the flow is increased. Vane flowmeters commonly are considered to be mechanical flowmeters because they have parts that move. However, the rotation of the vane is dependent upon the flow rate and the differential pressure produced by the (non-fixed) restriction that the vane imposes on the flow, so the position of the vane can be used to determine the cooling water flow rate. The mechanical nature of vane flowmeters often lends itself to cooling water applications where inexpensive local flow indication is required and where maintenance personnel can handle mechanical devices. However, the moving parts associated with vane flowmeters are subject to failure due to plugging and sticking.

Variable area flowmeters, or rotameters, have a float in the flowmeter tube that moves as flow is increased.

Variable Area Flowmeters. Often called rotameters, variable area flowmeters have a float in the flowmeter tube that moves as flow is increased. As the float rises, there is an increase in the annular area around the float where the cooling water flows. Float movement changes the area through which the cooling water flows and, as such, maintains the float in static balance where the upward force generated by the rising cooling water offsets the downward force of gravity (or of a spring). The position of the float (in static balance) can be used to determine the cooling water flow rate.

Variable area flowmeters have a moving float that is subject to plugging. In addition, variable area flowmeters with glass tubes are subject to damage due to breakage as well as internal coating that may adversely affect the ability to see the float. Nonetheless, variable area flowmeters are widely applied as economical flow indicators in many cooling water applications.

Vortex shedding flowmeters have an internal bluff body (shedder) that generates vortices in the flow stream as the cooling water passes.

Vortex Shedding Flowmeters. Vortex shedding flowmeters have an internal bluff body (shedder) that generates vortices in the flow stream as the cooling water passes through the flowmeter. Similar examples of vortices in nature include the waving of a flag in wind and the vortices that form downstream of a rock or branch in a flowing river. The number of vortices formed is proportional to the cooling water flow rate. Vortex shedding flowmeters have no moving parts and are not subject to plugging, but they do not measure low flow rates when no vortices are formed.

Utilizing flowmeters to measure and subsequently monitor cooling water flow can be paramount to maintaining the integrity and on-stream reliability of equipment by ensuring that certain components are not damaged or degraded excessively due to high temperature. There are many flowmeter technologies available to measure cooling water flow, the more common of which are described above. Flowmeter selection for a given application is dependent upon factors such as the desired accuracy, cooling water quality, desired features and cost. In making this selection, be sure to remember the importance of keeping equipment cool because the cost of equipment damage is often many times more expensive than the cooling water flowmeter.  

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