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Sample handling is comprised of the process tap and process return point, the sample transport system, and the sample conditioning system. This article will cover the basics of sample handling, lag time calculations, sample conditioning, and the principles that govern sample conditioning devices. These subjects are covered with the intent to furnish a short concise overview that will be beneficial to the analyzer technician.
The sample transport
system is installed as either a fast loop or a single line configuration. The
fast loop system as the name implies creates considerable velocity by taking advantage
of a high-pressure sample tap, routing the flow to the sample conditioning system,
and then returning the product to a lower pressure return point in the process
unit. The fast loop allows fresh process sample at the sample conditioning system,
and only the slipstream flow must be conditioned. The fast loop is representative
of the process conditions if the pressure and temperature is maintained within
specific limits and the sample tap is installed correctly at the correct location.
lag for a sample handling system is dependent upon the total volume of the sample
handling system including all tubing, fittings, and conditioning devices between
the sample tap and the analyzer and is a function of the flow rate. Another significant
consideration in total system lag time is first-order lag which is the delay caused
from mixing of process in sample conditioning devices.
time calculations can be very detailed but to stay within the scope of this article
the equations used are easily calculated and applied to field situations. The
lag time for various tubing sizes and for conditioning devices will be calculated.
See article: Total Lag Time.
process analyzers require filtration. Because the filter must remove impurities
without changing the composition of the sample, inert materials such as glass,
stainless steel, ceramics and fluorocarbon are recommended. A Small filter housing
designed for sample conditioning in the slipstream is used to minimize filter
element replacement and prevent excess lag time. Inside to outside flow direction
is recommended for particulate removal, coalescing liquid from gas or liquid from
liquid. An outside to inside flow direction is used with membrane type filters,
slip stream applications, and for the removal of bubbles. Either a manual, automatic,
or continuous flow drain is necessary for all but the particulate filter application.
filters have bowl housing which have too much volume for analyzer application
unless a bypass is installed. The miniature version of this type filter will have
good results when used with a vaporizer if a low particulate concentration is
Back-Purged filter probes
are used when the sample contains a heavy concentration of solids. This should
only be attempted with a good back flush design. "The use of a filter in or
on the probe is best avoided because of the difficulty of access."¹
Bypass filter element is mounted in a cylindrical steel body. "The major
sample flow passes axially through the filter unimpeded. Filtered sample exits
at a side connection. The main flow provides a scrubbing action to keep the inside
of the tube clean. By ensuring that the major flow is over ten times the filtered
sample flow, particles are made to impact the filter element at a shallow angle.
This together with the momentum effect of the fast stream, helps to increase the
efficiency of filtration without clogging the element."¹
bypass filters utilize the velocity of the process in a circular motion to clean
a single surface membrane filter. This filter can also use a hydrophobic membrane
to remove water droplets. The optimum flow rate (2-3gpm) should always be observed
to furnish the flow necessary to clean the membrane. Excessive particulate loading
will increase pad replacement intervals or result in device failure.
filter applications create a tortuous path whereby small droplets of liquid impact
an obstruction due to the inertia of the sample and join previously impacted droplets
to form a larger droplet, which separate by gravity. Generally a wire mesh such
as steel wool is used to create the tortuous path. There are also custom fitted
devices, which are constructed of one wire with a uniform 60-80 micron mesh, which
will precisely fit a small enclosure. In less demanding processes a filter can
be used by reversing the inlet flow from the particle removal orientation of outside
to inside to a coalescing orientation of inside to outside. The water will accumulate
on the bottom of the bowel. Another application for coalescing filters, which
will separate liquid emulsions, uses an element similar to the membrane elements,
which will allow one liquid to permeate the membrane, and the other will not.
A hydrophobic material such as a PTFE filter will allow a liquid hydrocarbon to
enter with an outside to inside orientation and the water will be coalesced on
the outside of the filter. The water will separate from the filter due to gravity.
It should be noted that sample velocity must be kept low and the higher the phase
density differential the greater the success rate.
Sample separators are
used when there is sufficient contaminates and/or immiscible liquids to warrant
separating the representative components from contaminates to protect the analytical
equipment. This is generally the case when filters alone fail to condition the
A Kinetic separator uses the inertia and gravity inherent in the flowing sample (kinetic energy) and reverses the flow direction of a relatively small volume of the slipstream. Lighter, representative components are separated from the contaminates. This is achieved using a device with one or more small chambers with the flow traveling from top to bottom and positioning a port at the upper end of the chamber to force the slipstream to reverse direction. Contaminates heavier than the process stream cannot negotiate a complete change of direction and will continue to a low-pressure return. Two chamber separators use the second chamber as a polisher chamber. This chamber employs a filter in a similar fashion to the Cylindrical Bypass Filter (CBF) with a vertical orientation. Like the CBF the filter extends the entire length of the chamber and the particulate impacts the walls at a shallow angle. The kinetic separators are also used in single line application with minimal lag time due in part to the lack of first order lag consideration.²
Basic sample handling and the majority of sample conditioning can be understood with an overview of the transport systems, lag time concerns, and sample conditioning devices that are basically physical in nature. Proper conditioning of analyzer samples is the single most important maintenance consideration in an analyzer installation. Of the maintenance problems with analyzers, over 80% are related to the sample handling system.¹ The majority of analyzer sample systems the analyzer technician will encounter can be explained by the above descriptions.
Kenneth J. Process Analyzer Technology
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