PD (POSITIVE DISPLACEMENT) FLOWMETERS FOR LIQUIDS - Operating principle


"Petrol" PD flowmeters are typical volumetric instruments which directly measure the quantity of flowing liquid by means of a couple of rotors. These instruments allow an accurate measurement either of the flow rate either of the volumes flown (integrated flow rate) of practically all the liquids industrially utilized.

They may be used for custody transfer purposes, operating controls and for the many other needs of the petroleum, chemical, pharmaceutical industry, etc.

The two rotors, engineered and manufactured in such a way they never touch each other nor the measuring unit components, are alternatively driven by the timing gears coupled on rotor's shaft just outside the measuring chamber.

As show in the operating scheme, the liquid flowing inside the PD flowmeter pushes the rotor on the right , which is in condition of unbalanced pressure, to rotate clockwise; contemporarly the rotor on the left, being driven by the timing gears, is forced to rotate counterclockwise.

After a 90 degrees rotation, the rotors are in a reciprocal position when compared to the initial one and therefore it is the rotor on the left which, pushed from the liquid, rotates counterclockwise driving, through the timing gears, the rotor on the right.

During the operating cycles the volumes which are generated between the rotors and the components of the measuring chamber determine the "base volume" of the instrument.

A complete rotors rotation generates four "base volumes". By transmitting the number of rotors rotations to the register the volume flown through the PD flowmeters is displayed.

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PD (POSITIVE DISPLACEMENT) FLOWMETERS FOR LIQUIDS - Main characteristics

"PETROL" is the unique european maker who has developped the Roots principle for the volumetric measurement of liquids.
Since its commercial debut this PD flowmeter has contributed to the automation and modernization of the industrial processes for its reliability and its exceptional operating characteristics coupled with a very high manufacturing technique. "PETROL" PD flowmeters are today available for the measurement of all the liquids industrially used in a range of models so wide, either in the standard version either in the jacketed version, to meet, even contemporarly, operating specifications as severe as:
- max operating pressures till over 150 BAR;
- max operating temperatures till over 250 C.
Flow rate ranges from few hundred of litres/hour to 1800 m3/h, with equipment sizes from 1 to 16".
Several manufacturing and operating features, hereinafter evidenced, have permitted to "PETROL" PD flowmeters to be considered a practically unreplaceable equipment in many fields of application and have also allowed its use in many different industrial sectors.

A) DOUBLE CASE CONSTRUCTION
The measuring unit is removable, as a single subass'y, from the outer housing flanged to the piping and may be maintained as well as functionally rechecked independently from the outer housing.

B) "FLOATING" TYPE ROTORS
Rotors never touch each other but are synchronized from the timing gears mounted outside the measuring chamber. Rotors are not therefore subject to wear and PD flowmeter does not need re-calibration with the time due to wearing.

C) CONSTANT PROFILE CUTTERS FOR ROTORS MACHINING
This type of rotors machining permits to realize, between the PD flowmeter "base volume" components, "tolerances" or "clearances" suitable for the accurate measurement either of extremely viscous liquids either of mediums at high operating temperatures.

D) CONSTRUCTION MATERIALS
Any type of construction material may be used as aluminum, cast iron, carbon steel, stainless steel in the various grades available and plastic materials as "moplen" either for the measuring unit either for the outer housing flanged to the piping, in accordance to specific needs.

E) MAGNETIC TRANSMISSION
With this type of movement transmission, used in the majority of equipment produced, the sealing between the wetted parts and the dry parts of PD flowmeter is of static type which fully guarantees against any leakage of the flowing liquid.

F) EXCELLENT ACCURACY
0,2% of reading for custody transfer purposes, 0,5% of reading for general purposes, anyway in accordance with the specific approvals issued by Italian Ministery of Industry and Commerce for their use in Italy and in the European Common Market Countries.

G) LONG LIFE AND VERY REDUCED MAINTENANCE
Rotors never touch each other and therefore are not subject to wear. More over double case construction allows to maintain the measuring unit without removing the entire PD flowmeter from the line.

H) WIDE RANGE OF ANCILLARY EQUIPMENT
Electronic or pneumatic pulse transmitters and frequency to current converters are available for remote totalization, recording and presetting of metered volumes. Local or remote preset counters, both with one or two stages closure cycles, togetHer with automatic temperature compensators, mechanic or electronic type, complete the range of PD flowmeter ancillary equipment. All the range of protective devices such as strainers, deaerators, max flow limiting and one or two stages closures valves are also available.

During the years "PETROL" has accumulated a very strong experience in the production of PD flowmeters suitable for the heaviest operating conditions in the more different industrial fields such as: the measurement of blood, of phtalic and of maleic anhydride, of crude oil on well-heads and of heavy oils at high temperatures.
Such a technical background and the investments yearly destined to the research in the field of the volumetric measurement of liquids have till now satisfied each particular need of the customers. Hereinafter are listed the most significant references for crude oil extration industry and for power generation plants.

CAST AN FABRICATED BODY BASKET TYPE STRAINERS

To protect the metering mechanism of the own PD (Positive Displacement) meters from the foreign solid particles eventually carried on by the liquid to be metered Petrol Metering Srl produces basket type strainers of sizes ranging from 1" through 16". The outer body of the strainers up to 4" size is cast while for 6" size and over is fabricated.
Standard body materials are cast steel and stainless steel Aisi 304 or Aisi 316 while for all the sizes the internal screener is in Aisi 304 or Aisi 316. Standard mesh of the screener are 40,80 and 200 but other mesh are available on request. Fabricated strainers are available in "tailored" versions for the input and output connections in accordance to specific needs. Both cast and fabricated strainers are available for operating pressure up and over 100 bar and operating temperature up and over 200 C. Both cast and fabricated strainers may be supplied with PED certification. Fabricated strainers may be engineered to meet ASME VIII Div. 1 requirements. Petrol Metering Srl strainers may be of course supplied as loose equipment to other metering skids and systems manufacturers.

METERING SKIDS AND SYSTEMS FOR LIQUIDS AND GASES

Due to the experience acquired with the production of PD (Positive Displacement) meters Petrol Metering Srl can supply since several years metering skids and metering systems for both liquids and gases as a result of the very close co-operation with engineering and manufacturing sister companies.
The most important item of both skids and systems is of course the flow-meter which can be of various typology in relation to the client needs or request but their manufacturing philosophy is practically indipendent by the flow-meter typology thus permitting to our Company to be in very favourable position in this industrial sector.
Petrol Metering Srl
Via Delle Valli, 25 - Italy 04011 Aprilia (LT) - Phone: ++39.06.92727658 - Fax: ++39.06.92860025
Info and general contact: petrolms@petrolms.it
P.I. 02068390596 - C.F. 06381091005 - REA 000000 - Reg. Imp. 00000 - Cap. Soc. 00,000.00 € i.v.