Pressure Measurement & Control


Process conditions affect pressure gauges

September 2014 Pressure Measurement & Control

All process and ambient factors should be considered when selecting and installing pressure measuring devices. The following is intended as a guideline from SA Gauge to assist with selecting the appropriate instrument.

The use of pressure transmitters as the primary source of pressure measurement in sophisticated process applications is very popular. However, as a backup to the electrical readings, and as a local display, mechanical dial indicating pressure gauges are still used on many systems.

Today’s mechanical pressure gauges mostly employ age-old proven technology in the form of a bourdon tube soldered or welded to a socket with the tip of the bourdon tube connected to a geared movement, indicating pressure applied via a pointer and dial.

Since the pressure gauge is a purely mechanical instrument, process conditions must be carefully considered before selection to ensure the best accuracy and to maximise the instrument’s service life.

To ensure accuracy and safety, best practice measuring recommends special consideration when dealing with process conditions such as extreme process or ambient temperatures, corrosive or solidifying media, vibration, pulsation and overpressure.

Temperature

Maximum permitted process temperatures for pressure gauges with brass wetted parts are 70°C. For stainless steel wetted parts maximum process temperature should not exceed 100°C. Ambient temperatures are permitted from -20 to 60°C. Errors in accuracy need to be considered when using gauges above or below calibration conditions – usually 22°C.

For applications where process temperatures exceed permissible levels, the media needs to be cooled down before entering the instrument. Accessories like cooling towers, siphon tubes, capillary assemblies and diaphragm seals are typical devices used to separate and protect the instrument from the process heat source.

Corrosive and solidifying media

Some aggressive or corroding process media will attack copper-alloy or stainless steel 316, or will solidify and obstruct the pressure port of the gauge. Diaphragm seals are designed to isolate the pressure gauge from the media to ensure accurate and reliable pressure readings. The wetted parts of the diaphragm seal are made from process appropriate materials such as stainless steel 316, hastelloy, tantalum or PTFE.

Vibration

Due to the purely mechanical nature of pressure gauges, vibration is one of the main reasons of premature gauge failure. Linkages, gears and bushings are all parts suffering from excessive wear when exposed to vibration. Pointer oscillation makes accurate reading difficult but can be avoided by filling the gauge with glycerine or silicone oil. Liquid filling of gauges lubricates all the friction points, ensuring longer instrument life.

Pulsation

Dynamic load cycles of pumps and valves that cause a rapid change in pressure in a system causes metal fatigue in the elastic bourbon sensing element, resulting in gauge failure. Whilst glycerine filling of the gauge will assist against pointer flutter, it cannot prevent the damage caused by pulsation to the bourdon tube. A flow restricting device is the best option here. Pulsation dampers such as inlet restricting or piston type snubbers restrict the pressure pulsation.

Over pressure

Over pressure typically occurs when a pressure gauge is subjected to a sudden rush of process pressure, such as when full bore valves are quickly and completely opened under load. This short period ‘spike’ in the system subjects the gauge to more pressure than it is designed for, resulting in permanent damage. To prevent this SA Gauge manufactures an overpressure protector that can be adjusted and locked to shut the process pressure off at a given set pressure. The spring loaded overpressure protector automatically opens again once the over pressure condition has passed. On certain ranges there are diaphragm operated gauges available, capable of coping with five times over pressure without any permanent damage.

For more information contact Chris du Plessis, SA Gauge, +27 (0)31 579 2216, chris@sagauge.com, www.sagauge.com



Credit(s)



Share this article:
Share via emailShare via LinkedInPrint this page

Further reading:

Ceramic and metallic measuring cells optimise additive production at BASF
September 2020, VEGA Controls SA , Pressure Measurement & Control
The combination of a ceramic Certec and a metallic Metec measuring cell proved to be the perfect combination from VEGA.

Read more...
Combination of digital pressure gauge and smartphone via Bluetooth
August 2020, WIKA Instruments , Pressure Measurement & Control
For optimal process efficiency in pharmaceutical applications, solutions featuring wireless communication are becoming first choice, especially in mobile applications.

Read more...
Thermometer guns on coronavirus front lines: accurate or not?
July 2020, SA Gauge , News
If you have not already had your temperature taken before entering a public building, there is a good chance you soon will. It has become an iconic image of the coronavirus outbreak worldwide: a masked ...

Read more...
Upgrade for PSM-700 pressure switch
May 2020, WIKA Instruments , Pressure Measurement & Control
WIKA’s PSM-700 pressure switch with large adjustable switch differential is now able to withstand higher process temperatures. Previously, the process media was limited to a temperature range of -30...

Read more...
Hygienic food production
June 2020, VEGA Controls SA , Pressure Measurement & Control
Food production is a matter of trust, and hygiene is the number one priority in the manufacturing process. Success is determined by both the reliability and the efficiency that make flawless production ...

Read more...
Upgrade for PSM-700 pressure switch
June 2020, WIKA Instruments , Pressure Measurement & Control
WIKA’s PSM-700 pressure switch with large adjustable switch differential is now able to withstand higher process temperatures. Previously, the process media was limited to a temperature range of -30ºC ...

Read more...
Vega’s solutions for pharmaceutical applications
May 2020, VEGA Controls SA , Pressure Measurement & Control
The special challenges faced by the pharmaceutical industry result from the wide diversity of its processes. Success depends on the consistency and continuity of the production processes, whether it be ...

Read more...
Neglected pressure gauges can lead to downtime and disaster
Technews Industry Guide: Maintenance, Reliability & Asset Optimisation 2020, SA Gauge , Pressure Measurement & Control
Pressure and temperature gauges are often the most used instruments in a plant. Gauges are relatively inexpensive and because of their large quantity, gauge maintenance is often neglected, resulting in ...

Read more...
VEGA extends portfolio for pharmaceuticals production
February 2020, VEGA Controls SA , Pressure Measurement & Control
The reliable and lasting performance of VEGA’s proven instrument portfolio has enabled plant operators to monitor and control with great precision chemical, thermal or biological processes, often under high temperatures and pressures.

Read more...
Intrinsically safe electronic pressure gauges
January 2020, Instrotech , Pressure Measurement & Control
Instrotech now offers Keller’s range of intrinsically safe electronic pressure gauges for use in areas subject to gas explosion risks. The type approvals are compliant with the ATEX Explosion Protection ...

Read more...