Industrial Thermowells: Sometimes Taken for Granted, but Critically Important

Thermowells come in a wide variety
of shapes, materials, and sizes.
(Courtesy of Ashcroft)

One of the most important accessories for any temperature-sensing element is a pressure-tight sheath known as a thermowell. This may be thought of as a thermally conductive protrusion into a process vessel or pipe allowing a temperature-sensitive instrument to detect process temperature without opening a hole in the vessel or pipe.

Thermowells are critically important for installations where the temperature element (RTD, thermocouple, etc.) must be replaceable without de-pressurizing the process.

Thermowells may be made out of any material that is thermally conductive, pressure-tight, and not chemically reactive with the process. Most thermowells are formed out of either metal (stainless steel or other alloy) or ceramic materials.

A simple diagram showing a thermowell in use with a temperature sensor (RTD) is shown here:

Typical RTD thermowell installation.

As useful as thermowells are, they are not without their caveats. All thermowells, no matter how well they may be installed, increase the first-order time lag of the temperature sensor by virtue of their mass and specific heat value. It should be intuitively obvious that a few pounds of metal will not heat up and cool down as fast as a few ounces’ worth of RTD or thermocouple, and therefore the addition of a thermowell to the sensing element will decrease the responsiveness of any temperature- sensing element. What is not so obvious is that such time lags, if severe enough, may compromise the stability of feedback control. A control system receiving a “delayed” temperature measurement will not see the live temperature of the process in real time due to this lag.

For more information on thermowells, contact Mead O'Brien by visiting https://meadobrien.com or by calling (800) 892-2769.

Five Important Criteria in Applying Pressure Gauges

Process pressure gauge.
(Ashcroft)

Pressure gauges are installed in countless industrial and commercial applications around the world. From hygienic pharmaceutical process lines, to the most unpleasant and hostile areas in chemical, power, and food processing plants.

While there are millions of possible combinations of shapes, sizes, options and materials, pressure gauges all share the five  following application criteria, required for safe use and long product life.

Diaphragm seal.
(Ashcroft)

1 - Process Media Properties: Media that is corrosive, sludgy, or that can solidify is a potential problem for pressure gauges. In non-corrosive, non-clogging media applications, a direct connection without intermediate protection can be applied. For process media that could potentially clog or chemically affect the gauge's wetted parts, a diaphragm seal should be used.

2 - Process Media Temperature: Very hot media, such as steam or hot water, can elevate the gauge's internal temperature leading to failure or an unsafe condition. For high temperature applications, the use of a "pigtail siphon" or diaphragm seal is recommended. Siphons act as a heat sink and lower the exposure temperature. Diaphragm seals isolate the gauge from the higher temperatures.

Pigtail siphon.

3 - Ambient Operating Temperature and Environment: It is important to know the ambient environmental rating for any process instrument. Elevated ambient temperatures, moisture, vibration, and corrosive atmospheres can all affect accuracy, calibration, and safety. Choose the proper housing and mechanism materials if oxidizing or reducing atmospheres exist, and consider the addition of ancillary devices, such as remote diaphragm seals to physically relocate the gauge away from the hostile area.

Snubber

4 - Severe Pressure Fluctuations: In applications where dramatic line pulsations or strong over-pressure conditions are a possibility, the use of pressure restrictors, snubbers, or liquid-filled gauges will extend the service life of the pressure gauge.

5 - Mounting: Pressure gauges are standardly available with bottom (radial) and back connections. NPT (National Pipe Thread Taper) threaded connections are generally the standard. Many other process connections are available though, such as straight threads, metric threads, and specialized fittings. Make sure you know how the gauge is being connected. When mounting, pressure gauges should be almost always be mounted upright.

For more information about pressure gauges, contact Mead O'Brien by visiting https://meadobrien.com or by calling (800) 892-2769.

Limitorque Fluid Power Systems (LFPS)

Limitorque Fluid Power Systems is a group of modular scotch yoke fluid power actuators designed to deliver maximum torque with the lowest possible displacement and overall size. These heavy-duty, fluid-powered valve actuators and control systems are design primarily for the oil and gas industry. The group is categorized into three major sub-groups:

• Gas Powered Actuators - The Limitorque LDG direct gas actuator is designed to operate on high pressure pneumatic supply, including pipeline gases, nitrogen and any other equivalent high pressure source.
• Hydraulic Actuators - LHS and LHH are Limitorque’s range of hydraulic, quarter-turn, scotch yoke actuators. Designed to meet or exceed the most current and stringent safety and reliability standards for application in the oil and gas industry LHS and LHH are suitable for on/off and modulating control of all quarter-turn valves. 
• Pneumatic Actuators - Limitorque’s LPS and LPC are pneumatic quarter turn scotch yoke actuators, featuring a robust design suitable for heavy duty services, and among the longest design lifespans and maintenance intervals in the industry.

Download the Limitorque Fluid Power Systems PDF here.

Mead O'Brien
www.meadobrien.com
(800) 892-2769