Pressure and temperature gauges are very widely used on process plants. Gauges are relatively inexpensive, and because they are installed in large numbers, maintenance can be neglected, resulting in unreliable and compromised instruments. Some plants choose not to replace failed gauges until they have to, and sometimes that is too late.
A properly maintained and calibrated gauge will accurately indicate how a system is performing. Part of that function is to enable early detection of potential plant failures or accidents. Poorly maintained, inaccurate gauges can compromise a plant’s ability to detect potential problems. Accidents, damage and injury to equipment and personnel is often the result.
Incorrect selection, installation, and neglecting to maintain pressure gauges can also result in unplanned production downtime, process issues and inferior product quality. In addition, allowing instruments that are critical to a process to drift out of specification can be a risk to employee safety. The cost of calibration is normally insignificant compared to the potential loss of production or injury costs. Ultimately, one needs to be able to trust the reading one gets from a pressure or temperature gauge.
The use of pressure transmitters as the primary source of pressure measurement in sophisticated process applications is very common. However, as a backup to these electrical readings, and as a local display, mechanical dials indicating pressure gauges are still used on many systems. Today’s mechanical pressure gauges still employ old and 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. This indicates the 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 optimum accuracy and safety, and to maximise the instrument’s service life. In addition, best practice in measurement requires that special precautions be taken when dealing with process conditions such as extreme ambient temperatures, corrosive or solidifying media, vibration, pulsation, and overpressure.
SA Gauge specialises in the design and manufacture of pressure and temperature measuring instruments. In addition to its absolute commitment to customer satisfaction, the company goes to great lengths to educate its customers so that they can make the best possible use of their pressure and temperature gauges. SA Gauge believes that all process and ambient factors should be considered when selecting and installing pressure measuring devices, and recommends that the following guidelines on selection and calibration be followed in order to prevent gauge failure.
Working pressure
Although pressure gauges will tolerate full-scale pressure for short periods, in general the working pressure should not exceed 70% of the full-scale value. For thermometers, media pressure should not exceed 2500 kPa without the use of a suitable thermowell.
Wetted parts
These must be compatible with the process media. Choose from Cu-alloy (brass) or stainless steel 316. With media that can corrode the wetted parts or obstruct the pressure port, a diaphragm-type chemical seal should be selected.
Temperature
The maximum permitted process temperature for pressure gauges with brass wetted parts is 70°C. For stainless steel wetted parts, the 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 can attack copper-alloy or stainless steel 316, or 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 materials appropriate to the process 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 for 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 cause metal fatigue in the elastic bourbon sensing element, resulting in gauge failure. While glycerine filling of the gauge will assist in countering 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 restriction or piston type snubbers restrict the pressure pulsation.
Overpressure
Overpressure typically occurs when a pressure gauge is subjected to a sudden rush surge 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 overpressure condition has passed. On certain ranges there are diaphragm-operated gauges available that are capable of coping with five times overpressure without any permanent damage.
Installation
Always secure the instrument by means of a suitable wrench on the hexagon/square of the threaded connection. Twisting the instrument by hand on the case can cause damage to the internals of the instrument. For gauges with flanges, support the gauge fitting with a suitably sized wrench to counter the force of tightening the process fitting. This will prevent damaging the gauge internals.
Calibration
Even the highest quality instruments are subject to drift over time, resulting in inaccurate measurements and substandard performance. It is important that all instruments are calibrated by approved personnel. Calibration intervals and error limits should be defined, and records of the calibration results should be kept, maintaining instrument integrity. SA Gauge’s calibration laboratories are SANAS ISO/IEC 17025 accredited, and maintain the highest accuracies to ensure international traceability.
SA Gauge is driven by a ‘customer satisfaction at all costs’ sales team, a ‘results driven’ engineering team, and a ‘first time right’ production team. Coupled with the company’s high internal standards for quality control, this philosophy ensures that its customers can be sure of receiving accurate and high-quality instruments that are made to their specifications at short notice.
| Tel: | +27 31 579 2216 |
| Fax: | +27 31 579 2301 |
| Email: | [email protected] |
| www: | www.sagauge.com |
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