Flow monitoring in a process sampling system
Regardless of the analyzer fluid sampling system configuration, flow monitoring is essential to ensure proper analyzer operation. There are numerous methods and technologies available to monitor sample flow on a real time basis. These can be mechanical spring or gravity loaded pistons, variable area types, differential pressure and thermal mass flow.
In most cases, a simple relay contact or solid state output change is all that is desired to indicate a reduction or absence of flow at a predetermined setting. A recent trend is driving the output requirements to include an analog or digital communication signal to monitor the sample flow rate throughout the flow range. This allows operators to better predict flow declines due to disruptions caused by clogging filters, line contamination from fouling fluids, leaky or failing pumps, and other time and wear susceptible components in the system.
Mechanical devices normally have the advantage of not requiring power to operate, however, a minimum amount of wires are still required to transmit an electronic signal (contact closure) back to the control system. As these devices are triggered by the force of the flow stream such that specific and fixed application details, including the sampled fluid’s density, viscosity, temperature, pressure, flow rate and, if non-adjustable type, the trip point, must be known and be specified when ordering them.
Furthermore, because springs, magnetic components and seals are all in the wetted flow stream, their material compatibility must also be evaluated by the specifying engineer. While mechanical devices with factory fixed trip points can be one of the lower priced solutions, those with adjustable trip points are often double the cost and approaching the price of some of the more robust and sophisticated technology solutions mentioned later.
The leading drawback and most frequent user issue with these mechanical devices is their susceptibility to sticking over time. As all mechanical designs have moving parts in the flow stream, contaminating fluids can be a serious source of future failure. Fouling contaminants can progressively build up over months or years and are only detected when other failures in the system are detected.
These devices also can have nooks and crannies that are exposed in the flow stream as they displace the flow path volume. In addition to being another area subject to clogging, they can trap previous samples or purge media which can contaminate subsequent samples. Piston actuated devices also have no continuous visual indication of normal or abnormal flow conditions for the operator, other than their preset trip point.
An ideal flow monitor for an analyzer sampling system would be one that can be adapted on site and adjusted specifically in the application. In addition, a more robust flow sensing technology married to flexible and user programmable electronics, which can be specified without detailed and finite validation of process variables such as density, viscosity, flow range, fluid type, etc., also would alleviate a pain point for the analyzer system supplier and site engineering team.
Flow monitors designed with thermal dispersion technology match up extremely well with the ideal universal use criteria to support an analyzer sampling system. Depending on the manufacturer's flow element and electronics design, they can be user set-up in situ to assure the field engineer a successful, first time correct installation. These devices may be configured in the field to operate in gases or liquids at most any density or viscosity with no special consideration at time of ordering.
There are two major types of thermal devices on the market. One type utilizes a capillary bypass technique and is better known as a Mass Flow Controller or MFC. MFCs divert a portion of the main flow into a small bypass and sense the heat transfer of flowing fluid in the bypass channel. This technique can be very effective; however, the capillary tube is highly susceptible to contamination and clogging and should only be considered for use with clean or pre-filtered fluids. Capillary bypass flow monitors are generally applied in lab use and agency approvals (e.g. FM, CSA, ATEX, IECEx, etc.) for use in hazardous, explosive environments are rare. Flow range selection is also important when specifying MFC type flow monitors.
This article is an excerpt from a FCI Technical Publication - "Verify Fluid Flow to Your Analyzer and Keep Your Plant Running" by Sam Kresh
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