Pressure Gauge Accessories and Attachments
Pressure gauge accessories are mechanical devices used in the hook up of a pressure gauge or transmitter to minimise any potentail damage to the pressure measuring instrument from the process fluid being measured.
Why use Pressure Gauge Accessories?
Pressure gauges and differentail pressure instruments are mechanical pressure measuring instruments used to measure gauge pressure, absolute pressure or differential pressure. Although relatively robust, they are susceptible to damage from the process being measured e.g. too much pressure, extreme temperatures, pulsating pressure etc. To help mitigate the likliehood of damage pressure gauge manufacturers and suppliers offer a wide range of accessories that can be installed with the gauge. Below is a description of the more common pressure gauge accessories.
Syphon Tube (aka Pigtail)
A pressure gauge siphon tube, also know as a pressure gauge pigtail, is a device installed to protect the gauge from high temperatures. Siphons are used in hot vapour applications and are particuarly common on steam systems.
The hot vapor condenses inside the coil of the siphon which prevents the high temperature vapors from reaching the sensing element of the pressure instrument. The condensed vapour transmits the pressure of the working fluid to the measuring element tube, enabling the gauge to be located some distance from the actual point where the pressure is being measured. In steam applications it is common for the syphon tube to be filled with water during gauge installation.
There are two common forms of syphon tube, namely: the "U", and the ring type. The ring type is frequently referred to as a pigtail. The choice of "U" or pigtail depends on pipe orientation and available space.
The ring tube is used on horizontal pipelines where there is sufficient space above the pipe, and the ‘U’ type pressure gauge syphon is used when mounting the gauge on a vertical pipeline, or on horizontal pipelines where there is insufficient space for a ring type siphon.
The mini-syphon is a relatively new accessory, designed to replace pigtail and "U" siphons. It is manufactured with an internal thermal barrier which protects the pressure measuring instrument from harmful steam, hot vapors and liquids.
It is also claimed that the mini–siphon helps eliminates gauge whip and vibration that is typically found on traditional syphons by mounting the instrument closer to the process.
Pressure Gauge Snubber
A pressure gauge snubber, or simply a gauge snubber, is a device used to dampen pressure pulses and fluctuations caused by reciprocating pumps and compressors. This smoothing effect provides more accurate pressure indication and extends the working life of the pressure gauge. Additionally a pressure gauge snubber can prolong the time between gauge calibrations.
Another method of smoothing out pulsations is to use a liquid filled gauge, typically glycerine. Snubbers are generally a cheaper option than using a glycerine filled gauge.
There are various types of snubber including porous snubbers, piston snubbers and throttling snubbers.
Porous snubbers are suitable for general purpose applications.
Piston snubbers are generaaly supplied with two or more pistons to adapt to varying applications.
Throttling snubbers have a built-in needle valve, or variable orifice that allows you to adjust the amount of snubbing externally. Anti-blow out stems are a desireable feature when specifying throttling gauge snubbers.
Over Pressure Protector
An overpressure protector is used to protect a pressure gauge against the effect of pressures that exceed the maximum pressure rating of the gauge. This prevents loss of calibration, damage to internal gauge components and rupture of the sensing element.
At a pre-set pressure, the over-pressure protector shuts-off the pressure to the gauge. The protector will reset when the pressure falls by typically 10%, though this will vary from manufacturer to manufacturer.
Although the set-point pressure is externally adjustable, it is usual to purchase over pressure protectors when purchasing a gauge. In this instance it is usual for the over pressure protector to be factory set at 1.1 x maximum scale value of the gauge being protected.
Over pressure protectors allow installation of several gauges with different pressure ranges in ascending stages and makes it possible to read even low ranges in a precise way, when the total range is in fact much higher.