Introduction to bypass rotameter :
The simplest & most common flow meter is a fixed Orifice in a
pipeline. This Orifice creates a pressure differential (i.e. pressure drop) which is proportional to flow. But the flow rate versus differential pressure is a square root function, thus at the high end of the scale the graduations are far apart and very easy to read. At the low end of the scale, the graduations are so close together that reading becomes impossible. The net result is a meter with only a 3 OR 4 to 1 usable range.
The limited rangeability of this type of meter can be overcome by adding a bypass leg around the Orifice, which has a pressure drop equal to the drop across the Orifice and bypassed flow is directly proportional to the mainline flow. This results in a meter, which has a linear function and not a square root. Since the flow meter has a 10 to 1 range, the Orifice meter will now have a 10 to 1 linear flow range, which is as easy to read at the bottom as the top of the scale. This principle can be used virtually any pipeline size, handling almost any flow rate and metering either liquid OR gas. It is one of the most versatile meters available. Its only limitation is that the solid particles OR slurries can clog the Orifice and prevent proper operation.
Bypass metering components:
The Bypass flow meter consists of a mainline Orifice plate, a set of Orifice Flanges Or a carrier ring, a flow meter, range Orifice, Bypass piping with Isolation Valves.
(i) Mainline Orifice Plate: - The purpose of the mainline orifice plate is to create the pressure differential. As the flow rate varies across this mainline orifice, the pressure differential across this plate also varies and it is this variation, which makes the meter function.
(ii) Orifice Flanges / Carrier Ring: - Orifice flanges / carrier rings serve two functions. First, they hold the mainline orifice plate in the proper position within the pipeline. Secondly they channel a portion of the flow out of the high-pressure side of the orifice and return it again to the low-pressure side. This function may be accomplished with other than flange taps. The Bypass piping may be connected directly to the pipeline in the form of pipe taps, radius taps OR vena contracta taps.
(iii) Variable Area Flow Meter: - As discussed above, a flow meter (Glass Tube Rotameter) is placed in the Bypass piping to measure the bypass flow.
(iv) Range Orifice: - The range orifice is a small orifice placed in the bypass piping either before OR after the flow meter. This orifice is sized so that the total pressure drop through the bypass-piping equals the pressure differential across the mainline orifice.
(v) Bypass Piping: - The bypass piping connects the upstream side of the mainline taps to the inlet side of the flow meter and the downstream side of the mainline taps to the outlet of the flow meter. Since the pressure drop through the bypass piping has a direct bearing on the accuracy of the flow meter, it is necessary to keep the bypass piping as short as possible with as few fittings as possible.
Contact Name: Nilesh Shah
Company Name: Flowtech Instruments Services
Street Address: 900/2/B GIDC Nr.
Pakona Engg, Makarpura Road, Vadodara,
390010 Gujarat, India
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