Process Instrumentation: Pressure gauge details

PRESSURE GAUGES:

The pressure gauge is one of the most common instruments used in voluminous number in any type of plant. There are different types of pressure gauges depending on their service and pressure ranges. Selection of the pressure gauges, their applications and construction are of prime importance.

SELECTION AND APPLICATION:

1) RANGE AND OVERRANGE:

Range:

A range of the instrument should be approximate twice the maximum operating pressure. Too low a range may result in low fatigue life of the elastic element due to high operating stress. Too high a range may yield insufficient resolution for the application.

Over range:

Pressure gauges shall have an integral over range protection of at least 130 % of design upper range value without affecting accuracy. An internal stop shall limit the movement of bourdon tube in case of over range pressure.

2) TEMPERATURE:

To ensure long life and accurate readings, pressure gauges should not be exposed to process or ambient temperature over 65°C.

3) STEAM SERVICE:

· To prevent steam from entering bourdon tube, a siphon filled with water should be installed between the gauge and the process line.

· Siphons can be applied up to 4000 psi at 540 °C.

· If freezing of condensate in the siphon is a possibility, a diaphragm seal should be chosen to isolate the gauge from the process steam.

· Siphons should also be used whenever condensing hot vapor is present.

4) Hot Lines:

· When a gauge is installed in a hot line, one way to get the hot liquid or gas cooled down is to simply include an extra length of piping or tubing between the tapping point and the gauge.

· The slow rate of heat transfer through the extra pipe / tube will generally protect the gauge.

· Long-term exposure to the temperature in excess of 65 °C may cause discoloration of dials and fill fluids, as well as hardening of elastomeric seals and possible fill leakage.

· Soft soldered, silver brazed and welded pressure joints are rated at 120 °C max, 230 °C max, and 400 °C max. Respectively. Plastic gauge cases including phenolic should not be exposed to temp. > 120 °C.

Maximum recommended service temperature for gauge windows:

· 177 °C FOR PLAIN GLASS.

· 150 °C FOR POLYSULFONE.

· 130 °C FOR POLYCARBONATE

· 95 °C FOR LAMINATED SAFETY GLASS

· 80 °C FOR ACRYLIC

5) COLD SERVICE:

· The minimum recommended operating temperature for all gauges is (–45 °C).

· A hermetically sealed gauge case can be selected to minimize condensate freezing on the movement, thus reduce hindering its action.

· Gage filled with silicon oil will have maximum resistance to the effects of operating in freezing conditions.

· Glycerine filled gauges may be unsuitable below (-7 °C) due to increasing in viscosity.

6) Diaphragm seals:

Diaphragm seals with filled, flexible line assemblies are the good solution to the problem of hot liquid or gas lines. Due to the small diameter of the flexible line and usually a 5-ft length is adequate to cool down the process temperature to an acceptable limit of the instrument.

A diaphragm seal is recommended for the following process media;

· which is corrosive to the process sensing element

· which contains heavy particulates (slurries)

· Which are very viscous including those that harden at room temperature

7) PULSATION / VIBRATION:

· To protect gauges from vibration, include a dampener such as a throttle plug / screw or porous metal snubber.

· A liquid filled gauge with dampener will last longer. If the vibration is extreme, then choose a remote mounted pressure gauge with flexible tubing.

8) GAUGE FILLS:

Following points shall be considered while selecting the filling liquid in the chemical seal for a pressure gauge.

· Glycerine meets most requirements and its usable temperature is -17°C to 120 °C.

· Silicon filled gauges have a broader service range - 40 to 120 °C.

· Oxidizing media such as pure oxygen, chlorine require the use of compatible gauge fill, Such as fluorolube with a service range of - 45 to 120 ° C.

ACCESSORIES:

Accessories used with process pressure gauges are Snubbers, siphons, diaphragm seals and mounting accessories etc.

SIPHONS:

Siphons shall be installed on locally mounted gauges for steam service, and for process fluids with a temperature over 200 Degree C, when the gauge is above the tap connection.

Pig tail siphon shall be used as the standard.

SNUBBERS:

Snubber design shall allow easy disassembly to facilitate cleaning.

It is used to avoid vibration of the instrument.

GAUGE SAVERS:

Gauge savers protecting instruments against pressures that are very high compared to the operating range may be used, except on switches in critical service.

DIAPHRAGM SEAL:

The diaphragm seal is used where the process material is highly corrosive. for example, if the service is H2S gas then we require diaphragm seal. There are some other cases as well in which diaphragm seal is used. In the case of highly corrosive service like H2S material for the diaphragm is hastalloy-c.

LIMIT SWITCHES:

Pressure gauges are also available with switches when alarm contacts are required. Both inductive and microswitches are available.

However microswitches are never specified as the force for the movement has to be derived from pressure gauge system.

INSTALLATION:

In the pressure measurement the means by which the instruments interfaces with the process is a prime consideration to achieve reliable operation. The measuring device must come in physical contact with the process.

Certain precautions need to be taken to prevent damage due to the following factors:

· Process Temperature may be above design limits.

· Process medium may be corrosive.

· Mechanical vibration or process pulsation detrimental to gauge element.

LOCATION:

· All local gauges shall be mounted close to the process connection as practical, consistent with required accessibility and visibility and not subject to vibration.

· Where considerable numbers of gauges are located in the same area, mounting them on a common panel may be considered. A sun shade may be considered to avoid drift.

Elevation:

Gauges shall be mounted at an elevation above the tap for low-pressure service.

SAFETY PATTERN GAUGES:

· These gauges are meant for ensuring safety to the operating personnel in case of a blowout.

· Process connected gauges of safety type shall have a solid front disc inserts and hydrocarbon resistant blow out backs to prevent bursting of the gauge case or window upon element rupture.

GLYCERINE FILLED GAUGES:

For damping of a pointer.