purge flow regulator and flow switches

PURGE FLOW

REGULATOR

When a variable area flow meter is combined with a differential pressure regulator, it becomes a self combined flow controller.

They are low flow rate of either gas or liquid

Usually serve to protect pressure taps from contacting hot or corrosive fluid.

It become a self contained flow controller .

Figure: purge flow regulator

.. The main components of the illustrated

purge flow regulator are a glass tube rota meter, an inlet needle valve, and a differential pressure regulator.

The purge flow is fixed by adjusting springs #1and #2 for a particular pressure difference, usually in the range of about 60–80 in. (150–200 cm) of water

This constant pressure drop (P2– Po) is then maintained across the flow control valve (V)

The configuration in Figure maintains the outlet pressure (Po) constant by compensating any variation in the inlet pressure Pi by changing the regulator valve opening.

ADVANTAGES

Controls the flow rate to instrument line to prevent obstructions or to keep corrosive media away from delicate instrument.

Some viscous fluid will coagulate and clog instrument lines when not in motion. It is very difficult to clean these line and even more difficult to service process instruments when this occurs. By keeping a steady purge flow of an inert gas or liquid, you can prevent this from happening .

if your process is corrosive and can damage your instruments over prolonged contact. You can purge these system with a steady flow and avoid the cost of specialty materials or custom made products

FLOW SWITCHES

Flow switches are used to determine if the flow rate are above or below a certain point. This value can be fixed or adjustable

Type of flow switches

1) Folding paddle switch 2) Thermal flow switch 3) By pass flow switch 4) Capacitor type flow

switch

Folding paddle switch

It is least expensive type flow switches

At no flow the paddle hang loosely in a pipe in which it is installed.

As flow is initiated, the paddle begins to swing upward in the direction of the flow stream , this direction of the paddle is translated in to mechanical motion by on of several technique, including a pivoting cam, a flexure type or a hallows assembly

Thermal flow switch

On of the most popular solid state design is the thermal flow switch .

All heat actuated flow switches senses the movement or stoppage of the process steam by

detecting the cooling effect (the temperature change ) on one or more probes.

Thermal flow switches consists of one heater probe and two sensor probe connected to wheat stone bridge.

When the flow stop . Imbalance occurs in the bridge .

Bypass flow switch

They have an externally adjustable vane that create a differential pressure in the flow stream .

This differential pressure forces a proportional flow through the tubing that bypass the vane .

A piston retained by a spring in this bypass tubing and will move laterally, this piston’s movement actuates a switch .

Advantages

Can be use for fairly low flow rate

Their ability to be externally adjusted is very desirable feature.

Capacitance-type flow switch

Capacitance type flow switch can detect the absence or presence of liquids or slurries , which usually relate to flow/ no flow condition.

This can prevent pumps from running dry.

The capacitor type flow switch is a capacitance switch that fit between two flat faced flange in standard pipe sizes between 2 to 8 inches.

When the ring sensor contain air, it detect a dielectric constant of 1 .

When the flow start, it detect the dielectric constant of the flowing fluid, which is greater then 1, which corresponding increase in radio frequency current flow and operate the switch accordingly .

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