Methods for Maintaining Media Phase in Process Instrumentation(PDF)
Michael J. Bequette, P.E., VP of Engineering SOR Inc.
Billy Terry, Product Manager SENSOR Sampling Systems
Maintaining the operating conditions of a fluid can be challenging but is extremely important so that systems operate safely and within the necessary conditions for system efficiency. Not having the right operating conditions can result in flaws within a process that can have a significant financial impact. Not maintaining the necessary operating conditions can also create significant process hazards to the environment as well as an operator. Unexpected rapid changes in process conditions can produce uncertainty in conditions that can catch operators unprepared.
Maintaining operating conditions of a process media is also important for obtaining a true representative sample to validate quality and reliability of the process or end-product. For example, gas chromatograph lines that are long and allowed to cool can change the consistency of a hydrocarbon mixture providing information to operations that is inconsistent. Grab sample specimens are another example where the structure of the media may vary with changes in temperature and pressure. It is very crucial that the sample maintains the exact conditions prior to collecting the sample.
There are several ways for maintaining operating conditions, whether for sampling or overall process measurement. These methods may be used individually or in combinations depending on the process. Insulation, heat trace, steam trace, coolers, and pressure control all work to help mitigate changes with operating conditions.
Insulation
Insulation comes in many different varieties but for industrial processes, we will be focusing on piping and vessel insulation. Insulation for piping and vessels may be used to maintain heat or cold depending on the specific process. Most of the insulation used in these applications tends to be custom made insulation blankets or insulation jackets that are made to fit the specific piping. These blankets are made to not only fit piping or vessels, but to fit over flanges and valves as well. The use of insulation blankets can result in lower energy costs, improved safety, and overall lower equipment costs. In addition, these blankets being made to fit the piping help reduce overall installation time resulting in large savings of labor.
In applications where high temperatures are common in piping systems, such as steam, covering the pipe reduces heat loss through the pipe to the atmosphere helping keep the application operating efficiently but also more cost effective. This reduces the potential for condensation and condensation build up in the piping system. In addition, the insulation helps prevent burns to personnel by exposure to the hot pipes. The insulation can also help reduce dangerous noise levels.
Image 1. Magnetic level indicators with insulation blankets attached to slug catchers in an Oil & Gas application. An insulation blanket has been added to both the primary float chamber as well as the side leg with instrumentation installation points.
Although insulation helps maintain the system temperatures, it is not completely without loss of heat. Heat is transferred from the interior of the pipe through the walls to the insulation and eventually to the exterior of the insulation.
The basic equation for heat transfer is as follows:
Eq (1) Q=U*A*ΔT (Heat Loss From Insulated Pipe to Air, n.d.)
Where Q is the heat loss, U is the heat transfer coefficient, ∆T is the difference in temperature between the interior surface and the surface on the exterior. When insulation is put on a pipe, additional thermal resistances are added to the system which help bring down Q.
Figure 1. (Heat Loss From Insulated Pipe to Air, n.d.)
From Figure 1, the overall heat transfer coefficient (U) is calculated based upon the geometry of the pipe and insulation as well as the thermal conductivity of the pipe, insulation, and the heat transfer coefficients of each boundary of the system.
Where r1 is the inside radius of the pipe, r2 is the outside radius of the pipe, r3 is the outside radius of the pipe along with the insulation thickness (radius), hi is the heat transfer coefficient on the inside of the pipe, ho is the heat transfer coefficient on the outside of the pipe, λ1 is the thermal conductivity of the pipe, λ2 is the thermal conductivity of the insulation. (Heat Loss From Insulated Pipe to Air, n.d.)
When designing a system to maintain a certain temperature, this equation can be utilized to determine the amount of insultation necessary.
In some cases, insulation by itself is not enough to maintain the temperature either due to unfeasible amount of insulation, or inconsistent temperature across the pipe.
Heat Trace and Steam Trace
Heat trace is used in conjunction with insulation blankets to help maintain consistent temperatures across the piping system. Heat trace is an electrical heater that is placed on the pipe beneath the insulation. This provides heat transfer into the pipe from the heat trace to help maintain a constant temperature in the piping system. In some cases, individual blankets can come with heated tape elements that also have temperature sensors for optimal temperature control and zoned heating within piping systems. In many cases, heat trace is attached directly to the pipe and controlled by a thermostat. Most electric heat trace systems are sized by power per foot and can be made for hazardous locations.
Heat trace installation can be accomplished with a single run, or multiple runs along the piping depending on the overall purpose. In some cases, simple freeze protection is all that is desired, so the installation is less challenging in terms of the piping geometry. A comprehensive heat tracing system usually consists of control and power distribution equipment, heating cables, connectors, thermostats, RTDs or thermocouples, splice kits, and termination kits. All these items are used in conjunction with insulation.
Image 2. Interior view of 2 different closed-loop grab sampling systems utilizing finned vertical electric heat coils. The heater orientation is dependent on the enclosure size, wattage, and other factors.
Image 3. The enclosure of a closed-loop grab sampling system with a horizontal heater and electric heat trace. The media being sampled is sour water, so the electric heat trace is used to prevent freezing. This system and those in Image 2 were destined for use in Canada, the Dakotas, or Minnesota where extreme cold exerts a constant influence on the process operating conditions.
Steam tracing is another proven technique to keep media temperature operating at desired temperatures. For years, steam tracing has been the dominant method for maintaining temperature in industrial settings because of its ability to provide heat to the overall piping system. Steam is a very efficient conductor of temperature, and many processes have steam readily available. The process for steam tracing on piping or a vessel such as a magnetic level indicator is very similar to electrical heat trace. Steam heat trace will typically have an inlet and an outlet that will transfer the heat from the steam through the vessel wall into the media to help maintain its state. As compared to electric heat trace, steam tracing is not as effective at controlling to a desired setpoint, as the amount of energy will be the gradient of temperature from the stem to the fluid. Getting consistent uniform temperature throughout the piping system is more challenging with steam tracing.
Image 4. Closed-loop grab sampling systems with steam heat trace. The system on the left was to be installed in Canada where it provides freeze protection while sampling sour water being maintained in the liquid phase. The system on the right is unique from the others pictured. Rather than preventing liquids from freezing, the enclosure interior is kept hot to prevent the vapor from condensing into the liquid phase.
Michael Bequette, P.E. – VP of Engineering SOR, Inc.
Michael Bequette has dual undergraduate degrees in Electrical Engineering, and Theoretical Physics from Kansas State University. He has a Master’s Degree in Electrical Engineering from the University of Kansas, and a Master of Business Administration from Park University. Michael has 29 years of experience in the oil and gas space, as well as aerospace, glass, pulp and paper, and water/wastewater. Michael is a licensed professional engineer in multiple states, holds 4 patents for fiber optic product development and capacitive fault location and is a senior member of IEEE.
Billy Terry – Product Manager SENSOR Sampling Systems
Billy Terry is a Product Manager for SOR Controls Group (SCG) team and is responsible for the overall product line management for SENSOR Sampling Systems. He has more than 30 years of experience working in various capacities focused primarily on sampling system product portfolios. Over the years Billy has held many roles, including Field Service Technician, Shop Supervisor, Head Quality Inspector, ISO9001:2008 Quality Administrator, Application Engineer, and Lead Sampling Engineer.
