As we already have discussed mechanical method let learn about gauge protection method :
Mainly three types of gauge protection method are used:
· Pulsation protection and vibration protection.
· Temperature protection.
· Chemical protection.
(1) PULSATION AND VIBRATION PROTECTION :
1. OIL FILLING (GLYCERIN)
The area surrounding the bourdon tube is filled with transparent liquid, normally glycerin. This protects the internal mechanisms against damage from vibration. This also damps the movement of the pointer from small transient fluctuations.
Pressure gauge snubbers are used to protect the pressure gauge from the effects of pulsating pressure. Pressure snubbers are the most common type.
Adjustable snubbers are provided with an adjustable needle valve that enables the operator to restrict the flow as operating conditions may demand even when the snubber is in service. Pressure gauge snubbers are effective for applications with pressure above 5 psi.
(2) TEMPERATURE PROTECTION
1. SYPHON TUBE (pig tail type, coil type)
As the bourdon tube may be damaged by high temperature, it is common practice on steam systems to install the gauge at the end of a syphon tube. The syphon tube is filled with water which transmits the pressure of the working fluid to the bourdon tube. The most common forms of syphon tubes are the U type and ring type. The ring type is used on horizontal pipelines where there is sufficient space above the pipe, and U type is used when mounting the gauge on a vertical pipe line or horizontal pipe line where there is not sufficient space for a ring syphon.
SS304 contains 18% chromium and 8% nickel. SS316 contains 16% chromium, 10% nickel and 2% molybdenum. The "moly" is added to help resist corrosion to chlorides (like sea water and de-icing salts).
(3) CHEMICAL PROTECTION
DIAPHRAGM SEALED PRESSURE GAUGE
A diaphragm seal is a flexible membrane that seals and isolates an enclosure. The flexible nature of this seal allows pressure effects to cross the barrier but not the material being contained. Common uses for diaphragm seals are to protect pressure sensors from the fluid whose pressure is being measured. Since diaphragm seals need to be highly flexible, elastomers are commonly used, and include a wide variety of both general purpose and specialty rubbers. Elastomers are limited to low pressure applications and those that are chemically compatible with the various plastics and rubbers used. This is a Diaphragm seal type pressure instrument in which liquid as a pressure transmitting media is filled between a diaphragm seal parts and bourdon tube as an element. In this catalogue, General type of Indicator, Pressure gauge with Electric contact, Pressure Switch, Differential Pressure Gauge, Differential Pressure Gauge with contact and Differential Pressure Switch are introduced. Diaphragm and the lower flange as wetted parts can be selected according to applications, so these instruments are appropriate for the measurement of highly corrosive fluid, high viscosity fluid, fluid which contains solid materials or fluid to be easily solidified. Liquid is filled between the diaphragm and the Bourdon tube. The pressure which is received by the diaphragm is transmitted to the Bourdon tube by the filled liquid as pressure transmitting medium, and the Bourdon tube is deformed under the pressure to rotate the pointer.
SELECTING PRESSURE GAUGE :
1. Accuracy requires.
2. Dial size.
3. Operating pressure gauge.
4. Chemical compatibility.
5. Operating temperature range.
7. Vibration, pulsation and shock.
8. Method of monitoring.