There are different types of accessories or mechanical components to
protect the pressure instruments against hazardous process fluids, from high/low
temperature, high/low pressure, corrosion, pulsation, etc, or isolate the instrument for removal or maintenance of process line or equipment.
They are mainly: Valve Manifolds, Pulsation Dampeners / Snubbers, Chemical
Seals, Water traps and pigtail Siphons, Bleed/Vent Fittings, Mounting Brackets,
Heated Enclosure, Impulse Lines, etc.
1. Valve Manifolds
Valve manifolds are generally used to isolate the pressure instruments
at the time of calibration or isolate the process connection during instrument
replacement on a continuous operation process. Some process fluids may be toxic, corrosive, radioactive, etc, and cause hazards to personnel or the
environment, so it is necessary to protect against the release of these fluids
during calibration or instrument replacement.
The most common type of manifold is the two-valve, three-valve,
and five-valve arrangements. Generally, the valves are termed as isolating,
block, bleed, equalizing, drain and vent valves.
When the valves and connections are preassembled with manifold,
results in time-saving, and the chances for leaks are reduced. Manifolds may be
preassembled to the instrument or available loose to bolt directly to standard
instruments.
2. Pulsation Dampeners / Snubbers
When the pressure gauge is in service, it affects by pulsation or
sudden fluctuation in the pressure of process fluids. It can be avoided by filling viscous
liquid like glycerin inside the gauge. The inherent properties to the friction of
this fill liquid act as a “shock absorber” which protects the gauge from pulsation
or external vibration. It cannot be in the instrument specifically to measure high-frequency pulsations.
A snubber consists of a fluid restriction installed between with
pressure sensor and the process to avoid damping pulsations in a pressure
instrument.
The simplest example of a snubber is a simple needle valve (an
adjustable valve designed for low flow rates) placed in a mid-open position,
restricting fluid flow in and out of a pressure gauge.
3. Chemical
Seals
Most pressure sensors come with chemical seals or diaphragm
protectors. They are using for the following reasons:
1. Use to avoid freezing or settling of the process fluid in the sensor
due to temperature variation.
2. Prevent entering hazardous materials into the pressure sensor
and slurries from entering, plugging the detector element.
3. Protect the sensing instrument from corrosive or harsh chemicals
we use the isolating diaphragm and fill fluid referred to as a chemical seal.
4. Water traps and pigtail Siphons
When sensing elements used in steam service, we must prevent the
stream from entering the element, which could cause temperature damage. For this reason,
to avoid damage to the sensing element, install a coil pipe siphon between the
gauge and the process connection.
For example, in a power plant to check the parameters of the steam at
the outlet of the boiler in the main steam line, we use a siphon to avoid damage to the sensing element.
5. Bleed/Vent Fittings
When removing the pressure transmitter from service we have to “bleed”
or “vent” stored fluid pressure to the atmosphere, prior to disconnecting the transmitter from the impulse lines. A common accessory for pressure-sensing
instruments (transmitters) is the bleed valve fitting or vent valve fitting,
installed on the instrument as a passive device that generally comes with ¼ inch
male NPT pipe threads.
These bleed valves are used to bleed unwanted fluids from the pressure
chamber when directly installed on the flanges of a pressure instrument. To sense
compressed air pressure, we bleed condensed water out of an instrument, and to
sense water pressure we bleed air bubbles from an instrument.
6. Mounting Brackets
In general, we use a 2-inch pipe mounting bracket to mount
transmitters. These brackets are manufactured from heavy sheet metal and equipped
with a U-bolt designed to clamp around a 2-inch iron pipe. To provide a mechanical stable of attaching the transmitter, the holes stamped in the bracket
match the mounting bolts.
7. Heated Enclosure
When the ambient temperature is too much cold or hot, a protective
measure against fluid inside a pressure transmitter is to house the transmitter
in an insulated, heated enclosure. Also use an enclosure, to protect all kinds of
temperature-sensitive instruments from extreme cold.
8. Impulse Lines
8.1 Purged Impulse Lines
The pressure instrument isolates from direct contact with the process
fluid by purging the line with a continuous flow of clean fluid particularly
when the impulse line is prone to plugging.
Generally, the purge fluid was clean water. But other than water,
Gases such as air, nitrogen, or carbon dioxide are often used in purged systems,
for both gas and liquid process applications.
8.2 Heat Traced Impulse Lines
In cold weather conditions, there is a possibility of liquid freezing that we used in impulse lines. To avoid this, we use active heating
mediums such as steam and electrical arrangement.
“Steam tracing” consists of a copper tube carrying low-pressure
steam, bundled alongside one or more impulse tubes, enclosed in a thermally
insulating jacket.
Electrically “tracing” uses a twin-wire cable (heat tape) that
acts as a resistive heater. When power is applied, the cable heats up, thus
imparting thermal energy to the impulse tubing it is bundled with.
List of
Prominent Manufacturers: Ametek, Anderson Greenwood, Dresser Measurements, Dwyer, Honeywell, Mid-West Instruments, MKS Instruments, Noshok, Parker, Rosemount, Siemens Moore, Smar International, Swagelok, Versa Gauge, Wallace
& Tiernan, Weiss Instruments
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