Measuring Differential Flow in Industrial Process Control

Measuring differential flow

The differential flow meter is the most common device for measuring fluid flow through pipes. Flow rates and pressure differential of fluids, such as gases vapors and liquids, are explored using the orifice plate flow meter in the video below.

The differential flow meter, whether Venturi tube, flow nozzle, or orifice plate style, is an in line instrument that is installed between two pipe flanges.

The orifice plate flow meter is comprised the circular metal disc with a specific hole diameter that reduces the fluid flow in the pipe. Pressure taps are added on each side at the orifice plate to measure the pressure differential.

According to the Laws of Conservation of Energy, the fluid entering the pipe must equal the mass leaving the pipe during the same period of time. The velocity of the fluid leaving the orifice is greater than the velocity of the fluid entering the orifice. Applying Bernoulli's Principle, the increased fluid velocity results in a decrease in pressure.

As the fluid flow rate increases through the pipe, back pressure on the incoming side increases due to the restriction of flow created by the orifice plate.

The pressure of the fluid at the downstream side at the orifice plate is less than the incoming side due to the accelerated flow.

With a known differential pressure and velocity of the fluid, the volume metric flow rate can be determined. The flow rate “Q”, of a fluid through an orifice plate increases in proportion to the square root the pressure difference on each side multiplied by the K factor. For example if the differential pressure increases by 14 PSI with the K factor of one, the flow rate is increased by 3.74.

Guide to Industrial Electrical Grounding

Electrical Grounding

Acromag, a manufacturer of industrial I/O solutions, recently produced a 3-part series on the best practices of grounding electrical equipment. For your convenience, this post combines all three parts of the white paper here into a single document below.

When wiring or connecting circuits, electrical equipment, and electrical instruments, there is a connection that you probably don’t give much thought to, and one that consequently reigns as one of the greatest sources of instrument error and malfunction. That connection is Ground.

Electrical systems must be grounded in order to work properly. The earth often serves as an ideal ground because of its large mass and ability to absorb charge, but ground can be any electrical connection that is able to freely conduct electricity, and grounding a circuit does not always refer to making a physical connection to earth ground.

To learn much more, please read the following document.

The Complete Acromag Guide to Industrial Electrical Grounding from Instrument Specialties, Inc.

Understanding Hot Tapping: Insertion Flowmeter Example

Hot tapping insertion probe.

The ability to remove an insertion flowmeter probe is important for service and calibration. In many situations, it is not desirable to shut down the process and drain the pipe. In these cases, a method for removal known as "hot tapping" is preferred. Hot tapping (also known as pressure tapping) is a technique where a connection is made to an existing pipe or pressure vessel without disturbing flow or having to empty the pipe or vessel. This allows a pipe or tank to be in operation while maintenance or modifications are being done.

This video demonstrates the steps (and precautions) to remove a Seametrics flowmeter insertion probe from a live process (hot tap).  Insertion type flowmeters use a ball valve as the shut-off device and as isolation from the process media in the pipe. The video outlines the sequence of loosening the lock nut, raising the probe, then shutting off the valve before removal.

Additionally, here is a good document with more specific information on the insertion flow meter / hot tapping process:

Flow Meter Hot Tap Instructions from Instrument Specialties, Inc.

For more information on any flowmeter installation, contact:

Instrument Specialties Inc.
3885 St. Johns Parkway
Sanford, FL 32771
phone 407.324.7800
fax 407.324.1104

www.isisales.com