Heated Impulse Lines on Pressure Gauges and Transmitters

Successive cutaway view of self-regulating heat trace

cable showing various layers of material

Courtesy BriskHeat

Temperature of the environment surrounding process equipment and instruments can sometimes have a deleterious impact on its function. A common example is cold weather impact on the impulse lines connecting pressure gauges or transmitters to process piping in outdoor or unheated locations. While the process lines may be large, with sufficient mass flow and insulation to prevent freezing, this may not be the case for small diameter impulse lines. Liquid freezing in cold weather conditions can be a threat to process operation, depending on the type of liquid being used. A safeguard exists for impulse lines where the lines can be traced with a heat source, allowing for counteraction of the environmental conditions and maintenance of proper operation.

There are a number of ways to deliver heat to an impulse line. Recognize two essential goals, with the first being to prevent freezing or other changes to the fluid in the line that would impact the response or accuracy of the instrument reading. The second goal is related to the heat tracing itself. The delivered heat must not be great enough to impact the fluid in the impulse line and generate a false pressure reading. Optimally, delivering heat in a fashion that is limited to what is necessary to maintain the impulse line fluid in an ideal working state is best.

One example of heat tracing an impulse line is through the placement of a tube or small diameter pipe, located in close proximity to line, through which low pressure steam flows. Insulation is applied to the bundle and the steam line serves as a heat source. The tube transfers heat to the impulse line when steam flows. After the steam heats the impulse line, a steam trap accompanying the system collects condensate for return to the boiler. It is also conceivable that the steam line could ultimately vent to atmosphere, with no condensate return. There are a number of concerns that must be addressed in the design of the steam portion of this scenario, since it would be necessary to keep any condensate from freezing under all anticipated operating conditions, including process shutdown.

A second common solution for freeze protection of impulse lines is through the installation of electric heat tracing. Two-wire cable serves as protection against the cold. When powered, the heat from the cable keeps the line warm. Electric heat tracing is available in a broad range of physical configurations, including cables, tape, blankets, and other flexible and solid shapes. Control of the electric heat system can be accomplished with an external controller and sensor, or a self-regulating heat trace cable can be used. As with a steam heating system, there are some specific considerations for electric heat tracing. Thermal insulation is still considered a best practice. Electric power must be delivered to the installation, and a means of monitoring heat trace performance for faults or failure should be included in the design.

Share your heat tracing requirements and challenges for process piping and other industrial applications with a product specialist. There are many options and product variants from which to choose. A consultation can help direct you to the best solution.

Water Quality Testing - Turbidity Standards

ProCal turbidity standards are suitable for use
with instruments from other manufacturers.
Image Courtesy HF Scientific

Turbidity is a commonly measured indicator of water quality. Regardless of the instrument being used, frequent and regular calibration is part of the procedure assuring accurate and traceable results that may be used as evidence of regulatory compliance.

Calibration requires the use of a prepared sample of a known value. HF Scientific, manufacturer of water quality instrumentation, reagents, and standards, provides high quality premixed turbidity standards that are suitable for use with their instruments, as well as those of several other manufacturers.

Share your water quality analysis requirements and challenges with process analytic specialists, combining your own experience and knowledge with their product application expertise to develop effective solutions.

Turbidity Standards for Turbidity Measurement from Alliance Technical Sales, Inc.

Liquid Level Measurement Using Hydrostatic Pressure

Hydrostatic pressure can be used to measure liquid level
in tanks or other vessels.

Pressure measurement is an inferential way to determine the height of a column of liquid in a vessel in process control. The vertical height of the fluid is directly proportional to the pressure at the bottom of the column, meaning the amount of pressure at the bottom of the column, due to gravity, relies on a constant to indicate a measurement. Regardless of whether the vessel is shaped like a funnel, a tube, a rectangle, or a concave polygon, the relationship between the height of the column and the accumulated fluid pressure is constant. Weight density depends on the liquid being measured, but the same method is used to determine the pressure.

A common method for measuring hydrostatic pressure is a simple gauge. The gauge is installed at the bottom of a vessel containing a column of liquid and returns a measurement in force per unit area units, such as PSI. Gauges can also be calibrated to return measurement in units representing the height of liquid since the linear relationship between the liquid height and the pressure. The particular density of a liquid allows for a calculation of specific gravity, which expresses how dense the liquid is when compared to water. Calculating the level or depth of a column of milk in a food and beverage industry storage vessel requires the hydrostatic pressure and the density of the milk. With these values, along with some constants, the depth of the liquid can be calculated.

The liquid depth measurement can be combined with known dimensions of the holding vessel to calculate the volume of liquid in the container. One measurement is made and combined with a host of constants to determine liquid volume. The density of the liquid must be constant in order for this method to be effective. Density variation would render the hydrostatic pressure measurement unreliable, so the method is best applied to operations where the liquid density is known and constant.

Interestingly, changes in liquid density will have no effect on measurement of liquid mass as opposed to volume as long as the area of the vessel being used to store the liquid remains constant. If a liquid inside a vessel that’s partially full were to experience a temperature increase, resulting in an expansion of volume with correspondingly lower density, the transmitter will be able to still calculate the exact mass of the liquid since the increase in the physical amount of liquid is proportional to a decrease in the liquid’s density. The intersecting relationships between the process variables in hydrostatic pressure measurement demonstrate both the flexibility of process instrumentation and how consistently reliable measurements depend on a number of process related factors.

Solutions to process instrumentation and measurement challenges are most effective when developed in concert with a product application specialist. The combination of user knowledge and experience with product application expertise will lead to a successful project.

Online Chlorine Monitor

Online chlorine and TRO analyzer
Photo courtesy HF Scientific

Online chlorine analyzers are utilized throughout industrial and commercial applications for the monitoring and control of chlorine in potable water, seawater, swimming pool water, process water, waste water, food processing, pulp and paper, and more. Every application benefits from instrumentation delivering accurate and reliable results with a minimum amount of human intervention.

Many instruments are available, with each possibly having a set of construction and operational features that will make it an advantageous choice for a particular application or installation.

The CLX-XT2 online chlorine monitor from HF Scientific is optimized for high temperature marine applications and provides extended reagent life and unattended operation of up to 90 days. The instrument includes communications and output signals that can be used to control chemical feed pumps or provide alarm function.

More detail on the unit is provided below. Share your analytical measurement challenges with application specialists, combining your own knowledge and experience with their instrumentation application expertise to develop effective solutions.

Online Chlorine and TRO Monitor from Classic Controls, Inc.