Overcome temperature measurement installation challenges with best practices

Your design looks great on paper. Now it has to work just as well in the real world, so make sure you install it using the same care.

05/06/2014


Figure 1: Be aware of connection head or integral transmitter housing dimensions added to the extension length relative to interference with nearby structures or equipment at the mounting site. Courtesy: Emerson Process ManagementYou have designed and purchased the optimal temperature measurement system to meet the performance requirements of your process. Your job is done, right? Wrong, because if it is not installed properly, the actual measurement results can fall far short of expectations. The field installation technician must understand the design decisions that were made to ensure that the "as installed" details agree with the "as designed" specification.

Best practices for proper installation

The level of effectiveness of a temperature system (a sensor with a connection head, a thermowell, and a transmitter) is dependent upon several factors including proper installation. If you were not the design engineer, you may not know what decisions were made about such things as mounting location, mounting style, what the immersion depth should be, or what the environmental conditions are at the intended mounting location. Therefore, several installation factors should be considered, including the installation, point-of-penetration, insertion length, mounting, and installation wiring.

Here are some considerations made in engineering a temperature system and the installation requirements that follow:

1. Locate the point of penetration—Start by locating a suitable measurement point that is representative of the desired measurement and is accessible. Determine the size of the pipe or vessel, the insulation thickness, and the presence of surrounding structures that may impede installation of the thermowell and access for future maintenance or replacement. Take into consideration the dimension of the entire assembly including an integrally mounted transmitter or connection head.

For installations downstream of static mixers, heat exchangers, or other turbulence producing elements, the insertion point must be far enough downstream where the streams have recombined into a homogeneous mixture that flows smoothly. Generally, a downstream distance equal to about 25 pipe diameters is sufficient.

There are special considerations for some other difficult applications like viscous fluids with laminar flow where the temperature at the pipe wall is different than that at the centerline. Insertion length is critical to get to a representative part of the flow at the centerline. Small pipe diameters present more of a challenge where tee fitting mounting or angled insertion may be considered.

After a suitable location is chosen, determine if it will be necessary to drain and clean the pipe or vessel before cutting into it to install the well. Ensure that the appropriate permits and approvals are secured.

2. Verify insertion length and other dimensions—Although the design engineer has made decisions about the thermowell mounting and insertion lengths based on the information at hand, it is incumbent upon the installation technician to verify pipe or tank diameter dimensions to determine that the thermowell provided has the correct insertion length. There is no standard formula to determine the insertion length of the thermowell. Rather, there are a few common practices that process industry plants follow along with good engineering judgment. Ideally, the tip of the thermowell should be located at an optimal process point, typically near the centerline, with flow conditions that represent the true process temperature. A general guideline for insertion length into pipe for optimal performance is 10 times the diameter of the thermowell for air or gas and 5 diameters for liquids.

The other dimensions of the thermowell may be verified by consideration of factors such as:

  • Insulation thickness
  • Connection type
  • Lagging length, and
  • Length of any required extensions to protrude through the insulation layer.

Be aware of connection head or integral transmitter housing dimensions added to the extension length relative to interference with nearby structures or equipment at the mounting site. (See Figure 1.)

Figure 2: There are several options to consider in choosing how to get the signal reliably from the field sensor to the control system with performance levels required by the application. Courtesy: Emerson Process Management3. Mounting the assembly—During the design phase, the engineer would have made mounting style decisions about using a threaded, welded, or flanged style of thermowell to meet the required process conditions of pressure rating, fluid velocity, type of fluid, conformance with codes and standards, and plant piping specifications and preferences. Consideration of speed of response, mechanical strength, and wake frequency concerns would have led to a decision of using a straight, tapered, or stepped thermowell profile.

4. Installation wiring—There are several options to consider in choosing how to get the signal reliably from the field sensor to the control system with performance levels required by the application. Figure 2 shows the most common choices.

The most common and preferred installation is a transmitter integrally mounted with the sensor and thermowell. In other cases, the transmitter is mounted separately but near the sensor-thermowell assembly.

Alternatively, sensors are connected to a marshaling cabinet using a twisted, shielded two-wire cable. From there a multi-pair bundle is typically run back to the control room. Ideally, the proper cable types have been specified during the design phase of the project. In all cases the conductors should be twisted and shielded with an outer insulated sheath selected in conformance with the environmental conditions where the wiring trays will be installed. For multi-conductor cables there are many designs. A common design has individually shielded pairs with an overall shield with drain wire for maximum noise protection and has an overall insulated sheath.

The sensor wires and output cables should be pulled through the conduits and fittings and into the transmitter housing and junction box through conduit seals.

Grounding and surge protection

Proper grounding and surge protection can pay huge dividends in enhanced performance. Here are some best practices to improve outcomes.

Follow proper grounding and shielding practices-Each process facility has its own guidelines for proper installation of grounds and shields. These guidelines should be followed where practical and appropriate. However, it may be prudent to verify that these guidelines are appropriate for your installation and, if in doubt about how to proceed, consult with the on-site electrical team leader and/or refer to the guidelines below.

Option 1. Remote mount with two separate grounding points-Connect the sensor shield, if supplied, only at the remount mount head and ensure that it is not connected at any other point and is electrically isolated from any grounded equipment. Ground the signal wiring shield only at the power supply end to an instrument system grounding point and ensure that the transmitter end is carefully isolated. (See Figure 3.)

Figure 3: Connect the sensor shield only at the remote mount head and ensure that it is not connected at any other point and is isolated from any grounded equipment. Courtesy: Emerson Process Management

Option 2. Remote mount with a continuous shield-Connect the sensor shield only to the signal cable shield and ensure that it is electrically isolated from the transmitter and all other field equipment. Alternatively, connect the signal cable shield to instrument system ground only at the power supply end. (See Figure 4.)

Figure 4: Connect the sensor shield only to the signal cable shield and ensure that it is electrically isolated from the transmitter and all other field equipment. Courtesy: Emerson Process Management

Option 3. Integral mount-Ground the signal wiring shield at the power supply end only to the instrument system ground ensuring that it is electrically isolated from the transmitter housing and all other field equipment. This is used for integral mount installations. (See Figure 5.)

Figure 5: Ground the signal wiring shield at the power supply end only to the instrument system ground ensuring that it is isolated from the transmitter housing. Courtesy: Emerson Process Management

One additional tip: The instrument system ground should not be connected to a power wiring ground which can carry noise, surges, and spikes that could interfere with measurement signals and/or destroy transmitters. An instrument system ground must be a very low resistance path to an earth grounding rod or grid.


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