Friday, April 10, 2015

Electrical Pressure Sensor Types

Here is an introduction to the basics of electrical pressure sensors, from Tony Kuphaldt's Lessons in Industrial Instrumentation.

This presentation explains how a variety of electrical pressure sensors work, including piezoresistive (strain gage), differential capacitance, and resonance sensors.

Sensor principles, sensor mounting and associated hardware design for pressure transmitters and differential pressure transmitters are also described.

Click on the image below to open the presentation (viewable on all devices).


pressure sensors
Click on image to see presentation.

Tuesday, March 31, 2015

Pressure Switches for Industrial Applications

pressure switch
Industrial
Pressure
Switch
(CCS Dualsnap)

A pressure switch is a device that detects the presence of fluid pressure. Pressure switches use a variety of sensing elements such as diaphragms, bellows, bourdon tubes, or pistons. The movement of these sensors, caused by pressure fluctuation, is transferred to a set of electrical contacts to open or close a circuit.
Pressure Switch Symbols

Normal status of a switch is the resting state with stimulation. A pressure switch will be in its “normal” status when it senses low or minimum pressure. For a pressure switch, “normal” status is any fluid pressure below the trip threshold of the switch.

One of the earliest and most common designs of pressure switch was the bourdon tube pressure sensor with mercury switch. When pressure is applied, the bourdon tube flex's enough to tilt the glass bulb of the mercury switch so that the mercury flows over the electrical contacts, thus completing the circuit. the glass bulb tilts far enough to cause the mercury to fall against a pair of electrodes, thus completing an electrical circuit. Many of these pressure switches were sold on steam boilers. While they became a de facto standard, they were sensitive to vibration and breakage of the mercury bulb.

Pressure switches using micro type electrical switches and force-balanced pressure sensors is another common design.  The force provided by the pressure-sensing element against a mechanical spring is balanced until one overcomes the other. The tension on the spring may be adjusted to set the tripping point, thus providing an adjustable setpoint.

One of the criteria of any pressure switch is the deadband or (reset pressure differential). This setting determines the amount of pressure change required to re-set the switch to its normal state after it has tripped.  The “differential” pressure of a pressure switch should not to be confused with differential pressure switch, which actually measures the difference in pressure between two separate pressure ports.

When selecting pressure switches you must consider the electrical requirements (volts, amps, AC or DC), the area classification (hazardous, non-hazardous, general purpose, water-tight), pressure sensing range, body materials that will be exposed to ambient contaminants, and wetted materials (parts that are exposed to the process media).

Thursday, February 26, 2015

High Performance Butterfly Valve Seating Principles - Metal Seats

high performance butterfly valve
High Performance Butterfly Valve
(courtesy of Flowseal)



PRINCIPLE OF METAL SEATING



Metal-to-metal sealing is accomplished by the “line contact” between a spherical surface and conical surface. Figure 1 illustrates a typical globe control valve seat and plug. The plug seating surface is the segment of a sphere; when engaged against the seat ring, a line contact seal is achieved.

In a metal seat design, it is necessary to apply enough force per linear inch to maintain a tight metal-to-metal contact between the seal- ing members; however, high linear thrust can cause a collapse of the seating members (“bearing failure”).




DISC CLOSED, Self-Energized Seal

In Figure 2, the Flowseal disc and seat are engaged, and the process fluid is under low pressure. The spherical edge of the disc, with a larger diameter than the conical seat tongue, imparts a thrust of approximately 600 pounds per linear inch against the seat. The mechanical properties and shape of the Inconel® seat allow it to both flex and maintain a constant thrust against the disc.

This controlled loading prevents the occurrence of bearing failure and reduces the leakage and wear between the components.

Wednesday, February 25, 2015

High Performance Butterfly Valve Seating Principles - Soft Seats

high performance butterfly valve
High Performance Butterfly Valve
(courtesy of Flowseal)

High performance butterfly valves (HPBV) are a standard in many industries including heating, ventilating and air conditioning, power generation, hydrocarbon processing, water and waste water treatment, and marine and commercial shipbuilding.

They are also installed in applications as diverse as food and beverage processing, snowmaking and pulp and paper production. Configurations are available for harsh conditions as well as applications requiring nominal pressure and temperature ratings.

The following describes the soft seating design principles for high performance butterfly valves:

soft seated HPBV valve
DISC OPEN 

In Figure 1, the disc and seat are not engaged. In this position, the shoulders of the seat are forced against the cavity shoulders by the compression of the o-ring.

The seat is recessed inside the seat cavity and acts as a gasket in the anchoring groove area.The seat cavity is sealed from exposure from the process fluid and protects the seat from abrasion and wear. The o-ring, which is completely encapsulated by the seat, is also isolated from exposure to the process fluid.

Tuesday, February 24, 2015

Applying Cooling Towers

Delta Cooling Tower
Delta Cooling Tower
Here is a presentation that explains how to apply cooling towers, including the basic cooling tower principles, design considerations, and construction issues.

The original presentation given in 2009 by Steve Lowe, PE to the Hampton Roads, VA Chapter of ASHRAE.






For more information on any cooling tower issue in Colorado, New Mexico, Wyoming, Montana, Utah, Nevada, Idaho, or the western Dakotas, contact:

Mountain States Engineering and Controls
1520 Iris Street
Lakewood, CO 80215
303.232.4100 Phone
303.232.4900 Fax
Email: info@mnteng.com
www.mnteng.com

Monday, February 16, 2015

Description of a Process Control Enclosure in a Hazardous, or Explosion Proof Location

This is a short video that explains what an explosion-proof enclosure looks like, how it works, and why it is safe to use in explosive or combustible atmospheres.

The definition of "explosion-proof" doesn't mean the enclosure can withstand the forces of an external explosion, but rather that it will cool any escaping hot gases (caused by an internal spark or arcing contacts) sufficiently enough as to not to allow the ignition of combustible gases or dusts in the surrounding area.

For more detailed information on electrical equipment enclosures in hazardous areas, visit this page.