Ashford Instrumentation Ltd - Installation & Maintenance of Pressure Gauges

Installation & Maintenance of Pressure Gauges

Installation and Maintenance of Pressure Gauges.

Introduction

When selecting a pressure or vacuum gauge for the application , the following points must be taken into consideration and must be acted upon where appropriate to remain in accordance with BS:EN 837/1-6 1997 and for the general safety of users and the protection of plant and equipment. Pressure gauges should also be selected and installed in accordance with the Pressure Equipment Directive which become mandatory on 29^th May 2002. Pressure gauges with a range of 200 bar and above must carry the CE mark if entering the distribution chain after 29^th May 2002. To satisfy the requirements of the Pressure Equipment Directive, pressure gauges with a scale above 0.5 bar and up to 200 bar are subject to Sound Engineering Practice and do not carry the CE mark.

Application

This pressure gauge is suitable for use on any liquid or gaseous (below 25 bar) media which is not highly viscous, which does not crystallize or solidify, which does not have any special hygienic or sterile connection requirements, which is within the normal temperature span of the instrument (see specification data sheet) and which is not corrosive to the selected wetted parts of the instrument (see material selection guide or contact the pressure gauge manufacturer if in any doubt). Steam should be regarded as a gas. Extreme caution should be taken when using a pressure gauge on explosive, flammable, toxic or oxidising materials and should only be specified and installed by a competent and qualified instrument engineer.

General

According to BS:EN837, on a static load, a pressure gauge should not be continuously used above 75% of its Full Scale Deflection (FSD). On cyclic pressures a pressure gauge should not be continuously used above 65% FSD.Gauges should never be pressurized over FSD, although, on certain models, our pressure gauges are built to withstand 1.3 times FSD (see individual data sheets). However, overpressure it is not recommended.The medium must not be of a nature to corrode or damage the selected wetted materials.Gauges for use on Oxygen, Acetylene and gasses or steam must be of the special type designated in BS:EN837 and are to be specifically manufactured for this purpose.If the atmosphere surrounding the pressure gauge is corrosive, suitable protection must be specified or a suitable material for construction must be selected.Where pulsation and vibration are present, suitable precautions must be taken as these conditions can seriously reduce the life of the pressure gauge if they are not compensated for.It is always recommended that a suitable isolation valve be fitted between the gauge and the ‘line’ to allow the pressure gauge to be removed for repair and calibration without disturbing the process.Where the gauge is fitted with it’s face other than vertical, this should be advised and compensated for during calibration.Pressure gauges should never be fitted to locations above or below the manufacturers recommended span for both medium or ambient temperature. Various devices are available to combat heat and should always be considered when ordering the gauge.

Installation of pressure gauges.

When fitting pressure gauges, the joint must always be tightened using the flats or hex below the gauge that are provided, the gauge must NEVER be tightened using the case. Where parallel threads are used, a tight joint must be made using a washer or bonded seal on the annular surface at the bottom of the gauge shank into a suitable fitting. A pressure joint should not be made with a parallel thread, tapered threads are suitable for pressure joints. Rigid piping should not be used on surface or flush mounting gauges. Where a pressure gauge is being used on liquid, the lines should be full of liquid and care should be taken to avoid loops where air or vapor can cause air locks or accumulate. Where a gauge is to be used on air or gas, the pipe-work should rise continuously to the gauge thus allowing any moisture or condensed liquid to drain away from the pressure gauge. Where a blow out device is fitted, this must not be obstructed in any way once installed and a minimum distance of 20mm clearance should always be provided. When used on any steam application, a suitable syphon or similar device must be fitted to the pressure gauge to allow the steam to condense. Care must always be taken to ensure that any process fluid is not allowed to freeze within the pressure gauge as this will cause damage to the instrument and may cause an indicating error on the dial. Special consideration must be taken when using a pressure gauge on exposed water applications.

Maintenance.

All pressure gauges should be regularly tested to ensure their accuracy. This work must be undertaken by qualified personnel. A ‘quick’ visual inspection should be frequently carried out in situation checking that the pressure gauge is reading within expected parameters for the application and that, if possible, upon pressure in the line being reduced to zero bar G, the pressure gauge also returns to zero. If these criteria are not met, the gauge should be removed from the line, repaired and re-calibrated.

Repair.

It is not recommended that repairs are carried out by unqualified personnel. Only either the site instrumentation specialist or other qualified persons should be permitted to carry out any gauge repairs on site if suitable equipment is available or the pressure gauges should be returned to the manufacturer for repair.Where chemical seals are in use, it is critical that the pressure gauge head and the chemical seal are never separated, they are vacuum filled systems which are calibrated in this state as a unit and must be repaired in the same manner.If there is any doubt regarding the repair of a pressure instrument, always return to manufacturer.

Spares.

As we have stated above, pressure gauges should only be repaired by qualified personnel in controlled conditions. With this in mind, we do not have a recommended spares list for pressure gauges. The only component that is valid as a ‘spare’ would be a replacement window (where it is possible to remove the window), most of which can be replaced using a glass or plastic supplier local to the site, we are happy to advise the relevant sizes of the glasses in our products.

Additional Information

More comprehensive information on the selection and installation of pressure gauges can be found in EN 837 Part 2: 1998, or from the pressure gauge manufacturer.