Pressure Gauges

Pressure Gauge Installation & Usage Guidelines

Users should become familiar with ASME B40.100 (Gauges – Pressure Indicating Dial Type – Elastic Element) before specifying pressure measuring gauges. This document – containing valuable information regarding gauge construction, accuracy, safety, selection and testing – may be ordered from: www.asme.org

PRESSURE RANGE SELECTION

To ensure proper operation and long service life, the proper pressure range should be selected. For applications with constant, steady pressure, the measured pressure should be no more than 75% of the full scale range of the gauge. For applications with fluctuating pressure, the measured pressure should be no more than two-thirds of the full scale range of the gauge. In general, it is best to choose a range that is roughly 2X the average measured pressure. This gives over pressure protection and the highest accuracy.

TEMPERATURE

Ambient Temperature: To ensure long life and accuracy, pressure gauges should preferably be used at an ambient temperature between –20 and +150°F (–30 to +65°C). At very low temperatures, standard gauges may exhibit slow pointer response. Above 150°F (65°C), the accuracy will be affected by approximately 1.5% per 100°F (38°C). The pressure gauge should not be used outside of its rated temperature limits as noted on the Data Sheet specific to that gauge. At temperatures above or below these limits, the gauge accuracy will be significantly reduced and the possibility of gauge failure may exist.

High Temperatures or Corrosive Process Media: In order to prevent hot media such as steam from entering the bourdon tube, a gauge siphon or pigtail filled with water should be installed between the gauge and the process line. A cooling tower may also be used to reduce the temperature effect on gauges. A chemical or a diaphragm seal should be used to protect gauges from corrosive media, or media that will plug the instrument.

INSTALLATION

The pressure gauge should be installed where exposure to heat and vibration are minimal and where the dial can be easily read. Whenever possible, gauges should be located to minimize the effects of vibration, extreme ambient temperatures and moisture.

Isolating Devices: A shut-off valve such as a needle valve or gauge cock should be installed between the gauge and the process in order to be able to isolate the gauge for inspection or replacement without shutting down the process. The use of such devices is critical in times where start up pressures may temporarily exceed normal operating pressure. All isolating devices shall be opened slowly to prevent “slamming” of the bourdon tube. Care not taken during this time may damage the instrument.

Overload Protection: An overload protector should be used in situations where the process media may spike or be susceptible to overpressure of the design range of the gauge. An overpressure device or overload protector may be installed to prevent damage to the instrument.

Threaded Connections: The tightening or loosening of gauge connections shall be done using the wrench flats on the gauge fitting. Using the gauge case to tighten or loosen pressure gauges will damage the gauge and may cause unrepairable damage to the instrument. Proper sealant tape or paste shall be used for sealing tapered threads like National Pipe Thread (NPT)

Vibration/Pulsation protection: If the pressure gauge is exposed to vibration or pulsating pressure or both, a liquid filled pressure gauge is recommended. The liquid dampens the effects of vibration making the pointer easier to read. Pressure dampeners, snubbers and or restrictor screws may be used to reduce pulsation. In extreme cases, a remotely mounted liquid filled gauge connected with a length of capillary line may be used.

Pressure Gauge Safety: Pressure media such as oxygen, acetylene, welding equipment, life support or diving equipment, boilers etc., may require pressure gauges of a construction complying with national standards or local codes. Selection of a pressure gauge for such media or applications must be carefully considered and specified when ordering.

Storage: Storage temperature should not exceed -4°F (-20°C) or 140°F (60°C) unless specified otherwise. Pressure gauges shall be stored in their original packaging until ready for use. Threads and gauge orifices shall be kept clean and free of debris until they are ready for installation.

Maintenance: If the accuracy of the gauge cannot be checked in place, the user can look for erratic or random pointer motion, readings that are suspect – especially indications of pressure when the user believes the true pressure is 0 psi. Any gauge which is obviously not working should be removed from service. Other indications include bent or unattached pointers due to extreme pressure pulsation or overpressure, broken windows which should be replaced to keep dirt out of the internals, leakage of gauge fill, case damage or cracks, any signs of process media leakage through the gauge, including its connection and/or discoloration of gauge fill that impedes readability.